## 4 Ternary and Multivariate Mn-Based Multi-Metal Oxide Catalysts
### Other Multi-Element Mn-Based Metal Oxide Catalysts
The Sm and Ce modified MnO\({}_{\rm x}\)-TiO\({}_{2}\) catalyst improves N\({}_{2}\) selectivity and suppresses the formation of N\({}_{2}\)O by inhibiting the formation of the N\({}_{2}\)O\({}_{4}\) intermediate, thereby reducing SO\({}_{2}\) adsorption and significantly enhancing SO\({}_{2}\) resistance. Ru modification markedly improves the low-temperature denitration activity of the CeO\({}_{\rm x}\)-MnO\({}_{\rm x}\)-TiO\({}_{2}\) catalyst. Ren et al. [87] discovered that doping with RuO\({}_{\rm x}\) significantly  
Figure 15: Effect of Nb/Fe co-doping on the MnCeO\({}_{\rm x}\) catalyst. (Reproduced with permission from Ref. [93]. Copyright 2023, Elsevier). promotes the dispersion of Ce, Mn, and Ti, bolsters the synergy among them, and notably enhances the redox performance of the CeO\({}_{\text{x}}\)-MnO\({}_{\text{x}}\)-TiO\({}_{\text{2}}\) catalyst, thereby strengthening the M-K mechanism. The addition of RuO\({}_{\text{x}}\) reduces the stability of NH\({}_{\text{x}}\) on the L acid sites (1283-1227 cm\({}^{-1}\)) of the catalyst. Upon introducing NO in the gas phase, the reaction rate increases, which facilitates the E-R reaction mechanism, thus significantly improving the low-temperature denitration performance of the CeO\({}_{\text{x}}\)-MnO\({}_{\text{x}}\)-TiO\({}_{\text{2}}\) catalyst. Fe doping can improve the specific surface area of the Mn-Ce-Ti catalyst and the adsorption capacity for NH\({}_{\text{3}}\) and NO\({}_{\text{x}}\). Shen et al. [88] studied the catalytic performance of the iron-doped Mn-Ce-Ti (Fe-Mn-Ce-Ti) catalyst, synthesized by the sol-gel method in the SCR of NO by NH\({}_{\text{3}}\) at low temperatures. Their results indicated that the NO conversion rate on Fe-Mn-Ce-Ti is notably higher compared to Mn-Ce-TiO\({}_{\text{2}}\) after Fe doping. The Fe-Mn-Ce-Ti catalyst, with an Fe/Ti molar ratio of 0.1, exhibits the highest activity. Fe (0.1)-Mn-Ce-Ti achieves a NO conversion rate of 96.8% at 180 \({}^{\circ}\)C and a space velocity of 50,000 h\({}^{-1}\). The Fe-Mn-Ce-Ti catalyst also demonstrates significantly greater resistance to H\({}_{\text{2}}\)O and SO\({}_{\text{2}}\) than Mn-Ce-Ti. Characterization results revealed that the specific surface area and the adsorption capacities for NH\({}_{\text{3}}\) and NO\({}_{\text{x}}\) are enhanced after Fe doping. The doping of Fe enhances the dispersion and oxidation state of manganese and cerium on the catalyst surface. Additionally, the oxygen concentration on the catalyst surface rises after Fe doping. Zhao et al. [89] prepared a series of Mn-Ce-V-WO\({}_{\text{x}}\)-TiO\({}_{\text{2}}\) composite oxide catalysts with different molar ratios (active component/TiO\({}_{\text{2}}\) = 0.1, 0.2, 0.3, 0.6) using the wet impregnation method. They carried out experiments on the SCR of NO by NH\({}_{\text{3}}\) over a broad temperature range. The results indicated that when the active component/TiO\({}_{\text{2}}\) molar ratio is 0.2, the catalyst achieves the highest NO conversion rate between 150 \({}^{\circ}\)C and 400 \({}^{\circ}\)C and exhibits excellent resistance to H\({}_{\text{2}}\)O and SO\({}_{\text{2}}\) at 250 \({}^{\circ}\)C. The gas space velocity (GHSV) is 40,000 h\({}^{-1}\). Different oxides are well dispersed and interact with each other. NH\({}_{\text{3}}\) and NO are strongly adsorbed on the catalyst surface, leading to a redox cycle and changes in the valence states of the surface oxides. The adsorption of SO\({}_{\text{2}}\) by Mn\({}^{4+}\) and Ce\({}^{4+}\) enhances the catalyst's resistance to water and SO\({}_{\text{2}}\), with Mn and Ce having a more pronounced effect. The diverse valence states of the four active components and their high redox performance are the main reasons for the high NO conversion rate in this system. Co and Ce doping significantly improves the NO\({}_{\text{x}}\) conversion efficiency of the Mn-TiO\({}_{\text{2}}\) catalyst. Qiu et al. [90] tested the low-temperature SCR of NO with NH\({}_{\text{3}}\) over Mn-TiO\({}_{\text{2}}\) catalysts doped with cobalt and cerium.