## 7 Low-Temperature NH\({}_{3}\)-SCR Reaction Mechanism
### Possible Mechanisms in Other Low-Temperature Mn-Based Catalysts
[127] found that the active V\({}^{5+}\) site exhibits significant reactivity for NH\({}_{3}\) in accordance with the mechanism proposed by Topsoe for V-based catalysts. The strong V\({}^{5+}\)/V\({}^{4+}\) redox cycle enhances the reactivity of gaseous NO. Based on the above discussion of in situ drift, the possible promotion mechanism of low-temperature NH\({}_{3}\)-SCR on the VO\({}_{\text{x}}\)-MnO\({}_{\text{x}}\)-CeO\({}_{2}\)-R catalyst is shown in \[\rm{2 NO}_{(gas)}+\rm{O}_{2(gas)}\to 2\rm{NO}_{2(gas)} \tag{18}\]1 The NH\({}_{3}\)-SCR catalytic performance over VO\({}_{\text{x}}\)-MnO\({}_{\text{x}}\)-CeO\({}_{2}\)-R correlates clearly with the amount of NH\({}_{3}\) (L), NH\({}_{4}\)\({}^{+}\) (B), and bridging nitrates, while bidentate nitrates are bystanders. During the NH\({}_{3}\)-SCR reaction, the well-dispersed vanadium monomer on the surface of CeO\({}_{2}\) nanorods {110} begins to adsorb NH\({}_{3}\), forming NH\({}_{3}\) (L) and NH\({}_{4}\)\({}^{+}\) (B). According to the L-H mechanism, the activated NH\({}_{3}\) (L) and NH\({}_{4}\)\({}^{+}\) (B) species then rapidly react with the bridged nitrate species to form N\({}_{2}\) and H\({}_{2}\)O. It was also noted that V\({}^{5+}\) reacts with bridging nitrates, releasing V\({}^{4+}\), which is subsequently reoxidized by NO or O\({}_{2}\). A higher Ce\({}^{3+}\)/(Ce\({}^{3+}\) + Ce\({}^{4+}\)) ratio accelerates the redox cycle. Moreover, the presence of Mn induces numerous oxygen vacancies and Ce\({}^{3+}\), facilitating the formation of more NH\({}_{3}\) (L) and bridging nitrate species. These findings also underscore the significant enhancement of NH\({}_{3}\)-SCR activity by MnO\({}_{\text{x}}\). Yang et al. [123] prepared Mn-(\(\gamma\)-Al\({}_{2}\)O\({}_{3}\)) and MnxMoy-(\(\gamma\)-Al\({}_{2}\)O\({}_{3}\)) catalysts using the incipient wetness impregnation (IWI) method. Their catalytic performance test revealed that Mn\({}_{3}\)Mo\({}_{1.2}\)-(\(\gamma\)-Al\({}_{2}\)O\({}_{3}\)) has a high SCR performance (approximately 96% NO conversion) over a broad low-temperature range (150\(\sim\)300 \({}^{\circ}\)C). Characterization indicated that the addition of Mo to the Mn-(\(\gamma\)-Al\({}_{2}\)O\({}_{3}\)) catalyst promotes the dispersion of MnO\({}_{\text{x}}\) on the \(\gamma\)-Al\({}_{2}\)O\({}_{3}\) surface. The adsorption of NO can form two different substances, nitrite and nitrate, on the catalyst surface. The presence of nitrite is beneficial for low-temperature SCR. Additionally, the presence of Mo in the catalyst promotes the formation of Mn\({}^{3+}\), which is crucial in NH\({}_{3}\) adsorption, thus improving the adsorption capacity of the MnO\({}_{\text{x}}\)-(\(\gamma\)-Al\({}_{2}\)O\({}_{3}\)) catalyst for NH\({}_{3}\). The low-temperature SCR of the Mn\({}_{3}\)Mo\({}_{1.25}\)-(\(\gamma\)-Al\({}_{2}\)O\({}_{3}\)) catalyst mainly follows the L-H mechanism, with the E-R mechanism also contributing. Consequently, the low-temperature SCR reaction involves four steps, as shown in \[\rm{2 NO}_{(gas)}+\rm{O}_{2(gas)}\to 2\rm{NO}_{2(gas)} \tag{18}\]2 In step one, NH\({}_{3}\) and NO are adsorbed on the catalyst surface; in step two, the adsorbed NH\({}_{3}\) is extracted with H to produce -NH\({}_{2}\) species, while the adsorbed NO species produce reactive nitrite; in step three, -NH\({}_{2}\) reacts with nitrite or gaseous NO through the L-H and E-R mechanisms to form the intermediate product -NH\({}_{2}\)NO; in step four, -NH\({}_{2}\)NO decomposes into N\({}_{2}\) and H\({}_{2}\)O.  
\[\rm{2 NO}_{(gas)}+\rm{O}_{2(gas)}\to 2\rm{NO}_{2(gas)} \tag{18}\]3 The possible mechanism on the VO\({}_{\text{x}}\)-MnO\({}_{\text{x}}\)-CeO\({}_{2}\)-R catalyst. (Reproduced with permission from Ref. [127]. Copyright 2020, Elsevier).