*Review* **Recent Advances of Cu-Based Catalysts for NO Reduction by CO under O2-Containing Conditions**

**Xiaoli Chen <sup>1</sup> , Yaqi Liu <sup>1</sup> , Yan Liu <sup>1</sup> , Dianxing Lian <sup>1</sup> , Mohaoyang Chen <sup>1</sup> , Yongjun Ji 1,\*, Liwen Xing 2,\* , Ke Wu 2,\* and Shaomian Liu 3,\***


**Abstract:** Selective catalytic reduction of NO*<sup>x</sup>* by CO (CO-SCR) to both N<sup>2</sup> and CO<sup>2</sup> is a promising way to simultaneously remove two harmful gases, CO and NO*x*, in automobile and factory exhaust gases. The development of efficient catalysts is the key challenge for the technology to be commercialized. The low-cost Cu-based catalysts have shown promising performance in CO-SCR, but there are some technical problems that obstruct their practical implementation, such as high reduction temperature and low O<sup>2</sup> , H2O, and SO<sup>2</sup> resistance. This paper provides a comprehensive overview and insights into CO-SCR under O<sup>2</sup> -containing conditions over the Cu-based catalysts, including catalytic performances of non-supported, supported mono-metallic, supported bimetallic, and supported multi-metallic Cu-based catalysts. In addition, the effects of O<sup>2</sup> concentration, reaction temperature, H2O, and SO<sup>2</sup> on the catalytic performance are discussed. Furthermore, the reaction mechanism of CO-SCR on Cu-based catalysts is briefly summarized. Lastly, challenges and perspectives with respect to this reaction are discussed. We hope this work can provide theoretical guidance for the rational design of efficient Cu-based catalysts in the CO-SCR reaction for commercial applications.

**Keywords:** Cu-based catalysts; CO + NO; catalytic performances; O<sup>2</sup> resistance; H2O and SO<sup>2</sup> resistance

#### **1. Introduction**

Nitrogen oxides (NOx), the major component of air pollutants, can cause a series of environmental issues, such as acid rain [1], photochemical smog [2], and the destruction of the ozone layer [3]. NO<sup>x</sup> are mainly from the exhaust emissions of automobiles [4] and factories [5]. With the enhanced environmental protection consciousness and tightening of the environmental regulations on emission control, NO<sup>x</sup> removal has become imperative. Among various denitrification technologies, selective catalytic reduction of NO<sup>x</sup> with NH<sup>3</sup> (the so-called NH3-SCR) is the most used. However, this technology still has many shortcomings [6], such as ammonia storage [7], leakage, and high cost [8]. In contrast, CO, which also widely exists in flue gas and automobile exhaust, has low corrosivity and low requirements for equipment [9]. Therefore, the use of CO as a reducing agent to selectively reduce NO<sup>x</sup> (CO-SCR) is more promising since it can simultaneously control the emissions of two harmful gases [10].

Noble metals (Pt, Pd, Rh, Ir, Ru, and Au), which have rich surface adsorption sites, high catalytic activity, and O<sup>2</sup> resistance, have been widely explored as CO-SCR catalytic active components. Among them, Pt [11], Pd [12], and Ir [13] are the most used. Recently, our group reported an efficient Ir1/m-WO<sup>3</sup> catalyst with single Ir atoms anchored on the mesoporous WO<sup>3</sup> [14]. This Ir1/m-WO<sup>3</sup> catalyst containing only 1 wt.% Ir loading exhibited exceptional catalytic performance in the presence of 2 vol.% O2, achieving 73% NO conversion and 100% N<sup>2</sup> selectivity at 350 ◦C. Its superior activity can be attributed

**Citation:** Chen, X.; Liu, Y.; Liu, Y.; Lian, D.; Chen, M.; Ji, Y.; Xing, L.; Wu, K.; Liu, S. Recent Advances of Cu-Based Catalysts for NO Reduction by CO under O2-Containing Conditions. *Catalysts* **2022**, *12*, 1402. https://doi.org/ 10.3390/catal12111402

Academic Editor: Carlo Santoro

Received: 12 October 2022 Accepted: 7 November 2022 Published: 9 November 2022

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**Copyright:** © 2022 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 (https:// creativecommons.org/licenses/by/ 4.0/).

to the following reasons: (i) isolated Ir single atoms and the maximized Ir-WO<sup>3</sup> interfaces promote the adsorption and activation of NO, and (ii) the accessible mesopores in m-WO<sup>3</sup> enhance the transport of both NO and CO. Despite the success achieved for noble metals, their high cost and poor low-temperature performance make them uncompetitive economically and technically in practical applications [15]. Therefore, low-cost transition metal-based catalysts have attracted widespread attention [16]. Among them, Cu-based [17,18], Fe-based [19], and Co-based [20] catalysts are widely studied. mote the adsorption and activation of NO, and (ii) the accessible mesopores in m-WO<sup>3</sup> enhance the transport of both NO and CO. Despite the success achieved for noble metals, their high cost and poor low-temperature performance make them uncompetitive economically and technically in practical applications [15]. Therefore, low-cost transition metal-based catalysts have attracted widespread attention [16]. Among them, Cu-based [17,18], Fe-based [19], and Co-based [20] catalysts are widely studied. Compared with other transition metal catalysts, Cu-based catalysts exhibit relatively

active components. Among them, Pt [11], Pd [12], and Ir [13] are the most used. Recently, our group reported an efficient Ir1/m-WO<sup>3</sup> catalyst with single Ir atoms anchored on the mesoporous WO<sup>3</sup> [14]. This Ir1/m-WO<sup>3</sup> catalyst containing only 1 wt.% Ir loading exhibited exceptional catalytic performance in the presence of 2 vol.% O2, achieving 73% NO conversion and 100% N<sup>2</sup> selectivity at 350 °C. Its superior activity can be attributed to the following reasons: (i) isolated Ir single atoms and the maximized Ir-WO<sup>3</sup> interfaces pro-

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Compared with other transition metal catalysts, Cu-based catalysts exhibit relatively better catalytic performance in the presence of O2, showing high application potential. Many supported Cu-based catalysts such as Cu/CeO2-Fe2O<sup>3</sup> [21], CuO/CeO2-Fe2O<sup>3</sup> [22], Cu/Fe2O3-CeO2/ZrO<sup>2</sup> [23], and CuOx/CeO<sup>2</sup> [24] exhibit excellent catalytic performance, with NO conversion usually surpassing 90% at 200 ◦C. Moreover, non-supported catalysts such as CuCeO<sup>x</sup> [25] and Cu1Ce0.5Fe1.5O4.25 [18] also have high denitrification efficiency at 100–200 ◦C. However, the reaction conditions of the above catalysts are O2-free. As we know, real exhaust gases contain a large amount of O<sup>2</sup> [10], which preferentially reacts with CO or NO to form CO<sup>2</sup> and NO2, resulting in poor catalytic activity. Additionally, SO<sup>2</sup> and H2O poisoning of Cu-based catalysts is another obstacle for CO-SCR technology [21,26]. Therefore, developing efficient Cu-based catalysts for CO-SCR under O2-containing conditions, especially with low reduction temperature and strong anti-SO<sup>2</sup> and H2O poisoning ability, is the key to promoting its industrial application on a large scale. better catalytic performance in the presence of O2, showing high application potential. Many supported Cu-based catalysts such as Cu/CeO2-Fe2O<sup>3</sup> [21], CuO/CeO2-Fe2O<sup>3</sup> [22], Cu/Fe2O3-CeO2/ZrO<sup>2</sup> [23], and CuOx/CeO<sup>2</sup> [24] exhibit excellent catalytic performance, with NO conversion usually surpassing 90% at 200 °C. Moreover, non-supported catalysts such as CuCeO<sup>x</sup> [25] and Cu1Ce0.5Fe1.5O4.25 [18] also have high denitrification efficiency at 100–200 °C. However, the reaction conditions of the above catalysts are O2-free. As we know, real exhaust gases contain a large amount of O<sup>2</sup> [10], which preferentially reacts with CO or NO to form CO<sup>2</sup> and NO2, resulting in poor catalytic activity. Additionally, SO<sup>2</sup> and H2O poisoning of Cu-based catalysts is another obstacle for CO-SCR technology [21,26]. Therefore, developing efficient Cu-based catalysts for CO-SCR under O2-containing conditions, especially with low reduction temperature and strong anti-SO<sup>2</sup> and H2O poisoning ability, is the key to promoting its industrial application on a large scale. In previous reviews, Du et al. [27] summarized research advances of CuO-CeO<sup>2</sup> cat-

In previous reviews, Du et al. [27] summarized research advances of CuO-CeO<sup>2</sup> catalysts for catalytic elimination of CO and NO, focusing on the analysis of the reaction mechanism and key factors influencing the catalytic activity. Wang et al. [28] systematically summarized the reaction mechanism and anti-inactivation measures of CO-SCR over Cubased catalysts and proposed some optimization strategies for designing catalysts with high catalytic activity. However, in these reviews, much attention is paid to the research progress of the Cu-based catalysts under O2-free conditions, and rarely to the case in the presence of oxygen, which represents the real industrial operating conditions. This review focuses on various types of Cu-based catalysts for NO reduction by CO under O2-containing conditions and systematically analyzes the research status, including their catalytic performances, influencing factors on the catalytic activity, and catalytic mechanism, as shown in Figure 1. Finally, challenges and perspectives with respect to this reaction are discussed. We believe this work will attract wide interest from both academia and industry, and promote the development of CO-SCR process catalysts and thus push this technology to be commercialized. alysts for catalytic elimination of CO and NO, focusing on the analysis of the reaction mechanism and key factors influencing the catalytic activity. Wang et al. [28] systematically summarized the reaction mechanism and anti-inactivation measures of CO-SCR over Cu-based catalysts and proposed some optimization strategies for designing catalysts with high catalytic activity. However, in these reviews, much attention is paid to the research progress of the Cu-based catalysts under O2-free conditions, and rarely to the case in the presence of oxygen, which represents the real industrial operating conditions. This review focuses on various types of Cu-based catalysts for NO reduction by CO under O2-containing conditions and systematically analyzes the research status, including their catalytic performances, influencing factors on the catalytic activity, and catalytic mechanism, as shown in Figure 1. Finally, challenges and perspectives with respect to this reaction are discussed. We believe this work will attract wide interest from both academia and industry, and promote the development of CO-SCR process catalysts and thus push this technology to be commercialized.

**Figure 1.** Schematic illustration of the review of Cu-based catalysts for NO reduction by CO under O2-containing conditions. **Figure 1.** Schematic illustration of the review of Cu-based catalysts for NO reduction by CO under O<sup>2</sup> -containing conditions.

## **2. Cu-Based Catalysts**

#### *2.1. Supported Catalysts*

Compared to non-supported catalysts, supported catalysts have obvious advantages, such as high dispersion of active components and low amount. Among multifarious catalysts reported for CO-SCR, supported Cu-based catalysts are the most extensively studied and have considerable application potential [29]. In this section, various types of supported Cu-based catalysts and their catalytic behavior in NO reduction by CO in the

presence of O<sup>2</sup> are summarized. Table 1 gives the catalytic performances of supported Cu-based catalysts for NO + CO reaction in the presence of O<sup>2</sup> reported in the literature.


**Table 1.** Catalytic activity of various supported Cu-based catalysts for the NO + CO + O<sup>2</sup> reaction.

<sup>a</sup> Tmax: Temperature corresponding to maximum NO conversion. <sup>b</sup> *ηNO*: The conversion of NO. <sup>c</sup> *SN2*: The selectivity of N2. <sup>d</sup> *ψ*(NO): The NO concentration in the feeding gas. <sup>e</sup> F: The volume flow rate of the feeding gas. <sup>f</sup> GHSV: Gas weight hourly space velocity.

#### 2.1.1. Mono-Metallic Catalysts

The loading state of the metal, the nature of the support, and the interaction between the metal and the support will all affect the catalytic performance. Yamamoto et al. [30] studied the effects of supported elements, supports, and calcination temperature on the catalytic performance of NO-CO-O<sup>2</sup> reaction. The Cu-based catalysts with different supports and loading amounts were investigated in detail, and the results suggest that 0.5 wt.% Cu/Al2O<sup>3</sup> exhibited the highest catalytic performance. The support γ-Al2O<sup>3</sup> itself can reduce NO to N2O but not N2, possessing a limited capability for the reduction of NO. Therefore, the addition of Cu to Al2O<sup>3</sup> promotes the formation of N2. Additionally, the calcination temperature of the catalyst and Cu loading could affect the catalytic activity. When the calcination temperature was too high (>500 ◦C), the Cu species on the surface could slowly form aggregated Cu species, which preferentially oxidize CO without reducing NOx, thereby resulting in a sharp decrease in catalytic performance. Moreover, when the Cu loading was 3 wt.%, the polymer-like Cu species were mainly present on the surface of the support. If the Cu loading was further increased to more than 5 wt.%, CuO species appeared on the catalyst surface. Both the polymer-like Cu species and CuO species mainly facilitate CO oxidation. With the 0.5 wt.% Cu loading, there was the generation of atomically dispersed Cu2+ species on γ-Al2O3. In this case, the oxidation activity of CO was weak, and a large amount of residual CO could interact with NO. Thus, the reduction activity of NO by CO is the highest. Nevertheless, Sierra-Pereira et al. [26] found that for CuO/TiO2, its activity increased with Cu loading from 2 wt.% to 10 wt.% in NO-CO-O<sup>2</sup> reaction, and 10 wt.% CuO/TiO<sup>2</sup> exhibited the highest catalytic performance, achieving 54% NO conversion at 500 ◦C.

In addition to the above single metal oxide support, metal oxide composite supports were also applied to optimize the denitration performance of Cu-based catalysts. AlPO4, which has two different types of surface hydroxyl groups, (AlOH) and (POH), is a kind of stable material with large specific surface area and acid properties [47]. Kacimi et al. [31] prepared a series of Cu/AlPO<sup>4</sup> catalysts with different Cu loadings by Cu(II) ion complexes

exchange, which leads to the formation of well-dispersed Cu(II) amino species. Among these catalysts, 5Cu/AlPO4, containing the largest amount of dispersed surface Cu(II) species, exhibited the best catalytic performance, achieving 90% NO conversion at 300 ◦C. Venegas et al. [32] reported that the Cu/SmCeO2@TiO<sup>2</sup> catalyst with Cu supported on core–shell-structured SmCeO2@TiO<sup>2</sup> achieved 50% NO conversion at 500 ◦C in the presence of 10 vol.% O2. Its superior catalytic performance was because CeO<sup>2</sup> possessed excellent redox properties through the transfer between Ce3+ and Ce4+, thus increasing the oxidation activity of the Cu/SmCeO2@TiO<sup>2</sup> catalyst. Moreover, the addition of Sm helped to maintain the thermal stability of the CeO<sup>2</sup> phase. Core–shell-structured CeO2@TiO<sup>2</sup> nanoparticles could also stabilize the involved Cu phase, preventing its migration and sinterization, and thus leading to higher activities [48]. Bai et al. [33] synthesized an efficient CuO/CeO2- Al2O<sup>3</sup> catalyst, which exhibited excellent catalytic performance and superior resistance to O<sup>2</sup> and SO<sup>2</sup> for CO-SCR. The incorporation of Ce4+ was conducive to the enrichment of Cu atoms and the generation of synergistic oxygen vacancies on the surface of the catalyst, which improved the redox performance of the catalyst. Moreover, Cu2+ was favorable for the CO adsorption, while the unpaired electrons in the CeO2-Al2O<sup>3</sup> support were favorable for the adsorption of NO.

#### 2.1.2. Bimetallic Catalysts

Chen et al. [43] synthesized a series of CuCoOx/TiO<sup>2</sup> catalysts and found that the CuCoOx/TiO<sup>2</sup> catalyst able to generate the CuCo2O<sup>4</sup> spinel exhibited the highest catalytic activity, reaching 98.9% NO conversion at 200 ◦C and in the absence of O2. However, when 2 vol.% O<sup>2</sup> was introduced, the NO conversion decreased sharply to 60%. Liu et al. [42] investigated the denitration performance of various transition metals supported on Al2O<sup>3</sup> pellets under O2-rich conditions (16 vol.%). Among these catalysts, Cu-Mn/Al2O<sup>3</sup> with a molar ratio Cu:Mn of 1.5 displayed the best catalytic activity, achieving nearly 78% NO conversion and 85% N<sup>2</sup> selectivity at 180 ◦C. Based on the density functional theory calculation, it was demonstrated that Mn had better O<sup>2</sup> resistance and Cu had better H2O resistance. López et al. [41] prepared a novel core–shell-structured K/Cu/SmCe@TiO<sup>2</sup> catalyst, giving 97% NO conversion at 330 ◦C in the presence of excess O<sup>2</sup> (10 vol.%). The interaction between highly dispersed Cu species and K promoted the reduction of NO. Gholami et al. [40] found that the catalytic activity of the Cu1:Ce3/CNT catalyst (carbon nanotubes) was much better than that of the Cu1:Ce3/AC catalyst (activated carbon) in the presence of O<sup>2</sup> (0.3 vol.%, O2/CO ≥ 0.6). The Cu1:Ce3/CNT catalyst displayed the highest NO conversion of 96% at 220 ◦C, attributed to its high concentration of surface oxygen vacancies (SOVs), high Cu<sup>+</sup> species content, superior reducing capability, and the synergistic effect between SOV and Cu<sup>+</sup> species. Furthermore, Gholami et al. [39] investigated the denitration performance of a string of Cu1:Ce3 catalysts supported on various supports (CNTs, AC, TiO2, γ-Al2O3, and SiC) in the presence of excess O<sup>2</sup> (5 vol.%), and found that Cu1:Ce3/Al2O<sup>3</sup> catalyst possessed the highest catalytic performance, with 71.8% NO conversion at 420 ◦C, mainly ascribed to the enrichment of catalytically active centers of Cu on the Al2O<sup>3</sup> support. Interestingly, it was observed that with the increase in O<sup>2</sup> concentration from 2% to 5%, the conversion of NO increased slightly. This was because the more O<sup>2</sup> was adsorbed on the catalyst surface, the more adsorbed O was provided. The adsorbed O then reacted with the adsorbed CO to form CO2, which thus led to the generation of oxygen vacancies for the adsorption and dissociation of NO further. Moreover, this adsorbed O could also react with NO to NO2, which was quickly reduced by CO to N2. Metal organic frameworks (MOFs) have broad application prospects in the field of catalysis, due to their huge surface area, tailored compositions, and variable structures [49,50]. Zhang et al. [37] prepared the Cu-BTC (BTC = benzene-1,3,5-tricarboxylate) and Ce-Cu-BTC catalysts, which are three-dimensional (3D) porous MOFs (their SEM images are shown in Figure 2a,b). Cu-BTC only exhibited 50% NO conversion at 250 ◦C, while Cemodified Cu-BTC catalysts could achieve much higher NO conversion of 91%. Owing to the incorporation of Ce3+, the Ce-Cu-BTC catalyst had more SOVs, conducive to enhancing

the adsorption of NO<sup>x</sup> on the surface of catalysts, as evidenced by the in situ DRIFTS spectrum (Figure 2c,d). The enhanced NO<sup>x</sup> adsorption ultimately improved the catalytic activity for CO-SCR. the adsorption of NOx on the surface of catalysts, as evidenced by the in situ DRIFTS spectrum (Figure 2c,d). The enhanced NO<sup>x</sup> adsorption ultimately improved the catalytic activity for CO-SCR.

concentration from 2% to 5%, the conversion of NO increased slightly. This was because the more O<sup>2</sup> was adsorbed on the catalyst surface, the more adsorbed O was provided. The adsorbed O then reacted with the adsorbed CO to form CO2, which thus led to the generation of oxygen vacancies for the adsorption and dissociation of NO further. Moreover, this adsorbed O could also react with NO to NO2, which was quickly reduced by CO to N2. Metal organic frameworks (MOFs) have broad application prospects in the field of catalysis, due to their huge surface area, tailored compositions, and variable structures [49,50]. Zhang et al. [37] prepared the Cu-BTC (BTC = benzene-1,3,5-tricarboxylate) and Ce-Cu-BTC catalysts, which are three-dimensional (3D) porous MOFs (their SEM images are shown in Figure 2a,b). Cu-BTC only exhibited 50% NO conversion at 250 °C, while Cemodified Cu-BTC catalysts could achieve much higher NO conversion of 91%. Owing to the incorporation of Ce3+, the Ce-Cu-BTC catalyst had more SOVs, conducive to enhancing

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**Figure 2.** SEM images of (**a**) Cu-BTC and (**b**) Ce-Cu-BTC. In situ DRIFTS spectra of CO + NO + O<sup>2</sup> co-adsorption on the surface of (**c**) Cu-BTC and (**d**) Ce-Cu-BTC at 150 °C, where NO = 1000 ppm, CO = 1000 ppm, 5 vol.% O2, and equilibrium gas was N2. Reproduced from [37] with permission. **Figure 2.** SEM images of (**a**) Cu-BTC and (**b**) Ce-Cu-BTC. In situ DRIFTS spectra of CO + NO + O<sup>2</sup> co-adsorption on the surface of (**c**) Cu-BTC and (**d**) Ce-Cu-BTC at 150 ◦C, where NO = 1000 ppm, CO = 1000 ppm, 5 vol.% O<sup>2</sup> , and equilibrium gas was N<sup>2</sup> . Reproduced from [37] with permission.

Recently, our group used a simple impregnation method followed by reduction with H<sup>2</sup> to synthesize a Pt-Cu@M-Y catalyst, which consists of sub-nanometric Pt on Cu nanoparticles confined in the NaOH-modified Y-zeolite. The Pt-Cu@M-Y catalyst with only 0.04 wt.% Pt loading showed superior catalytic activity for NO + CO reaction, with NO conversion and N<sup>2</sup> selectivity nearly 100% at 250 °C. This enhanced activity originated from the synergistic catalysis of Pt and Cu, in which NO was mainly adsorbed on subnanometric Pt, and the generated interfaces between Cu nanoparticles and surface CuO<sup>x</sup> species served as the dissociation sites of NO. However, when 1 vol.% O<sup>2</sup> was introduced (O2/CO = 10), the NO conversion decreased to 43% and N<sup>2</sup> selectivity dropped to 53% at Recently, our group used a simple impregnation method followed by reduction with H<sup>2</sup> to synthesize a Pt-Cu@M-Y catalyst, which consists of sub-nanometric Pt on Cu nanoparticles confined in the NaOH-modified Y-zeolite. The Pt-Cu@M-Y catalyst with only 0.04 wt.% Pt loading showed superior catalytic activity for NO + CO reaction, with NO conversion and N<sup>2</sup> selectivity nearly 100% at 250 ◦C. This enhanced activity originated from the synergistic catalysis of Pt and Cu, in which NO was mainly adsorbed on subnanometric Pt, and the generated interfaces between Cu nanoparticles and surface CuO<sup>x</sup> species served as the dissociation sites of NO. However, when 1 vol.% O<sup>2</sup> was introduced (O2/CO = 10), the NO conversion decreased to 43% and N<sup>2</sup> selectivity dropped to 53% at 350 ◦C, due to the preferential oxidation of CO and NO by O<sup>2</sup> at high temperatures.

#### 350 °C, due to the preferential oxidation of CO and NO by O<sup>2</sup> at high temperatures. 2.1.3. Multi-Metallic Catalysts

Pan et al. [46] synthesized a series of Cu-based and Mn-based catalysts by the wet impregnation method and applied them to the CO-SCR reaction. It was found that Cu-Ce-Fe-Co/TiO<sup>2</sup> and Mn-Ce-Fe-Co/TiO<sup>2</sup> exhibited better catalytic activity in the absence of O2, both reaching full NO conversion at 250 ◦C. However, the presence of O<sup>2</sup> largely restricts the NO reduction efficiency. Comparatively, Cu-Ce-Fe-Co/TiO<sup>2</sup> showed better tolerance to O<sup>2</sup> than Mn-Ce-Fe-Co/TiO2. When 6 vol % O<sup>2</sup> was fed, the Cu-Ce-Fe-Co/TiO<sup>2</sup> catalyst still exhibited 93% NO conversion and 74.3% NO<sup>x</sup> conversion at 200 ◦C ([NO] = 200 ppm, [CO] = 200 ppm), indicating that only a part of NO was oxidized. The enhanced catalytic

performance of Cu-Ce-Fe-Co/TiO<sup>2</sup> may owe to its superior reducibility, more oxygen vacancies, and better oxygen mobility. Wang et al. [45] synthesized a Cu-Ni-Ce/AC catalyst by the ultrasonic equal volume impregnation method. This catalyst exhibited extremely high catalytic activity in the presence of O<sup>2</sup> (5 vol.%), reaching 99.8% NO conversion at 150 ◦C. In this case, the doping of Ce promoted the uniform dispersion of Cu and Ni and formed many reaction units on the surface of the catalyst, enhancing the adsorption abilities of CO and NO and thus improving the catalytic performance. Two-dimensional (2D) vermiculite (VMT) is a natural layered clay mineral with a unique two-dimensional structure and high-temperature stability, widely used as a support and applied in the fields of photocatalysis and heterogeneous catalysis. Liu et al. [44] synthesized a CuCoCe/2D-VMT catalyst by the impregnation method. It exhibited superior catalytic activity in the coexistence of 1 vol.% O<sup>2</sup> and 5 vol.% H2O, reaching 70% NO conversion and 97% N<sup>2</sup> selectivity at 200 ◦C. They found that the doping of Ce could reduce the reduction temperature and promote the formation of oxygen vacancies, giving the CuCoCe/2D-VMT sample more active centers, thus improving its catalytic performance. This deduction was confirmed by the Raman spectrum (Figure 3a), in which the CuCoCe/2D-VMT sample had a higher concentration of oxygen vacancies than the CuCo/2D-VMT sample. Similar results were also obtained by XPS characterization. As shown in Figure 3b, CuCoCe/2D-VMT had more adsorbed oxygen (denoted as Oβ). catalytic performance of Cu-Ce-Fe-Co/TiO<sup>2</sup> may owe to its superior reducibility, more oxygen vacancies, and better oxygen mobility. Wang et al. [45] synthesized a Cu-Ni-Ce/AC catalyst by the ultrasonic equal volume impregnation method. This catalyst exhibited extremely high catalytic activity in the presence of O2 (5 vol.%), reaching 99.8% NO conversion at 150 °C. In this case, the doping of Ce promoted the uniform dispersion of Cu and Ni and formed many reaction units on the surface of the catalyst, enhancing the adsorption abilities of CO and NO and thus improving the catalytic performance. Two-dimensional (2D) vermiculite (VMT) is a natural layered clay mineral with a unique two-dimensional structure and high-temperature stability, widely used as a support and applied in the fields of photocatalysis and heterogeneous catalysis. Liu et al. [44] synthesized a CuCoCe/2D-VMT catalyst by the impregnation method. It exhibited superior catalytic activity in the coexistence of 1 vol.% O<sup>2</sup> and 5 vol.% H2O, reaching 70% NO conversion and 97% N<sup>2</sup> selectivity at 200 °C. They found that the doping of Ce could reduce the reduction temperature and promote the formation of oxygen vacancies, giving the CuCoCe/2D-VMT sample more active centers, thus improving its catalytic performance. This deduction was confirmed by the Raman spectrum (Figure 3a), in which the CuCoCe/2D-VMT sample had a higher concentration of oxygen vacancies than the CuCo/2D-VMT sample. Similar results were also obtained by XPS characterization. As shown in Figure 3b, CuCoCe/2D-VMT had more adsorbed oxygen (denoted as Oβ).

Pan et al. [46] synthesized a series of Cu-based and Mn-based catalysts by the wet impregnation method and applied them to the CO-SCR reaction. It was found that Cu-Ce-Fe-Co/TiO<sup>2</sup> and Mn-Ce-Fe-Co/TiO<sup>2</sup> exhibited better catalytic activity in the absence of O2, both reaching full NO conversion at 250 °C. However, the presence of O<sup>2</sup> largely restricts the NO reduction efficiency. Comparatively, Cu-Ce-Fe-Co/TiO<sup>2</sup> showed better tolerance to O2 than Mn-Ce-Fe-Co/TiO2. When 6 vol % O<sup>2</sup> was fed, the Cu-Ce-Fe-Co/TiO<sup>2</sup> catalyst still exhibited 93% NO conversion and 74.3% NO<sup>x</sup> conversion at 200 °C ([NO] = 200 ppm, [CO] = 200 ppm), indicating that only a part of NO was oxidized. The enhanced

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2.1.3. Multi-Metallic Catalysts

**Figure 3.** (**a**) Raman spectra of CuCoCe/2D-VMT, CuCo/2D-VMT, and 2D-VMT catalysts and (**b**) XPS spectra of O 1*s*. Reproduced from [44] with permission. **Figure 3.** (**a**) Raman spectra of CuCoCe/2D-VMT, CuCo/2D-VMT, and 2D-VMT catalysts and (**b**) XPS spectra of O 1*s*. Reproduced from [44] with permission.

#### *2.2. Non-Supported Catalysts 2.2. Non-Supported Catalysts*

Besides a large number of reported Cu-based supported catalysts, some non-supported Cu-based catalysts also have certain O<sup>2</sup> resistance in the CO-SCR reaction. Mehandjiev et al. [51] first reported that CuCo2O<sup>4</sup> had the ability to reduce NO by CO in the presence of O2. Furthermore, Panayotov et al. [52] found that CuxCo3−xO<sup>4</sup> spinels possessed excellent catalytic performance for CO-SCR under O2-containing conditions than CuO Besides a large number of reported Cu-based supported catalysts, some non-supported Cu-based catalysts also have certain O<sup>2</sup> resistance in the CO-SCR reaction. Mehandjiev et al. [51] first reported that CuCo2O<sup>4</sup> had the ability to reduce NO by CO in the presence of O2. Furthermore, Panayotov et al. [52] found that CuxCo3−xO<sup>4</sup> spinels possessed excellent catalytic performance for CO-SCR under O2-containing conditions than CuO and Co3O4. Additionally, in the presence of 650 ppm O2, the catalytic activity increased with the Cu content. Ivanka et al. [53] prepared CuO-MnO<sup>x</sup> (1.5 < x < 2) catalysts by coprecipitation and studied their catalytic performance in the presence of O2. They found that the degree of NO conversion to N<sup>2</sup> achieved by CuO-MnO<sup>x</sup> (Cu/Cu + Mn up to 0.53) under O2-containing conditions was similar to that under O2-free conditions. This could be explained as follows. After the introduction of O2, NO quickly reacted with it to produce NO2. Moreover, the reduction of NO<sup>2</sup> by CO was faster than CO oxidation. Therefore, N<sup>2</sup> and CO<sup>2</sup> were finally generated. Sun et al. [54] synthesized the CuCe mixed metal oxides, which showed superior NO conversion and N<sup>2</sup> selectivity, both maintaining more than 90% in a wide temperature window in the absence of O2. Nevertheless, when 1% O<sup>2</sup> was introduced, the

NO conversion dropped rapidly to 0 within 3.5 h. The NO conversion could gradually recover to the initial value after the O<sup>2</sup> was stopped. This result indicated that CO preferentially reacts with O2, resulting in the decrease in NO conversion in the presence of O2. Wen et al. [55] synthesized mixed CuCeMgAlO oxides by coprecipitation, which possessed higher NO conversion than CuMgAlO and CeMgAlO for NO + CO + O<sup>2</sup> reaction. The superior catalytic performance of CuCeMgAlO can be explained by the synergistic effect generated by the interaction of Cu and Ce. In addition, when 1% H2O was introduced, the NO conversion over CuCeMgAlO was significantly improved from 50% to 100% at 250 ◦C, but both CuMgAlO and CeMgAlO lost their catalytic activity completely. Moreover, when 500 ppm SO<sup>2</sup> was introduced, the NO conversion dropped rapidly over CuMgAlO and CeMgAlO; however, CuCeMgAlO still maintained 100% NO conversion at 720 ◦C. This suggests that CuCeMgAlO possesses high activity for NO + CO + O<sup>2</sup> reaction and excellent resistance to H2O and SO<sup>2</sup> poisoning. presence of O2. Wen et al. [55] synthesized mixed CuCeMgAlO oxides by coprecipitation, which possessed higher NO conversion than CuMgAlO and CeMgAlO for NO + CO + O<sup>2</sup> reaction. The superior catalytic performance of CuCeMgAlO can be explained by the synergistic effect generated by the interaction of Cu and Ce. In addition, when 1% H2O was introduced, the NO conversion over CuCeMgAlO was significantly improved from 50% to 100% at 250 °C, but both CuMgAlO and CeMgAlO lost their catalytic activity completely. Moreover, when 500 ppm SO<sup>2</sup> was introduced, the NO conversion dropped rapidly over CuMgAlO and CeMgAlO; however, CuCeMgAlO still maintained 100% NO conversion at 720 °C. This suggests that CuCeMgAlO possesses high activity for NO + CO + O<sup>2</sup> reaction and excellent resistance to H2O and SO<sup>2</sup> poisoning. **3. Influencing Factors on Catalytic Activity**

and Co3O4. Additionally, in the presence of 650 ppm O2, the catalytic activity increased with the Cu content. Ivanka et al. [53] prepared CuO-MnO<sup>x</sup> (1.5 < x < 2) catalysts by coprecipitation and studied their catalytic performance in the presence of O2. They found that the degree of NO conversion to N<sup>2</sup> achieved by CuO-MnO<sup>x</sup> (Cu/Cu + Mn up to 0.53) under O2-containing conditions was similar to that under O2-free conditions. This could be explained as follows. After the introduction of O2, NO quickly reacted with it to produce NO2. Moreover, the reduction of NO<sup>2</sup> by CO was faster than CO oxidation. Therefore, N<sup>2</sup> and CO<sup>2</sup> were finally generated. Sun et al. [54] synthesized the CuCe mixed metal oxides, which showed superior NO conversion and N<sup>2</sup> selectivity, both maintaining more than 90% in a wide temperature window in the absence of O2. Nevertheless, when 1% O<sup>2</sup> was introduced, the NO conversion dropped rapidly to 0 within 3.5 h. The NO conversion could gradually recover to the initial value after the O<sup>2</sup> was stopped. This result indicated that CO preferentially reacts with O2, resulting in the decrease in NO conversion in the

#### **3. Influencing Factors on Catalytic Activity** There is a certain amount of O2, SO2, and H2O in flue gas and automobile exhaust,

*Catalysts* **2022**, *12*, x FOR PEER REVIEW 7 of 14

There is a certain amount of O2, SO2, and H2O in flue gas and automobile exhaust, which will affect the activity of the catalysts. In addition, the temperature is also an important factor. In this section, the effects of O2, temperature, SO<sup>2</sup> and H2O on the catalytic activity of Cu-based catalysts for NO reduction by CO are summarized, as shown in Figure 4. which will affect the activity of the catalysts. In addition, the temperature is also an important factor. In this section, the effects of O2, temperature, SO<sup>2</sup> and H2O on the catalytic activity of Cu-based catalysts for NO reduction by CO are summarized, as shown in Figure 4.

**Figure 4.** Effect of O2, reaction temperature, SO2, and H2O on the catalytic activity of Cu-based catalysts for NO reduction by CO. **Figure 4.** Effect of O<sup>2</sup> , reaction temperature, SO<sup>2</sup> , and H2O on the catalytic activity of Cu-based catalysts for NO reduction by CO.

## *3.1. Effect of O<sup>2</sup>*

Currently, the published literature mainly studies the CO-SCR mechanism of Cu-based catalysts in the absence of O2. Research on the catalytic performance of Cu-based catalysts in the presence of O<sup>2</sup> is rare and mainly focuses on the reaction conditions of low O<sup>2</sup> concentration (≤1%). For instance, Wen et al. [35] prepared the Cu/Ce/Mg/Al/O mixed oxides catalyst, which could enable full NO conversion at 315 ◦C in the presence of 0.5% O<sup>2</sup> (O2/CO = 0.36). Yamamoto et al. [30] investigated the effect of O<sup>2</sup> concentration on

the denitration performance of the 0.5 wt.% Cu/Al2O<sup>3</sup> catalyst for CO-SCR. The results showed that in the absence of O2, the amount of N<sup>2</sup> generated was high, but when the O<sup>2</sup> concentration increased, the amount of N<sup>2</sup> decreased gradually. At the same time, after O<sup>2</sup> was introduced, the conversion of CO increased sharply, reaching nearly 100% at 0.5% O2. Moreover, with the increase in CO conversion, the amount of N<sup>2</sup> generated decreased, but the amount of N2O and NO<sup>2</sup> remained unchanged, which indicates that O<sup>2</sup> preferentially reacted with CO, so the amount of reducing agent CO decreased, resulting in the decrease in NO conversion. When the O<sup>2</sup> concentration exceeded 0.5%, the amount of N<sup>2</sup> generated was also less and less, while the NO<sup>2</sup> amount increased with increasing O<sup>2</sup> concentration, which indicates that more and more NO is oxidized by O2. However, the amount of NO<sup>2</sup> generated was much less than that of N<sup>2</sup> generated in the absence of O2, indicating that most NO has not reacted with O2, which may be because O<sup>2</sup> occupies the adsorption site of NO. Sun et al. [54] also observed the same phenomenon that CuCe mixed metal oxide catalysts enabled nearly full NO conversion at 250 ◦C in the absence of O2, and after 1% O<sup>2</sup> was introduced, the NO conversion gradually dropped to 0. Therefore, O<sup>2</sup> existing in the CO-SCR reaction system has an inhibitory effect on NO reduction. Gholami et al. [39] investigated the effect of O<sup>2</sup> (2% or 5%) on the denitration performance of the Cu1:Ce3/Al2O<sup>3</sup> catalyst for the CO-SCR reaction system, and found that compared with the case without O2, the conversion of NO decreased from 59.3% to nearly 40% after 2% O<sup>2</sup> was introduced at 300 ◦C, but increased to 43.3% when 5% O<sup>2</sup> was introduced. They believe that this is because more O<sup>2</sup> is adsorbed on the surface of the catalyst and then cleaved to O(ad), thereby promoting the formation of NO2, which can further rapidly react with CO to form N<sup>2</sup> and CO2. However, they did not provide specific information on the selectivity of N2. In addition, some scholars also explored the CO-SCR denitration activity of the Cu-based catalysts in the presence of excess O2. Venegas et al. [32] found that the NO conversion of the Cu/SmCeO2/TiO<sup>2</sup> catalyst reached 50% in the presence of 10% O<sup>2</sup> at 300 ◦C. López et al. [41] reported that the K/Cu/SmCe@TiO<sup>2</sup> catalyst delivered 97% NO conversion at 330 ◦C in the presence of 10% O2. However, none of them provided details on the selectivity of N2, which directly reflects the effectiveness of the catalyst. Therefore, it is necessary to study the adsorption and dissociation centers of O<sup>2</sup> on the surface of Cu-based catalysts and reveal its inhibition mechanism in combination with existing characterization and analysis methods. Furthermore, the relationship between the catalytic activity and support, surface active oxygen species, and defect structure should be further investigated to improve the O<sup>2</sup> resistance of Cu-based catalysts.

#### *3.2. Effect of Reaction Temperature*

In addition to O<sup>2</sup> resistance, CO-SCR also faces the challenge of low-temperature performance. As described, although noble metals have superior O<sup>2</sup> resistance [56–58], their high cost and poor low-temperature performance make them unsuitable for the treatment of large-scale industrial tail gas [59,60]. In contrast, transition metal-based catalysts, especially Cu-based catalysts, have been widely studied for their low-temperature activity and low cost [61]. Under O2-containing conditions, the minimum temperature at which the Cu-based catalyst achieves the best catalytic effect is mainly 200–300 ◦C, as shown in Table 1. Most published studies did not provide data on the selectivity of N2. Even in only a few studies reporting N<sup>2</sup> selectivity, it requires a relatively high reduction temperature to achieve a better catalytic effect. For example, under the reaction conditions of 0.1% CO, 0.05% NO, and 1% O2, the Pt-Cu@M-Y catalyst [36] could achieve 43% NO conversion and 53% N<sup>2</sup> selectivity at 350 ◦C. In the presence of 0.65 vol.% O2, the Cu/AlPO<sup>4</sup> catalyst [31] could achieve 78% NO conversion and 48% N<sup>2</sup> selectivity at 400 ◦C. Therefore, it is necessary to improve the low-temperature activity and N<sup>2</sup> selectivity of the Cu-based catalysts under O2-rich conditions in the future.

#### *3.3. SO<sup>2</sup> and H2O Poisoning* sary to improve the low-temperature activity and N<sup>2</sup> selectivity of the Cu-based catalysts under O2-rich conditions in the future.

*Catalysts* **2022**, *12*, x FOR PEER REVIEW 9 of 14

The presence of SO<sup>2</sup> and H2O in the flue gas also has a certain impact on the catalytic performance of the catalysts. Some studies have revealed that SO<sup>2</sup> could adsorb on the surface of the catalysts and compete with NO to form sulfate [62]. The deposition of sulfate led to a decrease in the NO reduction efficiency, and this process is mostly irreversible. On the other side, the effect of H2O on the catalyst activity is minor [21]. Sierra Pereira and Urquieta-González studied the effect of SO<sup>2</sup> or H2O on the catalytic activity of Cu/TiO<sup>2</sup> for CO + NO reaction [26]. In the absence of SO<sup>2</sup> and H2O, Cu/TiO<sup>2</sup> exhibited high NO conversion. After SO<sup>2</sup> was introduced for 30 min, the activity of Cu/TiO<sup>2</sup> decreased from approximately 55% to 0. Even if SO<sup>2</sup> was removed from the feed gas, the activity cannot be recovered at all, as shown in Figure 5a. The results indicate that SO<sup>2</sup> poisoning of Cu/TiO<sup>2</sup> catalyst is irreversible. This phenomenon was attributed to the formation of metal sulfates on the surface of catalysts, which blocked the active site. Compared with SO2, H2O has a relatively weak effect on the catalytic activity, as shown in Figure 5b. The introduction of 10 wt.% H2O led to a reduction in NO conversion from 60% to 40%. After the elimination of H2O, the activity of Cu/TiO<sup>2</sup> could be recovered. Liu et al. [44] reported that CuCoCe/2D-VMT exhibited superior catalytic activity in the presence of O<sup>2</sup> (1 vol.%) and H2O (5 vol.%), enabling full NO conversion to N<sup>2</sup> at 300 ◦C. When a small amount of SO<sup>2</sup> (50 ppm or 100 ppm) was introduced, the catalytic activity decreased slightly, and soon reached a plateau. After SO<sup>2</sup> was removed, the activity gradually returned to the initial value. It is known that SO<sup>2</sup> can occupy some active sites, resulting in a slight decrease in NO conversion. However, adding a certain amount of SO<sup>2</sup> can achieve the balance between the SO<sup>2</sup> adsorption and the SO<sup>2</sup> reduction by CO, so the conversion of NO remains stable. Moreover, the 2D-VMT has a high specific surface area, which can provide storage space for the generated sulfate. Furthermore, Ce preferentially adsorbs SO<sup>2</sup> to protect the Cu active site. When a large amount of SO<sup>2</sup> is introduced, the balance between SO<sup>2</sup> adsorption and reduction is broken, so the catalytic activity continues to decrease. It may be that a large amount of SO<sup>2</sup> reacts with H2O to produce stable sulfate, which occupies the active site, resulting in the irreversible deactivation of the CuCoCe/2D-VMT catalyst. *3.3. SO<sup>2</sup> and H2O Poisoning* The presence of SO<sup>2</sup> and H2O in the flue gas also has a certain impact on the catalytic performance of the catalysts. Some studies have revealed that SO<sup>2</sup> could adsorb on the surface of the catalysts and compete with NO to form sulfate [62]. The deposition of sulfate led to a decrease in the NO reduction efficiency, and this process is mostly irreversible. On the other side, the effect of H2O on the catalyst activity is minor [21]. Sierra Pereira and Urquieta-González studied the effect of SO<sup>2</sup> or H2O on the catalytic activity of Cu/TiO<sup>2</sup> for CO + NO reaction [26]. In the absence of SO<sup>2</sup> and H2O, Cu/TiO<sup>2</sup> exhibited high NO conversion. After SO<sup>2</sup> was introduced for 30 min, the activity of Cu/TiO<sup>2</sup> decreased from approximately 55% to 0. Even if SO<sup>2</sup> was removed from the feed gas, the activity cannot be recovered at all, as shown in Figure 5a. The results indicate that SO<sup>2</sup> poisoning of Cu/TiO<sup>2</sup> catalyst is irreversible. This phenomenon was attributed to the formation of metal sulfates on the surface of catalysts, which blocked the active site. Compared with SO2, H2O has a relatively weak effect on the catalytic activity, as shown in Figure 5b. The introduction of 10 wt.% H2O led to a reduction in NO conversion from 60% to 40%. After the elimination of H2O, the activity of Cu/TiO2 could be recovered. Liu et al. [44] reported that CuCoCe/2D-VMT exhibited superior catalytic activity in the presence of O<sup>2</sup> (1 vol.%) and H2O (5 vol.%), enabling full NO conversion to N<sup>2</sup> at 300 °C. When a small amount of SO<sup>2</sup> (50 ppm or 100 ppm) was introduced, the catalytic activity decreased slightly, and soon reached a plateau. After SO<sup>2</sup> was removed, the activity gradually returned to the initial value. It is known that SO<sup>2</sup> can occupy some active sites, resulting in a slight decrease in NO conversion. However, adding a certain amount of SO<sup>2</sup> can achieve the balance between the SO<sup>2</sup> adsorption and the SO<sup>2</sup> reduction by CO, so the conversion of NO remains stable. Moreover, the 2D-VMT has a high specific surface area, which can provide storage space for the generated sulfate. Furthermore, Ce preferentially adsorbs SO<sup>2</sup> to protect the Cu active site. When a large amount of SO<sup>2</sup> is introduced, the balance between SO<sup>2</sup> adsorption and reduction is broken, so the catalytic activity continues to decrease. It may be that a large amount of SO<sup>2</sup> reacts with H2O to produce stable sulfate, which occupies the active site, resulting in the irreversible deactivation of the CuCoCe/2D-VMT catalyst.

0.05% NO, and 1% O2, the Pt-Cu@M-Y catalyst [36] could achieve 43% NO conversion and 53% N<sup>2</sup> selectivity at 350 °C. In the presence of 0.65 vol.% O2, the Cu/AlPO<sup>4</sup> catalyst [31] could achieve 78% NO conversion and 48% N<sup>2</sup> selectivity at 400 °C. Therefore, it is neces-

**Figure 5.** The effect of O<sup>2</sup> or SO<sup>2</sup> (**a**) and H2O (**b**) on the catalytic performance for the CO-SCR reaction over Cu/TiO2. Reproduced from [26] with permission. **Figure 5.** The effect of O<sup>2</sup> or SO<sup>2</sup> (**a**) and H2O (**b**) on the catalytic performance for the CO-SCR reaction over Cu/TiO<sup>2</sup> . Reproduced from [26] with permission.

Some studies also reported that SO<sup>2</sup> and/or H2O have no significant effect on catalytic activity. Pan et al. investigated the catalytic activity of Cu-Ce-Fe-Co/TiO<sup>2</sup> for CO + NO reaction in the presence of SO<sup>2</sup> or H2O, and found that after 50 ppm SO<sup>2</sup> or 10% H2O addition to the feed gas, NO conversion on Cu-Ce-Fe-Co/TiO<sup>2</sup> did not exhibit significant changes and remained stable. This phenomenon is attributed to the special spinel structure of the Cu-Co-based catalysts, which preferentially adsorbs NO and CO compared with SO2. This is also consistent with the findings of Chen et al. [44]. However, after simultaneously introducing SO<sup>2</sup> and H2O, the activity of Cu-Ce-Fe-Co/TiO<sup>2</sup> decreased from approximately 95% to 60%. After SO<sup>2</sup> and H2O were removed, the activity gradually recovered to about 80%.

#### **4. Reaction Mechanism**

At present, the widely accepted redox reaction mechanisms of CO-SCR catalysts are the Langmuir–Hinshelwood (L-H) mechanism and the Eley–Rideal (E-R) mechanism [63]. The E-R mechanism means that the reaction proceeds through the interaction between the adsorbed species and the gaseous reactant, while the L-H mechanism is when the reaction is performed by the adsorbed components [64]. The L-H mechanism is usually preferred because the reaction attempts may be enormous [65], which is generally accepted by researchers for metal oxide catalysts [63]. First, CO is adsorbed to the surface of the catalyst and reacts with lattice oxygen to form oxygen vacancies (CO catalytic oxidation). Next, the o-terminal of NO is adsorbed on the oxygen vacancy, and then N-O breaks to form N(ad) (ad stands for an adsorbed state) and new lattice oxygen. However, oxygen vacancies can also be occupied by O2, leading to catalyst deactivation in the presence of O2. Finally, N(ad) transforms into N<sup>2</sup> or N2O by reacting with another N(ad) or another NO. N2O can be subsequently adsorbed to oxygen vacancies and then form new lattice oxygen and N2. The reaction formula is as follows:

$$\text{MO} + \text{CO}\_{\text{(ad)}} \rightarrow \text{M} + \text{CO}\_{2(g)} \tag{1}$$

$$\rm{M} + \rm{NO}\_{\rm{(ad)}} \rightarrow \rm{MO} + \rm{N}\_{2(ad)}\tag{2}$$

$$\text{M} + \text{NO}\_{\text{(ad)}} + \text{O}\_{\text{2(ad)}} \rightarrow \text{MO} + \text{N}\_{\text{2(ad)}}\tag{3}$$

$$\text{M} + \text{O}\_{2(ad)} \rightarrow \text{MO} \tag{4}$$

Zhang et al. [37] found that the model reaction NO + CO + O<sup>2</sup> over Ce-Cu-BTC follows the L-H mechanism, as shown in Figure 6a. NO is preferentially adsorbed on the active site of Ce-Cu-BTC to form various adsorbed NO species. CO is mainly adsorbed to the Ce3+ sites on the catalyst surface. The adsorbed NO species can react with the adsorbed CO species to form different intermediates. The interaction between Cu and Ce is conducive to the formation of more Cu<sup>+</sup> , which can promote the conversion of the N2O intermediate to N2. *Catalysts* **2022**, *12*, x FOR PEER REVIEW 11 of 14 Figure 6b. However, when the loading of Cu reaches 3 wt.%, Cu2+ is more easily reduced to metallic Cu<sup>0</sup> ; the latteris more conducive to CO oxidation than NO reduction.

**Figure 6.** The reaction mechanism of Ce-Cu-BTC (**a**) and low-loaded CuO/Al2O<sup>3</sup> (0.5 wt.% Cu) (**b**) for CO-SCR in the presence of O2. Reproduced from [34,37] with permission. **Figure 6.** The reaction mechanism of Ce-Cu-BTC (**a**) and low-loaded CuO/Al2O<sup>3</sup> (0.5 wt.% Cu) (**b**) for CO-SCR in the presence of O<sup>2</sup> . Reproduced from [34,37] with permission.

On the other hand, it is also reported that O<sup>2</sup> has a positive effect on CO-SCR because the dissociated O(ad) can combine with the adsorbed NO to form NO2, which can further rapidly react with CO to form N<sup>2</sup> and CO2. Fukuda et al. [68] found that after introducing O2, the catalytic performance of Cu2O/γ-Al2O<sup>3</sup> for the CO+NO reaction was improved. Spassova et al. [53] deem that O<sup>2</sup> is adsorbed to the surface of the catalyst and then cleaved to O(ad), which then reacts with CO(ad) and NO to CO<sup>2</sup> and NO2, and finally, NO<sup>2</sup> reacts with CO to produce CO<sup>2</sup> and N2. Similarly, Gholami [29] also believes that under O2-containing conditions, O<sup>2</sup> is adsorbed on the catalyst surface and then cleaved to O(ad), which reacts with CO to CO2, and then NO is adsorbed on the generated oxygen vacancy and decomposed into N(ad) and O(ad). Subsequently, N(ad) reacts with another N(ad) to generate N2, and O(ad) reacts with another NO form NO2. Finally, NO<sup>2</sup> reacts with CO to produce N<sup>2</sup> and CO2. Many researchers [66,67] believe that O<sup>2</sup> is preferentially adsorbed and dissociated at the active site, which hinders the further adsorption and dissociation of NO, so the catalytic performance of the catalysts decreases with the increase in O<sup>2</sup> concentration. In addition, the increased surface coverage of O(ad) after dissociation increases the contact probability with CO(ad), and directly consumes part of the reductant CO, which is also regarded as an important reason for the decline in NO conversion. Amano et al. [34] found that under O2-containing conditions, the reaction mechanism of CuO/Al2O<sup>3</sup> with different Cu loading is significantly different. When the loading amount of Cu is very small (0.5 wt.%), there is a single electron redox behavior, which is the cycle of Cu2+ and Cu<sup>+</sup> . It is considered as the reaction mechanism of NO reduction by CO in the presence of O2, as shown in Figure 6b. However, when the loading of Cu reaches 3 wt.%, Cu2+ is more easily reduced to metallic Cu<sup>0</sup> ; the latter is more conducive to CO oxidation than NO reduction.

one of the research hotspots in the denitrification field. Research on the application of Cubased catalysts in NO-CO reaction system has increased year by year, mainly focusing on O2-free or hypoxic conditions. When the O<sup>2</sup> concentration increases, the NO conversion or N<sup>2</sup> selectivity decreases significantly, and the selective catalytic activity of the catalysts decreases. This stems from the following reasons: (i) O<sup>2</sup> is preferentially adsorbed and dissociated at the active site, which hinders the adsorption and dissociation of NO; (ii) O(ad) reacts with CO, which directly consumes part of the reductant CO; and (iii) O(ad) reacts with NO, resulting in the decrease in N<sup>2</sup> selectivity. Therefore, it is necessary to investigate the relationship between the structural change in crystal defects and the type of oxygen species on the surface of Cu-based catalysts, and the type of NO and CO adsorbed species, so as to deeply understand the reaction process of CO-SCR. The influence of O<sup>2</sup> on the crystal structure of the Cu-based catalysts, the electron transfer of metal elements, as well as the dissociation of N-O bonds and O-O bonds should also be studied to explore the reaction mechanism of CO-SCR, so as to improve the O<sup>2</sup> resistance of the catalysts. Follow-up research can be implemented from the following aspects: (1) some in situ methods can be employed to study the real-time structure–activity relationship of the Cu-based catalysts in the future, because in terms of this reaction, to date, there is a lack of in situ characterization methods to monitor the structural changes of the catalysts in the reaction process; (2) it is imperative to develop green and energy-saving strategies to synthesize the Cu-based catalysts, i.e., ball milling, and mechanical mixing, used for CO-SCR. The currently used preparation methods for the Cu-based catalysts are mainly solution-based strategies, such as wet impregnation, precipitation, and sol-gel. These preparation

**5. Conclusions and Perspectives**

On the other hand, it is also reported that O<sup>2</sup> has a positive effect on CO-SCR because the dissociated O(ad) can combine with the adsorbed NO to form NO2, which can further rapidly react with CO to form N<sup>2</sup> and CO2. Fukuda et al. [68] found that after introducing O2, the catalytic performance of Cu2O/γ-Al2O<sup>3</sup> for the CO+NO reaction was improved. Spassova et al. [53] deem that O<sup>2</sup> is adsorbed to the surface of the catalyst and then cleaved to O(ad), which then reacts with CO(ad) and NO to CO<sup>2</sup> and NO2, and finally, NO<sup>2</sup> reacts with CO to produce CO<sup>2</sup> and N2. Similarly, Gholami [29] also believes that under O2-containing conditions, O<sup>2</sup> is adsorbed on the catalyst surface and then cleaved to O(ad), which reacts with CO to CO2, and then NO is adsorbed on the generated oxygen vacancy and decomposed into N(ad) and O(ad). Subsequently, N(ad) reacts with another N(ad) to generate N2, and O(ad) reacts with another NO form NO2. Finally, NO<sup>2</sup> reacts with CO to produce N<sup>2</sup> and CO2.

#### **5. Conclusions and Perspectives**

In recent years, the selective catalytic reduction of NO with CO as the reductant is one of the research hotspots in the denitrification field. Research on the application of Cu-based catalysts in NO-CO reaction system has increased year by year, mainly focusing on O2-free or hypoxic conditions. When the O<sup>2</sup> concentration increases, the NO conversion or N<sup>2</sup> selectivity decreases significantly, and the selective catalytic activity of the catalysts decreases. This stems from the following reasons: (i) O<sup>2</sup> is preferentially adsorbed and dissociated at the active site, which hinders the adsorption and dissociation of NO; (ii) O(ad) reacts with CO, which directly consumes part of the reductant CO; and (iii) O(ad) reacts with NO, resulting in the decrease in N<sup>2</sup> selectivity. Therefore, it is necessary to investigate the relationship between the structural change in crystal defects and the type of oxygen species on the surface of Cu-based catalysts, and the type of NO and CO adsorbed species, so as to deeply understand the reaction process of CO-SCR. The influence of O<sup>2</sup> on the crystal structure of the Cu-based catalysts, the electron transfer of metal elements, as well as the dissociation of N-O bonds and O-O bonds should also be studied to explore the reaction mechanism of CO-SCR, so as to improve the O<sup>2</sup> resistance of the catalysts. Followup research can be implemented from the following aspects: (1) some in situ methods can be employed to study the real-time structure–activity relationship of the Cu-based catalysts in the future, because in terms of this reaction, to date, there is a lack of in situ characterization methods to monitor the structural changes of the catalysts in the reaction process; (2) it is imperative to develop green and energy-saving strategies to synthesize the Cu-based catalysts, i.e., ball milling, and mechanical mixing, used for CO-SCR. The currently used preparation methods for the Cu-based catalysts are mainly solution-based strategies, such as wet impregnation, precipitation, and sol-gel. These preparation methods involved complex operations, enormous energy, and are not conducive to industrialization. In short, in view of the superior catalytic performance and low cost of Cu-based catalysts, it is urgent to further study how to simultaneously achieve a suitable antioxidant effect, lowtemperature activity, and high SO2/H2O tolerance to promote the industrial application of the CO-SCR technology.

**Author Contributions:** Investigation, X.C.; Data curation, X.C.; Writing-review & editing, X.C. and Y.J.; Visualization, Y.L. (Yaqi Liu); Review, Y.L. (Yan Liu) and M.C.; Resources, D.L. and L.X.; Conceptualization, Y.J. and S.L.; Methodology, Y.J. and S.L.; Supervision, Y.J. and S.L.; Project administration, Y.J. and S.L.; Funding acquisition, Y.J. and K.W. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Research Foundation for Youth Scholars of Beijing Technology and Business University grant number QNJJ2022-23 from K.W.; the Project for Improving the Research Ability of Postgraduate from Beijing Technology and Business University grant number 19008022056 from X.C.; the Research Foundation for Youth Scholars of Beijing Technology and Business University grant number QNJJ2022-22 from L.X.; the National Natural Science Foundation of China grant number 21978299 from Y.J.; the Research Foundation for Advanced Talents of Beijing Technology and Business University grant number 19008020159 from Y.J.

**Data Availability Statement:** Data sharing not applicable.

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