Research Progress on Metal Oxides for the Selective Catalytic Reduction of NO_x with Ammonia

Abstract

Nitrogen oxides emitted from diesel vehicle exhaust seriously endanger the atmospheric environment and human health, which have attracted people’s attention. Among numerous nitrogen oxide (NO_x) removal technologies, photocatalytic removal of NO_x and SCR have received widespread attention. The photocatalytic treatment of NO_x technology is a good choice due to its mild reaction conditions and low costs. Moreover, NH₃-SCR has been widely used in denitration technology and plays an important role in controlling NO_x emissions. In NH₃-SCR technology, the development of high-efficiency catalysts is an important part. This paper summarizes the research progress of metal oxide catalysts for NH₃-SCR reactions, including V-based catalysts, Mn-based catalysts, Fe-based catalysts, Ce-based catalysts, and Cu-based catalysts. Meanwhile, the detailed process of the NH₃-SCR reaction was also introduced. In addition, this paper also describes a possible SO₂ poisoning mechanism and the stability of the catalysts. Finally, the problems and prospects of metal oxide catalysts for NO_x removal were also proposed.

1. Introduction

With an increase in the number of vehicles globally, there has been an increase in pollution from vehicle exhaust [1]. Of the gases present in vehicle exhaust, nitrogen oxides (NO_x) are one of the most serious pollutants, causing air pollution that leads to acid rain, ozone depletion, and the greenhouse effect [2,3]. Moreover, NO_x also poses a huge threat to human health as it damages the human body through contact, inhalation, etc. [4]. Moreover, it has been reported that people with asthma may experience chronic pulmonary dilation and exacerbation of respiratory symptoms within 60 min of being exposed to nitrogen dioxide (NO₂) [5]. As NO_x gases pose such a serious threat to the human body and environment, the removal of NO_x is essential. Removal technologies for NO_x mainly include selective catalytic reduction (SCR), selective non-catalytic reduction (SNCR), activated carbon adsorption methods, photocatalytic degradation, and advanced oxidation processes (AOPs) based on ozone oxidation [6]. Generally, SCR technology involves reacting NO with reducing agents, such as gaseous ammonia (NH₃), liquid NH₃, and urea in the presence of a catalyst, whereas SNCR involves injecting a reducing agent into a system in a suitable “temperature window” to promote a denitration reaction without a catalyst to reduce NO_x to nitrogen and water. Activated carbon has a high adsorption capacity for a low concentration of NO_x. However, due to the possibility of spontaneous combustion of the activated carbon at >300 °C, its use poses significant difficulties for adsorption and regeneration. Photocatalytic degradation is a pollutant control technology that has the advantages of mild reaction conditions and producing a low amount of secondary pollution [7]. The photocatalytic reactor is the core equipment in photocatalytic technology and its efficiency is influenced by many factors, such as light source, catalyst properties, and temperature. Therefore, designing a perfect reactor is crucial for enhancing NO_x removal efficiency. This also indicates that the design of reactors plays a very important role in photocatalytic technology. Moreover, the catalytic removal of NO_x using the semiconductor TiO₂ as a photocatalyst is very effective. Khanal et al. [8] prepared ultrafine Ta₂O₅ nanoparticles and tested the photodegradation of NO_x under UV–vis irradiation at room temperature. Compared with commercially available TiO₂, which is traditionally considered to be the most advanced NO_x degradation material, the ultrafine Ta₂O₅ nanoparticles exhibited a higher NO conversion. AOPs based on ozone oxidation for denitration mainly utilize ozone to oxidize NO_x, which is then absorbed by a washing tower for removal. This denitration system efficiently removes pollutants such as NO_x, sulfur dioxide, and particulate matter simultaneously, without affecting other pollutant control technologies. Thus, it is an efficient supplementary or alternative technology to traditional denitration technology. Alumina, as a representative ozone catalyst, is highly efficient, has a long lifespan, is cheap, and can remove more harmful gases from the exhaust in a shorter time than other catalysts.

Table 1. The NO_x removal technologies as well as representative catalysts and performance.

Removal Technologies of NOx	Representative Catalysts and Their Performance
SCR	V2O3-WO3/TiO2 catalysts exhibit the satisfactory catalytic performance in the medium temperature range
SNCR	-
Activated carbon adsorption	Activated carbon adsorbs low concentration NOx at temperatures below 300 °C
Photocatalytic degradation	Commercially available TiO2 has the excellent NO removal effect
Advanced oxidation processes (AOPs) ozone oxidation	Al2O3 catalyst has the higher oxidation efficiency and longer lifespan

summarizes the methods of NO_x removal mentioned above, as well as their catalysts and performance.

Currently, hybrid systems involving the simultaneous use of multiple techniques are also a hot research topic, such as technology for the simultaneous removal of soot and NO_x [9], and of SO₂ and NO_x. Liu et al. [10] used an O₂/H₂O/H₂O₂ system activated by vacuum ultraviolet (VUV) in a wet VUV spray reactor for the simultaneous removal of NO and SO₂. This hybrid system exhibited good simultaneous removal performance for NO and SO₂. The effects of different process variables on the removal of NO and SO₂ were also investigated. The VUV power, acidity, and alkalinity of the solution, in addition to O₂ and H₂O₂ concentrations, were found to affect the removal of NO from the exhaust, with this study providing a sufficient research foundation for future studies of this technology. In addition, promising and eco-friendly integrated processes involving adsorption, photolysis, and photocatalysis to remove low concentrations of gaseous pollutants from the air have been investigated. Generally, carbon materials, zeolites, and metal–organic framework (MOF) materials are used as adsorbents or carriers in these hybrid systems, while TiO₂ is used as a photocatalyst [7].

However, it should be noted that the NO_x from diesel vehicles’ exhaust exceeds 80% of the total gas emitted from gasoline vehicle exhaust. Therefore, controlling the emissions of NO_x from diesel vehicles has become one of the main issues that need to be addressed. At present, post-treatment technology is the most effective method for controlling NO_x emissions [11]. From the perspective of economic and technological feasibility for diesel vehicles’ aftertreatment systems, SCR technology is the most suitable choice for controlling diesel vehicle exhaust emissions. Among SCR technologies, NH₃-SCR is the most effective emissions control technology [12,13], wherein the design and preparation of highly efficient catalysts that can effectively utilize NH₃ to remove NO_x are crucial [14]. At present, the catalysts for the NH₃-SCR reaction have been extensively studied [15,16]. SCR catalysts can be classified into categories based on their active components: noble metal catalysts [17,18], metal oxide catalysts [19,20], and transition metal ion exchange zeolite catalysts [21]. Noble metal catalysts exhibit satisfactory low-temperature performance, but they are expensive, exhibit a narrow operating temperature window, and their poor sulfur resistance limits their widespread application [22]. The transition metal ion exchange zeolite catalysts exhibit a wide operating window at a high NO conversion and poor low-temperature catalytic activity. However, with the international regulations on diesel vehicle pollutant emissions, there are certain constraints on the NO_x gases that are generated under cold-start diesel-engine conditions, and the SCR performance at low temperatures is receiving increased research attention. Therefore, metal oxides have become a focus of research due to their advantages of being cheap, as well as exhibiting high thermal stability and excellent low-temperature catalytic activity. Among the metal-oxide catalysts, traditional V₂O₃–WO₃/TiO₂ catalysts that exhibit satisfactory catalytic performance at 300–450 °C have been commercialized for many years [23]. In addition to the V₂O₃–WO₃/TiO₂ catalysts, metal-oxide catalysts, such as Mn-based oxides, have received increasing research attention due to their diverse valence states and excellent low-temperature SCR activity, and exhibit a good research future in the field of low-temperature denitration. Iron-based catalysts are also cheap, environmentally friendly, and exhibit excellent redox properties and good denitration activity. In addition, Ce- and Cu-based catalysts are widely used in NH₃-SCR reactions. This review provides an in-depth introduction to SCR and the application and role of metal oxides, such as V-, Mn-, Fe-, Ce-, and Cu- oxides, in SCR reactions. Moreover, the development and issues associated with the metal oxides used in the SCR reactions are also summarized.

2. The Main Reactions of the NH₃-SCR System

The NH₃-SCR technology is currently the most popular technology used for removing NO_x from vehicle exhaust. The main reaction of NH₃-SCR is shown in Equation (1), wherein NH₃ reduces NO to H₂O and N₂, which is referred to as the “standard SCR” reaction. When the content of NO₂ increases and reaches the same molar equivalent as NO, the “fast SCR” reaction (Equation (2)) occurs, which was first proposed in the early 1980s and can improve the denitration efficiency at low temperatures as the reaction rate of the “standard SCR” is much lower than that of the “fast SCR”. [24]. And excess NO₂ may partially depend on the relatively slow “NO₂-SCR” reaction (Equation (3)) [25]. Moreover, other reactions that occur during the SCR reaction can affect the overall activity and selectivity of the NH₃-SCR catalyst, including NO and NH₃ oxidation (Equations (4) and (5)) and N₂O generation (Equation (6)). Also, N₂O is more likely to contribute towards greenhouse effects than carbon dioxide [26,27,28].

In diesel exhaust gas, the NO content is much higher than that of the NO₂. However, it is possible to improve the upstream oxidation catalyst to increase the content of NO₂ entering the SCR device, promoting the reduction of NO_x in the presence of NO and NO₂, that is, “fast SCR.” The “fast SCR” reaction at low temperatures exhibits higher reaction activity than the “standard SCR” reaction, which can effectively improve the NO_x removal efficiency. According to Equations (1) and (2), it can be inferred that the adsorption of NH₃ and NO/NO₂ on the surface of the SCR catalyst as well as the adsorption and activation of oxygen are important determining factors in the SCR reaction.

4NO + 4NH₃ + O₂ → 4N₂ + 6H₂O

(1)

NO + 2NH₃ + NO₂ → 2N₂ + 3H₂O

(2)

8NH₃ + 6NO₂ → 7N₂ + 12H₂O

(3)

2NO + O₂ → 2NO₂

(4)

4NH₃ + 5O₂ → 4NO + 6H₂O

(5)

2NH₃ + 2NO₂ → N₂ + N₂O + 3H₂O

(6)

3. SCR Catalysts

3.1. V-Based Catalysts

V₂O₅–WO₃/TiO₂ is the earliest-known commercial NH₃-SCR catalyst, which has a sufficient number of acidic sites, good redox ability, and exhibits high NH₃-SCR catalytic activity in the medium-temperature range (300–450 °C) [29]. However, the exhaust emissions of diesel vehicles contain SO₂, which is easily oxidized to SO₃. NH₄HSO₄ is then formed as a result of the reaction between NH₃ and SO₃, which can deposit on the catalyst surface and cover the active sites, which is the reason for sulfur poisoning of the V₂O₅–WO₃/TiO₂ catalyst (Figure 1) [30]. In addition, the catalyst is also unstable at high temperatures; it may be due to the TiO₂ transformation from anatase to rutile at high temperatures and the separation or volatilization of vanadium species. Therefore, many researchers have turned their attention to the modification of V-based oxide catalysts to address the above issues [31,32].

The addition of active components is an appropriate method to enhance the sulfur resistance of a catalyst and promote its high-temperature stability. A V₂O₅/TiO₂ catalyst modified using Sb has been shown to exhibit high SO₂ tolerance in an NH₃-SCR reaction. Xu et al. [33] also reported a V₂O₅–Sb₂O₃/TiO₂ monolithic catalyst for the NH₃-SCR reaction, the operating temperature window of which was 225–375 °C when the NO_x conversion was >90%. The low-temperature activity of the catalyst significantly decreased after the introduction of SO₂, which was caused by the deposition of NH₄HSO₄. The catalyst modified with Sb₂O₃ promoted the reaction between NH₄HSO₄ and NO, reducing the deposition of NH₄HSO₄, and therefore exhibited better NH₃-SCR performance than the non-modified catalyst in the presence of SO₂. Moreover, a Nb-modified V₂O₅–WO₃/TiO₂ catalyst was also studied for the NH₃-SCR. It was shown that the SO₂ conversion in the presence of the Nb₂O₅–V₂O₅–WO₃/TiO₂ catalyst was the lowest (0.6%) of the tested catalysts at 350 °C at a Nb₂O₅ loading of 2%, while the NO_x conversion was still >95% (Figure 2A). This was because the surface area of the catalyst decreased upon the introduction of Nb, resulting in a decrease in the amount of SO₂ adsorbed. Moreover, the adsorbed oxygen on the catalyst was also significantly reduced after the modification of Nb, indicating that the redox performance of the modified catalyst is weakened, which limited the oxidation of SO₂ [34]. Jung et al. [35] also studied the effects of doping Nb on the NO_x removal from the exhaust in V₂O₅–WO₃/TiO₂ (VWT) catalysts. The results indicated that Nb enhanced the low-temperature performance of the VWT catalyst. Doping of the VWT catalyst with Nb also enhanced its SO₂ and H₂O resistance. After catalyst poisoning for 24 h, the NO conversion of the Nb-doped VWT catalyst at 240 °C was 12% higher than that of the VWT catalyst. This is because, compared to VWT, less ammonium bisulfate (ABS) is formed in the V₂O₅–WO₃–Nb₂O₅/TiO₂ catalyst. In addition, the V₂O₅/WO₃–TiO₂ catalyst modified with Zr showed better performance than the unmodified catalyst after hydrothermal aging of the catalysts, which was attributed to the inhibition of the shrinkage of the catalyst surface area and the growth of the TiO₂ crystal size upon addition of Zr [36]. Kang et al. [37] physically mixed V₂O₅–WO₃/TiO₂ and several Fe₂O₃ samples with different textural properties for the NH₃-SCR. In the presence of SO₂, the physically mixed catalyst showed higher catalytic stability and SO₂ resistance compared with the V₂O₅–WO₃/TiO₂ catalyst. The V₂O₅–WO₃/TiO₂ catalyst modified by Fe₂O₃ showed relatively stable catalytic performance and high SO₂ resistance. As shown in Figure 2B, Fe₂O₃ effectively prevented catalyst deactivation by inhibiting the formation of ammonium sulfate species. Moreover, Fe₂O₃ also increased the number of acidic adsorption sites of the reactants. In addition, compared to commercial V-based catalysts, the physical mixing of alumina and V-based catalysts resulted in better stability during the SO₂ aging process. This is because the ABS formed on the surface of alumina is obtained through physical contact and migration rather than the direct adsorption of gaseous SO₂. The mixed alumina still exhibited excellent ABS affinity and stability after undergoing SO₂ aging and regeneration processes, which was attributed to its physicochemical properties, such as acidity and pore size [38]. In addition, Ni et al. [39] reported that Ce improved the SO₂ resistance of a V₂O₅–WO₃/TiO₂ catalyst in a low-temperature NH₃-SCR reaction. The ABS on the surface of the catalyst preferentially deposited on CeO₂ to form cerium sulfate, which protected the active species V₂O₅ and TiO₂, maintaining better NH₃-SCR performance in the presence of SO₂. Also, it has been reported that CeO₂ promotes the decomposition of ABS [40]. Wang et al. [41] tested the activity of MO_x–WO₃/TiO₂ catalysts (M = Fe, Mn, Cu, V) treated with SO₂ at 200 °C for 24 h. The results showed that the low-temperature activity of the catalyst significantly decreased after treatment under an SO₂ atmosphere. The catalyst treated with SO₂ inhibited the adsorption of NH₃ and NO on the surface of the catalysts, which was the main reason for its deactivation. Among the prepared catalysts, FeO_x–WO₃/TiO₂ was found to be the best catalyst for the NH₃-SCR at medium to low temperatures due to the easy decomposition of sulfates on its surface.

In addition to adding a single component to modify V-based catalysts, dual component addition can further improve the SCR performance of a catalyst. The Ce and Sb co-modification of a V₂O₅/TiO₂ catalyst was shown to improve SO₂ and H₂O resistance. At a NO_x conversion of >90%, its operating temperature window was 220–450 °C. The excellent catalytic performance of the modified catalyst was attributed to its strong adsorption of NO, large number of acidic sites, and low SO₂ adsorption capacity [42]. Cao et al. [43] reported a Ce⁴⁺ and Zr⁴⁺ co-doped V₂O₅–WO₃/TiO₂ catalyst. The addition of Ce and Zr promoted the removal of NO_x at 300–400 °C and showed the best H₂O + SO₂ resistance owing to the resulting catalyst having more acidic sites as well as exhibiting strong water and sulfur resistance (Figure 2C). In addition, the promotional effects of Cu and Fe on the V₂O₅–WO₃/TiO₂ catalyst for the removal of NO by NH₃ in the temperature range of 280–360 °C were also studied by Wang et al. [44]. They reported that the addition of Cu led to an increase in the Cu²⁺ content of the catalyst, which is the main species of Cu-containing catalysts, while Fe formed FeVO₄ with V. The addition of Cu or Fe improved the SCR performance of the catalyst, which was attributed to a good dispersion of active substances, good redox performance, and abundant active oxygen species.

Figure 2. (A) Catalytic performance of various catalysts in the simultaneous denitration and SO₂ oxidation at 350 °C. Reprinted with permission from Ref. [34]. Copyright 2022, Elsevier B.V. (B) Effects of Fe₂O₃ on V₂O₅-TiO₂ catalyst. Reprinted with permission from Ref. [37]. Copyright 2021, Elsevier B.V. (C) The effects poisoning of Ce⁴⁺ and Zr⁴⁺ doping. Reprinted with permission from Ref. [43]. Copyright 2017, Elsevier B.V. (D) The effects of SiO₂ doping. Reprinted with permission from Ref. [45]. Copyright 2016, Elsevier B.V.

Figure 2. (A) Catalytic performance of various catalysts in the simultaneous denitration and SO₂ oxidation at 350 °C. Reprinted with permission from Ref. [34]. Copyright 2022, Elsevier B.V. (B) Effects of Fe₂O₃ on V₂O₅-TiO₂ catalyst. Reprinted with permission from Ref. [37]. Copyright 2021, Elsevier B.V. (C) The effects poisoning of Ce⁴⁺ and Zr⁴⁺ doping. Reprinted with permission from Ref. [43]. Copyright 2017, Elsevier B.V. (D) The effects of SiO₂ doping. Reprinted with permission from Ref. [45]. Copyright 2016, Elsevier B.V.

In addition, the high-temperature instability of V-based catalysts can be improved through doping with Si. The addition of SiO₂ to a WO₃–TiO₂ support improves the resistance of the catalyst to hydrothermal aging. As shown in Figure 2D, the introduction of SiO₂ inhibits the phase transition of TiO₂, the growth of TiO₂ crystal size, and the shrinkage of the catalyst surface area [45]. Similarly, Shao et al. also reported that a Si-doped V₂O₅–WO₃/TiO₂ catalyst showed higher thermal stability than a V₂O₅–WO₃/TiO₂ catalyst as Si doping prevents the anatase to rutile phase transition and contributes to good dispersion of VO_x and WO₃ [46].

Based on the above research, V-based catalysts have been improved and optimized, achieving significant results.

Table 2. The catalytic performances of V-based catalysts for NH₃-SCR reaction in reported the literature.

Catalysts		Feed Composition				GHSV (h−1)	Conversion (Corresponding Temperature Window)	Ref.
	NO (ppm)	NH3 (ppm)	O2 (vol%)	SO2 (ppm)	H2O (vol%)
V2O5-Sb2O3/TiO2 monolithic catalyst	1000	1000	5	1000	10	5000	>90% (225–375 °C)	[33]
V1WT	500	500	5	-	-	120,000	>90% (300–500 °C)	[36]
VW/Ti + Fe	1200	1200	13	5%CO2	5.2	120,000	>95% (300–400 °C)	[37]
7Ce-VW/Ti	500	500	5	-	-	160,000	100% (250–400 °C)	[39]
	500	500	5	200	5.5		65% (250 °C)
3V6Mo10CeTi	1000	1000	8	-	-	30,000	97% (200 °C)	[40]
SbV10Ce/TiO2	800	800	3	-	-	60,000	>90% (220–450 °C)	[42]
	800	800	3	800	6		>85% (220–500 °C)
VWTCZ	500	500	5	-	-	60,000	>97% (300–400 °C)	[43]
	500	500	5	100	5		75% (350 °C)
VWTS	500	500	5		10	100,000	>80% (350–550 °C)	[45]
VWTSi10-550 °C	500	500	5			300 mL min−1	>80% (205–520 °C)	[46]

shows a good catalytic performance of V-based catalysts for the NH₃-SCR reaction that has been reported in the literature. However, there are still some issues with using V-based catalysts, such as low SCR activity at low temperatures, a narrow activity temperature window, and biological toxicity. Therefore, V-free catalysts are receiving increased research attention.

3.2. Mn-Based Catalyst

Manganese oxide (MnO_x) catalysts exhibit excellent low-temperature SCR catalytic performance (Table 3) due to their various valence states and good redox capabilities. Therefore, the study of MnO_x has attracted widespread attention. The SCR catalytic activity of MnO_x is related to the valence, specific surface area, and crystallinity of the manganese. Among these attributes of manganese, the crystalline structures of MnO₂ catalysts feature structural defects, and therefore the SCR activity of MnO₂ is higher compared to that of other oxidized forms of manganese [47,48]. Thirupathi et al. [49] showed that the reducibility of MnO₂ catalysts was improved and their NH₃-SCR catalytic performance also was enhanced, indicating that MnO₂ is the active component with the highest SCR reaction activity. Moreover, MnO_x, which exhibits low crystallinity, shows good low-temperature performance. In addition, the catalytic performance of MnO_x catalysts can be improved by adjusting their crystal structure and morphology. Yang et al. [50] adjusted the crystal phase of MnO_x to enhance the denitration performance and found that α-MnO₂ exhibited better catalytic activity compared to β-MnO₂, γ-MnO₂, and δ-MnO₂. Meanwhile, the morphology of the catalyst also affects its redox properties. Liang et al. [51] introduced W and Mo as new acidic centers into α-, β-, γ-, and δ-MnO₂. When Mn–O–W or Mn–O–Mo bonds formed on the surface of α-MnO₂, the best ability to capture NH₃ and NO on the surface of the catalyst was observed. The results showed that the high activity of MnO₂ is related to the strong interaction between H and the surface O_2c site and that this site is prone to forming oxygen vacancies.

Moreover, the type of MnO_x and the valence state of Mn play crucial roles in the SCR reaction [52,53]. The SCR catalyst activity of MnO_x varies according to the valence state. It has been reported that MnO₂ exhibits the best SCR catalytic performance, which is related to Mn⁴⁺ promoting the redox cycle and enhancing the oxidation of NO to NO₂ [54,55]. Zhan et al. [56] prepared ordered mesoporous MnO₂ and Mn₂O₃ using the KIT-6 template (Figure 3A). The mesoporous manganese oxide has more chemically adsorbed oxygen and stronger acid sites, as well as strong reducibility. As shown in Figure 3B, mesoporous MnO₂ removed NO with an efficiency of 100% in the temperature range of 150–250 °C, which was higher than the activity of mesoporous Mn₂O₃ due to mesoporous MnO₂ having more Mn⁴⁺. The results indicate that the mesoporous structure and oxidation state of MnO_x have a major impact on the SCR. Liu et al. [57] constructed MnO₂ nanosheets with a large number of oxygen vacancies via a solvent-free synthesis method, which exhibited the best low-temperature activity among tested samples, with a denitration efficiency of ≤100% at 100 °C (space velocity 700,000 h⁻¹) (Figure 3C). Moreover, the NH₃-SCR mechanism of the MnO₂ nanosheet catalyst was further studied and the results showed its catalysis to be underpinned by the Langmuir–Hinshelwood (L–H) mechanism. Hollow spherical MnO₂, Mn₂O₃, Mn₃O₄, and MnO catalysts of similar sizes were obtained by decomposing MnCO₃ in air at 440 and 500 °C, and under H₂ and Ar at 430 °C, respectively. Within different reaction temperature ranges, as shown in Figure 3D, the SCR performance order of the MnO_x catalysts was different and the NO conversion of MnO₂ was the highest [58].

However, undoped MnO_x catalysts exhibit a narrow temperature window for the NH₃-SCR, as well as low N₂ selectivity and SO₂ resistance, which limit their practical applications [59,60]. Therefore, doping or mixing MnO_x with other metal oxides can greatly improve their SCR activity and SO₂ resistance due to the interaction between the Mn and doping element [61,62]. Table 3 summarizes the catalytic performances of Mn-based catalysts for the NH₃-SCR reaction reported in the literature.

CeO₂ can store and release oxygen and is added to Mn-based oxides as a promoter to enhance the performance and SO₂ resistance of catalysts in the SCR reactions. After the introduction of Ce, SO₂ preferentially reacts with Ce to form cerium sulfate, which acts as a sacrificial site to reduce the sulfation of MnO_x, thereby improving the sulfur resistance of the catalyst. Jin et al. [63] studied the mechanism of sulfur poisoning in Mn–Ce composite oxides in detail. As shown in Figure 4A, the introduction of Ce inhibits the generation of manganese sulfate on the surface of the catalyst. Moreover, it has been reported that CeO₂ can store NO_x as a SO₂ trap to prevent sulfation of the active sites when SO₂ is exposed [64,65]. Chen et al. [66] studied the SCR performance of doped CeO₂ catalysts. The results showed that the NO conversion of the catalysts doped with different crystal forms of MnO_x and CeO₂ was always >95% in the range of 75–250 °C, indicating that MnO₂-doped CeO₂ catalysts have excellent low-temperature NH₃-SCR performance. In addition, catalysts doped with CeO₂ also produce less N₂O and the generated N₂O mainly arises from the SCR reaction rather than NH₃ oxidation. This is because compared with an undoped manganese oxide catalyst, the reduction and acidity of the catalyst doped with CeO₂ are improved, which is conducive to the adsorption of NH₃ and NO, and reduces the reactivity of O₂ and NH₃.

In addition, different Mn and Ce contents also affect the catalytic performance of the NH₃-SCR. Wang et al. [67] studied the SCR catalytic performance of Mn₄CeO_x and MnCe₄O_x catalysts, reporting that the Mn₄CeO_x catalyst exhibited higher catalytic activity than the MnCe₄O_x catalyst in the temperature range of 75–250 °C and that the NO_x removal of the Mn₄CeO_x catalyst was 100% at 150 °C. Within the tested temperature range, the N₂ selectivity of the Mn₄CeO_x catalyst was always higher than that of the MnCe₄O_x catalyst due to the Mn₄CeO_x catalyst exhibiting better redox capacity and stronger acidity. Moreover, the relative content of Ce⁴⁺ in the Mn₄CeO_x catalyst was relatively high, indicating that it produces more Mn⁴⁺ and generates more oxygen vacancies, thereby promoting the SCR reaction.

The rare earth metal Sm is also a good dopant, as it is non-toxic, cheap, stable, and can control the chelation of transition state intermediates [68]. Meng et al. [69] first discovered that the doping of Sm enhanced the sulfur and water tolerance of MnO_x catalysts. Moreover, Chen et al. [70] revealed the promotional effects of Sm doping on the denitration and sulfur resistance of a Sm-doped MnFeO_x catalyst, the results of which are shown in Figure 4B. The strong electron transfer between Sm²⁺ and Mn⁴⁺ preferentially induced Fe and Sm to act as sacrificial sites to react with SO₂, effectively suppressing the formation of metal sulfates and ammonium salts on the catalyst surface. In addition, adding transition-metal oxides to MnO_x also enhances the catalytic performance of the catalysts. Li et al. [71] prepared cobalt-doped MnO_x catalysts via a coprecipitation method. The catalyst with a Mn/Co = 1 exhibited the best catalytic activity, with a NO conversion of >90% at 100–275 °C. The doping of MnO_x with Co greatly increased the high-valence-metal ion content and chemically adsorbed oxygen on the surface of the MnO_x catalyst and reduced the apparent activation energy of the catalyst, which contributed to the excellent SCR catalytic performance of the catalyst. Ni-doped MnO_x catalysts also showed good results in the NH₃-SCR; it is related to the synergistic effect between Mn and Ni. As shown in Figure 4C, Ni³⁺ traps electrons from Mn³⁺ and they are converted into Ni²⁺ and Mn⁴⁺, respectively. The activation process of converting Mn³⁺ to Mn⁴⁺ is related to the dehydration of NH₃ to NH₂, which helps to improve SCR activity and the stability of Ni–MnO_x catalysts. According to the reaction Mn³⁺ + Ni³⁺ ↔ Mn⁴⁺ + Ni²⁺, Ni³⁺ converts to Ni²⁺, with the generated Ni²⁺ providing electrons to O₂ before reverting back to Ni³⁺. Finally, NH₂ and O²⁻ react with NO gas to produce non-polluting products (Figure 4C) [72]. Li et al. [73] also reported Co-, Ni-, and Cu-doped MnO_x catalysts for the SCR, and it was found that it was easier to use Co to produce Mn-enriched oxides, with Co–MnO_x exhibiting the highest NO_x removal activity and SO₂ resistance among the synthesized catalysts. The high activity of Co–MnO_x contributes to its special Mn-enriched surface (Co²⁺)_tet(Mn³⁺Co³⁺)_octO₄ structure. In addition, Co–MnO_x exists in lower density surrounding the Mn. Meanwhile, Co–MnO_x benefits from the electron transfer between Mn and Co, and surface nitrate can react with the adsorbed NH₃ using the L–H mechanism (Figure 4D).

Based on the above research, Mn-based catalysts exhibit excellent low-temperature catalytic performance, but the N₂ selectivity and water/sulfur resistance at high NO conversion still need to be further improved. In addition, the reaction mechanism of Mn-based catalysts should be further studied.

Table 3. The catalytic performances of Mn-based catalysts for NH₃-SCR reaction in reported the literature.

Catalysts		Feed Composition				GHSV (h−1)	Conversion (Corresponding Temperature Window)	Ref.
	NO (ppm)	NH3 (ppm)	O2 (vol%)	SO2 (ppm)	H2O (vol%)
MnO2 Mn2O3 Mn3O4	500	500	11			36,000	100% (150 °C) 100% (250 °C) 100% (200 °C)	[48]
Mn–Ni/TiO2 (Ni/Mn = 0.4)	400	400	2	-	--	50,000	100% (200 °C)	[49]
α-MnO2 γ-MnO2 β-MnO2	500	500	19	-	-	36,000	100% (120 °C) 100% (120 °C) 40% (120 °C)	[50]
mesoporous MnO2	500	500	3	-	-	28,000	100% (150–250 °C)	[56]
Mn3CeW0.3Ox	500	500	5	-	-	100,000	>70% (100–275 °C)	[59]
	500	500	5		5		>72% (150–300 °C)
Cr(0.4)–MnOx	1000	1000	3	-	-	30,000	>98% (120–220 °C)	[61]
	1000	1000	3	100			85% (120 °C)
CeO2@α-MnO2	500	500	11	-	-	36,000	100% (75–250 °C)	[66]
SmMnFe-0.1	500	500	5			60,000	100% (75–200 °C)	[70]
	500	500	5	100	5		90% (200 °C)
Ni(0.4)-MnOx	500	500	5			64,000	100% (150–240 °C)	[72]
	500	500	5	100			87% (230 °C)
Co-MnOx	500	500	5	200	10	32,000	86% (200 °C)	[73]

Table 3. The catalytic performances of Mn-based catalysts for NH₃-SCR reaction in reported the literature.

Catalysts		Feed Composition				GHSV (h−1)	Conversion (Corresponding Temperature Window)	Ref.
	NO (ppm)	NH3 (ppm)	O2 (vol%)	SO2 (ppm)	H2O (vol%)
MnO2 Mn2O3 Mn3O4	500	500	11			36,000	100% (150 °C) 100% (250 °C) 100% (200 °C)	[48]
Mn–Ni/TiO2 (Ni/Mn = 0.4)	400	400	2	-	--	50,000	100% (200 °C)	[49]
α-MnO2 γ-MnO2 β-MnO2	500	500	19	-	-	36,000	100% (120 °C) 100% (120 °C) 40% (120 °C)	[50]
mesoporous MnO2	500	500	3	-	-	28,000	100% (150–250 °C)	[56]
Mn3CeW0.3Ox	500	500	5	-	-	100,000	>70% (100–275 °C)	[59]
	500	500	5		5		>72% (150–300 °C)
Cr(0.4)–MnOx	1000	1000	3	-	-	30,000	>98% (120–220 °C)	[61]
	1000	1000	3	100			85% (120 °C)
CeO2@α-MnO2	500	500	11	-	-	36,000	100% (75–250 °C)	[66]
SmMnFe-0.1	500	500	5			60,000	100% (75–200 °C)	[70]
	500	500	5	100	5		90% (200 °C)
Ni(0.4)-MnOx	500	500	5			64,000	100% (150–240 °C)	[72]
	500	500	5	100			87% (230 °C)
Co-MnOx	500	500	5	200	10	32,000	86% (200 °C)	[73]

Figure 4. (A) The formation pathway of bulk-like sulfate on Mn-Ce/TiO₂ samples. Reprinted with permission from Ref. [63]. Copyright 2014, Elsevier B.V. (B) Mechanism model of Sm promoting SCR activity and SO₂ resistance over MnFeO_x catalysts. Reprinted with permission from Ref. [70]. Copyright 2022, Elsevier B.V. (C) The electrons transfer between Mn ions and Ni ions over the catalyst for NH₃-SCR. Reprinted with permission from Ref. [72]. Copyright 2014, Elsevier B.V. and (D) The proposed mechanism of the SCR reaction over the Ni(0.4)-MnO_x catalyst and the synergetic catalytic effect between Mn and Ni cations. Reprinted with permission from Ref. [73]. Copyright 2021, Elsevier B.V.

Figure 4. (A) The formation pathway of bulk-like sulfate on Mn-Ce/TiO₂ samples. Reprinted with permission from Ref. [63]. Copyright 2014, Elsevier B.V. (B) Mechanism model of Sm promoting SCR activity and SO₂ resistance over MnFeO_x catalysts. Reprinted with permission from Ref. [70]. Copyright 2022, Elsevier B.V. (C) The electrons transfer between Mn ions and Ni ions over the catalyst for NH₃-SCR. Reprinted with permission from Ref. [72]. Copyright 2014, Elsevier B.V. and (D) The proposed mechanism of the SCR reaction over the Ni(0.4)-MnO_x catalyst and the synergetic catalytic effect between Mn and Ni cations. Reprinted with permission from Ref. [73]. Copyright 2021, Elsevier B.V.

3.3. Fe-Based Catalysts

The catalytic effect of Fe₃O₄ and FeO is very weak for the SCR, so Fe-based catalysts are generally based on Fe₂O₃ as the main active substance. In recent years, Fe₂O₃ has attracted attention in the field of developing Fe-based SCR catalysts due to its excellent redox properties, acidity, and sulfur resistance [74]. The conversion of the valence state of iron in iron oxide between Fe²⁺ and Fe³⁺ is helpful for generating lattice oxygen. Fe³⁺ converts NO into NO₃⁻ and NO₂⁻, improving the SCR activity of Fe-based catalysts [75]. Different crystal forms of iron oxide, such as α-Fe₂O_3, β-Fe₂O₃, and γ-Fe₂O₃, exhibit different NH₃-SCR catalytic activities. The α-Fe₂O₃ is more abundant in nature compared to β-Fe₂O₃ and γ-Fe₂O₃, and exhibits low toxicity, high resistance to SO₂, and has a suitable electronic band structure [76]. Zhou et al. [77] reported an Fe₂(SO₄)₃/CeO₂ catalyst for NH₃-SCR, which exhibited good NH₃-SCR performance and enhanced N₂O selectivity as a result of the synergetic interaction of Fe, Ce, and SO₄^2–. As shown in Figure 5, Yu et al. [78] conducted thermal stability experiments over an Fe₂(SO₄)₃/TiO₂ catalyst. It was found that Fe₂(SO₄)₃/TiO₂ decomposed into α-Fe₂O₃ at >350 °C, which led to a slight decrease in the number of Brønsted acid sites and NO_x conversion. α-Fe₂O₃ bound gaseous NO_x more effectively than NH₃, resulting in the production of abundant surface nitrate species, which may have had a negative impact on its catalytic performance in the SCR. Therefore, great efforts have been devoted to adjusting the redox properties and surface acidity of α-Fe₂O₃. Cerium oxide is added to Fe oxide due to its unique ability to store and release oxygen, which promotes lattice distortion to generate oxygen vacancies and regulates its chemical and physical properties, such as reducibility and surface acidity, leading to a “fast SCR” that improves the activity of the catalyst at low temperature [79]. Chen et al. [80] prepared an α-Fe₂O₃ with a single-atom Ce-doping catalyst. It was reported that the NO conversion of the Fe_0.93Ce_0.07O_x catalyst was 93% at 250 °C, which was maintained for 168 h when 200 ppm SO₂ and 5 vol% H₂O were introduced. This can be explained by the doping of Ce in the FeO_x catalyst generating Fe–O–Ce sites, which were beneficial to the NO oxidation of the catalyst and the reduction of NO_x by NH₃, resulting in the high activity of the catalyst. Therefore, the presence of Fe–O–Ce sites in this catalyst is crucial. Meantime, the catalyst also exhibited good resistance to SO₂/H₂O, as the NH₄HSO₄ deposited on the surface by the catalyst decomposes at lower temperatures.

In addition, compared to α-Fe₂O₃, γ-Fe₂O₃ has more surface defects and hydroxyl groups, therefore, improving its adsorption performance for NH₃ [81]. Mou et al. [82] reported that γ-Fe₂O₃ nanorods featuring reactive (110) and (100) crystal planes with iron sites and adjacent oxygen sites can jointly adsorb NO and NH₃. Yu et al. [83] also studied a γ-Fe₂O₃ catalyst regarding its resistance to SO₂. At >300 °C, the NO conversion fell to 50% within the first 3 h. Then, with time, the conversion of NO gradually recovered to approximately 80%. It was speculated that SO₂ may hinder the adsorption of NO_x; the formed surface sulfate can terminate the L–H reaction within the first 3 h, but promotes the Eley–Rideal (E–R) reaction to occur after 3 h. However, the conversion of NO continued to decrease without rebounding at <300 °C. This also means that beneficial reactive surface sulfates may form at a higher temperature. Therefore, different sulfate coverage at different temperatures may be triggered following different reaction mechanisms related to the NH₃-SCR reaction over sulfation catalysts. Moreover, as the balance between acidity and redox performance of the catalyst in the NH₃-SCR reaction is crucial, the doping effects of Ti⁴⁺, Ce^3+/4+, and Al³⁺ were systematically studied due to their different acidity and redox properties [84]. Among the doped catalysts, the catalyst doped with Ti⁴⁺ exhibited the best catalytic performance, with a four-fold NO_x conversion increase (100 °C) compared to Fe₂O₃, which is related to its acidity and reducibility. The samples doped with Ce^3+/4+ exhibited excellent redox properties but had fewer acidic sites. Meanwhile, after the introduction of SO₂ and H₂O, the Fe₉Ti₁O_x catalyst achieved a NO_x conversion of 80% at 250 °C and maintained this performance for 80 h. The excellent water and sulfur resistance of the Fe₉Ti₁O_x catalyst makes it a good candidate catalyst for the SCR reaction. In addition, the sulfation and doping of Ti reduced the oxidation of Fe³⁺ and inhibited NH₃ oxidation. Fan et al. prepared Ti-doped Fe₂O₃ (Ti-Fe₂O₃) nanoparticles [85], which showed a significant NO conversion of 80% at 250 °C under 100 ppm SO₂. The addition of Ti⁴⁺ resulted in the migration of electrons around Fe³⁺, leading to electron deficiency in Fe³⁺. As a result, electron-donating NH₃ was preferentially adsorbed by Fe³⁺, which is beneficial for enhancing the low-temperature resistance of the Ti-Fe₂O₃ catalyst to SO₂. Moreover, doping of the Fe₂O₃ catalyst with Ti⁴⁺ resulted in the conversion of a large number of Brønsted acid sites into Lewis acid sites. In addition, a Sm-doped Fe₂O₃ catalyst was successfully prepared via a citric acid assisted sol–gel method. The results showed that the reaction rate of NO_x over the Fe_0.94Sm_0.06O_x catalyst was almost 11 times that of an undoped Fe₂O₃ catalyst. The Fe_0.94Sm_0.06O_x catalyst maintained a NO_x conversion of >85% at 250 °C for 168 h after SO₂ and H₂O were introduced. The reason for the high activity and stability of the catalyst in the presence of SO₂+H₂O is that the doping of Sm promoted the decomposition of NH₄HSO₄ on the catalyst surface. [86].

Zhang et al. [87] prepared a Cu-modified W/Fe₂O₃ catalyst, and Fe₂O₃ and Fe₂CuO_z supports were successfully prepared by adjusting the glucose content. The introduction of Cu prevented the growth of γ-Fe₂O₃ crystals and the generation of α-Fe₂O₃ crystals in addition to promoting the formation of highly dispersed CuFe₂O₄, which is beneficial for the reaction Cu²⁺ + Fe²⁺ ↔ Cu⁺ + Fe³⁺, thereby increasing its O_α/(O_α + O_β) value and redox properties and exhibiting the best NH₃-SCR catalytic performance based on a large amount of Cu⁺ and Fe³⁺. Fan et al. [88] also used three-dimensional (3D) hierarchical dandelion-like TiO₂ microspheres and reduced graphene oxide (rGO) as carriers to synthesize Cu/Fe-TiO₂-rGO catalysts for NH₃-SCR that exhibited good catalytic performance. The temperature window of the Cu_0.4Fe_0.6-TiO₂-rGO catalyst at 90% NO conversion was 250–350 °C. Moreover, the presence of Fe species enhanced the activity at a low temperature. In addition, there were a large number of Lewis acid sites and a small number of Brønsted acid sites on the catalyst surface, indicating that the Fe in the catalyst follows the L–H mechanism to promote the removal of NO_x. Moreover, Liu et al. [89] reported a WO₃-doped Fe₂O₃ catalyst for NH₃-SCR. The 10W/Fe catalyst showed excellent catalytic performance, with a temperature window of 275–425 °C when the NO conversion was >90%. This result was attributed to the interaction between WO₃ and Fe₂O₃. The doping of Fe₂O₃ with WO₃ enhances the acidity of the catalyst and increases its specific surface area, which is conducive to the adsorption of reactants and thus enhances the catalytic performance of the catalyst. The catalytic activity of the catalyst decreased after the introduction of SO₂ and H₂O, which was caused by the deposition of sulfate species. However, after the removal of SO₂ and H₂O from the reaction atmosphere, the catalytic activity recovered, indicating that the catalyst showed high H₂O and SO₂ tolerance. Wang et al. reported [90] that a 30% WO₃–FeO_x catalyst showed high catalytic activity, with 100% NO_x removal between 225 and 500 °C. The introduction of WO₃ promoted Fe₂O₃ to adsorb more oxygen. As shown in Figure 6A, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements showed that the introduction of WO₃ increased the number of acidic sites; moreover, it also helped to form adsorbed NO_x species, which explained the enhanced performance of the WO₃-FeO_x catalysts. It was reported that the catalyst followed E–R and L–H mechanisms (Figure 6B). Based on this, adding Ce increased the number of active oxygen atoms and the position of moderate alkalinity. Meanwhile, the addition of W significantly reduced the number of alkaline sites and increased the number of Brønsted acid sites. The addition of Ce and W was beneficial for balancing the surface acidity and alkalinity of the catalyst, which was also the reason why the catalyst exhibited excellent low-temperature SCR activity (Figure 6C) [79]. Kang et al. [91] also prepared Fe₂O₃-modified CeO₂-WO₃ catalysts for NO_x reduction in the presence of SO₂, which exhibited excellent SCR catalytic performance in the range of 270–420 °C. The addition of Fe₂O₃ increased the proportion of Ce³⁺ and the number of surface oxygen vacancies in the catalyst. In addition, after introducing SO₂, the addition of Fe₂O₃ inhibited sulfur poisoning and prevented the irreversible binding of SO₂ with active ingredients, which also explained the good SO₂ resistance of the catalyst.

Based on the above research, Fe-based SCR catalysts have been shown to exhibit higher activity at medium to high temperatures, as shown in

Table 4. The catalytic performances of Fe-based catalysts for NH₃-SCR reaction in reported the literature.

Catalysts		Feed Composition				GHSV (h−1)	Conversion (Corresponding Temperature Window)	Ref.
	NO (ppm)	NH3 (ppm)	O2 (vol%)	SO2 (ppm)	H2O (vol%)
6FeSCe	500	500	5	-	-	118,000	>90% (300–450 °C)	[77]
	500	500	5	50	5		85% (300 °C)
FeTi	500	500	4			60,000	>90% (350–450 °C)	[78]
Fe0.93Ce0.07Ox	500	500	5	-	-	90,000	>95% (175–325 °C)	[80]
	500	500	5	200	5		93% (250 °C)
γ-Fe2O3-FM	500	500	3	-	-	30,000	96% (230 °C)	[81]
	500	500	3	300			79% (230 °C)
Fe9Ti1Ox	500	500	5	-	-	60,000	>80% (150–350 °C)	[84]
Fe0.94Sm0.06Ox	500	500	5	-	-	60,000	>95% (175–325 °C)	[86]
	500	500	5	200	5		85% (250 °C)
10W/Fe	500	500	5	-	-	100,000	>90% (275–425 °C)	[89]
	500	500	5	50	5		90% (350 °C)
0.3W-Fe	600	600	5			60,000	100% (250–500 °C)	[90]
	600	600	5	100	10		90% (250 °C)
Fe(4)@CeW/H	500	500	5			40,000	>96% (270–420 °C)	[91]
	500	500	5	100	8		82% (300 °C)

, but their low-temperature activity is not ideal and needs to be further improved. Therefore, modified Fe-based catalysts are particularly important for enhancing low-temperature catalytic performance and N₂ selectivity.

3.4. Ce-Based Catalyst

Cerium oxide has unique advantages in the NH₃-SCR reactions due to its excellent capacity for storing and releasing oxygen [92,93], good redox performance [94,95], and strong NO_x and NH₃ adsorption capabilities [96,97]. However, undoped CeO₂ catalysts exhibit poor SCR activity and SO₂ resistance at <200 °C. The presence of SO₂ results in the formation of NH₄HSO₄, Ce₂(SO₄)₃, and Ce(SO₄)₂ on Ce-based catalysts [98]. NH₄HSO₄ blocks the active sites and reduces the activity of the catalysts. Ce(SO₄)₂ and Ce₂(SO₄)₃ inhibit the formation and adsorption of nitrate, disrupting the Ce³⁺/Ce⁴⁺ redox cycle and leading to deactivation of the catalyst, which greatly limits the application of CeO₂ [99,100]. Adding other active components to CeO₂ can effectively resolve these issues. A good interaction between Ce and Mn endows them with good redox properties and denitration performance, as well as sulfur resistance [101,102]. Compared with a single-oxide catalyst, MnO_x–CeO₂ has been shown to exhibit higher activity for NO oxidation, which is due to its higher oxygen mobility and stronger reducibility [103]. Therefore, Ce–Mn composite oxide catalysts have been widely applied for NH₃-SCR reactions. Qi et al. reported, for the first time, that a 30 wt% Mn/70 wt% Ce exhibited excellent SCR activity at low temperatures [104]. Gao et al. [105] synthesized Mn/CeO₂ catalysts and studied their catalytic activity. The results showed that the Mn/CeO₂ catalyst exhibited high activity, with a NO_x conversion of 100% at 150–240 °C. Moreover, it was also reported that Mn–Ce composite oxides exhibited excellent low-temperature SCR catalytic performance, with a temperature window of 150–310 °C at >90% NO_x conversion [106]. It is worth noting that the Mn–Ce composite oxide exhibited good SO₂ resistance, with the NO_x conversion of the catalyst increasing from 92.6% to 97.8% at 150 °C in the presence of 200 ppm SO₂ (Figure 7A). In the presence of SO₂ in the reaction atmosphere, the NH₃ adsorption sites of the active component separated from the oxidant, inhibiting the oxidation of NH₃ (Figure 7B). In addition, a prepared Mn_xCe_y binary catalyst with a 3D network structure also exhibited good catalytic performance, which was attributed to the rich pore structure of the catalyst being conducive to gas diffusion and promoting the NH₃-SCR reaction. Among them, the Mn₁Ce₁ catalyst showed the best denitrification properties, with a NO conversion of 97% at 180 °C [107]. Lin et al. also studied MnO_x-CeO₂/TiO₂ catalysts for the NH₃-SCR reaction, with the catalysts exhibiting excellent low-temperature catalytic performance for the NH₃-SCR due to the large differences in radii; Mn atoms can enter lattice CeO₂ to form a MnCeO_x solid solution structure, which improved the redox performance of the catalyst. Moreover, the presence of abundant Mn⁴⁺ and Ce³⁺ on the catalyst surface generated more active oxygen [108].

In addition to manganese, the doping of the catalysts with other metals can also improve their denitration activities. Wang et al. [109] prepared WO₃/CeO₂ catalysts, which exhibited excellent SCR performance due to the strong interaction between W and Ce. It was also reported that W–Ce composite metal-oxide catalysts loaded with different W/Ce content were prepared via a coprecipitation method, which had a significant impact on the surface acidity, especially the number of Brønsted acid sites, with W_0.18CeO_x exhibiting the best NH₃-SCR performance at 190–490 °C for 100% NO_x conversion, while still maintaining a wide active temperature window from 235 to 460 °C when treated with 5% H₂O at high temperature for 12 h [110]. Yao et al. [111] developed TiO₂/CeO₂ nanocubes and nanorods for the effective reduction of NO_x. It was speculated that the strong interaction between the (110) plane of the CeO₂ nanorods and highly dispersed TiO₂ may have contributed to the improvement in the activation and reduction behavior of chemically adsorbed oxygen. Moreover, Liu et al. [112] prepared Co-, Cu-, and Fe-doped Ce-Ti mixed oxides, and improved the low-temperature SCR activity of the catalysts. Among the tested samples, Co-Ce-Ti catalysts exhibited the best SCR activity and the widest temperature window at low temperatures. This may be due to the different ionic sizes of Co²⁺ and Ce⁴⁺, which greatly promote the lattice distortion of Ce-Ti mixed oxides. Subsequently, the proportion of Ce³⁺ to surface adsorbed oxygen increased, which was conducive to the generation of adsorbed NO_x species and to improving the L-H reaction. Meanwhile, the coordination of unsaturated cation sites on the Co-Ce-Ti samples induced the adsorption of more NH₃ at Lewis acid sites. It is considered to be key in the E-R mechanism, thereby promoting a reaction that follows the E-R mechanism. The enhancement in the reaction via the L-H and E-R mechanisms seemed to be directly related to an improvement in the denitration performance of the Co-Ce-Ti samples, and the L-H mechanism at low temperatures may have been the main mechanism due to its fast reaction rate (Figure 8). In addition, Ce-Cu composite oxides have attracted research attention due to the interaction of Ce and Cu in the oxidation reaction. However, even during mild heat treatment, deactivation of Ce-Cu composite oxides may occur. This deactivation may be due to the high migration rate over Cu and the sensitivity of CeO₂ to thermal damage, leading to the formation of Cu_xO_y clusters [113,114]. Therefore, developing more heat-resistant Ce-Cu composite oxides is the most important step in the application of CeCu composite oxides in the SCR processes [115,116,117]. In further research, among the Co-, Mn-, and Cu-modified Ce-W-Ti catalysts, the temperature range of Cu/Ce-W-Ti catalysts with 5 wt.% Cu was 260–400 °C at a NO_x conversion of >90%. The catalysts also exhibited higher SO₂ and water vapor resistance, which may be attributed to the highly dispersed Cu species, in addition to the higher Ce³⁺ and active oxygen contents of the catalysts [118]. In addition, Li et al. [119] reported novel ternary CeNbVO catalysts for NH₃-SCR. The prepared CeNbVO catalysts exhibited high NH₃-SCR activity, where among them the temperature window of CeNbVO-2 was 210–420 °C for a NO conversion of >90%. The excellent catalytic performance of the catalyst was attributed to its appropriate acidity and redox properties. Moreover, this catalyst showed better H₂O/SO₂ resistance than the other tested catalysts.

Based on the above research, progress has been made in the study of Ce-based catalysts for NH₃-SCR. Among them, Mn-Ce composite oxide catalysts showed the best SCR activity. However, for further applications, the temperature window at high NO conversion, N₂ selectivity, and stability of Mn-Ce composite oxides still need to be further improved. In addition, the catalytic mechanism of Ce-based catalysts is relatively complex and further research on this is required.

3.5. Cu-Based Catalyst

The Cu-based catalysts are widely applied in the NH₃-SCR reaction and feature Cu²⁺/Cu⁺, which is both an oxidation–reduction and acidic site [120]. Usually, an undoped CuO catalyst exhibits poor SCR catalytic performance compared with modified Cu-based catalysts. Yu et al. prepared a series of CuO–V₂O₅/TiO₂ catalysts for the SCR of NO with NH₃ and found that the addition of the Cu species promoted the production of more V⁴⁺ and chemisorbed oxygen in the V-Ti catalysts, resulting in enhanced NH₃-SCR catalytic performance [121]. Wang et al. [122] reported that the addition of copper in the V/Ti catalysts enhanced the number of acidic sites and active oxygen species on the catalyst and promoted good redox properties. Zheng et al. also studied the SCR performance of CuV/Ti catalysts and found that the NO conversion of the CuV/Ti catalysts reached 100% in the range of 220–330 °C with excellent N₂ selectivity. This is because the addition of CuO transformed the V/Ti catalyst from a single active center (V) to a dual active center (V + Cu) [123]. From this, it can be seen that CuO is a good additive for V-Ti catalytic systems. Moreover, Raja et al. [124] also reported TiO₂–carbon nanotube (CNT) loaded MnO_x–CuO catalysts for the NH₃-SCR of NO at low temperatures, and it was found that the addition of Cu improved the catalytic activity of the catalyst. Different Cu loadings had different effects on the SCR performance. The catalyst with 5 wt% Cu content was found to be the most active catalyst. Even in the presence of 200 ppm SO₂, the temperature window for the Mn-Cu₅/Ti-CNTs for >90% NO conversion was still the widest at 200–300 °C, which was related to the increasing content of Mn⁴⁺ and chemical adsorption on the surface of the catalyst as a result of Cu loading. Researchers have also developed a new type of Mo-doped CuO catalyst for the low-temperature NH₃-SCR reaction. The NO_x conversion of a CuO catalyst doped with Mo was reported to be >80% at 175 °C. Mo doping into the CuO lattice forms Mo-O-Cu species, improving the adsorption of NH₃ and NO_x and promoting the formation of oxygen vacancies, which are important factors in promoting the SCR activity [125]. In addition, the sulfated Cu-based catalyst also exhibited excellent catalytic performance and sulfur resistance in the NH₃-SCR reaction.

A Cu-based catalyst surface sulfated by SO₂ forms new acidic sites, which inhibit the oxidation of NH₃ and enhance its resistance to SO₂. Yu et al. [126] prepared CuSO₄/TiO₂ catalysts with different CuSO₄ loading amounts (0–20 wt%). The effect of the copper(II) sulfate content on the catalytic activity was investigated. A high CuSO₄ loading was shown to be beneficial for the interaction between CuSO₄ and TiO₂. With an increase in CuSO₄ loading, the activity of the catalyst was significantly improved at <340 °C, however, the oxidation of NH₃ was also enhanced at >340 °C. A novel Cu-Ce-S catalyst was prepared by Shi et al. for the NH₃-SCR [127], where the S and Cu originated from CuSO₄. The NO_x conversion of the 3% Cu-Ce-S catalyst decreased by only 3% in the presence of H₂O and SO₂ due to the Cu²⁺ being doped into the CeO₂ lattice. During the reaction, SO₄²⁻ preferentially connected with Cu²⁺, protecting the active site of Ce⁴⁺. The Cu²⁺ ↔ Cu⁺ and Ce^{4 +} ↔Ce³⁺ redox cycles also enhanced the activity of the catalyst.

In addition to modifying Cu-based catalysts, special structures can also be designed to improve catalytic performance. Cu-based catalysts with a core–shell structure have been extensively studied for the SCR of NO_x by NH₃. Cu-based catalysts with a core–shell structure can achieve a dynamic equilibrium between NH₃ oxidation and NO_x reduction. Generated NO and NO₂ can diffuse from the core and react with NH₃ to form N₂, reducing the formation of the by-product N₂O. Yu et al. [128] designed and synthesized a CuO@Cu-MOF core-shell catalyst with CuO as the core and Cu₃(BTC)₂ as the shell via an in situ growth method for the NH₃-SCR reaction (Figure 9A). The CuO core in the catalyst exhibited excellent NO adsorption performance and reducibility, while the Cu₃(BTC)₂ shell also featured a large number of acidic sites. This material with abundant acidic sites and NO_x intermediate species promoted the low-temperature NH₃-SCR reaction. In addition, the CuO@Cu-MOF also showed satisfactory stability, making it a promising low-temperature SCR catalyst. In addition, the core–shell structure also prevented the low-temperature activity of the catalyst from being affected by SO₂ and water vapor. This new type of titanium dioxide-supported copper-doped phosphomolybdic acid (Cu-HPMo) was studied by Jiang et al. [129] As shown in Figure 9B, the Cu is the core and HPMo is the shell. The results show that when the Cu/Mo molar ratio is three, the Cu(3)-HPMo/TiO₂ catalyst shows the best SCR performance, with a denitrification efficiency of ≥99% at 200 °C, which does not decrease in the presence of 200 ppm SO₂ and 4 × 10⁴ ppm H₂O. This indicates that strong acidity inhibits SO₂ adsorption and exhibits good resistance to SO₂. According to density functional theory calculation results, SO₂ is not chemisorbed on the HPMo surface, indicating that the catalyst with this structure exhibited the best sulfur resistance.

Based on the above research, modified Cu-based catalysts exhibit enhanced SCR catalytic performance. However, the operating temperature window for high NO conversion over Cu-based catalysts is relatively narrow, which is an important issue that needs to be resolved so that Cu-based oxide catalysts can be practically used. In addition, the low-temperature SCR activity of Cu-based oxide catalysts needs to be further improved.

4. The Stability of the SCR Catalyst

SCR devices are usually placed downstream of the diesel particulate filter (DPF), and therefore denitration catalysts need to be able to withstand the temperature of the DPF regeneration. Thus, it can be seen that improving the high-temperature stability of denitration catalysts is of great significance for diesel exhaust aftertreatment systems. The important factor that affects the high-temperature resistance of the catalyst is its surface acidity. Therefore, the high-temperature resistance of catalysts is generally enhanced by increasing their surface acidity. Acidic oxides, such as WO₃ and ZrO₂, are considered to be the optimal materials for improving high-temperature stability [130]. Chen et al. [131] prepared W-Zr-O_x/TiO₂ catalysts for the NH₃-SCR via a precipitation method. The loading of W-Zr-O_x resulted in having more acidic sites on the catalyst surface, better redox performance, and a higher proportion of oxygen vacancies, thereby improving the high-temperature activity of the catalyst. It has also been found that the interaction between WO₃ and ZrO₂ can improve the surface acidity and redox performance of a catalyst, which is beneficial to its excellent performance at high temperatures. Feng et al. [132] used different methods to prepare W-Zr-ZSM-5 catalysts, among which catalysts with rich acidic sites and excellent redox properties exhibited better deNO_x performance at high temperatures. In addition, cerium(IV) oxide, as an additive, formed a Ti-Ce-O_x catalyst with Ti and significantly improved the catalytic activity and thermal stability of the TiO₂ catalyst support. After high-temperature treatment for 24 h, the catalytic activity of Ti-Ce-O_x-500 remained unchanged at 400 °C. This excellent catalytic performance and stability were attributed to its high specific surface area, in addition to its abundant acidic sites and active oxygen species. In addition, cerium atoms have been shown to prevent grain growth and protect the pore structure of catalysts at high temperatures [133].

However, when a diesel engine is cold started or operated under low engine load, there are also requirements for the low-temperature catalytic performance of denitration catalysts. Currently, the low-temperature activity of denitration catalysts is usually improved by adding metal oxides. Among them, Mn-based catalysts exhibit high low-temperature activity due to their variable valence states and excellent redox properties. However, the operating temperature window for high NO conversion over MnO_x is relatively narrow and the water- and sulfur-resistant performance needs to be improved (see Section 3.2 for details).

5. Conclusions and Outlook

NO_x, as one of the main pollutants in diesel vehicle exhaust, seriously endangers the atmospheric environment and human health. Therefore, the elimination of NO_x has become the focus of research. At present, the most effective and widely used technology for controlling NO_x from diesel vehicle exhaust is NH₃-SCR technology. This paper introduced the reactions that existed in the NH₃-SCR reaction process and their relationships. And the research progress of metal oxide catalysts for the NH₃-SCR reaction was also described, mainly including V-based catalysts, Mn-based catalysts, Fe-based catalysts, Ce-based catalysts, and Cu-based catalysts. V-based catalysts as mature commercial catalysts were studied earlier. Mn-based catalysts have the best low-temperature SCR performance. The Fe-based catalysts, Ce-based catalysts and Cu-based catalysts with good catalytic performance have also attracted attention. However, there are still many problems and challenges in the application of metal oxide catalysts for NO_x control, and further research is needed. Therefore, the following issues need to be addressed in future research on metal oxide catalysts:

(1)

The metal oxide catalysts have disadvantages, such as narrow operating temperature windows basically within the low or medium–low temperature range and poor N₂ selectivity. Therefore, modifying catalysts to enhance their NH₃-SCR catalytic performance is the main focus of future research.

(2)

The presence of H₂O and SO₂ are definite in diesel vehicle exhaust and metal oxides can easily react with them to deactivate the catalyst, therefore, improving the hydrothermal stability and SO₂ tolerance of the catalyst remains the major factor of the SCR technology.

(3)

Further research needs to be focused on the reaction mechanisms of sulfur dioxide and water poisoning processes for catalysts. The studies of catalyst poisoning mechanisms cannot determine the process of catalyst poisoning only through in situ characterization technology. The synchrotron-radiation, theoretical calculations, and isotopic tracer techniques should also be considered to fully explain the mechanisms of the catalyst poisoning process.

(4)

The injection of NH₃ is a major component of the SCR device, but there may be blockages in the NH₃ injection on occasion and the NH₃ amount is imprecise. Therefore, optimizing the injection way and accurately controlling the injection amount will be one of the important goals.