A Review of Low Temperature NH 3 -SCR for Removal of NO x

: The importance of the low-temperature selective catalytic reduction (LT-SCR) of NO x by NH 3 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 H 2 O or/and SO 2 . 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.


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, NO 2 , and N 2 O 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 NH 3 (NH 3 -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). V 2 O 5 -WO 3 (MoO 3 )/TiO 2 is the typical and efficient catalyst and has been commercialized for NH 3 -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 N 2 selectivity in the high-temperature range [3,10,11]. Therefore, many researchers continue to develop highly active catalysts for low-temperature NH 3 -SCR in a wide temperature window.
With this perspective, several transition metal oxide-based catalysts have been extensively investigated for low-temperature NH 3 -SCR reaction due to their excellent redox properties, low price, 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 NOx 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 m 2 g -1 ) that exposes more active sites (such as surface Mn 4+ 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). Han and co-workers [53] fabricated triple-shelled NiMn 2 O 4 hollow spheres (Figure 2a,b) by using a solvothermal method and tested their ability for low-temperature NH 3 -SCR reaction. As shown in Figure 2c, the prepared NiMn 2 O 4 hollow spheres (NiMn 2 O 4 -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 NiMn 2 O 4 nanoparticles (NiMn 2 O 4 -P). The triple-layer shell structure of the NiMn 2 O 4 -S catalyst generates a larger surface area (165.3 m 2 g -1 ) that exposes more active sites (such as surface Mn 4+ and surface adsorbed oxygen species), which are responsible for its superior activity. Additionally, the NiMn 2 O 4 -S catalyst displayed outstanding stability and good tolerance to H 2 O and SO 2 (Figure 2d). Li et al. [58] developed hollow MnOx-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 MnOx-CeO2 hollow nanotube catalyst with 3.75 g of Ce(OH)CO3 template (denoted it as MnOx-CeO2-B) exhibited outstanding performance with more than 96% NOx conversion in the temperature range of 100-180 °C. The best activity of the MnOx-CeO2-B catalyst was due to its ample number of surface Mn 4+ and O species, and hollow and porous structures that provide abundant Lewis acid sites and large surface area. Additionally, MnOx-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.  . Reprinted from Reference [57]. Copyright 2018, with Permission from Elsevier.
Li et al. [58] developed hollow MnO x -CeO 2 binary nanotubes as efficient low-temperature NH 3 -SCR catalysts via an interfacial oxidation-reduction process using KMnO 4 aqueous solution and Ce(OH)CO 3 nanorod as both template and reducing agent without any other intermediate. They reported that the MnO x -CeO 2 hollow nanotube catalyst with 3.75 g of Ce(OH)CO 3 template (denoted it as MnO x -CeO 2 -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 -CeO 2 -B catalyst was due to its ample number of surface Mn 4+ and O species, and hollow and porous structures that provide abundant Lewis acid sites and large surface area. Additionally, MnO x -CeO 2 -B catalyst showed an excellent resistance to H 2 O and SO 2 ( Figure 4) and especially, the great SO 2 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 SO 2 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 Fe0.75V0.25Oδ 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 deNOx 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 CoaMnbOx (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 CoaMnbOx mixed oxides showed higher NH3-SCR activity than either MnOx or CoOx 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 (Co7Mn3Ox) exhibited the greatest activity (>80% NOx conversion) in a temperature window of 116-285 °C as compared to the catalysts with Co/Mn molar ratio of 5:5 (Co5Mn5Ox) and 3:7 (Co3Mn7Ox). They considered that the high NO + O2 adsorption ability and enhanced redox properties of the Co7Mn3Ox catalyst, emerging from its MnCo2O4.5 spinel phase and higher surface area, were beneficial to augment the NH3-SCR performance by forming nitrate species on the catalyst surface. Although Co7Mn3Ox catalyst had Fe-based binary catalysts have also been studied as NH 3 -SCR catalysts due to their high activity, excellent resistance to H 2 O and SO 2 , outstanding environmentally friendly performance, lower cost, and higher abundance [59][60][61][62]. Mu et al. [63] synthesized a series of vanadium-doped Fe 2 O 3 catalysts and evaluated the effect of V on the low-temperature NH 3 -SCR activity of hematite. The NH 3 -SCR activity and N 2 selectivity are greatly enhanced after the incorporation of vanadium into Fe 2 O 3 and the Fe 0.75 V 0.25 O 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 H 2 O and SO 2 . 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 NH 3 -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 H 2 O 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 H 2 O, but also enhances the adsorption of bridging nitrate and NH 3 species on Lewis acid sites. Thus, the authors concluded that the CTAB acted as a "structural" and "chemical" promoter in improving the NH 3 -SCR activity and H 2 O resistance at low-temperature.
Recently, Co-based spinel catalysts have shown to exhibit a remarkable low-temperature NH 3 -SCR activity, N 2 selectivity, and tolerance to SO 2 /H 2 O [30,[65][66][67]. Meng et al. [48] synthesized a highly efficient Co a Mn b O 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 NH 3 -SCR reaction. The Co a Mn b O x mixed oxides showed higher NH 3 -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 (Co 7 Mn 3 O 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 (Co 5 Mn 5 O x ) and 3:7 (Co 3 Mn 7 O x ). They considered that the high NO + O 2 adsorption ability and enhanced redox properties of the Co 7 Mn 3 O x catalyst, emerging from its MnCo 2 O 4.5 spinel phase and higher surface area, were beneficial to augment the NH 3 -SCR performance by forming Based on their results, the authors also proposed the NH3-SCR reaction mechanism over the Co7Mn3Ox 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 O 2− (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 Mn 4+ (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 Mn 3+ to Co 3+ (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 Co7Mn3Ox catalyst. Scheme 1. The proposed mechanism of the NH3-SCR reaction over the Co7Mn3Ox catalyst and the synergetic catalytic effect between Mn and Co cations . Reprinted from Reference [48]. Copyright 2018, with Permission from Elsevier.
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 (NOx conversion above 80%). They ascribed this outstanding performance to the strong and abundant acid sites, the strong adsorption of NOx, 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, CuOx 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 Cux-Nb1.1-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 Cux-Nb1.1-x oxides exhibited significantly higher activity than the CuOx and Nb2O5, and among the Cux-Nb1.1-x samples, Cu0.25-Nb0.85 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 Cu0.25-Nb0.85 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 Cu0.25-Nb0.85, still the Scheme 1. The proposed mechanism of the NH 3 -SCR reaction over the Co 7 Mn 3 O x catalyst and the synergetic catalytic effect between Mn and Co cations. Reprinted from Reference [48]. Copyright 2018, with Permission from Elsevier.
Mesoporous materials have been proved as promising catalysts for NH 3 -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 NH 3 -SCR reaction. It was found that the synthesized Mn-Co-O samples showed excellent NH 3 -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 NH 3 -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 NH 3 -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 Nb 2 O 5 , 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 SO 2 /H 2 O streams in the feed gas have some adverse impact on Cu 0.25 -Nb 0.85 , still the catalyst showed excellent resistance to SO 2 /H 2 O with reversible deactivation (Figure 5d). The superior NH 3 -SCR performance and SO 2 /H 2 O tolerance of Cu 0.25 -Nb 0.85 catalyst were attributed to its high acid amount and NO adsorption capacity. catalyst showed excellent resistance to SO2/H2O with reversible deactivation (Figure 5d). The superior NH3-SCR performance and SO2/H2O tolerance of Cu0.25-Nb0.85 catalyst were attributed to its high acid amount and NO adsorption capacity.

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 Fe0.3Mn0.5Zr0.2 catalyst and found that it showed an excellent deNOx activity with 100% NO conversion in the temperature range of 200-360 °C as compared to the Fe0.5Zr0.5 and Mn0.5Zr0.5 samples. Moreover, the Fe0.3Mn0.5Zr0.2 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.

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 ternaryor multi-metal-based low-temperature NH 3 -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 NH 3 -SCR reaction over the Fe 0.3 Mn 0.5 Zr 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 0.5 Zr 0.5 and Mn 0.5 Zr 0.5 samples. Moreover, the Fe 0.3 Mn 0.5 Zr 0.2 catalyst had outstanding stability and good tolerance to SO 2 ( 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 SO 2 and H 2 O 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 MnTiOx catalyst for NH3-SCR reaction. The results showed that Sb modification has greatly improved the NH3-SCR performance of MnSbTiOx catalysts in comparison to the MnTiOx and SbTiOx samples. Particularly, the MnSbTiOx-0.2 (Sb/Mn molar ratio = 0.2) catalyst exhibited the best activity with above 90% NOx conversion in the temperature range of 138-367 °C as it had good adsorption and activation properties for NH3 and NOx reactants in SCR. It can be seen from Figure 7 that the addition of Sb dramatically improved the SO2 and H2O resistance of MnTiOx catalyst. Although the NOx conversion over the MnSbTiOx-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 NiyCo1-yMn2Ox 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 Guo and co-workers [89] studied the effect of Sb doping on the activity of MnTiO x catalyst for NH 3 -SCR reaction. The results showed that Sb modification has greatly improved the NH 3 -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 NH 3 and NO x reactants in SCR. It can be seen from Figure 7 that the addition of Sb dramatically improved the SO 2   Guo and co-workers [89] studied the effect of Sb doping on the activity of MnTiOx catalyst for NH3-SCR reaction. The results showed that Sb modification has greatly improved the NH3-SCR performance of MnSbTiOx catalysts in comparison to the MnTiOx and SbTiOx samples. Particularly, the MnSbTiOx-0.2 (Sb/Mn molar ratio = 0.2) catalyst exhibited the best activity with above 90% NOx conversion in the temperature range of 138-367 °C as it had good adsorption and activation properties for NH3 and NOx reactants in SCR. It can be seen from Figure 7 that the addition of Sb dramatically improved the SO2 and H2O resistance of MnTiOx catalyst. Although the NOx conversion over the MnSbTiOx-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 NiyCo1-yMn2Ox 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 Shi et al. [90] synthesized a series of Ni y Co 1-y Mn 2 O x microspheres (MSs) (y = 0.1, 0.3, 0.5, 0.7, 0.9) for NH 3 -SCR using a hydrothermal method. It was observed that the activity of all ternary MSs was greater than binary CoMn 2 O x and NiMn 2 O x , and Ni 0.7 Co 0.3 Mn 2 O x showed the best NH 3 -SCR performance among the Ni y Co 1-y Mn 2 O x catalysts. Although the Ni 0.7 Co 0.3 Mn 2 O x catalyst exhibited good resistance to H 2 O, it had poor SO 2 tolerance which needs to be improved. Wu and co-workers [91] compared the DeNO x performance of MnO 2 /CoAl-LDO and CoMnAl-LDO mixed metal oxides prepared from CoAl-MnO 2 -LDH and CoMnAl-LDH templates by ion-exchange/redox reaction and hexamethylenetetramine (HMT) hydrolysis methods, respectively. The CoAl-MnO 2 -LDH showed higher NH 3 -SCR activity in a broad temperature window of 90-300 • C (Figure 8a) as well as better stability and SO 2 /H 2 O 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.
Catalysts 2019, 9, x FOR PEER REVIEW 10 of 34 greater than binary CoMn2Ox and NiMn2Ox, and Ni0.7Co0.3Mn2Ox showed the best NH3-SCR performance among the NiyCo1-yMn2Ox catalysts. Although the Ni0.7Co0.3Mn2Ox catalyst exhibited good resistance to H2O, it had poor SO2 tolerance which needs to be improved. Wu and co-workers [91] compared the DeNOx 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. Reprinted from Reference [91]. Copyright 2019, with Permission from Elsevier.
Leng and collaborators [92] synthesized Mn 0.2 TiO x , Ce 0.3 TiO x and series of Mn a Ce 0.3 TiO x (a = 0.1, 0.2, 0.3) catalysts and investigated their applicability for low-temperature NH 3 -SCR reaction. They demonstrated that the low-temperature NH 3 -SCR activity of Mn a Ce 0.3 TiO x was greatly improved after incorporation of Mn, and the Mn 0.1 Ce 0.3 TiO x catalyst displayed the best performance (with 100% NO conversion and above 90% N 2 selectivity) in the temperature range of 175-400 • C even at high GHSV of 80,000 h −1 . The outstanding performance of Mn 0.1 Ce 0.3 TiO x catalyst in NH 3 -SCR resulted from its enhanced acidity and chemisorbed oxygen, and suitable redox property derived from Ce 3+ + Mn 4+ ↔ Ce 4+ + Mn 3+ reaction. Furthermore, the NO conversion over the Mn 0.1 Ce 0.3 TiO x decreased and stabilized at 82% after the introduction of 100 ppm SO 2 and 6% H 2 O and restored to almost 100% NO conversion after stopping the supply of SO 2 and H 2 O (Figure 9), suggesting that the catalyst had excellent resistance to SO 2 /H 2 O and the effects were reversible. Reprinted from Reference [91]. Copyright 2019, with Permission from Elsevier.
Leng and collaborators [92] synthesized Mn0.2TiOx, Ce0.3TiOx and series of MnaCe0.3TiOx (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 MnaCe0.3TiOx was greatly improved after incorporation of Mn, and the Mn0.1Ce0.3TiOx 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 Mn0.1Ce0.3TiOx catalyst in NH3-SCR resulted from its enhanced acidity and chemisorbed oxygen, and suitable redox property derived from Ce 3+ + Mn 4+ ↔ Ce 4+ + Mn 3+ reaction. Furthermore, the NO conversion over the Mn0.1Ce0.3TiOx 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 Fex-Nb0.5-x-Ce0.5 (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 Fe0.4-Nb0.1-Ce0.5 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. 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 NH 3 -SCR. The best activity (>90% NO conversion and near 100% N 2 selectivity) in the broad temperature window of 180-400 • C as well as excellent SO 2 /H 2 O 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 MnOx-TiO2 catalysts for NH3-SCR reaction. As shown in Figure 11, the Sm and Zr co-doped MnOx-TiO2 (MSZTOx) 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 MSTOx (MnOx-SmOx-TiO2), MZTOx (MnOx-ZrOx-TiO2) and MTOx (MnOx-TiO2) catalysts. The authors claimed that the enhanced redox properties and acidic sites play a crucial role in improving the NH3-SCR performance of MSZTOx catalyst. Yan and co-workers [95] fabricated Cu0.5Mg1.5Mn0.5Al0.5Ox 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 Cu0.5Mg1.5Mn0.5Al0.5Ox 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 Co0.2CexMn0.8-xTi10 (x = 0, 0.05, 0.15, 0.25, 0.35, and 0.40) oxides catalysts and observed that the Co0.2Ce0.35Mn0.45Ti10 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 NOx conversion decreased to some extent after introducing SO2 and H2O, the Co0.2Ce0.35Mn0.45Ti10 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, NOx adsorption sites and modest redox ability which could play a crucial role to improve the NH3-SCR activity of Co0.2Ce0.35Mn0.45Ti10. Sun and co-workers [94] reported the multimetallic Sm-and/or Zr-doped MnO x -TiO 2 catalysts for NH 3 -SCR reaction. As shown in Figure 11, the Sm and Zr co-doped MnO x -TiO 2 (MSZTO x ) catalyst had better activity (≈100% NO conversion and >95% N 2 selectivity) in a wide temperature range (125- and MTO x (MnO x -TiO 2 ) catalysts. The authors claimed that the enhanced redox properties and acidic sites play a crucial role in improving the NH 3 -SCR performance of MSZTO x catalyst. Yan and co-workers [95] fabricated Cu 0.5 Mg 1.5 Mn 0.5 Al 0.5 O x catalyst from layered double hydroxides and found that it showed better activity in a wide temperature range together with superior SO 2 and H 2 O tolerance than conventional Mn/γ-Al 2 O 3 . The improved performance of Cu 0.5 Mg 1.5 Mn 0.5 Al 0.5 O x was attributed to the high specific surface area, high reducibility of MnO 2 and CuO species, an abundance of acid sites, and the good dispersion of MnO 2 and CuO species.
Chen et al. [96] investigated the NH 3 -SCR reaction over a series of Co 0.2 Ce x Mn 0.8-x Ti 10 (x = 0, 0.05, 0.15, 0.25, 0.35, and 0.40) oxides catalysts and observed that the Co 0.2 Ce 0.35 Mn 0.45 Ti 10 catalyst exhibited the best catalytic performance with 100% NO conversion and over 91% N 2 selectivity in a broad temperature window of 180-390 • C. Although NO x conversion decreased to some extent after introducing SO 2 and H 2 O, the Co 0.2 Ce 0.35 Mn 0.45 Ti 10 catalyst showed excellent resistance to SO 2 /H 2 O 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 NH 3 -SCR activity of Co 0.2 Ce 0. 35    Reproduced from Reference [96]. Copyright 2018, with Permission from Elsevier.

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 NH 3 -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 TiO 2 , Al 2 O 3 , SiO 2 , carbon nanotubes (CNTs), etc. TiO 2 supported transition metal oxides, especially manganese oxides, have been widely reported as promising catalysts for NH 3 -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 TiO 2 for NH 3 -SCR reaction at low-temperature. Among the investigated samples, the Mn/Hombikat TiO 2 catalyst found to exhibit the highest activity even in the presence of water. They also studied the effect of different supports on the NH 3 -SCR performance and observed that the Mn/Hombikat TiO 2 (anatase, high surface area) had the best activity as compared to the Kemira TiO 2 (rutile), Degussa P25 TiO 2 (anatase, rutile), Aldrich TiO 2 (anatase, low surface area), Puralox γ-Al 2 O 3 , Aldrich SiO 2 supported Mn catalysts. It was concluded that the Lewis acidity, redox behavior, and a high surface concentration of MnO 2 could play a key role in improving the NH 3 -SCR activity. Later, they investigated the effect of Mn loading on the NH 3 -SCR performance of Mn/Hombikat TiO 2 and reported that the catalyst with 16.7 wt% Mn had optimal activity and excellent tolerance to H 2 O during 10 days of the reaction [108]. In another work, they also proposed the NH 3 -SCR reaction mechanism over the Mn/TiO 2 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 NH 3 and NO species were first adsorbed onto the Mn 4+ sites (Lewis acid sites), followed by the formation of nitrosamide and azoxy intermediate species.
Finally, these intermediates converted into N 2 and H 2 O products [24].

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-NOx 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 Mn 4+ sites (Lewis acid sites), followed by the formation of nitrosamide and azoxy intermediate species. Finally, these intermediates converted into N2 and H2O products [24]. Figure 13. Plausible SCR mechanism over the surface of Mn/TiO 2 catalyst. Adapted from Reference [24]. Copyright 2012, with Permission from Elsevier.
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 TiO 2 precursors (X = Ishihara (I), Kemira (K), Degussa P25 (P25), Sigma-Aldrich (SA), Hombikat (H), TiO 2 synthesized from titanium oxysulfate (TOS)) for low-temperature NH 3 -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 m 2 /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/TiO 2 ), 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% H 2 O in the feed (Figure 15), which might be attributed to the preferential existence of highly active and redox potential pairs of Mn 4+ and Mn 3+ in the tubular framework.
Catalysts 2019, 9, x FOR PEER REVIEW 15 of 34 Figure 13. Plausible SCR mechanism over the surface of Mn/TiO2 catalyst. Adapted from Reference [24]. Copyright 2012, with Permission from Elsevier.
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 NOx 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 m 2 /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 Mn 4+ and Mn 3+ 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 MnOx/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 MnOx/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 MnOx/TiO2, MnOx/ZrO2, and MnOx/ZrO2-TiO2 catalysts, and found that MnOx/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 Mn 4+ ions. Recently, Boningari et al. [109] extended this work by comparing the NH 3 -SCR activity of various metal oxide confined titania nanotubes M/TNT (M = Mn, Cu, Ce, Fe, V, Cr, and Co) based on the Hombikat TiO 2 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 NH 3 -SCR reaction.
Sheng et al. [100] synthesized core-shell MnO x /TiO 2 nanorod catalyst, showed high activity, stability, and N 2 selectivity in NH 3 -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 /TiO 2 catalyst exhibited excellent resistance to H 2 O, it was deactivated in the presence of SO 2 and SO 2 /H 2 O. Jia et al. [110] reported the low-temperature NH 3 -SCR efficiency of MnO x /TiO 2 , MnO x /ZrO 2 , and MnO x /ZrO 2 -TiO 2 catalysts, and found that MnO x /ZrO 2 -TiO 2 obtained good activity at a temperature of 80-360 • C and excellent resistance to H 2 O at 200 • C. However, all the catalysts showed poor tolerance to SO 2 and SO 2 /H 2 O 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 NH 3 -SCR performance in a wide temperature range due to its high surface area, Lewis acid sites, and surface Mn 4+ 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. Carbon nanotubes (CNTs) have been reported as promising catalyst support for NH 3 -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 NH 3 -SCR performance of Fe 2 O 3 was dramatically enhanced when it supported on CNTs (Figure 17a). It was concluded that the large surface area, fine dispersion of Fe 2 O 3 , and interaction between Fe 2 O 3 and CNTs were important factors to improve the NH 3 -SCR activity. In addition, the Fe 2 O 3 /CNTs catalyst showed an excellent tolerance to H 2 O/SO 2 . Interestingly, SO 2 stream in the feed had promoting effect on the NO conversion (Figure 17b), which could be attributed to the increased acid sites for NH 3 adsorption and activation on the catalyst surface in presence of SO 2 . Bai et al. [117] developed CNTs supported copper oxide catalysts, and found that the 10 wt% CuO/CNTs showed good NH 3 -SCR activity and excellent stability at 200 • C. The 10 wt% CuO/CNTs also had greater performance in comparison to 10 wt% CuO/TiO 2 . However, it exhibited poor resistance to SO 2 and moderate tolerance to H 2 O. 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.

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], MnOx-CeO2/graphene [120], Mg-MnOx/TiO2 [121], CeOx-MnOx/TiO2-graphene [122], Fe-Mn/Al2O3 [123], Mn-Fe/W-Ti [124], MnOx-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-NOx 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 Mn 4+ /Mn 3+ , Ce 3+ /Ce 4+ ratios including surface labile oxygen and small pore openings [25]. They also investigated the influence of Ni loading on the activity of Mn/TiO 2 catalyst, and found that the 5wt%Mn-2wt%Ni/TiO 2 (Mn-Ni(0.4)/TiO 2 , 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 MnO 2 claimed to be responsible for the best activity and stability of Mn-Ni/TiO 2 catalyst [5,22,23]. In another study, they compared the de-NO x performance of high surface texture hydrated titania and Hombikat TiO 2 supported Mn-Ce bimetallic catalysts, and observed that the Mn-Ce/TiO 2 (Hombikat) showed the better activity and excellent resistance to H 2 O (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 Mn 4+ /Mn 3+ , Ce 3+ /Ce 4+ ratios including surface labile oxygen and small pore openings [25].  Xu and co-workers [127] reported Ce-Mn/TiO 2 catalysts with different Ce loadings, and the Ce(20)-Mn/TiO 2 found to show high activity with >90% NO conversion in the temperature range of 140-260 • C (Figure 21a). Their SO 2 tolerance results showed that the resistance ability was decreased in the order of Ce(20)-Mn/TiO 2 > Ce(30)-Mn/TiO 2 > Ce(10)-Mn/TiO 2 (Figure 21b). Although the Ce(20)-Mn/TiO 2 catalyst had reasonable resistance to 100 ppm SO 2 at different reaction temperatures (Figure 21c), it exhibited moderate tolerance to SO 2 poisoning when added higher than 100 ppm SO 2 to the reaction feed (Figure 21d). They ascribed the good SO 2 resistance of Ce(20)-Mn/TiO 2 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. . Influence of inlet water concentrations (7 vol%) on NOx conversion in the SCR reaction over Mn-Ce(5.1)/TiO2-Hk (Hombikat) catalyst at 175 °C; feed: NO = 900 ppm, NO2 = 100 ppm NH3/NOx (ANR) = 1.0, O2 = 10 vol%, He carrier gas, catalyst. 0.08 g, GHSV. 80,000 h -1 . Reprinted from Reference [25]. Copyright 2015, with Permission from Elsevier.
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. . Adapted from Reference [127]. We thank the Royal Society Open Science for this Contribution.
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  (10), Ce (20) and Ce(30), respectively. Pure TiO 2 was also used for comparison; (b) The effect of various Ce concentrations using a Ce-Mn/TiO 2 catalyst on SO 2 resistance; (c) The effects of reaction temperature on NO conversion of the Ce(20)-Mn/TiO 2 catalyst in the presence of SO 2 . The above three types of reactions were performed at: 500 ppm NO, 500 ppm NH 3 , SO 2 100 ppm, 3% O 2 , N 2 balance. gas, GHSV = 10 000 h −1 ; and (d) the effects of SO 2 concentration on NO conversion of Ce(20)-Mn/TiO 2 catalysts (T = 180 • C, 500 ppm NO, 500 ppm NH 3 , 3%O 2 , N 2 balance gas, GHSV = 10 000 h −1 ). Adapted from Reference [127]. We thank the Royal Society Open Science for this Contribution.
Lin et al. [128] synthesized Me-Fe/TiO 2 (SD) catalyst via an aerosol-assisted deposition method, showing an excellent NH 3 -SCR performance and good tolerance to SO 2 /H 2 O 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 NH 3 /NO x of Mn-Fe/TiO 2 (SD) catalyst could be responsible for its superior activity. Lee and co-workers [129] investigated the poisoning effect of SO 2 as metal sulfate and/or ammonium sulfate deposits on the low-temperature NH 3 -SCR activity of MnFe/TiO 2 catalysts. They found that the metal sulfates had a more serious deactivation effect than that of ammonium salts on the MnFe/TiO 2 catalysts. Their results showed that metal sulfates poisoning resulted in lower crystallinity, lower specific surface area, a lower ratio of Mn 4+ /Mn 3+ , higher surface acidity, and more chemisorbed oxygen, which in turn led to an adverse effect on the NH 3 -SCR activity of the catalyst. Mu et al. [130] prepared Fe-Mn/Ti catalyst by ethylene glycol-assisted impregnation method, showing high NH 3 -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 NH 3 -SCR activity.
Liu and co-workers [131] reported that the addition of Eu had noticeably improved the NH 3 -SCR performance of Mn/TiO 2 catalyst even after sulfation process under SCR conditions (Figure 22a). However, both the Mn/TiO 2 and MnEu/TiO 2 catalysts showed poor activity when they sulfated only with SO 2 + O 2 (Figure 22a). Further, the MnEu/TiO 2 catalyst found to show better SO 2 tolerance as compared to the Mn/TiO 2 (Figure 22b). Their results revealed that Eu modification could inhibit the formation of surface sulfate species on the Mn/TiO 2 catalyst during the NH 3 -SCR in the presence of SO 2 , which could be the reason for improved SO 2 resistance. that the enhanced surface reducibility and adsorption ability of NH3/NOx 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 Mn 4+ /Mn 3+ , 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 NOx 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 NOx 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 Mn 4+ 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. Sun et al. [132] investigated the NH 3 -SCR activity over the Nb-doped Mn/TiO 2 catalysts with different Nb/Mn molar ratios, and found that the MnNb/TiO 2 -0.12 (where Nb/Mn = 0.12) catalyst had optimal NO x conversion and N 2 selectivity in the temperature range of 100-400 • C (Figure 23a,b). The optimal MnNb/TiO 2 -0.12 catalyst also exhibited greater SO 2 resistance than Mn/TiO 2 catalyst (Figure 23c). The incorporation of Nb into Mn/TiO 2 catalyst led to increase surface acidity and reducibility as well as generate more surface Mn 4+ and chemisorbed oxygen species along with more NO 2 , which results in the better NH 3 -SCR activity. In situ DRIFT studies over the Mn/TiO 2 and MnNb/TiO 2 -0.12 catalysts disclosed that the NH 3 -SCR took place through Eley-Rideal mechanism even in presence of SO 2 , in which the reaction mainly occurred between adsorbed NO 2 and gaseous NH 3 . Hence, it was concluded that the higher SO 2 tolerance of the MnNb/TiO 2 -0.12 catalyst could be due to the existence of more adsorbed NO 2 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]. In another study, they reported the Mo-modified Mn/TiO 2 catalysts, exhibiting improved NH 3 -SCR activity from 50 to 400 • C in comparison to Mn/TiO 2 catalyst. Particularly, the optimal MnMo/TiO 2 -0.04 (where molar ratio of Mo/Mn = 0.04) catalyst better tolerance to SO 2 poisoning compared with Mn/TiO 2 ( Figure 24) [133].
Fan and co-workers [134] fabricated ordered mesoporous titania supported CuO and MnO 2 composites (CuO/MnO 2 -mTiO 2 ) through a facile acetic acid-assisted one-pot synthesis approach, showing high deNO x efficiency (>90% NO conversion) and N 2 selectivity (>95%) in a wide operating temperature range of 120-300 • C. They considered that the superior NH 3 -SCR performance could be attributed to the unique structure and highly integrated mesoporous TiO 2 supported by the multicomponent system with high surface areas, accessible and homogenously dispersed CuO and MnO 2 with multivalent nature and good redox activity. Although the CuO/MnO 2 -mTiO 2 catalyst had good tolerance to H 2 O, the resistance to SO 2 and H 2 O/SO 2 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 NH 3 -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, Mn 4+ /Mn 3+ ratio, the concentration of adsorbed oxygen, and the redox activity are important features to enhance the NH 3 -SCR performance of the Fe-Mn/SBA-15 catalyst. In their subsequent study, the authors investigated the mechanisms of NO reduction and N 2 O 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 NH 3 -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 N 2 O 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 N 2 selectivity [136]. 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 deNOx 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, Mn 4+ /Mn 3+ 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 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 NOx conversion over the optimal catalyst decreased significantly (96% to 53%), and the NOx conversion was recovered to original level by water-washing after stopping the supply of SO2. However, the decreased activity (100% to 82% NOx 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 Figure 25. Low-temperature NH 3 -SCR reaction mechanism on Fe-Mn/SBA-15 catalyst. Adapted from Reference [136]. Copyright 2018, with Permission from American Chemical Society.
Tang and co-workers [137] reported that Mn 2 CoO 4 /reduced graphene oxide (Mn 2 CoO 4 /rGO) catalyst with an optimal amount of CoCl 2 ·6H 2 O of 0.3 (millimole) showed excellent NH 3 -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 SO 2 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 SO 2 . However, the decreased activity (100% to 82% NO x conversion) in the presence of H 2 O was restored to the original level after removing H 2 O from the feed gas. Wang et al. [138] investigated the honeycomb cordierite-based Mn-Ce/Al 2 O 3 catalyst for NH 3 -SCR reaction and found that it showed good activity and reasonable resistance to SO 2 /H 2 O. The catalyst deactivation in the presence of SO 2 was ascribed to the deposition of ammonium hydrogen sulfate and sulfated CeO 2 on the catalyst surface during the NH 3 -SCR process.
The CuAlO x /CNTs catalyst was found to exhibit higher NO x conversion (>80%) and N 2 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 NH 3 -SCR activity of CuAlO x /CNTs catalyst. As shown in Figure 26b, the CuAlO x /CNTs catalyst displayed excellent resistance to SO 2 /H 2 O at 240 • C during the NH 3 -SCR. The authors attributed this outstanding SO 2 /H 2 O tolerance to the presence of CNTs that could promote the reaction of NH 4 HSO 4 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 TiO 2 /CeO 2 catalysts and observed that the catalyst with a Cu/Ce molar ratio of 0.005 exhibited the high NH 3 -SCR performance and good tolerance to SO 2 . Their characterization results disclosed that the addition of Cu into TiO 2 /CeO 2 lead to enhance the Brønsted acid sites, amount of surface adsorbed oxygen and Ce 3+ species, redox, and surface acidic properties, which in turn improve the NH 3 -SCR activity. Figure 25. Low-temperature NH3-SCR reaction mechanism on Fe-Mn/SBA-15 catalyst. Adapted from Reference [136]. Copyright 2018, with Permission from American Chemical Society.
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 NOx conversion over the optimal catalyst decreased significantly (96% to 53%), and the NOx conversion was recovered to original level by water-washing after stopping the supply of SO2. However, the decreased activity (100% to 82% NOx 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 CuAlOx/CNTs (CNTs = carbon nanotubes) catalyst by facile one-step carbothermal reduction decomposition method for low-temperature NH3-SCR. The CuAlOx/CNTs catalyst was found to exhibit higher NOx conversion (>80%) and N2 selectivity (>90%) than the CuAlOx 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 CuAlOx/CNTs catalyst. As shown in Figure 26b, the CuAlOx/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 Ce 3+ 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 NH 3 -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 y CCT, where y represented the atomic ratio of Nb to Ti) catalysts for low-temperature NH 3 -SCR reaction. It was found that Nb y CCT catalysts demonstrated a better activity than the Cu-Ce-Ti (CCT) and Ce-Ti (CT) samples (Figure 27a). Among all the Nb y CCT catalysts, Nb 0.05 CCT 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 NH 3 , which in turn improved the NH 3 -SCR activity and N 2 selectivity in a wide temperature window. DRIFTS studies revealed that the introduction of Nb promoted the generation of NO 2 , which could improve the activity via "fast" SCR reaction process (Langmuir-Hinshelwood reaction pathway). As shown in Figure 27b, the optimal Nb 0.05 CCT catalyst exhibited higher resistance to SO 2  represented the atomic ratio of Nb to Ti) catalysts for low-temperature NH3-SCR reaction. It was found that NbyCCT catalysts demonstrated a better activity than the Cu-Ce-Ti (CCT) and Ce-Ti (CT) samples (Figure 27a). Among all the NbyCCT catalysts, Nb0.05CCT 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 Nb0.05CCT 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 (Mn0.4Ce0.07Ho0.1/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 Mn 4+ /Mn 3+ ratio, and acidity, as well as large specific surface area. Although the Mn0.4Ce0.07Ho0.1/TiO2 showed higher resistance to SO2 and H2O than the Mn0.4Ce0.07/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 FexCoyCezOm catalysts for low-temperature NH3-SCR, and found that the 3%Fe0.6Co0.2Ce0.2O1.57/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 Lu group [148] synthesized a series of activated coke (AC) supported Fe x Co y Ce z O m catalysts for low-temperature NH 3 -SCR, and found that the 3%Fe 0.6 Co 0.2 Ce 0.2 O 1.57 /AC catalyst had the best activity at 250-350 • C and good tolerance to H 2 O/SO 2 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 NH 3 , 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 /TiO 2 composite oxide catalysts, exhibiting greater NH 3 -SCR activity than the TiO 2 supported single-component catalysts (Figure 28a,b). Particularly, the catalyst with a molar ratio of active components/TiO 2 = 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 /TiO 2 (molar ratio of Mn-Ce-V-WO x /TiO 2 = 0.2) showed excellent stability and outstanding tolerance to H 2 O/SO 2 at 250 • C. The authors concluded that the better performance of Mn-Ce-V-WO x /TiO 2 mainly attributed to the variety of valence states of the four active components and their high oxidation-reduction ability. 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-WOx/TiO2 (molar ratio of Mn-Ce-V-WOx/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-WOx/TiO2 mainly attributed to the variety of valence states of the four active components and their high oxidation-reduction ability.

Conclusions
Since the emission standards for NOx are becoming more stringent to keep our atmosphere clean, the widespread use of fossil fuel in automobiles and industries require advanced catalytic materials for NOx emission control. The low-temperature SCR of NOx with NH3 would be a promising solution to mitigate the NOx emissions from mobile and stationary sources. Hence, the development of efficient catalysts for the low-temperature NH3-SCR with high deNOx 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-NOx efficiency and SO2/H2O tolerance of transition metal-based oxides in NH3-SCR at low-temperatures. Various

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 NH 3 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 NH 3 -SCR with high deNO x activity, N 2 selectivity, and high resistance toward SO 2 /H 2 O 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 NH 3 -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 NH 3 -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 SO 2 /H 2 O tolerance of transition metal-based oxides in NH 3 -SCR at low-temperatures. Various transition metal-based mixed oxides with and without support have been extensively studied for NH 3 -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 NH 3 -SCR performance at low-temperature. The loading of single and multi-transition metal-based oxides on the surface of supports (TiO 2 , TiO 2 nanotubes, carbon nanotubes, etc.) could also enhance the NO x conversion and N 2 selectivity in NH 3 -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 SO 2 /H 2 O tolerance of the catalysts, the durability of catalysts in the presence of both SO 2 and H 2 O still needs to be improved. Most transition metal-based catalysts suffered from low resistance when the reaction feed contains both SO 2 and H 2 O 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 NH 3 -SCR catalysts in terms of SO 2 /H 2 O tolerance at low-temperature. The understanding of the inhibition mechanism of SO 2 and H 2 O could be a promising strategy to develop high SO 2 /H 2 O resistance catalysts for NH 3 -SCR reaction. However, the SO 2 /H 2 O 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 N 2 selectivity in a wide operation temperature window and good resistance to SO 2 /H 2 O have attracted paramount attention, but it is still challenging task. The scope of NH 3 -SCR research is quite vast and a large number of improvements need to be achieved in the near future.