## 7 Low-Temperature NH\({}_{3}\)-SCR Reaction Mechanism
### Possible Mechanisms in Other Low-Temperature Mn-Based Catalysts  
Numerous studies and chemical calculations have demonstrated that the reaction mechanisms of different Mn-based catalysts differ significantly. This variation is primarily attributed to the different redox performance and surface acidity of different Mn-based catalysts, which lead to the formation of different NH\({}_{\text{x}}\)/NO\({}_{\text{x}}\) active intermediates during the reaction. Liu et al. [134] observed that a high relative content of Mn\({}^{4+}\) in the Mn-Ni/ordered mesoporous alumina (OMA) catalyst promotes the oxidation of unstable adsorbed NOx to bidentate nitrate. In situ drift analysis indicated that the effect of NH\({}_{3}\) adsorbed on Lewis acid sites and bidentate nitrates on SCR performance is more significant. The reaction mechanism on the catalyst follows both L-H and E-R mechanisms, with the E-R mechanism being predominant. Li et al. [135] studied the desulfurization and denitrification performance of activated coke (AC), Mn-AC, Fe-AC, and Fe-Mn-AC catalysts and proposed the possible mechanism of the Fe-Mn-AC catalyst (as shown in Figure 18). XPS analysis revealed that the presence of Mn\({}^{4+}\) enhances the catalytic activity for NO\({}_{\rm x}\) reduction at low temperatures. This enhancement is likely due to the redox cycle of Mn\({}^{4+}\), which promotes the oxidation of NO to NO\({}_{2}\), thereby supporting the 'fast SCR' of the Fe-Mn-AC catalyst. Considering the structure, morphology, and redox performance of the Fe-Mn-AC catalyst, a possible denitration reaction process was proposed, as shown in Equations (15)-(21):  
\[\rm{O}_{2(gas)}\to 2\rm{O}_{(ads)}\to 2\rm{O}_{(a)} \tag{15}\]  
\[\rm{NO}_{(gas)}\to\rm{NO}_{(ads)}\to\rm{NO}_{(a)} \tag{16}\]  
\[\rm{NH}_{3(gas)}\to\rm{NH}_{3(ads)}\to\rm{NH}_{3(a)} \tag{17}\]  
\[\rm{2 NO}_{(gas)}+\rm{O}_{2(gas)}\to 2\rm{NO}_{2(gas)} \tag{18}\]  
\[\rm{2 NO}_{2(gas)}\to 2\rm{NO}_{2(ads)}\to 2\rm{NO}_{2(a)} \tag{19}\]  
\[\rm{2 NH}_{3}+\rm{NO}_{2}+\rm{NO}\to 2\rm{N}_{2}+3\rm{H}_{2}O \tag{20}\]  
\[\rm{4 NH}_{3}+\rm{4 NO}+\rm{O}_{2}\to 4\rm{N}_{2}+6\rm{H}_{2}O \tag{21}\]  
Xin et al. [46] proposed a precise molecular mechanism aimed at improving low-temperature SCR activity using a bifunctional catalyst. This catalyst comprises a highly active oxide (Mn\({}_{2}\)O\({}_{3}\)) for NH\({}_{3}\) activation and a highly selective vanadate (Mn\({}_{2}\)V\({}_{2}\)O\({}_{7}\)) that promotes N\({}_{2}\) formation. Initially, Mn\({}_{2}\)O\({}_{3}\) activates NH\({}_{3}\) to form a NH\({}_{2}\) intermediate. This NH\({}_{2}\) intermediate is subsequently transferred to Mn\({}_{2}\)V\({}_{2}\)O\({}_{7}\), which aids in capturing gaseous NO, leading to the formation of NH\({}_{2}\)NO on Mn\({}_{2}\)V\({}_{2}\)O\({}_{7}\). NH\({}_{2}\)NO is then effectively  
Figure 18: Schematic for possible mechanisms of denitrification and desulfurization over the Fe-Mn/AC catalyst. (Reproduced with permission from Ref. [135]. Copyright 2024, Elsevier). converted into the desired N\({}_{2}\) instead of the undesired by-product N\({}_{2}\)O. The SCR reaction on this bifunctional catalyst mainly follows the pathway described in Figure 19.  
Meng et al. [38] studied the adsorption of NH\({}_{3}\) and NO on the SmMnO\({}_{\mathrm{x}}\) catalyst and its surface reaction. Their results indicated that incorporating Sm into the Sm\({}_{0.1}\)-MnO\({}_{\mathrm{x}}\) catalyst enhances the adsorption of NH\({}_{3}\) and NO, without altering the SCR reaction mechanism on the MnO\({}_{\mathrm{x}}\) catalyst. This implies that both the E-R mechanism and the L-H mechanism coexist (with bidentate nitrates as active intermediates), and the E-R mechanism is dominant. Liu et al. [99] proposed a dual redox cycle for the activation of NO\({}_{\mathrm{x}}\) and NH\({}_{3}\) on the Mn-Ce-Ti catalyst, as shown in Figure 20. NO\({}_{2}\) reacts with NH\({}_{4}\)\({}^{+}\) to form NH\({}_{4}\)NO\({}_{2}\), which further reacts to form NH\({}_{2}\)NO. This NH\({}_{2}\)NO then decomposes into N\({}_{2}\) and H\({}_{2}\)O.