## 7 Low-Temperature NH\({}_{3}\)-SCR Reaction Mechanism
### Possible Mechanisms in Other Low-Temperature Mn-Based Catalysts
In addition to NO\({}_{2}\), the adsorbed monodentate nitrates and bridging nitrates can also undergo an L-H reaction with activated NH\({}_{4}\)\({}^{+}\) or NH\({}_{3}\). Moreover, NH\({}_{3}\) adsorbed on Lewis acid sites reacts with gaseous NO (via the E-R mechanism) to form N\({}_{2}\). The dual redox cycle (Mn\({}^{4+}\) + Ce\({}^{3+}\leftrightarrow\) Mn\({}^{3+}\) + Ce\({}^{4+}\), Mn\({}^{4+}\) + Ti\({}^{3+}\leftrightarrow\) Mn\({}^{3+}\) + Ti\({}^{4+}\)) underpins the synergistic catalytic performance of the Mn\({}_{0.2}\)Ce\({}_{0.1}\)Ti\({}_{0.7}\)O\({}_{\mathrm{x}}\) catalyst, which promotes NO and NH\({}_{3}\) adsorption and activation, thus boosting NH\({}_{3}\)-SCR performance. \[\rm{NO}_{(gas)}\to\rm{NO}_{(ads)}\to\rm{NO}_{(a)} \tag{16}\]0 Dual redox cycles for the activation of NO and NH\({}_{3}\) on the Mn-Ce-Ti catalyst. (Reproduced with permission from Ref. [99]. Copyright 2014, American Chemical Society). Fan et al. [39] discovered through in situ drift spectroscopy that E-R and L-H reactions take place in Mn-Gd catalysts (Figure 21). Gd doping enhances the adsorption of NH\({}_{3}\) on the catalyst that has pre-adsorbed NO\({}_{\text{x}}\). Furthermore, Gd doping facilitates the participation of reactive NH\({}_{4}\)\({}^{+}\) substances in the SCR reaction to generate NH\({}_{2}\) intermediates. Consequently, the enhancement of denitration performance by Gd doping can be attributed to the processes shown in Equations (22)-(24):  
\[\rm{NO}_{(gas)}\to\rm{NO}_{(ads)}\to\rm{NO}_{(a)} \tag{16}\]1  
\[\rm{NO}_{(gas)}\to\rm{NO}_{(ads)}\to\rm{NO}_{(a)} \tag{16}\]2  
\[\rm{NO}_{(gas)}\to\rm{NO}_{(ads)}\to\rm{NO}_{(a)} \tag{16}\]3  
Sun et al. [103] developed MnO\({}_{\text{x}}\)-TiO\({}_{2}\) catalysts doped with Sm and/or Zr and analyzed the factors contributing to their high N\({}_{2}\) selectivity and SO\({}_{2}\) resistance. Through XPS and DFT calculations, it was revealed that the electron transfer of the Mn\({}^{4+}\) + Sm\({}^{2+}\)\(\leftrightarrow\) Mn\({}^{3+}\) + Sm\({}^{3+}\) redox cycle occurs between manganese and samarium (Figure 22). Additionally, the electron transfer from Sm\({}^{2+}\) to Mn\({}^{4+}\) suppresses the electron transfer from NH\({}_{3}\) to Mn\({}^{4+}\), thereby inhibiting the formation of NH\({}_{2}\) or NH. Thus, the NH formation pathway is blocked, preventing the reaction 2 NH + 4 NO \(\rightarrow\) 3N\({}_{2}\)O + H\({}_{2}\)O (Figure 23). Consequently, the N\({}_{2}\) selectivity in the NH\({}_{3}\)-SCR reaction is improved. \[\rm{NO}_{(gas)}\to\rm{NO}_{(ads)}\to\rm{NO}_{(a)} \tag{16}\]4 NH\({}_{3}\)-SCR reaction mechanism on the Gd-doped MnO\({}_{\text{x}}\) catalyst. (Reproduced with permission from Ref. [39]. Copyright 2018, Elsevier). \[\rm{NO}_{(gas)}\to\rm{NO}_{(ads)}\to\rm{NO}_{(a)} \tag{16}\]5 Redox catalytic cycle of low-temperature SCR reaction over the MSTO\({}_{\text{x}}\) catalyst. (Reproduced with permission from Ref. [103]. Copyright 2018, Elsevier). Fang et al. [104] found that Fe-Mn-Zr catalyst samples possess both Bronsted and Lewis acid sites. It seems that the substitution of Fe for Mn promotes the adsorption capacity of Fe\({}_{0.3}\)Mn\({}_{0.5}\)Zr\({}_{0.2}\) for NH\({}_{3}\), which leads to the activation of NH\({}_{3}\). The adsorbed NO can only consume the lattice oxygen in the metal oxide to form NO\({}_{2}\) without O\({}_{2}\). When O\({}_{2}\) is present, more NO\({}_{2}\) is produced, which favors the 'fast SCR' reaction. This occurs because with sufficient NO\({}_{2}\), NO is more readily reduced through the reaction: 2 NH\({}_{3}\) + NO + NO \({}_{2}\) \(\rightarrow\) 2N\({}_{2}\) + 3H\({}_{2}\)O. Therefore, the 'fast SCR' reaction partially inhibits the oxidation of NH\({}_{3}\) at higher temperatures. Additionally, NO\({}_{2}\) is more easily adsorbed on the catalyst surface than NO, significantly enhancing NO adsorption during the 'fast SCR' process. When NO is oxidized to NO\({}_{2}\), the generated NO\({}_{2}\) is reduced by H, produced from the N-H cleavage of adsorbed NH\({}_{3}\), to form N\({}_{2}\) and H\({}_{2}\)O.