## 7 Low-Temperature NH\({}_{3}\)-SCR Reaction Mechanism
### Possible Mechanisms in Other Low-Temperature Mn-Based Catalysts
Moreover, the H atom can also form NH\({}_{4}\)\({}^{+}\) with the absorbed NH\({}_{3}\). It is believed that NH\({}_{4}\)\({}^{+}\) is the main active ammonia species for low-temperature SCR, as NH\({}_{4}\)\({}^{+}\) can react with NO\({}_{2}\) to form [NH\({}_{4}\)\({}^{+}\)]2 NO\({}_{2}\), which then decomposes into N\({}_{2}\) and H\({}_{2}\)O (L-H mechanism). When one H atom in NH\({}_{3}\) is used, the resulting intermediate NH\({}_{2}\) reacts with NO to form NH\({}_{2}\)NO, which decomposes into N\({}_{2}\) and H\({}_{2}\)O (E-R mechanism). Fe\({}_{0.3}\)Mn\({}_{0.5}\)Zr\({}_{0.2}\) exhibits the highest NH\({}_{3}\) desorption amount and strongest surface adsorption of NH\({}_{3}\), promoting the dehydrogenation and activation of NH\({}_{3}\), thus benefiting the E-R and L-H processes. These factors collectively contribute to its high catalytic ability. Liu et al. [136] discovered that the NH\({}_{3}\)-SCR of NO\({}_{\rm x}\) on a 15 WMmZr catalyst follows the reaction pathway detailed in Formulas (25)-(30). The bridging nitrate formed on the 15 WMmZr surface is inactive; thus, it is inferred that the in situ formed NO\({}_{2}\), along with coordinated NH\({}_{3}\) and ion NH\({}_{4}\)\({}^{+}\), engage in the NH\({}_{3}\)-SCR reaction via the L-H mechanism. Furthermore, NH\({}_{3}\) adsorbed on Lewis acid sites and NH\({}_{4}\)\({}^{+}\) adsorbed on Bronsted acid sites can react with gaseous NO to form N\({}_{2}\) through the E-R mechanism. The reaction equations illustrate that the type of W and the redox process of Mn\({}^{4+}\)/Mn\({}^{3+}\) play a vital role in the NH\({}_{3}\)-SCR process. \[\rm{NO}_{(gas)}\to\rm{NO}_{(ads)}\to\rm{NO}_{(a)} \tag{16}\]6  
\[\rm{NO}_{(gas)}\to\rm{NO}_{(ads)}\to\rm{NO}_{(a)} \tag{16}\]7  
\[\rm{NO}_{(gas)}\to\rm{NO}_{(ads)}\to\rm{NO}_{(a)} \tag{16}\]8  
\[\rm{NO}_{(gas)}\to\rm{NO}_{(ads)}\to\rm{NO}_{(a)} \tag{16}\]9  
\[\rm{NH}_{3(gas)}\to\rm{NH}_{3(ads)}\to\rm{NH}_{3(a)} \tag{17}\]0  
\[\rm{NH}_{3(gas)}\to\rm{NH}_{3(ads)}\to\rm{NH}_{3(a)} \tag{17}\]1  
Wan et al. [44] proposed the existence of a synergistic catalytic effect (redox cycle) between manganese and nickel ions on Ni(0.4)-MnO\({}_{\rm x}\) catalysts, which maintains dynamic equilibrium through electron transfer between Mn and Ni ions, as shown in \[\rm{NH}_{3(gas)}\to\rm{NH}_{3(ads)}\to\rm{NH}_{3(a)} \tag{17}\]2  
\[\rm{NH}_{3(gas)}\to\rm{NH}_{3(ads)}\to\rm{NH}_{3(a)} \tag{17}\]3 Adding Sm to the MTO\({}_{\rm x}\) catalyst changes the electron transfer pathway. (Reproduced with permission from Ref. [103]. Copyright 2018, Elsevier). In Figure 24a, Ni\({}^{3+}\) acquires an electron from Mn\({}^{3+}\), transforming into Ni\({}^{2+}\) and Mn\({}^{4+}\), respectively. This process is similar to Ni\({}^{4+}\) + Co\({}^{3+}\leftrightarrow\) Ni\({}^{3+}\) + Co\({}^{4+}\), Fe\({}^{3+}\) + Ni\({}^{4+}\leftrightarrow\) Fe\({}^{4+}\) + Ni\({}^{3+}\), Cr\({}^{5+}\) + 2 Mn\({}^{3+}\leftrightarrow\) Cr\({}^{3+}\) + 2 Mn\({}^{4+}\), and Cu\({}^{2+}\) + Mn\({}^{3+}\leftrightarrow\) Cu\({}^{+}\) + Mn\({}^{4+}\). During this process, Mn\({}^{3+}\) is activated by Ni ions to the activation state of Mn\({}^{4+}\), which is responsible for the dehydration of NH\({}_{3}\) to NH\({}_{2}\) (Figure 24b) and finally used for NH\({}_{3}\)-SCR. This synergistic effect also helps to improve both the NH\({}_{3}\)-SCR activity and the high stability and self-regeneration performance of the Ni(0.4)-MnO\({}_{\text{x}}\) catalyst. At the same time, Ni\({}^{3+}\) is activated to Ni\({}^{2+}\) through electron transfer from Mn\({}^{3+}\) + Ni\({}^{3+}\) to Mn\({}^{4+}\) + Ni\({}^{2+}\), with the resulting Ni\({}^{2+}\) reducing O\({}_{2}\) to O\({}_{2}\)\({}^{-}\). By giving an electron to O\({}_{2}\) (Figure 24b), it is reconverted back to Ni\({}^{3+}\). Finally, -NH\({}_{2}\) and O\({}_{2}\)\({}^{-}\) react with NO to form N\({}_{2}\) and H\({}_{2}\)O (Figure 24c). The catalytic reaction continues with the subsequent manganese ion activation cycle of Mn\({}^{3+}\) + Ni\({}^{3+}\leftrightarrow\) Mn\({}^{4+}\) + Ni\({}^{2+}\). Yan et al.