## 4 Ternary and Multivariate Mn-Based Multi-Metal Oxide Catalysts
### MnO\({}_{\text{x}}\)-TiO\({}_{\text{2}}\)-Y Catalysts
XPS combined with DFT calculations revealed that the electron transfer of the Mn\({}^{4+}\) + Sm\({}^{2+}\leftrightarrow\) Mn\({}^{3+}\) + Sm\({}^{3+}\) redox cycle occurs between manganese and samarium. Additionally, the electron transfer from Sm\({}^{2+}\) to Mn\({}^{4+}\) inhibits the electron transfer from NH\({}_{3}\) to Mn\({}^{4+}\), thereby preventing the formation of NH\({}_{2}\) or NH. This elimination of the NH formation pathway prevents the 2 NH + 4 NO \(\rightarrow\) 3N\({}_{2}\)O + H\({}_{2}\)O reaction, thus enhancing the N\({}_{2}\) selectivity of the NH\({}_{3}\)-SCR reaction. Fe doping can reduce the apparent activation energy of the MnO\({}_{\rm x}\)-TiO\({}_{2}\) catalyst, enhancing its dispersion. Mu et al. [104] developed an Fe\({}_{2}\)O\({}_{3}\)-MnO\({}_{2}\)-TiO\({}_{2}\) catalyst using an ethylene glycol-assisted impregnation technique for NH\({}_{3}\)-SCR. This catalyst exhibits excellent low-temperature activity, reduced apparent activation energy, and superior sulfur resistance across a broad temperature range (100-325 \({}^{\circ}\)C). Characterization results revealed notable dispersion and smaller particle sizes within the catalyst. Fe doping partially integrates into the TiO\({}_{2}\) lattice, forming an Fe-O-Ti structure, which enhances electron induction and increases the proportion of surface chemisorbed oxygen. Improved NO oxidation through the 'fast SCR' process benefits low-temperature SCR activity. This study confirmed that changing dispersion can increase the number of active sites on the catalyst surface. Shi et al. [77] investigated the effect of iron and tungsten on the NO-SCR activity and water-induced deactivation in the Mn-Ti catalyst system. The results indicated that iron addition promotes surface material dispersion, elevates the manganese valence state, and improves low-temperature activity, while tungsten addition increases acid sites and enhances thermal stability. Cu doping can increase the specific surface area and surface acidity of the MnO\({}_{\rm x}\)-TiO\({}_{2}\) catalyst. Yan et al. [78] developed a new low-temperature NH\({}_{3}\)-SCR catalyst with the composition of Cu\({}_{\rm w}\)Mn\({}_{\rm y}\)Ti\({}_{1-y}\)O\({}_{\rm x}\). At 200 \({}^{\circ}\)C, the NO\({}_{\rm x}\) conversion rate of the Cu\({}_{1}\)Mn\({}_{0.5}\)Ti\({}_{0.5}\)O\({}_{\rm x}\) catalyst reaches 90%, which is significantly higher than that of Cu-Mn-TiO\({}_{2}\) (86.1%) and Mn-TiO\({}_{2}\) (80.7%). The excellent catalytic performance of the Cu\({}_{1}\)Mn\({}_{0.5}\)Ti\({}_{0.5}\)O\({}_{\rm x}\) catalyst is attributed to its large specific surface area and surface acidity, along with a higher content of MnO\({}_{2}\) and CuO active substances. Moreover, the introduction of copper oxide imparts substantial resistance to 100 ppm SO\({}_{2}\) and 5% H\({}_{2}\)O to the catalyst. Raja et al. [105] discovered that incorporating Cu and/or CNTs into Mn-TiO\({}_{2}\) positively influences the catalyst's activity. The addition of CNTs increases the specific surface area and total pore volume while reducing the average pore size. Simultaneously, Cu enhances the Mn\({}^{4+}\) species and chemisorbed oxygen species on the catalyst's surface. Furthermore, adding Cu and CNTs lowers the reduction temperature of the catalyst and boosts the number and strength of the acidic sites. These factors contribute to the catalyst's excellent NH\({}_{3}\)-SCR activity and SO\({}_{2}\)/H\({}_{2}\)O resistance. W doping can increase the acidity of MnO\({}_{\rm x}\)-TiO\({}_{2}\) catalysts by providing strong acid sites, thus boosting their high-temperature activity. Wang et al. [79] found that an increase in manganese content within MnO\({}_{\rm x}\)-TiO\({}_{2}\) catalysts raises the Mn\({}^{4+}\) and O\({}_{\rm\alpha}\) ratios, thereby improving the NH\({}_{3}\)-SCR activity at lower temperatures. Tungsten doping contributes to more robust acid sites and enhances high-temperature activity by preventing ammonia oxidation. Wang et al.