A Review of Low Temperature NH₃-SCR for Removal of NO_x

Abstract

The importance of the low-temperature selective catalytic reduction (LT-SCR) of NO_x by NH₃ is increasing due to the recent severe pollution regulations being imposed around the world. Supported and mixed transition metal oxides have been widely investigated for LT-SCR technology. However, these catalytic materials have some drawbacks, especially in terms of catalyst poisoning by H₂O or/and SO₂. Hence, the development of catalysts for the LT-SCR process is still under active investigation throughout seeking better performance. Extensive research efforts have been made to develop new advanced materials for this technology. This article critically reviews the recent research progress on supported transition and mixed transition metal oxide catalysts for the LT-SCR reaction. The review covered the description of the influence of operating conditions and promoters on the LT-SCR performance. The reaction mechanism, reaction intermediates, and active sites are also discussed in detail using isotopic labelling and in situ FT-IR studies.

1. Introduction

The non-renewable fossil fuels are continuing to remain the dominant energy source in power plants and automobiles to satisfy the ever-growing energy demands. However, the combustion of fossil fuels mainly generates nitrogen oxide (NO_x) pollutants (NO, NO₂, and N₂O and their derivatives) which can cause acid rain, photochemical smog, ozone depletion, and eutrophication problems [1,2,3,4]. Due to the negative impacts of NO_x, the mitigation of NO_x emissions is of paramount importance for environmental protection. Several technologies are available to reduce NO_x emissions by using catalytic materials and among them, selective catalytic reduction of NO_x with NH₃ (NH₃-SCR) has been widely applied due to its high NO_x removal efficiency [5,6,7]. Usually, the flue gas temperature of the industrial process is as low as 300 °C and, thus, the SCR catalyst must be active in the low-temperature regime (100–300 °C). V₂O₅–WO₃(MoO₃)/TiO₂ is the typical and efficient catalyst and has been commercialized for NH₃-SCR technology for medium temperature process [8,9]. However, this catalyst has some intrinsic drawbacks such as narrow and high working temperature window (350–400 °C), and low N₂ selectivity in the high-temperature range [3,10,11]. Therefore, many researchers continue to develop highly active catalysts for low-temperature NH₃-SCR in a wide temperature window.

With this perspective, several transition metal oxide-based catalysts have been extensively investigated for low-temperature NH₃-SCR reaction due to their excellent redox properties, low price, and high thermodynamic stability. Especially, the easy gain and loss of electrons in the d shell of the transition metal ions could be responsible for the facile redox properties [12,13,14]. For example, the Cr/TiO₂ [15], Cr-MnO_x [8], Fe-MnO_x [16], Mn/TiO₂ [17,18], Fe_xTiO_y [19], MnO_x/CeO₂ [20], and Cu/TiO₂ [21] catalysts were shown to exhibit good SCR activity in the low temperature range. In our earlier work, we investigated the low-temperature NH₃-SCR in the presence of excess O₂ on the TiO₂ supported V, Cr, Mn, Fe, Co, Ni, and Cu oxides and found the catalytic performance decreased in the following order of Mn > Cu ≥ Cr ≫ Co > Fe ≫ V ≫ Ni [16]. Particularly, manganese-containing catalysts have attracted much attention due to its variable valence states and excellent redox ability [2,5]. In the recent past, we published a series of papers on Mn-based SCR catalysts that showed a highly promising deNO_x potential in the low-temperature region [5,22,23,24,25,26]. However, these catalysts are very sensitive to the presence of SO₂ in the feed and exhibit lower N₂ selectivity [8,27,28,29]. Hence, the development of catalysts that show both good low-temperature activity and high SO₂/H₂O durability is of great importance for the NH₃-SCR reaction. In general, there are two plausible strategies available to enhance low-temperature NH₃-SCR performance. One strategy is to modify the transition metal oxide with one or multiple metal oxides, which could enhance the active sites for the reaction by inducing the synergistic effect [30,31,32,33]. The other approach is to synthesize the supported materials to disperse the transition metal-based oxides which can increase the activity by metal-support interactions [26,34,35,36,37]. Recently, many supported and mixed transition metal catalyst formulations have been studied to improve the low-temperature SCR performance, as well as resistance to SO₂/H₂O.

In this study, we systematically reviewed the recent advancements in developing the transition metal-based catalysts for low-temperature NH₃-SCR reaction. This review also demonstrated the action of different promoters and supports on the catalytic performance and SO₂/H₂O tolerance of the transition metal-based catalysts in NH₃-SCR of NO_x. The reported catalysts were divided into four categories, such as binary, ternary/multi, supported single, and supported binary/multi-transition metal-based catalysts.

2. Binary Transition Metal-Based Catalysts

Various transition-metal oxides have been proved to be active for the NH₃-SCR at low-temperature. However, the catalytic performance on single transition metal oxide is far from satisfactory due to their low specific surface area and thermal instability [38,39,40,41]. The addition of dopants is a common method to improve the drawbacks associated with pure transition metal oxide. Hence, much progress has done to improve the low-temperature SCR activity of transition metal oxides by mixing or doping with other metal oxides. In recent years, Mn, Fe, Co, Ni, and Cu-based binary oxide catalysts have been extensively studied for low-temperature NH₃-SCR reaction due to their attractive catalytic performance [19,33,41,42,43,44,45,46,47,48,49]. Particularly, Mn-based binary oxides are popular and proven to be effective catalysts for low-temperature NH₃-SCR reaction [42,50,51]. Recently, Xin et al. [52] designed bifunctional V_a-MnO_x (where a represents the molar ratios of V / (V + Mn)) catalysts composed of Mn₂O₃ and Mn₂V₂O₇ phases that significantly improved both NO_x conversion and N₂ selectivity in comparison with Mn₂O₃ at low-temperature (Figure 1). Although Mn₂V₂O₇ showed an excellent N₂ selectivity, the NO_x conversion is much lower on it. Especially, above 90% NO_x conversion and 80% N₂ selectivity was observed in the temperature region of 120–240 °C over the V_0.05-MnO_x catalyst. The V_0.05-MnO_x catalyst also found to be exhibit higher NO_x conversion to N₂ as compared to the mechanically mixed Mn₂O₃ + Mn₂V₂O₇ sample which has the same component content to V_0.05-MnO_x (Figure 1). This finding indicated that the synergism between Mn₂O₃ and Mn₂V₂O₇ exists in the chemically prepared V_0.05-MnO_x rather than the mechanically mixed Mn₂O₃ + Mn₂V₂O₇ sample. Moreover, the mechanically mixed Mn₂O₃ + Mn₂V₂O₇ sample showed higher activity in comparison to mechanically mixed MoO₃ + Mn₂V₂O₇ sample, suggesting that the presence of Mn₂O₃ phase in the catalyst is necessary for NH₃-SCR reaction. In conjunction with in situ IR characterization and DFT (density functional theory) calculation results, the authors concluded that the Mn₂O₃ phase of the catalyst could activate NH₃ into NH₂ intermediate, which then transferred to the Mn₂V₂O₇ phase of the catalyst and reacted with gaseous NO into NH₂NO. Finally, the generated NH₂NO intermediate on the Mn₂V₂O₇ phase exclusively decomposed to the N₂ rather than the undesired byproduct, N₂O, which is formed due to the deep oxidation of adsorbed NH₃ on Mn₂O₃.

Han and co-workers [53] fabricated triple-shelled NiMn₂O₄ hollow spheres (Figure 2a,b) by using a solvothermal method and tested their ability for low-temperature NH₃-SCR reaction. As shown in Figure 2c, the prepared NiMn₂O₄ hollow spheres (NiMn₂O₄-S) showed the best catalytic activity with NO_x conversion of above 90% over a wide temperature range from 100 °C to 225 °C as compared to the NiMn₂O₄ nanoparticles (NiMn₂O₄-P). The triple-layer shell structure of the NiMn₂O₄-S catalyst generates a larger surface area (165.3 m² g^‒1) that exposes more active sites (such as surface Mn⁴⁺ and surface adsorbed oxygen species), which are responsible for its superior activity. Additionally, the NiMn₂O₄-S catalyst displayed outstanding stability and good tolerance to H₂O and SO₂ (Figure 2d).

Gao et al. [54] investigated the low-temperature NH₃-SCR reaction over the hydroxyl-containing Me-Mn binary oxides (Me = Co, Ni) prepared by a combined complexation–esterification method. It was found that the NO_x conversion decreased in the order of Mn₃O₄-Co₃O₄-OH (Co-MnO_x binary oxide) > Mn₂O₃-NiMnO₃-OH (Ni-MnO_x binary oxide) > Mn₂O₃-OH, while the N₂ selectivity increased in the sequence of Mn₃O₄-Co₃O₄-OH < Mn₂O₃-OH < Mn₂O₃-NiMnO₃-OH. Although the Co and Ni elements in the catalysts delay the poisoning of SO₂ as compared to MnO_x sample, the Co-MnO_x and Ni-MnO_x binary oxides are deactivated by SO₂ over the postponement due to the formation of metal sulfate and ammonia hydrogensulfite species. In another study, Sun and co-workers [55] prepared Mn_0.66M_0.33O_x catalysts (M = Fe, Zn, Cu) and a series of Fe_αMn_1−αO_x (α = 1, 0.25, 0.33, 0.50, 0 mol%) catalysts and examined for NH₃-SCR at low-temperatures. The results demonstrated that the Fe_0.33Mn_0.66O_x catalyst displayed the superior NH₃-SCR activity (NO_x removal efficiency > 90%) in a wide temperature range (75–225 °C) among the Cu_0.33Mn_0.66O_x, Zn_0.33Mn_0.66O_x, and Fe_αMn_1−αO_x (α = 1, 0.25, 0.50, 0 mol%) catalysts. The authors proposed that the distortion of the catalyst structure by Fe doping could play a key role in improving the NH₃-SCR performance over the Fe_0.33Mn_0.66O_x catalyst.

Rare-earth metal oxides have been frequently adopted to modify the MnO_x as an efficient low-temperature NH₃-SCR catalyst due to their incomplete 4f and empty 5d orbitals [50,51,56]. Fan et al. [57] synthesized Gd-modified MnO_x catalysts with Gd/Mn molar ratio of 0.05, 0.1, and 0.3 to improve the catalytic performance and sulfur resistance in the NH₃-SCR reaction at low-temperature. The MnGdO-2 catalyst (the mole ratio of Gd/Mn = 0.1) found to show the optimal NO conversion and N₂ selectivity among the investigated catalysts. The addition of a proper amount of Gd into MnO_x could enhance the concentrations of surface Mn⁴⁺ and chemisorbed oxygen species, and increase the amount and the strength of surface acid sites, which lead to better low-temperature catalytic performance than the others. Furthermore, the MnGdO-2 catalyst had an excellent tolerance to SO₂/H₂O as compared to pure MnO_x sample (Figure 3). Their results demonstrate that the doping of Gd could restrains the transformation of MnO₂ to Mn₂O₃ and the generation of MnSO₄, obstructs the decrease in Lewis acid sites and the increase in Brønsted acid sites, and eases the competitive adsorption between the NO and SO₂ and, thus, improves the resistance to SO₂.

Li et al. [58] developed hollow MnO_x-CeO₂ binary nanotubes as efficient low-temperature NH₃-SCR catalysts via an interfacial oxidation-reduction process using KMnO₄ aqueous solution and Ce(OH)CO₃ nanorod as both template and reducing agent without any other intermediate. They reported that the MnO_x-CeO₂ hollow nanotube catalyst with 3.75 g of Ce(OH)CO₃ template (denoted it as MnO_x-CeO₂-B) exhibited outstanding performance with more than 96% NO_x conversion in the temperature range of 100–180 °C. The best activity of the MnO_x-CeO₂-B catalyst was due to its ample number of surface Mn⁴⁺ and O species, and hollow and porous structures that provide abundant Lewis acid sites and large surface area. Additionally, MnO_x-CeO₂-B catalyst showed an excellent resistance to H₂O and SO₂ (Figure 4) and especially, the great SO₂ tolerance was ascribed to the hierarchically porous and hollow structure that inhibits the deposition of ammonium sulfate species, and the doping of ceria that acts as an SO₂ trap to limit sulfation of the main active phase.

Fe-based binary catalysts have also been studied as NH₃-SCR catalysts due to their high activity, excellent resistance to H₂O and SO₂, outstanding environmentally friendly performance, lower cost, and higher abundance [59,60,61,62]. Mu et al. [63] synthesized a series of vanadium-doped Fe₂O₃ catalysts and evaluated the effect of V on the low-temperature NH₃-SCR activity of hematite. The NH₃-SCR activity and N₂ selectivity are greatly enhanced after the incorporation of vanadium into Fe₂O₃ and the Fe_0.75V_0.25O_δ catalyst with a Fe/V mole ratio of 3/1 showed the best catalytic performance over a wide temperature window and strong resistance to H₂O and SO₂. They found that the charge transfer from Fe to V due to the electron inductive effect between Fe and V which could enhance the redox ability and surface acidity thereby superior NH₃-SCR activity at low-temperature. The in situ DRIFTS and kinetic studies suggested that the SCR reaction followed the Langmuir–Hinshelwood mechanism below 200 °C, while an Eley−Rideal mechanism dominated at and above 200 °C. Li and co-workers [64] reported novel iron titanium (CT-FeTi) catalyst, prepared by a CTAB-assisted process, showing good deNO_x efficiency and H₂O resistance at low-temperature as compared to the FeTi catalyst that prepared without adding CTAB. The addition of CTAB during the CT-FeTi catalyst preparation not only promotes to form the uniform mesoporous structure to avoid being excessively enlarged in the presence of H₂O, but also enhances the adsorption of bridging nitrate and NH₃ species on Lewis acid sites. Thus, the authors concluded that the CTAB acted as a “structural” and “chemical” promoter in improving the NH₃-SCR activity and H₂O resistance at low-temperature.

Recently, Co-based spinel catalysts have shown to exhibit a remarkable low-temperature NH₃-SCR activity, N₂ selectivity, and tolerance to SO₂/H₂O [30,65,66,67]. Meng et al. [48] synthesized a highly efficient Co_aMn_bO_x (where a/b is the molar ratio of Co/Mn) mixed oxide catalysts and investigated the effects of the Co/Mn molar ratio on the low-temperature NH₃-SCR reaction. The Co_aMn_bO_x mixed oxides showed higher NH₃-SCR activity than either MnO_x or CoO_x alone due to their improved redox properties and surface acid sites by the synergistic effects between the Co and Mn species. Particularly, the catalyst with Co/Mn molar ratio of 7:3 (Co₇Mn₃O_x) exhibited the greatest activity (>80% NO_x conversion) in a temperature window of 116–285 °C as compared to the catalysts with Co/Mn molar ratio of 5:5 (Co₅Mn₅O_x) and 3:7 (Co₃Mn₇O_x). They considered that the high NO + O₂ adsorption ability and enhanced redox properties of the Co₇Mn₃O_x catalyst, emerging from its MnCo₂O_4.5 spinel phase and higher surface area, were beneficial to augment the NH₃-SCR performance by forming nitrate species on the catalyst surface. Although Co₇Mn₃O_x catalyst had better resistance to H₂O/SO₂ than the Co₃Mn₇O_x and MnO_x, the tolerance to SO₂ poisoning still need to be improved for practical use. Nevertheless, it was found that the deactivated Co₇Mn₃O_x, Co₃Mn₇O_x, and MnO_x catalysts in SO₂ stream can be regenerated simply by washing with water. Based on their results, the authors also proposed the NH₃-SCR reaction mechanism over the Co₇Mn₃O_x catalyst, which is shown in Scheme 1. The reaction was initiated by adsorption and activation of gaseous oxygen on oxygen vacancies (symbol □), which was then transformed into lattice oxygen O²⁻ (Step 1); This lattice oxygen was diffused to the catalyst surface and then it had become surface active oxygen (O*) (Step 2); Gaseous NO was adsorbed and subsequently reacted with O* to form NO₂/NO₃⁻ intermediates (Step 3); Meanwhile, NH₃ was activated to −NH₂ and NH₄⁺ species by Mn⁴⁺ (Step 4); Finally, NO₂/NO₃⁻ intermediates reacted with the NH species to produce reaction products, N₂, and H₂O (Step 5); By the electron transfer from Mn³⁺ to Co³⁺ (Step 6); the catalyst was recovered to its original state (Step 7); Thus, the synergistic effect between the Co and Mn plays a key role in improving the NH₃-SCR activity over Co₇Mn₃O_x catalyst.

Mesoporous materials have been proved as promising catalysts for NH₃-SCR reaction since they can facilitate to promote effective diffusion of reactants towards the active sites [30,65,66,68]. With this perspective, Hu et al. [47] developed mesoporous 3D nanosphere-like Mn-Co-O catalysts through a template-free approach and evaluated for low-temperature NH₃-SCR reaction. It was found that the synthesized Mn-Co-O samples showed excellent NH₃-SCR activity in a broad working temperature window of 75 to 325 °C (NO_x conversion above 80%). They ascribed this outstanding performance to the strong and abundant acid sites, the strong adsorption of NO_x, robust redox properties, the formation of more oxygen vacancies and metal-metal interactions between the cobalt and manganese species.

Besides Mn, Fe, and Co oxides, CuO_x has also been considered in the bimetallic catalyst formulations for low-temperature NH₃-SCR reaction [69,70]. For instance, Ali et al. [71] reported the Cu_x-Nb_1.1-x (x = 0.45, 0.35, 0.25, 0.15) bimetal oxides catalysts and found that the Cu/Nb ratio was crucial in enhancing the NH₃-SCR activity. As shown in Figure 5a,b, all binary Cu_x-Nb_1.1-x oxides exhibited significantly higher activity than the CuO_x and Nb₂O₅, and among the Cu_x-Nb_1.1-x samples, Cu_0.25-Nb_0.85 catalyst displayed the best performance in a wide temperature window of 180–330 °C (>90% NO conversion). Even at a high GHSV of 105,000 h⁻¹, the optimal Cu_0.25-Nb_0.85 catalyst showed a good NO removal efficiency (above 90% NO conversion) from 210 °C to 360 °C (Figure 5c). Although the SO₂/H₂O streams in the feed gas have some adverse impact on Cu_0.25-Nb_0.85, still the catalyst showed excellent resistance to SO₂/H₂O with reversible deactivation (Figure 5d). The superior NH₃-SCR performance and SO₂/H₂O tolerance of Cu_0.25-Nb_0.85 catalyst were attributed to its high acid amount and NO adsorption capacity.

3. Ternary and Multi-Transition Metal-Based Catalysts

The catalytic performance of single transition metal oxides can also be improved by mixing with two or multi other metal oxides. Thus, transition metal oxides are widely reported to fabricate ternary- or multi-metal-based low-temperature NH₃-SCR catalysts, which could improve the catalytic activity by the enlarged synergetic interactions [14,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87]. Fang et al. [88] investigated the low-temperature NH₃-SCR reaction over the Fe_0.3Mn_0.5Zr_0.2 catalyst and found that it showed an excellent deNO_x activity with 100% NO conversion in the temperature range of 200–360 °C as compared to the Fe_0.5Zr_0.5 and Mn_0.5Zr_0.5 samples. Moreover, the Fe_0.3Mn_0.5Zr_0.2 catalyst had outstanding stability and good tolerance to SO₂ (Figure 6), which they attributed to the strong interactions among Fe, Mn, and Zr species. However, the durability of the catalyst in the presence of both SO₂ and H₂O need to be tested to investigate its feasibility in practical use.

Guo and co-workers [89] studied the effect of Sb doping on the activity of MnTiO_x catalyst for NH₃-SCR reaction. The results showed that Sb modification has greatly improved the NH₃-SCR performance of MnSbTiO_x catalysts in comparison to the MnTiO_x and SbTiO_x samples. Particularly, the MnSbTiO_x-0.2 (Sb/Mn molar ratio = 0.2) catalyst exhibited the best activity with above 90% NO_x conversion in the temperature range of 138–367 °C as it had good adsorption and activation properties for NH₃ and NO_x reactants in SCR. It can be seen from Figure 7 that the addition of Sb dramatically improved the SO₂ and H₂O resistance of MnTiO_x catalyst. Although the NO_x conversion over the MnSbTiO_x-0.2 catalyst slightly decreased in the presence of SO₂ and H₂O, it recovered to almost the original level after stopping the SO₂/H₂O supply.

Shi et al. [90] synthesized a series of Ni_yCo_1-yMn₂O_x microspheres (MSs) (y = 0.1, 0.3, 0.5, 0.7, 0.9) for NH₃-SCR using a hydrothermal method. It was observed that the activity of all ternary MSs was greater than binary CoMn₂O_x and NiMn₂O_x, and Ni_0.7Co_0.3Mn₂O_x showed the best NH₃-SCR performance among the Ni_yCo_1-yMn₂O_x catalysts. Although the Ni_0.7Co_0.3Mn₂O_x catalyst exhibited good resistance to H₂O, it had poor SO₂ tolerance which needs to be improved. Wu and co-workers [91] compared the DeNO_x performance of MnO₂/CoAl-LDO and CoMnAl-LDO mixed metal oxides prepared from CoAl-MnO₂-LDH and CoMnAl-LDH templates by ion-exchange/redox reaction and hexamethylenetetramine (HMT) hydrolysis methods, respectively. The CoAl-MnO₂-LDH showed higher NH₃-SCR activity in a broad temperature window of 90–300 °C (Figure 8a) as well as better stability and SO₂/H₂O resistance (Figure 8b) than the CoMnAl-LDO, which was attributed to its larger specific surface area, stronger redox ability, more quantitative acid sites, and abundant active components.

Leng and collaborators [92] synthesized Mn_0.2TiO_x, Ce_0.3TiO_x and series of Mn_aCe_0.3TiO_x (a = 0.1, 0.2, 0.3) catalysts and investigated their applicability for low-temperature NH₃-SCR reaction. They demonstrated that the low-temperature NH₃-SCR activity of Mn_aCe_0.3TiO_x was greatly improved after incorporation of Mn, and the Mn_0.1Ce_0.3TiO_x catalyst displayed the best performance (with 100% NO conversion and above 90% N₂ selectivity) in the temperature range of 175–400 °C even at high GHSV of 80,000 h⁻¹. The outstanding performance of Mn_0.1Ce_0.3TiO_x catalyst in NH₃-SCR resulted from its enhanced acidity and chemisorbed oxygen, and suitable redox property derived from Ce³⁺ + Mn⁴⁺ ↔ Ce⁴⁺ + Mn³⁺ reaction. Furthermore, the NO conversion over the Mn_0.1Ce_0.3TiO_x decreased and stabilized at 82% after the introduction of 100 ppm SO₂ and 6% H₂O and restored to almost 100% NO conversion after stopping the supply of SO₂ and H₂O (Figure 9), suggesting that the catalyst had excellent resistance to SO₂/H₂O and the effects were reversible.

Ali et al. [93] developed a series of Nb-promoted Fe_x-Nb_0.5-x-Ce_0.5 (x = 0.45, 0.4, 0.35) oxides for NH₃-SCR. The best activity (>90% NO conversion and near 100% N₂ selectivity) in the broad temperature window of 180–400 °C as well as excellent SO₂/H₂O resistance (Figure 10) was observed for Fe_0.4-Nb_0.1-Ce_0.5 catalyst. The authors considered the strong interaction among Nb, Fe, and Ce oxides leading to the enhancement of BET surface area, redox ability, acid amount, and NO adsorption capacity, which could be responsible for the outstanding performance of the catalyst.

Sun and co-workers [94] reported the multimetallic Sm- and/or Zr-doped MnO_x-TiO₂ catalysts for NH₃-SCR reaction. As shown in Figure 11, the Sm and Zr co-doped MnO_x-TiO₂ (MSZTO_x) catalyst had better activity (≈100% NO conversion and >95% N₂ selectivity) in a wide temperature range (125–275 °C) with an excellent H₂O/SO₂ tolerance than the MSTO_x (MnO_x-SmO_x-TiO₂), MZTO_x (MnO_x-ZrO_x-TiO₂) and MTO_x (MnO_x-TiO₂) catalysts. The authors claimed that the enhanced redox properties and acidic sites play a crucial role in improving the NH₃-SCR performance of MSZTO_x catalyst. Yan and co-workers [95] fabricated Cu_0.5Mg_1.5Mn_0.5Al_0.5O_x catalyst from layered double hydroxides and found that it showed better activity in a wide temperature range together with superior SO₂ and H₂O tolerance than conventional Mn/γ-Al₂O₃. The improved performance of Cu_0.5Mg_1.5Mn_0.5Al_0.5O_x was attributed to the high specific surface area, high reducibility of MnO₂ and CuO species, an abundance of acid sites, and the good dispersion of MnO₂ and CuO species.

Chen et al. [96] investigated the NH₃-SCR reaction over a series of Co_0.2Ce_xMn_0.8-xTi₁₀ (x = 0, 0.05, 0.15, 0.25, 0.35, and 0.40) oxides catalysts and observed that the Co_0.2Ce_0.35Mn_0.45Ti₁₀ catalyst exhibited the best catalytic performance with 100% NO conversion and over 91% N₂ selectivity in a broad temperature window of 180–390 °C. Although NO_x conversion decreased to some extent after introducing SO₂ and H₂O, the Co_0.2Ce_0.35Mn_0.45Ti₁₀ catalyst showed excellent resistance to SO₂/H₂O with reversible inhibition effect (Figure 12). It was concluded that the interactions among Ce, Co, Mn, and Ti oxides led to more surface Brønsted acid and Lewis acid sites, NO_x adsorption sites and modest redox ability which could play a crucial role to improve the NH₃-SCR activity of Co_0.2Ce_0.35Mn_0.45Ti₁₀.

4. Supported Single Transition Metal-Based Catalysts

Support materials have proved to be highly beneficial for enhancing the activity and durability of catalysts as they possess high surface area and good thermal stability. In virtue of the fine dispersion of the active component on the surface of the support and the synergistic effect between active component and support, supported catalysts exhibits improved NH₃-SCR performance than the unsupported transition metal oxide. Therefore, a lot of attention has been focused to increase the de-NO_x efficiency by dispersing the transition metal oxides over different support materials such as TiO₂, Al₂O₃, SiO₂, carbon nanotubes (CNTs), etc. TiO₂ supported transition metal oxides, especially manganese oxides, have been widely reported as promising catalysts for NH₃-SCR reaction at low temperature [17,24,97,98,99,100,101,102,103,104,105]. Smirniotis and co-workers [17,106,107] first reported the transition metal oxides (V, Cr, Mn, Fe, Co, Ni, and Cu) supported on Hombikat TiO₂ for NH₃-SCR reaction at low-temperature. Among the investigated samples, the Mn/Hombikat TiO₂ catalyst found to exhibit the highest activity even in the presence of water. They also studied the effect of different supports on the NH₃-SCR performance and observed that the Mn/Hombikat TiO₂ (anatase, high surface area) had the best activity as compared to the Kemira TiO₂ (rutile), Degussa P25 TiO₂ (anatase, rutile), Aldrich TiO₂ (anatase, low surface area), Puralox γ-Al₂O₃, Aldrich SiO₂ supported Mn catalysts. It was concluded that the Lewis acidity, redox behavior, and a high surface concentration of MnO₂ could play a key role in improving the NH₃-SCR activity. Later, they investigated the effect of Mn loading on the NH₃-SCR performance of Mn/Hombikat TiO₂ and reported that the catalyst with 16.7 wt% Mn had optimal activity and excellent tolerance to H₂O during 10 days of the reaction [108]. In another work, they also proposed the NH₃-SCR reaction mechanism over the Mn/TiO₂ catalyst using transient isotopic labeled and in-situ FT-IR studies. As shown in Figure 13, the reaction proceeds via a Mars-van-Krevelen-like mechanism, in which NH₃ and NO species were first adsorbed onto the Mn⁴⁺ sites (Lewis acid sites), followed by the formation of nitrosamide and azoxy intermediate species. Finally, these intermediates converted into N₂ and H₂O products [24].

In the aspect of catalyst structure design, Smirniotis and co-workers [26] developed a series of manganese confined titania nanotube (Mn/TNT-X) catalysts using different TiO₂ precursors (X = Ishihara (I), Kemira (K), Degussa P25 (P25), Sigma–Aldrich (SA), Hombikat (H), TiO₂ synthesized from titanium oxysulfate (TOS)) for low-temperature NH₃-SCR reaction. As can be observed from the Figure 14, all Mn/TNT-X catalysts exhibited an excellent NO_x conversion in broad temperature window, and especially, Mn(0.25)/TNT-H sample obtained the superior activity in the temperature range of 100–300 °C as compared to other catalysts. They believed that the better performance of Mn(0.25)/TNT-H catalyst was due to the high surface area (421 m²/g) of the TNT-H support, and high dispersion of active components. The Mn(0.25)/TNT-H catalyst also showed greater catalytic performance than the conventional Mn-loaded titania nanoparticles (Mn/TiO₂), suggesting that the unique multiwall nanotube with open-ended structure could be advantageous to promote the reaction. Besides, the Mn (0.25)/TNT-H displayed outstanding tolerance to 10 vol% H₂O in the feed (Figure 15), which might be attributed to the preferential existence of highly active and redox potential pairs of Mn⁴⁺ and Mn³⁺ in the tubular framework.

Recently, Boningari et al. [109] extended this work by comparing the NH₃-SCR activity of various metal oxide confined titania nanotubes M/TNT (M = Mn, Cu, Ce, Fe, V, Cr, and Co) based on the Hombikat TiO₂ support. As shown in Figure 16, the Mn-, V-, Cr-, and Cu-oxide confined titania nanotubes had excellent low-temperature activity and meanwhile, vanadium oxide confined titania nanotubes showed a broad operation temperature window for the NH₃-SCR reaction.

Sheng et al. [100] synthesized core-shell MnO_x/TiO₂ nanorod catalyst, showed high activity, stability, and N₂ selectivity in NH₃-SCR. They concluded that the abundant mesopores, Lewis-acid sites, and high redox capability could be beneficial to improve catalytic performance. Although the MnO_x/TiO₂ catalyst exhibited excellent resistance to H₂O, it was deactivated in the presence of SO₂ and SO₂/H₂O. Jia et al. [110] reported the low-temperature NH₃-SCR efficiency of MnO_x/TiO₂, MnO_x/ZrO₂, and MnO_x/ZrO₂-TiO₂ catalysts, and found that MnO_x/ZrO₂-TiO₂ obtained good activity at a temperature of 80–360 °C and excellent resistance to H₂O at 200 °C. However, all the catalysts showed poor tolerance to SO₂ and SO₂/H₂O that caused irreversible deactivation. Similar findings were also observed by Zhang et al. [111] over the Mn/Ti, Mn/Zr, and Mn/Ti-Zr catalysts, in which Mn/Ti-Zr sample exhibited an excellent NH₃-SCR performance in a wide temperature range due to its high surface area, Lewis acid sites, and surface Mn⁴⁺ ions.

Carbon nanotubes (CNTs) have been reported as promising catalyst support for NH₃-SCR catalysis due to their excellent stability and unique electronic and structural properties [36,112,113,114,115]. Qu and co-workers [116] reported that the NH₃-SCR performance of Fe₂O₃ was dramatically enhanced when it supported on CNTs (Figure 17a). It was concluded that the large surface area, fine dispersion of Fe₂O₃, and interaction between Fe₂O₃ and CNTs were important factors to improve the NH₃-SCR activity. In addition, the Fe₂O₃/CNTs catalyst showed an excellent tolerance to H₂O/SO₂. Interestingly, SO₂ stream in the feed had promoting effect on the NO conversion (Figure 17b), which could be attributed to the increased acid sites for NH₃ adsorption and activation on the catalyst surface in presence of SO₂. Bai et al. [117] developed CNTs supported copper oxide catalysts, and found that the 10 wt% CuO/CNTs showed good NH₃-SCR activity and excellent stability at 200 °C. The 10 wt% CuO/CNTs also had greater performance in comparison to 10 wt% CuO/TiO₂. However, it exhibited poor resistance to SO₂ and moderate tolerance to H₂O.

5. Supported Binary and Multi Transition Metal-Based Catalysts

Given that the dispersion of two active components on support enhances the active sites further, researchers have been widely reported the supported binary transition metal-based oxides to improve the performance and SO₂/H₂O tolerance in NH₃-SCR reaction. With this perspective, several composites, such as MnCe/CNTs [118], Mn-Fe/TiO₂ [119], MnO_x-CeO₂/graphene [120], Mg-MnO_x/TiO₂ [121], CeO_x-MnO_x/TiO₂-graphene [122], Fe-Mn/Al₂O₃ [123], Mn-Fe/W-Ti [124], MnO_x-CeO₂/TiO₂-1%NG (NG = N-doped grapheme) [125], Mn-Ce/CeAPSO-34 [126], etc., were investigated for the NH₃-SCR reaction at low-temperature. Smirniotis et al. [22] studied the promotional effect of co-doped metals (Cr, Fe, Co, Ni, Cu, Zn, Ce, and Zr) on the NH₃-SCR performance of Mn/TiO₂. As shown in Figure 18, except Zn and Zr, all other co-doped metals had a positive impact on the activity of Mn/TiO₂, and particularly, the Mn-Ni/TiO₂ exhibited the highest NO conversion and N₂ selectivity among the other titania-supported bimetallic catalysts.

They also investigated the influence of Ni loading on the activity of Mn/TiO₂ catalyst, and found that the 5wt%Mn-2wt%Ni/TiO₂ (Mn-Ni(0.4)/TiO₂, where Ni/Mn = 0.4) had the optimal activity with complete NO conversion at the temperature range of 200–250 °C (Figure 19a) and outstanding stability even in the presence of 10 vol% water (Figure 19b,c) [5,23]. The enhanced reducibility of manganese oxide and dominant phase of MnO₂ claimed to be responsible for the best activity and stability of Mn–Ni/TiO₂ catalyst [5,22,23]. In another study, they compared the de-NO_x performance of high surface texture hydrated titania and Hombikat TiO₂ supported Mn‒Ce bimetallic catalysts, and observed that the Mn–Ce/TiO₂ (Hombikat) showed the better activity and excellent resistance to H₂O (Figure 20). The superior performance could be attributed to the enhancement in reduction potential of active components, broadening of acid sites distribution, and the promotion of Mn⁴⁺/Mn³⁺, Ce³⁺/Ce⁴⁺ ratios including surface labile oxygen and small pore openings [25].

Xu and co-workers [127] reported Ce-Mn/TiO₂ catalysts with different Ce loadings, and the Ce(20)-Mn/TiO₂ found to show high activity with >90% NO conversion in the temperature range of 140–260 °C (Figure 21a). Their SO₂ tolerance results showed that the resistance ability was decreased in the order of Ce(20)-Mn/TiO₂ > Ce(30)-Mn/TiO₂ > Ce(10)-Mn/TiO₂ (Figure 21b). Although the Ce(20)-Mn/TiO₂ catalyst had reasonable resistance to 100 ppm SO₂ at different reaction temperatures (Figure 21c), it exhibited moderate tolerance to SO₂ poisoning when added higher than 100 ppm SO₂ to the reaction feed (Figure 21d). They ascribed the good SO₂ resistance of Ce(20)-Mn/TiO₂ to the widely distributed elements of Mn and Ce which in turn led to the inability of the sulfate material to remain on the surface.

Lin et al. [128] synthesized Me-Fe/TiO₂ (SD) catalyst via an aerosol-assisted deposition method, showing an excellent NH₃-SCR performance and good tolerance to SO₂/H₂O as compared to its counterparts prepared by co-precipitation and wet impregnation methods. The authors concluded that the enhanced surface reducibility and adsorption ability of NH₃/NO_x of Mn-Fe/TiO₂ (SD) catalyst could be responsible for its superior activity. Lee and co-workers [129] investigated the poisoning effect of SO₂ as metal sulfate and/or ammonium sulfate deposits on the low-temperature NH₃-SCR activity of MnFe/TiO₂ catalysts. They found that the metal sulfates had a more serious deactivation effect than that of ammonium salts on the MnFe/TiO₂ catalysts. Their results showed that metal sulfates poisoning resulted in lower crystallinity, lower specific surface area, a lower ratio of Mn⁴⁺/Mn³⁺, higher surface acidity, and more chemisorbed oxygen, which in turn led to an adverse effect on the NH₃-SCR activity of the catalyst. Mu et al. [130] prepared Fe-Mn/Ti catalyst by ethylene glycol-assisted impregnation method, showing high NH₃-SCR efficiency over a broad temperature window (100−325 °C) and outstanding tolerance to sulfur poisoning. The formation of the Fe-O-Ti structure with strong interaction strengthened the electronic inductive effect and increased the ratio of surface chemisorption oxygen, thereby the enhancement of NO_x adsorption capacity and NO oxidation performance, which could be beneficial to improve the NH₃-SCR activity.

Liu and co-workers [131] reported that the addition of Eu had noticeably improved the NH₃-SCR performance of Mn/TiO₂ catalyst even after sulfation process under SCR conditions (Figure 22a). However, both the Mn/TiO₂ and MnEu/TiO₂ catalysts showed poor activity when they sulfated only with SO₂ + O₂ (Figure 22a). Further, the MnEu/TiO₂ catalyst found to show better SO₂ tolerance as compared to the Mn/TiO₂ (Figure 22b). Their results revealed that Eu modification could inhibit the formation of surface sulfate species on the Mn/TiO₂ catalyst during the NH₃-SCR in the presence of SO₂, which could be the reason for improved SO₂ resistance.

Sun et al. [132] investigated the NH₃-SCR activity over the Nb-doped Mn/TiO₂ catalysts with different Nb/Mn molar ratios, and found that the MnNb/TiO₂-0.12 (where Nb/Mn = 0.12) catalyst had optimal NO_x conversion and N₂ selectivity in the temperature range of 100–400 °C (Figure 23a,b). The optimal MnNb/TiO₂-0.12 catalyst also exhibited greater SO₂ resistance than Mn/TiO₂ catalyst (Figure 23c). The incorporation of Nb into Mn/TiO₂ catalyst led to increase surface acidity and reducibility as well as generate more surface Mn⁴⁺ and chemisorbed oxygen species along with more NO₂, which results in the better NH₃-SCR activity. In situ DRIFT studies over the Mn/TiO₂ and MnNb/TiO₂-0.12 catalysts disclosed that the NH₃-SCR took place through Eley–Rideal mechanism even in presence of SO₂, in which the reaction mainly occurred between adsorbed NO₂ and gaseous NH₃. Hence, it was concluded that the higher SO₂ tolerance of the MnNb/TiO₂-0.12 catalyst could be due to the existence of more adsorbed NO₂ on its surface.

In another study, they reported the Mo-modified Mn/TiO₂ catalysts, exhibiting improved NH₃-SCR activity from 50 to 400 °C in comparison to Mn/TiO₂ catalyst. Particularly, the optimal MnMo/TiO₂-0.04 (where molar ratio of Mo/Mn = 0.04) catalyst better tolerance to SO₂ poisoning compared with Mn/TiO₂ (Figure 24) [133].

Fan and co-workers [134] fabricated ordered mesoporous titania supported CuO and MnO₂ composites (CuO/MnO₂-mTiO₂) through a facile acetic acid-assisted one-pot synthesis approach, showing high deNO_x efficiency (>90% NO conversion) and N₂ selectivity (>95%) in a wide operating temperature range of 120–300 °C. They considered that the superior NH₃-SCR performance could be attributed to the unique structure and highly integrated mesoporous TiO₂ supported by the multicomponent system with high surface areas, accessible and homogenously dispersed CuO and MnO₂ with multivalent nature and good redox activity. Although the CuO/MnO₂-mTiO₂ catalyst had good tolerance to H₂O, the resistance to SO₂ and H₂O/SO₂ poisoning, as well as high space velocity (GHSV), still need to be enhanced for practical use. Li et al. [135] synthesized fly ash-derived SBA-15 mesoporous molecular sieves supported Fe and/or Mn catalysts, and reported that Fe-Mn/SBA-15 catalyst showed notably greater NH₃-SCR activity than Mn/SBA-15 or Fe/SBA-15 in the temperature range of 150–250 °C. Moreover, the Fe-Mn/SBA-15 catalyst exhibited good time-on-stream stability (200 h) and water tolerance at 200 °C. The high metal dispersion, Mn⁴⁺/Mn³⁺ ratio, the concentration of adsorbed oxygen, and the redox activity are important features to enhance the NH₃-SCR performance of the Fe-Mn/SBA-15 catalyst. In their subsequent study, the authors investigated the mechanisms of NO reduction and N₂O formation using in-situ DRIFT and transient reaction studies and proposed a possible denitration mechanism over the Fe-Mn/SBA-15 catalyst which is shown in Figure 25. The NH₃-SCR reaction over the Fe-Mn/SBA-15 catalyst proceeded through Langmuir-Hinshelwood, Eley-Rideal, and Mars-van Krevelen mechanisms. Their results also revealed that a large amount of nitrate thereby N₂O being produced over the Fe-Mn/SBA-15 during the reaction due to its strong oxidation ability, low acidity, and high basicity, which resulted in the lower N₂ selectivity [136].

Tang and co-workers [137] reported that Mn₂CoO₄/reduced graphene oxide (Mn₂CoO₄/rGO) catalyst with an optimal amount of CoCl₂·6H₂O of 0.3 (millimole) showed excellent NH₃-SCR activity and stability at low-temperature due to its large specific surface area, abundant Lewis acid sites, and special three-dimensional architecture. When 100 ppm SO₂ added to reaction feed, the NO_x conversion over the optimal catalyst decreased significantly (96% to 53%), and the NO_x conversion was recovered to original level by water-washing after stopping the supply of SO₂. However, the decreased activity (100% to 82% NO_x conversion) in the presence of H₂O was restored to the original level after removing H₂O from the feed gas. Wang et al. [138] investigated the honeycomb cordierite-based Mn-Ce/Al₂O₃ catalyst for NH₃-SCR reaction and found that it showed good activity and reasonable resistance to SO₂/H₂O. The catalyst deactivation in the presence of SO₂ was ascribed to the deposition of ammonium hydrogen sulfate and sulfated CeO₂ on the catalyst surface during the NH₃-SCR process.

Meng et al. [139] synthesized a novel CuAlO_x/CNTs (CNTs = carbon nanotubes) catalyst by facile one-step carbothermal reduction decomposition method for low-temperature NH₃-SCR. The CuAlO_x/CNTs catalyst was found to exhibit higher NO_x conversion (>80%) and N₂ selectivity (>90%) than the CuAlO_x in the temperature range of 180–300 °C (Figure 26a). They concluded that more favorable formation of Cu⁺ active sites, better dispersion of active CuO species and higher surface adsorbed oxygen were beneficial to enhance the NH₃-SCR activity of CuAlO_x/CNTs catalyst. As shown in Figure 26b, the CuAlO_x/CNTs catalyst displayed excellent resistance to SO₂/H₂O at 240 °C during the NH₃-SCR. The authors attributed this outstanding SO₂/H₂O tolerance to the presence of CNTs that could promote the reaction of NH₄HSO₄ and NO continuously to avoid the formation and accumulation of excess ammonium sulfate salts on the catalyst surface. Li group [140] reported a series of ultra-low content copper-modified TiO₂/CeO₂ catalysts and observed that the catalyst with a Cu/Ce molar ratio of 0.005 exhibited the high NH₃-SCR performance and good tolerance to SO₂. Their characterization results disclosed that the addition of Cu into TiO₂/CeO₂ lead to enhance the Brønsted acid sites, amount of surface adsorbed oxygen and Ce³⁺ species, redox, and surface acidic properties, which in turn improve the NH₃-SCR activity.

Recently, supported multi-metal oxide catalysts have been considered as the very promising candidates for low-temperature NH₃-SCR reaction because of the enlarged synergetic catalysis effects of different components as well as improved metal-support interactions [141,142,143,144,145]. Wang and co-workers [146] reported a series of Nb modified Cu-Ce-Ti mixed oxide (Nb_yCCT, where y represented the atomic ratio of Nb to Ti) catalysts for low-temperature NH₃-SCR reaction. It was found that Nb_yCCT catalysts demonstrated a better activity than the Cu-Ce-Ti (CCT) and Ce-Ti (CT) samples (Figure 27a). Among all the Nb_yCCT catalysts, Nb_0.05CCT showed a higher NO conversion (>90%) in a broad temperature range of 180–360 °C under the GHSV of 40,000 h⁻¹ (Figure 27a). Results indicated that the incorporation of Nb to Cu-Ce-Ti led to strong interactions among the active phases that increased the oxygen vacancies and inhibited the over-oxidation of NH₃, which in turn improved the NH₃-SCR activity and N₂ selectivity in a wide temperature window. DRIFTS studies revealed that the introduction of Nb promoted the generation of NO₂, which could improve the activity via “fast” SCR reaction process (Langmuir–Hinshelwood reaction pathway). As shown in Figure 27b, the optimal Nb_0.05CCT catalyst exhibited higher resistance to SO₂/H₂O as compared to the Nb free catalyst. Li et al. [147] investigated the effect of Ho doping on the NH₃-SCR performance and the SO₂/H₂O resistance of Mn-Ce/TiO₂ catalyst. Among the catalysts tested, the catalyst with Ho/Ti of 0.1 (Mn_0.4Ce_0.07Ho_0.1/TiO₂) showed the best performance with >90% NO conversion in the temperature range of 150–220 °C, which was attributed to high concentration of chemisorbed oxygen, surface Mn⁴⁺/Mn³⁺ ratio, and acidity, as well as large specific surface area. Although the Mn_0.4Ce_0.07Ho_0.1/TiO₂ showed higher resistance to SO₂ and H₂O than the Mn_0.4Ce_0.07/TiO₂ catalyst, it was deactivated some extent in presence of SO₂/H₂O which is irreversible.

Lu group [148] synthesized a series of activated coke (AC) supported Fe_xCo_yCe_zO_m catalysts for low-temperature NH₃-SCR, and found that the 3%Fe_0.6Co_0.2Ce_0.2O_1.57/AC catalyst had the best activity at 250–350 °C and good tolerance to H₂O/SO₂ at 250 °C. The superior performance of the catalyst was ascribed to the co-participation of Fe, Co, and Ce species with different valence states, high concentration of chemisorbed oxygen, well dispersed active components, increase of weak acid sites, good redox properties of metallic oxides, and abundant functional groups on the catalyst surface. Their mechanistic and kinetic studies also indicated that the enhanced active sites for the adsorption of NO and NH₃, and the redox cycle among Fe, Co and Ce were responsible for the improved activity. Zhao et al. [149] reported a series of Mn-Ce-V-WO_x/TiO₂ composite oxide catalysts, exhibiting greater NH₃-SCR activity than the TiO₂ supported single-component catalysts (Figure 28a,b). Particularly, the catalyst with a molar ratio of active components/TiO₂ = 0.2 showed the best performance (>90% NO conversion) from 150 to 400 °C (Figure 28a). As shown in Figure 28c, the optimal Mn-Ce-V-WO_x/TiO₂ (molar ratio of Mn-Ce-V-WO_x/TiO₂ = 0.2) showed excellent stability and outstanding tolerance to H₂O/SO₂ at 250 °C. The authors concluded that the better performance of Mn-Ce-V-WO_x/TiO₂ mainly attributed to the variety of valence states of the four active components and their high oxidation-reduction ability.

6. Conclusions

Since the emission standards for NO_x are becoming more stringent to keep our atmosphere clean, the widespread use of fossil fuel in automobiles and industries require advanced catalytic materials for NO_x emission control. The low-temperature SCR of NO_x with NH₃ would be a promising solution to mitigate the NO_x emissions from mobile and stationary sources. Hence, the development of efficient catalysts for the low-temperature NH₃-SCR with high deNO_x activity, N₂ selectivity, and high resistance toward SO₂/H₂O poisonings is the subject of increasing interest in the field of environmental catalysis. Transition metal-based oxide catalysts have drawn much attention for low-temperature NH₃-SCR due to their excellent redox properties, high activity, durability, and relatively low manufacturing costs. In this review, we have summarized the recent progress in the low-temperature NH₃-SCR technology over the various transition metal-based catalysts. Over the past decades, significant research efforts have been made to improve the de-NO_x efficiency and SO₂/H₂O tolerance of transition metal-based oxides in NH₃-SCR at low-temperatures. Various transition metal-based mixed oxides with and without support have been extensively studied for NH₃-SCR reaction and, particularly, MnO_x-based catalyst formulations have caught much attention because of their excellent de-NO_x efficiency at low-temperatures. The modification of transition metal oxides by doping with other metal oxides led to high redox ability and acidic sites, and consequently, better NH₃-SCR performance at low-temperature. The loading of single and multi-transition metal-based oxides on the surface of supports (TiO₂, TiO₂ nanotubes, carbon nanotubes, etc.) could also enhance the NO_x conversion and N₂ selectivity in NH₃-SCR reaction by the fine dispersion of active component/s and its/their strong interaction with the support. The choice of metal loading and the support could play a key role in the catalytic function of the supported transition metal-based catalysts. The synergistic redox interaction between the active components of mixed metal oxide/supported metal oxide catalysts was also found to be an important factor to design the efficient denitration catalysts. In spite of the significant progress on the SO₂/H₂O tolerance of the catalysts, the durability of catalysts in the presence of both SO₂ and H₂O still needs to be improved. Most transition metal-based catalysts suffered from low resistance when the reaction feed contains both SO₂ and H₂O streams simultaneously. Hence, researchers have continuously explored the different options of transition metal-based mixed oxides and active transition metal/s-support combinations in order to develop the better NH₃-SCR catalysts in terms of SO₂/H₂O tolerance at low-temperature. The understanding of the inhibition mechanism of SO₂ and H₂O could be a promising strategy to develop high SO₂/H₂O resistance catalysts for NH₃-SCR reaction. However, the SO₂/H₂O inhibition mechanism was not very clear that needs to be investigated deeply. Especially, the design of transition metal-based catalysts with a combination of high NO_x conversion and N₂ selectivity in a wide operation temperature window and good resistance to SO₂/H₂O have attracted paramount attention, but it is still challenging task. The scope of NH₃-SCR research is quite vast and a large number of improvements need to be achieved in the near future.