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

Abstract

Selective catalytic reduction of NO_x by CO (CO-SCR) to both N₂ and CO₂ 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 O₂, H₂O, and SO₂ resistance. This paper provides a comprehensive overview and insights into CO-SCR under O₂-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 O₂ concentration, reaction temperature, H₂O, and SO₂ 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.

1. Introduction

Nitrogen oxides (NO_x), 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_x 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_x removal has become imperative. Among various denitrification technologies, selective catalytic reduction of NO_x with NH₃ (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_x (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 O₂ 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-WO₃ catalyst with single Ir atoms anchored on the mesoporous WO₃ [14]. This Ir₁/m-WO₃ catalyst containing only 1 wt.% Ir loading exhibited exceptional catalytic performance in the presence of 2 vol.% O₂, achieving 73% NO conversion and 100% N₂ selectivity at 350 °C. Its superior activity can be attributed to the following reasons: (i) isolated Ir single atoms and the maximized Ir-WO₃ interfaces promote the adsorption and activation of NO, and (ii) the accessible mesopores in m-WO₃ 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 better catalytic performance in the presence of O₂, showing high application potential. Many supported Cu-based catalysts such as Cu/CeO₂-Fe₂O₃ [21], CuO/CeO₂-Fe₂O₃ [22], Cu/Fe₂O₃-CeO₂/ZrO₂ [23], and CuO_x/CeO₂ [24] exhibit excellent catalytic performance, with NO conversion usually surpassing 90% at 200 °C. Moreover, non-supported catalysts such as CuCeO_x [25] and Cu₁Ce_0.5Fe_1.5O_4.25 [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 O₂ [10], which preferentially reacts with CO or NO to form CO₂ and NO₂, resulting in poor catalytic activity. Additionally, SO₂ 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-SO₂ 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-CeO₂ 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 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.

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 O₂ are summarized. Table 1 gives the catalytic performances of supported Cu-based catalysts for NO + CO reaction in the presence of O₂ reported in the literature.

Table 1. Catalytic activity of various supported Cu-based catalysts for the NO + CO + O₂ reaction.

Type	Sample	Tmax a (°C)	ηNOb (%)	SN2c (%)	ψ(NO) d (%)	ψ(CO) (%)	ψ(O2) (%)	F (mL min−1) e	GHSV (mL gAg−1 h−1) f	Ref.
Mono- metallic catalysts	0.5%Cu/Al2O3	500	---	---	1	1	0.5	100	30,000	[30]
	CuO/TiO2	500	54	---	5	5	0.12	50	75,000 h−1	[26]
	5Cu/AlPO4	400	78	48	0.2	1.5	0.65	60	36,000	[31]
	Cu/SmCeO2/TiO2	300	50	---	0.05	1	10	50	10,000 h−1	[32]
	Cu/Ce0.1Al	400	100	---	0.06	0.12	0.1	30	18,000	[33]
	CuO/Al2O3	500	30		1	1	0.5	100	30,000 h−1	[34]
	Cu/Al2O3	500	---	---	1	1	0.5	100	30,000	[35]
Bimetallic catalysts	Pt-Cu@M-Y	350	43	53	0.05	0.1	1	667	10,000 h−1	[36]
	Ce-Cu-BTC	250	90	---	0.1	0.1	5	---	---	[37]
	Fe2Cu1/RHA	100	100	100	0.056	0.056	8.9	2400	11,220 h−1	[38]
	Cu1:Ce3/Al2O3	420	71.8	---	0.025	0.5	5	300	360,000	[39]
	Cu1:Ce3/CNT	220	96	---	0.025	0.5	0.3	300	12,600 h−1	[40]
	K/Cu/SmCe@TiO2	330	97	---	0.05	1	10	50	10,000 h−1	[41]
	Cu-Mn/Al2O3	180	78	85	0.055	0.9	16	---	10,000 h−1	[42]
Multi- metallic catalysts	CuCoOx/TiO2	200	60	---	0.1	1	2	1200	20,000 h−1	[43]
	CuCoCe/2D-VMT	200	70	97	0.05	0.1	1	---	102,000 h−1	[44]
	Cu-Ni-Ce/AC	150	99.8	---	0.4	4	5	1000	30,000 h−1	[45]
	Cu-Ce-Fe-Co/TiO2	200	93	---	0.02	1	6	---	10,000 h−1	[46]
	Cu-Ce-Fe-Mn/TiO2	200	82	---	0.02	0.02	1	---	10,000 h−1	[46]

^a T_max: Temperature corresponding to maximum NO conversion. ^b η_NO: The conversion of NO. ^c S_N2: The selectivity of N₂. ^d ψ(NO): The NO concentration in the feeding gas. ^e F: The volume flow rate of the feeding gas. ^f GHSV: Gas weight hourly space velocity.

Table 1. Catalytic activity of various supported Cu-based catalysts for the NO + CO + O₂ reaction.

Type	Sample	Tmax a (°C)	ηNOb (%)	SN2c (%)	ψ(NO) d (%)	ψ(CO) (%)	ψ(O2) (%)	F (mL min−1) e	GHSV (mL gAg−1 h−1) f	Ref.
Mono- metallic catalysts	0.5%Cu/Al2O3	500	---	---	1	1	0.5	100	30,000	[30]
	CuO/TiO2	500	54	---	5	5	0.12	50	75,000 h−1	[26]
	5Cu/AlPO4	400	78	48	0.2	1.5	0.65	60	36,000	[31]
	Cu/SmCeO2/TiO2	300	50	---	0.05	1	10	50	10,000 h−1	[32]
	Cu/Ce0.1Al	400	100	---	0.06	0.12	0.1	30	18,000	[33]
	CuO/Al2O3	500	30		1	1	0.5	100	30,000 h−1	[34]
	Cu/Al2O3	500	---	---	1	1	0.5	100	30,000	[35]
Bimetallic catalysts	Pt-Cu@M-Y	350	43	53	0.05	0.1	1	667	10,000 h−1	[36]
	Ce-Cu-BTC	250	90	---	0.1	0.1	5	---	---	[37]
	Fe2Cu1/RHA	100	100	100	0.056	0.056	8.9	2400	11,220 h−1	[38]
	Cu1:Ce3/Al2O3	420	71.8	---	0.025	0.5	5	300	360,000	[39]
	Cu1:Ce3/CNT	220	96	---	0.025	0.5	0.3	300	12,600 h−1	[40]
	K/Cu/SmCe@TiO2	330	97	---	0.05	1	10	50	10,000 h−1	[41]
	Cu-Mn/Al2O3	180	78	85	0.055	0.9	16	---	10,000 h−1	[42]
Multi- metallic catalysts	CuCoOx/TiO2	200	60	---	0.1	1	2	1200	20,000 h−1	[43]
	CuCoCe/2D-VMT	200	70	97	0.05	0.1	1	---	102,000 h−1	[44]
	Cu-Ni-Ce/AC	150	99.8	---	0.4	4	5	1000	30,000 h−1	[45]
	Cu-Ce-Fe-Co/TiO2	200	93	---	0.02	1	6	---	10,000 h−1	[46]
	Cu-Ce-Fe-Mn/TiO2	200	82	---	0.02	0.02	1	---	10,000 h−1	[46]

^a T_max: Temperature corresponding to maximum NO conversion. ^b η_NO: The conversion of NO. ^c S_N2: The selectivity of N₂. ^d ψ(NO): The NO concentration in the feeding gas. ^e F: The volume flow rate of the feeding gas. ^f 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-O₂ reaction. The Cu-based catalysts with different supports and loading amounts were investigated in detail, and the results suggest that 0.5 wt.% Cu/Al₂O₃ exhibited the highest catalytic performance. The support γ-Al₂O₃ itself can reduce NO to N₂O but not N₂, possessing a limited capability for the reduction of NO. Therefore, the addition of Cu to Al₂O₃ 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 NO_x, 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-O₂ reaction, and 10 wt.% CuO/TiO₂ 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/AlPO₄ 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@TiO₂ catalyst with Cu supported on core–shell-structured SmCeO₂@TiO₂ achieved 50% NO conversion at 500 °C in the presence of 10 vol.% O₂. Its superior catalytic performance was because CeO₂ possessed excellent redox properties through the transfer between Ce³⁺ and Ce⁴⁺, thus increasing the oxidation activity of the Cu/SmCeO₂@TiO₂ catalyst. Moreover, the addition of Sm helped to maintain the thermal stability of the CeO₂ phase. Core–shell-structured CeO₂@TiO₂ 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-Al₂O₃ catalyst, which exhibited excellent catalytic performance and superior resistance to O₂ and SO₂ 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-Al₂O₃ support were favorable for the adsorption of NO.

2.1.2. Bimetallic Catalysts

Chen et al. [43] synthesized a series of CuCoO_x/TiO₂ catalysts and found that the CuCoO_x/TiO₂ catalyst able to generate the CuCo₂O₄ spinel exhibited the highest catalytic activity, reaching 98.9% NO conversion at 200 °C and in the absence of O₂. However, when 2 vol.% O₂ was introduced, the NO conversion decreased sharply to 60%. Liu et al. [42] investigated the denitration performance of various transition metals supported on Al₂O₃ pellets under O₂-rich conditions (16 vol.%). Among these catalysts, Cu-Mn/Al₂O₃ with a molar ratio Cu:Mn of 1.5 displayed the best catalytic activity, achieving nearly 78% NO conversion and 85% N₂ selectivity at 180 °C. Based on the density functional theory calculation, it was demonstrated that Mn had better O₂ resistance and Cu had better H₂O resistance. López et al. [41] prepared a novel core–shell-structured K/Cu/SmCe@TiO₂ catalyst, giving 97% NO conversion at 330 °C in the presence of excess O₂ (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 O₂ (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⁺ species content, superior reducing capability, and the synergistic effect between SOV and Cu⁺ 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 O₂ (5 vol.%), and found that Cu1:Ce3/Al₂O₃ 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 Al₂O₃ support. Interestingly, it was observed that with the increase in O₂ concentration from 2% to 5%, the conversion of NO increased slightly. This was because the more O₂ 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 Ce-modified 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_x on the surface of catalysts, as evidenced by the in situ DRIFTS spectrum (Figure 2c,d). The enhanced NO_x adsorption ultimately improved the catalytic activity for CO-SCR.

Recently, our group used a simple impregnation method followed by reduction with H₂ 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 N₂ 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 sub-nanometric Pt, and the generated interfaces between Cu nanoparticles and surface CuO_x species served as the dissociation sites of NO. However, when 1 vol.% O₂ was introduced (O₂/CO = 10), the NO conversion decreased to 43% and N₂ selectivity dropped to 53% at 350 °C, due to the preferential oxidation of CO and NO by O₂ 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/TiO₂ and Mn-Ce-Fe-Co/TiO₂ exhibited better catalytic activity in the absence of O₂, both reaching full NO conversion at 250 °C. However, the presence of O₂ largely restricts the NO reduction efficiency. Comparatively, Cu-Ce-Fe-Co/TiO₂ showed better tolerance to O₂ than Mn-Ce-Fe-Co/TiO₂. When 6 vol % O₂ was fed, the Cu-Ce-Fe-Co/TiO₂ catalyst still exhibited 93% NO conversion and 74.3% NO_x 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/TiO₂ 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.% O₂ and 5 vol.% H₂O, reaching 70% NO conversion and 97% N₂ 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 O_β).

2.2. Non-Supported Catalysts

Besides a large number of reported Cu-based supported catalysts, some non-supported Cu-based catalysts also have certain O₂ resistance in the CO-SCR reaction. Mehandjiev et al. [51] first reported that CuCo₂O₄ had the ability to reduce NO by CO in the presence of O₂. Furthermore, Panayotov et al. [52] found that Cu_xCo_3−xO₄ 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_x (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 N₂ achieved by CuO-MnO_x (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 NO₂ by CO was faster than CO oxidation. Therefore, N₂ and CO₂ were finally generated. Sun et al. [54] synthesized the CuCe mixed metal oxides, which showed superior NO conversion and N₂ selectivity, both maintaining more than 90% in a wide temperature window in the absence of O₂. Nevertheless, when 1% O₂ 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 O₂ 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 + O₂ 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 SO₂ 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 + O₂ reaction and excellent resistance to H₂O and SO₂ poisoning.

3. Influencing Factors on Catalytic Activity

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, SO₂ and H₂O on the catalytic activity of Cu-based catalysts for NO reduction by CO are summarized, as shown in Figure 4.

3.1. Effect of O₂

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 O₂ is rare and mainly focuses on the reaction conditions of low O₂ 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% O₂ (O₂/CO = 0.36). Yamamoto et al. [30] investigated the effect of O₂ concentration on the denitration performance of the 0.5 wt.% Cu/Al₂O₃ catalyst for CO-SCR. The results showed that in the absence of O₂, the amount of N₂ generated was high, but when the O₂ concentration increased, the amount of N₂ decreased gradually. At the same time, after O₂ 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 N₂ generated decreased, but the amount of N₂O and NO₂ remained unchanged, which indicates that O₂ preferentially reacted with CO, so the amount of reducing agent CO decreased, resulting in the decrease in NO conversion. When the O₂ concentration exceeded 0.5%, the amount of N₂ generated was also less and less, while the NO₂ amount increased with increasing O₂ concentration, which indicates that more and more NO is oxidized by O₂. However, the amount of NO₂ generated was much less than that of N₂ generated in the absence of O₂, indicating that most NO has not reacted with O₂, which may be because O₂ 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% O₂ was introduced, the NO conversion gradually dropped to 0. Therefore, O₂ existing in the CO-SCR reaction system has an inhibitory effect on NO reduction. Gholami et al. [39] investigated the effect of O₂ (2% or 5%) on the denitration performance of the Cu1:Ce3/Al₂O₃ 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% O₂ was introduced at 300 °C, but increased to 43.3% when 5% O₂ was introduced. They believe that this is because more O₂ 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 N₂ 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/TiO₂ catalyst reached 50% in the presence of 10% O₂ at 300 °C. López et al. [41] reported that the K/Cu/SmCe@TiO₂ 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 O₂ 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 O₂ resistance of Cu-based catalysts.

3.2. Effect of Reaction Temperature

In addition to O₂ resistance, CO-SCR also faces the challenge of low-temperature performance. As described, although noble metals have superior O₂ resistance [56,57,58], 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 N₂ 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% N₂ selectivity at 350 °C. In the presence of 0.65 vol.% O₂, the Cu/AlPO₄ catalyst [31] could achieve 78% NO conversion and 48% N₂ selectivity at 400 °C. Therefore, it is necessary to improve the low-temperature activity and N₂ selectivity of the Cu-based catalysts under O₂-rich conditions in the future.

3.3. SO₂ and H₂O Poisoning

The presence of SO₂ and H₂O in the flue gas also has a certain impact on the catalytic performance of the catalysts. Some studies have revealed that SO₂ 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 SO₂ or H₂O on the catalytic activity of Cu/TiO₂ for CO + NO reaction [26]. In the absence of SO₂ and H₂O, Cu/TiO₂ exhibited high NO conversion. After SO₂ was introduced for 30 min, the activity of Cu/TiO₂ decreased from approximately 55% to 0. Even if SO₂ was removed from the feed gas, the activity cannot be recovered at all, as shown in Figure 5a. The results indicate that SO₂ poisoning of Cu/TiO₂ 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 O₂ (1 vol.%) and H₂O (5 vol.%), enabling full NO conversion to N₂ at 300 °C. When a small amount of SO₂ (50 ppm or 100 ppm) was introduced, the catalytic activity decreased slightly, and soon reached a plateau. After SO₂ was removed, the activity gradually returned to the initial value. It is known that SO₂ can occupy some active sites, resulting in a slight decrease in NO conversion. However, adding a certain amount of SO₂ can achieve the balance between the SO₂ adsorption and the SO₂ 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 SO₂ to protect the Cu active site. When a large amount of SO₂ is introduced, the balance between SO₂ adsorption and reduction is broken, so the catalytic activity continues to decrease. It may be that a large amount of SO₂ reacts with H₂O to produce stable sulfate, which occupies the active site, resulting in the irreversible deactivation of the CuCoCe/2D-VMT catalyst.

Some studies also reported that SO₂ and/or H₂O have no significant effect on catalytic activity. Pan et al. investigated the catalytic activity of Cu-Ce-Fe-Co/TiO₂ for CO + NO reaction in the presence of SO₂ or H₂O, and found that after 50 ppm SO₂ or 10% H₂O addition to the feed gas, NO conversion on Cu-Ce-Fe-Co/TiO₂ 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 SO₂ and H₂O, the activity of Cu-Ce-Fe-Co/TiO₂ decreased from approximately 95% to 60%. After SO₂ 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 N₂ 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:

MO + CO_(ad) → M + CO_2(g)

(1)

M + NO_(ad) → MO + N_2(ad)

(2)

M + NO_(ad) + O_2(ad) → MO + N_2(ad)

(3)

M + O_2(ad) → MO

(4)

Zhang et al. [37] found that the model reaction NO + CO + O₂ 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⁺, which can promote the conversion of the N₂O intermediate to N₂.

Many researchers [66,67] believe that O₂ 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 O₂ 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/Al₂O₃ 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⁺. 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 Cu⁰; the latter is more conducive to CO oxidation than NO reduction.

On the other hand, it is also reported that O₂ 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 N₂ and CO₂. Fukuda et al. [68] found that after introducing O₂, the catalytic performance of Cu₂O/γ-Al₂O₃ for the CO+NO reaction was improved. Spassova et al. [53] deem that O₂ is adsorbed to the surface of the catalyst and then cleaved to O_(ad), which then reacts with CO_(ad) and NO to CO₂ and NO₂, and finally, NO₂ reacts with CO to produce CO₂ and N₂. Similarly, Gholami [29] also believes that under O₂-containing conditions, O₂ 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, NO₂ reacts with CO to produce N₂ 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 O₂ concentration increases, the NO conversion or N₂ selectivity decreases significantly, and the selective catalytic activity of the catalysts decreases. This stems from the following reasons: (i) O₂ 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 N₂ 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 O₂ 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 O₂ 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 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, low-temperature activity, and high SO₂/H₂O tolerance to promote the industrial application of the CO-SCR technology.