Search Results (31)

It has been found out that Pd-Co-based catalyst, supported on anodized aluminum, possesses very high activity in combustion reactions of C₁–C₆ alkanes and toluene. The catalyst characterization has been made by N₂-pysisorption, XRD, SEM, XPS, FTIR, TEM, and EPR methods. In view of the great interest, methane combustion was investigated in detail. It is ascertained that the complete oxidation of methane proceeds by dissociative adsorption on PdO and formation of hydroxyl and methyl groups, the former being highly reactive, and it undergoes further reaction to oxygen-containing intermediates, whereupon HCHO is one of them. The presence of Co²⁺ cations promotes greatly oxygen adsorption. The dissociative adsorption is favored on neighboring Co²⁺ cations, leading to the formation of bridging peroxides. Further, the oxygen dissociates on the nearest Pd²⁺ cations. According to the results from the experimental data, instrumental methods, and the observed kinetics and DFT model calculations, it can be concluded that the reaction pathway over Pd+Co/anodic alumina support (AAS) catalyst proceeds most probably through Mars–van Krevelen. The obtained data on the kinetics were used for simulation of the methane combustion in a full-scale adiabatic reactor. Full article

The conversion of molecular dinitrogen into ammonia under mild conditions is a significant pursuit in chemistry due to its potential for sustainable and clean ammonia production. The electrochemical reduction of N₂ offers a promising route for achieving this goal with reduced energy consumption, utilizing renewable energy sources. However, the exploration of effective electrocatalysts for this process, particularly at room temperature and atmospheric pressure, remains under exploration. This study addresses this gap by conducting a comprehensive investigation of potential catalysts for nitrogen electro-reduction to ammonia under ambient conditions. Using density functional theory calculations, we explore the (110) facets of rock salt structures across 11 transition metal carbides. Catalytic activity is evaluated through the construction of free energy diagrams for associative, dissociative, and Mars–van Krevelen reaction mechanisms. Additionally, we assess material stability against electrochemical poisoning and decomposition of parent metals during operation. Our findings suggest that a few of the candidates are promising for nitrogen reduction reactions, such as TaC and WC, with moderate onset potentials (−0.66 V and −0.82 V vs. RHE) under ambient conditions. Full article

VOCs can be used instead of ammonia as a reducing agent to remove NO, achieving the effect of removing VOCs and NO simultaneously. Due to the high energy consumption and low photocatalytic efficiency required for conventional thermocatalytic purification, photothermal coupled catalytic purification can integrate the advantages of photocatalysis and thermocatalysis in order to achieve the effect of pollutants being treated efficiently with a low energy consumption. In this study, samples loaded with Co and Mn catalysts were prepared using the hydrothermal method on Fe-MOF with various morphologies. The catalytic performance of each catalyst was analyzed by studying the effects of their physicochemical properties through various characterizations, including XRD, SEM, BET, XPS, H₂-TPR, TEM and O₂-TPD. The characterization results demonstrated that the specific surface area, pore volume, high valence Co and Mn atoms, surface adsorbed oxygen and the abundance of oxygen lattice defects in the catalysts were the most critical factors affecting the performance of the catalysts. Based on the results of the performance tests, the catalysts prepared with an octahedral-shaped Fe-MOF loaded with Co and Mn showed a better performance than those loaded with Co and Mn on a rod-shaped Fe-MOF. The conversions of acetone and NO reached 50% and 64%, respectively, at 240 °C. The results showed that the catalysts were capable of removing acetone and NO at the same time. Compared with the pure Fe-MOF without Co and Mn, the loaded catalysts showed a significantly higher ability to remove acetone and NO simultaneously under the combination of various factors. The key reaction steps for the catalytic conversion of acetone and NO on the catalyst surface were investigated according to the Mars–van Krevelen (MvK) mechanism, and a possible mechanism was proposed. This study presents a new idea for the simultaneous removal of acetone and NO_x by photothermal coupling. Full article

CO₂ oxidative dehydrogenation of propane (CO₂-ODHP), being not only favorable for olefin production but also beneficial for CO₂ emission control, has recently attracted great attention. Here, a series of single metal (Cr) and bimetal (Zr, La, Fe) modified ZSM-5 zeolites were prepared via an impregnation method. It was found that the bimetal modified ZSM-5 possessed much higher C₃H₈ and CO₂ conversion than that of monometallic modified Cr_3%-ZSM-5 (Cr_3%-Z5), especially for Cr_3%Zr_2%-ZSM-5 (Cr_3%Zr_2%-Z5), which displayed the highest activity (65.4%) and olefin yield (1.65 × 10³ μmol·$g_{cat}^{-1}$ h⁻¹). Various characterizations were performed, including XRD, N₂ adsorption-desorption, H₂-TPR, Raman, XPS, HAAD-STEM, and TEM. It was revealed that Zr not only favored an improvement in the redox ability of Cr, but also contributed to the surface dispersion of loaded Cr species, constituting two major reasons explaining the superior activity of Cr_3%Zr_2%-Z5. To further improve CO₂-ODHP catalytic behavior, a series of Cr_3%-ZSM-5@SBA-15-n composite zeolite catalysts with diverse (ZSM-5/SBA-15) mass ratios were prepared (Cr_3%-ZS-n, n = 0.5, 2, 6, 16), which screened out an optimum mass ratio of six. Based on this, the Cr_3%Zr_2%-ZS-6 compound was further prepared, and it eventually achieved even higher CO₂-ODHP activity (76.9%) and olefin yield (1.72 × 10³ μmol·$g_{cat}^{-1}$ h⁻¹). Finally, the CO₂-ODHP reaction mechanism was further investigated using in situ FTIR, and it was found that the reaction followed the Mars–van Krevelen mechanism, wherein CO₂ participated in the reaction through generation of polydentate carbonates. The Cr⁶⁺ constituted as the active site, which was reduced to Cr³⁺ after the dihydrogen reaction, and was then further oxidized into Cr⁶⁺ by CO₂, forming polydentate carbonates, and thus cycling the reactive species Cr⁶⁺. Additionally, assisted by a Brönsted acid site (favoring breaking of the C-C bond), C₂H₄ and CH₄ were produced. Full article

In this study we perform a comparison of the reaction mechanism and the activation barrier for the rate-determining step in various metal nitrides (Ta₃N₅, Mn₆N₅, Fe₃Mo₃N, Co₃Mo₃N) for the ammonia synthesis reaction. The reactions are explained with simplified schematics and the energy profiles for the various reaction mechanisms are given in order to screen the catalytic activity of the catalysts for the ammonia synthesis reaction. We find that the catalytic activity ranks in the following order: Co₃Mo₃N > Fe₃Mo₃N > Ta₃N₅ > Mn₆N_5. We also find that the reaction mechanism proceeds either by a Langmuir–Hinshelwood and an Eley–Rideal/Mars–van Krevelen mechanism. This is an overview of about 10 years of computational research conducted to provide an overview of the progress established in this field of study. Full article

There is an urgent need to develop novel and high-performance catalysts for chlorinated volatile organic compound oxidation as a co-benefit of NO_x. In this work, HSiW/CeO₂ was used for chlorobenzene (CB) oxidation as a co-benefit of NO_x reduction and the inhibition mechanism of NH₃ was explored. CB oxidation over HSiW/CeO₂ primarily followed the Mars–van–Krevelen mechanism and the Eley-Rideal mechanism, and the CB oxidation rate was influenced by the concentrations of surface adsorbed CB, Ce⁴⁺ ions, lattice oxygen species, gaseous CB, and surface adsorbed oxygen species. NH₃ not only strongly inhibited CB adsorption onto HSiW/CeO₂, but also noticeably decreased the amount of lattice oxygen species; hence, NH₃ had a detrimental effect on the Mars–van–Krevelen mechanism. Meanwhile, NH₃ caused a decrease in the amount of oxygen species adsorbed on HSiW/CeO₂, which hindered the Eley-Rideal mechanism of CB oxidation. Hence, NH₃ significantly hindered CB oxidation over HSiW/CeO₂. This suggests that the removal of NO_x and CB over this catalyst operated more like a two-stage process rather than a synergistic one. Therefore, to achieve simultaneous NO_x and CB removal, it would be more meaningful to focus on improving the performances of HSiW/CeO₂ for NO_x reduction and CB oxidation separately. Full article

We used ab initio molecular dynamics simulations to compare the activation of 2-propanol on the low index Co₃O₄ (111), (110) and (001) surfaces in dry conditions. The thermal and surface assisted decomposition of a film of 2-propanol to 2-propoxide on the B-termination of each surface was monitored and analyzed. The investigations suggest an activity order of Co₃O₄ (111) > (110) > (001). On all surfaces, the Co³⁺ serve as adsorption sites. On the B-terminated (111) surface, full dissociation of all 2-propanol molecules at the interface is observed, accompanied by a Mars-van Krevelen-type mechanism upon pre-hydroxylation of the surface. The active regions show Co³⁺–O_2-propoxide–Co²⁺ bridges where the coordinatively unsaturated Co²⁺ ions also participate in the adsorption and decomposition of 2-propanol. On the (110) surface, 2-propanol dissociation is driven by temperature, which activates the two-fold coordinatively unsaturated surface oxygens. The (001) surface on which almost no dissociation occurs is the least active. No formation of acetone is observed in the simulations conditions on all surfaces. Full article

Aromatic volatile organic compounds (VOCs) are toxic to public health and contribute to global air pollution; thus, it is urgent to control VOC emissions. Catalytic oxidation technology has been widely investigated to eliminate aromatic VOCs; this technology exhibits high catalytic efficiency even at low temperatures. However, the reaction mechanism of aromatic VOCs’ total oxidation over metal-oxide-based catalysts, which is of great significance in the design of catalysts, is not yet clear. In this study, we systemically calculated the catalytic oxidation mechanism of toluene over the Ce_0.875Zr_0.125O₂ catalyst using density functional theory (DFT). The results show that toluene first loses hydrogen from the methyl group via oxy-dehydrogenation and is gradually oxidized by lattice or adsorbed oxygen to benzyl alcohol, benzaldehyde, and benzoic acid following the Mars-van Krevelen (MVK) mechanism. Afterwards, there is a decarboxylation step to produce phenyl, which is further oxidized to benzoquinone. The rate-determining step then proceeds via the ring-opening reaction, leading to the formation of small molecule intermediates, which are finally oxidized to CO₂ and H₂O. This work may provide atomic-scale insight into the role of lattice and adsorbed oxygen in catalytic oxidation reactions. Full article

New results on the effect of TiO₂ on Pd/La₂O₃-CeO₂-Al₂O₃ systems for catalytic oxidation of methane in the presence of H₂O and SO₂ have been received. Low-temperature N₂-adsorption, XRD, SEM, HRTEM, XPS, EPR and FTIR techniques were used to characterize the catalyst. The presence of Ce³⁺ on the catalytic surface and in the volume near the lantana was revealed by EPR and XPS. After aging, the following changes are observed: (i) agglomeration of the Pd-clusters (from 8 nm to 12 nm); (ii) transformation of part of the TiO₂ from anatase to larger particles of rutile; and (iii)—the increase in PdO/Pd—ratio above its optimum. The modification by Ti of the La₂O₃-CeO₂-Al₂O₃ system leads to higher resistance towards the presence of SO₂ most likely due to the prevailing formation of unstable surface sulfites instead of thermally stable sulfates. Based on kinetic model calculations, the reaction pathway over the Pd/La₂O₃-CeO₂-TiO₂-Al₂O₃ catalyst follows the Mars–van Krevelen mechanism. For evaluation of the possible practical application of the obtained material, a sample of Pd/La₂O₃-CeO₂-TiO₂-Al₂O₃, supported on rolled aluminum-containing stainless steel (Aluchrom VDM^®), was prepared and tested. Methane oxidation in an industrial-scale monolithic reactor was simulated using a two-dimensional heterogeneous reactor model. Full article

The chemical looping oxidative dehydrogenation of propane to propylene (CL-ODHP) replaces molecular oxygen with lattice oxygen (O_latt) in oxygen carriers. This method boosts propylene selectivity by avoiding the deep oxidation of propane. Herein, a series of 10V-XCe/Al oxygen carriers with different Ce contents were prepared to realize different VO_x-CeO_y interactions. The effect of the Ce content in 10V-XCe/Al oxygen carriers on the CL-ODHP reaction was studied and the optimal Ce content was determined. CeO₂ prevents the outward diffusion and evolution of O_latt in VO_x carriers to the adsorbed electrophilic oxygen species (O_elec), effectively inhibiting the loss of O_latt, improving the selectivity of propylene, and extending the lifetime and activity of the oxygen carriers. After characterizing and analyzing the oxygen carriers, it was found that 10V-3Ce/Al has the highest specific surface area, highest oxygen capacity, and lowest reducibility. The 10V-3Ce/Al also delivers the highest oxidative dehydrogenation performance. At 550 °C, the average propylene and CO_x selectivity values of 10V-3Ce/Al were 81.87% and 7.28%, respectively (vs. 62.79% and 25.64% respectively, for 10V/Al). It is demonstrated that 10V-3Ce/Al exhibits good cycle stability with no significant decrease in catalytic performance after 15 cycles. In situ diffuse-reflectance infrared Fourier-transform spectroscopy indicates that CL-ODHP on 10V-3Ce/Al undergoes the Mars-van Krevelen mechanism. The migration and evolution of O_latt in oxygen carriers is controlled by reasonably modifying the metal oxide interactions to improve propylene yield. This work will thus guide the subsequent development of novel and efficient CL-ODHP oxygen carriers. Full article

Oxidative dehydrogenation (ODH) is an alternative for styrene (ST) production compared to the direct dehydrogenation process. However, ODH with O₂ or CO₂ suffers from either over-oxidation or endothermic property/low ethylbenzene conversion. Herein, we proposed an ODH process with a CO₂-O₂ mixture atmosphere for the efficient conversion of ethylbenzene (EB) into styrene. A thermoneutral ODH is possible by the rationalizing of CO₂/O₂ molar ratios from 0.65 to 0.66 in the temperature range of 300 to 650 °C. K modification is favorable for ethylbenzene dehydrogenation, and 10%K/CeO₂ achieved the highest ethylbenzene dehydrogenation activity due to the enhanced oxygen mobility and CO₂ adsorbability. The catalyst achieved 90.8% ethylbenzene conversion and 97.5% styrene selectivity under optimized conditions of CO₂-4O₂ oxidation atmosphere, a temperature of 500 °C, and a space velocity of 5.0 h⁻¹. It exhibited excellent catalytic and structural stability during a 50 h long-term test. CO₂ induces oxygen vacancies in ceria and promotes oxygen exchange between gaseous oxygen and ceria. The ethylbenzene dehydrogenation in CO₂-O₂ follows a Mars-van Krevelen (MvK) reaction mechanism via Ce³⁺/Ce⁴⁺ redox pairs. The proposed ODH strategy by using oxygen vacancies enriched catalysts offers an important insight into the efficient dehydrogenation of ethylbenzene at mild conditions. Full article

Electrochemical nitrogen reduction (NRR) has attracted much attention as a promising technique to produce ammonia at ambient conditions in an environmentally benign and less energy-consuming manner compared to the current Haber–Bosch process. However, even though much research on the NRR catalysts has been conducted, their low selectivity and reaction rate still hinder the practical application of the NRR process. Among various catalysts, transition metal nitride (TMN)-based catalysts are expected to be promising catalysts for NRR. This is because the NRR process can proceed via the unique Mars–Van Krevelen (MvK) mechanism with a compressed competing hydrogen evolution reaction. However, a controversial issue exists regarding the origin of ammonia produced on TMN-based catalysts. The instability of the TMN-based catalysts can lead to ammonia generation from lattice nitrogen instead of supplied N₂ gas. Thus, this review summarizes the recent progress of TMN-based catalysts for NRR, encompassing the NRR mechanism, synthetic routes, characterizations, and controversial opinions. Furthermore, future perspectives on producing ammonia electrochemically using TMN-based catalysts are provided. Full article

This work presents a systematic study of the kinetic aspects of CO oxidation reaction catalyzed by platinum nanoparticles (NPs) supported on the surface of multiwalled carbon nanotubes (MWCNTs). The investigation presented is closely related to the actual practical task of air purification in enclosed spaces. Therefore, the catalytic reaction was carried out in the presence of an excess of oxygen (5 vol.%) and over a wide range of CO concentrations from 50 ppm to 1600 ppm. For the catalyst characterization, transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) were applied. Kinetic modelling based on the Langmuir–Hinshelwood and Mars-van Krevelen mechanisms was taken as a basis, using the results obtained on Pt foil. Simulation of CO oxidation reaction on platinum NPs at temperatures above 90 °C was carried out using a kinetic model describing the reaction mechanism on bulk platinum. The description of the kinetics of CO oxidation reaction on Pt NPs over the entire temperature range, including the low temperatures down to −40 °C, required the introduction of the steps characterizing an additional concerted mechanism related to CO-assisted O₂ dissociation. Using the presented model, some predictions of the kinetic behaviour of the system were made. Full article

One of the current problems in the environmental catalysis is the design of an effective and less costly catalytic system for the oxidation of CO. The nano-sized α-Mn₂O₃ oxide has been prepared and modified with 0.5 wt.% Pt. The catalysts have been characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), temperature-programmed reduction (TPR) and diffuse-reflectance infrared spectroscopy (DRIFTS). Finely divided PtO and Pt(OH)₂ are being formed on the Mn₂O₃ surface as a result of the strong interaction between platinum and the nano-oxide. Based on DRIFTS investigations and the model calculations, a Langmuir–Hinshelwood type of mechanism is supposed for CO oxidation on Pt/Mn₂O₃. The CO and oxygen are adsorbed on different types of sites. The Mars–van Krevelen mechanism is the most probable one over pure Mn₂O₃, thus suggesting that CO₂ is adsorbed on the oxidized sites. The CO adsorption in the mixture CO + N₂ or in the presence of oxygen (CO + N₂ + O₂) leads to a partial reduction in the Pt⁺ surface species and the formation of linear Pt¹⁺−CO and Pt⁰−CO carbonyls. Both of them take part in the CO oxidation reaction. Full article

In this work, a series of CuCo₂O₄-x (x = N, A and C) catalysts were synthesized using different metal salt precursors by urea hydrothermal method for catalytic soot combustion. The effect of CuCo₂O₄-x catalysts on soot conversion and CO₂ selectivity in both loose and tight contact mode was investigated. The CuCo₂O₄-N catalyst exhibited outstanding catalytic activity with the characteristic temperatures (T₁₀, T₅₀ and T₉₀) of 451 °C, 520 °C and 558 °C, respectively, while the CO₂ selectivity reached 98.8% during the reaction. With the addition of NO, the soot combustion was further accelerated over all catalysts. Compared with the loose contact mode, the soot conversion was improved in the tight contact mode. The CuCo₂O₄-N catalysts showed better textural properties compared to the CuCo₂O₄-A and CuCo₂O₄-C, such as higher specific surface areas and pore volumes. The XRD results confirmed that the formation of a CuCo₂O₄ crystal phase in all catalysts. However, the CuO crystal phase only presented in CuCo₂O₄-N and CuCo₂O₄-A. The relative contents of Cu²⁺, Co³⁺ and O_ads on the surface of CuCo₂O₄-x (x = N, A and C) catalysts were analyzed by XPS. The CuCo₂O₄-N catalyst displayed the highest relative content of Cu²⁺, Co³⁺ and O_ads. The activity of catalytic soot combustion showed a good correlation with the order of the relative contents of Cu²⁺, Co³⁺ and O_ads. Additionally, the CuCo₂O₄-N catalyst exhibited lower reduction temperature compared to the CuCo₂O₄-A and CuCo₂O₄-C. The cycle tests clarified that the copper–cobalt spinel catalyst obtained good stability. In addition, based on the Mars–van Krevelen mechanism, the process of catalytic soot combustion was described combined with the electron transfer process and the role of oxygen species over CuCo₂O₄ spinel catalysts. Full article