Recent Advances in NO Reduction with NH₃ and CO over Cu-Ce Bimetallic and Derived Catalysts

Abstract

Sintering flue gas contains significant amounts of harmful gases, such as carbon monoxide and nitrogen oxides (NO_x), which pose severe threats to the ecological environment and human health. Selective catalytic reduction (SCR) technology is widely employed for the removal of nitrogen oxides, with copper-cerium-based bimetallic catalysts and their derivatives demonstrating excellent catalytic efficiency in SCR reactions, primarily due to the significant synergistic effect between copper and cerium. This paper summarizes the main factors affecting the catalytic performance of Cu-Ce-based bimetallic catalysts and their derivatives in the selective catalytic reduction of ammonia and carbon monoxide. Key considerations include various preparation methods, doping of active components, and the effects of loading catalysts on different supports. This paper also analyzes the influence of surface oxygen vacancies, redox capacity, acidity, and specific surface area on catalytic performance. Additionally, the anti-poisoning performance and reaction mechanisms of the catalysts are discussed. Finally, the paper proposes strategies for designing high-activity and high-stability catalysts, considering the development prospects and challenges of Cu-Ce-based bimetallic catalysts and their derivatives, with the aim of providing theoretical guidance for optimizing Cu-Ce-based catalysts and promoting their industrial applications.

1. Introduction

As China’s economy continues to grow and social productivity improves, the demand for energy rises. In China’s energy landscape, fossil fuels remain the primary driving force behind economic development. However, the environmental pollution caused by burning fossil fuels cannot be underestimated. The iron and steel industry, a major consumer of fossil energy, emits significant amounts of sintering flue gas, which has low smoke temperatures and a complex composition containing numerous atmospheric pollutants, primarily nitrogen oxides (NO_x) and carbon monoxide (CO). These pollutants have detrimental effects on human health and the ecological environment. NO_x in the atmosphere is easily converted into nitrogen dioxide (NO₂), which reacts with water molecules to produce nitric acid, contributing to acid rain that corrodes buildings and harms human beings. Additionally, NO_x serves as a precursor to PM2.5 and particulate matter, leading to haze and photochemical smog [1]. CO has a high affinity for hemoglobin, impairing its ability to transport oxygen and endangering human health [2]. Therefore, it is crucial to develop effective measures for removing NO_x and CO from industrial emissions [3,4].

SCR technology is an effective method for treating NO_x that utilizes a reductant to react with NO_x, converting it into harmless nitrogen gas [5]. Compared to other NO_x removal technologies, such as direct decomposition, storage reduction (NSR), and selective non-catalytic reduction (SNCR) [6], SCR offers significant advantages, including high purification efficiency, low secondary pollution, and well-established technological applications [7]. The core of SCR technology lies in the selection of a suitable reductant, which can include hydrogen selective catalytic reduction (H₂-SCR), hydrocarbon selective catalytic reduction (HC-SCR), ammonia selective catalytic reduction (NH₃-SCR), and carbon monoxide selective catalytic reduction (CO-SCR) [8]. H₂-SCR employs hydrogen as the reductant, effectively removing NO_x and generating water at low temperatures. Although H₂ is a clean energy source that does not produce secondary pollution, this technology incurs higher operating costs, requires less catalyst, and lacks stability and durability, limiting its large-scale application. Additionally, H₂ is flammable and explosive, necessitating stringent safety measures for its storage and transportation, which adds to operational complexity [9]. HC-SCR reduces NO_x using hydrocarbons such as methane, ethylene, and propylene, leading to efficient conversion of NO_x and hydrocarbons. However, this technology has limitations, as it is prone to side reactions that decrease selectivity for NO_x and reduce catalytic performance. Moreover, using hydrocarbons as reductants may increase the emission of volatile organic compounds, contributing to secondary pollution [10]. NH₃-SCR, widely used in industry, harnesses the reducing function of NH₃ to convert NO_x into environmentally benign N₂ and H₂O. This technology efficiently removes NO_x under flexible reaction conditions, offering a wide and selective temperature window. The denitrification reactions in NH₃-SCR can be divided into two types, one of which is 17 times faster than the standard reaction (Equation (1)) at low temperatures and is referred to as the “fast SCR reaction”(Equation (2)). However, some side reactions may occur, with the main by-product being N₂O (Equations (3) and (4)) [11]. Conversely, CO-SCR employs CO as a reducing agent, reacting with NO_x to produce harmless substances such as N₂ and CO₂. Compared to NH₃-SCR, the use of CO-SCR avoids the problem of ammonia escape and enables the simultaneous removal of both CO and NO_x (Equations (5) and (6)) [12], embodying the concept of “waste for waste”. In summary, SCR technology has become a crucial method for managing NO_x due to its high efficiency and flexibility. However, the various reductant technologies each have their own characteristics, which should be considered based on specific application scenarios.

4NH₃ + 4NO + O₂ → 4N₂ + 6H₂O

(1)

4NH₃ + 2NO + 2NO₂ → 4N₂ + 6H₂O

(2)

4NH₃ + 4NO + 3O₂ → 4N₂O + 6H₂O

(3)

2NH₃ + 2O₂ → N₂O + 3H₂O

(4)

NO + CO → 1/2N₂ + CO₂

(5)

NO₂ + 2CO → 1/2N₂ + 2CO₂

(6)

The catalytic reactions on the surface of SCR catalysts generally follow the Eley–Rideal (ER) and Langmuir–Hinshelwood (LH) mechanisms [13]. Different catalytic systems and reaction conditions dictate which mechanism is followed, and the reaction mechanism for the same catalyst may also shift with changes in the reaction temperature. In the NH₃-SCR reaction, the ER mechanism posits that the reduced gas NH₃ is first adsorbed on the acidic sites of the catalyst surface, forming a highly active adsorbed state of NH₃ (NH₃(a)). Subsequently, NH₃ reacts with NO_X (NO(g)) in the gas phase to produce the intermediate product NH_X-NO_X, which is further decomposed into N₂ and H₂O. In contrast, the LH mechanism suggests that NO_X and NH₃ are simultaneously adsorbed on the same or adjacent active sites of the catalyst surface, where the adsorbed NO_X (NO_X(a)) interacts with NH₃ to form intermediate products that also decompose into N₂ and H₂O [14]. Mechanistic studies for CO-SCR are equally crucial, as this process is also typically understood to follow the ER and LH mechanisms. In the ER mechanism, one component of NO or CO is chemisorbed on the catalyst surface and reacts with the gas-phase (or physisorbed) component. Conversely, the LH mechanism refers to the simultaneous chemisorption and interaction of both NO and CO on the catalyst surface. Thus, the ER mechanism involves the participation of a single molecular species in the reaction, while the LH mechanism entails bimolecular participation [15].

The SCR catalyst is the core component of the denitrification system, and its performance directly impacts the efficiency of the denitrification reaction [16]. Currently, vanadium-based catalysts are widely used in the industry due to their stable denitrification performance and adaptability to various reducing agents, making them suitable for different types of industrial waste gas treatment. However, these catalysts operate within a narrow temperature range, with optimal activity occurring between 300–450 °C. Additionally, V₂O₅ is biotoxic, posing potential threats to the ecological environment and human health [17]. Consequently, many plants install denitrification units after desulfurization and dust removal systems to enhance energy utilization and NO_x removal; this leads to relatively clean treated flue gas, which minimizes damage to the SCR catalyst. However, the lower temperature of the flue gas entering the denitrification unit necessitates the development of non-toxic catalysts that perform well at temperatures below 300 °C, with environmental friendliness and high tolerance to SO₂ and H₂O. Early denitrification catalysts were primarily based on noble metals (e.g., Pd [18,19], Pt [20,21], Rh [22], Ir [23]), which, while exhibiting superior catalytic performance, were poorly selective, expensive, and had limited natural reserves, thereby restricting their large-scale application. In contrast, transition metal atoms or ions, with their more active outermost electrons and varied valence states, favor redox cycling. This characteristic enables transition metal oxide catalysts to enhance low-temperature catalytic activity while also reducing reaction costs, resulting in their widespread use in SCR applications. Commonly utilized transition metal oxide catalysts include MnO_x [24,25,26], Fe₂O₃ [27,28], CuO [29,30], and CeO₂ [31,32,33], which generally demonstrate efficient NO_x conversion performance. However, studies indicate that CuO [34,35], Fe₂O₃, and CeO₂ typically achieve complete NO conversion only at temperatures above 300 °C [36]. To address this issue, researchers have introduced the concept of composite metal oxide catalysts. By doping other metals or incorporating supports into a single metal oxide to create binary or multicomposite oxides, the limitations of individual components can be mitigated, resulting in composite catalysts with strong redox capacity, excellent low-temperature activity, and improved resistance to toxicity. The synergistic effect among the active components in these multicomposite catalysts facilitates electron transfer and redistribution, enhancing the adsorption of reactants and their participation in reactions. Furthermore, the diverse active sites provided by different metals contribute to the formation of acidic sites, promote the transfer of oxygen species, and increase the oxygen concentration in the reaction, thereby improving reaction efficiency. This synergistic effect may also alter the reaction mechanism, allowing the reaction between NO and NH₃ to proceed via a lower energy pathway, reducing activation energy and boosting catalytic activity. Consequently, bimetallic and polymetallic catalysts are garnering increasing attention from researchers.

Among the binary composite metal catalysts, copper-cerium catalysts exhibit significant potential due to their excellent synergistic effects [36].

Table 1. Cu-Ce bimetallic and derived catalysts for the NH₃-SCR and CO-SCR reaction.

Catalysts	Preparation Method	Reaction Condition	GHSV (h−1)	NO Conversion	Ref.
CuCeO	co-precipitation	[NO] = [NH3] = 1000 ppm, [O2] = 5%	40,000	~100% (160–200 °C)	[40]
CuO-CeO2-TiO2	sol-gel	[NO] = [NH3] = 500 ppm, [O2] = 5%	30,000	>90% (150–250 °C)	[41]
CuO/TiCe	impregnation	[NO] = [NH3] = 500 ppm, [O2] = 5%	120,000	>80% (200–300 °C)	[42]
CuO-CeO2-ZrO2	co-precipitation	[NO] = [NH3] = 600 ppm, [O2] = 5%	60,000	~100% (125–180 °C)	[43]
CeMnO/Cu-SSZ-39	ion exchange	[NO] = [NH3] = 500 ppm, [O2] = 5.3%	100,000	~100% (225–600 °C)	[44]
NbCuCeTiO	wet impregnation.	[NO] = [NH3] = 600 ppm, [O2] = 5%	40,000	>90% (180–360 °C)	[45]
W/Cu-CeO2	wet impregnation	[NO] = [NH3] = 500ppm, [O2] = 5%	60,000	>85% (200–400 °C)	[46]
CuO-Nb2O5/CeO2	microwave hydrothermal	[NO] = 500 ppm, [NH3] = 600 ppm, [O2] = 8%	150,000	~100% (225–450 °C)	[47]
Cu/CeO2	impregnation	[NO] = [CO] = 5000 ppm, He	32,000	~100% (300–400 °C)	[48]
Rh/CeCuO	co-crystallization	[NO] = [CO] = 1500 ppm, Ar	150,000	~100% (162–400 °C)	[49]
3DOM CuCe	colloidal crystal template	[NO] = 1000 ppm, [CO] = 2000 ppm, N2	60,000	~100% (450–700 °C)	[50]
CuO-CeO2	supercritical hydrothermal	[NO] = 600 ppm, [CO] = 1200 ppm, N2	40,000	~100% (150–300 °C)	[51]
Cu/MgO-CeO2	citric acid sol–gel	[NO] = 5000 ppm, [CO] = 6000 ppm, He	160,000	>80% (250–400 °C)	[52]

shows the efficient catalytic performance of Cu-Ce-based bimetallic and derived catalysts for the NH₃-SCR and CO-SCR reactions that have been reported in the literature. Cerium is abundant and inexpensive, and when it interacts with metal ions, CeO₂ can generate oxygen vacancies and demonstrate high oxygen storage capacity. The outermost electron of elemental cerium is unstable and easily shifts between Ce⁴⁺ and Ce³⁺, resulting in excellent redox properties. Additionally, Ce-based catalysts are favored for their low cost, high selectivity, and resistance to water and sulfur. However, pure CeO₂ suffers from insufficient catalytic activity and thermal stability during catalytic reactions [37]. CuO is a crucial active component that demonstrates good catalytic activity for SCR in the temperature range of 150–300 °C. Due to the valence changes of Cu²⁺ and Cu⁺ in CuO, it is commonly used as a promoter for Ce-based catalysts. The strong interaction between highly dispersed CuO and its support enhances the adsorption and activation of active substances [38]. Copper-cerium catalysts exhibit unique and compelling properties closely related to the characteristics of the copper-cerium oxide interface, particularly the chemical bonding patterns of copper species on the cerium oxide surface. The interaction between copper and cerium oxide is primarily achieved through a synergistic effect: cerium oxide effectively stabilizes the copper species, while the highly dispersed copper enhances the redox reaction of cerium oxide. This process involves the interplay between Cu²⁺/Cu⁺/Cu⁰ and Ce³⁺/Ce⁴⁺ ions. Notably, cerium oxide not only improves the dispersion of copper oxide but also structurally contributes to the overall catalytic performance [39]. The construction of a Cu-Ce composite metal catalytic system not only maximizes the properties of each component but also promotes the synergistic effects between them, thereby increasing the number of acidic sites and oxygen vacancies, which accelerates the catalytic reaction.

Cu-based and Ce-based metal oxides have been extensively investigated in the fields of NH₃-SCR and CO-SCR [53]; however, specialized studies on Cu-Ce-based bimetallic catalysts and their derivatives remain insufficient. To promote the industrial application of SCR technology and provide a solid theoretical basis and practical guidance for the design and optimization of future catalysts, a comprehensive review from a fresh perspective is necessary. This paper reviews the research progress of Cu-Ce bimetallic catalysts and their derivatives for NH₃-SCR and CO-SCR, focusing on summarizing the reaction mechanisms and the factors influencing catalyst performance. Specifically, the morphology, particle size, and formation of specific crystalline phases of the catalysts can be effectively regulated by selecting suitable preparation methods, thereby optimizing their catalytic activities. Additionally, doping other metals into Cu-Ce-based catalysts can improve reaction pathways, reduce the occurrence of side reactions, and enhance both the activity and selectivity of the catalysts. The use of high-quality supports not only improves the dispersion of the active Cu-Ce metals and increases the surface area but also provides necessary mechanical support, thereby enhancing catalyst stability. This paper also summarizes relevant studies on the antitoxicity and reaction mechanisms of the catalysts. Finally, the future development of Cu-Ce bimetallic catalysts and their derivatives is envisioned, with the aim of providing theoretical references for further optimizing their catalytic performance.

2. NH₃-SCR Catalysts

2.1. Effect Factors of NH₃-SCR Performance

2.1.1. Effects of Preparation Methods

In the synthesis of Cu-Ce-based catalysts, the preparation method is crucial. Key details include the synthesis technique, morphology, and the Cu/Ce ratio, all of which significantly influence the dispersion of the catalyst’s active components, the redox interactions between Cu and Ce, the formation of oxygen vacancies on the surface, and the specific surface area. These factors, in turn, have a substantial impact on the catalyst’s activity [54].

Commonly used catalyst preparation methods include co-precipitation, hydrothermal, impregnation, and sol-gel methods [55]. The co-precipitation method is easy to operate and inexpensive, but its post-treatment process is complicated. The hydrothermal method is environmentally friendly and suitable for synthesizing multi-metal catalysts, though it requires high-quality equipment and incurs higher costs. The impregnation method is straightforward and can enhance metal dispersion and catalytic activity, but its baking step is complex. Lastly, the sol-gel method offers high homogeneity and chemical stability, is energy-efficient, but it involves a cumbersome process. Thus, the choice of synthesis method is crucial for catalyst activity.

In recent years, researchers have optimized catalysts through various preparation methods to enhance their catalytic activity under different temperature conditions. For instance, Xing et al. [40] prepared CuCe catalysts using the co-precipitation method for the NH₃-SCR reaction, achieving a test speed of 40,000 h⁻¹ and a NO_x conversion of 100% within the temperature range of 160–200 °C. Chen et al. [41] employed the sol-gel method to synthesize CuO-CeO₂-TiO₂ catalysts, attaining over 90% NO_x conversion between 150–250 °C at an airspeed of 30,000 h⁻¹. Yao et al. [42] investigated the catalytic performance of CuO/Ti_0.95Ce_0.05 catalysts prepared via the impregnation method for low-temperature NH₃-SCR, achieving more than 80% NO_x conversion in the range of 200–300 °C with an airspeed of 120,000 h⁻¹. Hao et al. [47] utilized a microwave hydrothermal method to prepare CuO-Nb₂O₅/CeO₂ catalysts, reaching a NO_x conversion of 100% at 225–450 °C with an airspeed of 150,000 h⁻¹. These studies provide important references for further optimizing catalyst design and enhancing catalytic performance under various reaction conditions.

Zhang et al. [43] investigated the activity of CuO-CeO₂-ZrO₂ catalysts prepared by different methods in the NH₃-SCR reaction. The results show that the catalyst activity follows this order: co-precipitation > hydrothermal method > impregnation method > sol-gel method. Compared to other preparation methods, the catalysts prepared via the co-precipitation method exhibit the largest surface area, total pore volume, and the highest amount of Cu²⁺. This enhances the redox cycle of Cu²⁺ + Ce³⁺ → Cu⁺ + Ce⁴⁺, demonstrating excellent low-temperature activity. For composite catalysts, the advantages of the co-precipitation method are even more pronounced, as it allows for full contact between the active components, promoters, and supports. This enhances interactions, facilitates oxygen cycling, and aids in the formation of acidic sites and the adsorption of reactant molecules. As shown in Figure 1, Shuang et al. [56] developed a novel melt-ice precipitation method to prepare Ce_xCuTi-Ice catalysts, which proved comparable to the conventional precipitation method. The Ce_xCuTi-Ice catalysts exhibit a 20% higher NO_x conversion in the same temperature range compared to the Ce_xCuTi-Con catalysts prepared using the conventional method. The melt-ice precipitation method promotes good dispersion of crystalline nanoparticles and enhances catalytic activity by mixing the precursor solution in ultrapure water and forming ice cubes in a refrigerator. This allows for a slower release of the precursor solution, improving the interaction between the active ingredients, influencing the crystal structure, limiting microcrystalline growth, and promoting the dispersion of crystalline nanoparticles. The enhanced catalytic activity can be attributed to the improved ability of NO to oxidize to NO₂ at low temperatures and the rapid reaction of NO species with ligand NH₃ at high temperatures.

Synthesis methods significantly affect the morphology and properties of catalysts. By employing different preparation techniques, catalysts with various morphologies, such as Cu-Ce nanorods, nanocubes, and nanopolyhedra, can be obtained, which in turn alter their physical and chemical properties and enhance the specific redox capabilities of Cu-Ce. Zhuang et al. [57] demonstrated that the reaction properties of Cu-CeO catalysts synthesized through impregnation, sol-gel, two-step core-shell nanorods, and one-step hydrothermal core-shell nanotubes exhibit notable differences in morphology and properties. Specifically, the temperature windows of these catalysts follow this order: Cu-Ce/TiO₂ (impregnation method nanoparticles) < Cu-Ce@TiO₂ (nanospheres) < Cu-Ce@TiO₂ (nanotubes) < Cu-Ce/TiO₂ (sol-gel method nanoparticles). Among them, the nanoparticles prepared by the sol-gel method exhibit the highest dispersion and the lowest crystallinity, indicating that catalyst morphology plays a crucial role in the NH₃-SCR reaction. The excellent performance of CuCe catalysts arises from their complex copper-cerium oxide interfacial properties and interaction modes, which can be significantly influenced by different synthesis methods. According to Guo et al. [58], the copper-cerium synergistic interactions can be categorized into two types based on the strength of the interactions with the oxygen vacancies on the surfaces of copper oxide and cerium oxide: Strong Interaction (SI) and Weak Interaction (WI). When employing the complexation method, a Strong Interaction is formed between copper and cerium oxide. In this case, the oxygen atoms of copper oxide preferentially bind to the oxygen vacancies on the surface of cerium oxide, resulting in the formation of stable Ce-O-Cu bonds. This process helps stabilize the reduction characteristics of copper. Conversely, when the co-precipitation method is used, a Weak Interaction predominates. Here, an induced effect occurs between the oxygen atoms of copper oxide and the oxygen vacancies on the surface of cerium oxide, leading to an interaction distance between copper and cerium oxide that exceeds that of an intermolecular bond. This finding underscores the importance of optimizing the synthesis method to enhance catalytic performance.

The ratio of Cu to Ce is a crucial factor influencing the activity of Cu-Ce catalysts. By adjusting the Cu/Ce molar ratio, the distribution of Cu in Cu-Ce composite oxides can be effectively controlled, thereby enhancing the catalyst’s ability to remove NO in the NH₃-SCR reaction. As shown in Figure 2A, Xing et al. [40] synthesized (CeO_x)_mCuO composite oxides with varying Ce/Cu molar ratios and discovered that the (CeO_x)_0.25CuO ratio exhibits the highest NO conversion and the broadest operating temperature range. As the CeO_x content increases, a significant decrease in the intensity of the CuO phase diffraction peaks is observed in the XRD plots, which correlates with the increasing Ce composition as shown in Figure 2B. This reduction in CuO phase intensity ultimately leads to a gradual decline in the catalytic performance of (CeO_x)_mCuO. Ali et al. [14] demonstrated that Cu_x-Ce_0.5-x-Zr_0.5 catalysts significantly enhance NO removal performance. Specifically, the Cu_0.2-Ce_0.3-Zr_0.5 catalysts exhibit excellent SCR activity, nitrogen selectivity, and resistance to H₂O/SO₂ at low temperatures ranging from 150 to 270 °C. These catalysts also show remarkable durability against H₂O/SO₂ exposure. However, further increasing the Cu content while decreasing the Ce content leads to a decline in the SCR activity of the catalyst. This study suggests that the strong interactions between Cu and Ce oxides result in the formation of highly dispersed metal oxides, which provide enhanced adsorption mobility and improve the redox properties of the Cu_x-Ce_0.5-x-Zr_0.5 catalysts.

Additionally, the results from Li et al. [59] demonstrated that loading a very small amount of Cu onto Ce can significantly enhance catalytic activity at low temperatures by over threefold. They also observed that the activity of Ti₁Cu_y/CeO₂ catalysts display a volcanic trend with respect to Cu content; specifically, an initial increase in Cu content leadd to a rise in activity, followed by a subsequent decline. When the Cu/Ce molar ratio is 0.005, the NO conversion at 150 °C significantly improves from approximately 20% to 60% compared to Ti₁/CeO₂, with the catalyst exhibiting optimal low-temperature activity and excellent resistance to sulfur poisoning.

In summary, optimizing the preparation method, morphology, and Cu/Ce ratio of the catalysts leads to different structures that affect particle size, surface properties, the generation of oxygen vacancies, and the regulation of the valence state of the active centers. These factors significantly enhance the activity of the catalysts in the NH₃-SCR reaction. Such studies provide an important theoretical foundation and experimental basis for designing future catalysts, promoting progress in the field of environmental catalysis. With the continuous development of novel synthesis technologies, we have reason to believe that catalyst performance will further improve, thereby advancing cleaner energy use and environmental protection.

2.1.2. Effects of Modification/Doping

Cu-Ce catalysts exhibit excellent DeNO_x activity in the NH₃-SCR reaction due to their remarkable synergistic effects. The SCR reaction is a typical gas–solid heterogeneous catalytic process, with most of the reaction occurring on the surface of the catalyst. Therefore, the physical and chemical properties of the catalyst surface are crucial for the catalytic performance in SCR. As shown in Figure 3A, two main active sites are involved: the acidic site, where gas-phase NH₃ is adsorbed onto the catalyst surface, and the redox site, where NH₃ reacts with NO_x [38]. By doping transition metals, the redox properties, surface acidity, and overall performance of Cu-Ce catalysts can be further enhanced, leading to improved activity, stability, and selectivity in the NH₃-SCR reaction.

The doping of transition metal oxides (e.g., Mn and Nb) exhibits excellent redox properties and significantly promotes ammonia activation, thereby enhancing the SCR activity at low temperatures. Manganese oxides, in particular, can effectively activate ammonia and improve the SCR performance of catalysts under low-temperature conditions due to their strong oxidizing and reducing abilities. As shown in Figure 3B, Tang et al. [44] demonstrated that modifying cerium and cerium-manganese oxides significantly enhances the low-temperature activity of Cu-SSZ-39 catalysts in the NH₃-SCR reaction. Compared to the unmodified catalysts, the modified ones exhibit superior low-temperature SCR performance. The CeMnO_x/Cu-SSZ-39 catalysts achieve nearly 100% conversion at 200 °C, outperforming conventional Cu-SSZ-39 catalysts. The introduction of these doped oxides not only improves the redox performance of the catalysts but also enhances their surface acidity, which is beneficial for the low-temperature SCR reaction.

Niobium (Nb) is widely used as a promoter in cerium dioxide-based NH₃-SCR catalysts to enhance their physicochemical properties through strong interactions with cerium dioxide while inhibiting the excessive oxidation of NH₃. Hao et al. [47] demonstrated that Nb-doped Cu-CeO₂ catalysts achieve 100% NO_x conversion over a broad temperature range of 250–550 °C. The efficiency of these Nb-doped Cu-CeO₂ catalysts can be attributed to the introduction of a moderate amount of Cu ions into the CeO₂ lattice, which induces structural distortion and further facilitates the doping of Nb ions. The synergistic interaction between embedded Nb and the Cu and Ce components generates new adsorption sites for NO and NH, forming abundant oxygen vacancies in the interfacial region and promoting redox reactions among Ce, Nb, and Cu. This significantly enhances the activity, N₂ selectivity, and operating temperature window of the NH₃-SCR process. Wang et al. [45] found that unmodified CuCeTi catalysts exhibit only 60% NO removal in the 240–300 °C range, while Nb-modified CuCeTi catalysts achieve over 90% NO conversion in the 180–360 °C range. They discovered that Nb acts as a binder, promoting strong interactions among the catalyst components, which leads to the formation of abundant oxygen vacancies. This not only significantly enhances SCR activity but also inhibits the over-oxidation of NH₃, effectively prolonging the operating temperature window of the SCR process and improving N₂ selectivity.

The introduction of acidic metal oxides, such as molybdenum (Mo) and tungsten (W), into the transition metals can significantly increase the Brønsted and Lewis acidic sites on the catalyst surface. This enhancement improves the dispersion of the Cu-Ce catalysts, increases the quantity of surface reactive oxygen species, and enhances their acidity. Ultimately, these changes contribute to improved performance in selective catalytic reduction.

Mo-based catalysts typically exhibit excellent acidity, which enhances their NH₃ adsorption capacity and subsequently improves the selectivity and efficiency of reactions. Shen et al. [60] demonstrated that Mo-modified CuCeO_x catalysts possess good catalytic activity at low temperatures, along with excellent long-term stability and resistance to H₂O/SO₂. Additionally, there is a significant enhancement in selectivity toward N₂. H₂-TPR results indicate that the doped Mo species could promote the redox performance of the catalysts at low temperatures, which is closely related to the structural effects and uniform dispersion of the smaller Cu particles. As shown in Figure 4A, the incorporation of Mo also improves the thermal stability of the catalyst at strong acid sites, leading to increased NH₃ conversion and NO removal efficiency. Furthermore, as shown in Figure 4B, Mo doping facilitates oxygen migration and enhances the formation of oxygen vacancies, all of which significantly contribute to the catalytic activity.

On the other hand, tungsten oxide has an abundance of chemical defects that have a significant impact on its electron-leaping behavior. Notably, the increase of ionic defects decreases the band gap of the catalyst. It has been shown that there is a linear relationship between the apparent activation energy of the oxidation reaction and the bandgap of the catalyst: the smaller the bandgap, the lower the activation energy of the redox reaction, which improves the catalytic activity [61]. In addition, WO₃ can accommodate a high concentration of oxygen vacancies, which creates conditions for the introduction of localized energy states within the band gap [62]. These localized states strongly interact with CeO₂, forming ligand-unsaturated cationic sites and increasing the number of Lewis acid sites. This modification promotes electron transfer and increases the proportion of antibonding orbitals, which leads to longer metal-oxygen bonds and weaker chemical bond strength. As a result, the activation of lattice oxygen is enhanced, accelerating oxygen cycling and significantly improving the redox properties of the catalyst [63]. Ultimately, these changes contribute to a substantial improvement in the efficiency of the NH₃-SCR reaction [46]. As shown in Figure 5, Hao [46] used an impregnation method to prepare highly dispersed WO_x-modified Cu-doped CeO₂ nanorods, which exhibit excellent DeNO_x catalytic activity across a wide range of operating temperatures. This remarkable performance is attributed to the high dispersion of WO_x species and the incorporation of Cu into the CeO₂ lattice, which results in an increased concentration of Ce³⁺ ions and oxygen vacancies on the surface, thereby providing more active sites for the trapping and dispersion of WO_x. DFT calculations indicate that the WO_x species gain very little energy on CeO₂, leading to a decrease in electron density and an increase in the electrophilicity of the Cu-CeO₂ surface, which could elevate Lewis acidity. Additionally, the introduction of WO_x could increase Brønsted acidity, further enhancing SCR activity.

In summary, the performance of Cu-Ce catalysts in the NH₃-SCR reaction is significantly enhanced by doping with transition metal oxides, such as Mn, Nb, Mo, and W. The introduction of these metals not only improves the redox capacity of the catalyst but also enhances its surface acidity, facilitating the adsorption and activation of NH₃. This, in turn, increases both catalytic activity and selectivity at low temperatures. Specifically, Mn and W effectively enhance the surface acidity of the catalyst and improve reaction selectivity, while Nb extends the working temperature window and enhances N₂ selectivity by enriching oxygen vacancies and promoting synergistic interactions among the components. Additionally, the doping of Mo remarkably enhances the long-term stability and resistance of the catalysts. In conclusion, these modification strategies offer new insights for optimizing the application of CuCe-based catalysts in NO_x removal, demonstrating significant potential in the field of environmental treatment. Future studies should further explore the combination of different transition metals and their synergistic effects in catalytic reactions to achieve more efficient and environmentally friendly catalyst designs.

2.1.3. Effects of Supports

The dispersion of CuCe-based catalysts on supports enhances both the catalytic performance of NH₃-SCR and the stability of the catalyst. By improving the pore structure, acidity, and mechanical stability of the support, these advancements facilitate an effective interaction between the support and the active material, thereby optimizing the structure and overall performance of the catalyst.

TiO₂ is the most commonly used support in NH₃-SCR catalysts, serving as the focal point of catalytic activity due to its ability to provide abundant acid sites, as well as a high specific surface area and pore volume. As shown in Figure 6, Zeng et al. [64] demonstrated that loading Ti onto Cu/Ce catalysts broadens the temperature window of these catalysts and enhances their catalytic activity. The introduction of Ti species induces the formation of a three-dimensional pore structure and facilitates the phase transition from crystalline to amorphous catalysts. This process significantly increases the specific surface area and the dispersion of CuO and CeO₂. The well-dispersed CuO and CeO₂ species provide more adsorption sites for CO and NO, promote stronger Cu-Ce interactions, and lead to the formation of more surface oxygen vacancies. These changes contribute to the generation of large amounts of reactive oxygen species, reducing the competition for adsorption and activation between CO and NO, thereby enhancing the efficiency of the Ti-modified CuO/CeO₂ catalysts in NH₃-SCR denitrification reactions. Furthermore, it has been pointed out that the enhancement of catalytic activity may be closely related to the morphology of TiO₂. For instance, Song et al. [65] synthesized TiO₂ supports with different structural units, including flower-like spheres with elongated nanorods, nanosheet-assembled spheres with broad and short nanorods, and particle-assembled spheres. Among these, the floral spherical TiO₂ morphology with elongated nanorods exhibits the best catalytic performance, which can be attributed to its larger pore structure and higher specific surface area. This morphology provides sufficient dispersion of the active components, enhances the adsorption of reactants, and exposes more active sites.

Pristine zeolites, with large specific surface areas and abundant acid sites, have attracted considerable attention [66,67,68,69]; however, their SCR catalytic activity is generally poor [70]. The incorporation of active CuCe-based catalysts into zeolites significantly enhances NH₃-SCR performance [71]. As shown in Figure 7, Wu et al. [72] investigated the effects of cerium and copper doping in SAPO-18 catalysts with an AEI-type framework (CuCe-SAPO-18), which exhibits a hybridized crystal structure. This hybridized structure facilitates a broader temperature operating range. XPS and DFT results indicate that the addition of small amounts of Ce influences the reactivity of NH₃-SCR by affecting either the copper species or the molecular sieve structure. Furthermore, the number of Cu²⁺ species and the acidity of the Cu-based zeolite SCR catalysts are critical factors in achieving efficient low-temperature denitrification.

The creation of mesopores can modify the structure of zeolites and enhance their catalytic efficiency. Compared to conventional microporous zeolites, mesoporous zeolites exhibit higher activity and long-term stability at low temperatures. Ma et al. [73] used Cu-Ce co-modified mesoporous ZSM-5 molecular sieves (MZ) for the NH₃-SCR reaction, comparing them with conventional ZSM-5 and SBA-15 molecular sieves. The results indicate that the superior catalytic activity of Cu-Ce/MZ could be attributed to the uniform distribution of active components on the surface of MZ, in contrast to the surfaces of microporous ZSM-5 and mesoporous SBA-15. Characterization analyses reveal a high specific surface area, more and uniformly distributed active sites, abundant surface oxygen vacancies, and strong acidity. These properties contribute to the excellent SCR performance of Cu-Ce/MZ.

Metal oxide-loaded carbonaceous materials exhibit high catalytic activity for NH₃-SCR, with the performance of these carbon-loaded catalysts being significantly influenced by the pore structure and surface properties of the loadings [74,75,76]. Ordered mesoporous carbon (OMC) is a novel type of carbon material characterized by excellent structural properties, including remarkable chemical stability, a large specific surface area, and uniform pore size. Consequently, OMCs are regarded as exceptional catalyst support materials. As shown in Figure 8, Chen et al. [77] synthesized bimetallic cerium-copper nanoparticles on OMCs with varying Ce/Cu ratios and investigated their activity in the ammonia selective catalytic reduction of NO. They found that an optimal Ce/Cu ratio could markedly enhance the catalytic performance of the OMCs, with Ce₅Cu₅-OMC demonstrating the best catalytic activity. All synthesized samples exhibit an ordered mesoporous structure. The incorporation of Ce or Cu into the OMCs increases the number of acidic oxygen functional groups on the surface, thereby enhancing the acidity of the OMCs, particularly in Cu-rich OMCs. The surface Cu²⁺ ions may facilitate the activation of NH₃, which is crucial for the SCR reaction.

In comparison to conventional carbon materials like activated carbon (AC) and activated carbon fibers (ACFs), carbon nanotubes (CNTs) possess unique electronic properties and adsorption selectivity. Yang et al. [78] prepared activated carbon and carbon nanotube composites (CAC-CNTs) as supports through in situ growth and introduced copper to create Cu_xCe/CAC-CNTs for studies of low-temperature NH₃-SCR catalytic reactions. The results demonstrate that Cu₀.₂Ce/CAC-CNTs achieves the highest performance, with 100% NO conversion and 95.8% N₂ selectivity at 150 °C. The incorporation of Cu into the Cu_0.2Ce/CAC-CNT catalysts enhances the Lewis acid sites and lattice oxygen, thereby accelerating the adsorption of NH₃ onto the acidic sites. Additionally, the presence of Ce facilitates the redox cycling of Ce³⁺ + Cu²⁺ ↔ Ce⁴⁺ + Cu⁺ electron transfer, which improves catalytic activity. Analysis of the pore structure of the Cu_0.2Ce/CAC-CNT catalyst reveals a diverse pore architecture that supports the uniform dispersion of the loaded metal oxide catalysts. This highly dispersed configuration provides numerous active sites that are favorable for surface catalytic reactions, ultimately enhancing the activity of the SCR reaction.

In summary, the performance enhancement of CuCe-based catalysts in the NH₃-SCR reaction primarily depends on the optimization and design of the supports. By rationally selecting and modifying supports such as titanium dioxide, zeolites, mesoporous materials, and carbonaceous materials, we can significantly improve the specific surface area, distribution of active sites, and acidity of the catalysts, thereby enhancing their catalytic performance and stability. Research has demonstrated that the pore structure and surface properties of these supports directly affect the dispersion of active substances and the adsorption capacity of reactants, making the catalytic reaction more efficient. These findings provide an important theoretical foundation and practical guidance for the future development of efficient and stable NH₃-SCR catalysts, which will aid in addressing environmental pollution problems and promoting sustainable development.

2.2. Anti-Toxicity Properties of NH₃-SCR Reactions

In industrial applications, impurities such as sulfur dioxide (SO₂) [79,80,81], alkali metals [82,83,84], and moisture [85,86] frequently present in flue gas can significantly affect the activity and stability of catalysts. Specifically, during combustion, flue gas often contains substantial amounts of water, which can lead to catalyst deactivation due to competitive adsorption. Some researchers have noted that the interaction between water and Lewis acid sites is weaker than that with Brønsted acid sites [87]; thus, increasing the proportion of Lewis acids may enhance the water resistance of the catalysts. For instance, as shown in Figure 9A, Ma et al. [88] demonstrated that the CuFeCeTiO_x catalysts, formed after loading Ce onto CuFeTiO_x catalysts, achieved a NO_x conversion of over 90% in the temperature range of 175 °C to 360 °C, comparable to that under anhydrous conditions. This phenomenon is attributed to the modification by Ce, which alters the acid distribution on the catalytic surface and improves the characteristics of Lewis acids, resulting in excellent water resistance of the catalyst.

In SO₂-containing flue gas, catalysts are prone to severe deactivation due to the deposition of (NH₄)₂SO₄, NH₄HSO₄, and metal sulfates, which block and damage the active sites. Shi et al. [89] found that, compared to CeWO_x catalysts, the NO_x conversion of CuCeWO_x decreases only slightly upon the addition of SO₂, indicating that CuCeWO_x exhibits better SO₂ tolerance. The XRD results show that the intensity of all peaks for CeWO_x is significantly weaker than that for the Cu-loaded CuCeWO_x catalysts, which is attributed to the generation of sulfate species that cover the surface of CeWO_x, thereby decreasing its SO₂ poisoning tolerance.

To address the issue of SO₂ poisoning, several strategies can be employed, including increasing surface acidity, designing adsorption structures, blocking the oxidation pathway, facilitating the conversion of reactive nitrate species, and introducing sacrificial sites. For instance, Mu et al. [90] investigated the effect of sulfate modification on the SO₃ resistance of the NH₃-SCR catalyst CeSnTiOx. Their results show that the catalytic performance of CeSnTiOx decreases following the introduction of SO₂. In contrast, the NO conversion of the copper sulfate-modified 5Cu-S/CeSnTiO catalyst remains stable at approximately 80% when SO₂ is passed through it at 240 °C and recovered to 80% after the SO₂ passage is stopped. These findings indicate that copper sulfate modification effectively inhibits the adsorption of SO₂, thereby enhancing the catalyst’s performance.

Figure 9. (A) NH₃-SCR activity on CuFeTiO and CuFeCeTiO_x with 10% H₂O [88]. (B) NH₃-SCR activity over K-poisoned catalysts Ti/Ce, TiCu/C [91].

Figure 9. (A) NH₃-SCR activity on CuFeTiO and CuFeCeTiO_x with 10% H₂O [88]. (B) NH₃-SCR activity over K-poisoned catalysts Ti/Ce, TiCu/C [91].

Alkali metals in flue gas can deposit on the surface of the catalyst, leading to a reduction in specific surface area, pore volume, and pore diameter, which weakens its adsorption capacity for the raw gas and inhibits the reaction [92]. Additionally, alkali metals can interact with the active sites of the catalyst, reducing its redox properties and acidity, further hindering the adsorption and activation of reactive species and affecting its denitrification activity. Modifying catalysts to increase the number of acidic sites and facilitate redox reactions is an important strategy to mitigate the adverse effects of alkali metal poisoning. Studies have shown that potassium oxide (K₂O) and potassium chloride (KCl) significantly reduce the activity of CeO₂ catalysts, while CuO, as an additive, can enhance the reducibility of the catalysts and protect the active CeO₂ species. As shown in Figure 9B, Zhao et al. [91] proposed a CuO modification method to improve the tolerance of CeTi catalysts towards K. They found that the NO conversion of the pristine Ti/Ce catalyst decreases from 95% to less than 10% after 1 wt.% K loading at 250 °C. In contrast, the CuO-modified Ti/Ce catalyst retains more than 70% activity in NO conversion under the same conditions. Their study concludes that CuO modification also inhibits the adsorption of inert nitrate after K loading. Furthermore, the Brønsted acid sites introduced by CuO act as capture sites for alkali metals, alleviating the poisoning of the active sites and enhancing the catalytic performance of the CuTi/Ce catalysts in the presence of K poisoning.

As shown in Figure 10, Ye et al. [93] investigated the resistance of metal-doped Ce/TiO₂(001) surfaces to potassium (K) poisoning, demonstrating that the seven transition metals exhibit varying degrees of resistance, ranked in the order of Cu > Co > Sb > Mo > Zr > Nb > W. This trend correlates with the number of oxygen vacancies formed, suggesting that these metals facilitate the decomposition of the by-product N₂O while enhancing the trapping ability of NH₃ and NO, thereby promoting the low-temperature NH₃-SCR reaction. The study reveals that the introduction of the transition metal Cu not only creates additional Lewis acid sites but also improves the adsorption capacity of NH₃ on the surface. This effectively inhibits K adsorption and reduces the sensitivity of CeTi catalysts to K poisoning. Furthermore, Cu doping lowers the energy barriers for NH₃ dehydrogenation, NH₂NO generation and decomposition, and NO₂ generation on the CeTi catalysts, which diminishes the inhibitory effects of K on these reactions.

In summary, the stability of catalysts used in flue gas treatment is influenced by various factors, including the presence of moisture, SO₂, and alkali metals. The tolerance and activity of these catalysts can be significantly enhanced through rational modifications, such as increasing Lewis acid sites, introducing beneficial metal components, and optimizing catalyst structures. These studies not only improve the performance of catalysts but also contribute to environmental protection and economic benefits, thereby promoting the goal of sustainable development.

2.3. NH₃-SCR Reaction Mechanism

The study of NH₃-SCR reactions, particularly the role of catalysts, holds significant scientific and practical importance. These studies can illuminate catalyst properties, optimize design and preparation processes, and drive the development of more efficient catalysts. Therefore, an in-depth exploration of the mechanisms involved in the reaction is crucial. Currently, two main mechanisms are recognized in the NH₃-SCR reaction: The ER mechanism and the LH mechanism [13,94]. In the ER mechanism, ammonia molecules are first adsorbed on the catalyst surface and then react directly with nitrogen oxides (e.g., NO or NO₂) in the gas phase, without first requiring the nitrogen oxides to be adsorbed on the catalyst surface. In contrast, the LH mechanism involves the separate adsorption of ammonia and NO_x onto the catalyst surface, followed by their interaction and reaction on the surface, ultimately producing nitrogen and water. The choice between these two mechanisms primarily depends on the adsorption states of ammonia and NO_x, with the nature of the catalyst and reaction environment potentially allowing for both mechanisms to operate simultaneously under different conditions.

Different catalytic systems and reaction conditions significantly influence the choice of reaction mechanism. Jia et al. [95] investigated the adsorption of NH₃ and NO on the catalyst surface at 175 °C using in situ infrared techniques, concluding that the catalyst follows the ER mechanism in the NH₃-SCR reaction. In this process, ammonia is adsorbed on the catalyst surface in the form of adsorbed NH₃ and NH₂, which then reacts with NO and O₂ to produce N₂ and H₂O. Conversely, Yang et al. [78] demonstrated that the SCR reaction of Cu-doped Ce/CAC-CNT catalysts primarily follows the LH mechanism. As shown in Figure 11A, gaseous NH₃ in this reaction is first adsorbed on Lewis and Brønsted acid sites as NH₃⁺ and NH₄⁺. Simultaneously, gaseous O₂ is adsorbed and activated through electron transfer between CuO and CeO₂. NO, adsorbed on the catalyst surface, is oxidized via surface-activated oxygen (O*) and subsequently reacts with NH₄⁺ to form the unstable intermediate product NH₄NO₂, which decomposes into N₂ and H₂O.

The same catalyst system may follow both ER and LH mechanisms. As shown in Figure 11B, Yan et al. [96] demonstrated that the NH₃-SCR reaction over CuMnTiO_x catalysts can be explained by both mechanisms. In the ER mechanism, adsorbed NH₃ is converted to NH₂, which then reacts with gaseous NO to form highly reactive NH₄NO₂/NH₂NO intermediates. In contrast, the LH mechanism involves a reaction between adsorbed NH₃ and NO. Similarly, as shown in Figure 11C, Qin et al. [97] investigated the NH₃-SCR reaction on CuCeO_x catalysts modified with Nb-doped species, finding that it also follows both ER and LH mechanisms. The Nb modification increases the concentration of NH₃(a) and NO_x(a) species on the CuCeNbO_x samples. Gaseous NO could react with either the adsorbed NH₃(a) or NH₂ formed by the oxidative dehydrogenation of NH₃(a) to produce environmentally friendly N₂ and H₂O, consistent with the ER mechanism. Additionally, after Nb modification, new species such as NH₄⁺-B and increased amounts of NH₃(a)-L and NO_x(a) are observed on CuCeNbO_x. The adsorbed ammonia species can then combine with gaseous NO₂ or adsorbed NO₃⁻ species to generate N₂ and H₂O via the LH mechanisms.

The reaction mechanism of the same catalyst can change with varying reaction temperatures. As shown in Figure 11D, Ali et al. [14] demonstrated, using DRIFTs, that the reduction of NO to N₂ by NH₃ at high temperatures (above 200 °C) mainly follows the ER mechanism. In this process, the SCR reaction occurs between NO(g) and NH₃(a). Initially, the absorbed NH₃ is converted to NH₂, which then reacts with NO(g) to form the reactive and highly unstable intermediates NH₂NO/NH₄NO₂, which rapidly decompose to produce N₂ and H₂O. Conversely, at low temperatures (below 200 °C), the reduction of NO to N₂ by NH₃ primarily follows the LH mechanism, where NH₃(a) and NO(a) undergo the SCR reaction. On Cu_0.2-Ce_0.3-Zr_0.5 catalysts, NH₃ coordinated to Lewis acid sites and NH₄⁺ on Brønsted acid sites can react with adsorbed NO (LH mechanism) and gaseous NO (ER mechanism).

The exploration of these mechanisms reveals the complexity of the reaction processes under varying catalyst and temperature conditions. A thorough understanding of these mechanisms not only deepens our insight into the reactions themselves but also provides a crucial theoretical foundation for the design and optimization of catalysts. This knowledge will ultimately promote the development of more efficient NO_x removal technologies.

Figure 11. (A) NH₃–SCR mechanism diagram of Cu_0.2Ce/CAC-CNTs [78]. (B) Schematic diagram of NH₃-SCR reaction mechanism [96]. (C) Promotion mechanism of doping Nb species into CuCeO_x catalyst for NH₃-SCR reaction [98]. (D) Reaction mechanism of NH₃-SCR over Cu_0.2Ce_0.3Zr_0.5 [14].

Figure 11. (A) NH₃–SCR mechanism diagram of Cu_0.2Ce/CAC-CNTs [78]. (B) Schematic diagram of NH₃-SCR reaction mechanism [96]. (C) Promotion mechanism of doping Nb species into CuCeO_x catalyst for NH₃-SCR reaction [98]. (D) Reaction mechanism of NH₃-SCR over Cu_0.2Ce_0.3Zr_0.5 [14].

3. CO-SCR Catalysts

3.1. Effect Factors of CO-SCR Performance

3.1.1. Effects of Preparation Methods

In the field of catalyst research, CuO-CeO₂ has garnered significant attention due to its excellent performance in the reduction of NO_x. By employing various preparation methods, the morphology and structure of the catalyst can be effectively adjusted, allowing for the selection of an optimal Cu to Ce ratio to enhance the composition and performance of the catalyst. This strategy not only significantly improves reaction efficiency but also reduces the generation of by-products, thus providing robust support for achieving the goals of green environmental protection.

The preparation method is a crucial factor in determining the phase composition, conformation, and dispersion of active components, which significantly impact the activity and selectivity of catalysts. For instance, the performance ranking of catalysts synthesized by various techniques, such as mechanical mixing and impregnation, indicates that well-dispersed CuO species and strong interactions through the CuCe surface are key to enhancing catalytic effectiveness. Yao et al. [99] synthesized CuO-CeO₂ catalysts using five different preparation methods: mechanical mixing method (MMM), impregnation method (IM), grinding method (GM), hydrothermal treatment method (HTM), and co-precipitation method (CPM). The catalytic performance of these catalysts was ranked as follows: CuCe-IM > CuCe-CPM > CuCe-GM > CuCe-HTM > CuCe-MMM. This ranking correlates with variations in surface oxygen vacancy concentration, reducibility, and Cu content in the catalysts, suggesting a close synergistic effect between Cu species and the surface oxygen vacancies of CuO-CeO₂. Additionally, these CuO-CeO₂ catalysts exhibit considerable activity at temperatures below 175 °C. Other researchers have prepared CuCe composite oxide catalysts using different synthesis methods. Catalysts featuring two-phase CuO-CeO₂ were produced via a simple milling method [100], achieving a NO conversion of 87.2% at 200 °C. Cu/CeO₂ catalysts prepared through the impregnation method attain a conversion of over 90% for NO and CO in the 300–400 °C range [48]. As shown in Figure 12, Rh/CeCuO catalysts prepared by the co-crystallization method reach a NO conversion of over 90% at 162 °C [49]. Furthermore, hollow spherical LaCeCuFeO catalysts were synthesized using solvothermal synthesis, achieving 100% NO conversion in the 250–500 °C range [101].

Recent studies have demonstrated that catalysts prepared using advanced techniques, such as the colloidal crystal template method and the supercritical hydrothermal method, can achieve nearly 100% conversion of NO_x within specific temperature ranges. These findings not only offer new insights into catalyst design but also advance the development of environmental treatment technologies. As shown in Figure 13A, Ye et al. [50] utilized colloidal crystal templates to create three-dimensionally ordered macroporous (3DOM) copper-cerium binary composite oxide catalysts aimed at the selective catalytic reduction of CO to NO. Their results indicate that the 3DOM Cu₁Ce₁ catalysts exhibit the broadest temperature window, achieving 100% NO_x conversion in the range of 450–700 °C. Similarly, Dai et al. [51] synthesized Cu-doped CeO₂ nanocatalysts using the supercritical hydrothermal method, achieving 100% NO_x conversion at 150 °C. The SCR activity of the supercritical hydrothermal (SC-H₂O) route significantly surpasses that of the conventional hydrothermal route, further confirming the advantages of the SC-H₂O approach. As shown in Figure 13B, Liu et al. [102] found that CuO/CeO₂ nanorods demonstrate higher surface reducibility, activity, and N₂ selectivity for the reduction of NO below 250 °C compared to polyhedral and cubic structures. This is attributed to the incorporation of Cu²⁺ ions into the pore and surface lattices by occupying vacancies in the CeO₂ nanorods, which enhances interactions between Cu and Ce. In contrast, some CuO₂ species tend to segregate on the surfaces of CeO₂ cubes with larger particle sizes.

Optimizing the ratio of Cu to Ce can significantly enhance the conversion efficiency of NO in the CO-SCR reaction while reducing by-product generation and improving selectivity. Deng et al. [100] investigated the intergrowth and symbiosis phenomena of CuO-CeO₂ catalysts, finding that introducing an appropriate amount of CuO increases catalyst activity. However, excessive CuO leads to surface agglomeration, resulting in decreased activity. In terms of selectivity, Sun et al. [103] demonstrated that an optimal proportion of Cu-doped CeO₂ catalysts increases the concentration of oxygen vacancies and facilitates the formation of (NO)₂ dimers, thereby inhibiting the production of N₂O at low temperatures. Ye et al. [50] studied 3DOM Cu-Ce bimetallic oxide catalysts, showing that performance is closely related to the molar ratio of Cu and Ce. A reasonable molar ratio optimizes the pore structure and specific surface area of the catalyst, enhancing contact between the active gas and the active sites. Additionally, the Cu to Ce ratio significantly influences the adsorption capacity of CO, which affects the consumption of reactive oxygen species and the formation of oxygen vacancies, ultimately impacting catalytic performance.

The NO conversion efficiency in the CO-SCR reaction can be significantly enhanced, and the generation of by-products can be effectively reduced by adopting diverse catalyst design methods, selecting the optimal morphology, and rationally regulating the ratio of Cu and Ce. These studies lay an important theoretical foundation for the development of catalysts and provide practical solutions for environmental management in real-world applications.

3.1.2. Effects of Modification/Doping

In the field of catalyst research, Cu-Ce catalysts have garnered significant attention due to their excellent performance in CO-SCR reactions. To enhance the performance of these catalysts, researchers have improved their structure and catalytic activity by doping them with various elements, such as magnesium (Mg) [104], manganese (Mn) [105], and iron (Fe) [106,107]. This doping strategy aims to increase the specific surface area of the catalyst and improve the dispersion of active substances, thereby enhancing the efficiency of the catalytic reaction. In this paper, we discuss the recent research progress of Cu-Ce catalysts, focusing particularly on the impact of transition metal doping on their catalytic performance.

The doping of transition metals such as Mg and Mn can increase the surface area of catalysts and further promote the dispersion of active substances, thereby improving the overall performance of the catalysts. Chen et al. [52] found that magnesium doping significantly enhances the performance of Cu-Ce catalysts in the CO-SCR reaction. Characterization results show that magnesium doping forms a Mg_xCe_1−x/₂O₂ solid solution with high defect density, which facilitates the efficient embedding of copper ions in the CeO₂ matrix. Additionally, the hydrolysis of magnesium oxides increases the pH of the impregnation solution, leading to the homogeneous formation of Cu₂(NO₃) (OH)₃ precipitates. This process promotes effective dispersion of CuO and enhances the penetration of copper ions. The evolution of magnesium oxide also improves the specific surface area and porosity of the catalysts, providing more channels for the diffusion of reactants. Furthermore, Deng et al. [108] demonstrated tha Mn-doped CuO/CeO₂ catalysts exhibit excellent catalytic activity in the CO-SCR reaction. At 300 °C, the NO_x conversion increases from about 10% without Mn-doped catalysts to 100% with Mn-doped catalysts, with a selectivity close to 100%. Adequate doping of MnO_x in the CeO₂ lattice enhances the specific surface area, which improves the dispersion of CuO and, in turn, boosts catalytic activity and thermal stability.

Fe₂O₃ exhibits excellent low-temperature catalytic activity due to its moderate adsorption strength for CO and NO. Most Fe₂O₃-containing catalysts demonstrate high denitrification efficiencies in the temperature range of 250–400 °C, along with exceptional resistance to H₂O and SO₂. As shown in Figure 14, Shi et al. [109] demonstrated that Ce species play a dominant role in the reaction process of CuFe/Ce catalysts by promoting the formation of active Cu species and enhancing their stability. The predominant Ce⁴⁺ species facilitate a redox equilibrium reaction (Ce⁴⁺+ Cu⁺ → Ce³⁺+ Cu²⁺), which drives the equilibrium to favor the Cu²⁺ state, thereby inhibiting the formation of metallic Cu(Cu^⁰) to some extent. This results in a cyclic transition between Cu²⁺ and Cu⁺ during reactions conducted below 400 °C, further promoting the formation and enhancing the stability of reactive Cu species. In higher temperature reaction environments (T ≥ 750 °C), Ce⁴⁺ species similarly engage in a reaction (Ce⁴⁺ + Fe²⁺ → Ce³⁺+ Fe³⁺) that converts Fe²⁺ to Fe³⁺, thereby inhibiting the formation of metallic iron (Fe⁰) under these conditions. In this scenario, the newly generated CeFeO₃ plays a critical role, indicating that Ce species can stabilize iron and maintain catalytic activity. Consequently, the synergistic stabilization of Cu and Fe by Ce species results in a catalyst with a broader activity window. Cheng et al. [110] found that the catalytic performance of the Cu/CeFe catalysts surpasses that of the Cu/CeO samples in the range of 100–200 °C, with NO conversion exceeding 95% at 150 °C. The catalysts are characterized by cubic Ce and cubic CeO₂, with results indicating that cubic CeO₂ can serve as a lattice framework for mixed oxides. In the CuFe sample, the Fe₂O₃ phase primarily forms on the surface of the CeO₂ lattice, with these “top” Fe₂O₃ phases contributing to the incorporation of Fe atoms into the CeO₂ lattice. In situ DRIFTs studies have demonstrated that Fe₂O₃ serves as a storage site for nitrite and nitrate, facilitating the adsorption of nitrogen oxides and promoting the adsorption and transformation of nitrite/nitrate on the Cu/CuFe catalyst. This may account for its excellent catalytic activity in the NO + CO reaction. It is important to note that variations in the dispersion states of Fe result in differing interactions among Cu, Fe, and Ce, leading to distinct mechanisms for enhancing CO-SCR catalytic activity.

The effect of different dispersion states of Fe on the catalytic performance was investigated for Fe-modified Cu/Ce₀.₁Al catalysts prepared by Bai et al. [111]. Both the Cu/Fe/Ce_0.1Al catalyst, prepared by impregnation, and the Cu/FeCe_0.1Al catalyst, prepared by the co-precipitation method, demonstrate significant performance enhancements compared to the initial state of Ce_0.1Al. Among these, the introduction of Fe via the co-precipitation method proves to be more effective, as it results in smaller particle sizes and higher specific surface areas, which in turn improve redox and adsorption performance. In contrast, when Fe is introduced by the impregnation method, the enhanced catalytic performance is primarily attributed to the enrichment of more active substances on the catalyst surface. This contributes to the formation of defects and reduces the amount of adsorbed oxygen on the surface.

In recent years, significant advances in materials preparation science and technology have enabled the controlled synthesis of materials with specialized structures, specific morphologies, and exposed crystalline surfaces. This is particularly evident in copper-based catalysts derived from metal-organic frameworks (MOFs) [112,113] and hydrotalcites (LDHs) [114], which demonstrate excellent catalytic performance. The specific surface area and pore structure of these catalysts are known to influence their SCR denitrification activity to a considerable extent. For instance, Qian et al. [115] investigated the application of Cu₃Ce_xAl_(1−x) catalysts, based on hydrotalcite-like compounds derived from LDHs, in the CO-SCR reaction. Their findings reveal that the layered structure facilitates electron transfer, preventing the recombination of electrons and holes. Additionally, a moderate addition of CeO₂ significantly improves the dispersion of CuO on the catalyst surface and the homogeneous distribution of active centers, thereby enhancing reduction capability. Similarly, Qin et al. [116] reported that the Cu-BTC catalyst achieves 100% NO conversion at 279 °C, while the Ce-Cu-BTC catalyst achieves the same conversion at a lower temperature of 253 °C. Scanning electron microscope (SEM) images confirm that both Cu-BTC and Ce-Cu-BTC exhibit distinct octahedral shapes. During treatment at 600 °C, some cracks appeared on the surface of CeO_x/CuO_y/C, and the specific surface area reached 170 m²/g. This large specific surface area and abundant pore structure facilitate mass transfer, adsorption, and activation of reactants during the reaction process. Furthermore, the uniform distribution of metal nodes within the MOF framework provides numerous potential active sites for the CO-SCR reaction. The incorporation of Ce atoms enhances the availability of activated Cu sites, thereby promoting the catalytic reaction.

In summary, doping transition metals such as magnesium, manganese, and iron into CuCe catalysts not only improves the structure and performance of the catalysts but also enhances the dispersion of active substances and catalytic activity. The doping of magnesium enhances the embedding ability of Cu ions in the CeO₂ matrix, while the introduction of manganese significantly improves the NO_x conversion of the catalysts. Additionally, iron contributes to the stability of the catalysts by strengthening the interactions between the metals. Furthermore, the use of advanced material preparation methods, such as MOFs and LDHs-derived catalysts, has broadened the application prospects of CuCe catalysts. These catalysts facilitate the mass transfer and activation of reactants by improving the specific surface area, pore structure, and uniform distribution of active sites, which in turn enhances catalytic performance [117]. With in-depth studies on catalyst mechanisms and performance enhancement strategies, future research is expected to achieve significant breakthroughs in increasing the catalytic efficiency and stability of these catalysts, thereby promoting the widespread application of CuCe catalysts in environmental treatment and industrial applications.

3.1.3. Effects of Supports

The selection of an optimal catalyst support is critical for enhancing denitrification activity. High-quality supports can significantly increase the specific surface area, provide abundant reaction sites, promote the uniform dispersion of active components, and enhance the interaction between the supports and these active components [118]. The composition of the supports has a substantial effect on the performance of CuCe composite metal oxide catalysts in the selective reduction of NO by CO. Zahra Gholami’s study [119] demonstrated that the catalytic activities of CuCe/Al₂O₃ and CuCe/TiO₂ were lower compared to those using carbon-based materials as supports. This reduced performance is primarily attributed to the strong interaction between the metal and the supports, which raises the reduction temperature. Consequently, this increases the amount of reducible metal on the catalyst surface and adversely affects catalytic activity.

Carbon materials facilitate the formation of copper on the support surface due to their high electrical conductivity and excellent electron transfer capabilities. As a result, they are considered exceptional supports in SCR denitrification catalysts, owing to their large specific surface area and favorable properties, including size distribution, pore structure, and surface morphology [120]. In recent years, activated carbon (AC) has garnered considerable attention in the study of low-temperature SCR catalysts, primarily due to its abundant oxygen-containing functional groups and large specific surface area. Wang et al. [121] found that doping with Ce metal ions results in the subdivision of the graphitic microcrystalline structure of AC into smaller graphene fragments, which significantly improves the dispersion of Cu and Ni. SEM images (Figure 15A) reveal that large spherical clusters are present on the surface of the activated carbon when the Ce loading is low; however, these spherical metal oxide particles gradually decrease in size as the Ce content increases. This change in spherical structure can be attributed to the mixed-crystal effect, where Ce doping reduces the tensile stress at the grain boundaries of CuNi metal oxides. This reduction leads to smaller particle sizes, enhanced dispersion on the activated carbon, and an increased number of denitrification active sites, thereby significantly improving denitrification efficiency. Furthermore, this doping increases the formation of reaction units on the catalyst surface, enhancing the adsorption of reaction gases such as CO and NO. The amount of surface-adsorbed oxygen from Cu²⁺/Cu⁺ and Ni³⁺/Ni²⁺ increased significantly with higher Ce doping levels in the Cu-Ni-Ce/AC catalysts. The specific surface area and pore volume of the supports also play crucial roles in influencing catalytic performance; larger pores facilitate greater interfacial contact between the catalyst and reactants, thereby enhancing catalytic effectiveness. According to Liu et al. [122], the Cu-Ce/VMT catalyst exhibits particularly impressive performance, achieving 100% catalytic conversion at 300 °C. As shown in the SEM image (Figure 15B), SEM images reveal that the surface of Cu-Ce/VMT is uniformly coated with nanoparticles and features a substantial number of clearly visible pores, these characteristics provide more effective active sites for the catalytic reaction, allowing a greater number of active substances to participate in the reaction between NO and CO, thus significantly improving catalytic performance.

In conclusion, the selection of catalyst supports is crucial for enhancing the efficiency and activity of denitrification reactions [123]. The physicochemical properties of different supports directly influence the performance of the catalysts [124], making it essential to carefully consider the nature and composition of the supports during the design and application of catalysts to achieve optimal catalytic effects. Future studies should further explore the development and optimization of novel support materials to meet increasingly stringent environmental requirements.

3.2. Anti-Toxicity Properties of CO-SCR Reactions

The study of resistance to H₂O, SO₂, and heavy metals is of great significance in CO-SCR catalysts [125,126], as the presence of these substances in actual flue gas atmospheres is inevitable. These components can negatively affect the performance of the catalysts, poison them, and compromise their oxidative properties and stability. As shown in Figure 16, the reasons for catalyst deactivation by H₂O and SO₂, as well as measures to solve the problem, are presented [38].

The synergistic interaction between copper and cerium has been shown to significantly improve the water and sulfur resistance of the catalysts. Wang et al. [127] found that in the absence of sulfur dioxide in the flue, the NO concentration is approximately 40 ppm, and the reaction remains stable. When 200 ppm of SO₂ is introduced, the NO concentration increases rapidly and stabilizes at about 120 ppm. A similar phenomenon is observed with the change in N₂O concentration: in the NO + CO reaction without SO₂, the N₂O concentration is about 70 ppm but rises to 110 ppm with the addition of SO₂. This indicates that the introduction of SO₂ reduces the efficiency of DeNO_x from 95% to 80%, while improving the selectivity of N₂ to about 0.34. After 7000 s of reaction, the SO₂ in the flue is expelled, and the catalytic efficiency is restored to its original state. Further studies showed that an appropriate increase in the proportion of iron can enhance the catalyst’s resistance to SO₂. Although Cu_0.5Ce_0.25Fe_2.25O_4.375 exhibits lower initial de-NO_x activity than Cu₁Ce_0.5Fe_1.5O_4.25, its catalytic performance is not significantly reduced in the presence of SO₂ when the NO concentration increases to about 110 ppm and the N₂O concentration reaches 100 ppm. This suggests that a moderate increase in the iron proportion enhances the water and sulfur resistance of the Cu-Ce catalysts. The addition of iron inhibits sulfate formation and prevents sulfate from converting large amounts of sulfur dioxide into chemisorbed sulfur dioxide and sulfite, thereby retaining highly active copper. Moreover, a higher proportion of iron generates more water-absorbing sites, further enhancing the catalyst’s water resistance.

Cu-Ce catalysts face significant challenges due to heavy metal toxicity, particularly from zinc (Zn), a major component of flue gas. The presence of zinc leads to the formation of ZnCl₂ and ZnSO₄, which contain chlorine (Cl⁻) and sulfate (SO₄²⁻). These compounds can physically or chemically react with the active components on the catalyst surface, primarily resulting in a decreased adsorption capacity for reactive gases [128]. Zinc salt particles occupy the pores of the catalyst surface, contributing to the crystallization and aggregation of active components. This aggregation reduces the content of active components and diminishes the catalyst’s chemical adsorption capacity for oxygen, ultimately weakening its redox capacity. To address this issue, acid washing has proven effective in removing toxic metals and regenerating deactivated SCR catalysts. Wen et al. [129] investigated the NO conversion of Cu-Ce/AC catalysts at 100 °C, achieving a conversion rate of 94.34%. However, this conversion significantly decreases in catalysts poisoned by zinc sulfate. Notably, after regeneration through acid washing, the catalyst conversion rapidly recovers. Sulfuric acid effectively removes solid particles deposited on the catalyst surface, restoring the specific surface area and pore structure while increasing the adsorption area for reactive gases. Additionally, the sulfate group generated from sulfuric acid impregnation reacts with zinc to form an S-O-S structure, which prevents the zinc salt from damaging the oxygen-containing functional groups. This process enhances the catalyst’s reduction ability and CO adsorption capacity, ultimately restoring its denitrification activity.

In summary, the Cu-Ce catalysts demonstrate excellent catalytic activity and resistance when exposed to water and sulfur dioxide. Furthermore, research into the toxicity issues associated with heavy metals has provided an effective solution for the regeneration of these catalysts.

3.3. CO-SCR Reaction Mechanism

In catalytic reactions, the mechanism of the CO-SCR reaction is complex and influenced by various factors. This mechanism can be primarily categorized into two modes: The ER mechanism and the LH mechanism.

Different catalytic systems and reaction conditions dictate that they follow distinct reaction mechanisms. Li et al. [130] thoroughly investigated the effects of Cu⁺-Ce catalysts on carbon monoxide (CO) adsorption and reaction mechanisms at varying valence states, revealing significant differences between Cu²⁺-Ce and Cu⁺-Ce catalysts at low temperatures. In Cu⁺-Ce catalysts, CO is primarily adsorbed as Cu⁺-CO species, which leads to the formation of abundant Cu⁺-CO species; these adsorbed CO molecules then react with reactive oxygen species on the catalyst surface to generate CO₂. In contrast, in Cu²⁺-Ce catalysts, CO mainly combines with a small amount of reactive oxygen species on the catalyst surface to form CO₂. Both catalysts primarily follow the LH mechanism. Furthermore, Deng et al. [108] demonstrated that surface Mn-doped CuO/CeO₂ catalysts adhere to the ER reaction mechanism in the CO-SCR reaction. The synergistic effect of Ce³⁺ + Cu²⁺ → Ce⁴⁺ + Cu⁺ facilitates the formation of additional Cu⁺, and during the heating process, Cu²⁺ is reduced to Cu⁺ due to the excess of CO, thereby increasing Cu⁺ and providing more CO adsorption sites. The CO molecules adsorbed on Cu⁺ can react with the O radicals generated by the dissociation of NO on the surface oxygen vacancies to form CO₂. The remaining N radicals can recombine with either NO or CO to yield N₂O or NCO, respectively, or with another N radical to produce N₂. Additionally, at higher temperatures, Cu²⁺ can further reduce to CuO metal, and N₂O can be further reduced to N₂.

The same catalyst system may follow both ER and LH mechanisms. As shown in Figure 17A, He et al. [131] proposed that competitive adsorption of NO and CO occurs over the catalyst, with NO_x preferentially occupying the active sites and inhibiting CO adsorption. As the reaction temperature increases, a portion of the adsorbed NO_x reacts with CO in the gas phase to produce oxygen vacancies and gaseous CO₂, while the remaining NO_x is either desorbed or decomposed, which is consistent with the ER mechanism. Additionally, some of the adsorbed CO reacts with NO_x or reactive oxygen species on the catalyst surface, reducing the reactive species and generating oxygen vacancies, a process that aligns with the LH reaction mechanism.

The reaction mechanism of the same catalyst transforms with the reaction temperature. As shown in Figure 17B, Li et al. [15] investigated the reaction mechanism of CuCe catalysts in CO-SCR reactions at different temperatures. At 150 °C, the adsorbed CO reacts with gaseous NO molecules, indicating that the reaction follows the ER mechanism at this temperature. In contrast, at 350 °C, nitrate and nitrite species on the surface of the CuCe catalyst interact with each other and with adsorbed CO, demonstrating that the CO-SCR reaction follows the LH mechanism. As shown in Figure 17C, Liu et al. [97] investigated the CO-SCR reaction mechanism on a three-dimensionally ordered porous Cu-doped Ce-Fe mixed oxide catalyst (3DOMCuCeFeO). At room temperature, NO is preferentially adsorbed on the catalyst surface due to its unpaired electrons, which limits the adsorption of CO. As the temperature increases, CO can more efficiently capture reactive oxygen species in the Cu²⁺-O-Cu²⁺ framework, resulting in the formation of additional oxygen vacancies. CO adsorbed on Cu⁺ (Cu-CO) can react with NO_x species to produce N₂O and CO₂. At even higher temperatures, gas-phase CO can react with O radicals generated by dissociation at the oxygen vacancies to form CO₂, while the remaining N radicals can combine with NO to form N₂O or combine with another N radical to produce N₂. Consequently, at lower temperatures, Cu-CO reacts with NO_x species (a reaction between adsorbed species); at higher temperatures, gas-phase CO reacts with radicals formed from oxygen vacancies (a reaction between gaseous molecules and adsorbed species), which aligns well with the LH and ER mechanisms.

The interactions and mechanisms of Cu-Ce catalysts in CO-SCR reactions are influenced not only by the catalyst type and valence state but also by the reaction temperature. Competitive adsorption of reactants on the catalyst surface, changes in reactive oxygen species, and a detailed analysis of the reaction pathways contribute to a better understanding and optimization of catalyst performance. These studies provide an important theoretical foundation and practical guidance for the further development of efficient Cu-Ce catalysts, as well as for enhancing the efficiency of CO-SCR reactions.

4. Conclusions and Outlook

This paper reviews new design strategies for enhancing the mechanisms, activity promotion, and toxicity resistance of copper-cerium bimetallic catalysts and their derivatives in NH₃-SCR and CO-SCR reactions. It demonstrates that significant synergistic effects exist in Cu-Ce-based multimetallic catalysts, primarily reflected in the specific surface area, surface concentration of Cu²⁺ and Ce³⁺, and concentration of chemisorbed oxygen. These factors enhance the dispersion of active components and boost the overall activity of the catalysts. However, practical applications still face many challenges. The key points and future perspectives include the following.

Catalyst Design and Performance Optimization: The NO_x conversion efficiency in NH₃-SCR and CO-SCR reactions can be significantly improved, and by-product generation can be effectively reduced through various catalyst design methods. Selecting an optimal catalyst morphology and rationally regulating the ratio of copper to cerium provides a theoretical basis for catalyst development and practical solutions for effective environmental management. Further exploration of new catalyst development methods and optimization of synthesis conditions for the Cu-Ce catalytic system is urgent. This includes adjusting the acidity and alkalinity of the catalyst and the content of active species, enhancing the adsorption and activation of reactants, and optimizing the morphology and crystalline surface of the catalyst to improve catalytic performance.

Application of Doping Technology: Doping technology is crucial for enhancing the performance of Cu-Ce catalysts. By adjusting the catalyst structure and improving the dispersion of active substances, different metal doping elements can effectively enhance the activity and stability of catalysts in NH₃-SCR and CO-SCR reactions. Future studies should explore optimal doping ratios of various metals to optimize the electronic structure and active sites of catalysts, as well as adjust their surface acidity and alkalinity to improve selective adsorption of reactants and optimize reaction pathways.

Role of Carriers: Carriers play an essential role in catalyst design. High-quality carrier materials, such as titanium dioxide, silica-aluminate, or metal-organic frameworks with high specific surface areas, can significantly enhance catalyst activity. Subsequent studies should focus on modifying carrier surfaces, including introducing functional groups to improve interactions between the carrier and the metal, or coating the carrier surface with thin films to enhance its adsorption capacity for reactants.

Research on Toxicity Mechanisms: Understanding the toxicity mechanisms of catalysts is crucial for developing efficient and stable SCR catalysts. Catalysts may be affected by toxic substances such as H₂O, SO₂, heavy metals, and alkali metals. Research in this area will provide insights into the poisoning process and serve as a reference for academia and industry to develop high-efficiency SCR catalysts for low-temperature denitrification with broad activity temperature ranges and high stability. Since catalyst poisoning typically results from multiple factors, an in-depth investigation into integrated anti-poisoning strategies is urgently needed. Additionally, exploring regeneration technologies and optimization of experimental conditions to prolong catalyst lifespan is particularly important. These comprehensive research directions will contribute to the development of efficient, stable, and economical catalysts for NO conversion.