**Emissions Control Catalysis**

Special Issue Editors

**Ioannis V. Yentekakis Philippe Vernoux**

MDPI • Basel • Beijing • Wuhan • Barcelona • Belgrade • Manchester • Tokyo • Cluj • Tianjin

*Special Issue Editors* Ioannis V. Yentekakis Technical University of Crete Greece

Philippe Vernoux Universite de Lyon ´ France

*Editorial Office* MDPI St. Alban-Anlage 66 4052 Basel, Switzerland

This is a reprint of articles from the Special Issue published online in the open access journal *Catalysts* (ISSN 2073-4344) (available at: https://www.mdpi.com/journal/catalysts/special issues/ emissions catalysis).

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### **Contents**




### **About the Special Issue Editors**

**Ioannis V. Yentekakis** is currently a Full Professor of Physical Chemistry and Catalysis, School of Environmental Engineering, Technical University of Crete (TUC), Greece. He received a Chemical Engineering Diploma (1983) and a Ph.D. (1988) from the University of Patras (UP). His academic career is associated with Princeton University (Post-Doc), ICE-HT/FORTH (Senior Researcher), the Department of Chemical Engineering UP (Assistant Professor), and Cambridge University, U.K. (Visiting Professor). His current interest concerns development of novel nano-structured catalysts and eco-friendly processes with applications in added-value chemicals production, environmental protection, renewable energy generation, fuel cells, hydrogen energy, CO2 hydrogenation, natural gas (NG), and biogas valorization. He has authorized more than 110 journal publications (>4500 citations, h-index = 38), 3 international patents, 10 books, and has Guest Edited 5 Special Issues. He has served as department chairman, member of the University Council and the Senate, and chairman or organizer of many scientific conferences. He has coordinated more than 30 funded research projects.

**Philippe Vernoux** is a senior researcher at CNRS (Ph.D. in Electrochemistry in 1998 and habilitation in catalysis in 2006). Dr. Vernoux has published more than 130 refereed papers, 18 patents in domains of solid state electrochemistry, environmental catalysis, and electrochemical promotion of catalysis (H-index = 32, >3400 citations). Dr. Vernoux has supervised 22 Ph.D. students and 20 post-docs. He has Guest Edited 5 Special Issues and gave 39 invited lectures and 180 oral communications. He has strong experience in the coordination of national and international projects (EFEPOC Marie Curie Project, ADEME PIREP, ANR PIREP2, EPOX, DYCAT, etc.). His current interest concerns development of catalysts and electrocatalysts for vehicle exhaust treatment, air cleaning, wastes valorization, and H2 production. He is currently the head of the CARE group (Catalytic and Atmospheric Reaction for the Environment) of the Institute of Researches for Catalysis and Environment of Lyon in France.

### *Editorial* **Emissions Control Catalysis**

#### **Ioannis V. Yentekakis 1,\* and Philippe Vernoux <sup>2</sup>**


Received: 1 October 2019; Accepted: 21 October 2019; Published: 31 October 2019

#### **1. Overview**

Important advances have been achieved over the past years in agriculture, industrial technology, energy, and health, which have contributed to human well-being. However, some of these improvements in our lives were accompanied by a great threat to the environment and public health, with photochemical smog, stratospheric ozone depletion, acid rain, global warming, and finally climate change being the most well-known major issues, as a result of a variety of pollutants emitted through these human activities. The indications are that we are already at a tipping point that might lead to non-linear, sudden environmental change on a global scale.

Aiming to ensure that "we live well, within the planet's ecological limits" the United Nation's Sustainable Development Goals [1] and the EU's Environmental Action Plan [2] include calls to action and priority objectives to protect life on land and water and tackling climate change. In a full harmony, scientists around the world are developing tools and techniques to understand, monitor, protect, and improve the environment, and to ensure that our achievements must not adversely affect our environment and at the same time we must mitigate any damage that has already occurred. In turn, new tools, technology, and advanced materials are continuously added to our quiver that enable us to effectively control emissions, either of mobile (e.g., cars) or stationary (industrial) sources, and to improve the quality of outdoor and indoor air, with catalysis to play a major role on these efforts, i.e., as a leading technology for improving quality of life, health, and environment.

Emissions Control Catalysis in the area of Environmental Catalysis [3,4] is continuously growing up, providing novel multifunctional, nano-structured materials with active metal species at sizes of the level of nanoparticles, nanoclusters or even single atoms [5–8]. These materials are also often promoted by several ways (i.e., by surface- or support-induced promotion [9–13] and by electrochemical promotion [14–16], among others) in order to be very active and selective for the abatement of a variety of pollutants and greenhouse gases. Regarding the latter, representative cases concern the abatement of CO, NOx, N2O, NH3, CH4, higher hydrocarbons, volatile organic compounds (VOCs), particulate matter (PM), and specific pollutants emitted by industry (e.g., SOx, H2S, dioxins, and aromatic hydrocarbons) or landfill and wastewater treatment plants (biogas).

The concept of Cyclic Economy is currently of growing attention in emission control catalysis strategies for the production of useful chemicals and fuels from the controlled pollutants. The CO or CO2 hydrogenation for the production of CH4 or liquid fuels, biogas or methane reforming for the production of syngas, and/or H2, oxidative coupling of methane for the production of ethylene are a few examples of such efforts.

#### **2. Special Issue Contributions and Highlights**

The present "Emissions Control Catalysis" Special Issue was successful in collecting 21 high-quality contributions by several research groups around the world, which covers recent research progress in the field of the catalytic control of air pollutants emitted by mobile or stationary sources, not limited only to the abatement of pollutants but also including research dedicated to cyclic economy strategies. These papers among others are ranging from the synthesis and physicochemical-textural-structural characterization of the materials, activity–selectivity–durability evaluation under environmentally important reactions, fundamental understanding of structure–activity relationships or other metal–metal and metal–support interactions on the multifunctional materials involved. In particular, three comprehensive review articles covering several major topics and directions in emissions control catalysis subject were published in this special issue. The review of the guest editors (Yentekakis I., and Vernoux P.) and co-workers [17] concerns mainly the of CO, CHs, and NOx emissions abatement from stoichiometric, lean burn, and diesel engines exhausts, addressing the literature that concerns the electropositive promotion by alkalis or alkaline earths of platinum group metals (PGMs) that have found to be extremely effective for the related three-way and lean-burn reactions and catalytic chemistry. The review of Smirniotis P. and co-workers [18] concerns the selective catalytic reduction of NOx with NH3 (NH3-SCR of NOx) focusing to low temperature applications that is a highly desirable perspective, and finally the review of Bogaerts A. and co-workers [19] covering a hot recent trend in emissions control implicating cyclic economy strategies, that is the conversion and utilization of CO2 for the production of value-added chemicals. On the other hand, a major part of contributions (9/21) concerns original research on nitrogen oxides reduction processes [20–28], reflecting the fact that this topic still remains hot among the targets of environmental catalysis. Five out of 21 studies concern CO and hydrocarbons oxidation processes [29–33] while the remained 4/21 concern CO2 capture/recycling processes under the view of cyclic economy [34–37].

#### *Review Contributions*

Alkali metals as surface-promoters of platinum group metals (PGMs) have shown remarkable promotional effects on the vast majority of reactions related to the emissions control catalysis either they are applied by traditional routes (e.g., impregnation) or electrochemically, via the so called, concept of the electrochemical promotion of catalysis (EPOC) or non-faradaic electrochemical modification of catalytic activity (NEMCA effect) [14,15]. In the comprehensive review of Yentekakis et al. [17] published in this Special Issue, more than 120 papers on the subject were collected and discussed comparatively after classification in groups, on the basis of the specific reaction promoted, e.g., (CO, HCs, or H2)-SCR of NOx, and CO or hydrocarbons oxidation. The authors also present and analyze, by means of indirect (kinetics) and direct (spectroscopic) evidences, a mechanistic model for the mode of action of electropositive promoters, which consistently interprets all the observed promoting phenomena. Concluding, they claim that this very pronounced (in some cases extraordinarily) alkali-induced promotion in emissions control catalysis prompts to the development of novel catalyst formulations for a more efficient and cost-effective control of the emissions of automotives and stationary combustion processes.

Since the emission standards for NOx are becoming more stringent to keep our atmosphere clean and severe pollution regulations being imposed around the world, the challenge of a cost-effective, i.e., low-temperature, selective catalytic reduction (LT-SCR) of NOx by NH3 with a combination of high NOx activity and N2 selectivity in a wide operation temperature window and good resistance to SO2/H2O have attracted paramount attention. Supported and mixed transition metal oxides have been widely investigated for LT-SCR technology. However, these catalytic materials have some drawbacks, especially in terms of catalyst poisoning by H2O or/and SO2. Hence, the development of catalysts for the LT-SCR process is still under active investigation throughout seeking better performance. Extensive research efforts have been made to develop new advanced materials for this technology. The comprehensive review of Smirniotis and co-workers [18] collected and comparatively analyses

more than 140 publications in the topic, covering the description of the influence of operating conditions, materials, and promoters on the LT-SCR performance, as well as active sites, reaction intermediates, and mechanistic implications evidenced by using isotopic labeling and in situ FT-IR studies.

Global climate change as a result of the greenhouse effects of the increasing emissions of the so-called greenhouse gases, with CO2 to be the most representative one, is of major concern currently. CO2 capture, conversion, and utilization is one of the possible solutions to reduce CO2 concentration in the atmosphere. Among other methods, this can be accomplished by direct catalytic hydrogenation of CO2, producing value-added products. In their comprehensive review, Bogaerts A. and co-workers [19], focused mainly in the last 5-years progress on the topic summarized the literature research (~125 papers) and the current priorities on CO2 hydrogenation to CO, CH4, CH3OH, DME, olefins, and higher hydrocarbons value-added chemicals by heterogeneous catalysis and plasma catalysis. Although, its energy efficiency is still deterring for commercialization, plasma catalysis has currently attracted sufficient attention, due to its simple operating conditions (ambient temperature and atmospheric pressure) and unique advantages in activating inert molecules; its potential advantages and current limitations on CO2 hydrogenation process have been accounted in the review.

#### **3. Original Contributions**

#### *3.1. NOx Abatement Related Results*

A major part of the contributions (9/21) involves research on the selective catalytic reduction (SCR) of NO and/or its direct catalytic decomposition. In particular:

Shen D. and co-workers [20] investigated the deNOx activity on a series of bimetallic Cu–Mn molecular sieve catalysts (Cu–Mn/SAPO-34) with different loadings of Cu and Mn components during the selective catalytic reduction (SCR) of NO with NH3 at low temperatures (ca. 120–330 ◦C), including the effects of H2O vapor and/or SO2. Among the catalysts tested, the performance of 2 wt% Cu-6 wt% Mn/SAPO-34 one found to be superior, achieving 72% NO conversion at 120 ◦C and even better (90%) at 180–330 ◦C. The reversible negative effect of H2O on NO conversion was attributed to the competitive adsorption of H2O and NH3 on Lewis acid sites; this poisoning was diminished upon increasing the reaction temperature to 300 ◦C. A permanent poisoning effect of SO2 on deNO*<sup>x</sup>* activity found is strongly dependent on the reaction temperature, becomes more pronounced at lower ones and is further enhanced by H2O co-feed; this is assigned to the formation of (NH4)2SO4, which results in the plug of active sites and a decrease of surface area.

Gao Y. and co-workers [21] synthesized Mn-Co/TiO2 and Mn-Fe/TiO2 nanocatalysts by a hydrothermal method which were characterized by a variety of methods including Brunner–Emmet–Teller (BET)/Barrett–Joyner–Halenda (BJH) analysis of N2 adsorption/desorption isotherms at −196 ◦C, transmission electron microscope (TEM), X-ray diffraction (XRD), H2-temperature-programmed reduction (TPR), NH3-temperature-programmed desorption (TPD), and X-ray photoelectron spectroscopy (XPS) that enabled the authors for a comprehensive comparison of the catalysts nanostructure characteristics and their de-NOx catalytic performance, gained insight into the structure-activity relationships. The Mn–Co/TiO2 catalyst offered superior structure characteristics than Mn–Fe/TiO2: higher surface area and active components distribution, diminished crystallinity, reduced nanoparticle size and also higher Mn4+/Mnn<sup>+</sup> ratio, confirming its better oxidation ability and larger amount of Lewis and Brønsted acid sites on the Mn–Co/TiO2 surface. As a result, Mn–Co/TiO2 nanocatalyst displayed superior SCR of NO with NH3 on both activity and selectivity in the temperature range of 75–250 ◦C. Kinetics data revealed that both Eley–Rideal (E–R) and Langmuir–Hinshelwood (L–H) mechanisms were implicated in NH3-SCR process over Mn–Fe/TiO2 and Mn–Co/TiO2 catalysts.

Han J. and co-workers [22] investigated the enhanced deNOx performance and stability of sulfated sintered ore catalysts (SSOC) during the selective catalytic reduction of NOx with NH3. The maximum deNOx efficiency found was about 92% at 300 ◦C, NH3/NO = 1 and 5000 h−<sup>1</sup> gas hourly space velocity (GHSV). A systematic characterization of the materials by means of X-ray

fluorescence spectrometry (XRF), Brunauer–Emmett–Teller (BET) analyzer, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and diffuse reflectance infrared spectroscopy (DRIFTS) was conducted to provide an in depth understanding of the NH3-SCR reaction mechanism and to explain the denitration performance and stability of SSOC. The existence of more Brønsted acid sites at the surface of SSOC found to be responsible for the improved adsorption capacity of NH3 and NO over the SSOC surface that accomplished the formation of amide species (–NH2), NH4 <sup>+</sup> species, NO2 molecules in a gaseous or weakly adsorbed state, and nitrates. The reaction between –NH2, NH4 <sup>+</sup>, and NO (E–R mechanism) and the reaction of the coordinated ammonia with the adsorbed NO2 (L–H mechanism) were attributed to NOx reduction.

Song C. and co-workers [23] studied the promotional effect of Ce and/or Zr incorporation by ion exchange on Cu/ZMS-5 catalysts for the NH3-SCR of NO. The cerium and zirconium addition promotes the activity of catalysts; the cerium-rich catalysts exhibiting superior SCR activities compared to the zirconium-rich ones. The improved low temperature activity of the CuCe*x*Zr1−*x*O*y*/ZSM-5 catalysts in comparison to the unpromoted Cu/ZSM-5 (the former achieving >95% NO conversion at 175–468 ◦C, the later at 209–405 ◦C) was attributed to an increase of the reactive lattice oxygen content and reducibility of the catalysts via the Ce<sup>3</sup>+/Ce4<sup>+</sup> redox couple and its interaction with cooper species. Moreover, the presence of zirconium in the catalysts promotes surface copper enrichment, prevents copper crystallization and causes suppression of N2O formation, increasing N2-selectivity of the system.

He H. and co-workers [24] synthesized CeZr0.5TiaO*<sup>x</sup>* (with a = 0, 1, 2, 5, and 10) catalysts by a stepwise precipitation approach, which were studied on the NH3-SCR of NOx. In all contents, Ti addition was beneficial to the deNOx catalytic performance. Superior behavior was obtained by the CeZr0.5Ti2O*<sup>x</sup>* particular catalyst composition. Controlling pH and precipitation time during the two steps involved in the preparation method enabled the authors to achieve a catalyst with enhanced acidity (favorable for NH3 adsorption in NH3-SCR processes) and high dispersion of CeO2 onto the surface of ZrO2-TiO2 synthesized first. The as-prepared CeZr0.5Ti2O*<sup>x</sup>* catalyst was characterized by superior redox properties, enhanced adsorption and activation of NOx and NH3 and enhanced surface adsorbed oxygen such as O2 <sup>2</sup><sup>−</sup> and O<sup>−</sup> belonging to defect-oxide or a hydroxyl-like group; all these factors positively affecting its SCR of NOx with NH3 performance.

Olson L. and co-workers [25] investigated the poisoning effect of a phosphorous containing atmosphere on the NOx storage capacity of a Pt/Ba/Al2O3 structured (i.e., washcoated on a ceramic monolith) catalyst. A significant loss of the NOx storage capacity was caused by phosphorous exposure characterized by a progressively decreasing axial distribution of phosphorous concentration from the inlet to the outlet of the monolith. The values of the specific surface area and pore volume of phosphorous-poisoned monolithic catalysts followed an inverse order: lower at the inlet, higher at the outlet of the monolith. Additional features of the axial phosphorous accumulation detected were: a higher surface accumulation at the inlet of the monolith mostly appeared in the form of P4O10, the presence of more metaphosphate (PO3 −) in the middle section of the monolith, and a less surface accumulation of phosphorous at the outlet of the monolith due to its extended diffusion into the washcoat. In respect to the poisoning effect of phosphorous on the SCR of NOx it was revealed that the formations of N2 and N2O were decreased in favor of NH3 production; the reaction is more influenced by the phosphorous poisoning than the ammonia formation from the stored nitrates.

The direct NO decomposition activity on PdO or PtO supported on Co3O4 spinel was studied by Reddy et al. [26] in an attempt to discover means of enhancing the activity of Co3O4 spinel, one of the most active single-element oxide catalysts for NO decomposition at high temperatures (typically > 650 ◦C). In fact, the authors demonstrated a four-fold higher promotion on the NO decomposition activity of PdO- rather than of PtO-modified Co3O4 at 650 ◦C, accompanied by superior selectivity towards N2 as well. Structural and surface analysis measurements using a variety of methods (e.g., XRD, XPS, H2-TPR, and in situ FT-IR) showed an enhanced reducibility of PdO/Co3O4 with an increased thermal stability of surface adsorbed NOx species, both considered to contribute on

the promotion observed. In opposite, PtO enters into the Co3O4 structure, without notable influences on the redox and NO adsorption properties of Co3O4, resulting in marginal promotion compared to PdO. The PdO promotion followed volcano behavior with an optimal PdO loading of 3 wt%.

Zhang et al. [27] investigated the NO*<sup>x</sup>* storage capacity of a series of Pd/BEA catalysts with various Pd loadings for cold-start applications. In situ FTIR measurements using CO and NH3 enable the authors to identify two isolated Pd2<sup>+</sup> species, Z−-Pd2+-Z<sup>−</sup> and Z−-Pd(OH)+, on exchanged sites of zeolites, as the main active sites for NO trapping. Among these active sites a superior NO*<sup>x</sup>* storage capacity of Z−-Pd2+-Z<sup>−</sup> was demonstrated, which is caused by the different resistance to H2O. Atom utilization of Pd can be improved by using lower Pd loading, with an optimum at 0.5 wt%, since this leads to a sharp decline of Z−-Pd(OH)<sup>+</sup> attributed to the 'exchange preference' for Z−-Pd2+-Z<sup>−</sup> in BEA.

Finally, Ingel et al. [28] in a different approach for controlling N2O emissions in ammonia oxidation process at high temperature, proposed the design of experiments and response surface methodology to study this process. The reactor's load, the temperature of reaction and the number of catalytic gauzes were selected as independent variables, whereas ammonia oxidation efficiency and N2O concentration in nitrous gases were assumed as dependent variables (response). Statistically significant mathematical models were developed from the achieved results, which describe the effect of independent variables on the analyzed responses. The ammonia oxidation efficiency value depends on the reactor's load and the number of catalytic gauzes but not on the temperature in the studied range (870–910 ◦C). The concentration of N2O in nitrous gases depends on all three parameters. The developed models were used for the multi-criteria optimization with the application of desirability function. Sets of parameters were achieved for which optimization assumptions were met: maximization of ammonia oxidation efficiency and minimization of the N2O amount being formed in the reaction. As authors claim, the presented methodology can be used to minimize the primary N2O emission at high ammonia oxidation efficiency. It can be applied for optimization of operating parameters of ammonia oxidation reactor with two types of catalysts: catalytic gauzes and catalyst for high temperature of N2O decomposition. As a result, it is possible to obtain the set of independent variables ensuring low N2O emission and to meet the binding environmental regulations.

#### *3.2. CO, CH4, and Other Hydrocarbons Oxidation Reactions*

Avgouropoulos and co-workers [29] synthesized a series of atomically dispersed copper-ceria nanocatalysts via appropriate tuning of a novel hydrothermal method and investigated their activity on CO oxidation, which was found to be strongly dependent on the nanostructured morphology, oxygen vacancy concentration, and nature of atomically dispersed Cu2<sup>+</sup> clusters. A number of techniques including electron paramagnetic resonance (EPR) spectroscopy, X-ray diffraction (XRD), N2 adsorption, scanning electron microscopy (SEM), Raman spectroscopy, and ultraviolet-visible diffuse reflectance spectroscopy (UV-Vis DRS) were employed in the characterization of the synthesized materials. The aim was to find the key factors that govern the physicochemical properties of the synthesized materials during preparation and then to provide convincing structure-activity correlations. Elevated temperatures and low concentrations of NaOH (≤0.1 M) during preparation have led to more active ceria-based catalysts in CO oxidation. This was explained with the obtained morphology, the nature of oxygen vacancies and dispersed copper species, and to a lesser extent, with the specific surface area of the materials and the concentration of defects.

Briois and co-workers [30] in an attempt to provide means of replacing expensive platinum group metals with cost-effective perovskite type materials for catalytic oxidation reactions, prepared thin catalytic coatings of Sr and Ag doped lanthanum perovskites, La1−*x*−*y*Sr*x*Ag*y*CoO3−<sup>α</sup> (*x* = 0.13–0.28, and *y* = 0.14–0.48), on alumina substrates by using the cathodic co-sputtering magnetron method in reactive condition. Such thin porous catalytic film arrangements can optimize the surface/bulk ratio by combining a large gas exposure surface area with an extremely low loading, thus saving raw materials. The sputtering method was optimized to generate crystallized and thin perovskite

films. The authors found that high Ag contents has a strong impact on the morphology of the coatings, favoring the growth of covering films with a porous wire-like morphology that showed a good catalytic activity for CO oxidation. Interestingly, the optimal composition (La0.40Sr0.1Ag0.48Co0.93O3) displayed similar catalytic performance than this of a Pt film, and was also efficient for CO and NO abatement in a simulated Diesel exhaust gas mixture, demonstrating the promising catalytic properties of such nanostructured thin sputtered perovskite films.

Methane, a substantially more potent greenhouse gas than CO2, is the main compound of natural gas, which is lately used with a continuous increased rate in various industrial processes and as an alternative fuel for heavy-duty transportation not excluding light-duty tracks. As a consequence, control of CH4 emissions via catalytic deep oxidation has attracted considerable renewed attention, given that methane appears the lowest reactivity among alkanes. Under this view, Iliopoulou and co-workers [31] synthesized a series of novel Co–Ce mixed oxide catalysts in an effort to enhance synergistic effects that could improve their redox and oxygen storage properties and, thus, their activity in methane deep oxidation. The effect of the synthesis method (hydrothermal or precipitation) and Co loading (0, 2, 5, and 15 wt%) on the catalytic efficiency and stability was investigated. Hydrothermally synthesized Co3O4/CeO2 catalysts appeared superior performance due to their improved physicochemical properties (smaller crystallite size, larger surface area, and enhanced reducibility). In respect to Co loading, the optimum performance was observed over a 15 wt% Co/CeO2 catalyst, which also presented sufficient tolerance to water presence.

Based on the fact that Pd is one of the most active catalysts for complete methane oxidation Baranova and coworkers [32] used the concept of electrochemical promotion of catalysis (EPOC) to further promote the reaction over palladium nano-structured catalysts deposited on yttria-stabilized zirconia (YSZ) solid electrolyte. Anodic polarization (O2<sup>−</sup> supply to the catalyst) resulted to a rate enhancement up to ~3 at 450 ◦C with an apparent Faradaic efficiency as high as 3000 (for a current application as low as 1 μA). Electrochemical promotion on this catalytic system showed persistent behavior the catalyst remained under promotion for a long period of time after interruption of the external bias induced EPOC. Increasing polarization time resulted in a longer-lasting persistent promotion (p-EPOC); more time was required for the reaction rate to reach its initial un-promoted value. The phenomenon was attributed to the continuing promotion by the stored oxygen in palladium oxide formed during the anodic polarization.

Taking into account that sulfur poisoning is one of the most important factors deteriorating the efficiency of diesel exhaust after-treatment systems and that bare TiO2 appears high sulfur resistivity, Zhang et al. [33] prepared TiO2–CeO2 composites by co-precipitation and studied their sulfur resistance and catalytic activity in the oxidation of diesel soluble organic fraction (SOF). They found that TiO2-modification of CeO2 significantly improves the catalytic SOF purification efficiency of CeO2 besides the fact that this ceria doping does not downgrade the excellent sulfur resistance of bare TiO2; the prepared TiO2–CeO2 exhibited superior sulfur resistance than e CeO2 and commercial CeO2–ZrO2–Al2O3. TiO2–CeO2 characterization by X-ray diffraction (XRD) and Raman spectroscopy indicate that cerium ions can enter into the TiO2 lattice, without forming complete CeO2 crystals. Moreover, as confirmed by XPS and H2-TPR, the synthesized TiO2–CeO2 composites appeared enhanced oxygen storage capacities (OSC) that considered responsible for their better SOF oxidation activity.

#### *3.3. CO2 Capture*/*Recycling: Combining Emissions Control with Added-Value Chemical Production (Cyclic Economy)*

De Lucas-Consuegra and co-workers [34] developed a low-temperature (below 90 ◦C) proton exchange membrane (Sterion) electrochemical cell for the electrocatalytic conversion of gaseous CO2 to liquid fuels. This novel system achieved gas-phase electrocatalytic reduction of CO2 over a Cu-based cathode by using water electrolysis-derived protons generated in-situ on an IrO2 anode. Three Cu-activated carbon cathodes with varying Cu loading (10, 20, and 50 wt% Cu–AC), and thus particle size, were tested. Products distribution was a function of the Cu loading and particle size

of the Cu–AC cathode; methyl formate, acetaldehyde, and methanol were being the main reaction products, respectively, over 50, 20, and 10 wt% Cu–AC. The membrane electrode assembly (MEA) containing the cathode with the largest Cu loading and particle size (50 wt% Cu–AC, 40 nm) showed the highest CO2 electrocatalytic activity per mole of Cu (and the lowest energy consumption values for the conversion of CO2, 119 kW·h·mol<sup>−</sup>1), which was attributed to the lower Cu–CO bonding strength over large Cu particles.

Wang H., Lu J.-X. and co-workers [35] fabricated an electrocatalytic cell consisting of a 0.5 M KHCO3 aqueous solution as electrolyte saturated with CO2 by bubbling, a CuO/TiO2-Nafion as working electrode, and Pt as counter and reference electrodes in order to study the electroreduction of CO2 to added-value multi-carbon oxygenate products (ethanol, acetone, and n-propanol). The non-noble metal electrocatalyst CuO/TiO2 was in situ reduced to Cu/TiO2, which efficiently catalyzed CO2 reduction, offering a maximum overall faradaic efficiency of 47.4% at a potential of −0.85 V vs. reversible hydrogen electrode (RHE). The catalytic activity for CO2 electroreduction was strongly dependent on the CuO contents of the catalysts as-prepared, resulting in different electroactive surface areas. The significantly improved CO2 reduction activity of CuO/TiO2 was attributed to the high CO2 adsorption ability of TiO2 component of the working electrode.

Since hydrogen is currently considered as an efficient and environmentally benign energy carrier, among others, considerable attention is played by scientists worldwide for its sufficient production from hydrocarbon feedstocks (natural gas (NG), liquefied petroleum gas (LPG), etc.), biogas and bio-alcohols. To this end, Tang D. and co-workers [36] applied chemical looping reforming (CLR) as a prospective alternative for hydrogen production via ethanol steam reforming, which is characterized by energy efficiency and inherent CO2 capture. Taking into account that oxygen carriers (OCs) with sufficient oxygen mobility and sintering resistance still remain the main challenges for the development of high-performance materials in the CLR process, the authors explore the performance of Ni/CeO2 nanorod (NR) synthesized by a hydrothermal method as an OC in the CLR of ethanol. Using a variety of characterization techniques, they showed that the as-prepared Ni/CeO2-NR possesses the desired properties for CLR, i.e., high Ni dispersion, abundant oxygen vacancies, and strong metal-support interaction, all factors improving catalytic activity. Testing the material in CLR process successfully offered a H2 selectivity of 80% in 10-cycle stability test. The authors concluded that the small particle size and abundant oxygen vacancies contributed to improve water gas shift reaction, the high oxygen mobility of CeO2–NR effectively eliminated surface coke on the Ni particle, and the covered interfacial Ni atoms closely anchored on the underlying surface oxygen vacancies on the (111) facets of CeO2–NR enhance the anti-sintering capability.

Ioannidou et al. [37] contributed with a detailed and comparative catalytic-kinetic study of the performance of modified X-Ni/GDC electrodes (where X = Au, Mo, and Fe), in the form of half-electrolyte supported cells, in the reverse water gas shift reaction (RWGS). The importance of the RWGS reaction (H2 + CO2 → H2O + CO) is well known, since it takes place in most of the hydrocarbon processing reactions (e.g., hydrocarbons reforming processes) as well as in co-electrolysis of H2O and CO2 (CO2 utilization) in solid oxide electrolysis cells (SOECs) yielding synthesis gas (CO + H2), as considered in the present study. Solid oxide electrolysis is a contemporary process for CO2 capture/recycling, which is proven as an attractive method to provide CO2 neutral synthetic hydrocarbon fuels. The X-Ni/GDC catalysts were tested at open circuit conditions in order to elucidate their catalytic activity towards the production of CO; one of the products of the H2O/CO2 co-electrolysis reaction. The CO production rate increases by increasing the operating temperature and the partial pressure of H2 in the reaction mixture. Fe and Mo modification enhances CO production, and 2 wt% Fe-Ni/GDC and 3 wt% Mo-Ni/GDC electrodes were superior compared to the other samples, in the whole studied temperature range (800–900 ◦C) reaching thermodynamic equilibrium. No carbon formation was detected.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

*Review*

### **Electropositive Promotion by Alkalis or Alkaline Earths of Pt-Group Metals in Emissions Control Catalysis: A Status Report**

### **Ioannis V. Yentekakis 1,\*, Philippe Vernoux 2, Grammatiki Goula <sup>1</sup> and Angel Caravaca <sup>2</sup>**


Received: 23 December 2018; Accepted: 29 January 2019; Published: 5 February 2019

**Abstract:** Recent studies have shown that the catalytic performance (activity and/or selectivity) of Pt-group metal (PGM) catalysts for the CO and hydrocarbons oxidation as well as for the (CO, HCs or H2)-SCR of NOx or N2O can be remarkably affected through surface-induced promotion by successful application of electropositive promoters, such as alkalis or alkaline earths. Two promotion methodologies were implemented for these studies: the Electrochemical Promotion of Catalysis (EPOC) and the Conventional Catalysts Promotion (CCP). Both methodologies were in general found to achieve similar results. Turnover rate enhancements by up to two orders of magnitude were typically achievable for the reduction of NOx by hydrocarbons or CO, in the presence or absence of oxygen. Subsequent improvements (ca. 30–60 additional percentage units) in selectivity towards N2 were also observed. Electropositively promoted PGMs were also found to be significantly more active for CO and hydrocarbons oxidations, either when these reactions occur simultaneously with deNOx reactions or not. The aforementioned direct (via surface) promotion was also found to act synergistically with support-mediated promotion (structural promotion); the latter is typically implemented in TWCs through the complex (Ce–La–Zr)-modified γ-Al2O3 washcoats used. These attractive findings prompt to the development of novel catalyst formulations for a more efficient and cost-effective control of the emissions of automotives and stationary combustion processes. In this report the literature findings in the relevant area are summarized, classified and discussed. The mechanism and the mode of action of the electropositive promoters are consistently interpreted with all the observed promoting phenomena, by means of indirect (kinetics) and direct (spectroscopic) evidences.

**Keywords:** platinum; palladium; Rhodium; iridium; NO; N2O; propene; CO; methane; alkali; alkaline earth; platinum group metals; deNOx chemistry; lean burn conditions; TWC; catalyst promotion; EPOC

### **List of Contents**



#### **4 Conclusions and Perspectives 68**

#### **References 69**

#### **1. Introduction**

The development of new catalyst formulations for a more efficient and cost-effective control of unburned hydrocarbons, CO and NOx pollutants emitted by mobile and stationary combustion processes is currently an urgent need [1–3]. Commercial three-way catalytic converters (TWCs) have been highly successful in controlling NOx, CO and hydrocarbon emissions from conventional stoichiometric gasoline engines. However, some problems, that emerged during their four decades of implementation, concerning the economy of their production process, their composition, their operational/life time behaviour and their recycling still need to be resolved. The most significant issues of TWC technology are the following:


In general, catalytic systems with suitable efficiency under oxygen rich conditions are a great challenge in emissions control technology. Besides lean-burn and diesel engines' effluent gases, such conditions are also typical in several stationary processes of significant environmental footprint, such as in soil fuels combustion processes for heat production in industry, in solid wastes combustion processes and certain chemical processes such as ammonia oxidation processes (e.g., nitric acid plants), adipic acid and other specific industries.

Prompted by these issues, numerous efforts are currently reported in open literature, by several research groups, on discovering means for enhancing the catalytic performance of Pt and Pd or even Rh, for reactions related to CO, NOx and hydrocarbons abatement under stoichiometric and/or oxygen rich conditions. A notable part of these studies is focussed on the reduction or replacement of the Rh usage due to its scarcity and consequently higher cost compared to Pt and Pd.

It has been recently shown that the catalytic performance of precious metals can be strongly promoted by alkalis and alkaline earths for reactions related to emissions control catalysis. Two promotion methods were implemented towards this aim: (i) the Electrochemical Promotion of Catalysis (EPOC) concept or NEMCA effect (Non-Faradaic Electrochemical Modification of Catalytic Activity), proposed three decades ago by Vayenas and co-workers (e.g., [6–13]) and (ii) the Conventional Catalysts' Promotion (CCP), applicable by means of highly dispersed (supported) catalysts, as typically used in industrial applications (e.g., [14,15]). On these bases, rational/more efficient catalyst formulations and catalytic systems have been designed with significantly widened operational and/or implementation windows.

The present review summarises and comparatively analyses these literature results, paying attention on both fundamental and practical issues emerged. Mechanistic considerations (involving the mode of action of electropositive promoters in emissions control catalytic chemistry) are described in confrontation with recent spectroscopic and surface analysis studies evidencing their rightness.

#### **2. Promotion Methodologies and Catalysts Formulations/Designs**

Materials formulations and designs, experimental setup and other specific characteristics and/or rules regarding the two promotion methods, EPOC and CCP, are briefly described below.

#### *2.1. Electrochemical Catalysts Formulations. The Electrochemical Promotion of Catalysis (EPOC) Concept*

It has been shown by Vayenas and co-workers (e.g., [6,7]) that the chemisorptive and catalytic properties of metal catalysts, in the form of thin polycrystalline porous metal films, interfaced with solid electrolytes (Xz-ionic conductors: X is the type of ion transferred, z is its positive or negative charge) are subjected to in situ controlled electrochemical promotion via external bias applications, that is, currents or voltages imposed between the catalyst metal film and a separate counter electrode deposited on the same solid electrolyte specimen, which constitute a solid state electrochemical galvanic cell schematically shown in Figure 1.

In this way, applying small electric currents (typically some tens of μA units) or potentials (typically −2 to +2 V) between the working (catalyst film) and the counter electrodes via a Galvanostat/Potentiostat (Figure 1), ions are supplied from (or to) the solid electrolyte to (or from) the catalyst-electrode surface. Ions flux direction is determined by the direction of the applied current in combination with their charge, while their flux rate by the Faraday's law, rz = I/zF (I is the applied current in A; z is the ion's charge; F is the Faraday's constant = 96,485 Cb/mol). As we shall see in detail in Section 3.3, there is compelling evidence that these ions (together with their compensating charge in the metal thus forming surface dipoles) spill over (migrate) onto the gas-exposed catalyst surface, establishing an effective electrochemical double layer on the catalytically active surface altering its electron availability (work function). This strongly affects the binding strength of chemisorbed reactants and intermediates resulting to a significant modification on catalytic reaction rate (and or selectivity), typically greatly larger than the rate of ions flux that created these modifications (i.e., non-Faradaic behaviour), the so-called NEMCA (Non-Faradaic Electrochemical Modification of Catalytic Activity) phenomenon or EPOC (Electrochemical romotion of Catalysis).

**Figure 1.** Schematic representation of a solid state electrochemical cell, through which the EPOC concept can be applied. X: type of ion transferred from the solid electrolyte to the catalyst surface and z: ion charge (e.g., O2−, Na+, K+, H+).

The EPOC was then applied to a large number of catalytic reactions [6,7,13], including reactions related to emissions control catalysis (e.g., [16–22]). Numerous EPOC studies regarding electrochemically imposed electropositive promotion by alkalis of Pt-group metals' catalysed emissions control reactions have also been reported, which are subject of the present review.

Alkali-conducting solid electrolytes such as Na<sup>+</sup> or K<sup>+</sup> substituted *β*- and *β*"-aluminas, (Na or K)1+XAl11O17+X/2 and (Na or K)1+XMXAl11-XO17, respectively [6], are used for the construction of electrochemical galvanic cells with the following configuration (Figure 1):

#### *PGM-film electrode / alkali-conducting solid electrolyte / inert counter electrode (e.g., Au)* (1)

These cells are exposed to the reacting gas mixture by means of a continuous flow well-mixed reactor, as that shown in Figure 2, described as "single-pellet" (reactor) configuration and behaved similarly to a classical Continuous-Stirred-Tank-Reactor (CSTR) [12]. Controlled amounts of promoting species (alkali cations) can therefore be in situ supplied through the solid electrolyte onto the catalyst surface, causing significant alterations on the catalytic properties of the latter.

For the sake of simplicity, inert (in respect to the studied reaction) counter and reference electrodes (e.g., Au) are typically applied in EPOC studies conducted in reactor configurations similar to this shown in Figure 2; avoiding their participation in the catalytic reaction vectors (activity, selectivity) simplifies the data analysis, interpretation and understanding. It is also worth noting that besides their role as an in situ and reversible promoting species sources, solid electrolytes involved in the configuration of EPOC galvanic cells do not actually play any additional role on the promotional phenomena obtained.

**Figure 2.** The single pellet reactor configuration for electrochemical promotion studies [12].


One can use the galvanostatic transient mode of EPOC operation, described in detail in ref. [16], in order to be able to calculate the coverage, *θ*, of the promoting species (electrochemically supplied) on the catalyst surface as a function of time. Briefly, transient galvanostatic operation concerns the application of a constant current, *I*, between the catalyst and counter electrodes via the galvanostat. In cells of the type described by the formula (1), imposition of a constant negative current corresponds to a constant flux of alkali cations to the catalyst surface at a rate equal *I*/*F* (in moles of alkali/s).

The obtained alkali coverage, *θ*Alk, can therefore be calculated from Faraday's law [6,16],

$$
\theta\_{\text{Alk}} \, \text{(\%)} = 100 \cdot (-\text{It} / \text{FN}\_{\text{O}}) \tag{2}
$$

where, *t* is the time of current (*I*) application, *F* is the Faraday's constant and *N*<sup>o</sup> is the number of catalyst active sites independently measured via surface titration. Recording the metal-solid electrolyte polarization transient during the galvanostatic experiment that is" the time variation of catalyst potential by means of a reference electrode, VWR (Figure 1), this permits establishment of the relationship *θ*Alk(VWR) between alkali coverage and catalyst potential VWR (Figure 3).

This relationship is in general dependent on gaseous composition but this dependence is relatively weak because the catalyst work function *eΦ* varies according to [6,7,11]:

$$
\varepsilon \Delta \mathbf{V}\_{\text{WR}} = \Delta (\varepsilon \Phi) \tag{3}
$$

and is determined primarily by the coverage of alkali, due to the large dipole moment of alkalis on the Pt-group metal surfaces.

Galvanostatic operation concerns a continuous supply and accumulation of alkali on the catalyst surface (Equation (2)). Current interruption at a desired alkali coverage can then provide an optimally alkali-promoted catalyst surface. Alkali species are not typically consumed during the catalytic reaction, which allows working under stable reaction conditions. Hence, the promotional effect induced by the alkali coverage remains practically constant, even after the interruption of the applied current.

**Figure 3.** Dependence of alkali coverage on catalyst potential VWR under certain reaction conditions (Reproduced with permission from Ref. [19]. Copyright 1997, Elsevier).

It is worth noting that galvanostatic EPOC operation provides a rapid method for assessing the response of the reaction rate to promoter coverage (Figure 4); it consequently offers a very quick and easy way to find, at any set of conditions, the optimal promoter loading. This information could then be used for the direct design of optimally promoted conventional catalysts formulations for practical applications (Figure 4) [21].

(ii) Potentiostatic operation (a steady-state mode of operation):

The potentiostatic mode of EPOC operation corresponds to catalyst operation under the imposition of a constant catalyst polarization (VWR) via the potentiostat. This is a steady-state mode of operation: since the desired catalyst potential VWR is imposed via the potentiostat, a very rapid transfer of the appropriate amount of alkali cations to the catalyst surface takes place, which supplies the required amount of alkali in order to develop and stabilize the externally imposed polarization value VWR; then the current is sharply vanished, asymptotically approaching the zero value.

Since the *θ*Alk(VWR) relationship between alkali coverage and catalyst potential can be obtained through a galvanostatic transient conducted at the desired reaction conditions (as described in (*i*); e.g., Figure 3), the potentiostatic mode of operation is more suitable and commonly used in EPOC studies. It allows to work under steady-state conditions at several constant catalyst potentials, that is, at several constant coverages of the promoter. The steady-state reaction rate response on promoted catalyst surfaces, upon varying of the other reaction parameters (e.g., temperature, reactants' composition, etc.) can then be studied (e.g., [6,7]).

**Figure 4.** Comparison of electrochemically promoted (EPOC) and conventionally promoted (CCP) Pt with respect to N2-selectivity under the NO + C3H6 reaction at T = 375 ◦C and [NO] = 1.3%, [C3H6] = 0.6% feed. Similar well-correlated dependences of the variation of N2-selectivity on nominal sodium coverage (*θ*Na, upper abscissa) were obtained by the two methods of promotion (EPOC and CCP) applied. EPOC was performed in a Pt/(Na)*β*"Al2O3/Au galvanic cell (Reprinted with permission from Ref. [22]. Copyright 2000, Elsevier).

2.1.2. Certain Characteristics of EPOC Concept:

(i) Reversibility of the electrochemically imposed promotional phenomena:

By reversing the direction of the applied current in the galvanic cell represented in Figure 1 or even by setting the potential value that corresponds to an alkali-free catalyst surface (Figure 3), the catalyst restores its initial un-promoted intrinsic properties. In other words, the electrochemically imposed promotional phenomena are totally reversible; EPOC provides an in situ, controlled and reversible way of catalyst promotion [6,7].

(ii) EPOC can be used as a fast probe for the optimal promoter loading determination:

Inter alia, EPOC can be successfully used as an effective research tool for evaluating the influence of a candidate promoter to a catalytic system and at the same time for determining the optimal promoter loading (coverage) at any reaction conditions of our interest. After the understanding that electrochemical and conventional promotions in catalysis, EPOC and CCP, are subjected to the same physicochemical rules (this has been proved both experimentally [21,22] and theoretically [23,24]), the findings and knowledge obtained by the application of the electrochemical promotion can be successfully applied for the design of effective conventional catalyst formulations for practical applications [21,22,25]. This strategy enabled us to design novel catalyst formulations extremely active and selective in emission control catalysis, for example, optimally alkali-promoted noble metal catalysts [21].

#### *2.2. Conventional Catalyst Formulations. The Conventional Catalysts Promotion (CCP) Method*

The wet impregnation method was used in most cases for the production of conventional type (i.e., highly dispersed on large surface area carriers), alkali-promoted noble metal catalysts. Typical carriers used were γ-Al2O3, SnO2, yttria stabilized zirconia (YSZ) and rare earth oxides (REOs: La2O3, CeO2) modified γ-Al2O3. The preparation of the conventional promoted catalysts was performed either by two subsequent impregnation steps (where the noble metal was first deposited, followed by that of the promoter) or by simultaneous, one-step impregnation, in a solution containing both the noble metal and the promoter precursors, typically nitrate but also chloride salts). When using only nitrate precursors, pre-treatment procedures typically involve a high temperature (ca. 500–600 ◦C) treatment step in air for the decomposition of the nitrates (active phase and promoter) and a subsequent treatment of the resulted catalysts under reaction conditions (i.e., under the studied reaction) for several hours or days in order to ensure stable operation. On the other hand, when a chloride precursor is used, pre-treatment in H2 flow at temperatures typically 400–450 ◦C for several hours (>2 h) is applied as a first step for the precursor decomposition and the removal of the residual chlorine (e.g., [14,15]); the inhibitory behaviour of the latter on the reactions related to emissions control catalysis is well known. Then, for performance stabilization purposes, operation at certain reaction conditions is applied, as well [14,15].

#### 2.2.1. Estimation of the Promoter Coverage

The coverage of the promoter species on the active catalyst surface is a significant factor not only for ranking but also for better understanding its promoting effects. Its knowledge is also useful for comparison purposes between electrochemically and conventionally promoted catalysts. For the former case (EPOC) we have already shown (Equation (2)) that the coverage of the alkali promoter can be directly estimated via the Faraday law applied to the data of a galvanostatic transient. In the latter case (CCP), a "nominal" percentage surface coverage of the alkali promoter (*θ*Alk) could be calculated from the alkali wt% content of the catalyst (e.g., [14,21]) by the assumption that all the promoter is present at the surface and distributed uniformly over the entire available area (noble metal + support), without any incorporation into the bulk and that an one-to-one correlation between alkali adatoms and active metal sites exists. Hence, the following equation can be used:

$$
\theta\_{\text{Allk}} \text{ (\%)} = \text{(wt\% of Alkali on the catalyst)} \cdot \text{(N}\_{\text{AV}} / M\_{\text{Allk}} \cdot A \cdot d) \tag{4}
$$

where *NAV* is the Avogadro number, *M*Alk is the molecular weight of alkali used, *A* is the BET catalyst surface area (m2/g) and *d* is the surface density (atoms/m2) of the active metal (PGM) (e.g., *dPt* = 1.53 × 1019, *dPd* = 1.27 × <sup>10</sup>19, *dRh* = 1.33 × <sup>10</sup><sup>19</sup> for {111} crystallites). Considering *<sup>d</sup>* in Equation (4) as the surface density of the alkali adatom used (i.e., 8.84 × <sup>10</sup>19, 3.314 × <sup>10</sup>19, 1.8 × <sup>10</sup>19, 1.115 × <sup>10</sup><sup>19</sup> or 1.45 × <sup>10</sup><sup>19</sup> for Li, Na, K, Cs or Rb, respectively; values calculated on the basis of the alkali ionic radius), one can probably be led to a better estimation of the nominal alkali coverage [22,25]. In any case and since the issues about the alkali distribution (preference between active metal or support surface, formation of 2D and/or 3D aggregates) are not clear yet, it is apparent that Equation (4) leads to a quite rough estimation of alkali coverage, which however, in the absence of more rigorous estimations or experimental measurements, can be of some interest and applicable as a first approximation upon comparing the EPOC and CCP methods of promotion on a catalytic system (e.g., [22,25]).

The fixed bed, single pass, quartz (or stainless steel) tube reactor configuration is typically used for testing the conventional type catalysts (in the form of powder, small particle clusters or even structured honeycomb monolithic configurations) at the desired reactions. On-line gas chromatography, on-line mass spectroscopy and continuous NOx chemiluminescence's analysis were used for the analysis of the influent and effluent reactor streams in most studies.

#### 2.2.2. EPOC and CCP Comparison Issues

In order to have a solid basis for comparison between electrochemically promoted or conventional promoted catalysts, the rate enhancement ratio (*ρ*), is introduced in both cases, defined as:

$$
\rho = \mathbf{r}\_{\text{x}} / \mathbf{r}\_0 \quad \text{ (x = EPOC or CCP)} \tag{5}
$$

ro is the unpromoted rate and rx is the promoted by alkali rate, where the alkali species are either supplied electrochemically (x = EPOC) or conventionally (x = CCP). *ρ* values as high as 420 (42,000% rate increase) have been reported for some important emissions control catalytic systems (e.g., NO + C3H6/Pt) by means of alkali promotion. It must be emphasized here that in literature studies (either EPOC or CCP) involving *ρ* values estimation, particular attention was devoted in order the catalytic activity data to be acquired in the intrinsic kinetic regime, not influenced by mass transfer limitations (i.e., under low conversions, no limiting reactant, differential reactor operation, small catalyst particles, etc.)

In the case of EPOC an additional parameter, the Faradaic efficiency (*Λ*) is also introduced [6], which can describe the magnitude of promotion; however, this has no meaning in CCP studies:

$$
\Delta = (\mathbf{r}\_{\rm EPOC} - \mathbf{r}\_{\rm o}) / (\mathbf{l}/\!\!\!\!\!\!\/\!F) = \Delta \mathbf{r} / (\mathbf{l}/\!\!\!\!\!\!F) \tag{6}
$$

where *F* is the Faradaic constant and *I* is the current. The term *I/zF* (*z* is the promoting ion charge; *z* = +1 in the case of alkali promotion) corresponds to the rate of promoting ions supplied electrochemically to the catalyst surface according to the Faraday's law.

Another useful parameter to quantify of the magnitude of the EPOC is the promotion index *Pi* [16,26]:

$$\dot{P}\mathbf{i} = (\mathbf{r}\_{\text{EF}\text{CC}} - \mathbf{r}\_{\text{O}}) / \Delta\theta\_{\text{i}}\tag{7}$$

where Δ*θ*<sup>i</sup> is the coverage of the promoting species such as Na+.

#### **3. Results and Discussion**

#### *3.1. Promotion of Simple, "Model" Reactions*

Literature studies concerning "model" (i.e., simple) reactions, catalysed by Pt-group metals under electropositive promotion by alkalis or alkaline earths, conducted either by EPOC or CCP, are listed in Table 1 and discussed below.

#### 3.1.1. CO Oxidation

Several publications concern the promotion of the PGMs-catalysed CO oxidation by alkalis. Significant promotional effects have been achieved either on thin metal film electrocatalysts interfaced with an alkali-contacting solid electrolyte (EPOC), playing the role of in situ source of alkali cations or on conventional type highly dispersed catalysts dosed with several amounts of alkali promoter (CCP). However, the promoting phenomena were found to be strongly depended on the CO/O2 gas phase composition and on the alkali loading of the catalyst.

In particular, Yentekakis et al. [16] using the continuous flow, well-mixed, single-pellet reactor described in Figure 2 have shown that the rate of CO oxidation can be markedly enhanced via EPOC by up to 600% (at T = 350 ◦C) under CO-rich conditions (i.e., at conditions where CO oxidation kinetics obey negative order in CO, positive order in O2) over a Pt film electrocatalyst interfaced with a Na+-conducting *β*"Al2O3 solid electrolyte (Table 1). Those rate enhancements were achieved at sodium coverages on Pt surface of about 2–4%, while higher sodium coverages were found to depress the catalyst activity, thus leading to a "volcano" type behaviour upon increasing the coverage of the promoting species (Figure 5, curve a). However, at O2-rich conditions (i.e., when the rate is of positive order in CO) only poisoning phenomena occurred upon increasing Na coverage on the catalyst surface (Figure 5 curve b). According to the rules of catalyst promotion, Na promoters modify the chemisorptive properties of CO and O2 reactants on Pt active sites. At CO-rich conditions and for Na coverages up to ~4%, the promotional effect was attributed to a Na-induced enhancement in oxygen chemisorption as a result of a strengthening of the Pt–O bond. The rate poisoning behaviour at higher Na coverages was attributed to active sites blocking phenomena due to the formation of a CO–Na–Pt surface complex.

**Figure 5.** Effect of Na coverage *θ*Na on the rate of CO oxidation at oxygen lean (a: T = 350 ◦C, [O2] = 6%, [CO] = 5.3%; filled symbols) and at oxygen rich ((**b**): T = 350 ◦C, [O2] = 6%, [CO] = 2.8%; open symbols) conditions. The volcano type behaviour upon increased Na coverage is obvious in case (**a**). Data was acquired in a Pt/(Na)*β*"Al2O3/Au galvanic cell (Reprinted with permission from Ref. [16]; Copyright 1994, Elsevier).

Another EPOC study was reported by De Lucas-Consuegra et al. [27] on the effect of electrochemically imposed potassium (K) on the Pt-catalysed CO oxidation at 200–350 ◦C. Rate enhancement ratios of about *ρ* = 11 (1100%) at T = 278 ◦C and light-off temperature decreases by ~40 ◦C were achieved at equimolar reactant composition, [CO] = [O2] = 5000 ppm (Figure 6 and Table 1).

**Figure 6.** Effect of catalyst overpotential (VWR) on the maximum rate enhancement ratio (*ρ*max) and on T50 of CO oxidation on Pt. Conditions: [CO] = [O2] = 0.5% He balance, Ft = 108 cm3/min. Data was acquired in a Pt/(K)*β*"Al2O3/Au galvanic cell. (Reprinted with permission from Ref. [27]; Copyright 2008, Elsevier).

The maximum promotional effect was found under the application of −2 V; the appropriate parameters necessary for the estimation of the K coverage corresponding to this catalyst polarization are not available. The reversible promotional phenomena were attributed to the formation and decomposition of potassium compounds on the Pt surface (probably potassium carbonates and/or oxides) that enhanced the adsorption of oxygen at the expense of CO on a surface predominately covered by CO under unpromoted conditions in the reaction temperature range studied.

Regarding CCP studies, CO oxidation activity enhancements were also reported by Mirkelamoglu and Karakas [28,29] over conventional type 0.1wt% Na-promoted 1wt% Pd/SnO2 catalyst tested at two different [CO]/[O] ratios, 1.25 and 0.5. The authors found that in the case of CO-rich gas composition ([CO]/[O] = 1.25), Pd can be up to 2.6-fold times (260%) more active on CO oxidation at 150 ◦C (Table 1), while non or moderate promotional effects were observed at O2-rich conditions ([CO]/[O] = 0.5). A surface segregation of Pd atoms increasing the reactive sites for CO oxidation, together with the formation of super-oxide species observed over Na-PdO/SnO2 were considered to be the origin of the observed promotional effects on the Na-dosed catalyst [29].

The effect of alkali oxide additives (Na2O, K2O) on the alumina support of Pt/Al2O3 catalysts on CO oxidation was investigated by Lee and Chen [30]. They concluded that the addition of alkalis on Al2O3 influences the basicity of the catalyst, which in turn results to higher oxidation activities. Higher promotional effect was achieved by potassium, which resulted in a decrease of CO light-off temperature by 90 ◦C under CO/O2 stoichiometric conditions. The study, however, was performed under simulated two-stroke motorcycle emissions (involving C3H6, H2, CO2 and H2O in the feed, besides CO and O2) at the stoichiometric point and oxygen-deficient environments. Therefore, this work will be further analysed in the proper Section 3.2.1.

The influence of alkaline earths on CO oxidation activity was also investigated over Pd catalysts supported on Ba-modified γ-Al2O3 or CeO2-ZrO2 carriers by Tanikawa and Egawa [31]. They found that the activity of Pd/γ-Al2O3 catalyst can be significantly improved by increasing the Ba promoter loading; an amount of 10–15wt% Ba decreases the light-off temperature by ~45 ◦C compared to the un-modified catalyst (Table 1). However, an adverse effect upon Ba addition was observed for Pd/CeO2-ZrO2 catalysts; an amount of 10 wt% Ba leads to an increase of light-off temperature by ~100 ◦C. The pronounced effect of Ba over Pd/γ-Al2O3 catalysts was ascribed to the weakening of CO adsorption strength, whereas its inhibiting role over Pd/CZ catalysts was attributed to the suppression of Pd interaction with ceria-zirconia support.

In addition, the pronounced effect of electropositive promoters on CO preferential oxidation (PROX) under H2-rich conditions has been recently demonstrated [32–41]. Minemura et al. [32,33] showed that the addition of alkalis (Li, Na, K, Rb and Cs) in a 2wt% Pt/γ-Al2O3 catalyst results in very significant promotional effects on the selective oxidation of CO under H2-rich environments (Table 1). The promotional effects followed "volcano" type behaviour upon increasing alkali loading, thus providing an optimal loading for each alkali; a 10–15 alkali/Pt loading ratio was found to optimize the promotional effects of Na and K, while the optimal alkali/Pt ratios were 3 and 5 for Cs and Rb, respectively. For a constant alkali/Pt loading equal to 3, the order on the promoting effect on CO conversion (and TOFs) was found to be Cs > Rb > K > Na > Li ~alkali-free Pt, while the order of the selectivity to CO2 was K > Rb, Na > Cs > Li. At higher alkali loadings (alkali/Pt = 10), the order of the turnover frequency rates of CO oxidation were K > Na > Rb > Cs~Li > alkali-free Pt, with a similar order for the selectivity towards CO2. The changes in the order of the activity of the catalysts were due to over-promotion, that is, for some alkalis (e.g., Rb and Cs) their optimal loading was exceeded under the conditions used. In these studies, turnover rate enhancement ratio values up to *ρ* = 10 for the optimal K promotion at T = 100 ◦C [32] and even better (up to *ρ* = 20) at 80 ◦C [33] were achieved. Further studies by the research group, focused on the optimal promoter (K) when applying different supports (Al2O3, SiO2, ZrO2, Nb2O5 and TiO2) for the dispersion of Pt, have shown that the most remarkable promoting effects are obtained on Pt/γ-Al2O3 [34] (Table 1). Corroborative characterization studies by means of a variety of techniques, e.g., Transmission Electron Microscopy (TEM), Extended X-ray Absorption Fine Structure (EXAFS), X-ray Absorption Near Edge Structure (XANES), Fourier-Tranform Infrared Spectroscopy (FTIR), enable the authors to consistently interpret the promotion features [35,36]. FTIR spectroscopic evidences for a K-induced weakening in the strength of CO adsorption on Pt and drastic changes on the adsorption site of CO (bridge and three-fold hollow CO species) found on the Pt (10wt% K)/γ-Al2O3 catalyst were considered responsible for the observed promotional effects attributed to the change in the electronic state of K-modified platinum [35,36]. OH co-adsorbed species (originating form H2 and O2 interaction under PROX conditions) that promote the CO oxidation were found by in situ FTIR on the highly active K-modified Pt/γ-Al2O3 catalyst [36]. The inhibiting effects on PROX reaction under over-promotion conditions (beyond the optimal promoter loading) were attributed to aggregation of Pt metal particles caused by the larger amount of alkali metal used [36].

CO preferential oxidation was also found to be very pronounced over Pt clusters dispersed on alkali (Na, Rb or Cs)-modified SiO2 by Pedrero et al. [37]. Optimum promotional effects were obtained with Cs at a surface concentration of 1.6 atoms/nm2; CO oxidation turnover rates (TOFs) more than one order of magnitude higher than those obtained on the un-promoted Pt/SiO2 catalysts were recorded, which were accompanied by CO oxidation selectivity >90% (Table 1). In accordance to the authors, the beneficial effect of alkalis can be interpreted based both on the electronic interactions between

alkali atoms and metals and on the inhibition of spill over-mediated H2 oxidation pathways induced by alkalis.

The effect of Na promoter on CO PROX reaction was also investigated over bimetallic PtCo/Al2O3 catalysts by Kwak et al. [38]. They found moderate promotion of PROX activity on monometallic Pt catalyst while substantial rate promotion on the bimetallic PtCo counterpart catalyst up to a sodium loading of 2.0wt%; the CO-PROX selectivity was not influenced in both cases (Table 1). They concluded that the formation of Na-O-Al bonds by incorporation of Na ions into the alumina lattice, that happens at low sodium content, suppresses the formation of the surface spinel cobalt species and promotes the formation of bimetallic Pt-Co species; at higher Na content (ca. 3wt%) bulk sodium particles on the catalyst surface interact with the active species inhibiting PROX activity.

On the other hand, a notable increase on both CO conversion and CO2 selectivity under PROX conditions was attained with Mg-promoter over Pt/Al2O3 catalysts by Cho et al. [39] (Table 1). The maximum values of CO conversion and CO2 selectivity for Pt-Mg/Al2O3 catalysts were 93.1 and 62%, respectively, at 170 ◦C, compared to 70.2 and 46.8% at 200 ◦C over the un-promoted Pt/Al2O3 catalysts. The effect of CO2 and H2O in the feed was also studied in this work under PROX conditions, as well as the CO oxidation reaction in the absence of excess H2 under Mg-promotion. The superior performance of Mg-modified catalysts was attributed to an increase in Pt electron density, as well as to an increase of hydroxyl groups on catalyst surface.

Finally, de Lucas-Consuegra and co-workers [40] demonstrated the pronounced effect of electropositive modifiers (K<sup>+</sup> ions) on the preferential oxidation of CO via EPOC by the use of a Pt/K+-conducting-*β*Al2O3/Au galvanic cell. At a 195 ◦C and a feed composition of CO/O2/H2 = 0.4%/0.2%/16%, the authors found up to *ρ* = 1.3 rate enhancements, achievable at a potassium coverage of *θ*K~2–4%. A concomitant increase in CO2 selectivity of ~10% was recorded as well (Table 1). The effect of the alkali on the chemisorptive bonds of CO, H and O on Pt surface was considered to be the origin of the observed promotional phenomena.

For more information, a short review has been recently published regarding the promotional effect of alkali promoters (CCP) on PGMs (Pt, Ru and Ir)-catalysed CO PROX reaction [41].

A comparative overview of the literature findings included in Section 3.1.1 and in Table 1 leads to the following general remarks: Among PGMs only Pt and Pd were investigated so far for CO oxidation under electropositive promotion (by alkalis or alkaline earths). No studies exist that were performed at similar conditions using both EPOC and CCP methods, preventing us from making a direct one-to-one comparison of the outputs of the two methods. The general view is that alkalines efficiently promote the Pt and Pd catalysed CO oxidation only at conditions where the reaction rate is negative order in CO, positive order in O2 (the so-called CO-rich conditions at which CO coverage predominates on the catalyst surface; not necessary at CO/O2 > 1), while non, moderate or only poisoning was found under O2-rich conditions. Even in the former case, the promotion follows volcano type behaviour upon increasing promoter loading, reversed to poisoning for high promoter loadings; optimal promoter loadings were typically low. The maximum rate enhancement ratios were reported for Pt/CO+O2 catalytic system under EPOC: *ρ* = 6 with Na at 350 ◦C [16] and *ρ* = 11 with K at 278 ◦C [27].

Volcano-type promotion was the main feature of the electropositive promotion of PGMs under the preferential CO oxidation (PROX), as well. Moreover, for similar reaction and promoter loading conditions larger alkalis (Cs, Rb, K) offered superior promotion. For this reaction two studies exist that concern usage of the same promoter (K) and active metal (Pt), one performed using CCP the other EPOC. Although *ρ* values obtained by CCP were significantly larger (*ρ* = 10 [32–36]) than that obtained by EPOC (*ρ* = 1.3 [40]), the applied conditions were quite different, preventing us from making a creative comparison.

The alkaline-induced pronounced enhancement on oxygen adsorption compared to that of CO was a key argument in most of the aforementioned studies upon explaining their findings, whereas the formation of large surface alkali complexes was considered responsible for the rate poisoning at high alkali loadings.


**Table1.**ElectropositivepromotionofPGMs-catalysedCOoxidation(CO+O2)andPreferentialCOoxidation(PROX:CO+O2+excess

**Table 1.** *Cont*.


**Table 1.** *Cont*.


#### 3.1.2. Light Hydrocarbons Oxidation

#### (i) Alkenes oxidation

The oxidation of ethylene over polycrystalline Pt films has been extensively studied under electropositive promotion (EPOC) by Na via a Na+-conducting *β*"Al2O3 solid electrolyte, by Vayenas and co-workers [18,42] (Table 2). Both promoting and poisoning effects induced by alkali have been observed: low Na coverages (typically <8%) were found to cause increases in the reaction rate (up to *ρ* = 2), while larger Na coverages led to a significant decrease of rate, which eventually falls well below the initial value corresponding to Na-free Pt surface (Figure 7). The data was interpreted in terms of (i) Na-enhanced oxygen chemisorption and (ii) poisoning of the surface by accumulation of Na compounds.

**Figure 7.** Rate as a function of Na coverage for both potentiostatic and galvanostatic experiments for [O2] = 8.0% [C2H4] = 4.2% (balance He) mixture at 352 ◦C. Data was acquired in a Pt/(Na)*β*"Al2O3/Au galvanic cell (Reprinted with permission from Ref. [18]; Copyright 1996, Elsevier).

According to the authors, low Na coverage gives rise to an enhanced chemisorption of O2 at the expense of ethylene, resulting in an increased rate. At high sodium coverages both (i) and (ii) factors operate synergistically to poison the system: the increased strength of the Pt–O bond and the coverage of the catalytic surface by compounds of Na strongly suppress the rate. Kinetic results obtained over a Pt(111)–Na model catalyst, dosed with several amounts of Na by vapour deposition (CCP), were found to mirror the behaviour of the Pt/*β*"Al2O3 catalyst promoted by the EPOC concept [18]. This strongly suggested that Na pumped from the solid electrolyte via electrical polarization is indeed the key species for the promoting or poisoning phenomena. In addition, these results demonstrated that electrochemical promotion of catalysis (EPOC) and surface promotion by conventional means (CCP) obey the same physiochemical rules in the case of alkaline promoting cations. This is well documented nowadays but it was unclear at those early steps of electrochemical promotion studies.

The Pt-catalysed propene oxidation has been also extensively studied by Filkin et al. [20] under in situ electrochemical promotion (EPOC) by sodium, using polycrystalline Pt films deposited on Na+-conducting *β*"Al2O3. Depending on propene and oxygen reactants partial pressures, fully reversible promotion and poisoning effects were observed as a function of sodium coverage as follows. As [O2]/[C3H6] decreases, the effects of Na promotion rapidly increase: for [O2]/[C3H6] = 5.7, where the activity of Na-free Pt is highest, the maximum enhancement ratio *ρ*max = rmax(on Na-promoted Pt)/ro(on Na-free Pt) is equal to 1.1, which then increases to *ρ*max = 2.3 at [O2]/[C3H6] = 2.0, where Na-free Pt activity is lower. These maxima enhancement ratios where obtained at moderate Na coverages (not specified) while higher Na coverages cause poisoning of the propene oxidation rate. The larger poisoning was obtained in the case of [O2]/[C3H6] = 5.7 (Table 2). A Na-modified chemisorption of the reactants considered (Na enhances oxygen chemisorption and inhibits propene chemisorption) on a Langmuir-Hinselwood type reaction accounted all the experimental findings. Using X-ray Photoelectron Spectroscopy (XPS), Auger Electron Spectroscopy (AES) and postreaction Na K edge XANES (X-ray Absorption Near Edge Structure) the authors have demonstrated that under reaction conditions the promoter phase consists of small amounts of 3D sodium carbonate crystallites, while thick layers of this sodium surface compound were responsible for rate inhibition in the poisoning regime. Notably, these promoter and poisoning phases, although stable at reaction temperatures investigated, were readily destroyed via electrochemical Na pumping away from the catalyst by the application of electrical polarizations of opposite sign [20].

Other studies involving the propene oxidation over Pt films using the EPOC concept as well, were reported by Vernoux and co-workers [43,44]. In these studies Pt was interfaced with NASICON (Na3Zr2Si2PO12), an alternative Na+-conductor, acting therefore as a Na source for its electrochemical pumping to the Pt catalyst surface. The authors used two distinguished propene/oxygen gas phase compositions, near stoichiometric (C3H6/O2 = 0.04%/0.2%) and excess oxygen (C3H6/O2 = 0.04%/8.3%) conditions, to investigate the effect of Na on the catalytic system under consideration at T ~ 300 ◦C (Table 2). They found that under oxygen excess Na coverages larger than 3% cause a slight poisoning of the propene oxidation rate (*ρ* ~ 0.8). However, under near-stoichiometric conditions, sodium had strong beneficial effect (*ρ* = 3.5) on the Pt-catalysed propene oxidation rate, which is maximized at a *θ*Na ~ 3.6%; the rate was then slightly decreased when higher (up to 6%) sodium coverages were supplied. Cyclic voltammetry studies indicated that sodium promoter exists in the form of Na2CO3 and NaHCO3 surface phases during reaction [43], in agreement with earlier studies by Filkin et al. [20].

In this line, de Lucas-Consuegra et al. [45,46] have more recently studied the propene oxidation under EPOC of a Pt film interfaced with a K+-conducting *β*Al2O3 solid electrolyte, that is, using potassium as promoter species. They found that the reaction can be strongly promoted at the temperature interval of ~200–300 ◦C, under both near-stoichiometric and oxygen-rich conditions. Rate enhancement ratios of the order of *ρ* = 7 were achieved for a catalyst polarization of about −2 V (Table 2); parameters' values necessary for the estimation of the corresponding alkali coverage are not available in the paper. Cyclic voltammetry, FTIR and SEM-EDX studies enable the authors to identify the formation of stable potassium oxide and superoxide phases upon electrical polarization. These species were responsible for the promotional phenomena. In addition, due to the stability of the potassium surface compounds formed, a permanent promotional effect was observed.

The effect of electropositive modifiers on hydrocarbons oxidation over conventional highly dispersed catalysts (CCP) was also investigated by Yentekakis and co-workers [47]. In particular, they studied the propene oxidation over Na-promoted Pt(Na)/γ-Al2O3 catalysts under oxygen excess

(1000 ppm propene + 5% O2). Significant activity enhancements were observed for a Na-loading of ~1.5–2.6 wt % (or 0.68–1.13 × <sup>10</sup>−<sup>3</sup> mmol effective Na/gcat) [47]. Optimal promotion was achieved for 2.6 wt% Na loading, corresponding to a nominal sodium coverage of *θ*Na~23% (Equation (4) by using *<sup>d</sup>*Na = 3.314 × <sup>10</sup><sup>19</sup> atoms/m2), causing a significant shift of the propene light-off temperature to about 60 ◦C lower values (Figure 8, Table 1). Rate enhancement ratios as high as *ρ*~10 can be calculated from these results. It is worth noting that the aforementioned promotional effects were recorded under oxygen excess conditions, typical of lean-burn combustion engines. The Na-induced enhancement of oxygen adsorption on the Pt surface, which is predominately covered by propene even at these excess oxygen gas phase conditions, was considered to be responsible for the observed promotional effect.

**Figure 8.** The effect of temperature on the conversion of C3H6 Na-dosed Pt(Na)/γ-Al2O3 catalysts under C3H6 + O2 reaction at oxygen rich conditions: 1000 ppm C3H6 + 5% O2; catalyst weight 70 mg; total flow rate was varying as Ft = 210, 126, 62.5 and 52 cm3/min for C1, C2, C3 and C4 catalysts, respectively, in order to keep the effective contact time of C3H6 constant at 4 s. (The effective contact time is defined as: surface Pt atoms/reactant molecules/s) (Reprinted with permission from Ref. [47]; Copyright 2005, Elsevier).

#### (ii) Alkanes oxidation

An EPOC study regarding the propane oxidation on Pt-based catalysts under Na-promotion supplied via a (Na)*β*"Al2O3 solid electrolyte has been reported by Kotsionopoulos and Bebelis [48]. The activity was dramatically inhibited by Na-addition even at low Na coverages, over a wide range of propane/oxygen ratios (Table 2). Under stoichiometric conditions ([O2]/[C3H8] = 1%/0.2%) and T = 320 ◦C, a Na coverage as small as *θ*Na~2.5% leads to a significant decrease of the reaction rate of 625 times. The poisoning effect was more pronounced at higher temperatures (e.g., 360, 400 and 440 ◦C). This poisoning effect of electropositive promoters on Pt for alkanes oxidation can be attributed to the weak adsorption of alkanes compared to oxygen. A latter study published by the same authors dealt with the electrochemical characterization of the aforementioned catalytic system [49]. In this work they observed, in good agreement with some of the studies mentioned above, that the sodium promoter is on the metal surface mainly forming carbonate, bicarbonate and oxide phases.



**Table 2.** Electropositive promotion of PGMs-catalysed alkenes and alkanes oxidation.


**Table 2.** *Cont*.


**Table 2.** *Cont*.

Olsson and co-workers [50] studied the effect of the addition of alkalis (Na, K) and alkaline earths (Ba, Ca) on Pd/Al2O3 catalyst for methane combustion (CCP). Both beneficial and detrimental effects on methane oxidation were observed. High loadings of alkaline modifiers caused reaction inhibition. No promotion was induced by Na and K modifiers, while Ba- and Ca- doping caused reaction inhibition. However, when using a specific catalyst pre-treatment, which leads to the removal of surface hydroxyl species, promotional effects of Ca were recorded at Ca loadings where they previously observed reaction inhibition. The Ca/Pd ratio was found to be critical for the aforementioned promotion. A Ca/Pd ratio of 0.15 was favourable. The modification of the redox properties of palladium and the formation of PdO were considered the origin of this promotion, which overcomes the alkaline-induced inhibition of methane adsorption (activation) on increased electronic density catalyst particles surfaces which occurs in parallel.

Comparatively overviewing the literature findings of Section 3.1.2 summarized in Table 2, the following general features and conclusions can emerge.

Alkali addition on PGMs-based catalyst is detrimental for alkanes oxidation reactions that was attributed to the low adsorption propensity of alkanes on PGMs surfaces which becomes worse due to the alkali induced promotion of oxygen adsorption and the concomitant O-poisoning of the surfaces.

On the opposite, alkenes oxidation on PGMs is substantially promoted by electropositive promoters, attributed to the opposite trend of their adsorption compared to alkanes: The alkali-induced enhancement on oxygen adsorption counterbalances its coverage on a PGM surface, predominantly covered by the alkene molecules on the unpromoted surface, resulting to an enhancement on reaction probability (promotion).

The alkenes oxidation followed volcano type promotion upon increasing promoter loading, resulting from the competitive adsorption of the reactants (Langmuir-Hinshelwood type reaction) and the formation of 3D alkali carbonates and their excessive accumulation on the surface at high promoter loadings.

The main promotion characteristics were qualitatively similar between EPOC and CCP studies, although the intensity of the promotion was larger in the latter case: maximum rate enhancement ratios (*ρ*) obtained were 3.5 and 10 by using EPOC and CCP, respectively. An ultimate contact between nanodispersed active phase particles and promoter species, which is favoured in CCP, is most probably responsible for this superiority.

#### 3.1.3. NO Reduction by CO

The first study of the effect of electropositive promoters on the Pt-catalysed NO reduction by CO was reported by Palermo et al. [17]. The EPOC was applied by means of a Pt/Na+-conducting *β*"Al2O3/Au cell. The promotional effect of Na on the Pt/NO + CO system was strongly dependent on the reaction temperature, gas composition and Na coverage. For instance, at high CO/NO ratios the effect of Na was marginal and according to the authors this was attenuated by CO island formation and limited availability of chemisorbed NO. At low CO/NO ratios the effect of Na was also relatively small, because, as authors conclude, the CO+Oreaction is limited by the low CO coverage. However, at the intermediate CO/NO regime, where the above constrains do not apply, Na strongly accelerates the reaction: rate gains of up to *ρ* = 13 for N2 production compared to the unpromoted Na-free Pt rate were achieved (Table 3). Significant increases to N2 selectivity of the system (up to 30%) were also obtained (Figure 9). Significant progressive changes in the apparent activation energy of the NO + CO reaction upon varying the promoter coverage, similar to that obtained with the Pt/O2 + CO catalytic system [16], were also recorded in this study. The observed promotional phenomena, on both rate and selectivity, were attributed to the enhanced NO versus CO chemisorption and the extended dissociation of the chemisorbed NO molecules, both induced by Na-promoter.


**Table 3.** Electropositive promotion of PGMs-catalysed NO reduction by CO (NO + CO).


**Table 3.** *Cont*.

**Figure 9.** Effect of catalyst potential on the CO2, N2 and N2O formation rates and the selectivity towards N2 during the EPOC by Na of the Pt-catalysed NO + CO reaction. Conditions: [NO] = [CO] = 0.75%, T = 348 ◦C. Data was acquired in a Pt/(Na)β"Al2O3/Au galvanic cell. (Reprinted with permission from Ref. [17]; Copyright 2005, Elsevier).

Another EPOC (by Na) study using a Rh/(Na)*β*"Al2O3/Au galvanic cell was performed by Williams et al. [51,52]. They showed that the NO reduction by CO can also be promoted over Rh catalyst. Rate enhancement ratios of *ρ*N2 = 3.1, *ρ*CO2 = 1.4 and *ρ*N2O = 0.3 were observed at T = 307 ◦C and stoichiometric CO/NO gas reaction conditions ([NO] = [CO] = 1%) for a sodium coverage as low as 2% (Table 3), together with a large increase in N2-selectivity: from 24% on the electrochemically cleaned Na-free Rh to 80% on Rh electrochemically dosed with *θ*Na = 2%. These promotional effects were attributed to the Na-induced NO dissociation, which was considered as the rate limiting step for the NO reduction by CO process. Using XPS and work function (Δ*Φ*) measurements the authors also demonstrated that the Na coverage on the catalyst surface can be controlled through the externally imposed catalyst potential; a linear relationship of catalyst work function and *θ*Na with catalyst potential was experimentally demonstrated in this study [51,52].

Konsolakis et al. [53] studied the NO + CO reaction on (K, Rb and Cs)-promoted, conventional (CCP), highly dispersed Pt/γ-Al2O3 catalysts, over a wide range of temperature (ca. 150–500 ◦C), partial pressures of reactants and promoter loadings (Table 3). All these alkalis were found to be very active promoters for the reaction, however, optimal promotion was achieved by rubidium: rate increases by factors as high as *ρ*N2 = 110 and *ρ*CO2 = 45 for the production of N2 and CO2, respectively, relative to alkali-free Pt were obtained, accompanied by substantial increases in N2-selectivity by up to ~60% (Figure 10) [53]. The effects of K and Cs-promotion mirror those of Rb-promotion but they were less pronounced. Light-off measurements in the temperature range of 150–500 ◦C and under stoichiometric conditions (1000 ppm NO + 1000 ppm CO) showed that the NO conversion performance of alkali-free Pt is very poor (never exceeding 60%), whereas the 0.5wt%Pt(Rb)/γ-Al2O3 catalyst

promoted with 9.7wt% Rb (optimally promoted catalyst), exhibited 100% NO and CO conversions with 100% N2-selectivity at ~350 ◦C. In addition, 115 ◦C decrease on the NO light-off temperature (T50) was obtained for K and Cs, while a larger decrease in T50 by ~150 ◦C was obtained by Rb-promotion (Figure 11).

**Figure 10.** (**a**) Effect of Rb or K content of Pt/γ-Al2O3 catalyst on the turnover (TOF) formation rates of N2, CO2 and N2O and on N2-selectivity at T = 352 ◦C, [NO] = 1%, [CO] = 0.5%. (**b**) Corresponding enhancement ratio *ρ* = TOF/TOF0 values. Solid lines and symbols refer to Rb-promotion, dashed lines and open symbols refer to K-promotion. (Reprinted with permission from Ref. [53]; Copyright 2001, Elsevier).

**Figure 11.** Effect of K-, Rb- or Cs-promotion on the NO light-off temperature (T50) for Pt/NO + CO system. Conditions: [NO] = 1000 ppm, [CO] = 1000 ppm, total gas flow rate Ft = 80 cm3STP/min; catalyst weight 8(±0.2) mg. (w/Ft = 6×10−<sup>3</sup> g s/cm3). (Reprinted with permission from Ref. [53]; Copyright 2001, Elsevier).

The observed promotional effects were ascribed to alkali-induced changes in the chemisorption bond strengths of CO, NO and NO dissociation products. The superior promotion of Rb compared to that of K was attributed to its larger size and thus to the greater electric field experienced by co-adsorbed species located at an adjacent site. On the other hand, the unexpected lower efficiency of Cs promotion compared to that of Rb was attributed to a change in the adsorption site of NO due to the very large Cs ion [53].

More recently Konsolakis and Yentekakis [54] reported on the promotional effect of Na on the Pd-catalysed NO + CO reaction (CCP). A series of Na-promoted Pd catalysts were prepared on yttria-stabilized zirconia (YSZ; 10mol% Y2O3/ZrO2) carrier (i.e., Pd(Na)/YSZ), with different loadings of alkali-promoter. Both promoting and poisoning effects, regarding the Na-loading, were found in a wide range of CO/NO ratios. For example at equimolar CO/NO composition ([CO] = [NO] = 1%) and T = 352 ◦C, the optimal promoter loading was found to be between 0.02–0.03wt% of Na (corresponding to a nominal sodium coverage of *<sup>θ</sup>*Na~3%; Equation (4) with *<sup>d</sup>*Na = 3.314 × 1019 atoms/m2) which leads to N2 and CO2 formation rate enhancements by up to ~200%, accompanied by a significant improvement in N2-selectivity of about 20% in comparison to the un-promoted Pd/YSZ catalyst (Table 3). The promotional effects were interpreted in a similar manner to the aforementioned studies [17,51–53].

Tanikawa and Egawa [55] studied the effect of Ba-promoter on NO + CO reaction in the presence of O2 over Pd dispersed on γ-Al2O3 and ceria-zirconia supports (CCP). They found that the activity of Pd/γ-Al2O3 was gradually enhanced by increasing the Ba loading up to 15wt%; the light-off

temperatures (T50) for CO oxidation and NO reduction were decreased by 40 and 33 ◦C, respectively, over the optimally modified Pd(15%Ba)/γ-Al2O3. However, Ba addition on Pd/CZ catalyst decreased its activity for both NO reduction and CO oxidation reactions (Table 3).

Finally, Lepage et al. [56] studied the effect of electropositive promoters (monovalent H+, Na+, Rb<sup>+</sup> and Cs<sup>+</sup> or divalent Mg2+, Ca2+, Sr2+ and Ba2+ cations) over Rh nanoparticles supported on a series of Y zeolites for the NO reduction by CO by means of infrared spectroscopy of NO adsorption to monitor the electronic modifications of Rh nanoparticles. The observed decrease in NO reduction ignition temperature (T10) with increasing ionic radius-to-charge ratio was related to the degree of electron transfer from Rh particles to the adsorbed NO molecules. The shift of the IR band of linearly adsorbed NO to lower wavenumbers with increasing the radius/charge ratio of promoter elements substantiated this argument.

A comparative overview of the literature involved in this section and in Table 3 gives prominence to the following general features: (i) Promotion either induced by EPOC or by CCP is substantial and has qualitatively similar characteristics, although CCP leads to much larger rate enhancements; *ρ* values in the region of 45–110 and around 13 were obtained by using CCP and EPOC, respectively. The ultimate contact and thus the higher interaction of the well dispersed active phase particles with the promoter species may be the cause of the mentioned differences on promotion intensity. (ii) Heavier alkalis were typically more effective than lighter alkalis, with Rb being the best. (iii) Among Pt, Pd and Rh precious metals, alkali promotion was significantly more effective on Pt than on Rh and Pd. (iv) Alkali-induced enhancement of the dissociative chemisorption of NO was considered as a key factor for the interpretation of the aforementioned pronounced promotion.

#### 3.1.4. NO Reduction by Hydrocarbons or H2

#### (i) NO reduction by alkenes

NO reduction by hydrocarbons under in situ electrochemical promotion (EPOC) by alkalis was firstly demonstrated by Harkness and Lambert [57] and Yentekakis et al. [58]. Using ethylene as reductant, Harkness and Lambert [57] showed that the rate of the NO + ethylene reaction over a Pt film interfaced with Na+-conducting *β*"Al2O3, linearly increased with *θ*Na al low and intermediate sodium coverages (typically *θ*Na < 0.3), achieving a plateau for higher values. Rate enhancement values *ρ* up to 15 were obtained (Table 4).

On the other hand Yentekakis et al. [58] showed very pronounced NO reduction rate enhancements (using propene as the reductant) for the Pt-catalysed NO+propene reaction upon Na promotion over similar Pt/(Na)*β*"Al2O3 electrocatalyst. This effect was studied at 375 ◦C in a wide range of PNO (0–7 kPa) and PC3H6 (0–0.4 kPa) partial pressures. The promotional effect of Na was found to be significant in the whole range of conditions studied. The coverage of the promoting species was determined via galvanostatic transients. These transients showed that the promotional effects maximized at a Na coverage of *θ*Na~0.4, for which value the maximum enhancements in CO2 and N2 rates were ~7 times larger than the rate of the unpromoted Pt, while the N2-selectivity increased from 60% on the clean Pt surface to 80% on the optimal Na-promoted Pt surface (Table 4). Higher sodium coverages (*θ*Na > 1) caused a gradual decrease of the promoting phenomena. This suggests that nominal sodium coverages greater than one exhibit activity much higher than clean Pt because a substantial amount of Na is present as 3D crystallites, as demonstrated by XPS. A Na-induced strengthening of the NO chemisorpting bond relative to propene was indicated by the kinetic data. The accompanied weakening of the N–O bond and the concomitant extended NO dissociation were considered for the interpretation of the observed promotional effects. XPS data confirmed that under reaction conditions the promoter phase consists of a mixture of NaNO2 and NaNO3 3D crystallites. At high Na loadings these surface promoter compounds inhibit reaction due to active sites blocking phenomena.

EPOC was also used by Williams et al. [59] for the study of NO reduction by propene over Rh film catalyst interfaced with (Na)*β*"Al2O3. They found that Na pumping onto Rh surface at a coverage as low as ~ 2% causes a very large increase in the N2-selectivity from 45% on the Na-free Rh surface to 82% on the Na-promoted surface together with beneficial effects on the reaction rate: rate enhancement ratio values of *ρ* = 2.4, 1.7 and 0.4 were recorded for the N2, CO2 and N2O productions, respectively (Table 4). In a subsequent publication [60] the authors examined the impact of O2 co-feed. It was found that upon increasing [O2], the promotional effects of Na were progressively attenuated and finally reversed to poisoning for [O2] = 2%. Similar considerations as in Reference [57] and [58] were used for the interpretation of the promotional effects, as well.

Using CCP, the group of Yentekakis [14,54] showed that the NO + propene reaction over Pd dispersed on 8mol% Y2O3 stabilized ZrO2 support (YSZ) can be substantially promoted by dosing the catalyst with alkalis. In Reference [14], the rate increased by an order of magnitude accompanied by selectivity increases from ~75% over the Na-free Pd/YSZ catalyst to >95% over the optimally Na-promoted Pd/YSZ catalyst in the temperature interval of 250–450 ◦C. The promotional effects were found to be optimized at a sodium content of 0.06wt% (corresponding to nominal sodium coverage of ~7%; Equation (4)), while over-promotion attenuated the rate (volcano type behaviour). These promotional effects caused a decrease in the NO light-off temperature (T50) of about 110 ◦C between the optimally promoted and unpromoted Pd catalyst (Table 4). A strengthening of the chemisorption bond of NO relative to propene that facilitates weakening of the N–O bond of the adsorbed NO molecules and thus their dissociative chemisorption was considered as the origin of the promotional effects. In ref. [54] the effects of Li, Na, K and Cs alkalis on Pd-catalysed NO + C3H6 reaction were comparatively studied: Four catalysts dosed with the same alkali/Pd = 1/1.5 atom/atom loading were prepared and their behaviour was compared in a wide range of [NO] at constant [C3H6] = 0.8% and T=380 ◦C. All these catalyst were found significantly better in terms of activity and selectivity in comparison to the unpromoted Pd/YSZ, in the whole range of conditions investigated; Na provided the most significant promotional effects (Table 4).

Systematically expanding their studies on conventional promoted catalysts (CCP), Yentekakis and co-workers showed that Pt nanoparticles dispersed on γ-Al2O3 are subjected to extraordinarily effective promotion by alkalis (Li, Na, K, Rb and Cs) [15,61] and alkaline earths (Ba) [62,63] in both activity and N2-selectivity during the NO+propene reaction in the temperature interval of TWCs interest (ca. 200–550 ◦C). In References [15,61] rate increases by two orders of magnitude were achieved, while the selectivity towards N2 was improved from ~20% over the alkali-free unpromoted Pt catalyst, to >95% over the optimally alkali-promoted catalysts. Best promotional effects were achieved by Rb-promotion, for which rate increases as high as 420-, 280- and 25-fold were obtained for the formation rates of N2, CO2 and N2O, respectively, in comparison to the performance of the unpromoted Pt/γ-Al2O3 catalyst (Table 4). Volcano type behaviour was found to be followed upon increases of promoter loadings for all cases. Thus, promotion was found to be maximized for a Li content of 1.25wt%, a K content of 7.1wt%, a Rb content of9.7 wt% a Cs content of 15wt% (Figure 12) [61] and a Na content of 4.18wt% [15].

This very spectacular promotion in light-off diagrams (see for example Figure 13) showed that the marginal (~10%) NO conversion efficiency of un-promoted Pt/γ-Al2O3 can be readily proved by alkalis to values up to 100% in the temperature window of TWCs interest, offering at the same time selectivities towards N2 >95% [15,61]. However, besides temperature, NOx removal efficiency of automotive catalytic converters is greatly affected by the presence of oxygen in the exhausts gases, in particular under excess O2 conditions. Therefore, the NO + C3H6 + excess O2 reaction is of higher practical significance than the NO + C3H6 one. Hence, it was decided to analyse this reaction and the effect of O2 in a distinguishable section of the article (Section 3.2).

**Figure 12.** The effect of Li (**a**), K (**b**), Rb (**c**) and Cs (**d**) content of 0.5wt%Pt/γ-Al2O3 catalyst on the turnover (TOF) formation rates of N2, CO2 and N2O and on N2-selectivity. Conditions: T = 375 ◦C, [NO] = 1.3%, [C3H6] = 0.3% (Reprinted with permission from Ref. [61]; Copyright 2001, Elsevier).

In the case of Ba-promotion the optimal Ba-loading was found to be 15.2wt% Ba (Table 4) [62]. Using a 1000 ppm NO + 1000 ppm C3H6 feed composition (these concentrations are close to those encountered in practical applications and commonly used in the literature as referred conditions) the authors showed that the very poor NO light-off behaviour of Ba-free Pt/γ-Al2O3 catalysts, which gave a lower than 10% NO conversion in the whole 200–550 ◦C temperature interval, can be dramatically improved (in a similar manner to alkali-promotion [15,61]), achieving 100% NO conversion at relatively low temperatures accompanied by N2-selectivities near to 100%. Further studying the Ba-promoted Pt/γ-Al2O3 system, the authors focussed on the effect of residual chloride, which concerns catalysts prepared from chlorine-containing precursors [63]; the detrimental effect of residual chloride on deNOx catalysis is well known. Very strikingly, it was found that when Ba is used to promote catalysts prepared from chloride-containing precursors, all the original chloride is retained, yet the catalysts exhibit very strong promotion and in every aspect their behaviours are identical to that of the chloride-free Ba-promoted catalyst [63]. This significant from the practical point of view attenuation of the inhibiting effect of chloride was rationalized in terms of formation of a stable 2D BaCl2 phase on the catalysts surface, where the promoting effect of Ba overwhelms the poisoning effect of Cl and a large net promotion is resulted.

**Figure 13.** The conversion of NO (**a**) and the corresponding selectivity towards N2 (**b**) for Na-promoted 0.5wt%Pt/γ-Al2O3 catalysts, as a function of temperature at constant reactor inlet conditions: [NO] = 1000 ppm, [C3H6] = 1000 ppm; total flow rate Ft = 80 cm3STP/min; catalyst weight 8(±0.2) mg. (Reprinted with permission from Ref. [15]; Copyright 1999, Elsevier).

Macleod et al. [64] studied the NO + C3H6 reaction over dispersed Rh/γ-Al2O3 catalysts (CCP). Rh is one of the noble metals used in TWCs and its use is related to the effective NO reduction in the three-way catalytic chemistry. They found that NO reduction chemistry on Rh can also be improved by alkalis. For instance, at 375 ◦C, a 7.3wt% Na content in the catalyst was found to be optimal promoter loading, leading to a ~3-fold increase in Rh activity (Table 4). Significant increases in N2-selectivity, up to 90% were also obtained, when the unpromoted Rh/γ-Al2O3 appeared to have only 53%. In addition, Na was found to suppress de formation of CO and HCN. Higher Na loadings were poisoning the catalyst bellow to its initial Na-free performance. However, it is worth noticing the substantially lower degree of promotional effect of alkalis on the NO reduction chemistry of Rh compared to that observed for Pt and Pd.

#### (ii) NO reduction by alkanes

Yentekakis and co-workers [65,66] studied the reduction of NO by methane over Na-dosed Pd catalyst (CCP) dispersed on yttria stabilized zirconia (YSZ) at temperatures between 350–500 ◦C and a wide range of reactants concentrations ([NO] = 0–1.5%, [CH4] = 0–25%). It was observed that the reaction clearly exhibits Langmuir-Hinshelwood type kinetics over Pd with characteristic rate maxima reflecting competitive adsorption of NO and methane; NO adsorption however was found to be much more pronounced than that of CH4 within the temperature range investigated. Sodium was found to cause strong poisoning of the reaction at any loading due to a Na-induced oxygen poisoning of the catalyst (derived by NO decomposition) (Table 4). The very different response of alkali promotion for the NO reduction by alkanes (strong poisoning effects) and by alkenes (strong promotional effects) is attributed to the different trends of the adsorption of the two kinds of hydrocarbons (alkanes and alkenes) on the Pt-group metal surfaces [67,68].

It is worth noting the similarities on the effect of electropositive promotion of Pt-group metals (PGM) between the alkanes or alkenes oxidation by dioxygen (Section 3.1.2) and by NO (Section 3.1.4): In both cases when the oxidation process involves alkenes, electropositive promoters induce strong promotion, whilst when the reductant agent is an alkane, electropositive promoters lead to poisoning. This is understandable in view of the different trend of alkenes and alkanes adsorption on the PGM surfaces (strong and weak, respectively [67,68]) and the influence of the promotion on it [15,20,47,61,65,66] and the fact that in both cases the oxidant species are mainly based on adsorbed atomic oxygen derived by the dissociative chemisorption of molecular oxygen or NO [47].

#### (iii) NO reduction by H2 or CO+H2

Besides hydrocarbons, hydrogen is also one of the gases present in automotive exhaust stream. It could also be externally supplied, by the use of a hydrocarbon reformer or another source of safe in situ hydrogen generator, for the reduction of NOx from stationary power sources and chemical plants. Notably, the NH3-Selective Catalytic Reduction process (NH3-SCR) which is currently on of the most performing technologies for deNO*<sup>x</sup>* processes in stationary power applications and chemical plants, experiences drawbacks related to the storage and slip of ammonia, although it may be pointed out that urea (as an NH3 carrier) is now commercially available for use on diesel truck and some lighter vehicles. These issues can explain the growing interest and efforts for the use of H2 as a reducing agent of NOx in the last years (e.g., [69–72]). Most of these studies have used Pt-group metals as catalysts.

The first EPOC work in this field was performed by Marina et al. [19], who studied the effect of Na on the Pt-catalysed NO reduction by H2 with (Na)*β*"Al2O3 as a source of Na promoter species. Rate enhancement ratio values up to *ρ*N2 = 30 were achieved for the formation of N2, accompanied by significant increases in N2-selectivity. At 375 ◦C, Na coverage of only 6% increased the N2-selectivity from ~36% on the clean Na-free Pt surface to ~75% on the Na-promoted Pt surface (Table 4). The promotional effects of Na were applicable in a wide range of H2/NO gas phase compositions (ca. 0.1–3.0) and temperature (ca. 300–450 ◦C).

Burch and Coleman [73] studied the effect of sodium for the selective catalytic reduction of NO by hydrogen in the presence of excess oxygen (lean-burn conditions) over Na2O-modified conventional, highly dispersed Pt/Al2O3 and Pt/SiO2 catalysts (CCP), at temperatures representative of automotive "*cold-start*" conditions (<200 ◦C). They found that small sodium loadings significantly increased the NO conversion rate while larger sodium loadings led to severe poisoning. Under oxygen excess and low temperatures (<~140 ◦C) the selectivity to N2 was practically unaffected by the addition of sodium, while an adverse effect was found at higher temperatures (Table 4).

Machida et al. [74] investigated the effect of alkalis (Na, K, Cs) and alkaline earths (Mg, Ca, Ba) on Pt/ZSM5 catalysts (CCP) for the selective reduction of NO by H2 under O2 excess (0.08% NO, 0.08–0.56% H2, 10% O2). The optimum promotional effects were obtained with Na promoter at loadings of 10–15wt%, which exhibited NOx conversion higher than 90% and N2 selectivity of ~50%. Based on in situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) studies, it was suggested that Na promotes the adsorption of NO as NO2-type species, which would be an intermediate towards N2 formation.

The H2-SCR process over Pt for automotive emissions control, however, exhibits an important disadvantage: CO, which also coexists in exhaust stream, is strongly adsorbed on Pt surface causing inhibition of NOx reduction. Pioneer CCP work by Lambert and co-workers [75–78] showed that Pd overcomes this poisoning effect of CO leading to good NOx conversions at mixed H2 + CO feeds. In addition, they found that Pd dispersed on 10wt% TiO2-modified Al2O3 offers even more improved deNOx efficiency in comparison to the Pd/Al2O3 catalyst. The effective temperature window of these new catalyst formulations was however relatively narrow (ca. 150–200 ◦C). In a similar manner, Yentekakis and co-workers [79] studied the lean NOx reduction by H2 + CO over potassium-modified Pd/Al2O3 and Pd/Al2O3-TiO2 catalysts. They found that potassium is a very active promoter of the Pd/Al2O3 catalyst for H2-SCR process leading to significant enhancements of both NOx conversion activity and N2-selectivity of Pd. A potassium loading of ~0.25 wt% was found to optimize the catalyst performance in both activity and selectivity, leading to a catalyst formulation that offered up to ~90% NOx conversion with a N2-selectivity of the order of >85% for the temperature range of 180–300 ◦C (Table 4).

(iv) Direct NO decomposition

Limited studies can be found on this topic, probably due to the necessity of high temperatures for the direct NO decomposition. Wang et al. [80] studied the direct decomposition of NO into nitrogen and oxygen over Na-modified Pd/Al2O3 catalysts (CCP) in a flow reactor. They found that the NO reduction activity of Pd was significantly promoted by pre-coating the Al2O3 support with NaOH before the impregnation with Pd. A loading of 13wt% NaOH on a 4.8wt% Pd/Al2O3 catalyst enhanced the nitrogen yield from 15% up to 60% at T = 627 ◦C (Table 4). The observed NO decomposition rate enhancement was attributed to the strengthening of the NO adsorption bond on Pd sites.

Overviewing the literature included in Section 3.1.4 and Table 4 the following general remarks can emerge:



**Table 4.** Electropositive promotion of PGMs-catalysed NO reduction by hydrocarbons (alkenes or alkanes), H2or H2+ CO.

**Table 4.** *Cont*.





**Table 4.** *Cont*.


#### 3.1.5. N2O Decomposition and/or Reduction

It is well known that nitrous oxide (N2O) is absent in automotive engines exhausts gases, although this can be a by-product (in low concentrations) of the NO reduction reactions taking place in the after-treatment devices of cars (catalytic converters), primarily formed during the "cold start" and "intermediate temperature" periods. However, N2O is a powerful greenhouse gas (global warming potential ~310) and also the dominant stratospheric ozone-depleting gas nowadays. These account for the currently devoted high attention on the N2O emissions control. Besides mobile sources, other anthropogenic sources of N2O emissions involve stationary fuels combustion processes, agricultural activities and industrial plants (e.g., adipic and nitric acid plants) where it is produced in fairly high concentrations; the latter rationalizes why a major portion of deN2O studies concerns gas mixtures with relatively high N2O concentrations (ca. 0.1–1 vol.%; Table 5), not actually mirror cars emissions.

#### (i) Direct N2O decomposition

No studies of electrochemical catalyst promotion (EPOC) were found in literature regarding the direct N2O decomposition. In the next paragraphs we summarize some examples of the CCP studies for this reaction.

The pronounced effect of electropositive promoters on the direct N2O decomposition performance (de-N2O) of noble metals supported catalysts was demonstrated by several groups [81–85] (Table 5). In particular, Haber et al. [81] investigated the effect of Li, Na, K and Cs on N2O decomposition over Rh catalysts supported on alkali-modified Al2O3 carrier. They found that the deposition of alkali metals on Al2O3 support results in a notable increase of Rh dispersion, which improved de-N2O efficiency. In almost all cases the optimum promotional effects were obtained at ~0.078 mol% alkali (Table 5).

Parres-Esclaped et al. [82] studied the N2O decomposition over Rh catalysts supported either on bare γ-Al2O3 or on alkaline earth modified alumina (5 wt% Mg- and Sr-doped γ-Al2O3). Enhanced deN2O performance was obtained with Rh/(Sr)Al2O3 catalysts, which achieved total N2O conversion at 300 and 350 ◦C, in the absence and presence of O2, respectively, compared to 375 and 450 ◦C over un-promoted catalysts (Table 5). However, Mg addition was not beneficial to deN2O Rh/Al2O3 activity. The latter was attributed to the formation of magnesium aluminate, while the former was attributed to the enhanced Rh dispersion (determined by TEM) on account of the Sr-modifier.

Combined structural and surface promotion was applied by Konsolakis et al. [83] to facilitate the deN2O performance of Pt/γ-Al2O3 catalyst. To this end, rare earth oxides (CeO2, La2O3) were used as structural promoters of the γ-Al2O3 support and potassium (K) as surface promoter of Pt, providing a doubly-promoted Pt(K)/Al2O3-(CeO2-La2O3) catalyst composite. The study was conducted on macro-structured, cordierite honeycomb monolith washcoated catalytic arrangements, suitable for practical implementations. The effect of K-promoter was studied in the range of 0–2wt% K loadings, while the impact of O2, CO and H2O in the feed stream was also investigated (Table 5). It was found that incorporation of 20wt% CeO2-La2O3 in the γ-Al2O3 support (ACZ) results in dramatic enhancement of the deN2O catalyst activity which is further improved by potassium addition. The promotional effects of K were found to monotonically increase upon increasing its loading on the structurally promoted Pt/ACZ catalyst in the range of 0-2 wt% studied. As a result, the final doubly-promoted catalyst with 2wt% K loading achieved 100% N2O conversion at a temperature of only 440 ◦C; notably the bare Pt/γ-Al2O3 did not exceed ~35% N2O conversion even at 600 ◦C (Figure 14a). In respect to the impact of CO, O2, H2O co-feed, the authors found the following deN2O efficiency sequence in all promoted or non-promoted catalysts: N2O + CO >> N2O>N2O+O2 > N2O+O2 + CO (Figure 14b). Water co-feed was detrimental to deN2O performance but in a partially reversible manner after its removal from the feed stream; the catalytic efficiency was totally restored by adding H2 into N2O+H2O feed.

Papista et al. [84] studied the promoting impact of potassium addition to Ir/γ-Al2O3 catalyst on N2O decomposition under both deficient and O2 excess conditions. Varying K loading in the range of 0–1wt%, the authors found poisoning effect on the deN2O performance of iridium in the absence of oxygen in the feed. The opposite was true in the case of oxygen excess conditions; a pronounced promotion was recorded showing that K prohibits the oxygen-poisoning of the catalysts; the latter self-poisoning is a typical problem in deN2O catalytic chemistry, since the removal of the N2O decomposition-derived adsorbed oxygen on the catalyst surface is a key reaction step of the process. The alkali promotion found in the case of oxygen excess conditions follows volcano behaviour providing therefore an optimal promoter loading of 0.5 wt% K, that resulted in a significant decrease of T50, equal to ΔT50 = −80 ◦C (Table 5). The effect of alkali on the adsorption properties of Ir involving strengthening of the Ir–O bond of adsorbed O species with concomitant changes on the oxidation state of the metal was considered upon interpretation of the promotional and/or poisoning effects observed.

Finally, Goncalves and Figueiredo [85] studied the effect of potassium on the catalytic activity of Pt supported on activated carbon (ROX 0.8) for the simultaneous reduction of NO + N2O mixtures. They found a synergistic effect between K and Pt which led to high activity at relatively low temperatures. For instance, at 350 ◦C, a very stable and close to 100% conversion of both reactants was recorded over a 5wt% K/0.1wt% Pt catalyst, in opposite to the monometallic 0.5wt% K or 0.1wt% Pt counterparts (Table 5). The latter showed significantly lower conversion activity of about 25% for NO and 90% and 0% for N2O, respectively. The improved redox properties of the bimetallic K/Pt formulation was responsible for this synergy on enhancing both deNO and deN2O reactions [85].

**Figure 14.** (**a**): N2O conversion profiles as a function of temperature (light-off curves) for bare Pt/γ-Al2O3, structurally promoted Pt/ACL and doubly-promoted Pt(K)/ACL catalysts under direct decomposition of N2O; Feed: 0.1% N2O, balance He; GHSV = 10,000 h<sup>−</sup>1. (**b**): N2O (and CO) conversion profiles obtained over the optimally doubly-promoted Pt(2wt%K)/ACL catalyst under direct N2O decomposition or N2O + CO reactions in the absence or presence of O2, that is, under the feeds: 0.1% N2O, balance He; 0.1% N2O + 2% O2, balance He; 0.1% N2O + 0.1% CO, balance He; 0.1% N2O + 0.1% CO + 2% O2, balance He. GHSV = 10,000 h<sup>−</sup>1. (Reprinted with permission from Ref. [83]; Copyright 2013, Elsevier).

#### (ii) N2O reduction by hydrocarbons

Very recently, the effect of alkali modifiers on another very important reaction related both to the TWC chemistry and N2O abatement, that is, the N2O reduction by hydrocarbons, was extensively studied by Pekridis et al. [25,86]. In particular, the role of potassium (K) promoter on the surface and catalytic properties of Pd/Al2O3 (CCP) catalysts for N2O reduction by alkanes (CH4, C3H8) or alkenes (C3H6) was investigated [86]. Potassium strongly enhanced the N2O reduction by propane or propene, resulting in notably lower N2O light off temperatures (~100 ◦C) compared to the un-promoted catalyst (Table 5). However, a slight inhibition upon K-promotion was obtained when CH4 was used

as a reducing agent, implying that the extent of K-promotion is strongly related with the nature of the reducing agent. A comprehensive surface characterization study, involving XPS, in situ DRIFT spectroscopy of CO adsorption and FTIR-pyridine adsorption, was undertaken to correlate the de-N2O performance with the catalyst surface characteristics. The results revealed that the electronic, acidic and structural properties of Pd/Al2O3 catalyst can be substantially modified by potassium, which in turn affects the reactants chemisorption bonds and as a consequence the overall surface activity. Specifically, addition of K on Pd/Al2O3 decreases the adsorption strength of electron-donor adsorbates, such as C3H8 and C3H6, enhancing the N2O adsorption through an electron transfer from metal sites to N2O antibonding orbitals. Both factors act synergistically towards increasing active sites for N2O adsorption/decomposition. However, in the case of CH4, K enhances the adsorption strength of N2O and its dissociation products (Oads), at the expense of weakly bonded CH4, leading to poisoning by strongly bonded oxygen atoms.

In correlation with the above study, the EPOC concept (through a Pd-film electrocatalyst interfaced with (K)*β*"Al2O3 solid electrolyte) was also used as a probe technique for exploring in detail the response of Pd metal to alkali (K) promotion in the same reaction [25]. The obtained results matched very well the promotion characteristics of the previous CCP study that involved conventional highly dispersed catalysts, demonstrating experimentally again that electrochemical promotion is an efficient tool for a rapid investigation of the promoter effect on a catalytic system. The obtained information could be further used to design and optimize efficient and "smart" catalysts formulations.

The N2O reduction by propene was also studied by de Lucas-Consuegra et al. [87,88], over Pt-based electro-catalysts, electrochemically promoted by potassium by the use of a (K)*β*"Al2O3 solid electrolyte. The authors found significant rate enhancement ratios (*ρ*max~10) at temperatures between 400–450 ◦C and a reaction mixture composition of N2O/C3H6/O2 = 1000 ppm/2000 ppm/2000 ppm (Table 5). Potassium coverages (estimated by appropriate galvanostatic transients) higher than *θ*K~70% were necessary in order to achieve such large rate enhancements. They also found that these pronounced promotional effects are similar for a wide range of O2 concentration in the reaction mixture (2000–7000 ppm), while at larger O2 concentrations (>9000 ppm) the promoting phenomena were practically vanished. A very significant observation of this work was also the demonstration that potassium addition strongly decreased the inhibiting effect of water vapor on the Pt-catalysed N2O/C3H6/O2 catalytic system.

Overviewing the literature findings included in Section 3.1.5 and Table 5 the following general conclusions can emerge:



**Table 5.** Electropositive promotion of PGMs-catalysed N2O decomposition and/or reduction reactions.

**Table 5.** *Cont*.


#### *3.2. Electropositive Promotion of PGMs Operated Under Simulated Practical Conditions*

#### 3.2.1. Simulated TWC Conditions

Given that alkalis and alkaline earths were found to strongly promote reactions related with the three-way catalytic (TWC) chemistry, testing of these new catalyst formulations at conditions that simulate automotive exhaust conditions was a reasonable consequence. To this end, Konsolakis et al. [89] studied the promotional effects of Na on Pt/γ-Al2O3 catalysts operated under simulated exhaust conditions at the stoichiometric point (SP), that is, at the value equal to 1 of the stoichiometric number (S), calculated as:

$$\text{S} = \left(2[\text{O}\_2] + [\text{NO}]\right) / \left([\text{CO}] + \text{\text{\textdegree C}\_3\text{H}\_6\text{I}\text{}]\right) \tag{8}$$

using a feed mixture consisting (v/v) 1000 ppm NO + 1067 ppm C3H6 + 7000 ppm CO + 7800 ppm O2.

A series of Na-promoted Pt/γ-Al2O3 catalysts were tested under these conditions at temperatures from 200–500 ◦C typical of TWCs. Typical results of the promoter effect on the overall NO, CO and C3H6 conversions and selectivity obtained at 400 ◦C, are shown in Figure 15. The conversions of all three pollutants, NO, CO and C3H6, reach 100% at sodium contents > 4.2wt%. In addition, Na promoter causes a significant decrease in the light off temperature of ~100 ◦C, together with significant improvements (~75%) in N2-selectivity (Table 6). The promotional effect was attributed to the modified relative adsorption strengths of reactants and reaction intermediates, as explained above in the previous sections [89].

In a similar motive, Macleod et al. [90,91] studied the Na promotion of Pd/γ-Al2O3 and Rh/γ-Al2O3 catalysts operated under simulated TWC conditions over a range of stoichiometries from fuel rich to fuel lean conditions (0.90 < S < 1.1). They observed significantly different responses of Na addition in the two cases (Table 6). Na addition to Pd/γ-Al2O3 significantly improves its overall performance: although the recorded promotional effects were not as high as those observed in Reference [89] for Na-promoted Pt/γ-Al2O3, results were again very spectacular.

A 7wt% Na loading (optimal promoter loading for the conditions used) offered a ~50 ◦C lower light off temperature than the unpromoted Pd when tested under stoichiometric feed conditions (1000 ppm NO + 1067 ppm C3H6 + 7000 ppm CO + 7800 ppm O2). Beneficial effects of sodium addition were also recorded under both fuel rich and fuel lean conditions (Table 6). In the former case, Na was found to prevent catalyst poisoning by carbon deposition therefore maintaining high NO conversions, while in the latter case Na reduced the formation of N2O therefore leading to enhanced selectivities toward N2. However, the authors found that Na addition to Rh has a detrimental effect causing severe activity poisoning and N2-selectivity reduction over the greater part of the temperature interval of 200–400 ◦C investigated [92]. It was attributed to the enhanced oxygen adsorption on the Rh active sites at the expense of the hydrocarbon, whilst in the case of Pd the beneficial effects were considered as a consequence of Na-promoted NO dissociation, together with inhibition of self-poisoning due to excessive propene adsorption [91]. It is worth noting that NO dissociative chemisorption takes place spontaneously on Rh but not on Pd.

Tanaka et al. [92] have also reported the beneficial effects of the addition of a small amount of Na2O to (1.67wt%Pt)(12.3wt%MoO3)/SiO2 catalysts investigated under simulated exhaust from automobile engine and three-way behaviour around the stoichiometric point (SP). They found that the active window of SP for the Na-promoted PtMoNa/SiO2 catalyst is wider than that of the Na-free counterpart, that is, PtMo/SiO2 and Pt/SiO2 catalysts (Table 6). The observed improvements were attributed to the wider redox ratio window and to the depressed (by Mo and Na) oxidation of Pt, even under oxygen excess.

**Figure 15.** Testing Na-promoted Pt/γ-Al2O3 catalysts at simulated exhaust conditions of conventional stoichiometric gasoline engines (T=400 ◦C, 1000 ppm NO, 1067 ppm CO and 7800 ppm O2; Ft = 80 cm3STP/min; mcat = 8 mg). Effect of Na loading on reactants conversions (**a**) and selectivity towards N2 (**b**). (Reprinted with permission from Ref. [89]; Copyright 2000, Elsevier).

Shinjon et al. [93] have investigated the effect of an alkaline earth (Ba) modifier on the catalytic activity of Pt/γ-Al2O3 and Rh/γ-Al2O3 catalysts under simulated automotive exhaust conditions at the stoichiometric point (1% CO, 0.3% H2, 0.1% C3H6, 0.1% NO, 0.75% O2, 3% H2O, 12% CO2). They found that the TWC-performance of Pt/γ-Al2O3 catalysts was improved by Ba, offering ~30 ◦C lower light off temperatures compared to those for the un-promoted catalysts (Table 6). However, the overall

performance of Rh catalysts was deteriorated with Ba addition. It was attributed to the fact that Ba addition to Pt catalysts suppressed the strong hydrocarbon chemisorption, enhancing simultaneously the oxygen adsorption. On the other hand, Ba addition to Rh catalysts results in strong adsorption of oxygen species, suppressing hydrocarbon chemisorption and thus the reaction between them.

Kobayashi et al. [94] investigated the influence of Ba and Sr alkaline earth metals on a commercial Pd-only TWC catalyst (Pd/Al2O3-based TWC catalyst of N.E. ChemCat Corp.). Improved three-way catalytic performance was obtained regarding CO and NOx conversions (Table 6). The enhanced basicity and the electron-donation ability of these basic elements were considered to play significant role in the improvement of TWC performance. A higher electron density around Pd(II) was revealed by XPS measurements in the Ba-promoted Pd/Al2O3 catalyst in comparison with the Ba-free counterpart. The stabilization of PdO that may suppress palladium species sintering and the electron enrichment palladium particles that endows Pd to behave like Rh were concluded as the main promoting factors in this study [94].

A NO-free gas mixture of CO, C3H6, H2, O2, CO2 and H2O, simulating two-stroke motorcycle emissions (Table 6), was considered by Lee and Chen [30] for the investigation of the impact of Na2O and K2O addition on a 0.4wt% Pt/γ-Al2O3 catalyst operated at 150–450 ◦C under stoichiometric and oxygen-deficient conditions (by varying the oxygen to reductants stoichiometric number; S = 1, 0.31 and 0.17). Significant enhancements on the CO and C3H6 conversions were achieved by the addition of both alkalis under operation at the stoichiometric point (S = SP = 1); superior promotion was the one induced by potassium. Under O2-deficient conditions alkalis addition enhanced CO conversion but not that of propene, which was reduced. The presence of water in the gas mixture was found to have a strong positive impact on catalyst performance, in particular on K-promoted catalyst and for CO conversion (Table 6).

Yentekakis and co-workers [95,96] developed monolithic type catalytic converters with a washcoat containing only one precious metal (Pt-only TWC), which was optimally promoted by Na. Catalytic performance tests of this novel TWC under simulated exhaust conditions in a wide temperature range (150–500 ◦C) exhibited similar performance and even better N2 selectivity (~100% in the whole temperature range) compared to that of a commercial bimetallic (Pt/Rh)-TWC catalyst (Figure 16A, Table 6); notably, the aforementioned TWC had a 4.5-fold lower noble metal loading in comparison to the commercial one. These novel material formulation and design for TWCs is subjected to much lower production cost (use of only one noble metal at much lower loading, without the necessity of scarce Rh in their constitution) and cost-effective recycling.

Most recent tests of these novel TWCs at more severe temperature conditions (>800 ◦C) have also shown a significant resistance to deactivation (Figure 16B) [97,98], implying that the promoter phase (Na) does not practically escape from the catalyst composite even at very high temperatures, due to its stabilization via the formation of new *β*' and *β*" sodium-alumina phases as verified by TEM and XRD studies [97,98]. These phases, in direct interaction with the active phase (Pt), then act as a spontaneous (thermal diffusion driven) Na promoter source during TWC operation causing permanent promotion [97,98].

Overviewing the literature findings presented in this section and in Table 6, one can readily conclude that electropositive promotion of PGM-catalysts under simulated automotive exhaust conditions follows, in general terms, the promotional characteristics corresponding to the simple (model) reactions of the complex reactions scheme taking place into the converter. However, other possible synergistic or competitive interactions between the reactants and reactions' intermediates on a densely populated surface with all these competitive species can be at work affecting the overall performance.

**Figure 16.** Three-way catalytic performance at 450 ◦C of "fresh" (**A**) and "aged" (**B**) TWC samples at simulated stoichiometric gasoline engines exhaust conditions (0.1% NO + 0.7% CO + 0.1067% C3H6 + 0.78% O2, balanced with He at 1 bar; Ft = 3200 cm3 (STP)/min). (Reproduced with permission from Ref. [98]; Copyright 2011, Elsevier).


**Table6.**ElectropositivepromotionofPGM-catalystsoperatedundersimulatedautomotiveexhaustconditions.


**Table 6.** *Cont*.


**Table 6.** *Cont*.

#### 3.2.2. Oxygen Excess Conditions (Simulated Lean-Burn and Diesel Exhausts Gases)

Some other EPOC studies regarding the electrochemical activation of Pt-based electrochemical catalysts for NOx reduction by propene under oxygen excess were performed by the groups of Vernoux and de-Lucas Consuegra. In this sense, Vernoux et al. observed a significant promotional effect of Na promoters electrochemically supplied from a NASICON solid electrolyte support [99]. The study was carried out under lean-burn conditions (oxygen excess, typical of Diesel engines). In such a system, electrochemical promotion is shown to strongly enhance both the catalytic activity and the selectivity to N2 (from 41% to 61%) using very low overpotentials (−100 mV) at 300 ◦C (Table 7).

In the same line, the group of de Lucas Consuegra performed several EPOC studies by using Na and K promoters by means of (Na+)β Al2O3 and (K+)βAl2O3 solid electrolytes, respectively. First, they studied the effect of reaction temperature and O2 concentration in the feed stream on Pt catalysts promoted with Na species. Under lean burn conditions, at low reaction temperatures (220 ◦C), rate enhancement ratios up to 1.4 were observed for the NO reduction rate (Table 7) [100,101]. Nevertheless, as the reaction temperature increased, the promotional effect decreased, even resulting in a transition to a regime where the alkali metal induced poisoning. This progressive suppression of the promotional effect was due to an increase of the oxygen coverage on the Pt surface with the temperature, which led to a C3H6 adsorption inhibition. However, at all explored reaction temperatures, the presence of sodium ions induced a large increase of the N2 selectivity (up to 90%), minimizing the N2O formation.

Regarding the effect of the O2 concentration, a study was performed at low temperatures (240 ◦C) under O2 concentrations ranging from 0.5% to 5%. The promotional effect of sodium on the overall catalytic activity for NO removal was progressively lowered with increasing oxygen concentrations, as a result of a strong inhibition of propene adsorption and a relative increase of the oxygen coverage. In all cases, the presence of sodium ions induced an increase in the nitrogen selectivity by promoting the NO adsorption and subsequent dissociation.

In a new investigation, de Lucas Consuegra et al. [102] studied for the first time the promotional effect of K (EPOC) under the presence of steam in the feed stream under lean burn conditions, as a new approach to the development of efficient catalysts under real working conditions of combustion engines. In this study, they demonstrated that the presence of K promoters could be practically used to decrease the inhibitory effect of water for the reduction of NO by propene. In addition, as previously demonstrated in other studies, they verified that the K species form stable nitrate species on the Pt surface. This discovery led to a new series of studies performed by the same research team, leading with the NOx-storage and Reduction technology (NSR) by means of alkali-based Pt electrochemical catalysts.

Briefly, the NSR process was developed in the early 1990s by Toyota. It is based on the ability of alkali and alkali earth elements to store Nox in form of nitrites/nitrates under lean burn conditions (oxygen excess). Then, by the injection of extra fuel, the stored NOx are released and reduced with hydrocarbons, CO or H2, to produce N2, CO2 and H2O. The alkali promoters function in two ways in these "smart" materials: first, they act as electronic promoters to enhance the NO oxidation to NO2 (which has been identified as the rate determining step of the overall process) and to store the NO2 produced in form of nitrites/nitrates (as previously demonstrated).

In these studies, de-Lucas Consuegra et al. demonstrated, for the very first time, the possibility to perform the NSR process via EPOC, by the use of Pt catalysts supported on K+-*β*Al2O3 solid electrolytes (to supply the K promoter species) [103,104]. The studies were performed under lean burn conditions and even under the presence of steam. The 2 main conclusions from these studies were:


**Figure 17.** Influence of the reaction temperature on the amount of NO*x* stored, potassium transferred and on the NO*<sup>x</sup>* conversion to N2 during NO*<sup>x</sup>* storage/reduction experiments. Lean phase (NO/C3H6/O2:1000 ppm/1000 ppm/5% O2), 6 min of duration, Vcell = −1.5 V. Rich phase (NO/C3H6/O2:1000 ppm/1000 ppm/0.5% O2), 5 min of duration, Vcell = 3V. Data was acquired in a Pt/(K)*β*Al2O3/Au galvanic cell (Reprinted with permission from Ref. [103]; Copyright 2011, Elsevier).

In respect to conventional highly dispersed catalysts and using CCP, early studies by Burch and Watling that involved the influence of a number of promoters, including alkalis (namely K and Cs) and alkaline earths (namely La, Mg, Ba), on the Pt-catalysed propene-SCR of NOx under lean-burn conditions have showed that addition of one of the following: 2.42wt% K2O, 7.25wt% Cs2O, 7.93wt% BaO, 2.07wt% MgO and 8.35wt% La2O3 in 1wt%Pt/γ-Al2O3 catalyst results in suppression of the catalytic activity of Pt (NOx conversion efficiency) and to marginal effects on N2/N2O selectivity (Table 7) [105]. However, later studies of the de-Nox activity of supported Pt catalysts under lean burn conditions have shown very pronounced promotional effects by alkalis or alkaline earths: Vernoux et al. [106] and Yentekakis et al. [47], systematically investigated the resulted promotional effects of Na addition on supported Pt/γ-Al2O3 catalysts (CCP) during NO + propene + excess O2 as a function of the Na-loading, have shown significant beneficial effects of the alkali on both de-Nox activity and N2/N2O selectivity at low and intermediate Na-loadings (Table 7). In both studies, the effects (promoting or poisoning, depending on promoter loading) were understandable in terms of the influence of the alkali promoter on the relative adsorption strengths of reactants and intermediates on a highly populated surface with electrophilic and electrophonic adsorbates. In particular, Yentekakis et al. [47] demonstrated that Na acts beneficially to de-Nox process in a narrow window of Na loading (ca. 0–2.6wt% Na), while overpromotion to the optimal value of ~2.6wt% Na causes significant inhibition of the catalyst performance. The recorded promotional effects can be summarized as follows: Na widened the temperature window of the C3H6 + NO + O2 reaction towards lower temperatures by ~50 ◦C, accompanied by an enhancement in N2-selectivity by ~40 additional percentage points; the promotion is optimized at a sodium loading of 2.6wt% Na on the 0.5wt%Pt/γ-Al2O3 catalyst (Figure 18). Obviously, the promoter loadings chosen by Burch and Watling were out from this optimal window, explaining the poisoning effects observed by the authors [105].


**Table 7.** Electropositive promotion of PGMs-catalysed lean-NOx reduction by C3H6 (the so-called simulated lean-burn or diesel exhaust conditions).


**Table 7.** *Cont*.

**Figure 18.** C3H6 + NO + O2 reaction on Na-promoted Pt/γ-Al2O3 catalysts. The effect of temperature on the conversion of NO to various N-containing products and the corresponding N2-selectivity: (**a**) NO conversion to N2; (**b**) NO conversion to N2O; (**c**) NO conversion to NO2; (**d**) N2-selectivity. Conditions: 1000 ppm NO, 1000 ppm C3H6, 5% O2; reactant contact time 4 s. (Reprinted with permission from Ref. [47]; Copyright 2005, Elsevier).

The performance of another noble metal, iridium, the three-way catalytic chemistry of which has several promising characteristics, have been studied under Na-promotion by Wogerbauer et al. [107]. A simulated exhaust gas mixture consisted of 300 ppm NO, 0.18% propene, 450 ppm CO, 8% O2, 10% H2O, 10.7% CO2, balance N2 was used, while the temperature and Na promoter loading were varied between 150–450 ◦C and 0–10wt%, respectively. They found that under the conditions used and for Na loadings higher than ~3wt%, Na caused an inhibition on the Ir-black catalytic activity for CO, propene and NOx conversions (Table 7). However, the selectivity towards N2 was significantly improved up to 100% for samples with Na contend > ~1wt% in the whole temperature range.

More recently, Goula et al. studied the effect of electrochemically supplied potassium on a porous Ir-film catalyst under the NO + C3H6 + O2 reaction at a variety of oxygen concentrations [108]. In this EPOC study the Ir-film was interfaced with a (K)*β*"Al2O3 solid electrolyte on which the catalyst was supported by spattering. It was found that K addition on the Ir surface is detrimental on catalyst performance (propene oxidation, NO reduction efficiency and N2-selectivity) for all oxygen concentrations investigated. Only at zero O2 concentration, that is" at net reducing conditions of the reactive mixture, a slight promoting effect of potassium on the N2-selectivity but not on the rates was observed (Table 7). This very different effect of alkali-promotion, compared to that observed on Pt

and Pd noble metals under similar conditions (on which strong promotional effects were recorded), was understandable in terms of the electronic influence of co-adsorbed potassium on the adsorption strengths of the neighbour reactants on the Ir surface: the excessive enhancement of oxygen adsorption on Ir sites in the expense of the hydrocarbon adsorption caused O-poisoning of the surface.

Overviewing Section 3.2.2 and the involved literature listed in Table 7, it is apparent that as we have seen in Section 3.1.4, alkali-promotion of the reduction of NO by propene remains very effective even under excess O2 conditions, at concentrations > ~5% which simulate lean-burn and diesel engines exhaust gases. However, at excess oxygen conditions promotional effects on both activity and selectivity are significantly attenuated and also limited in quite narrows temperature and/or promoter loading windows. Volcano type behaviour of promotion is once again recorded upon increasing alkali loading and EPOC or CCP promotion characteristics were qualitatively similar.

#### *3.3. Mechanistic Implications: The mode of Action of Electropositive Promoters*

#### 3.3.1. Main Promotion Characteristics and Mechanistic Implications

The above described results show that electropositive promoters (alkalis and alkaline earths) can markedly affect the catalytic properties of Pt-group metals during CO or hydrocarbons oxidation and NO reduction by CO or hydrocarbons in the absence or presence of oxygen, as well as of other environmentally important reactions (see Tables 1–7). The promotional benefits on the catalytic performance (activity, selectivity) achieved were similar, independently of the promotion method used, that is" the electrochemical promotion of catalysis (EPOC) or the conventional catalyst promotion one (CCP). The intensity of promotion, the optimal promoter loading and other characteristics of the induced promotional effects are subjected to the specific reaction/catalyst system and the individual reaction conditions applied (Tables 1 and 2).

This electropositive promotion (by alkalis and alkaline earths) of PGMs for emissions control catalysis reaction was found to be very pronounced not only when applied to model reaction systems (Tables 1–5) but also under complex reaction systems that mirror practical applications, that is, stoichiometric gasoline, lean-burn and diesel engines exhaust conditions (Tables 6 and 7). The latter boosts the practical importance of the subject and direct implementations on practical systems appear attractive and promising.

Model reactions studies and their individual characteristics (Section 3.1) enabled us to better understand the mechanism of the promoters' action. EPOC has also a significant role on this issue, since it gave us the opportunity of in situ adjustment of the promoter loading, recording at the same time its influence on the catalytic performance. Recall that some studies were explicitly focussed on experimentally establishing that EPOC and CCP are similar in origin and are subjected to the same physicochemical rules; the only practical different is the procedure used for the ptomoter supply [21,22,25]. To this end, Yentekakis and co-workers [21,22] have first shown that the promotional effects of sodium, electrochemically induced (via EPOC) on the NO + C3H6/Pt catalytic system, mirror those obtained on a highly dispersed Pt/γ-Al2O3 catalyst, promoted by means of CCP method at similar conditions. Therefore, EPOC and CCP will be hereafter considered as identical phenomena; the mechanistic model described below that interprets the mode of action of electropositive promoters, may be considered identical valid for both promotion methods. The vast majority of the publications reviewed here have been based on this model to consistently interpret individual promotional phenomena appeared at the specific reaction systems investigated. The main body of the model is described below and several direct spectroscopic evidences supporting its consistency are presented.

Using EPOC (via a galvanic cell as it was described by the formula (1) and Figure 1) one can electrochemically control (Faraday law) the migration of promoting species (e.g., Na+, Figure 19) from the solid electrolyte onto the catalyst/gas interface, where an effective and overall neutral double layer is formed through this ions back-spill over imposed electrochemically (Figure 19).

Actually, the electrocatalytic reaction taking place at the three-phase-boundary metal-solid electrolyte-gas that leads to the formation of the effective double layer is [6,7,16,109]:

$$\text{Na}^+ \text{(from } \beta'' \text{Al}\_2\text{O}\_3) + \text{e}^- \text{(from metal)} \rightarrow \text{[Na}^{\delta+} \text{-} \delta^- \text{](on metal)}\tag{9}$$

where [Na<sup>δ</sup>+–δ−] denotes the overall neutral Na species present at the metal-gas interface (effective double layer): the Na adatom on the metal surface carries a charge δ<sup>+</sup> and δ<sup>−</sup> is the image charge in the metal; the charge δ is about 0.8–0.5, decreasing with increasing Na coverage [7].

**Figure 19.** Schematic of the effective double layer approach of catalyst promotion [6,7,110]. An electron enriched (lower work function) catalyst surface is developed due to the uniformly dispersed Naδ<sup>+</sup> adatoms.

It is worth noting that an identical configuration is expected on the metal catalyst surface when Na is introduced to the catalyst by conventional methods (e.g., impregnation, vapor deposition, etc.), as recently demonstrated by Lambert and co-workers [110,111] who showed using XPS that the electrochemically-supplied sodium is identical with gas-supplied (by evaporation) of Naδ<sup>+</sup> cations on the catalyst surface.

Since alkali ad-atoms are imposed on the catalyst surface, they are spread over the entire metal surface due to strong repulsive dipole-dipole interactions, establishing a homogeneous, neutral, effective double layer. This double layer corresponds to an electrically modified metal surface, practically an electron enriched (lower work function) metal surface due to the δ− charge located at the metal side interface half-layer. This electrically modified surface interacts with the co-adsorbed reactants, reaction intermediates and products, changing their binding energies (in respect to those on a promoter-free metal surface), producing pronounced alterations in catalytic performance. Therefore, the main feature of the electropositive promotion of PGM by alkalis and alkaline earths is the resulted electron enriched, lower work function, PGM surface.

Therefore, the concomitant alterations on the adsorption characteristics of the co-adsorbed reactants on such a modified metal surface are of course subjected to the electronic properties of the reactants: electrophilic (electron acceptor) or electrophobic (electron donor): both theory [112] and experiment (e.g., [106,113]) have shown that electropositively promoted (by alkalis) Pt-group metal (PGM) surfaces appear strengthening in the bonds of the metal—electron acceptor (electrophilic) adsorbates, for example, PGM–NO, PGM–O2, PGM–N, PGM–O, PGM–CO and weakening in the bonds of the metal-electron donor (electrophobic) adsorbates, for example, PGM-hydrocarbons and their fragments, PGM-CO. Donation and/or backdonation issues of electron charge between adsorbates and metal surfaces during the formation of a chemisorptive bond and their impact on the bond strength have been thoroughly explained by Vayenas and Brosda in Ref. [114] and in brief summarized in the following statements: "*Electron backdonation to bonding singly occupied orbitals of an electron acceptor reactant lying below the metal Fermi level (EF) results in strengthening of the metal-adsorbate bond, while electron backdonation to antibonding orbitals, leads to weakening of the chemisorptive bond and destabilization of the adsorbate. Also electron donation to a metal from a singly occupied orbital on an electron donor adsorbate lying above Fermi level of the metal leads in general to strengthening of the chemisorptive bond. There are cases, such as the CO chemisorption on transition metals, where both donation of electrons (from the adsorbate to the metal) and backdonation of electronic charge (from the metal to the adsorbate) play an important role in the chemisorptive bond formation (e.g., Blyholder model for CO chemisorption [115])*"; (The latter explains why someone can find CO to be considered as an electron-acceptor or even as an electron-donor adsorbate). These modifications on the reactants chemisorptive bonds are accompanied by alterations in the activation energies of the catalytic reactions and in some cases on their mechanism (e.g., on the rate determination step), resulted to dramatic changes on their intrinsic activity and/or selectivity. The specific features of the promotion are subjected to the specific catalytic systems under consideration and to the reaction conditions imposed.

Taking into account that the vast majority of the reactions involved in emission control catalysis processes obey Langmuir-Hinshelwood mechanism (competitive adsorption of the reactants), the following example could be of usefulness for a better understanding of the mechanism of action of the promoters under the concept of the effective double layer approach. Let's suggest a starting rate at point 1 on the rate versus [A] curve I (Figure 20), as determined by the applied reaction conditions on the unpromoted reaction A + B, that are considered to follow a typical Langmuir-Hinshelwood (LH) mechanism with the well-known characteristic rate maximum (point 2) due to the competitive adsorption of the reactants. The reaction rate is proportional to the product of the reactants' coverages, *θ*A·*θ*<sup>B</sup> (Equation (10)), maximized at equilibrated coverages *θ*<sup>A</sup> and *θ*<sup>B</sup> on the catalyst surface:

$$\mathbf{r} = \mathbf{k}\_{\mathrm{o}} \exp(-\mathrm{E}\_{\mathrm{a}}/\mathrm{RT}) \boldsymbol{\theta}\_{\mathrm{A}} \cdot \boldsymbol{\theta}\_{\mathrm{B}} \tag{10}$$

where koexp(−Ea/RT) = k is the temperature dependence of the rate constant k with a pre-exponential factor ko and an apparent activation energy Ea.

Applying (chemically or electrochemically) a promoter, the created effective double layer (Figure 19) causes alterations on the adsorption strengths of the reactants, modifying their coverages *θ*<sup>A</sup> and *θ*<sup>B</sup> thus giving the possibility of their adjustment at the equilibrated values *θ*A\* and *θ*B\* that minimizes reaction probability and consequently rate (point 2):

$$\mathbf{r}^\* = \mathbf{k}\_\mathbf{O} \exp(-\mathbf{E}\_\mathbf{a} / \mathbf{RT}) \theta\_\mathbf{A} ^\* \cdot \theta\_\mathbf{B} ^\* \tag{11}$$

However, this is the minimum effect that can be offered by this course and actually Equation (11) and consequently path 1→2 are not real: Due to the induced changes on the chemisorptive bonds of the reactant, alteration on the apparent activation energy of the reaction is also expected from the unpromoted value Ea to a modified value Ea\*, leading the reaction rate to follow a new substantially enhanced behaviour (curve II, Figure 20, for example, point 3), following Equation (12):

$$\mathbf{r}^{\ast \ast} = \mathbf{k}\_{\mathrm{O}} \exp(-\mathrm{E\_{a}}^{\ast}/\mathrm{RT}) \boldsymbol{\theta}\_{\mathrm{A}} \,^{\ast} \cdot \boldsymbol{\theta}\_{\mathrm{B}} \,^{\ast} \tag{12}$$

Notably, in the cases where reaction mechanism modifications are induced by the promoter as well (e.g., change of the rate determination step), promotion can receive unprecedented enhancements (promoted rate r\*\*\* on curve III, Figure 20, e.g., point 4).

**Figure 20.** Schematic representation of the mechanism of promotion on a Langmuir-Hinshelwood type reaction.

An example which mirrors all the aforementioned issues is the alkali-promoted NO + C3H6 reaction on Pt [15,58,61]; reaction rate enhancements up to 40,000% were achieved accompanied with dramatic chances on the selectivity towards N2, as well (Table 4). The promotion mechanism on this system is described as follow (Figure 21):

First, it is well established that NO + C3H6 reaction over Pt obeys Langmuir-Hinshelwood type kinetics with characteristic rate maxima reflecting competitive adsorption of the two NO and C3H6 reactants on Pt active sites; these rate maxima occurred at very low C3H6/NO ratios (<0.08), that is, very high NO concentrations relative to C3H6 are necessary in order to achieve a comparable coverage of both reactants on the Pt surface, thus maximizing the rate [58,62], reflecting a weaker adsorption of NO on Pt sites relative to C3H6 [58,62,67,68] mainly due to the presence of π-electrons in the alkene structure. As a consequence, in a wide range of reactants partial pressures and temperatures conditions the un-promoted (alkali-free) Pt surface is predominantly covered by propene and propene-derived fragments (Figure 21a), therefore the un-promoted reaction rate, ro, is quite low.

Addition of alkali, which is accompanied by the formation of the [Alkali<sup>δ</sup>+–δ−] effective double layer, provides an electron enriched (lower work function) metal surface that in effect favours the adsorption of electrophilic co-adsorbates (NO) and inhibit that of electrophobic co-adsorbates (C3H6 and its fragments). According to the following Reaction (13)–(18) scheme,

$$\text{NO(g)} \rightarrow \text{NO}\_{\text{ads}} \qquad \text{(enhanced by alkali addition)} \tag{13}$$

$$\text{NO}\_{\text{ads}} \rightarrow \text{N}\_{\text{ads}} + \text{O}\_{\text{ads}} \quad \text{(enhundred by alkali addition)} \tag{14}$$

$$\text{C}\_3\text{H}\_6(\text{g}) \rightarrow \text{C}\_3\text{H}\_{6,\text{ads}} \qquad \text{(inhibited by alkali addition)} \tag{15}$$

$$\rm N\_{ads} + N\_{ads} \to N\_2(g) \tag{16}$$

$$\rm{N\_{ads}} + \rm{NO\_{ads}} \to \rm{N\_2O(g)}\tag{17}$$

$$\text{CO}\_{\text{ads}} + \text{hydrocarbon peroxide (or CO)} \rightarrow \text{CO}\_2\text{(g)} + \text{H}\_2\text{O(g)}\tag{18}$$

the resulted effects are (Figure 21b): (i) strengthening of the Pt-NO bond with a concomitant enhancement population of NO molecules on the alkali-modified surface (Reaction (13)) –the surface populations of NO and propene are therefore equilibrated; (ii) strong enhancement of the dissociative adsorption of NO (Reaction (14)) as a result of the Pt–NO bonds strengthening and the concomitant weakening of the N–O bond in the adsorbed NO molecules—the surface is enriched with very active atomic oxygen species; (iii) weakening of the strength of the Pt–C3H6 bond—an equilibrated population of more active (due to their weaker adsorption) hydrocarbonaceous species is resulted (Reaction (15), Figure 21b).

**Figure 21.** Schematic of the mechanism of action of alkalis as promoters on the Pt-catalysed NO + C3H6 reaction. Expected distribution of reactants and/or intermediates over the unpromoted surface (**a**) and the Na-promoted surface (**b**).

All these factors operate together in increasing the reaction probability of NO and propene molecules, leading to an enhanced promoted rate, rp, over the alkali-modified surface. These changes on adsorbed species distribution and/or population (coverage), their bond-strengths and activation energies and on reaction rate-determination-step (rds) were successfully used to convincingly interpret the rate enhancement values, *ρ* = rCCP/ro, as high as 420 and 280 for N2 and CO2 productions have been reported for this catalytic system under electropositive promotion by alkalis or alkaline earths [15,61,62]. Notably, these gains in catalytic activity did not reflect the maxima of the L-H rate curves, *that is*, were not restricted by the optimal balance of the competitive reactants' coverages. These were extraordinarily larger, due to the synergy of the aforementioned factors.

The observed increase in N2-selectivity upon electropositive promotion can be also readily understood in terms of the above considerations. According to the reaction network (13)–(18), the production rates of N2 and N2O depend on the extent of NO dissociation (reaction (14)), which is followed by the elementary Reactions (16) and (17). The observed increase in the N2-selectivity upon promoter addition is a consequence of increased NO dissociation, that is, less molecular NO and more atomic N on the surface. According to Lang et al. [112], the electrostatic field produced by Na<sup>+</sup> can shift the π∗-orbital energy of NO adsorbed in the vicinity of an alkali ion below the Pt Fermi level. Then, valence electrons from the metal can populate the NO π∗-orbital energy, resulting in weakening of the N–O bond and a strengthening of the Pt–N bond. Both factors favour Reaction (16) over Reaction (17), leading to higher N2 selectivity.

In the case of CO + O2 reaction the strong promotion observed under CO-rich reaction conditions (e.g., [16,27]), was understandable by means of the pronounced strengthening of the O adsorption bond, compared to that of CO: at such conditions where CO coverage predominates and the formation of CO islands suppresses reaction probability, the alkali-induced pronounced adsorption of O species causes destruction of CO islands thus increasing reaction probability [16].

In the case of alkanes oxidation, it is well known that alkanes, in particular CH4, have low propensity for adsorption (activation) on PGM surfaces. The adsorption of these electrophobic adsorbates is being even worse on alkali-modified electron enriched surfaces. This makes the generally observed alkali-induced poisoning of alkanes oxidation reactions readily understandable, no matter of what oxidant agent (oxygen or NO) used [48,49,65,66].

The opposite was true for alkenes oxidation (by O2 or NO), due to their strong propensity to adsorb on PGMs. Thus, weakening their adsorption and at the same time strengthening the adsorption of the electrophilic limiting reactant (O) on the alkali modified PGM surfaces are factors that operate synergistically creating ideal promotion conditions (e.g., [18,20,43–47]).

Comparing the alkali promotion of the NO reduction by alkenes, the Ir~Rh < Pd < Pt increasing order can be observed: marginal promoting effects have been obtained during Rh- or Ir-catalysed NO reduction by propene [59,60,64,107,108], in opposite to the very large on Pd [14] and to the extraordinarily large on Pt (e.g., [15,58,61,62,106]) This is related to the different propensity of NO adsorption on these metals: on Ir and Rh, NO has high propensity of adsorption, which is also dissociative; on Pd less and partly dissociative, while on Pt it is even less and non-dissociative. Therefore, enhancing NO adsorption by alkali-promotion on Ir and Rh surfaces has not further practical value (NO adsorption/dissociation is close to ideal on the unpromoted Ir and Rh), while on Pt the non-dissociative and quite limited NO adsorption can become much stronger and fully dissociative, enabling Pt to behave like Rh or Ir (e.g., [15,47,61,62,89]).

Comparing the amount of an alkali necessary to optimize promotion for a certain reaction on different Pt group metals, the general trend is that only few surface concentrations are needed in the case of Rh and Ir (ca. *θ*Alkali~1–3%), quite more for Pd (*θ*Alkali~5–10%) and much more for Pt (*θ*Alkali~15–40%) (e.g., [14,15,18,22,51,57,58,108]). This is readily understandable in similar terms used above: Rh and Ir electron availability (work function, WF) is by its own close to the optimal value for the reactions under consideration and only few amounts of alkalis are necessary for the optimal promoter loading adjustment. The opposite is true for Pd and in particular for Pt; these two metals showing a higher WF than Rh and Ir.

For a certain catalytic system and operating conditions, the amount of alkali that is needed to optimize promotion depends on the chemical identity of the alkali: heavier alkalis appear to be more effective that the lighter ones (e.g., [61]). This is fully consistent to the effective double layer approach considered and to the theoretical predictions of Lang et al. [112], which have shown that the larger the alkali cation the greater the effect its electric field has on an electron acceptor adsorbate (e.g., NO).

In the cases of more complex reactions systems, as for instance those simulating automotive exhaust gas mixtures, the explanation of the promotional effects is typically more complicated and may involve all the possible effects, synergistic or competitive, induced through the modification of the metal work function on all electrophilic and/or electrophobic reactant species and intermediates on such a densely populated surface with all these competitive species (e.g., [47,79,89,106]).

It is also of worth noting that: (i) Alkalis and alkaline earths are typically permanent promoters as a result of the fact that they do not participate in the reactions networks. This is of high technological importance: since the optimal alkali amount is incorporated on the catalyst and independently of the method used for its supply (EPOC or CCP), its amount remains practically constant for a very long time offering promotion. This explains why Faradaic Efficiency values as large as *Λ* > 105 have been measured in EPOC by alkalis studies (e.g., [16–18,42]). (ii) Volcano type behaviour of the promotional effects as a function of the promoter loading was an additional common feature of the titled promotion, that is, outgoing of the optimal amount of the promoter (the so-called over-promotion) the reaction is gradually inhibited rather than promoted (e.g., [15–17,47,58]). Two main factors were considered to be responsible for this behaviour: (a) the extended strengthening of the adsorption bond of the electrophilic adsorbate (electronic effect) and (b) the formation of extended surface complexes of the promoter with the reactant species and/or reaction intermediates that can block active sites (geometric effect). These factors can operate together suppressing the rate in case of over-promoted catalysts. However, it must be noticed that the formation of such 2D or 3D surface alkali complex compounds (alkali nitrites, nitrates, carbonates, oxides or even superoxides, depending on the reaction atmosphere), that can be valid even at low promoter loadings, do not actually cancel the role of alkali as a promoter (e.g., [20,43–46,51,52,57–60]). On the opposite, stabilization on the catalysts surfaces, even at elevated temperatures of catalytic interest, of normally volatile alkali metals is achieved by the formation of such stable surface alkali complex compounds.

#### 3.3.2. Direct Spectroscopic and Other Analytical Technique Evidences

Besides of the numerous kinetic studies demonstrating the usefulness of the electropositive promotion of PGM in emissions control catalysis, particular emphasis to direct evidences for the origin and mechanism of this promotion have been devoted by several research groups using a variety of surface science or other analytical techniques, including: in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) [95–98,113], x-ray photoelectron spectroscopy (XPS) (e.g., [51,52,111]), temperature programmed desorption (TPD) [106,116–120], scanning tunnelling microscopy (STM) [121–123] and work function [51,52] measurements.

To this end, Koukiou et al. [113], studying the interaction of NO with Na-modified Pt surfaces over a conventional, highly dispersed, Pt catalyst, namely Pt(Na)/γ-Al2O3, by means of in situ DRIFTS, have shown that increasing sodium loading causes a pronounced and progressive red shift of the N–O stretching frequency (Figure 19) associated with molecular NO adsorbed on the Pt component of the supported catalyst.

At the highest sodium loading (10wt% Na; corresponding to an upper limit of about *θ*Na = 0.5 nominal sodium fractional coverage on Pt particles [102]) a 1680 cm−<sup>1</sup> species is observed that corresponds to an activated NOδ<sup>−</sup> species with bond order 2, that is, a negatively charged adsorbate with increased electron density in the π\* antibonding orbital. The rigorous model developed by Lang et al. [112] was considered for understanding the effect of alkali on Pt surface. They have shown

that the inhomogeneous electric field associated with adsorbed alkali ions acts to depress the energy of the antibonding π\* orbital of electron-accepting (electrophilic) co-adsopbates (e.g., CO, NO) bellow the metal Fermi level. Therefore, the red shifts showed in Figure 22, as authors argue [113], are due to the resulting metal→adsorbate charge transfer that acts to weaken the N–O bond, red-shifting its vibration frequency and promoting its dissociation, rather than to alkali-induced changes in the relative populations of various forms of adsorbed NO.

**Figure 22.** DRIFT spectra (expanded scale for the region 2000–1600 cm−<sup>1</sup> and subtracting spectrum of pure Al2O3) obtained with 10wt%Na/γ-Al2O3 (**B**), Pt/γ-Al2O3 (**C**), Pt(Na5)/γ-Al2O3 (**D**) and Pt(Na10)/γ-Al2O3 (**E**) samples after NO adsorption for 1 min at 27 ◦C. (Reprinted with permission from Ref. [113]; Copyright 2007, Elsevier).

The chemisorption characteristics of NO on Na-dosed Pt{111} surface has been also studied by Harkness and Lambert [116] by means of temperature-programmed desorption (TPD). It was clearly demonstrated that the presence of Na on the Pt{111} single crystal causes an increase in the adsorption energy of NO (i.e., strengthening of the metal-NO bond) accompanied by a monotonically increased

NO dissociation (i.e., N2 desorption) as a function of Na coverage. In particular, NO-TPD spectra on Na-dosed Pt{111} (from 0 to 1.2ML of Na coverage) have showed two characteristic peaks, indicated as features (a) and (b); sodium caused an increase in the desorption peak (a) temperature from 360 K on the clean Pt{111} to 380 K at 0.25 ML Na-dosed Pt{111}. Even at very low Na coverages (>0.1 ML), the second NO desorption feature (b) from sodium affected sites is appeared at ~520K which is grew at the expense of the feature (a) and progressively shifted to higher desorption temperature, up to 610 K, upon increasing Na coverage to 1.2 ML. At the same time, the feature (a) is progressively attenuated to zero (practically vanished at~>1ML of Na); A progressively increased N2 peak is concomitant to the aforementioned TPD experiment, with the N2 desorption peak to remain constant at 540K up to 1ML of Na, after which it is shifted to 540 K for 1.2 ML (i.e., 100% dissociation of adsorbed NO can be obtained at sufficiently high, ca 1 ML, sodium coverage) [116].

Similar TPD studies by Garfungel et al. [117], involving the interaction of NO with K-dosed Pt{111}, yielded the same evidences: an alkali-induced increase in the Pt–NO bond strength accompanied by alkali-promoted NO dissociation was demonstrated.

Temperature-programmed desorption (TPD) studies after adsorption of oxygen on Na-dosed Pt/Al2O3 catalysts where performed by Vernoux et al. [106] in order to elucidate possible influences of the Na addition on the Pt–O bond strength. They demonstrated that the adsorption strength of the atomic oxygen (an electron acceptor adsorbate) on the Pt component of the catalyst is enhanced by Na addition; their data clearly showed that the temperature of the oxygen desorption peak correspond to Pt–O interaction increases with increasing Na loading of the catalyst, directly evidencing a strengthening of the Pt–O bond [106].

A number of early and recent studies concern the chemisorption of CO on alkali-modified PGM surfaces [95–97,117–120]. To this end, Bertolini et al. [118] studied the chemisorption of CO on K-modified Pt(100) surfaces by means TPD and Auger Electron Spectroscopy (AES). The progressive increase on the CO desorption peak temperature (i.e., strengthening of the Pt–CO) as a function of K coverage, observed at low K coverages *θ*<sup>K</sup> (ca. 0–0.4), was attributed to long range uniform electronic modifications, that is, work function changes, due to charge transfer; Kδ<sup>+</sup> (<sup>δ</sup> <sup>≈</sup> 1) adspecies appear on the K-dosed surface, uniformly distributed due to repulsion between these charged species. Such an electron enriched (lower work function) Pt surface strengthens the adsorption of CO via substantial charge donation form the K-modified Pt surface into the 2π\* orbitals of CO [118]. At high K coverages (ca. 0.45–1) short range interactions between the CO and K adspecies, that is, an even more direct bonding of CO with K (formation of CO-K-Pt surface complexes), were considered.

Pitchon et al. studied the effect of (Li, Na, K and Cs)-dosed Pd/SiO2 on CO adsorption characteristics [119]. They demonstrated drastic changes in the infrared spectrum of CO adsorbed on Pd sites; a significant weakening on the strength of the C–O bond of adsorbed CO molecules caused the appearance of a new *ν*CO band at low infrared frequency—the extent of the interaction was depended by the nature of the alkali. These influences were assigned to a localized interaction "Pd–CO–alkali" rather than to long range electronic modifications.

Using FTIR spectroscopy, Liotta et al. studied the effect of sodium on the adsorption of CO on Pd-based catalysts supported on two different supports (SiO2 and model or natural pumices) [120]. Depending on the support used, the appearance of both electronic and geometric effects—that attributed to the different localization of sodium ions in the catalysts- were evidenced; the geometric effects were predominant in the Pd/SiO2 catalysts, whereas electronic effects were most important than the geometric ones in the Pd/pumise catalysts. It was again considered in this study that an electronic density transfer to Pd is the common effect of the presence of alkali either it is subsequently added to the catalysts or is present as a structural component of the support, although, the authors observed important geometric effects in the former case. The electronic effect, that produces a red shift (towards lower frequencies) of the CO bands of the IR spectra of chemisorbed CO, owing to an enhanced transfer of electron density from the metal to the π\* molecular orbitals of CO, was again invoked [120].

Electron enriched Pt surfaces resulted by combined application of alkali-induced surface promotion and support-mediated promotion by Ce-based mixed oxides were also recently demonstrated via in situ DRIFTS studies by Yentekakis and co-workers [95–97]. Over doubly promoted Pt(Na)/Al2O3-CeO2-La2O3 catalysts operated under TWC conditions, the authors demonstrated a substantial population of adsorbed species at 2060 cm−<sup>1</sup> attributed to CO on reduced Pt0 sites when CeO2-La2O3 was incorporated into the Al2O3 support. This feature was substantially exacerbated when Na promoter was also incorporated in the catalyst formulation, while at the same time the formation of active Pt–NCO intermediates at about 2180 cm<sup>−</sup>1, which resulted from an enhanced NO decomposition, was facilitated by alkali-promotion on the Pt(Na)/Al2O3-CeLa catalyst (Figure 21). On the un-modified Pt/Al2O3 catalyst counterpart, CO species adsorbed on positively charged Pt sites (Pt<sup>δ</sup>+–CO) were assigned (Figure 23). These features, fully compatible with the way of action of alkali promotion analysed so far, were considered as a convincing explanation of the substantially enhanced TWC performance of this complex reactions/catalyst system.

**Figure 23.** In-situ DRIFT spectra during simulated TWC reaction conditions (1000 ppm NO + 1067 ppm C3H6 + 7000 ppm CO + 7800 ppm O2; Ft = 80 cm3/min; T = 200 ◦C). (Reproduced with permission from Ref. [96]. Copyright 2008, Elsevier).

Finally, a number of studies involving x-ray photoelectron spectroscopy (XPS) published by the groups of Lambert and Yentekakis (e.g., [20,51,52,57–60]) have doubtlessly shown that in EPOC studies that concern PGMs films interfaced with an alkali conducting solid electrolyte, the promoting species are indeed alkaline ions reversibly supplied by the external bias onto the catalyst surface.

The authors have also shown that under reaction conditions the electrochemically supplied alkali ions form stable surface compounds (e.g., alkali nitrites, nitrates and carbonates) that can act as promoters even under such formulations and which, in excess (over-promotion), induce geometric, active sites blocking, phenomena and thus poisoning effects on PGMs activity [20,51,52,57–60]. Combining XPS and ultraviolet photoelectron spectroscopy (UPS), the authors demonstrated a linear relationship of the catalyst work function (Δ*Φ*) and the alkali coverage (*θ*Na) with the catalyst potential overpotential (ΔVWR) (Figure 24) [51,52] and confirmed that electrochemically supplied alkali ions on the catalyst surface are identical in behaviour (and chemical state) with the alkali supplied by vacuum deposition [21,22]. This makes the close similarities found between EPOC and CCP methodologies of catalyst promotion readily understandable [15,21,22,25,58] and prompts for the use of EPOC as an effective and rapid method for exploring the effects of a range of promoters and for assessing the response of the reaction rate/selectivity to promoter coverage (finding out its optimal loadings at any set of conditions) before applying them to the design of efficient conventional catalyst formulations.

**Figure 24.** Na 1s XPS spectra versus catalyst potential (VWR) for Rh film interfaced with (Na)*β*"Al2O3 Na+-ions conductor, at 307 ◦C and UHV conditions. The inset shows the integrated Na1s XPS intensity due to sodium on the Rh surface and associated work function change of the Rh film as a function of VWR. Data was acquired using a Rh/(Na)*β*"Al2O3/Au galvanic cell (Reproduced with permission from Ref. [51]. Copyright 2001, American Chemical Society).

A recent review of the literature using surface analysis techniques to shed light on the electropositive promotion via the EPOC concept has recently provided by Gonzalez-Cobos and de Lucas-Consuegra [124].

All the aforementioned in this chapter studies, unambiguously lead to the main conclusion that alkalis adatoms on PGM surfaces provide electron enriched metal sites (lower work function surfaces), which favour the adsorption of electron donor (electrophilic) co-adsorbates and inhibit the adsorption of electron acceptor (electrophobic) co-adsorbates. The formation of alkali-reactants-metal surface complexes and therefore geometric type influences, besides the above electronic ones, can be at work

in cases where the conditions assist such a course. These issues were considered in the vast majority of electropositive promotion studies by alkalis or alkaline earths to explain promotion phenomena observed on PGM-catalysed emissions control catalysis reactions.

#### **4. Conclusions and Perspectives**

The electropositive promotion, by alkalis or alkaline earths, has been found to be an effective tool for promoting the catalytic performance of Platinum Group Metals for the vast majority of important reactions involved in emissions control catalysis systems, such as CO and alkenes oxidations, NO reduction by CO, alkenes or H2, N2O decomposition and/or reduction; the only exceptions were the alkanes oxidation by dioxygen or NO.

Up to two orders of magnitude rate increases were achievable, in particular with electropositive-promoted Pt, while in de-Nox reactions the selectivity towards N2 was simultaneously improved towards values approaching 100% over the "optimally promoted" catalysts.

Promotional achievements were large not only for model reactions but also for gas mixtures that simulate practical systems, such as stoichiometric gasoline, lean-burn or diesel engines exhaust streams.

The intensity of promotion was generally followed the order Pt > Pd > Rh~Ir, thus providing Pt or Pd-based catalysts to operate like Rh or even better in emissions control catalytic reactions. These achievements were lead to the synthesis of novel simple in synthesis and cost-effective monometallic (Pt or Pd-only) with low precious metal loading, extremely active, stable and readily recycling, three-way catalyst formulations that were compared well with commercial bi- or three-metallic (Rh, Pt, Pd)-TWCs for the control of stoichiometric gasoline engines exhaust emissions. The absence of rare and expensive Rh in the formulations of three-way converters has therefore been introduced with the concomitant environmental and economic gains.

Electropositive promotion of PGM was also found to be effective at excess oxygen conditions (e.g., lean-burn of diesel engines exhaust gases), without however in that case to obtain solutions fully satisfying the requirements.

Latterly, doubly promoted catalyst formulations, combing electropositive promotion by alkalis and support-mediated promotion by mixed oxide supports (including combinations of Al2O3 with CeO2, ZrO2, La2O3 or TiO2) have been found to provide even better promotion and stability for emissions control implementations. Such synergistic promotion catalysts' designs could lead to control systems readily adapted on a number of specific emissions of stationary sources, that have high environmental footprint and their control is an urgent issue.

From the theoretical point of view, electropositive promotion of PGM was successfully understood and interpreted in terms of the effective double layer approach, that is, a homogeneous, neutral, effective double layer established by the alkali (or alkaline earths) adatoms that are spread over the entire PGM surface providing an electron enriched (lower work function) metal surface due to the δ− charge located at the metal side interface half-layer, which interacts with the co-adsorbed reactants, reaction intermediates and products, changing their binding energies producing pronounced alterations in catalytic performance. To this end, strengthening in the bonds of the metal—electron acceptor adsorbates and weakening in the bonds of the metal—electron donor adsorbates are resulted, which are accompanied by alterations in the activation energies of the reactions between them, leading to dramatic changes of the intrinsic catalytic performance (activity and/or selectivity). In particular, in emissions control catalysis processes involving NOx reduction, the promotion of the dissociative chemisorption of the NO molecules, which is a key reaction initiating step in deNOx processes, facilitated by the alkali-induced strengthening of the PGM–NO and the accompanied weakening of the N–O bond of the adsorbed NO molecules, convincingly interpret the enormous effective promotion of both deNOx activity and N2-selectivity offered via electropositive promotion of these catalytic systems.

**Author Contributions:** I.V.Y. was the primary author, writing this review and contributing to the conception, design and comparative interpretation of the literature. P.V., A.C. and G.G. provided assistance in writing, discussing and interpreting results and updating the article. All authors have a significant research contribution

in the field, publishing a number of papers that are included and analysed in the present review. All authors read and approved the final version of the manuscript.

**Acknowledgments:** I.V.Y. and G.G. acknowledge support of the European Union and Greek national funds through the Operational Program "Competitiveness, Entrepreneurship and Innovation," under the call "RESEARCH-CREATE-INNOVATE" (project code: T1EΔK-00782). The authors thank all co-workers, colleagues and researchers contributed in the field. This work is dedicated to Professor Costas G. Vayenas, to whom the authors express their gratitude for discovering and introducing to science the EPOC phenomenon (described also as "the NEMCA effect"), which obviously is of high fundamental and practical importance and has opened new horizons in the frontiers of Catalysis and Electrochemistry.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

### *Review* **A Review of Low Temperature NH3-SCR for Removal of NOx**

### **Devaiah Damma 1, Padmanabha R. Ettireddy 2, Benjaram M. Reddy <sup>3</sup> and Panagiotis G. Smirniotis 1,\***


Received: 6 March 2019; Accepted: 4 April 2019; Published: 10 April 2019

**Abstract:** The importance of the low-temperature selective catalytic reduction (LT-SCR) of NOx by NH3 is increasing due to the recent severe pollution regulations being imposed around the world. Supported and mixed transition metal oxides have been widely investigated for LT-SCR technology. However, these catalytic materials have some drawbacks, especially in terms of catalyst poisoning by H2O or/and SO2. Hence, the development of catalysts for the LT-SCR process is still under active investigation throughout seeking better performance. Extensive research efforts have been made to develop new advanced materials for this technology. This article critically reviews the recent research progress on supported transition and mixed transition metal oxide catalysts for the LT-SCR reaction. The review covered the description of the influence of operating conditions and promoters on the LT-SCR performance. The reaction mechanism, reaction intermediates, and active sites are also discussed in detail using isotopic labelling and in situ FT-IR studies.

**Keywords:** low-temperature selective catalytic reduction; NH3-SCR; de-NOx catalysis; SO2/H2O tolerance; transition metal-based catalysts

#### **1. Introduction**

The non-renewable fossil fuels are continuing to remain the dominant energy source in power plants and automobiles to satisfy the ever-growing energy demands. However, the combustion of fossil fuels mainly generates nitrogen oxide (NOx) pollutants (NO, NO2, and N2O and their derivatives) which can cause acid rain, photochemical smog, ozone depletion, and eutrophication problems [1–4]. Due to the negative impacts of NOx, the mitigation of NOx emissions is of paramount importance for environmental protection. Several technologies are available to reduce NOx emissions by using catalytic materials and among them, selective catalytic reduction of NOx with NH3 (NH3-SCR) has been widely applied due to its high NOx removal efficiency [5–7]. Usually, the flue gas temperature of the industrial process is as low as 300 ◦C and, thus, the SCR catalyst must be active in the low-temperature regime (100–300 ◦C). V2O5–WO3(MoO3)/TiO2 is the typical and efficient catalyst and has been commercialized for NH3-SCR technology for medium temperature process [8,9]. However, this catalyst has some intrinsic drawbacks such as narrow and high working temperature window (350–400 ◦C), and low N2 selectivity in the high-temperature range [3,10,11]. Therefore, many researchers continue to develop highly active catalysts for low-temperature NH3-SCR in a wide temperature window.

With this perspective, several transition metal oxide-based catalysts have been extensively investigated for low-temperature NH3-SCR reaction due to their excellent redox properties, low price, and high thermodynamic stability. Especially, the easy gain and loss of electrons in the *d* shell of

the transition metal ions could be responsible for the facile redox properties [12–14]. For example, the Cr/TiO2 [15], Cr-MnOx [8], Fe-MnOx [16], Mn/TiO2 [17,18], FexTiOy [19], MnOx/CeO2 [20], and Cu/TiO2 [21] catalysts were shown to exhibit good SCR activity in the low temperature range. In our earlier work, we investigated the low-temperature NH3-SCR in the presence of excess O2 on the TiO2 supported V, Cr, Mn, Fe, Co, Ni, and Cu oxides and found the catalytic performance decreased in the following order of Mn > Cu ≥ Cr Co > Fe V Ni [16]. Particularly, manganese-containing catalysts have attracted much attention due to its variable valence states and excellent redox ability [2,5]. In the recent past, we published a series of papers on Mn-based SCR catalysts that showed a highly promising deNOx potential in the low-temperature region [5,22–26]. However, these catalysts are very sensitive to the presence of SO2 in the feed and exhibit lower N2 selectivity [8,27–29]. Hence, the development of catalysts that show both good low-temperature activity and high SO2/H2O durability is of great importance for the NH3-SCR reaction. In general, there are two plausible strategies available to enhance low-temperature NH3-SCR performance. One strategy is to modify the transition metal oxide with one or multiple metal oxides, which could enhance the active sites for the reaction by inducing the synergistic effect [30–33]. The other approach is to synthesize the supported materials to disperse the transition metal-based oxides which can increase the activity by metal-support interactions [26,34–37]. Recently, many supported and mixed transition metal catalyst formulations have been studied to improve the low-temperature SCR performance, as well as resistance to SO2/H2O.

In this study, we systematically reviewed the recent advancements in developing the transition metal-based catalysts for low-temperature NH3-SCR reaction. This review also demonstrated the action of different promoters and supports on the catalytic performance and SO2/H2O tolerance of the transition metal-based catalysts in NH3-SCR of NOx. The reported catalysts were divided into four categories, such as binary, ternary/multi, supported single, and supported binary/multi-transition metal-based catalysts.

#### **2. Binary Transition Metal-Based Catalysts**

Various transition-metal oxides have been proved to be active for the NH3-SCR at low-temperature. However, the catalytic performance on single transition metal oxide is far from satisfactory due to their low specific surface area and thermal instability [38–41]. The addition of dopants is a common method to improve the drawbacks associated with pure transition metal oxide. Hence, much progress has done to improve the low-temperature SCR activity of transition metal oxides by mixing or doping with other metal oxides. In recent years, Mn, Fe, Co, Ni, and Cu-based binary oxide catalysts have been extensively studied for low-temperature NH3-SCR reaction due to their attractive catalytic performance [19,33,41–49]. Particularly, Mn-based binary oxides are popular and proven to be effective catalysts for low-temperature NH3-SCR reaction [42,50,51]. Recently, Xin et al. [52] designed bifunctional Va-MnOx (where a represents the molar ratios of V / (V + Mn)) catalysts composed of Mn2O3 and Mn2V2O7 phases that significantly improved both NOx conversion and N2 selectivity in comparison with Mn2O3 at low-temperature (Figure 1). Although Mn2V2O7 showed an excellent N2 selectivity, the NOx conversion is much lower on it. Especially, above 90% NOx conversion and 80% N2 selectivity was observed in the temperature region of 120–240 ◦C over the V0.05-MnOx catalyst. The V0.05-MnOx catalyst also found to be exhibit higher NOx conversion to N2 as compared to the mechanically mixed Mn2O3 + Mn2V2O7 sample which has the same component content to V0.05-MnOx (Figure 1). This finding indicated that the synergism between Mn2O3 and Mn2V2O7 exists in the chemically prepared V0.05-MnOx rather than the mechanically mixed Mn2O3 + Mn2V2O7 sample. Moreover, the mechanically mixed Mn2O3 + Mn2V2O7 sample showed higher activity in comparison to mechanically mixed MoO3 + Mn2V2O7 sample, suggesting that the presence of Mn2O3 phase in the catalyst is necessary for NH3-SCR reaction. In conjunction with in situ IR characterization and DFT (density functional theory) calculation results, the authors concluded that the Mn2O3 phase of the catalyst could activate NH3 into NH2 intermediate, which then transferred to the Mn2V2O7 phase of

the catalyst and reacted with gaseous NO into NH2NO. Finally, the generated NH2NO intermediate on the Mn2V2O7 phase exclusively decomposed to the N2 rather than the undesired byproduct, N2O, which is formed due to the deep oxidation of adsorbed NH3 on Mn2O3.

**Figure 1.** (**a**) NOx conversion and (**b**) N2 selectivity for Va-MnOx, Mn2O3, Mn2V2O7, and reference samples. Reprinted from Reference [52]. Copyright 2018, with Permission from American Chemical Society.

Han and co-workers [53] fabricated triple-shelled NiMn2O4 hollow spheres (Figure 2a,b) by using a solvothermal method and tested their ability for low-temperature NH3-SCR reaction. As shown in Figure 2c, the prepared NiMn2O4 hollow spheres (NiMn2O4-S) showed the best catalytic activity with NOx conversion of above 90% over a wide temperature range from 100 ◦C to 225 ◦C as compared to the NiMn2O4 nanoparticles (NiMn2O4-P). The triple-layer shell structure of the NiMn2O4-S catalyst generates a larger surface area (165.3 m<sup>2</sup> g-1) that exposes more active sites (such as surface Mn4+ and surface adsorbed oxygen species), which are responsible for its superior activity. Additionally, the NiMn2O4-S catalyst displayed outstanding stability and good tolerance to H2O and SO2 (Figure 2d).

**Figure 2.** (**a**) Scheme of the preparation of triple shelled NiMn2O4 hollow spheres and (**b**) TEM image of NiMn2O4-S; (**c**) NOx conversion over the NiMn2O4 catalysts and (**d**) durability tests of the NiMn2O4-S catalyst at 150 ◦C. Reaction conditions: [NO] = [NH3] = 500 ppm, [O2] = 5 vol%, [SO2] = 100 ppm (when used), [H2O] = 5 vol% (when used), balanced with Ar, GHSV = 68,000 h-1. Adapted from Reference [53]. Copyright 2018, with Permission from Royal Society of Chemistry.

Gao et al. [54] investigated the low-temperature NH3-SCR reaction over the hydroxyl-containing Me-Mn binary oxides (Me = Co, Ni) prepared by a combined complexation–esterification method. It was found that the NOx conversion decreased in the order of Mn3O4-Co3O4-OH (Co-MnOx binary oxide) > Mn2O3-NiMnO3-OH (Ni-MnOx binary oxide) > Mn2O3-OH, while the N2 selectivity increased in the sequence of Mn3O4-Co3O4-OH < Mn2O3-OH < Mn2O3-NiMnO3-OH. Although the Co and Ni elements in the catalysts delay the poisoning of SO2 as compared to MnOx sample, the Co-MnOx and Ni-MnOx binary oxides are deactivated by SO2 over the postponement due to the formation of metal sulfate and ammonia hydrogensulfite species. In another study, Sun and co-workers [55] prepared Mn0.66M0.33Ox catalysts (M = Fe, Zn, Cu) and a series of FeαMn1−<sup>α</sup>Ox (α = 1, 0.25, 0.33, 0.50, 0 mol%) catalysts and examined for NH3-SCR at low-temperatures. The results demonstrated that the Fe0.33Mn0.66Ox catalyst displayed the superior NH3-SCR activity (NOx removal efficiency > 90%) in a wide temperature range (75–225 ◦C) among the Cu0.33Mn0.66Ox, Zn0.33Mn0.66Ox, and FeαMn1−αOx (α = 1, 0.25, 0.50, 0 mol%) catalysts. The authors proposed that the distortion of the catalyst structure by Fe doping could play a key role in improving the NH3-SCR performance over the Fe0.33Mn0.66Ox catalyst.

Rare-earth metal oxides have been frequently adopted to modify the MnOx as an efficient low-temperature NH3-SCR catalyst due to their incomplete 4f and empty 5d orbitals [50,51,56]. Fan et al. [57] synthesized Gd-modified MnOx catalysts with Gd/Mn molar ratio of 0.05, 0.1, and 0.3 to improve the catalytic performance and sulfur resistance in the NH3-SCR reaction at low-temperature. The MnGdO-2 catalyst (the mole ratio of Gd/Mn = 0.1) found to show the optimal NO conversion and N2 selectivity among the investigated catalysts. The addition of a proper amount of Gd into MnOx could enhance the concentrations of surface Mn4+ and chemisorbed oxygen species, and increase the amount and the strength of surface acid sites, which lead to better low-temperature catalytic performance than the others. Furthermore, the MnGdO-2 catalyst had an excellent tolerance to SO2/H2O as compared to pure MnOx sample (Figure 3). Their results demonstrate that the doping of Gd could restrains the transformation of MnO2 to Mn2O3 and the generation of MnSO4, obstructs the decrease in Lewis acid sites and the increase in Brønsted acid sites, and eases the competitive adsorption between the NO and SO2 and, thus, improves the resistance to SO2.

**Figure 3.** The (**a**) resistance to water vapor poisoning test and (**b**) resistance to sulfur poisoning test. (Reaction condition: [NO] = [NH3] = 500 ppm, [O2] = 5 vol%, balanced with N2, [H2O] = 5 vol%, [SO2] = 100 ppm, and GHSV = 36,000 h−1). Reprinted from Reference [57]. Copyright 2018, with Permission from Elsevier.

Li et al. [58] developed hollow MnOx-CeO2 binary nanotubes as efficient low-temperature NH3-SCR catalysts via an interfacial oxidation-reduction process using KMnO4 aqueous solution and Ce(OH)CO3 nanorod as both template and reducing agent without any other intermediate. They reported that the MnOx-CeO2 hollow nanotube catalyst with 3.75 g of Ce(OH)CO3 template (denoted it as MnOx-CeO2-B) exhibited outstanding performance with more than 96% NOx conversion in the temperature range of 100–180 ◦C. The best activity of the MnOx-CeO2-B catalyst was due to its ample number of surface Mn4+ and O species, and hollow and porous structures that provide abundant Lewis acid sites and large surface area. Additionally, MnOx-CeO2-B catalyst showed an excellent resistance to H2O and SO2 (Figure 4) and especially, the great SO2 tolerance was ascribed to the hierarchically porous and hollow structure that inhibits the deposition of ammonium sulfate species, and the doping of ceria that acts as an SO2 trap to limit sulfation of the main active phase.

**Figure 4.** H2O tolerance and SO2 tolerance of the MnOx-CeO2-B hollow nanotube. Reaction conditions: [NOx] = [NH3] = 1000 ppm, [O2] = 5%, N2 as balance gas, and GHSV = 30,000 h−1. Reprinted from Reference [58]. Copyright 2018, with Permission from Elsevier.

Fe-based binary catalysts have also been studied as NH3-SCR catalysts due to their high activity, excellent resistance to H2O and SO2, outstanding environmentally friendly performance, lower cost, and higher abundance [59–62]. Mu et al. [63] synthesized a series of vanadium-doped Fe2O3 catalysts and evaluated the effect of V on the low-temperature NH3-SCR activity of hematite. The NH3-SCR activity and N2 selectivity are greatly enhanced after the incorporation of vanadium into Fe2O3 and the Fe0.75V0.25O<sup>δ</sup> catalyst with a Fe/V mole ratio of 3/1 showed the best catalytic performance over a wide temperature window and strong resistance to H2O and SO2. They found that the charge transfer from Fe to V due to the electron inductive effect between Fe and V which could enhance the redox ability and surface acidity thereby superior NH3-SCR activity at low-temperature. The in situ DRIFTS and kinetic studies suggested that the SCR reaction followed the Langmuir–Hinshelwood mechanism below 200 ◦C, while an Eley−Rideal mechanism dominated at and above 200 ◦C. Li and co-workers [64] reported novel iron titanium (CT-FeTi) catalyst, prepared by a CTAB-assisted process, showing good deNOx efficiency and H2O resistance at low-temperature as compared to the FeTi catalyst that prepared without adding CTAB. The addition of CTAB during the CT-FeTi catalyst preparation not only promotes to form the uniform mesoporous structure to avoid being excessively enlarged in the presence of H2O, but also enhances the adsorption of bridging nitrate and NH3 species on Lewis acid sites. Thus, the authors concluded that the CTAB acted as a "structural" and "chemical" promoter in improving the NH3-SCR activity and H2O resistance at low-temperature.

Recently, Co-based spinel catalysts have shown to exhibit a remarkable low-temperature NH3-SCR activity, N2 selectivity, and tolerance to SO2/H2O [30,65–67]. Meng et al. [48] synthesized a highly efficient CoaMnbOx (where a/b is the molar ratio of Co/Mn) mixed oxide catalysts and investigated the effects of the Co/Mn molar ratio on the low-temperature NH3-SCR reaction. The CoaMnbOx mixed oxides showed higher NH3-SCR activity than either MnOx or CoOx alone due to their improved redox properties and surface acid sites by the synergistic effects between the Co and Mn species. Particularly, the catalyst with Co/Mn molar ratio of 7:3 (Co7Mn3Ox) exhibited the greatest activity (>80% NOx conversion) in a temperature window of 116–285 ◦C as compared to the catalysts with Co/Mn molar ratio of 5:5 (Co5Mn5Ox) and 3:7 (Co3Mn7Ox). They considered that the high NO + O2 adsorption ability and enhanced redox properties of the Co7Mn3Ox catalyst, emerging from its MnCo2O4.5 spinel phase and higher surface area, were beneficial to augment the NH3-SCR performance by forming

nitrate species on the catalyst surface. Although Co7Mn3Ox catalyst had better resistance to H2O/SO2 than the Co3Mn7Ox and MnOx, the tolerance to SO2 poisoning still need to be improved for practical use. Nevertheless, it was found that the deactivated Co7Mn3Ox, Co3Mn7Ox, and MnOx catalysts in SO2 stream can be regenerated simply by washing with water. Based on their results, the authors also proposed the NH3-SCR reaction mechanism over the Co7Mn3Ox catalyst, which is shown in Scheme 1. The reaction was initiated by adsorption and activation of gaseous oxygen on oxygen vacancies (symbol ), which was then transformed into lattice oxygen O2<sup>−</sup> (Step 1); This lattice oxygen was diffused to the catalyst surface and then it had become surface active oxygen (O\*) (Step 2); Gaseous NO was adsorbed and subsequently reacted with O\* to form NO2/NO3 − intermediates (Step 3); Meanwhile, NH3 was activated to −NH2 and NH4 <sup>+</sup> species by Mn4+ (Step 4); Finally, NO2/NO3 − intermediates reacted with the NH species to produce reaction products, N2, and H2O (Step 5); By the electron transfer from Mn3+ to Co3+ (Step 6); the catalyst was recovered to its original state (Step 7); Thus, the synergistic effect between the Co and Mn plays a key role in improving the NH3-SCR activity over Co7Mn3Ox catalyst.

**Scheme 1.** The proposed mechanism of the NH3-SCR reaction over the Co7Mn3Ox catalyst and the synergetic catalytic effect between Mn and Co cations. Reprinted from Reference [48]. Copyright 2018, with Permission from Elsevier.

Mesoporous materials have been proved as promising catalysts for NH3-SCR reaction since they can facilitate to promote effective diffusion of reactants towards the active sites [30,65,66,68]. With this perspective, Hu et al. [47] developed mesoporous 3D nanosphere-like Mn-Co-O catalysts through a template-free approach and evaluated for low-temperature NH3-SCR reaction. It was found that the synthesized Mn-Co-O samples showed excellent NH3-SCR activity in a broad working temperature window of 75 to 325 ◦C (NOx conversion above 80%). They ascribed this outstanding performance to the strong and abundant acid sites, the strong adsorption of NOx, robust redox properties, the formation of more oxygen vacancies and metal-metal interactions between the cobalt and manganese species.

Besides Mn, Fe, and Co oxides, CuOx has also been considered in the bimetallic catalyst formulations for low-temperature NH3-SCR reaction [69,70]. For instance, Ali et al. [71] reported the Cux-Nb1.1-x (x = 0.45, 0.35, 0.25, 0.15) bimetal oxides catalysts and found that the Cu/Nb ratio was crucial in enhancing the NH3-SCR activity. As shown in Figure 5a,b, all binary Cux-Nb1.1-x oxides exhibited significantly higher activity than the CuOx and Nb2O5, and among the Cux-Nb1.1-x samples, Cu0.25-Nb0.85 catalyst displayed the best performance in a wide temperature window of 180–330 ◦C (>90% NO conversion). Even at a high GHSV of 105,000 h−1, the optimal Cu0.25-Nb0.85 catalyst

showed a good NO removal efficiency (above 90% NO conversion) from 210 ◦C to 360 ◦C (Figure 5c). Although the SO2/H2O streams in the feed gas have some adverse impact on Cu0.25-Nb0.85, still the catalyst showed excellent resistance to SO2/H2O with reversible deactivation (Figure 5d). The superior NH3-SCR performance and SO2/H2O tolerance of Cu0.25-Nb0.85 catalyst were attributed to its high acid amount and NO adsorption capacity.

**Figure 5.** (**a**) NO conversion; (**b**) N2 selectivity over Cux-Nb1.1-x (x = 0.45, 0.35, 0.25, 0.15) as a function of temperature under a GHSV of 35,000 h<sup>−</sup>1; (**c**) effect of GHSV on NO conversion over Cu0.25-Nb0.85, and (**d**) effect of SO2, H2O, and SO2 + H2O on NO conversion over Cu0.25-Nb0.85 at 200 ◦C under a GHSV of 35,000 h-1. (Reaction conditions: [NO] = [NH3] = 1000 ppm, [O2] = 3% and N2 balance), and Effect of SO2, H2O, and SO2 + H2O on NO conversion over Cu0.25-Nb0.85 at 200 ◦C under a GHSV of 35,000 h<sup>−</sup>1. Reprinted from Reference [71]. Copyright 2018, with Permission from Elsevier.

#### **3. Ternary and Multi-Transition Metal-Based Catalysts**

The catalytic performance of single transition metal oxides can also be improved by mixing with two or multi other metal oxides. Thus, transition metal oxides are widely reported to fabricate ternaryor multi-metal-based low-temperature NH3-SCR catalysts, which could improve the catalytic activity by the enlarged synergetic interactions [14,72–87]. Fang et al. [88] investigated the low-temperature NH3-SCR reaction over the Fe0.3Mn0.5Zr0.2 catalyst and found that it showed an excellent deNOx activity with 100% NO conversion in the temperature range of 200–360 ◦C as compared to the Fe0.5Zr0.5 and Mn0.5Zr0.5 samples. Moreover, the Fe0.3Mn0.5Zr0.2 catalyst had outstanding stability and good tolerance to SO2 (Figure 6), which they attributed to the strong interactions among Fe, Mn, and Zr species. However, the durability of the catalyst in the presence of both SO2 and H2O need to be tested to investigate its feasibility in practical use.

**Figure 6.** (**a**) NO removal efficiency of Fe0.3Mn0.5Zr0.2 as a function of time at 200 ◦C and (**b**) effect of SO2 on the NO removal over Fe0.3Mn0.5Zr0.2 at 200 ◦C. Reprinted from Reference [88]. Copyright 2017, with Permission from Elsevier.

Guo and co-workers [89] studied the effect of Sb doping on the activity of MnTiOx catalyst for NH3-SCR reaction. The results showed that Sb modification has greatly improved the NH3-SCR performance of MnSbTiOx catalysts in comparison to the MnTiOx and SbTiOx samples. Particularly, the MnSbTiOx-0.2 (Sb/Mn molar ratio = 0.2) catalyst exhibited the best activity with above 90% NOx conversion in the temperature range of 138–367 ◦C as it had good adsorption and activation properties for NH3 and NOx reactants in SCR. It can be seen from Figure 7 that the addition of Sb dramatically improved the SO2 and H2O resistance of MnTiOx catalyst. Although the NOx conversion over the MnSbTiOx-0.2 catalyst slightly decreased in the presence of SO2 and H2O, it recovered to almost the original level after stopping the SO2/H2O supply.

**Figure 7.** SO2 resistance of MnTiOx and MnSbTiOx-0.2 catalysts at 150 ◦C. Reaction conditions: 600 ppm NO, 600 ppm NH3, 5% O2, 5% H2O, 100 ppm SO2, balance Ar, GHSV = 108,000 h<sup>−</sup>1. Reprinted from Reference [89]. Copyright 2018, with Permission from Elsevier.

Shi et al. [90] synthesized a series of NiyCo1-yMn2Ox microspheres (MSs) (y = 0.1, 0.3, 0.5, 0.7, 0.9) for NH3-SCR using a hydrothermal method. It was observed that the activity of all ternary MSs was greater than binary CoMn2Ox and NiMn2Ox, and Ni0.7Co0.3Mn2Ox showed the best NH3-SCR performance among the NiyCo1-yMn2Ox catalysts. Although the Ni0.7Co0.3Mn2Ox catalyst exhibited

good resistance to H2O, it had poor SO2 tolerance which needs to be improved. Wu and co-workers [91] compared the DeNOx performance of MnO2/CoAl-LDO and CoMnAl-LDO mixed metal oxides prepared from CoAl-MnO2-LDH and CoMnAl-LDH templates by ion-exchange/redox reaction and hexamethylenetetramine (HMT) hydrolysis methods, respectively. The CoAl-MnO2-LDH showed higher NH3-SCR activity in a broad temperature window of 90–300 ◦C (Figure 8a) as well as better stability and SO2/H2O resistance (Figure 8b) than the CoMnAl-LDO, which was attributed to its larger specific surface area, stronger redox ability, more quantitative acid sites, and abundant active components.

**Figure 8.** (**a**) NOx conversion and N2 selectivity over MnO2/CoAl-LDO and CoMnAl-LDO catalysts prepared by calcination at 500 ◦C; and (**b**) the stability and SO2/H2O resistance test (inset) of MnO2/CoAl-LDO and CoMnAl-LDO catalysts at 240 ◦C. Reaction conditions: [NH3] = 600 ppm, [NO] = 600 ppm, [O2] = 5 vol%, 100 ppm SO2 (when used), 10 vol% H2O. (when used) balanced by N2. Reprinted from Reference [91]. Copyright 2019, with Permission from Elsevier.

Leng and collaborators [92] synthesized Mn0.2TiOx, Ce0.3TiOx and series of MnaCe0.3TiOx (a = 0.1, 0.2, 0.3) catalysts and investigated their applicability for low-temperature NH3-SCR reaction. They demonstrated that the low-temperature NH3-SCR activity of MnaCe0.3TiOx was greatly improved after incorporation of Mn, and the Mn0.1Ce0.3TiOx catalyst displayed the best performance (with 100% NO conversion and above 90% N2 selectivity) in the temperature range of 175–400 ◦C even at high GHSV of 80,000 h−1. The outstanding performance of Mn0.1Ce0.3TiOx catalyst in NH3-SCR resulted from its enhanced acidity and chemisorbed oxygen, and suitable redox property derived from Ce3+ + Mn4+ ↔ Ce4+ + Mn3+ reaction. Furthermore, the NO conversion over the Mn0.1Ce0.3TiOx decreased and stabilized at 82% after the introduction of 100 ppm SO2 and 6% H2O and restored to almost 100% NO conversion after stopping the supply of SO2 and H2O (Figure 9), suggesting that the catalyst had excellent resistance to SO2/H2O and the effects were reversible.

**Figure 9.** Effect of H2O and SO2 on NO conversion over the Mn0.1Ce0.3TiOx catalyst at 200 ◦C (1000 ppm NO, 1000 ppm NH3, 3% O2, balance N2, GHSV = 40,000 h<sup>−</sup>1). Reprinted from Reference [92]. Copyright 2018, with Permission from Elsevier.

Ali et al. [93] developed a series of Nb-promoted Fex-Nb0.5-x-Ce0.5 (x = 0.45, 0.4, 0.35) oxides for NH3-SCR. The best activity (>90% NO conversion and near 100% N2 selectivity) in the broad temperature window of 180–400 ◦C as well as excellent SO2/H2O resistance (Figure 10) was observed for Fe0.4-Nb0.1-Ce0.5 catalyst. The authors considered the strong interaction among Nb, Fe, and Ce oxides leading to the enhancement of BET surface area, redox ability, acid amount, and NO adsorption capacity, which could be responsible for the outstanding performance of the catalyst.

**Figure 10.** Effect of SO2, H2O, and SO2 + H2O on NO conversion over the Fe0.4-Nb0.1-Ce0.5 catalyst at 220 ◦C. Reprinted from Reference [93]. Copyright 2018, with Permission from Elsevier.

Sun and co-workers [94] reported the multimetallic Sm- and/or Zr-doped MnOx-TiO2 catalysts for NH3-SCR reaction. As shown in Figure 11, the Sm and Zr co-doped MnOx-TiO2 (MSZTOx) catalyst had better activity (≈100% NO conversion and >95% N2 selectivity) in a wide temperature range (125–275 ◦C) with an excellent H2O/SO2 tolerance than the MSTOx (MnOx-SmOx-TiO2), MZTOx (MnOx-ZrOx-TiO2) and MTOx (MnOx-TiO2) catalysts. The authors claimed that the enhanced redox properties and acidic sites play a crucial role in improving the NH3-SCR performance of MSZTOx catalyst. Yan and co-workers [95] fabricated Cu0.5Mg1.5Mn0.5Al0.5Ox catalyst from layered double hydroxides and found that it showed better activity in a wide temperature range together with superior SO2 and H2O tolerance than conventional Mn/*γ*-Al2O3. The improved performance of Cu0.5Mg1.5Mn0.5Al0.5Ox was attributed to the high specific surface area, high reducibility of MnO2 and CuO species, an abundance of acid sites, and the good dispersion of MnO2 and CuO species.

Chen et al. [96] investigated the NH3-SCR reaction over a series of Co0.2CexMn0.8-xTi10 (x = 0, 0.05, 0.15, 0.25, 0.35, and 0.40) oxides catalysts and observed that the Co0.2Ce0.35Mn0.45Ti10 catalyst exhibited the best catalytic performance with 100% NO conversion and over 91% N2 selectivity in a broad temperature window of 180–390 ◦C. Although NOx conversion decreased to some extent after introducing SO2 and H2O, the Co0.2Ce0.35Mn0.45Ti10 catalyst showed excellent resistance to SO2/H2O with reversible inhibition effect (Figure 12). It was concluded that the interactions among Ce, Co, Mn, and Ti oxides led to more surface Brønsted acid and Lewis acid sites, NOx adsorption sites and modest redox ability which could play a crucial role to improve the NH3-SCR activity of Co0.2Ce0.35Mn0.45Ti10.

**Figure 11.** (**a**) NO conversions and (**b**) N2 selectivities of the catalysts in the NH3-SCR reaction as a function of temperature; (**c**) SO2 (100 ppm) resistance tests, and (**d**) H2O + SO2 (2.5 vol%, 100 ppm) resistance tests at 200 ◦C over the catalysts. Reproduced from Reference [94]. Copyright 2018, with Permission from Elsevier.

**Figure 12.** Effects of H2O and/or SO2 on NOx conversion over the Co0.2Ce0.35Mn0.45Ti10 catalyst. Reproduced from Reference [96]. Copyright 2018, with Permission from Elsevier.

#### **4. Supported Single Transition Metal-Based Catalysts**

Support materials have proved to be highly beneficial for enhancing the activity and durability of catalysts as they possess high surface area and good thermal stability. In virtue of the fine dispersion of the active component on the surface of the support and the synergistic effect between active component and support, supported catalysts exhibits improved NH3-SCR performance than the unsupported transition metal oxide. Therefore, a lot of attention has been focused to increase the de-NOx efficiency by dispersing the transition metal oxides over different support materials such as TiO2, Al2O3, SiO2, carbon nanotubes (CNTs), etc. TiO2 supported transition metal oxides, especially manganese oxides, have been widely reported as promising catalysts for NH3-SCR reaction at low temperature [17,24,97–105]. Smirniotis and co-workers [17,106,107] first reported the transition metal oxides (V, Cr, Mn, Fe, Co, Ni, and Cu) supported on Hombikat TiO2 for NH3-SCR reaction at low-temperature. Among the investigated samples, the Mn/Hombikat TiO2 catalyst found to exhibit the highest activity even in the presence of water. They also studied the effect of different supports on the NH3-SCR performance and observed that the Mn/Hombikat TiO2 (anatase, high surface area) had the best activity as compared to the Kemira TiO2 (rutile), Degussa P25 TiO2 (anatase, rutile), Aldrich TiO2 (anatase, low surface area), Puralox *γ*-Al2O3, Aldrich SiO2 supported Mn catalysts. It was concluded that the Lewis acidity, redox behavior, and a high surface concentration of MnO2 could play a key role in improving the NH3-SCR activity. Later, they investigated the effect of Mn loading on the NH3-SCR performance of Mn/Hombikat TiO2 and reported that the catalyst with 16.7 wt% Mn had optimal activity and excellent tolerance to H2O during 10 days of the reaction [108]. In another work, they also proposed the NH3-SCR reaction mechanism over the Mn/TiO2 catalyst using transient isotopic labeled and in-situ FT-IR studies. As shown in Figure 13, the reaction proceeds via a Mars-van-Krevelen-like mechanism, in which NH3 and NO species were first adsorbed onto the Mn4+ sites (Lewis acid sites), followed by the formation of nitrosamide and azoxy intermediate species. Finally, these intermediates converted into N2 and H2O products [24].

**Figure 13.** Plausible SCR mechanism over the surface of Mn/TiO2 catalyst. Adapted from Reference [24]. Copyright 2012, with Permission from Elsevier.

In the aspect of catalyst structure design, Smirniotis and co-workers [26] developed a series of manganese confined titania nanotube (Mn/TNT-X) catalysts using different TiO2 precursors (X = Ishihara (I), Kemira (K), Degussa P25 (P25), Sigma–Aldrich (SA), Hombikat (H), TiO2 synthesized from titanium oxysulfate (TOS)) for low-temperature NH3-SCR reaction. As can be observed from the Figure 14, all Mn/TNT-X catalysts exhibited an excellent NOx conversion in broad temperature window, and especially, Mn(0.25)/TNT-H sample obtained the superior activity in the temperature range of 100–300 ◦C as compared to other catalysts. They believed that the better performance of Mn(0.25)/TNT-H catalyst was due to the high surface area (421 m2/g) of the TNT-H support, and high dispersion of active components. The Mn(0.25)/TNT-H catalyst also showed greater catalytic performance than the conventional Mn-loaded titania nanoparticles (Mn/TiO2), suggesting that the unique multiwall nanotube with open-ended structure could be advantageous to promote the reaction. Besides, the Mn (0.25)/TNT-H displayed outstanding tolerance to 10 vol% H2O in the feed (Figure 15), which might be attributed to the preferential existence of highly active and redox potential pairs of Mn4+ and Mn3+ in the tubular framework.

**Figure 14.** Catalytic evaluation of the Mn(0.25)/TNT-X (X = Hombikat, Ishihara, P25 Degussa, Kemira, Sigma–Aldrich, and Titania oxysulfate) family of catalyst for the SCR of NOx by NH3, in the presence of 900 ppm NO, 100 ppm NO2, 1000 ppm NH3, 10 vol% O2 with He balance under a GHSV of 50,000 h-1 in the temperature range from 100–300 ◦C. Reproduced from Reference [26]. Copyright 2016, with Permission from Elsevier.

**Figure 15.** Influence of inlet water concentrations (10 vol%) on NOx conversion in the SCR reaction over Mn(0.25)/TNT-H catalyst at 140 ◦C; feed: NO = 900 ppm, NO2 = 100 ppm, NH3/NOx (ANR) = 1.0, O2 = 10 vol%, He carrier gas, GHSV = 50,000 h-1. Reproduced from Reference [26]. Copyright 2016, with Permission from Elsevier.

Recently, Boningari et al. [109] extended this work by comparing the NH3-SCR activity of various metal oxide confined titania nanotubes M/TNT (M = Mn, Cu, Ce, Fe, V, Cr, and Co) based on the Hombikat TiO2 support. As shown in Figure 16, the Mn-, V-, Cr-, and Cu-oxide confined titania nanotubes had excellent low-temperature activity and meanwhile, vanadium oxide confined titania nanotubes showed a broad operation temperature window for the NH3-SCR reaction.

Sheng et al. [100] synthesized core-shell MnOx/TiO2 nanorod catalyst, showed high activity, stability, and N2 selectivity in NH3-SCR. They concluded that the abundant mesopores, Lewis-acid sites, and high redox capability could be beneficial to improve catalytic performance. Although the MnOx/TiO2 catalyst exhibited excellent resistance to H2O, it was deactivated in the presence of SO2 and SO2/H2O. Jia et al. [110] reported the low-temperature NH3-SCR efficiency of MnOx/TiO2, MnOx/ZrO2, and MnOx/ZrO2-TiO2 catalysts, and found that MnOx/ZrO2-TiO2 obtained good activity at a temperature of 80–360 ◦C and excellent resistance to H2O at 200 ◦C. However, all the catalysts showed poor tolerance to SO2 and SO2/H2O that caused irreversible deactivation. Similar findings were also observed by Zhang et al. [111] over the Mn/Ti, Mn/Zr, and Mn/Ti-Zr catalysts, in which Mn/Ti-Zr sample exhibited an excellent NH3-SCR performance in a wide temperature range due to its high surface area, Lewis acid sites, and surface Mn4+ ions.

**Figure 16.** Catalytic activity evaluation of metal oxides confined titania (made of Hombikat titania) nanotube catalytic formulations M/TNT where M = Mn, Cu, Ce, Fe, V, Cr, and Co for the selective catalytic reduction of NOx by NH3 in the presence of 900 ppm NO, 100 ppm NO2, 1000 ppm NH3, 10 vol% O2 in He balance, under a GHSV = 50,000 h-1. Reprinted from Reference [109]. Copyright 2018, with Permission from Elsevier.

Carbon nanotubes (CNTs) have been reported as promising catalyst support for NH3-SCR catalysis due to their excellent stability and unique electronic and structural properties [36,112–115]. Qu and co-workers [116] reported that the NH3-SCR performance of Fe2O3 was dramatically enhanced when it supported on CNTs (Figure 17a). It was concluded that the large surface area, fine dispersion of Fe2O3, and interaction between Fe2O3 and CNTs were important factors to improve the NH3-SCR activity. In addition, the Fe2O3/CNTs catalyst showed an excellent tolerance to H2O/SO2. Interestingly, SO2 stream in the feed had promoting effect on the NO conversion (Figure 17b), which could be attributed to the increased acid sites for NH3 adsorption and activation on the catalyst surface in presence of SO2. Bai et al. [117] developed CNTs supported copper oxide catalysts, and found that the 10 wt% CuO/CNTs showed good NH3-SCR activity and excellent stability at 200 ◦C. The 10 wt% CuO/CNTs also had greater performance in comparison to 10 wt% CuO/TiO2. However, it exhibited poor resistance to SO2 and moderate tolerance to H2O.

**Figure 17.** (**a**) NOx conversion as a function of temperature over different catalysts and (**b**) SO2/H2O tolerance of the Fe2O3/CNTs catalyst. Reprinted from Reference [116]. Copyright 2015, with Permission from Royal Society of Chemistry.

#### **5. Supported Binary and Multi Transition Metal-Based Catalysts**

Given that the dispersion of two active components on support enhances the active sites further, researchers have been widely reported the supported binary transition metal-based oxides to improve the performance and SO2/H2O tolerance in NH3-SCR reaction. With this perspective, several composites, such as MnCe/CNTs [118], Mn-Fe/TiO2 [119], MnOx-CeO2/graphene [120], Mg-MnOx/TiO2 [121], CeOx-MnOx/TiO2-graphene [122], Fe-Mn/Al2O3 [123], Mn-Fe/W-Ti [124], MnOx-CeO2/TiO2-1%NG (NG = N-doped grapheme) [125], Mn-Ce/CeAPSO-34 [126], etc., were investigated for the NH3-SCR reaction at low-temperature. Smirniotis et al. [22] studied the promotional effect of co-doped metals (Cr, Fe, Co, Ni, Cu, Zn, Ce, and Zr) on the NH3-SCR performance of Mn/TiO2. As shown in Figure 18, except Zn and Zr, all other co-doped metals had a positive impact on the activity of Mn/TiO2, and particularly, the Mn-Ni/TiO2 exhibited the highest NO conversion and N2 selectivity among the other titania-supported bimetallic catalysts.

**Figure 18.** N2 selectivity and catalytic performance of Mn-M /TiO2 anatage (M = Cr, Fe, Co, Ni, Cu, Zn, Zr, and Ce) catalysts: NH3 = 400 ppm; NO = 400 ppm; O2 = 2.0 vol%; GHSV = 50,000 h−1; catalyst wt. = 0.1 g; reaction temperature = 200 ◦C. Reproduced from Reference [22]. Copyright 2011, with Permission from Elsevier.

They also investigated the influence of Ni loading on the activity of Mn/TiO2 catalyst, and found that the 5wt%Mn-2wt%Ni/TiO2 (Mn-Ni(0.4)/TiO2, where Ni/Mn = 0.4) had the optimal activity with complete NO conversion at the temperature range of 200–250 ◦C (Figure 19a) and outstanding stability even in the presence of 10 vol% water (Figure 19b,c) [5,23]. The enhanced reducibility of manganese oxide and dominant phase of MnO2 claimed to be responsible for the best activity and stability of Mn–Ni/TiO2 catalyst [5,22,23]. In another study, they compared the de-NOx performance of high surface texture hydrated titania and Hombikat TiO2 supported Mn-Ce bimetallic catalysts, and observed that the Mn–Ce/TiO2 (Hombikat) showed the better activity and excellent resistance to H2O (Figure 20). The superior performance could be attributed to the enhancement in reduction potential of active components, broadening of acid sites distribution, and the promotion of Mn4+/Mn3+, Ce3+/Ce4+ ratios including surface labile oxygen and small pore openings [25].

**Figure 19.** (**a**) Influence of Ni/Mn atomic ratio on NO conversion in the SCR reaction at a temperature range (160–240 ◦C) over Mn-Ni/TiO2 catalysts (XNO% = conversion of NO at 6 h on stream); (**b**) SCR of NO with NH3 at 200 ◦C temperature over Mn/TiO2 and Mn-Ni/TiO2 catalysts; (**c**) Influence of inlet water concentrations (10 vol%) on NO conversion in the SCR reaction over Mn–Ni(0.4)/TiO2 catalyst at 200 ◦C (GHSV = 50,000 h-1; feed: NO = 400 ppm, NH3 = 400 ppm, O2 = 2 vol%, He carrier gas, catalyst = 0.1 g, total flow = 140 mL min-1). Reprinted from Reference [5]. Copyright 2012, with Permission from Elsevier.

**Figure 20.** Influence of inlet water concentrations (7 vol%) on NOx conversion in the SCR reaction over Mn–Ce(5.1)/TiO2-Hk (Hombikat) catalyst at 175 ◦C; feed: NO = 900 ppm, NO2 = 100 ppm NH3/NOx (ANR) = 1.0, O2 = 10 vol%, He carrier gas, catalyst. 0.08 g, GHSV. 80,000 h-1. Reprinted from Reference [25]. Copyright 2015, with Permission from Elsevier.

Xu and co-workers [127] reported Ce-Mn/TiO2 catalysts with different Ce loadings, and the Ce(20)-Mn/TiO2 found to show high activity with >90% NO conversion in the temperature range of 140–260 ◦C (Figure 21a). Their SO2 tolerance results showed that the resistance ability was decreased in the order of Ce(20)-Mn/TiO2 > Ce(30)-Mn/TiO2 > Ce(10)-Mn/TiO2 (Figure 21b). Although the Ce(20)-Mn/TiO2 catalyst had reasonable resistance to 100 ppm SO2 at different reaction temperatures (Figure 21c), it exhibited moderate tolerance to SO2 poisoning when added higher than 100 ppm SO2 to the reaction feed (Figure 21d). They ascribed the good SO2 resistance of Ce(20)-Mn/TiO2 to the widely distributed elements of Mn and Ce which in turn led to the inability of the sulfate material to remain on the surface.

**Figure 21.** (**a**) Catalytic activity of Ce-Mn/TiO2 catalyst for NH3–SCR. Catalysts were loaded with 10%, 20% and 30% Ce and denoted as Ce(10), Ce(20) and Ce(30), respectively. Pure TiO2 was also used for comparison; (**b**) The effect of various Ce concentrations using a Ce-Mn/TiO2 catalyst on SO2 resistance; (**c**) The effects of reaction temperature on NO conversion of the Ce(20)-Mn/TiO2 catalyst in the presence of SO2. The above three types of reactions were performed at: 500 ppm NO, 500 ppm NH3, SO2 100 ppm, 3% O2, N2 balance. gas, GHSV = 10 000 h−1; and (**d**) the effects of SO2 concentration on NO conversion of Ce(20)-Mn/TiO2 catalysts (T = 180◦C, 500 ppm NO, 500 ppm NH3, 3%O2, N2 balance gas, GHSV = 10 000 h<sup>−</sup>1). Adapted from Reference [127]. We thank the Royal Society Open Science for this Contribution.

Lin et al. [128] synthesized Me-Fe/TiO2 (SD) catalyst via an aerosol-assisted deposition method, showing an excellent NH3-SCR performance and good tolerance to SO2/H2O as compared to its counterparts prepared by co-precipitation and wet impregnation methods. The authors concluded that the enhanced surface reducibility and adsorption ability of NH3/NOx of Mn-Fe/TiO2 (SD) catalyst could be responsible for its superior activity. Lee and co-workers [129] investigated the poisoning effect of SO2 as metal sulfate and/or ammonium sulfate deposits on the low-temperature

NH3-SCR activity of MnFe/TiO2 catalysts. They found that the metal sulfates had a more serious deactivation effect than that of ammonium salts on the MnFe/TiO2 catalysts. Their results showed that metal sulfates poisoning resulted in lower crystallinity, lower specific surface area, a lower ratio of Mn4+/Mn3+, higher surface acidity, and more chemisorbed oxygen, which in turn led to an adverse effect on the NH3-SCR activity of the catalyst. Mu et al. [130] prepared Fe-Mn/Ti catalyst by ethylene glycol-assisted impregnation method, showing high NH3-SCR efficiency over a broad temperature window (100−325 ◦C) and outstanding tolerance to sulfur poisoning. The formation of the Fe-O-Ti structure with strong interaction strengthened the electronic inductive effect and increased the ratio of surface chemisorption oxygen, thereby the enhancement of NOx adsorption capacity and NO oxidation performance, which could be beneficial to improve the NH3-SCR activity.

Liu and co-workers [131] reported that the addition of Eu had noticeably improved the NH3-SCR performance of Mn/TiO2 catalyst even after sulfation process under SCR conditions (Figure 22a). However, both the Mn/TiO2 and MnEu/TiO2 catalysts showed poor activity when they sulfated only with SO2 + O2 (Figure 22a). Further, the MnEu/TiO2 catalyst found to show better SO2 tolerance as compared to the Mn/TiO2 (Figure 22b). Their results revealed that Eu modification could inhibit the formation of surface sulfate species on the Mn/TiO2 catalyst during the NH3-SCR in the presence of SO2, which could be the reason for improved SO2 resistance.

**Figure 22.** (**a**) NH3-SCR activities of the fresh and sulfated catalysts; and (**b**) SO2 tolerances of Mn/TiO2 and MnEu/TiO2 in NH3-SCR reaction. Reprinted from Reference [131]. Copyright 2018, with Permission from Elsevier.

Sun et al. [132] investigated the NH3-SCR activity over the Nb-doped Mn/TiO2 catalysts with different Nb/Mn molar ratios, and found that the MnNb/TiO2-0.12 (where Nb/Mn = 0.12) catalyst had optimal NOx conversion and N2 selectivity in the temperature range of 100–400 ◦C (Figure 23a,b). The optimal MnNb/TiO2-0.12 catalyst also exhibited greater SO2 resistance than Mn/TiO2 catalyst (Figure 23c). The incorporation of Nb into Mn/TiO2 catalyst led to increase surface acidity and reducibility as well as generate more surface Mn4+ and chemisorbed oxygen species along with more NO2, which results in the better NH3-SCR activity. In situ DRIFT studies over the Mn/TiO2 and MnNb/TiO2-0.12 catalysts disclosed that the NH3-SCR took place through Eley–Rideal mechanism even in presence of SO2, in which the reaction mainly occurred between adsorbed NO2 and gaseous NH3. Hence, it was concluded that the higher SO2 tolerance of the MnNb/TiO2-0.12 catalyst could be due to the existence of more adsorbed NO2 on its surface.

**Figure 23.** (**a**) SCR activities and (**b**) N2 selectivities over different catalyst samples as a function of reaction temperature. Reaction conditions: [NO] = [NH3] = 600 ppm, [O2] = [H2O] = 5%, balance Ar, GHSV = 108,000 h-1; (**c**) Effect of SO2 on the SCR activities over Mn/TiO2 and MnNb/TiO2-0.12 catalyst samples. Reaction conditions: [NO] = [NH3] = 600 ppm, [O2] = [H2O] = 5%, [SO2] = 100 ppm, balance Ar, GHSV = 108,000 h-1, reaction temperature = 150 ◦C. Reproduced from Reference [132]. Copyright 2018, with Permission from Elsevier.

In another study, they reported the Mo-modified Mn/TiO2 catalysts, exhibiting improved NH3-SCR activity from 50 to 400 ◦C in comparison to Mn/TiO2 catalyst. Particularly, the optimal MnMo/TiO2-0.04 (where molar ratio of Mo/Mn = 0.04) catalyst better tolerance to SO2 poisoning compared with Mn/TiO2 (Figure 24) [133].

Fan and co-workers [134] fabricated ordered mesoporous titania supported CuO and MnO2 composites (CuO/MnO2-mTiO2) through a facile acetic acid-assisted one-pot synthesis approach, showing high deNOx efficiency (>90% NO conversion) and N2 selectivity (>95%) in a wide operating temperature range of 120–300 ◦C. They considered that the superior NH3-SCR performance could be attributed to the unique structure and highly integrated mesoporous TiO2 supported by the multicomponent system with high surface areas, accessible and homogenously dispersed CuO and MnO2 with multivalent nature and good redox activity. Although the CuO/MnO2-mTiO2 catalyst had good tolerance to H2O, the resistance to SO2 and H2O/SO2 poisoning, as well as high space velocity (GHSV), still need to be enhanced for practical use. Li et al. [135] synthesized fly ash-derived SBA-15 mesoporous molecular sieves supported Fe and/or Mn catalysts, and reported that Fe-Mn/SBA-15

catalyst showed notably greater NH3-SCR activity than Mn/SBA-15 or Fe/SBA-15 in the temperature range of 150–250 ◦C. Moreover, the Fe-Mn/SBA-15 catalyst exhibited good time-on-stream stability (200 h) and water tolerance at 200 ◦C. The high metal dispersion, Mn4+/Mn3+ ratio, the concentration of adsorbed oxygen, and the redox activity are important features to enhance the NH3-SCR performance of the Fe-Mn/SBA-15 catalyst. In their subsequent study, the authors investigated the mechanisms of NO reduction and N2O formation using in-situ DRIFT and transient reaction studies and proposed a possible denitration mechanism over the Fe-Mn/SBA-15 catalyst which is shown in Figure 25. The NH3-SCR reaction over the Fe-Mn/SBA-15 catalyst proceeded through Langmuir-Hinshelwood, Eley-Rideal, and Mars-van Krevelen mechanisms. Their results also revealed that a large amount of nitrate thereby N2O being produced over the Fe-Mn/SBA-15 during the reaction due to its strong oxidation ability, low acidity, and high basicity, which resulted in the lower N2 selectivity [136].

**Figure 24.** SO2 tolerance of Mn/TiO2 and MnMo/TiO2-0.04 catalysts at 150 ◦C, Reaction conditions: 600 ppm NO, 600 ppm NH3, 100 ppm SO2, 5% O2, balance Ar, GHSV = 108,000 h-1. Reprinted from Reference [133]. Copyright 2018, with Permission from Elsevier.

**Figure 25.** Low-temperature NH3-SCR reaction mechanism on Fe-Mn/SBA-15 catalyst. Adapted from Reference [136]. Copyright 2018, with Permission from American Chemical Society.

Tang and co-workers [137] reported that Mn2CoO4/reduced graphene oxide (Mn2CoO4/rGO) catalyst with an optimal amount of CoCl2·6H2O of 0.3 (millimole) showed excellent NH3-SCR activity and stability at low-temperature due to its large specific surface area, abundant Lewis acid sites, and special three-dimensional architecture. When 100 ppm SO2 added to reaction feed, the NOx conversion over the optimal catalyst decreased significantly (96% to 53%), and the NOx conversion was recovered to original level by water-washing after stopping the supply of SO2. However, the decreased activity (100% to 82% NOx conversion) in the presence of H2O was restored to the original level after removing H2O from the feed gas. Wang et al. [138] investigated the honeycomb cordierite-based Mn-Ce/Al2O3 catalyst for NH3-SCR reaction and found that it showed good activity and reasonable resistance to SO2/H2O. The catalyst deactivation in the presence of SO2 was ascribed to the deposition of ammonium hydrogen sulfate and sulfated CeO2 on the catalyst surface during the NH3-SCR process.

Meng et al. [139] synthesized a novel CuAlOx/CNTs (CNTs = carbon nanotubes) catalyst by facile one-step carbothermal reduction decomposition method for low-temperature NH3-SCR. The CuAlOx/CNTs catalyst was found to exhibit higher NOx conversion (>80%) and N2 selectivity (>90%) than the CuAlOx in the temperature range of 180–300 ◦C (Figure 26a). They concluded that more favorable formation of Cu+ active sites, better dispersion of active CuO species and higher surface adsorbed oxygen were beneficial to enhance the NH3-SCR activity of CuAlOx/CNTs catalyst. As shown in Figure 26b, the CuAlOx/CNTs catalyst displayed excellent resistance to SO2/H2O at 240 ◦C during the NH3-SCR. The authors attributed this outstanding SO2/H2O tolerance to the presence of CNTs that could promote the reaction of NH4HSO4 and NO continuously to avoid the formation and accumulation of excess ammonium sulfate salts on the catalyst surface. Li group [140] reported a series of ultra-low content copper-modified TiO2/CeO2 catalysts and observed that the catalyst with a Cu/Ce molar ratio of 0.005 exhibited the high NH3-SCR performance and good tolerance to SO2. Their characterization results disclosed that the addition of Cu into TiO2/CeO2 lead to enhance the Brønsted acid sites, amount of surface adsorbed oxygen and Ce3+ species, redox, and surface acidic properties, which in turn improve the NH3-SCR activity.

**Figure 26.** (**a**) NH3-SCR activity and N2 selectivity as a function of temperature from 150 ◦C to 330 ◦C; and (**b**) SO2/H2O resistance test of CuAlOx/CNTs catalyst at 240 ◦C. Reaction conditions: 600 ppm NH3, 600 ppm NO, 5.0 vol% O2, 100 ppm SO2 (when used), 10 vol% H2O (when used) balanced by N2 with a GHSV was 45,000 h−1. Reprinted from Reference [139]. Copyright 2019, with Permission from Elsevier.

Recently, supported multi-metal oxide catalysts have been considered as the very promising candidates for low-temperature NH3-SCR reaction because of the enlarged synergetic catalysis effects of different components as well as improved metal-support interactions [141–145]. Wang and co-workers [146] reported a series of Nb modified Cu-Ce-Ti mixed oxide (NbyCCT, where y represented the atomic ratio of Nb to Ti) catalysts for low-temperature NH3-SCR reaction. It was found that NbyCCT catalysts demonstrated a better activity than the Cu-Ce-Ti (CCT) and Ce-Ti (CT) samples (Figure 27a). Among all the NbyCCT catalysts, Nb0.05CCT showed a higher NO conversion (>90%) in a broad temperature range of 180–360 ◦C under the GHSV of 40,000 h−<sup>1</sup> (Figure 27a). Results indicated that the incorporation of Nb to Cu-Ce-Ti led to strong interactions among the active phases that increased the oxygen vacancies and inhibited the over-oxidation of NH3, which in turn improved the NH3-SCR activity and N2 selectivity in a wide temperature window. DRIFTS studies revealed

that the introduction of Nb promoted the generation of NO2, which could improve the activity via "fast" SCR reaction process (Langmuir–Hinshelwood reaction pathway). As shown in Figure 27b, the optimal Nb0.05CCT catalyst exhibited higher resistance to SO2/H2O as compared to the Nb free catalyst. Li et al. [147] investigated the effect of Ho doping on the NH3-SCR performance and the SO2/H2O resistance of Mn-Ce/TiO2 catalyst. Among the catalysts tested, the catalyst with Ho/Ti of 0.1 (Mn0.4Ce0.07Ho0.1/TiO2) showed the best performance with >90% NO conversion in the temperature range of 150–220 ◦C, which was attributed to high concentration of chemisorbed oxygen, surface Mn4+/Mn3+ ratio, and acidity, as well as large specific surface area. Although the Mn0.4Ce0.07Ho0.1/TiO2 showed higher resistance to SO2 and H2O than the Mn0.4Ce0.07/TiO2 catalyst, it was deactivated some extent in presence of SO2/H2O which is irreversible.

**Figure 27.** (**a**) NH3-SCR activities of different catalysts {Reaction conditions: [NH3] = [NO] = 600 ppm, [O2] = 3%, [H2O] = 5 vol%, N2 as balance. GHSV = 40 000 h<sup>−</sup>1}; and (**b**) SO2 and H2O resistance of the catalysts at 250 ◦C in SCR reaction process {Reaction conditions: [NH3] = [NO] = 600 ppm, [O2] = 3%, [SO2] = 50 ppm, [H2O] = 5 vol%, N2 balance, GHSV = 40 000 h<sup>−</sup>1}. Reproduced from Reference [146]. Copyright 2018, with Permission from Elsevier.

Lu group [148] synthesized a series of activated coke (AC) supported FexCoyCezOm catalysts for low-temperature NH3-SCR, and found that the 3%Fe0.6Co0.2Ce0.2O1.57/AC catalyst had the best activity at 250–350 ◦C and good tolerance to H2O/SO2 at 250 ◦C. The superior performance of the catalyst was ascribed to the co-participation of Fe, Co, and Ce species with different valence states, high concentration of chemisorbed oxygen, well dispersed active components, increase of weak acid sites, good redox properties of metallic oxides, and abundant functional groups on the catalyst surface. Their mechanistic and kinetic studies also indicated that the enhanced active sites for the adsorption of NO and NH3, and the redox cycle among Fe, Co and Ce were responsible for the improved activity. Zhao et al. [149] reported a series of Mn-Ce-V-WOx/TiO2 composite oxide catalysts, exhibiting greater NH3-SCR activity than the TiO2 supported single-component catalysts (Figure 28a,b). Particularly, the catalyst with a molar ratio of active components/TiO2 = 0.2 showed the best performance (>90% NO conversion) from 150 to 400 ◦C (Figure 28a). As shown in Figure 28c, the optimal Mn-Ce-V-WOx/TiO2 (molar ratio of Mn-Ce-V-WOx/TiO2 = 0.2) showed excellent stability and outstanding tolerance to H2O/SO2 at 250 ◦C. The authors concluded that the better performance of Mn-Ce-V-WOx/TiO2 mainly attributed to the variety of valence states of the four active components and their high oxidation-reduction ability.

**Figure 28.** (**A**) Selective catalytic reduction (SCR) activity of Mn-Ce-V-WOx/TiO2 composite catalysts with molar ratio of active components/TiO2 at different values; (a) 0.1; (b) 0.2; (c) 0.3; (d) 0.6; (**B**) SCR activity of V2O5/TiO2, WO3/TiO2, MnO2/TiO2, CeO2/TiO2 and TiO2. Reaction conditions: [NO] = [NH3] = 1500 ppm, O2 = 3%, gas hourly space velocity (GHSV) = 40,000 h-1; and (**C**) the lifetime of Mn-Ce-V-WOx/TiO2 catalyst with molar ratio of 0.2 at 250 ◦C: inset (a-c) H2O and SO2 resistance at 250 ◦C. Reaction conditions: [NO] = [NH3] = 1500 ppm, [O2] = 3%, [H2O] = 5%, [SO2] = 100 ppm, GHSV = 40,000 h-1. Adapted from Reference [149].

#### **6. Conclusions**

Since the emission standards for NOx are becoming more stringent to keep our atmosphere clean, the widespread use of fossil fuel in automobiles and industries require advanced catalytic materials for NOx emission control. The low-temperature SCR of NOx with NH3 would be a promising solution to mitigate the NOx emissions from mobile and stationary sources. Hence, the development of efficient catalysts for the low-temperature NH3-SCR with high deNOx activity, N2 selectivity, and high resistance toward SO2/H2O poisonings is the subject of increasing interest in the field of environmental catalysis. Transition metal-based oxide catalysts have drawn much attention for low-temperature NH3-SCR due to their excellent redox properties, high activity, durability, and relatively low manufacturing costs. In this review, we have summarized the recent progress in the low-temperature NH3-SCR technology over the various transition metal-based catalysts. Over the past decades, significant research efforts have been made to improve the de-NOx efficiency and SO2/H2O tolerance of transition metal-based oxides in NH3-SCR at low-temperatures. Various transition metal-based mixed oxides with and without support have been extensively studied for NH3-SCR reaction and, particularly, MnOx-based catalyst formulations have caught much attention because of their excellent de-NOx efficiency at low-temperatures. The modification of transition metal oxides by doping with other metal oxides led to high redox ability and acidic sites, and consequently, better NH3-SCR performance at low-temperature. The loading of single and multi-transition metal-based oxides on the surface of supports (TiO2, TiO2 nanotubes, carbon nanotubes, etc.) could also enhance the NOx conversion and N2 selectivity in NH3-SCR reaction by the fine dispersion of active component/s and its/their strong interaction with the support. The choice of metal loading and the support

could play a key role in the catalytic function of the supported transition metal-based catalysts. The synergistic redox interaction between the active components of mixed metal oxide/supported metal oxide catalysts was also found to be an important factor to design the efficient denitration catalysts. In spite of the significant progress on the SO2/H2O tolerance of the catalysts, the durability of catalysts in the presence of both SO2 and H2O still needs to be improved. Most transition metal-based catalysts suffered from low resistance when the reaction feed contains both SO2 and H2O streams simultaneously. Hence, researchers have continuously explored the different options of transition metal-based mixed oxides and active transition metal/s-support combinations in order to develop the better NH3-SCR catalysts in terms of SO2/H2O tolerance at low-temperature. The understanding of the inhibition mechanism of SO2 and H2O could be a promising strategy to develop high SO2/H2O resistance catalysts for NH3-SCR reaction. However, the SO2/H2O inhibition mechanism was not very clear that needs to be investigated deeply. Especially, the design of transition metal-based catalysts with a combination of high NOx conversion and N2 selectivity in a wide operation temperature window and good resistance to SO2/H2O have attracted paramount attention, but it is still challenging task. The scope of NH3-SCR research is quite vast and a large number of improvements need to be achieved in the near future.

**Author Contributions:** The original draft was prepared by D.D., reviewed and edited by D.D., P.G.S., B.M.R, and P.R.E.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

### *Review* **Hydrogenation of Carbon Dioxide to Value-Added Chemicals by Heterogeneous Catalysis and Plasma Catalysis**

#### **Miao Liu 1, Yanhui Yi 1,2,\*, Li Wang 3, Hongchen Guo <sup>1</sup> and Annemie Bogaerts <sup>2</sup>**


Received: 3 February 2019; Accepted: 8 March 2019; Published: 18 March 2019

**Abstract:** Due to the increasing emission of carbon dioxide (CO2), greenhouse effects are becoming more and more severe, causing global climate change. The conversion and utilization of CO2 is one of the possible solutions to reduce CO2 concentrations. This can be accomplished, among other methods, by direct hydrogenation of CO2, producing value-added products. In this review, the progress of mainly the last five years in direct hydrogenation of CO2 to value-added chemicals (e.g., CO, CH4, CH3OH, DME, olefins, and higher hydrocarbons) by heterogeneous catalysis and plasma catalysis is summarized, and research priorities for CO2 hydrogenation are proposed.

**Keywords:** carbon dioxide; hydrogenation; heterogeneous catalysis; plasma catalysis; value-added chemicals; methanol synthesis; methanation

#### **1. Introduction**

Climate changes are mostly induced by the greenhouse effect, and carbon dioxide (CO2) accounts for a dominant proportion of this greenhouse effect. Indeed, one can barely ignore the connection between the emission of CO2 and climate changes [1]. The CO2 concentration in the atmosphere has actually climbed to 405 ppm in 2017, as shown in Figure 1a. As the global energy consumption is still mainly based on burning coal, oil, and natural gas, and this situation will last until the middle of the century (see Figure 1b), experts predict that the CO2 concentration in the atmosphere will continue to rise to ~570 ppm by the end of the century if no measures are taken [2]. Hence, it is urgent to control the CO2 emissions by taking effective measures to capture and utilize CO2.

In principle, there are three strategies to reduce CO2 emissions, i.e., reducing the amount of CO2 produced, storage of CO2, and utilization of CO2 [3]. Recently, some comprehensive reviews have discussed the technological state-of-the-art of carbon capture and storage (CCS) [4–6]. Direct air capture technology (DAC) draws people's attention to mitigate climate change by taking advantage of chemical sorbents (e.g., basic solvents, supported amine and ammonium materials, etc.) [4]. In addition, Bui et al. also considered the economic and political obstacles in terms of the large-scale deployment of CCS [6]. Significantly reducing the amount of CO2 produced is unrealistic in view of the current energy structure dominated by fossil energies, as shown in Figure 1b. CO2 storage seems to be a potential approach, but there are some challenges, such as efficiency of capture and sequestration of CO2, cutting down the operation costs for capture and separation of CO2, and the long-term stability of underground storage [7,8]. Utilization of CO2 is a promising approach, since CO2 is a cheap and

attractive carbon source, which can be used to yield a variety of industrial raw materials, which can be further converted into value-added chemicals and fuels. Interestingly, CO2 can also be used as a desired trigger for stimuli-responsive materials. Darabi et al. summarized the synthesis, self-assembly, and applications of CO2-responsive polymeric materials [9].

**Figure 1.** (**a**) Trends in atmospheric CO2 concentrations (ppm). (**b**) Projected global energy consumption (Mt) from 1990 to 2040. Data from International Energy Agency.

One of the options to convert CO2, is catalytic hydrogenation, as illustrated in Figure 2. The production of H2, however, is also a vital problem to realize direct hydrogenation of CO2 to oxygenates and hydrocarbons. Hydrogen production can be either based on renewable or non-renewable sources, such as electrical, thermal, photonic, and hybrid [10,11]. The main methods of hydrogen production include electrolysis, thermolysis, photo-electrolysis, and hybrid thermochemical cycles [10]. According to some evaluation criteria—such as global warming potential (GWP), social cost of carbon (SCC), acidification potential (AP), energy and exergy efficiencies, and production cost—the hybrid hydrogen production methods seems a promising route. On the other hand, the production cost evaluation shows that coal gasification (\$0.92/kg H2) and fossil fuel reforming (\$0.75/kg H2) are relatively low-cost methods compared to early R&D phase methods (e.g., photo-electrochemical: \$10.36/kg H2 from water dissociation). However, fossil fuel is non-renewable, being a major drawback. Therefore, reducing the cost of hydrogen production is an urgent and challenging issue, since we have to balance both the economy and sustainability.

**Figure 2.** Catalytic hydrogenation of carbon dioxide.

Various methods have been adopted—including photo-catalysis, electro-catalysis, heterogeneous catalysis, and plasma catalysis [12,13]—to realize hydrogenation of CO2. Dalle et al. systematically presented the activity of the first-row transition metal complexes (i.e., Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn) in CO2 reduction by electro-catalysis, photo-catalysis and photoelectron-catalysis [14]. Li et al. discussed the important roles of co-catalysts (e.g., biomimetic, metal-based, metal-free, and multifunctional) in selective photo-catalytic CO2 reduction [15]. Besides thermochemical approaches, Mota et al. made a detailed retrospective based on electrochemical and photo-chemical approaches for CO2 hydrogenation to oxygenates and hydrocarbons [16]. In this review, we focus on the latter two methods. Heterogeneous catalysis has been widely studied, while plasma catalysis is still an emerging technology. Some excellent reviews were recently published for heterogeneous catalytic CO2 hydrogenation [12,17,18]. Jadhav et al. focused on methanol production [17], Porosoff et al. on the synthesis of CO, CH3OH, and hydrocarbons [18], while Alvarez et al. discussed the greener preparation of formates (formic acid), CH3OH, and DME [12]. With regard to plasma catalysis for CO2 hydrogenation, there are only a handful reports (as discussed below). Nevertheless, it exhibits great potential since plasma can operate at ambient temperature and atmospheric pressure.

In this review, we summarize the progress of mainly the last five years in CO2 hydrogenation to value-added chemicals (e.g., CO, CH4, CH3OH, DME, olefins, and higher hydrocarbons) driven by both heterogeneous catalysis and plasma catalysis. The literature bibliography range is shown in Figure 3. Indeed, on the one hand, the insights obtained by heterogeneous catalysis can be useful for the further development of the emerging field of plasma catalysis. On the other hand, we also want to pinpoint the differences between heterogeneous and plasma catalysis, and thus the need for dedicated design of catalytic system tailored to the plasma environment.

**Figure 3.** Literature bibliography included in this review.

#### **2. Heterogeneous Catalysis**

In heterogeneous catalysis, support materials, active metals, promoters, and the preparation methods of catalysts are major adjustable factors determining the catalytic activity. As far as the support materials are concerned, the catalytic performance is influenced by the metal-support interaction since it usually induces specific physicochemical properties for catalysis, e.g., active cluster, active oxygen vacancy, and acid-based property. Metal oxides and zeolites with special channel structures are usually selected. For the active metal components, research focuses on searching cheap and available metals to replace precious metals or to reduce the amount of precious metals in industrial heterogeneous catalysis. Promoters (structural-type and electron-type) can also significantly influence the catalytic activity by regulating the adsorption and desorption behavior of molecules (reactant, intermediate, and product) on the catalyst surface. Preparation methods usually determine the catalyst morphology including metal dispersion (particle size distribution), specific surface area, and channel structure, which also influences the catalytic performance [12]. In this section, we summarize recent progress of CO2 hydrogenation to CO, CH4, CH3OH, and some other products in terms of rational design of heterogeneous catalysts.

#### *2.1. CO2 to CO*

CO can be used as feedstock to produce liquid fuels and useful chemicals by Fischer–Tropsch (F–T) synthesis reaction. Therefore, the catalytic hydrogenation of CO2 to CO via the reverse water–gas shift (RWGS) reaction, reaction (1), is promising to account for the shortage of energy. Accordingly, catalyst design for the RWGS reaction have attracted extensive attention.

$$\text{CO}\_2 + \text{H}\_2 \leftrightarrow \text{CO} + \text{H}\_2\text{O}\_\prime\\\triangle\text{H}\_{298\text{K}} = 41.2\text{ kJ mol}^{-1} \tag{1}$$

Generally, noble metal catalysts (e.g., Pt, Ru, and Rh) exhibit effective ability towards H2 dissociation, and thus precious metals have been investigated extensively for the RWGS reaction. However, these catalysts yield, methane as dominant product, mainly caused by the higher rate of C-H bond formation than CO desorption [19]. To change the product distribution, Bando et al. applied Li-promoted Rh ion-exchanged zeolites for CO2 hydrogenation, achieving 87% CO selectivity for an atomic ratio of Li/Rh higher than 10/1 [19]. Although the catalytic performance of noble metals can be improved via promoters, the high price and instability of noble metals (aggregation of nano-particles) limit the industrial applicability. In order to properly decrease the operation cost and improve the life cycle of catalysts, non-noble metal carbides have been developed. Porosoff et al. synthesized a molybdenum carbide (Mo2C) catalyst for the RWGS reaction [20], yielding 8.7% CO2 conversion and 93.9% CO selectivity (at 573 K reaction temperature). The catalytic performance was much better than those of bimetallic catalysts (e.g., Pt-Co, Pt-Ni, Pd-Co, Pd-Ni) supported on CeO2 (~5% CO2 conversion and 83.3% CO selectivity at the same reaction temperature). The catalytic mechanism of Mo2C in the RWGS reaction was further studied by ambient pressure X-ray photoelectron spectroscopy (AP-XPS) and in situ X-ray absorption near edge spectroscopy (XANES), which demonstrate that Mo2C not only broke the C=O bond, but also dissociated hydrogen. Hence, Mo2C has a dual functional and is an ideal catalytic material for the RWGS reaction. Interestingly, the authors also found that by modifying the catalyst with Co, the catalytic activity of the Co-Mo2C catalyst was further improved (9.5% CO2 conversion, 99% CO selectivity at 573 K), probably attributed to the existence of the CoMoCyOz phase confirmed by X-ray Diffraction (XRD) (see Figure 4).

**Figure 4.** XRD pattern of fresh and reduced Co-Mo2C with Mo2C as a reference. The symbols correspond to the following: —β-Mo2C, ♦—CoMoCyOz, —MoO2, reproduced with permission from [20]. Copyright Wiley-VCH, 2014.

The support plays an important role in CO2 hydrogenation to CO through the RWGS reaction. Kattel et al. prepared Pt, Pt/SiO2 and Pt/TiO2 materials as catalysts for the RWGS reaction [21], and showed that Pt nanoparticles alone cannot catalyze the RWGS reaction, while using SiO2 and TiO2 as support, the overall CO2 conversion can be significantly improved, pointing towards a synergy effect between Pt and the oxide support. To reveal the synergy effect, they combined density functional theory (DFT), kinetic Monte Carlo (KMC) simulations and other experimental measurements. They found that the hydrogenation of CO2 to CO is promoted once CO2 is stabilized by the Pt-oxide interface. In the case of Pt nanoparticles (NP) alone, the conversion was close to 0, since the ability of Pt NP in binding CO2 is weak. When SiO2 and defected TiO2 with oxygen vacancies served as supports, however, the CO2 conversion was enhanced (to 3.35% and 4.51%, respectively) on the Pt-oxide interface. The synergy effect between Pt and oxide supports in activating and hydrogenating CO2 is shown in Figure 5, and possible reaction pathway [21] for the RWGS reaction is illustrated in Scheme 1.

**Figure 5.** DFT optimized geometries. (**1**) (**a**) Pt25/hydroxylated SiO2 (111), and (**b**) \*CO2 species, (**c**) \*CO species, (**d**) \*HCO species, (**e**) \*H2COH species, (**f**) \*CH3OH species, (**g**) \*CH2 species, and (**h**) \*OH species adsorbed on Pt/SiO2 (111). The dashed lines show hydrogen bonds. (**2**) (**a**) Pt25/TiO2 (110) with oxygen vacancy, and side (**top**) and top (**bottom**) views of (**b**) \*CO2 species, (**c**) \*CO species, (**d**) \*HCO species, (**e**) \*H2COH species, (**f**) \*CH3OH species, (**g**) \*CH2 species, and (**h**) \*OH species adsorbed on Pt/TiO2 (110). The black circle in (a) depicts the position of oxygen vacancy on TiO2 (110). Note: Si: green, Ti: light blue, Pt: light gray, C: dark gray, O: red, and H: blue, reprinted with permission from [21]. Copyright Elsevier, 2016.

**Scheme 1.** Reaction pathways for the RWGS reaction, where '\*X' represents species X adsorbed on a surface site, reproduced with permission from [21]. Copyright Elsevier, 2016.

Yan et al. investigated the effect of the Ru-Al2O3 interfaces on the catalytic activity of the RWGS reaction, and proposed Ru35/Al2O3 and Ru9/Al2O3 catalyst models to explain the experimental observations [22]. The product selectivity switched between CH4 and CO over the Ru/Al2O3 catalyst, i.e., monolayer Ru sites favored the production of CO, while Ru nano-clusters preferred the production of CH4, during CO2 reduction reaction. Confirmed by kinetic analysis, characterization of the surface structures and real-time monitoring of the active intermediate species, the product selectivity of CH4 and CO was regulated by the Ru sites and Ru-Al2O3 interfacial sites. Furthermore, based on the combination of theoretical calculations and isotope-exchange experimental results, the authors found that the O\* species derived from the dissociative adsorption of CO2 at interfacial Ru sites easily bridge with the Al sites from the γ-Al2O3 support, and new Ru-O-Al bonds are formed via the oxygen-exchange process. The interfacial O species existing in Ru-O-Al bonds was responsible for the CO2 activation via oxygen-exchange with the O atoms of CO2. Therefore, this experimental work is a good inspiration to further explore the influence of metal-support interfaces for the effective activation of CO2.

Besides the widely used impregnation method, Yan et al. also reported a doping-segregation method for the preparation of Rh-doped SrTiO3 [23]. Precursors with a molar ratio of Sr:Ti:Rh = 1.10:0.98:0.02. First, TiO2 was suspended in deionized water, and then Sr(OH)2·8H2O and Rh(NO3)3 were introduced. Subsequently, the sample was poured in a stain steel acid digestion vessel, which was kept at 473 K for 24–48 h. Finally, the reaction product was dried at 343 K overnight after it was centrifuged, and washed via deionized water. As confirmed by in situ X-Ray-Diffraction (XRD) and X-ray Absorption Fine Structure (XAFS) measurements, the Rh-doped SrTiO3 catalysts produce sub-nanometer Rh clusters, which are highly active for the conversion of CO2 compared to the supported Rh/SrTiO3 prepared by wetness impregnation. The better catalytic performance (7.9% CO2 conversion and 95% CO selectivity at 573 K) could be ascribed to the cooperative effect between sub-nanometer Rh clusters and the reconstructed SrTiO3 which is active for dissociation of H2 and is favorable for adsorption/activation of CO2. Therefore, the novel approach, doping-segregation method, maybe a novel strategy to tune the size of active metals and the physicochemical properties of supports for rational design of catalysts for the RWGS reaction.

Dai et al. studied CeO2 catalysts which were prepared by the hard-template method (Ce-HT), the typical complex method (Ce-CA), and the typical precipitation method (Ce-PC) for the RWGS reaction [24]. The experimental results show that catalysts prepared by Ce-CA, Ce-HT, and Ce-PC methods exhibit a 100% CO selectivity and the CO2 conversions were 9.3, 15.9, and 12.7% respectively at 853 K. Obviously, the hard-template (Ce-HT) method is beneficial for the RWGS reaction in view of the catalytic performance. The Ce-HT method comprises the following steps: (1) Ce(NO3)3·6H2O was dissolved in ethanol, and KIT-6 mesoporous silica was added; (2) The mixture was stirred until a dry power was obtained; (3) The powder was calcined; (4) The obtained samples were treated with NaOH to remove the template. To reveal the relationship between the preparation method and the catalytic activity, the authors carried out XRD, Transmission Electron Microscopy (TEM) and Brunauer–Emmett–Teller (BET) characterization, and the characterization results show that CeO2 catalysts which were prepared by the Ce-HT method have a porous structure and a high specific surface area, while CeO2 catalysts prepared by the other methods (Ce-CA, Ce-PC) have an agglomerated structure (Ce-CA) and overlapped bulk structure (Ce-PC) with low porosity. Moreover, in the CeO2 catalysts, oxygen vacancies as active sites were formed by H2 reduction at 673 K, which were confirmed by in situ X-ray photoelectron spectroscopy (XPS) and H2-temperature-programmed reduction (H2-TPR). Therefore, the improvement of the catalytic activity for the RWGS reaction can be ascribed to the change of catalyst structure and oxygen vacancies.

Except for the above monometallic catalysts, some bimetallic catalysts—i.e., Pt-Co, Fe-Mo and Ni-Mo [25–27]—were also prepared and tested in the RWGS reaction. In general, the preparation method of bimetallic catalysts generally can be divided into two categories: (1) the bi-metal was first prepared and then supported on the carrier [25]; (2) the bi-metal as formed in the preparation progress [26,27]. Compared with pure Co catalyst, Pt-Co catalyst showed a better catalytic activity (mainly CO, close to 100%) for the RWGS reaction. Ambient pressure X-ray photoelectron spectroscopy (AP-XPS) and environmental transmission electron microscopy (ETEM) showed that Pt migrates on the catalyst surface, and Pt aids the reduction of Co to its metallic state under appropriate reaction conditions confirmed by near edge X-ray absorption fine structure (NEXAFS) spectroscopy [25]. Abolfazl et al. investigated Mo/Al2O3 and Ni–Mo/Al2O3 catalysts prepared by the impregnation method used for the RWGS reaction. The experimental results showed that Ni–Mo/Al2O3 catalyst is a promising catalyst (35% CO2 conversion, i.e., close to the 38% equilibrium conversion) compared with Mo/Al2O3 catalyst (15% CO2 conversion). As demonstrated by XPS, the electronic effect, which transfers electrons from Ni to Mo and leads to an electron-deficient state of the Ni species, is beneficial for CO2 adsorption, and thus improves the CO2 conversion. Furthermore, the XRD and H2-TPR profiles indicated that the Ni–O–Mo structure crystallizes into the NiMoO4 phase which can improves the adsorption and dissociation of H2 on the Ni-Mo/Al2O3 catalyst [27]. Similarly, they also found that Fe-Mo/Al2O3 catalyst synthesized by the impregnation method is an efficient catalyst with high CO yield, almost no by-products and relatively stable (60 h) for the RWGS reaction. The enhancement of the catalytic activity may be attributed to better Fe dispersion with the addition of Mo and smaller particle size of active Fe species, which was confirmed by the BET method and scanning electron microscopy (SEM) [26].

Overall, to some extent, the reaction activity and stability of bimetallic catalysts for the direct hydrogenation of CO2 to CO are excellent compared to mono-metallic catalysts. The reduction of the active metal, the formation of alloy metal, the dispersion and particle size of the catalyst are possible factors for the improvement of the catalytic performance in the RWGS reaction. Details of the conversion and product selectivity, along with the reaction conditions of several representative RWGS catalytic systems, are compared in Table 1. Obviously, precious metal catalysts (e.g., Pt, Rh, Ni) are still advantageous for the formation of CO compared to non-noble metal catalysts (e.g., Fe, Co, Mo), and upon increasing the gas hourly space velocity (GHSV), the selectivity of CO slightly decreases to some extent.


**Table 1.** Catalytic performance of several catalytic systems for CO2 hydrogenation into CO, in terms of CO2 conversion and CO selectivity, along with the reaction conditions

<sup>a</sup> mL gcat−<sup>1</sup> h−1; <sup>b</sup> h−1; N/A: not available.

#### *2.2. CO2 to CH4*

Methane, a high value carbon source, is used to produce syngas via steam reforming, and subsequently the syngas is usually converted into chemicals and/or fuels through F–T synthesis. The direct hydrogenation of CO2 to CH4 (also called CO2 methanation), reaction (2), is a feasible approach, if the production technology of H2 becomes widespread and at low-cost.

$$\text{CO}\_2 + 4\text{H}\_2 \leftrightarrow \text{CH}\_4 + 2\text{H}\_2\text{O}, \triangle \text{H}\_{298\text{K}} = -252.9 \text{ kJ mol}^{-1} \tag{2}$$

Ni-based [29], Co-based [30], and Ru-based [31] catalysts have been used in CO2 methanation, and Ni-based catalysts are considered to be the most effective and the lowest cost alternative. However, coke formation is a serious problem for Ni-based catalytic systems [2]. Therefore, researchers are trying to seek appropriate promoters to improve the activity and stability of Ni-based catalytic systems for CO2 methanation. Yuan et al. investigated the effect of Re on the catalytic activity of Ni-based catalysts in CO2 methanation [32]. Based on DFT calculations, they found that, attributed to the strong affinity of Re to O (see Figure 6), the presence of Re markedly lowered the energy barrier of C-O bond cleavage, which benefits the activation of CO2. Moreover, CH4 selectivity can be enhanced owe to the presence of Re, which was confirmed by analysis of surface coverage of the adsorbed species on Ni (111) and Re@Ni (111). In addition, micro-kinetic analysis showed that, in addition to CO\* and H\*, a suitable amount of Oad atoms were present on Re@Ni (111), and thus a possible reaction network of CO2 methanation was proposed as shown in Scheme 2, in which a red line represents the preferable steps in each pathway.

**Figure 6.** Adsorption sites existed in the (**a**) Re@Ni(111) and (**b**) Ni(111) surface. Ni: blue, Re: green, reprinted with permission from [32]. Copyright Elsevier, 2018.

**Scheme 2.** Reaction network for CO2 methanation. The preferable steps in each pathway are marked with a red line, reproduced with permission from [32]. Copyright Elsevier, 2018.

Besides Re, La is also an excellent promoter of Ni-based catalysts for CO2 methanation. Quindimil et al. [29] applied Ni-La2O3/Na-BETA catalysts for CO2 methanation. They found that the presence of La2O3 created more CO2 adsorption sites and more hydrogenation sites, mainly attributed to the improved surface basicity and Ni dispersion by the co-catalyst effect of La. Under the optimized

reaction conditions, 65% CO2 conversion and nearly 100% CH4 selectivity were achieved over a Ni-10%La2O3/Na-BETA catalyst with a good stability for more than 24 h at 593 K.

CeO2, TiO2, and SiO2 have been used as the supports of methanation catalysts [33]. Reactions over Ni/CeO2 catalyst performed full selectivity to CH4 with higher TOF (up to forty-fold) compared to TiO2 and SiO2 supported Ni nanoparticles, and in CO2 methanation, the catalytic stability of Ni/CeO2 catalyst lasted for 50 h at 523 K. HRTEM analysis indicated that different supports induced distinctive crystal structure. Ni/CeO2 catalyst presented hexagonal Ni nanocrystallites, while TiO2 and SiO2 favored the formation of pseudo-spherical Ni nanoparticles. Demonstrated by TPR, XPS, and UV Raman analysis, characterization results revealed partial reduction of the CeO2 surface, and the partial reduction of the CeO2 surface contributed to the generation of oxygen vacancies, which is beneficial for the formation of a strong metal-support interaction (SMSI) between Ni and CeO2, while no SMSI was observed over Ni/SiO2 and Ni-TiO2 catalyst. Furthermore, pulse reaction by temporal analysis products (TAP) demonstrated the capacity of CO2 adsorption following the order: Ni/SiO2 < Ni/TiO2 < Ni/CeO2. Therefore, metal particle morphology and surface oxygen vacancies were used to anchor/stabilize Ni nanoparticles, and SMSI of Ni/CeO2 catalyst contributed to the remarkable catalytic activity for CO2 methanation.

Lin et al. investigated the influence of TiO2 phase structure on the degree of dispersion of Ru nanoparticles [31]. Experiments showed that Ru/r-TiO2 (rutile-type TiO2) catalysts have a fast rate of CO2 conversion, more than twice as fast as Ru/a-TiO2 (anatase-type TiO2) for CO2 methanation. Meanwhile, compared to Ru/a-TiO2 catalysts, Ru/r-TiO2 catalysts exhibited a much higher thermal stability. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images showed that r-TiO2 supported Ru nanoparticles displayed a narrower particle size distribution (1.1 ± 0.2 nm) compared with a-TiO2 supported Ru nanoparticles (4.0 ± 2.4 nm). As confirmed by XRD measurements and H2-TPR experiments, a strong interaction existed in RuO2 and r-TiO2 contributed to the formation of the Ru–O–Ti bond. Experimental tests, revealed that the strong interaction between RuO2 and r-TiO2, not only promotes the highly dispersion of Ru nanoparticles, but also prevents nanoparticles' aggregation, which is responsible for the enhancement of the catalytic activity and thermal stability.

Furthermore, the influence of Al2O3, ZrO2, SiO2, KIT-6, and GO supports on Ni-based catalysts [30,34,35] has also been reported recently (Table 2). The specific area of support, the metal-support interaction and particle size of active sites are still mainly adjustable factors. Interestingly, Ni-SiO2/GO-Ni-foam catalyst which was synthesized via intercalation of graphene oxide (GO) exhibited excellent activity compared with Ni-SiO2/Ni-foam catalyst (see Figure 7) at 743 K for CO2 methanation. Furthermore, the formation of nickel silicates on GO is responsible for the uniform dispersion of Ni active sites, which is favorable for inhibition of catalyst sintering.


**Table 2.** Catalytic performance of several catalysts for CO2 methanation, in terms of CO2 conversion and CH4 selectivity, along with the reaction conditions

<sup>a</sup> mL gcat−<sup>1</sup> h−1; <sup>b</sup> h−1; N/A: not available; pressure: 0.1 MPa.

**Figure 7.** TOF of Ni-SiO2/Ni-foam and Ni-SiO2/GO-Ni-foam catalysts for CO2 methanation, reprinted with permission from [35]. Copyright Elsevier, 2019.

Besides the traditional methods (co-impregnation method and deposition–precipitation method), some novel preparing methods, such as the sequential impregnation and a change of the preparation progress (e.g., the metal incorporation order, the reduction temperature, and the selection of different types of precursors), have been recently reported for preparing methanation catalysts.

Romero-Sáez et al. synthesized Ni-ZrO2 catalysts supported on CNTs via the sequential and co-impregnation methods for CO2 methanation [36]. The catalyst prepared by co-impregnation was apparently less active and selective to CH4, compared with the catalyst synthesized by the sequential impregnation method. The preparation approach of the sequential impregnation method is as follows: (1) the appropriate amount of ZrO(NO3)2 xH2O was dissolved in acetone; (2) the CNTs were added to the solution; (3) removal of the solvent, drying and heat treatment at 623 K; (4) in a rotatory evaporator, the same procedure for Ni(NO3)2 impregnation, followed drying and heat treatment was applied. As characterized by TEM analysis, NiO nanoparticles surrounded by ZrO2 in core–shell structures were formed via co-impregnation method.The existence of core–shell structure reduced reactant access to Ni and Ni–ZrO2 interface. However, when the catalyst was prepared via a sequential impregnation method, NiO nanoparticles were available and deposited either on the surface or next to the ZrO2 nanoparticles, which improved the extent of the Ni–ZrO2 interface. The ratio of Ni–O–Zr exposed species thus increases in the sequential impregnation method, and the interaction between H atoms (produced upon H2 dissociation on the Ni surface) and the CO2 molecule (activated by ZrO2), can also be enhanced. The schematic representation is shown in Figure 8.

**Figure 8.** Schematic representation of the disposition of NiO, ZrO2 and interface Ni-O-Zr at the surface of CNTs for (**A**) Ni-Zr-CNT-COI catalytic system, and (**B**) Ni-Zr-CNT-SEQ catalytic system, reproduced with permission from [36]. Copyright Elsevier, 2018.

Interestingly, Bacariza et al. investigated the influence of the metal incorporation order in the preparation of Ni-Ce/Y(USY) zeolite catalysts [37]. In their experiments, three ways were used for the preparation of catalysts: Ni before Ce (Ce/Ni), Ce before Ni (Ni/Ce), and co-impregnation (Ni-Ce). Experimental tests showed that the catalytic activity follows the order: Ce/Ni ≈ Ni/Ce < Ni-Ce. TEM and H-TPR characterizations demonstrated that the Ni0 average size decreases to approximately 2.5 nm, and in terms of Ni/Ce or Ni-Ce catalysts, stronger interactions between Ni and Ce species are established. However, the CO2 adsorption capacity is smaller for Ni/Ce catalyst. In contrast, even

though larger Ni0 particles (13.3 nm) are formed for Ce/Ni catalyst, CO2 adsorption capacity can be enhanced. Finally, Ce-Ni was found as the best preparation method for CO2 methanation using Ni-based catalysts supported on CeO2.

In addition, it has been reported that the reduction temperature and a variety of different precursors (e.g., nitrate, chlorate, and oxalate) affect the number of active centers for CO2 methanation [30,38]. As discussed above, the preparation methods showed beneficial influences on the catalytic performance, suggesting that reasonable adjustment of the preparation process is a strategy to optimize the experiments for the conversion of CO2 to CH4. Details of CO2 conversion and product selectivity, along with the reaction conditions of several representative catalytic systems, are compared in Table 2. From Table 2, we can see that Ni-based catalysts are the main catalytic systems for the conversion of CO2 to CH4. Furthermore, the optimal temperature for CH4 production is 300–400 ◦C.

#### *2.3. CO2 to CH3OH*

Methanol is not only an important industrial raw material, but also a stable hydrogen storage compound, which is convenient for transport and reserve. Recently, the concept of "methanol economy" advocated by the Nobel Laureate George Olah revealed the importance of methanol in energy structure [41]. Hence, a large amount of heterogeneous catalysts for the direct hydrogenation of CO2 to CH3OH, reaction (3), have emerged in recent years. Herein, we summarize some catalytic systems for the direct hydrogenation of CO2 to CH3OH (see Figure 9).

$$\text{CH}\_2 + 3\text{H}\_2 \leftrightarrow \text{CH}\_3\text{OH} + \text{H}\_2\text{O}, \triangle \text{H}\_{298\text{K}} = -49.5 \text{ kJ mol}^{-1} \tag{3}$$

**Figure 9.** Catalytic system for the direct hydrogenation of CO2 to CH3OH in this review.

Cu-ZnO-based catalysts, developed by ICI (Imperial Chemical Industries) in the 1960s, are still widely used in CH3OH synthesis from syngas (CO and H2), and Cu<sup>0</sup> is considered to be the active site. Li et al. reported the synthesis of Cu/ZnO catalysts using a unique method, i.e., facile solid-phase grinding, using mixture of oxalic acid, copper nitrate, and zinc nitrate as raw materials [42]. Appropriate amount of these compounds were physically mixed, and then manually ground for 0.5 h. Subsequently, at 393 K, the obtained precursor was dried for 12 h, and at 623 K, calcined for 3 h in N2 flow, followed by passivation in 1% O2/N2 flow for 5 h. In contrast, by the following steps: (1) at 623 K, calcining the dried precursor at for 3 h in air; (2) at 503 K, reducing the oxide for 10 h in 5% H2/N2 flow; and (3) at room temperature, passivating the catalyst in 1% O2/N2 flow for 5 h, the catalyst can be obtained via H2 reduction. As demonstrated by XRD, H2-TPR and thermal-gravity-differential thermal analysis (TG-DTA), in the facile solid-phase grinding process, the decomposition of oxalate complexes and the reduction of CuO took place simultaneously when the samples were calcined in N2, which prevented the growth of active Cu<sup>0</sup> species and the aggregation of catalyst particles. In contrast, in the

conventional H2 reduction process, the growth of active species or the aggregation of catalyst particles were inevitable. Notably, Cu/ZnO catalysts prepared by a facile solid-phase grinding method achieved 29.2% CO2 conversion (at 523 K and 3 MPa), which is even higher than the thermodynamic equilibrium conversion. Indeed, the equilibrium conversion of CO2 at 473 K and 3 MPa is 25.78% according to the Benedict–Webb–Rubin equation [43], and it is even lower at 523 K, since CO2 hydrogenation to CH3OH is an exothermic reaction. Generally, tandem reaction can break the limit of thermodynamic on reaction result. Inspiring from which, the above unusual experimental results (higher than thermal equilibrium value) could be most likely achieved through tandem reactions, in which every stepwise reaction was accelerated by different metal sites. Therefore, the simple and solvent-free method based on solid-phase grinding, opened a new approach to synthesize bimetallic or multimetallic catalysts without further reduction.

To improve the catalytic activity of Cu/ZnO/Al2O3 catalysts, ZnO was replaced by g-C3N4-ZnO hybrid material, as reported by Deng et al. [44]. The experimental results showed that, at 12 bar and 523 K, the methanol space time yield (STY) reached 5.73 mmol h−<sup>1</sup> gCu−<sup>1</sup> for Cu- g-C3N4-ZnO/Al2O3, which is superior to the methanol yield (5.45 mmol h−<sup>1</sup> gCu<sup>−</sup>1) of industrial catalyst (Cu-ZnO-Al2O3) under the same reaction pressure. As confirmed by time-resolved photoluminescence (TRPL) and electronic spin resonance (ESR), the electron-richness of ZnO was enhanced via the formation of type-II hetero-junction between g-C3N4 and ZnO. In addition, in the TPR curve, enhanced SMSI was observed, and the SMSI between electron-rich ZnO and Cu could boost the catalytic performance in CH3OH production. The study provided a viable and economic method to modify traditional catalysts for improving CH3OH production.

For CuO-ZnO-ZrO2 catalysts, doping graphene oxide (GO) is a good strategy for improving the activity of CO2 to CH3OH [45]. The highest methanol selectivity reached up to 75.88% (473 K, 20 bar) at 1 wt % GO content, while under the same conditions, the methanol selectivity was found to be 68% over the GO-free catalyst. Furthermore, CuO-ZnO-ZrO2-GO catalyst exhibited an almost constant space-time yield (STY) of methanol after 96 h time-on stream experiment. As demonstrated by H2-TPD and CO2-TPD, the CO2 and H2 adsorption capacity is enhanced over the CuO-ZnO-ZrO2-GO catalysts. Additionally, to explain the improvement of catalytic activity, the authors proposed that GO nano-sheet can serve as a bridge between mixed metal oxides, which strengthens a hydrogen spillover (see Figure 10). That is, H species migrating from the copper surface to the carbon species are adsorbed on the isolated metal oxide particles.

**Figure 10.** Graphene oxide (GO) nanosheet as a bridge promoting hydrogen spillover from the surface of copper to the surface of other metal oxides, reproduced with permission from [45]. Copyright Elsevier, 2018.

It is also reported that modification by small amounts (2 and 5 at %) of WO3 can improve the CO2 conversion and CH3OH selectivity of the CuO-ZnO-ZrO2 catalyst [46]. The optimal catalytic performance can be attributed to the specific surface area of metallic Cu, basic sites, and the reducibility of catalysts. However, if we want to effectively tune the catalytic reaction, a better understanding of the reaction mechanisms is essential. Up to now, the possible reaction pathways for the hydrogenation of CO2 to methanol over Cu-ZnO-based catalysts are shown in Scheme 3.

**Scheme 3.** Possible reaction pathways for CO2 hydrogenation to methanol, where '\*X' represents species X adsorbed on a surface site, reprinted with permission from [21]. Copyright Elsevier, 2016.

Besides Cu-based catalysts, In2O3 catalyst with surface oxygen vacancies has attracted more and more attention from researchers. Using density functional theory (DFT) calculations, Ye et al. investigated a In2O3 catalyst for the hydrogenation of CO2 to CH3OH [47]. On a perfect In2O3 (110) surface, six possible surface oxygen vacancies (Ov1 toOv6) were investigated (see Figure 11a), and the D4 surface with the Ov4 defective site was found to be most beneficial for CO2 activation and further hydrogenation. Potential energy profiles of CO2 hydrogenation and protonation on the D4 defective In2O3 (110) surfaces are shown in Figure 11b. In addition, the simulation results showed that the formation of CH3OH replenishes the oxygen vacancy sites, while H2 contributes to generate the vacancies, and this cycle between perfect and defective states of the surface is responsible for the formation of CH3OH from CO2 hydrogenation. To demonstrate this hypothesis, In2O3 catalyst was used for the hydrogenation of CO2 to CH3OH in practice, which showed the superior catalytic activity (7.1% CO2 conversion, 39.7% CH3OH selectivity at 603 K). This experimental result is better than for many other reported catalytic systems, which generally show low selectivity of CH3OH at 603 K. Confirmed by thermo-gravimetric analysis (TGA), XRD, and HR-TEM, the authors found that In2O3 catalyst had satisfactory thermal and structural stability for CO2 conversion to CH3OH below 773 K [48]. Furthermore, to reveal the effect of oxygen vacancies, Martin et al. synthesized bulk In2O3 catalyst, and at a wide range of reaction conditions, the CH3OH selectivity could be tuned up to 100%. XRD, H2-TPR, CO2-TPD, XPS, operando diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and electron paramagnetic resonance (EPR) characterization confirmed that oxygen vacancies on the In2O3 catalyst are active sites for the reduction of CO2. Additionally, to further improve the stability of In2O3 catalyst for the production of CH3OH, the authors investigated a variety of supports (i.e., ZrO2, TiO2, ZnO2, SiO2, Al2O3, C, SnO2, MgO). ZrO2 showed the best catalytic performance since it prevented the sintering of the In2O3 phase, which was demonstrated by the enduring stability of In2O3/ZrO2 catalyst over 1000 h [49]. Overall, In2O3 is a potential catalyst for CO2 hydrogenation to CH3OH with high selectivity.

**Figure 11.** (**a**) Optimized structure of the In2O3 (110) surface. Red: O atoms; brown: In atoms. (**b**) Potential energy profiles of CO2 hydrogenation and protonation on the D4 defective In2O3 (110) surfaces. Red line: hydrogenation; black line: protonation. A\* represents the adsorption state of A on the surface, while [A+B]\* represents the co-adsorption state of A and B on the surface, reproduced with permission from [47]. Copyright American Chemical Society, 2013.

Interestingly, Wang et al. synthesized a series of x% ZnO-ZrO2 solid solution catalysts (x% represents the molar ratio of Zn) for CO2 direct hydrogenation to CH3OH [50]. Under the specified reaction conditions (5.0 MPa, H2/CO2 = 3:1 to 4:1, 593 K to 588 K), ZnO-ZrO2 catalyst can achieve 86–91% methanol selectivity with single-pass CO2 conversion more than 10%, better than the results reported by other researchers. Moreover, in the presence of 50 ppm SO2 or H2S in the reaction stream, no deactivation was observed. Experimental results confirmed that ZrO2 and ZnO alone showed little activity in methanol synthesis, while the catalytic performance was significantly enhanced and CO2 conversion reached the maximum value when the Zn/(Zn + Zr) molar percentage is close to 13%. Demonstrated by CO2-TPD, most of the CO2 adsorbed on the Zr sites of ZnO-ZrO2 catalyst, and the H2-D2 exchange experiment indicated that ZnO had much higher activity than ZrO2. Therefore, in ZnO-ZrO2 solid solution catalyst, the synergetic effect between the Zn and Zr sites markedly promotes the activation of H2 and CO2. To explain the reaction mechanism on the solid solution catalyst (ZnO-ZrO2), in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and density functional theory (DFT) calculations were conducted. It was concluded that CO2 direct hydrogenation to CH3OH is dominated via the formate pathway, and CH3OH was formed by H3CO\* protonation on the surface of ZnO-ZrO2 catalyst.

Although Cu-ZnO-based catalysts are highly selective for CO2 conversion to methanol, there are still some serious problems during industrial operation, such as deactivation of active sites at high temperature and agglomeration of catalytic particles under industrial reaction conditions. To overcome these issues, researchers designed a series of bimetallic catalysts for the direct hydrogenation of CO2 to CH3OH. Jiang et al. prepared a series of Pd-Cu bimetallic catalysts supported on SiO2 with a wide range of total metal loading (i.e., 2.4–18.7 wt %) and evaluated the catalytic performance for the reduction of CO2 to CH3OH. With a decrease in metal loadings, the CO2 conversion dropped stepwise, namely from 6.6 to 3.7%. However, the CH3OH STY was slightly enhanced by 31% from 0.16 to 0.21 mmol mol−<sup>1</sup> s−<sup>1</sup> [51]. To unclose the reaction mechanisms, the authors carried out in situ diffuse reflectance FT-IR (DRIFTS) measurements on Pd(0.34)-Cu/SiO2 catalyst [52]. The resulting spectra identified that the dominant species on a bimetallic surface were formate and carbonyl species on a bimetallic surface, which were dependent on the catalyst composition. Therefore, they proposed that on Pd-Cu catalysts, the surface coverage of formate species was correlated to the methanol promotion, implying its vital role in CH3OH synthesis. Bahruji et al. prepared PdZn/TiO2 bimetallic catalysts by a solvent-free chemical vapor impregnation method [53]. According to the order in which the metals were impregnated, the catalysts could be classified as 2Pd-1Zn-TiO2, 2Zn-1Pd-TiO2, and PdZn/TiO2 (1 and 2 represent the order of sequential metal impregnations). The experimental results showed that PdZn/TiO2 catalyst achieved optimal catalytic performance, 10.1% CO2 conversion and 40% CH3OH selectivity. The formation of ZnTiO3 and PdZn nanoparticles was confirmed via XPS, XRD, and TEM. Combining the experimental results, the authors proposed that PdZn nanoparticles were beneficial for methanol formation and catalytic stability. Although it is hard to assign the formation of the PdZn alloy to the differences between the preparation methods, an interaction between Pd(acac)2 and Zn(acac)2 precursors might be responsible for the production of the PdZn active site in terms of PdZn/TiO2 catalyst. Additionally, Yin et al. reported the preparation of Pd@zeolitic imidazolate framework-8 (ZIF-8) catalyst for the conversion of CO2 to CH3OH [54]. The optimal methanol yield can reach 0.65 g gcat−<sup>1</sup> h−<sup>1</sup> over a PdZn catalyst. As demonstrated by XPS, XRD, TEM and electron paramagnetic resonance (EPR), after H2 reduction, PdZn alloy particles formed, and abundant oxygen defects existed on the ZnO surface. The experimental results revealed that the active site is a PdZn alloy rather than metallic Pd, in terms of CH3OH formation.

Numerous studies suggested that promoters have an indispensable role in the catalytic performance of CuZn catalysts [55], although the mechanism of action still remains unclear. Pd-doped CuZn catalysts were prepared to evaluate the surface modification effect. An interesting observation was made from the volcano-shaped relationship between CH3OH STY and Pd loading, which implied that an appropriate amount of Pd loading is beneficial for methanol synthesis. The chemisorption analyses further revealed a strong interaction between Pd and Cu surface, where a hydrogen spillover has taken place, generating more activated Cu sites. Hence, the CH3OH STY and the methanol turnover frequency (TOF) improved a lot with Pd loading of 1 wt % [55]. In addition, a variety of bimetallic catalysts (e.g., Cu-Fe, Cu-Co, Cu-Ni, Co-Ga, Ni-Ga) have been evaluated by many researchers [56–59]. A possible catalytic reaction mechanism of CO2 hydrogenation over Cu−Ni/CeO2−NT is shown in

Figure 12. Details of the CO2 conversion and product selectivity, along with reaction conditions of several representative catalytic systems are compared in Table 3. In terms of hydrogenation of CO2 to CH3OH, In-based and Pd-based catalysts have gradually drawn researchers' attention, besides traditional Cu-based catalysts. Additionally, many researchers demonstrated that 250 ◦C is the optimal temperature to produce CH3OH with the high selectivity. According to the current experimental results, increasing the reaction pressure is also a good strategy to improve CO2 conversion, but a high pressure means a high cost of operation and equipment.

**Figure 12.** Possible catalytic mechanism of CO2 hydrogenation over Cu−Ni/CeO2−NT, reproduced with permission from [59]. Copyright American Chemical Society, 2018.

**Table 3.** Catalytic performance of several catalytic systems for CO2 hydrogenation into CH3OH, in terms of CO2 conversion and CH3OH selectivity, along with the reaction conditions.


<sup>a</sup> mL gcat−<sup>1</sup> h−1; <sup>b</sup> h−1; N/A: not available.

#### *2.4. CO2 to Other Products*

Besides CO, CH4, and CH3OH, dimethyl ether (DME), light olefins [71–74], alcohol [75], isoparaffins [76], and aromatics [77,78] have also been produced by CO2 hydrogenation. Clearly, in terms of CO2 conversion, the coupling of C-C bond to produce higher hydrocarbons and oxygenates is a technological barrier. However, taking advantage of tandem catalysis to realize one-step synthesis of hydrocarbons via hydrogenation of CO2 is feasible. The extensive studies can be mainly categorized into two categories: methanol-mediated and non-methanol-mediated reactions [79,80]. In the methanol-mediated approach, DME is usually produced as main product, while for the non-methanol-mediated approach, alkenes and alkanes are generally produced as main products. The possible reaction mechanism for CO2 hydrogenation to DME and light olefins is shown in Scheme 4.

**Scheme 4.** Possible reaction pathways for CO2 hydrogenation to DME and light olefins. Note: ---H, ---O derived from H-ZSM5 zeolite, reprinted with permission from [21]. Copyright Elsevier, 2016.

Cu-based catalysts are in practice used for methanol synthesis, and HZSM-5 zeolites are widely employed for methanol dehydration due to its solid acid catalysis. Therefore, a variety of studies have been reported with regard to the bi-functional catalysts consisting of Cu and HZSM-5 used for the direct hydrogenation of CO2.

Zhang et al. reported a Cu-ZrO2/HZSM-5 catalyst promoted by Pd/CNT for direct synthesis of DME from CO2/H2 [79]. Although a minor amount of the Pd-decorated CNTs into the CuZr/HZSM-5 catalytic system caused little change in the activation energy for CO2 conversion compared with the CuZr/HZSM-5 catalyst, the former created a micro-environment including higher concentration of active H species and adsorbed CO2 species on the surface of the catalyst system. Owing to the increase of hydrogenation reactions, under reaction conditions of 5 MPa and 523 K, the specific rate of CO2 hydrogenation-conversion was 1.22 times higher than the pure CuZr/HZSM-5 catalyst. Furthermore, Zhang et al. reported a series of V-modified CuO-ZnO-ZrO2/HZSM-5 catalysts which were prepared via an oxalate co-precipitation method for the synthesis of DME from CO2/H2 [81]. The catalytic performances of the catalysts were strongly dependent on the content of V, which was confirmed by XRD, N2O chemisorption, and X-ray photoelectron spectroscopy (XPS). Similarly, the influence of the promoter—i.e., La and W—has also been examined. The optimal catalytic activity was obtained (43.8% CO2 conversion and 71.2% DME selectivity) when the amount of La was 2 wt %. In contrast, the Cu-ZnO-ZrO2 catalyst admixed with WOx obtained 18.9% CO2 conversion and 15.3% DME selectivity. Obviously, La is a better promoter compared with W, and this can be explained by the fact that the reducibility and dispersion of bi-functional catalysts (CuO-ZnO-Al2O3-La2O3/HZSM-5) were largely dependent on the modification of La, while the hybrid catalyst (Cu-ZnO-ZrO2-WOx/Al2O3) modified by W strongly adsorbed water molecules, resulting in a lower catalytic performance. Moreover, as shown in Figure 13, the STY of DME of different WOx/Al2O3 catalysts (i.e., on supports with different pore sizes) as a function of W surface density shows a volcanic curve relation. Details of CO2 conversion and DME selectivity reported recently are shown in Table 4 [82,83]. The above studies indicate that the current catalysts used for DME preparation from CO2/H2 are still dominated by Cu-based-HZSM-5 catalysts. Some other zeolite catalysts (SBA-15, SAPO-5, SUZ-4) with similar acid properties (acid content and acid strength) as HZSM-5 zeolite may be a direction of future research.

**Figure 13.** STY of DME of different WOx/Al2O3 catalysts, with different support pore size as a function of W surface density. AS, AM, and AL refer to the mean pore size of alumina supports with small, medium and large pores, respectively, reproduced with permission from [83]. Copyright Elsevier, 2018.

Among the large-scale technologies with industrial potential, the conversion of CO2 to DME is promising and relatively mature [84]. Korea Gas Corporation (KOGAS) has developed a DME plant, with CO2 as a raw material. The schematic diagram of the KOGAS tri-reforming process is shown in Figure 14 [85,86]. Kansai Electric Power Co. and Mitsubishi Heavy Industries have realized a bench-scale (100 cm<sup>3</sup> catalyst loading) experiment for DME synthesis [87,88]. However, during the reaction process, water formation decreased the yield of DME [89–91]. Catizzone et al. and Bonura et al. evidenced that zeolites or ferrierite could effectively mitigate the influence of water and avoid catalyst sintering [89,90]. In a fixed bed reactor, kinetic modeling confirmed the negative effect of water (formed during the reaction process) on DME production, which is consistent with the experimental results. Therefore, Falco et al. suggested that hydrophilic membranes could be promising for industrial production of DME [92]. Recently, Fang et al. advocated CO2 capture and conversion by using a membrane reactor system, where a high-temperature mixed electronic and carbonate-ion conductor (MECC) membrane was used for CO2 capture and a solid oxide electrolysis cell (SOEC) was used for CO2 reduction [93]. Furthermore, based on membrane technology, Sofia et al. carried out a techno-economic analysis project of power and hydrogen co-production via an integrated gasification combined cycle (IGCC) plant with CO2 capture [94]. The development of membrane technology would be an advantageous factor for the capture and utilization of CO2.

**Figure 14.** Schematic diagram of KOGAS tri-reforming process, reproduced with permission from [86]. Copyright Elsevier, 2009.

Li et al. synthesized a novel ZnZrO/SAPO tandem catalyst, combined by a ZnO-ZrO2 solid solution and a Zn-modified SAPO-34 zeolite, which achieved 80–90% olefin selectivity among the hydrocarbon products [95]. Based on the surface reaction kinetics, they proposed that the tandem reaction process proceeded as follows: (1) generation of CHxO species on ZnZrO via CO2 reduction; (2) olefins production from the derived CHxO species which migrate/transfer onto SAPO zeolite pore structure. Moreover, experimental results confirmed that the excellent selectivity can be ascribed to the effective synergy between ZnZrO and SAPO for the tandem catalyst. In addition, this catalyst (ZnZrO and SAPO-34) shows an excellent stability toward thermal and sulfur treatments, indicating the potential value for industrial application.

Recently, CO2 was converted to aromatics with a selectivity up to 73%, at 14% CO2 conversion over ZnZrO/HZSM-5 catalyst [77]. Demonstrated by operando infrared (IR) characterization, Li et al. proposed that CHxO, as an intermediate species, transformed from the ZnZrO surface into the pore structure of HZSM-5, which is responsible for C-C bond formation for aromatics production. The reaction scheme is shown in Figure 15. Interestingly, the presence of H2O and CO2 markedly suppressed the generation of polycyclic aromatics, consequently, enhanced the stability (100 h in the reaction stream) of the tandem catalyst, which showed potential industrial application prospects. Similarly, Ni et al. synthesized a tandem catalyst of ZnAlOx and HZSM-5, which yields 73.9% aromatics selectivity and 0.4% CH4 selectivity among the carbon products without CO [78]. Confirmed by XRD, SEM, TEM, and element distribution analysis, ZnAlOx was formed and uniformly dispersed in the tandem catalyst. Furthermore, demonstrated by 2,6-di-tert-butyl-pyridine absorption (DTBPy-FTIR), the external Brønsted acid of HZSM-5 can be shielded by ZnAlOx, which is beneficial to aromatization. According to the operando diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS)

study, they proposed the following possible reaction mechanism: MeOH and DME, produced by hydrogenation of formate species, are transmitted to HZSM-5 and then converted into olefins and finally aromatics.

**Figure 15.** Reaction scheme of CO2 hydrogenation to aromatics, reproduced with permission from [77]. Copyright Elsevier, 2019.

Non-methanol-mediated reactions, i.e., the direct hydrogenation of CO2 to light olefins is even more significant than CH3OH synthesis from CO2 hydrogenation, since a large proportion of methanol is used for the synthesis of olefins in industry, through the methanol-to-olefins (MTO) reaction by using SAPO-34 zeolite catalysts. CO2 to lower olefins can be realized by coupling of the RWGS reaction and F–T synthesis, as shown in Schemes 1 and 3.

Iron-based catalysts has been considered as an excellent option for the synthesis of light olefins from CO2/H2, mainly because of their excellent activity, high selectivity, and low price. Using honeycomb-structure graphene (HSG) as the support and K as a promoter, Wu et al. prepared an iron-based catalyst [80]. They found out that the confinement effect of the porous HSG was beneficial for the sintering of the Fe active sites, and within 120 h stability test, no significant deactivation occurred. In addition, as confirmed by CO2 temperature programmed desorption (CO2-TPD) and H2-TPD, they found out that potassium as a promoter effectively enhanced the chemisorption and activation of the reactants CO2 and H2. Moreover, as revealed by 57Fe Mossbauer absorption spectroscopy, for the Fe/HSG catalyst, there were two doublets with isomer shift (IS) of 0.96 mm s−<sup>1</sup> and quadrupole splitting (QS) of 0.28 mm s−<sup>1</sup> and IS of 1.21 mm s−<sup>1</sup> and QS of 1.82 mm s−1, which corresponded to the Fe (II) species in low- and high-coordination environments. Instead, for the FeK1.5/HSG catalyst, only one doublet with IS of 0.31 mm s−<sup>1</sup> and QS of 1.05 mm s−<sup>1</sup> ascribed to the Fe (III) species, which implies that K is capable of stabilizing high valence-state iron (Fe (III)) during CO2 hydrogenation to light olefins (CO2–FTO). The above mentioned three factors contributed to the excellent catalytic performance, i.e., 56% olefins selectivity and a 120-h stability testing experiment (as shown in Figure 16).

**Figure 16.** CO2−FTO results over the FeK1.5/HSG catalyst during 120 h on stream (Fe time yield to hydrocarbons, termed as FTY). Reaction conditions: 0.15 g catalyst, T = 613 K, P = 20 bar, H2/CO2 = 3 by volume, and the space velocity of 26 L h−<sup>1</sup> <sup>g</sup>−1. The CO selectivity is in the range of 39−43%, reproduced with permission from [80]. Copyright American Chemical Society, 2018.

Zhang et al. used an impregnation method to prepare Fe-Zn-K catalysts, in which K was used as a promoter [71]. The experimental results showed that the Fe-Zn-K catalyst with H2/CO reduction showed the best catalytic activity, with 51.03% CO2 conversion and 53.58% C2–C4 olefins selectivity, at the reaction conditions of 593 K and 0.5 MPa. XRD, H2-TPR, and XPS characterization results revealed that, in the Fe-Zn-K catalyst, ZnFe2O4 spinel phase and ZnO phase were formed. Among them, ZnFe2O4 spinel phase strengthens the interaction between iron and zinc, and changed the reduction and CO2 adsorption behaviors. In addition, the H2-TPR profiles show that the catalyst modified by K contributed to a slight shift of the initial reduction peak to higher temperature, indicating the reduction of Fe2O3 and formation of Fe phase was inhibited by K modification.

You et al. investigated the catalytic activity of non-supported Fe catalysts (bulk Fe catalysts) modified by alkali metal ions (i.e., Li, Na, K, Rb, Cs) for the conversion of CO2 to light olefins [72]. By calcining ammonium ferric citrate, non-supported Fe catalyst was prepared. Compared with Fe catalyst without modification (5.6% CO2 conversion, 0% olefins), the modification of the Fe catalyst with an alkali metal ion markedly enhanced the catalytic activity for CO2 conversion to light olefins. For Fe catalyst modified by K and Rb, the conversion of CO2 and the olefin selectivity (based on only the hydrocarbon compounds, without CO) increased to about 40% and 50%, respectively, and the yield of light (C2–C4) olefins can reach 10–12%. Further investigation via XRD showed that over the alkali-metal-ion-modified Fe catalysts, Fe5C2 was formed, while in the unmodified catalyst, iron carbide species were not observed after the reaction. Accordingly, they proposed that in the presence of an alkali metal ion, the generation of iron carbide species was one possible reason for the enhanced catalytic activity. Therefore, modification by alkali metal ions remains a good strategy to tune the product distribution.

Additionally, Wei et al. reported a highly efficient, stable and multifunctional Na–Fe3O4/HZSM-5 catalytic system [76], for direct hydrogenation of CO2 to gasoline-range (C5–C11), in which the selectivity of C5–C11 hydrocarbons reached 78% (based on total hydrocarbons), while under industrial conditions only reached 4% methane selectivity at a 22% CO2 conversion. Characterization by high resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), and Mossbauer spectroscopy showed that two different types of iron phase—i.e., Fe3O4 and x-Fe5C2—were discerned in the spent Na–Fe3O4 catalyst, which cooperatively catalyzed a tandem reaction (RWGS and FT). The possible reaction scheme for CO2 hydrogenation to gasoline-range hydrocarbons is shown in Figure 17. Furthermore, acid sites existing in the HZSM-5 zeolite were favorable for acid-catalyzed reactions (oligomerization, isomerization, and aromatization). Notably, the multifunctional catalyst exhibited a significant stability for 1000 h on stream, showing the potential as promising industrial catalyst material for CO2 conversion to liquid fuels. Similarly, Gao et al. investigated a bi-functional catalytic system composed of In2O3 and HZSM-5, which can achieve 78.6% C5+ selectivity with only 1% CH4 at a 13.1% CO2 conversion [73]. As demonstrated by Ye et al. [47], CO2 and H2 can be activated in the oxygen vacancies on the In2O3 surfaces, and catalyzed CH3OH formation. Subsequently, C−C coupling occurred inside the zeolite pores structure (HZSM-5) to synthesize gasoline-range hydrocarbons with a relatively high octane number (C5+).

**Figure 17.** Reaction scheme for CO2 hydrogenation to gasoline-range hydrocarbons, reproduced with permission from [76]. Copyright Nature Publishing Group, 2017.

Details of the conversion and selectivity for the hydrogenation of CO2 into other products, along with the reaction conditions of several representative catalytic systems, are compared in Table 4. Fe-based catalysts are beneficial for the production of olefins, while Cu-based + HZSM-5 is still a dominant catalytic system for CO2 conversion to DME. Similarly, 250 ◦C is the optimal temperature for the production of DME based on the current experimental results, and 300–400 ◦C, which is close to industrial production conditions, seems to be better for the direct conversion of CO2 to olefins. On the other hand, the selectivity of DME can be maintained upon increasing GHSV (>10,000 mL gcat−<sup>1</sup> h<sup>−</sup>1), but the high selectivity of olefins generally relies on a relatively low GHSV (<5000 mL gcat−<sup>1</sup> h<sup>−</sup>1).

**Table 4.** Catalytic activity of several catalysts for CO2 hydrogenation into other products (e.g., DME, olefins, alcohol, isoparaffins, gasoline, aromatics), in terms of CO2 conversion and product selectivity, along with the reaction conditions.



**Table 4.** *Cont.*

<sup>a</sup> mL gcat−<sup>1</sup> h−1; <sup>b</sup> h−1; N/A: not available.

#### *2.5. Opportunities of Heterogeneous Catalysis for CO2 Conversion*

The relationship between products selectivity and CO2 conversion by heterogeneous catalytic hydrogenation is shown in Figure 18. It is obvious that, in the RWGS reaction, although the CO selectivity reaches up to nearly 100%, the CO2 conversion is low. In the methanation reaction of CO2, the CH4 selectivity is high enough, and the CO2 conversion also exceeds 50%. With regard to the synthesis of CH3OH, DME, and light olefins, the relationship between conversion and selectivity is less clear.

**Figure 18.** Selectivity and conversion distribution of different products according to recent reports in heterogeneous catalysis.

In the RWGS reaction, CO is mainly produced over Fe-, Co-, Mo-, and Pt-based catalysts (Table 1), while CO2 methanation is typically carried out on Co- and Ni-based catalysts (Table 2). Although CO is a main component of syngas and CH4 is an important energy resource, neither is the best product for CO2 conversion from a relatively economic point of view. Furthermore, the conversion and utilization of CO and CH4 is also an important research field, which also implies that CO and CH4 are not the

optimal choice as end-products of CO2 conversion. Therefore, the coupling reaction is a potential direction for CO2 conversion to some higher value products, i.e., CH3OH and light olefins.

Cu-ZnO-based catalyst is still the main catalytic material for direct conversion of CO2 to CH3OH under industrial reaction condition, while highly innovative methodologies are awaited to achieve low-pressure and low-temperature CH3OH synthesis processes. Considering the industrial application value of methanol, exploring novel catalytic systems and designing rational reactors to improve the catalytic activity should be targeted in the future. DME, up to now, is synthesized via Cu-based and H-ZSM5 catalyst (Table 4), and this process essentially remains a two-step tandem reaction. Hence, the bottleneck of DME production could be the deactivation of CH3OH dehydration catalyst, due to water poisoning. The formation of coke is also the major reason of catalyst deactivation, especially in a relatively long-term operation process. To maintain high activity of catalyst for the production of DME, water- and coke-resistant catalysts need to be developed for CH3OH dehydration.

As shown in Table 4, the direct hydrogenation of CO2 to light olefins is mainly catalyzed by Fe-based catalysts. However, the starting point of current research work is mainly the coupling of RWGS, F–T and/or methanol-to-olefins (MTO) reactions. Therefore, suitable catalytic materials are sought for the coupling of C-C bonds, which is an effective strategy for improving the catalytic activity.

#### **3. Plasma Catalysis**

Plasma, the 'fourth state of matter', consists of electrons, neutral species (i.e., molecules, radicals, and excited species) and ions. Plasma can be in so-called thermal equilibrium or not, based on which it is subdivided into 'thermal plasma' and 'non-thermal plasma' (NTP) [13,112,113]. In non-thermal plasma, the gas temperature remains near room temperature, while electrons temperature is extremely high, usually in the range of 1–10 eV (~10,000–100,000 K). The latter is enough to activate stable gas molecules into reactive species (e.g., radicals, excited atoms, molecules, and ions). These reactive species, especially the radicals, can trigger reactions at low temperature. That is, NTP offers a unique approach to enable thermodynamically unfavorable chemical reactions to proceed at low temperature by breaking thermodynamic limits. Nevertheless, the control of selectivity of desired products in plasma is extremely difficult, since the reactions in plasma are mainly triggered through nonselective collision between active species (radicals, molecules, atoms, and ions).

To improve the desired product selectivity of the reactions in plasma, the combination of catalysts with plasma technology (so-called plasma catalysis) is a promising strategy, since catalysts usually have a special feature of regulating product distribution.

NTP can be generated through various types of discharges—i.e., microwave discharges, glow discharges, gliding arc discharges, dielectric barrier discharges (DBD), etc.—but DBD are the best option to be used in plasma catalysis. Indeed, DBDs are usually operated at atmospheric pressure and ambient temperature, and the integration of DBD plasma with catalysts has the advantages of simple operation and low cost. Thus plasma-driven direct hydrogenation of CO2 is mostly based on DBD plasmas. The possible plasma/catalyst synergism is illustrated in Figure 19 [13].

A lot of research has been performed for pure CO2 splitting, in various types of plasma reactors, including DBD, microwave discharge and gliding arc discharge, without catalysts [114–117]. A typical experimental set-up of a DBD plasma for CO2 decomposition is shown in Figure 20. Paulussen et al. studied the conversion of CO2 to CO and oxygen in DBD [114], and they found that the gas flow rate is the most crucial parameter affecting the CO2 conversion. At 0.05 L min−<sup>1</sup> flow rate, 14.75 W cm−<sup>3</sup> power density and 60 kHz discharge frequency, 30% CO2 conversion was achieved. The performance might be further enhanced by optimizing the discharge parameters (i.e., power, frequency, dielectric material) or by implementing parallel reactors.

**Figure 19.** Overview of the possible effects of the catalyst on the plasma and vice versa, possibly leading to synergism in plasma-catalysis, reproduced with permission from [13]. Copyright Royal Society of Chemistry, 2017.

**Figure 20.** Schematic diagram of the experimental set-up used in the decomposition of CO2, reproduced with permission from [116]. Copyright IOP Publishing, 2016.

Dry reforming of methane (DRM), i.e., the combined conversion of CH4 and CO2 by plasma and/or plasma catalysis, has attracted extensive attention in recent years. For instance, Li et al. studied CO2 reforming of CH4 by taking advantage of atmospheric pressure glow discharge plasma [118]. Liu et al. reported high-efficient conversion of CO2 and CH4 in AC-pulsed tornado gliding arc plasma [119]. Kolb et al. investigated DRM in a DBD reactor [120]. In the above-mentioned studies [118–120], syngas (CO and H2) was produced as the main product. Recently, Wang et al. reported a novel one-step reforming of CO2 and CH4 into liquid oxygenate products, dominated by acetic acid, at room temperature by the coupling of DBD plasma and catalysts [121]. They examined the effect of CH4/CO2 molar ratio and of various catalysts (γ-Al2O3, Cu-γ-Al2O3, Au-γ-Al2O3, Pt-γ-Al2O3). Interestingly, compared with plasma-only mode, the coupling of plasma and catalysts

tuned the selectivity of liquid chemicals, and oxygenates selectivity was achieved up to approximately 60% when Cu catalyst was used. Details about the effects of operating mode and catalysts on the CO2 conversion reaction results are shown in Figure 21. Although much efforts need to be made to reveal the unknown mechanisms, the results are attractive and show an excellent application prospect.

**Figure 21.** Effect of operating modes and catalysts on the reaction: (**a**) selectivity of liquid oxygenates, (**b**) selectivity of gaseous products, (**c**) conversion of CH4 and CO2 (total flow rate 40 mL min−1, discharge power 10 W, catalyst ca. 2 g), reproduced with permission from [121]. Copyright Wiley online library, 2017.

Besides the above-mentioned metal catalysts, zeolites have also been used in plasma catalytic DRM. Zhang et al. studied the catalytic performance of zeolite catalysts (i.e., NaA, NaY, and HY) for the direct conversion of CH4 and CO2 at relatively low temperature range and ambient pressure via DBD plasma [122], and the products were dominated by syngas and C4 hydrocarbons. Form the investigated catalysts (NaA, NaY, and HY), HY zeolite catalyst exhibited the best performance (26.7% CO2 conversion and 52.1% C4 hydrocarbon selectivity), which was mainly attributed to the appropriate pore size and electrostatic properties of HY zeolite.

Although direct hydrogenation of CO2 to CH3OH is an exothermic reaction, studies of heterogeneous catalysis for CO2 hydrogenation to CH3OH were usually operated at high temperature and high pressure, mainly caused by the high stability of CO2 and the low equilibrium constant at atmospheric pressure. Plasma catalysis, however, is a promising approach to enable CO2 conversion to CH3OH at ambient conditions, and has gradually attracted more and more interest. For instance, Eliasson et al. reported the direct hydrogenation of CO2 to CH3OH by coupling of DBD plasma and a discharge-activated catalyst (CuO-ZnO-Al2O3) [123]. By comparison of the experiments, with catalyst only, discharge only, and discharge + catalyst, they found that DBD plasma effectively lowered the

optimal reaction temperature corresponding to the best catalytic performance. Indeed, the optimal reaction temperature was 493 K for catalyst only, while in terms of discharge only and discharge + catalyst, the optimal reaction temperature for CO2 conversion was 373 K. The maximum selectivity for the methanol formation (10%) was achieved at a temperature of 373 K, which implies that the plasma improved the catalytic activity of CuO-ZnO-Al2O3 catalyst at low temperature. Additionally, the authors found that low input power and high pressure are beneficial for the improvement of the methanol selectivity.

Zeng et al. studied CO2 hydrogenation by combining various catalysts (i.e., Cu/*γ*-Al2O3, Mn/*γ*-Al2O3, and Cu–Mn/*γ*-Al2O3) with DBD plasma in a coaxial packed-bed [124]. The experimental results showed that the addition of catalysts in the reactor improved the conversion of CO2. At the same time, with the increase of the H2/CO2 molar ratio, the CO2 conversion was improved, and the CO yield was also enhanced. It is also worth mentioning that, compared with plasma only experiments, the energy efficiency was enhanced by adding catalysts, although the synergetic mechanism between catalysts and plasma is still unknown.

Recently, Wang et al. examined the influence of plasma reactor structure and catalysts on CO2 conversion and CH3OH selectivity for plasma catalytic CO2 hydrogenation [125], and a schematic diagram of the experimental setup and images of the H2/CO2 discharge are shown in Figure 22. They investigated three kinds of reactors, i.e., a cylindrical reactor (aluminum foil sheet as ground electrode), a double dielectric barrier discharge reactor (water as ground electrode), and a single dielectric barrier discharge reactor (water as a ground electrode). The single DBD reactor equipped with a special water-electrode showed the optimal reaction performance (21.2% CO2 conversion and 53.7% CH3OH selectivity). In addition, they tested the catalytic performance of Pt/γ-Al2O3 catalysts and Cu/γ-Al2O3 catalysts in the optimized reactor for direct conversion of CO2 to CH3OH. As shown in Figure 23, both Pt/γ-Al2O3 catalysts and Cu/γ-Al2O3 catalysts improved not only the CO2 conversion, but also the CH3OH selectivity, and Cu/γ-Al2O3 catalysts showed a better catalytic performance (21.2% CO2 conversion and 53.7% methanol selectivity). That is, a strong synergistic effect between the plasma and Cu/γ-Al2O3 catalysts promoted the hydrogenation of CO2 to methanol, although the reaction temperature in the optimized reactor remained near room temperature.

**Figure 22.** (**a**) Schematic diagram of the experimental setup of a DBD plasma catalytic reactor. (**b**) Images of H2/CO2 discharge generated in DBD reactor without catalyst, reprinted with permission from [125]. Copyright American Chemical Society, 2017.

**Figure 23.** Effect of H2/CO2 molar ratio and catalysts on the reaction performance of the plasma hydrogenation process, reprinted with permission from [125]. Copyright American Chemical Society, 2017.

It is obvious that the combination of the plasma and catalysts can enhance the catalytic reaction at room temperature and atmospheric pressure. The maximum methanol selectivity of 53.7% was achieved with a CO2 conversion of 21.2% via the plasma-catalysis process (see Figure 23). All the above studies show a strong synergistic effect between plasma and catalysts.

Using one-dimensional fluid modeling [126], De Bie et al. proposed a chemical reaction network in the plasma (i.e., without catalysts) for conversion of CO2 to value-added chemicals, i.e., CO, CH4, CH2O, CH3OH, and hydrocarbons. The simulation results indicated the dominant reaction pathways for the conversion of CO2 and H2, as illustrated in Scheme 5. According to the model, the combination between H atoms and CHO radicals is the most important reaction to form CO, while this reaction is counterbalanced by the reorganization of H with CO into CHO radicals. Therefore, the most effective net CO formation reaction is dissociation of CO2 influenced by electrons. The production of CH4 was generally driven by two reactions, i.e., three-body recombination reaction between CH3 and H radicals, and charge transfer reaction between CH5 <sup>+</sup> and H2O. However, the latter reaction is partly balanced by the loss of CH4, resulting from a charge transfer reaction with H3 +. The production of CH2O is closely related to the initial CO2 fraction in the gas mixture. At a low initial fraction of CO2, the reaction between CO2 and CH2 radicals seem to be the most important channel for the formation of CH2O, while at higher initial CO2 fractions, CH2O is also produced out of two CHO radicals to some extent. In addition, as predicted by the model, the most important channel for the formation of CH3OH is the three-body reaction between CH3 and OH radicals, while the three-body reaction between CH2OH and H radicals is also an effective production channel for CH3OH. Furthermore, they also found that a higher density of CH3 and CH2 radicals would be essential to tune the distribution of end products. Therefore, it can be predicted that the degree of hydrogenation in the reaction has a significant influence for the targeted products.

Figure 24 summarizes the selectivity of CO, CH4, or CH3OH, as a function of the CO2 conversion, obtained from the recent reports discussed above. Several main trends are clear. Firstly, the main products are CO and CH4 for direct hydrogenation of CO2 in plasma catalysis, while the selectivity of CH3OH is relatively low. Moreover, it is obvious that researchers have paid more attention to the reduction of CO2 to CO and CH4 up to now. However, the production of other hydrocarbons, such as olefins and gasoline hydrocarbons, would also be a promising direction, to achieve the maximum utilization of CO2 by plasma catalysis. Therefore, some insights from heterogeneous catalysis, especially the combination of metal catalysts, metal oxide catalysts, and zeolite catalysts could be helpful to develop a methodology for plasma catalytic hydrogenation of CO2. On the other hand, there is no guarantee that good catalysts in thermal processes would also perform well in plasma

catalysis, because of the clearly different operating conditions (e.g., lower temperature, abundance of reactive species, excited species, charges, and electric field present in plasma).

**Scheme 5.** Dominant reaction pathways for the plasma-based conversion (without catalysts) of CO2 and H2 into various products, in a 50/50 CO2/H2 gas mixture. The thickness of the arrow lines is proportional to the rates of the net reactions. The stable molecules are indicated with black rectangles, reprinted with permission from [126]. Copyright American Chemical Society, 2016.

**Figure 24.** Overview of selectivity into CO, CH4, and CH3OH, as a function of CO2 conversion, based on all reports available in literature about plasma catalysis (all in DBD reactors). The references where these data are adopted from are discussed in the text.

Up to now, in view of the complexity of this interdisciplinary field, the development of plasma catalysis still needs major research efforts. On one hand, plasma catalysis has potential for industrial application, because it drives the CO2 conversion reaction at ambient temperature and atmospheric pressure, breaking thermodynamic equilibrium to make full use of feedstock. On the other hand, the excessive energy consumption—caused by high input power and heat loss—is an important disadvantage, but it can be mitigated with the further development of renewable energy (i.e., wind, solar, and tidal energy). Additionally, to improve the reaction activity for CO2 conversion, it is crucial to explore the reaction mechanisms by in situ characterization and computer modeling, to improve the synergistic effect between plasma and catalysts. Finally, we need to search for suitable catalytic materials to strength the reaction performance and decrease the production costs.

#### **4. Outlook and Conclusions**

Currently, fuels and base chemicals are nearly all produced from non-renewable fossil energy (oil, natural gas, and coal), and CO2 is generally the end product (e.g., upon burning fossil fuels) or a waste product in chemical industry. This indicates that we should use CO2 as the main carbon source when fossil energy would get depleted in the future. Thus, in the long term, hydrogenation of CO2 (as well as CO2 conversion with other H-sources) into value-added chemicals and fuels is very significant, since it can close the carbon cycle, as shown in Figure 25. However, some crucial issues should be addressed in advance for application of CO2 hydrogenation in industry.

**Figure 25.** CO2 conversion into fuels, which release CO2 again upon burning, aiming a closed carbon cycle.

From a relatively economical point of view, the direct hydrogenation of CO2 is not yet a viable approach. On one hand, CO2 of a certain purity generally depends on the development of capture and separation techniques. On the other hand, compared with the price of the feedstock (H2, 10,000 \$/ton), the price of the main products (i.e., liquefied natural gas, 770 \$/ton, and CH3OH, 340 \$/ton) obtained by CO2 hydrogenation is too low to make this an economically viable process. However, once the production of H2 can be realized with fully-fledged solar power technology, the production cost for CO2 hydrogenation will be greatly reduced. Meanwhile, the CO2 conversion driven by solar energy (artificial photosynthesis) is also a promising routing.

Technologically, the direct hydrogenation of CO2 to CO is an endothermic reaction (ΔH = 41.2 kJ mol<sup>−</sup>1), and a higher reaction temperature is beneficial for the production of CO according to Le Chatelier's principle. Based on the available experimental results (Table 1), the CO selectivity can reach nearly 100% under conditions of 873 K and 1 MPa. On the other hand, the production of other products (i.e., CH4, CH3OH, DME, and light olefins) from CO2 is exothermic, and in theory a low temperature favors the equilibrium conversion. However, the inert CO2 molecule generally needs relatively high reaction temperature to be activated. The competition between these two factors makes the reaction performance not very satisfactory. Up to now, from the perspective of technology readiness level, methanol synthesis is a far more advanced production process, with a high selectivity up to 80–90% with Cu-based catalyst (Table 3). However, there are some limitations in heterogeneous catalysis for direct hydrogenation of CO2, such as catalyst sintering at high temperature, the influence of water produced in the reaction process and low CO2 conversion caused by thermo-dynamical restriction in terms of the formation of CH3OH and hydrocarbons. Hence, we should explore novel catalytic materials and reactors to break the thermodynamic equilibrium, as well as design bi-functional and/or multi-functional catalysts (metal, metal oxides, and zeolites) to couple the chemical reactions (RWGS, methanation, methanol synthesis, and/or MTO).

Plasma catalysis, a new field of catalysis, has attracted sufficient attention in recent years, due to its simple operating conditions (ambient temperature and atmospheric pressure) and unique advantages in activating inert molecules. However, the energy efficiency is still too high for commercial exploitation. On the other hand, renewable energy (e.g., wind, solar, and tidal energy) will further develop in the near future, and it will be a perfect match with plasma catalysis (because plasma is generated by electricity and can simply be switched on/off—allowing storage of fluctuating energy), hence making the problem of limited energy efficiency less dramatic. Nevertheless, significant efforts must be devoted to elucidate the reaction mechanisms in plasma catalysis, to improve not only the energy efficiency, but certainly also the selectivity towards value-added products. Indeed, plasma catalysis is complicated from chemistry and physics point of view, but the potential of plasma technology for industrial applications deserves major research efforts. A combination of computer simulations with experiments will be needed for an in-depth understanding of the reaction mechanisms, responsible for the synergy between plasma and catalysts. Although the development process in plasma catalysis may be slow due to its complex character, it would bring great benefits to human society, once it is developed mature enough, and once appropriate catalysts for plasma catalysis can be designed. Therefore, future research should definitely emphasize on a better understanding and rational screening of highly active catalysts.

Compared with the numerous studies on CO2 hydrogenation by heterogeneous catalysis, much more research should be carried out in the field of plasma catalysis, to improve the CO2 conversion and target products selectivity (and even to exploit new products, such as olefins, gasoline hydrocarbons, and aromatics), since the results achieved by plasma catalysis (Figure 24) are far from those of heterogeneous catalysis (Figure 18). To achieve this goal, the advantage of plasma should be exploited in full, and at the same time, insights from heterogeneous catalysis (e.g., catalyst combination, reaction combination, active sites design, etc.) can help to further improve the potential of this promising field of catalysis.

**Author Contributions:** Conceptualization: M.L., Y.Y., L.W. and A.B.; Validation: M.L., Y.Y., L.W., H.G. and A.B.; Formal analysis: M.L., Y.Y., L.W., H.G. and A.B.; Resources: Y.Y. and A.B.; Data curation: Y.Y., L.W. and A.B.; Writing—original draft preparation: M.L. and Y.Y.; Writing—review and editing: Y.Y. and A.B.; Supervision: Y.Y., L.W. and A.B.; Funding acquisition: Y.Y. and A.B.

**Funding:** This research was funded by the Fundamental Research Funds for the Central Universities of China (DUT18JC42), the National Natural Science Foundation of China (21503032), PetroChina Innovation Foundation (2018D-5007-0501), and the TOP research project of the Research Fund of the University of Antwerp (32249).

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

### *Article* **H2O and/or SO2 Tolerance of Cu-Mn/SAPO-34 Catalyst for NO Reduction with NH3 at Low Temperature**

**Guofu Liu 1,†, Wenjie Zhang 1,†, Pengfei He 2, Shipian Guan 2, Bing Yuan 3, Rui Li 3, Yu Sun <sup>3</sup> and Dekui Shen 1,\***


Received: 2 February 2019; Accepted: 19 March 2019; Published: 21 March 2019

**Abstract:** A series of molecular sieve catalysts (Cu–Mn/SAPO-34) with different loadings of Cu and Mn components were prepared by the impregnation method. The deNO*<sup>x</sup>* activity of the catalyst was investigated during the selective catalytic reduction (SCR) of NO with NH3 in the temperature range of 120 ◦C to 330 ◦C, including the effects of H2O vapors and SO2. In order to understand the poisoning mechanism by the injection of H2O and/or SO2 into the feeding gas, the characteristics of the fresh and spent catalyst were identified by means of Brunner−Emmet−Teller (BET), X-ray Diffraction (XRD), Scanning Electronic Microscopy (SEM) and Thermal Gravity- Differential Thermal Gravity (TG-DTG). The conversion of NO by the catalyst can achieve at 72% under the reaction temperature of 120 ◦C, while the value reached more than 90% under the temperature between 180 ◦C and 330 ◦C. The deNO*<sup>x</sup>* activity test shows that the H2O has a reversible negative effect on NO conversion, which is mainly due to the competitive adsorption of H2O and NH3 on Lewis acid sites. When the reaction temperature increases to 300 ◦C, the poisoning effect of H2O can be negligible. The poisoning effect of SO2 on deNO*<sup>x</sup>* activity is dependent on the reaction temperature. At low temperature, the poisoning effect of SO2 is permanent with no recovery of deNO*<sup>x</sup>* activity after the elimination of SO2. The formation of (NH4)2SO4, which results in the plug of active sites and a decrease of surface area, and the competitive adsorption of SO2 and NO should be responsible for the loss of deNO*x* activity over Cu/SAPO-34.

**Keywords:** SCR; Catalyst; (NH4)2SO4; deNO*x*; H2O and SO2 poisoning

#### **1. Introduction**

Nitrogen oxides were estimated as one of the major air pollutants released from the combustion of fossil fuels (especially coal), being hazardous for the ecological and environmental system [1–4]. Currently, selective catalytic reduction (SCR) was regarded as a widely-used deNOx technology for the purification of flue gas, where the performance of the catalyst plays a significant role in the process [5]. Most of the commercial catalysts (e.g., V-W-Ti system) exhibited the effective activity with temperature located in a narrow and relatively high window as 300–400 ◦C, accelerating the deactivation of catalyst through sintering and occlusion of salts produced from H2O or SO2. The high deNOx activity of SCR catalyst at relatively low temperature is highly required without the formation of salts from H2O and

SO2 in the flue gas. The stability of the air-preheating system can be improved along with the secure low-temperature SCR system, leading to a full-time deNOx for the power plant under different power loadings. Therefore, it is of great significance to develop efficient and stable low-temperature SCR catalyst [6].

Among all these catalysts, transition metal loaded on zeolite materials with CHA structure have been widely focused due to the broad operating temperature range and the high deNOx activity [7–9]. A number of research works have been conducted on the deNOx performance of molecular sieve catalysts, such as the ZSM-5, BEA, USY, SSZ-13 and so on [10–12]. Kim [13] prepared Mn–Fe/ZSM-5 by the impregnation method, exhibiting the NO*<sup>x</sup>* conversion ratio as high as 95% at 175 ◦C, and the conversion of NO*x* nearly to 100% during the temperature between 200 and 350 ◦C. SAPO-34 possesses pear-shaped cages with 8-membered ring (8MR) openings, and double 6-membered ring (D6MR) units linked by 4-membered ring (4MR) units, while most of the P is replaced by Si to generate a Si–O–Al linkage, resulting in remarkable SCR performance. Compared with Fe-zeolites and vanadia-zeolites, Cu-zeolites exhibited a superior deNOx activity and high N2 selectivity [14,15]. Wang [16,17], Ye [18] and Deka [19] prepared Cu/SAPO-34 by different methods, exhibiting an excellent low-temperature SCR activity in the temperature range from 150 ◦C to 500 ◦C. Among all these researches, Cu-SAPO-34 based catalysts showed a remarkable deNOx activity compared to other zeolites [8,20].

However, Cu-SAPO-34 catalyst is proved to be sensitive to SO2 poisoning due to the strong chemical binding strength and the oxidative conditions in the gas and the deactivation effect is more pronounced at low temperatures [21]. Zhang et al. [7] have used DRIFT and Temperature Programmed Desorption (TPD) method to study the poisoning effect of SO2 over Cu/SAPO-34 catalyst, they reported that low-temperature deactivation is caused by the formation of ammonium sulfates. Shen et al. [22] observed no obvious sulfur species on the Cu/SAPO-34 catalyst and they concluded that the reduction of the number of isolated Cu2+ caused by SO2 might induce the loss of SCR activity. Wijayanti et al. [23] studied the SO2 poisoning effects and found that the main reason for the deactivation is the formation of copper sulfates, resulting in the loss of redox properties. Jangjou et al. [24] reported the S species formed on Cu2+ at 6MR by DRIFT study. On the basis of such observations, the formation of ammonium sulfates in a complex with Cu is claimed as the main mechanism for the loss of low-temperature deNOx activity. In general, different SO2 poisoning mechanisms have been proposed by different researchers.

In addition, H2O is also one of the main components in the flue gas and often causes the loss of low-temperature deNOx activity [3]. In this work, for a better and specific understanding of the SO2 poisoning mechanism and the synergistic effect of H2O and SO2 over Cu/SAPO-34 catalyst, the influence of SO2 or/and H2O with different concentrations at different reaction temperatures on deNOx activity and physicochemical properties over Cu/SAPO-34 catalyst was studied. A series of Cu-Mn/SAPO-34 catalysts were prepared through the impregnation method. The low-temperature deNOx activity of the catalysts was estimated by means of a self-designed apparatus, where the effects of H2O and SO2 on the reaction activity were also investigated. BET, XRD, SEM and TG-DTG were employed to determine the characteristics of the fresh and spent catalyst, in order to understand the poisoning mechanism of the catalyst by H2O and SO2 during the low-temperature SCR process.

#### **2. Results and Discussion**

#### *2.1. DeNOx Activity of the Catalysts Without H2O and SO2*

Figure 1a illustrates the deNOx activity of Cu/SAPO-34, Mn/SAPO-34, Cu–Mn/SAPO-34 catalysts under different temperatures without H2O and SO2. The deNOx activity of Cu–Mn/SAPO-34 catalyst (bimetallic composite molecular sieve) was much higher than that of Cu/SAPO-34 and Mn/SAPO-34 catalysts (monomeric molecular sieve). Even when the reaction temperature is lower than 100 ◦C the NO conversion by Cu–Mn/SAPO-34 catalyst can be achieved at about 60%, while for the other two catalysts was around 20%.

**Figure 1.** DeNOx performance of mono-component and multi-component catalysts (**a**); deNOx performance of molecular catalysts with different *Mn* loadings (**b**); outlet N2O concentration over Cu(2)-Mn(6)/SAPO-34 catalyst (**c**); outlet NO2 concentration over Cu(2)-Mn(6)/SAPO-34 catalyst (**d**).

The NO conversion over Cu–Mn/SAPO-34 catalyst could reach 90% at around 180 ◦C, compared to that of 220 ◦C for Cu/SAPO-34 and 270 ◦C for Mn/SAPO-34. It can be concluded that the Cu–Mn/SAPO-34 catalyst gives a better deNOx performance under the relatively low-temperature range from 100 to 200 ◦C. This can be attributed to the promotion of NH3 adsorption on the surface of the catalyst by the bimetallic system on SAPO-34 [25–27]. In addition, the deNOx reaction energy might be declined by the bimetallic interaction on the catalyst, improving the low-temperature activity of the catalyst and broadening its SCR temperature range.

The effect of metal loading on the activity of Cu-Mn/SAPO-34 catalyst is shown in Figure 1b. The deNOx activity of Cu(2)–Mn(6)/SAPO-34 performed as the best one among the catalysts at the temperature from 120 to 210 ◦C and from 270 to 330 ◦C, while Cu(2)–Mn(8)/SAPO-34 performs the best one at the temperature from 210 to 270 ◦C. It needs to be noted that the NO conversion over Cu–Mn/SAPO-34 with the Mn content more than 8% was notably declined when the reaction temperature is higher than 270 ◦C. Under high temperatures, the oxidation of the catalyst can be enhanced with the increased loading of Mn [28]. This leads to the oxidation of NH3 to NO and then the decline of the NO conversion (Figure 1b). In addition, the nonselective catalytic reduction (NSCR) reaction and catalytic oxidation reaction (i.e., the C–O reaction) happens simultaneously during the NH3-SCR reaction. The outlet N2O and NO2 concentration over Cu(2)–Mn(6)/SAPO-34 catalyst are shown in Figure 1c,d. It can be seen that the N2O concentration increases with the increase of reaction temperature, which may contribute to the decrease of deNOx activity over Cu(2)–Mn(6)/SAPO-34 catalyst at 330 ◦C. With the increase of reaction temperature, the NO2 concentration decreases. These

results demonstrate that N2 is the main product of NH3-SCR reaction over Cu(2)–Mn(6)/SAPO-34 catalyst according to the "standard SCR " reaction [29].

#### *2.2. Effect of H2O on the deNOx Activity of the Catalyst*

The effect of the injected H2O concentration into feeding gas on the deNOx activity of Cu(2)–Mn(6)/SAPO-34 catalyst at a reaction temperature of 240 ◦C was shown in Figure 2a. The NO conversion decreased with an increased H2O concentration. When the volume concentration of 2% water vapor was injected into the original feeding gas after 8 h, the deNOx activity of the catalyst was declined from 93% to 91%. While the volume concentration of the vapor injected into the feeding gas was increased to 10%, after 8 h, the activity of the catalyst decreased to 86%. It needs to be noted that the activity of catalyst after the 8-hour injection of vapor was recovered to its original level after a while of the vapor cut-off in spite of the concentration of vapor. This indicates that the poisoning of the catalyst by the water is due to the competitive adsorption between H2O vapor and NH3 or NO, which is reversible. When the water vapor concentration was increased, the activity of the spent catalyst (after the cut-off of vapor) was a little bit lower than that of the original catalyst. It is demonstrated that the hydroxyl may be created due to the adsorption and decomposition of H2O on the surface of Cu/SAPO-34 with the increase of H2O concentration, resulting in the irreversible loss of deNOx activity [3].

**Figure 2.** Effect of the injected H2O concentration on the deNOx activity of the catalyst (240 ◦C) (**a**); effect of H2O on the deNOx activity of catalyst under different temperatures (10% H2O) (**b**).

Figure 2b shows the effect of H2O on the deNOx activity of the catalyst Cu(2)–Mn(6)/SAPO-34 under different temperatures with the concentration of water vapor as 10%. The deNOx activity of the catalyst was significantly influenced at low reaction temperature with the presence of H2O. A sharp decline of the deNOx activity of catalyst (from 86% to 68%) can be observed under the reaction temperature of 180 ◦C after the 4-hour injection of vapor into the feeding gas. However, no significant change in the activity of the catalyst can be found for the reaction under 300 ◦C in the presence of vapor. After 8 h injection of the vapor under 180 ◦C, the activity of the catalyst was all recovered to its original level in 2 h, while the activity recovery duration for the catalyst was decreased with the increased reaction temperature after the cut-off of vapor. The adsorption capacity of H2O on the surface of the catalyst can be enhanced under the lower temperatures, occupying more active sites than that of NH3, NO and other reaction gases [30]. It can be also concluded that poisoning performance of water on the activity of the catalyst can be ignored while the reaction temperature is increased over 300 ◦C.

#### *2.3. Effect of SO2 on the deNOx Activity of the Catalyst*

The effect of the injected SO2 concentration (from 500 ppm to 1500 ppm) on the deNOx activity of the catalyst Cu(2)–Mn(6)/SAPO-34 under the reaction temperature of 240 ◦C was shown in Figure 3a. It is obvious that the deNOx activity of the catalyst was remarkably and abruptly decreased with the injection of SO2 for the three different concentrations. After 2 h injection of SO2, the deNOx activity of catalyst was reduced from 97% to 72%, 68%, 63% for the SO2 concentration of 500, 1000 and 1500 ppm. Compared to that of the injected H2O, the high concentration of the injected SO2 could accelerate the decline of the activity of the catalyst. The formation of SO3 can be promoted by the high concentration of the injected SO2, consequently enhancing the formation of ammonium sulfate with NH3 covering the catalyst active sites on the surface. After the cut-off (8 h) of the injected SO2, the activity of the catalyst was increased, but much lower than its initial activity. It is indicated that the competitive adsorption between SO2 and NH3 or NO is not the main reason for the loss of deNOx activity in the presence of SO2. Part of active sites on the catalyst was occupied by SO2 over NH3 and NO leading to the temporary poisoning, while a great number of active sites was covered by the formed sulfate salts (such as ammonium sulfate) for the permanent deactivation of the catalyst [31,32].

**Figure 3.** Effect of the injected SO2 concentration of on deNOx activity of catalyst (240 ◦C) (**a**); Effect of the injected SO2 (500 ppm) on deNOx activity of catalyst under different temperatures (**b**).

The effect of the injected SO2 on the deNOx activity of the catalyst Cu(2)–Mn(6)/SAPO-34 under different reaction temperatures was shown in Figure 3b. The effect of the injected SO2 on the activity of the catalyst was greatly influenced by the reaction temperature. It needs to be noted that the activity of the catalyst was decreased from 90% to 29% after 8-hour injection of 500 ppm SO2 under 180 ◦C. Comparatively, under the reaction temperature of 300 ◦C, the activity of the catalyst was reduced from 99% to 88% after 8-hour injection of 500 ppm SO2. This result demonstrates that the effect of SO2 on deNOx activity is greatly related to the reaction temperature and a much more rapid decrease of deNOx activity happened with the addition of SO2 at a lower temperature. It should be noted that no obvious recovery of deNOx activity at the reaction temperature of 180 ◦C can be observed. The more detailed mechanism of SO2 poisoning over Cu/SAPO-34 at different reaction temperatures will be further discussed.

#### *2.4. Effect of Both H2O and SO2 Injection on the deNOx Activity of the Catalyst*

Figure 4 shows the effect of injection of H2O and SO2 on the catalytic activity of Cu(2)–Mn(6)/SAPO-34. NO conversion was decreased at the presence of H2O and SO2, compared to the performance the individual injection of H2O or SO2. Moreover, the co-existence of H2O and SO2 in the feeding gas gave a much more serious decline on the deNOx activity of the catalyst compared to the sum of the effect of H2O and SO2 respectively, which is labeled as the estimated value as shown in Figure 4. The existence of H2O could enhance the deactivation of the catalyst by SO2 through two ways: (1) SO2 in gas phase reacted with the H2O adsorbed on active site of catalyst to generate sulfuric acid or sulfurous acid which is easily reacted with NH3; (2) the thermal decomposition of the formed ammonium sulfate on the active sites was confined at the presence of H2O.

**Figure 4.** Effect of the injected H2O and SO2 on the deNOx activity of catalyst (240 ◦C).

#### *2.5. Mechanism of the Catalyst Poisoning by H2O and/or SO2*

In order to identify the change of crystalline phases for the fresh and spent Cu(2)–Mn(6)/SAPO-34 catalyst after SO2 and/or H2O poisoning. Powder X-ray diffraction (XRD) measurements were carried out and the patterns are shown in Figure 5. For the fresh Cu/SAPO-34 catalyst, sharp diffraction peaks corresponding to CHA phases are obtained. After poisoned with SO2 or H2O, the intensity of each peak decreases, especially for the catalyst after SO2 + H2O treatment, indicating the obvious skeleton damage of SAPO-34 after adding SO2 + H2O for 8 h [33]. No obvious crystalline change can be observed in the presence of 10% H2O, this indicates that the phase poisoned by H2O existed either in amorphous form or in the particle beyond the limited size of XRD detection [34]. For the catalyst after SO2 poisoning for 8 h, A number of new diffraction peaks at 2θ = 11.9◦, 29.8◦and 40.7◦ can be observed. The peaks at 11.9◦ and 29.8◦ can be attributed to NH4HSO4 and (NH4)2SO4, while the latter peak at 40.7◦ is assigned to MnSO4. For the catalyst after SO2 + H2O poisoning for 8h, new diffraction peaks at 2θ = 29.8◦ and 34.1◦ can be observed, which can be assigned to (NH4)2SO4. In addition, the new peak at 2θ = 21.1◦ can be assigned to the formation of CuSO4. It can be deduced that the newly formed NH4HSO4, (NH4)2SO4 and MnSO4 result in the loss of deNOx activity in the presence of SO2. When H2O and SO2 were added simultaneously, SO2 in gas phase may react with adsorbed H2O to generate sulfuric acid or sulfurous acid, which is easily reacted with NH3 to form (NH4)2SO4. Thus, the formation of (NH4)2SO4 and CuSO4 may cause the loss of deNOx activity in the presence of SO2 + H2O.

In order to determine the morphology before and after SO2 or/and H2O poisoning, the fresh Cu/SAPO-34 and poisoned catalyst by SO2 or/and H2O were characterized by SEM method shown in Figure 6. No significant change of the surface morphology can be found between the catalysts before and after the H2O poisoning. The active particles on the surface of fresh catalyst were replaced by the bulks of ammonium sulfate after the poisoning by individual SO2, inhibiting the deNOx activity of the catalyst. Similar phenomena can be observed for the spent catalyst in presence of H2O and SO2, where the agglomeration of ammonium sulfate lead to the decline in the surface active sites of the catalyst and thus the activity of the catalyst.

**Figure 5.** X-ray diffraction spectra of the fresh and spent Cu(2)–Mn(6)/SAPO-34.

**Figure 6.** SEM micrographs of fresh and spent Cu(2)-Mn(6)/SAPO-34: (**a**) fresh; (**b**) 10% H2O 240 ◦C 8 h; (**c**) 500 ppm SO2 240 ◦C 8 h; (**d**) 10% H2O and 500 ppm SO2 240 ◦C 8 h.

The TG curves of the poisoned catalyst in the presence of individual SO2 and both H2O and SO2 were shown in Figure 7. Three mass loss stages can be observed for the two kinds of the poisoned catalyst: the first mass loss in the temperature between 50 ◦C and 100 ◦C was attributed to the dehydration of the catalyst; the second stage between 200 ◦C and 400 ◦C can be designated to the thermal decomposition of NH4HSO4 and (NH4)2SO4 is 200 ◦C [35]; the third stage between 600 ◦C and 800 ◦C can be attributed to the decomposition of the sulfate metal salts [36]. The peak value of the second mass loss stage of the poisoned catalyst in the presence of H2O and SO2 is notably higher than that of the poisoned catalyst in presence of individual SO2, indicating that more content of (NH4)SO4 deposited on the surface of the catalyst. This confirms that the formation of ammonium sulfate can be facilitated and promoted with the injection of H2O in the feeding gas, which is consistent with the XRD and TG-DTG analysis. While the peak value of the third mass loss stage of the poisoned catalyst in the presence of H2O and SO2 is lower than that in the presence of SO2. It might be because the formation of (NH4)SO4 can inhibit the formation of sulfate metal salts due to the consumption of SO2.

The specific surface area, pore volume and pore size of fresh and spent Cu(2)–Mn(6)/SAPO-34 were determined by N2 adsorption and summarized in Table 1. The specific surface area and pore volume of Cu(2)–Mn(6)/SAPO-34 catalyst was decreased after the all different NO reduction experiments with or without the injection of H2O and/or SO2. It can be seen that the damage of the surface area is related to the concentration of SO2 or H2O and the reaction temperature. With the increase of reaction temperature, the damage of the surface area caused by SO2 is lightening. The low reaction temperature could confine the thermal decomposition of the formed NH4HSO4 and (NH4)2SO4 on the surface, resulting in a decline in surface area and the blockage of pore channel [37]. With the increase of the concentration of SO2, a change of which (from 331 m2/g to 320 m2/g) can be observed for different SO2 concentrations (from 500 ppm to 1500 ppm) under the temperature of

240 ◦C. For the poisoning effect of H2O, with the increase of reaction temperature, the existence of H2O has a more severe effect on the surface area of Cu/SAPO-34, which may result from the inhibited adsorption capacity of H2O. With the increase of the concentration of H2O, the surface area decreases from 425 m2/g to 408 m2/g as the concentration of H2O increased from 2% to 10% as well as the pore volume of catalyst. When H2O and SO2 added into the gas stream simultaneously for 8 h, the surface area decreases from 457 m2/g to 276 m2/g, which is a dramatically decrease compared to the injection of SO2 for 8 h at 240 ◦C (from 457 m2/g to 331 m2/g) and the injection of H2O for 8 h at 240 ◦C (from 457 m2/g to 408 m2/g). It is demonstrated that the synergistic poisoning effect of SO2 and H2O is enhanced, ascribed to the large amount of deposited (NH4)2SO4 on the pore channel of Cu/SAPO-34, which is consistent with the TG-DTG results.

**Figure 7.** TG-DTG curves of the thermal decomposition for the Cu(2)–Mn(6)/SAPO-34 catalyst after 8 h poisoning in presence of 500 ppm SO2 and/or 10% H2O.


**Table 1.** Brunner−Emmet−Teller (BET) analysis of the fresh and spent Cu(2)–Mn(6)/SAPO-34 catalyst.

Based on the activity tests and characterizations, the poisoning effect of SO2 or/and H2O can be described as follows. The poisoning effect of H2O, especially at low temperatures, can be ascribed to the competitive adsorption between H2O and NH3 on Lewis acid sites by occupying the metal sits [38], which can be recovered to nearly the original activity after H2O was removed. The XRD patterns of the catalyst after H2O poisoning did not show ant obvious changes, indicating no change with the active metal sites.

While for the poisoning effect of SO2 over Cu/SAPO-34 catalyst, the deNOx activity decreases from 90% to 29% at 180 ◦C in a short time of SO2 injection. In addition, the deNOx activity could not be recovered after the elimination of SO2, indicating the permanent deactivation of SO2 on Cu/SAPO-34 at 180 ◦C. The poisoning mechanism could be summarized as three aspects. Firstly, SO2 in the gas may be oxidized to SO3 and further react with NH3 to form NH4HSO4, which is a drying powdery decomposed at 280 ◦C [39]. The formation of NH4HSO4 causes the plug of active sites of the catalyst and the decline in surface area. Ammonium sulfate crystallite is observed on the XRD spectra and the weight loss peak ascribed to the decomposition of ammonium sulfates is also observed from the TG curves. Secondly, the active sites (i.e., MnO2 or CuO species) may react with SO2 or SO3 to form MnSO4 or CuSO4, which inhibited the redox properties [23]. The diffraction peaks assigned to MnSO4 and CuSO4 are observed in XRD spectra. Thirdly, the competitive adsorption of SO2 and NO on metal sites may be a part of the reason for the loss of deNOx activity at the reaction temperature of above 180 ◦C [40]. With regards to the synergistic effect of SO2 + H2O, the deNO*<sup>x</sup>* activity tests and characterizations show that the existence of H2O could enhance the deactivation of the catalyst by SO2 through two ways: (1) SO2 in gas phase reacted with the H2O adsorbed on active site of catalyst to generate sulfuric acid or sulfurous acid which is easily reacted with NH3 to form large amount of (NH4)2SO4; (2) the thermal decomposition of the formed ammonium sulfate on the active sites was confined at the presence of H2O. Both of the two explanations can be assigned to the deposition of (NH4)2SO4, which further plug the pore channel of catalyst and cause the rapid decrease of surface area as shown in Table 1.

#### **3. Materials and Methods**

#### *3.1. Catalyst Preparation*

The molecular sieve was modified by impregnation method. A certain amount of zeolite molecular sieve H-SAPO-34 (n(P2O5):n(SiO2):n(Al2O3) = 1:1:1), provided by the catalyst factory of Nankai University, was weighed and dried in a drying oven at 105 ◦C for 30 min. A certain amount of Cu(NO3)2•3H2O powder was mixed with manganese nitrate solution with the 50 wt.% in the beaker of 200 mL, and then added 50 mL of deionized water into the immersion liquid. The beaker with a magnetic stirrer inside was immersed in a water bath at a constant temperature of 40 ◦C. After 12 h of immersion, the solution was thoroughly mixed and heated until the moisture was completely evaporated. The powder was then put into a dry oven at about 100 ◦C for 12 h. The dried powder was ground and sieved by the 40 to 60 mesh. The obtained powders were placed in a tube furnace and calcined at 450 ◦C for 6 h to obtain the catalyst sample for the experiment. The catalyst sample Cu(2)-Mn(6)/SAPO-34(450) indicated that the 2 wt.% Cu and 6 wt.% Mn were loaded on the molecular sieve SAPO-34 with the calcination temperature of 450 ◦C.

#### *3.2. DeNOx Activity Measurements*

DeNOx activity measurements using NH3 were carried out in a fixed-bed stainless steel tubular flow reactor (Inner diameter: 16 mm) (Figure 8). The tube furnace of the reactor can be heated from the room temperature to 800 ◦C The gas feeding system was composed of five gas feeding pipes controlled by the mass flow meter (0–1.5 L) and one liquid feeder a microinjection pump (0.001 μL/min–127 mL/min), equipped with the reactor for adjusting the composition of the feeding gas. The reaction temperature in the experiments was set to be changed from 90 to 330 ◦C.

Two milliliter catalyst was placed on the holder of the reactor, and the feeding gas consisted of 350 ppm NO, 350 ppm NH3, 3 vol.% O2 and N2 as balanced. H2O (0–10 vol.%) and/or 500–1500 ppm SO2 would be injected into the feeding gas for investigating the effect of H2O and/or SO2 on the catalyst poisoning. The GHSV was set to be 15,000h<sup>−</sup>1. The NO and NO2 concentration of the reactor inlet and outlet was collected by an airbag and analyzed by the flue gas analyzer (Testo 350, Testo, Inc., Lenzkirch, Germany). The outlet N2O concentration is collected by the N2O analyzer (Medi-Gas G200, Bedfont Scientific Ltd., Bedfont, United Kingdom). The NO conversion was obtained from the equation as follows:

$$\eta = \frac{\text{C}\_{\text{NO}}^{\text{in}} - \text{C}\_{\text{NO}}^{\text{out}}}{\text{C}\_{\text{NO}}^{\text{in}}} \times 100\%$$

where *η*, *Cin NO*, *<sup>C</sup>out NO* represented the NO conversion, inlet and outlet NO concentration.

**Figure 8.** Schematics for the fixed-bed tubular flow reactor.

#### *3.3. Catalyst Characterization*

A micromeritics ASAP 2010M micropore size analyzer was used to measure the N2 adsorption isotherms of the catalyst sample at liquid N2 temperature (−196 ◦C. Specific surface area, pore volume and pore diameter can be determined by N2 adsorption using the BET and BJH methods.

The XRD measurement for the catalyst was carried out on a Rigaku D/Smartlab(III) system (Rigaku, Neu-Isenburg, Germany) with Cu Ka radiation. The X-ray source was operated at 40kV and 40mA. The diffraction patterns were taken in the 2θ range of 5–50◦ at a scan speed of 10◦ min−<sup>1</sup> and a resolution of 0.02◦.

SEM was performed using a SIRION-50 scanning electron microscope from Field Electron and Ion Company, the Netherlands, with a resolution of 150 eV.

Thermal gravimetric analysis of the catalyst samples was performed using TGA-101 type produced by the Nanjing Exhibition Electrical and Mechanical Technology Company (Nanjing, China). The accuracy of the instrument is 0.2 μg. For the TG experiments, the spent catalyst samples were measured under the temperature from room temperature to 900 ◦C at the heating rate of 15 ◦C min<sup>−</sup>1.

#### **4. Conclusions**

Cu-Mn/SAPO-34 with the loading of 2 wt.% Cu and 6 wt.% Mn exhibited remarkable low-temperature NO reduction activity among the prepared Cu-Mn/SAPO-34 catalysts. The NO conversion could achieve as high as 72% under the reaction temperature of 120 ◦C, while the value reached more than 90% under the temperature between 180 ◦C and 330 ◦C. The reversibly poisoning effect of H2O is mainly due to the competitive adsorption between H2O and NH3 on Lewis acid sites by occupying the metal sits. With the increase of reaction temperature, the poisoning effect is less important. The poisoning effect of SO2 on deNOx activity is dependent on the reaction temperature. At low temperature, the poisoning effect of SO2 is permanent with no recovery of deNOx activity after the elimination of SO2. XRD, SEM and BET analysis suggested that the deposition of (NH4)2SO4 on active sites may be the main reason for the loss of deNO*<sup>x</sup>* activity. TG-DTG analysis shows that

some metal sulfates are formed on the surface of Cu/SAPO-34 catalyst, which may inhibit the redox properties and cut off the redox cycle during the low-temperature SCR reaction. The addition of H2O into the SO2-containing atmosphere promotes the formation of (NH4)2SO4 on the surface of the catalyst, causing the damage of surface area and the rapid decrease of deNO*<sup>x</sup>* activity. At a higher reaction temperature, the formation of ammonium sulfate might be inhibited by the high reaction temperature, or the formed ammonium sulfate from SO2 and NH3 can be easily decomposed under the high temperature.

**Author Contributions:** Funding acquisition, D.S.; Investigation, S.G., B.Y., R.L. and Y.S.; Methodology, G.L., W.Z., P.H., S.G., B.Y., R.L., Y.S. and D.S.; Supervision, D.S.; Writing–original draft, G.L., W.Z. and P.H.; Writing–review & editing, G.L., W.Z. and P.H.

**Funding:** This work was supported by National Natural Science Foundation of China [grant number 51676047], the international collaboration project from Department of Science and Technology of Jiangsu Province [grant number BZ2017014], Key Technology R and D Program of Jiangsu Province [grant number BE2015677], and the Science and Technology Project of Jiangsu Power Design Institute Co., Ltd. [grant number 32-JK-2019-017].

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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