## 7 Low-Temperature NH\({}_{3}\)-SCR Reaction Mechanism
### Possible Mechanisms in Other Low-Temperature Mn-Based Catalysts
[78] discovered that the CuMnTiO\({}_{\text{x}}\) catalyst exhibits high low-temperature activity in the NH\({}_{3}\)-SCR reaction. This performance is likely attributed to the high specific surface area dispersion of manganese and copper oxides, along with the abundant acid sites. The electron transfer process can enhance the NO oxidation reaction, as shown in Equations (31)-(33). Moreover, the addition of Cu increases the number of acid sites and improves the SO\({}_{2}\) resistance of Mn-based NH\({}_{3}\)-SCR catalysts. Based on these findings, a low-temperature redox cycle for the NH\({}_{3}\)-SCR reaction on this new CuMnTiO\({}_{\text{x}}\) catalyst can be proposed, as shown in \[\rm{NH}_{3(gas)}\to\rm{NH}_{3(ads)}\to\rm{NH}_{3(a)} \tag{17}\]4 The reaction mechanism of NH\({}_{3}\)-SCR on CuMnTiO\({}_{\text{x}}\) can be explained by both the E-R and L-H mechanisms. On the CuMnTiO\({}_{\text{x}}\) catalyst, the coordinated NH\({}_{3}\) on the Lewis acid sites and NH\({}_{4}\)\({}^{+}\) on the Brensted acid sites can react with gaseous NO (E-R mechanism) and adsorbed NO (L-H mechanism). In the E-R mechanism, the adsorbed NH\({}_{3}\) species are converted into NH\({}_{2}\) species, which then react with gaseous NO to form a highly active NH\({}_{4}\)NO\({}_{2}\)/NH\({}_{2}\)NO intermediate. Subsequently, NH\({}_{4}\)NO\({}_{2}\)/NH\({}_{2}\)NO quickly decomposes into N\({}_{2}\) and H\({}_{2}\)O. In the L-H mechanism, the NH\({}_{3}\)-SCR reaction occurs between the adsorbed NH\({}_{3}\) species and the adsorbed NO species. \[\rm{NH}_{3(gas)}\to\rm{NH}_{3(ads)}\to\rm{NH}_{3(a)} \tag{17}\]5  
\[\rm{NH}_{3(gas)}\to\rm{NH}_{3(ads)}\to\rm{NH}_{3(a)} \tag{17}\]6  
\[\rm{NH}_{3(gas)}\to\rm{NH}_{3(ads)}\to\rm{NH}_{3(a)} \tag{17}\]7  
Drawing from the analysis of structural properties and reaction mechanisms, Ji et al. [94] concluded and summarized the evident effect of sulfate and sulfite on MnCe catalysts at ultra-low temperatures (Figure 26). This limits the redox capacity and significantly hinders the adsorption of NH\({}_{3}\) on the surface Ce and Mn sites on the Lewis acid site NO\({}_{\text{x}}\). The E-R mechanism of NH\({}_{4}\)\({}^{+}\) anchoring on BASs (sulfate species) occurs solely in MnCe-SO\({}_{2}\), resulting in a poor catalytic performance in the presence of MnCe-SO\({}_{2}\). For MnCe-SO\({}_{2}\)  
\[\rm{NH}_{3(gas)}\to\rm{NH}_{3(ads)}\to\rm{NH}_{3(a)} \tag{17}\]8 The mechanism of SCR reaction on the Ni(0.4)-MnO\({}_{\text{x}}\) catalyst and the synergistic catalytic effect between Mn and Ni cations. (**a**) step 1, (**b**) step 2, (**c**) step 3. (Reproduced with permission from Ref. [44]. Copyright 2014, Elsevier). H\({}_{2}\)O, the dissociation of hydroxyl at the defect site from H\({}_{2}\)O markedly reduces the electron transfer between sulfur species and the active Mn site, thereby inhibiting the oxidation of sulfite and protecting the active Mn site. Sulfite maintains the NH\({}_{3}\) adsorption on LASs, and the reaction rate is 50% higher than that of NH\({}_{4}\)\({}^{+}\) provided by sulfate. Furthermore, sulfite can also induce the formation of ad-NO\({}_{\text{x}}\) (bidentate nitro and cis-N\({}_{2}\)O\({}_{2}\)\({}^{2-}\)) with high reactivity. Hence, the reaction on MnCe-SO\({}_{2}\) + H\({}_{2}\)O follows the E-R and L-H mechanisms. \[\rm{NH}_{3(gas)}\to\rm{NH}_{3(ads)}\to\rm{NH}_{3(a)} \tag{17}\]9 The significant effects of sulfate and sulfite on the adsorption performance and reaction mechanism of the MnO\({}_{\text{x}}\)-CeO\({}_{2}\) catalyst. (Reproduced with permission from Ref. [94]. Copyright 2023, Elsevier). \[\rm{2 NO}_{(gas)}+\rm{O}_{2(gas)}\to 2\rm{NO}_{2(gas)} \tag{18}\]0 The preparation process of CuMnTiO\({}_{\text{x}}\) and the low-temperature redox catalytic cycle of NH\({}_{3}\)-SCR over the CuMnTiO\({}_{\text{x}}\) catalyst. (Reproduced with permission from Ref. [78]. Copyright 2018, Elsevier). Wu et al.