## 4 Ternary and Multivariate Mn-Based Multi-Metal Oxide Catalysts
### Other Multi-Element Mn-Based Metal Oxide Catalysts  
In addition to binary and ternary Mn-based catalysts, multi-component metal oxide catalysts with more complex compositions have also garnered considerable attention. Most reports are based on multi-component Mn-based catalysts co-doped with other metal oxides, such as Ce and/or Ti. Research has demonstrated that Mn-Ce-TiO\({}_{2}\) low-temperature NH\({}_{3}\)-SCR catalysts doped with Fe and Co exhibit strong SO\({}_{2}\) resistance. Wang et al. [84] studied the SO\({}_{2}\) resistance of Fe and Co-doped Mn-Ce-TiO\({}_{2}\) low-temperature NH\({}_{3}\)-SCR catalysts. Their results indicated that, compared to the Mn-Ce-TiO\({}_{2}\) catalyst, the Fe and Co co-doped catalyst (2 Fe\({}_{4}\)Co-MCT) with an Fe/Co molar ratio of 2:4 achieves excellent SO\({}_{2}\) resistance (98% NO conversion) at 200 \({}^{\circ}\)C. This improvement is attributed to the addition of Fe and Co, which significantly reduces sulfur adsorption on Mn-Ce-TiO\({}_{2}\). The uniform distribution of Fe and Co on the Mn-Ce-TiO\({}_{2}\) surface prevents SO\({}_{2}\) from diffusing into the inner layers of the catalyst. Figure 14 shows the mechanism for the SCR process of the Fe/Co co-doped MnCeTi catalyst. Figure 14: Mechanism for the SCR process of the Fe/Co co-doped MnCeTi catalyst. (Reproduced with permission from Ref. [84]. Copyright 2019, Elsevier). Fe and Nb co-doping can improve the low-temperature denitrification activity and N\({}_{2}\) selectivity of the MnCeO\({}_{\rm x}\) catalyst. Long et al. [93] found that co-doping with Nb\({}^{5+}\) and Fe\({}^{3+}\) improves the low-temperature denitrification activity, N\({}_{2}\) selectivity, and SO\({}_{2}\) resistance of the MnCeO\({}_{\rm x}\) catalyst by enhancing surface acidity, surface adsorbed oxygen, and electron interaction (Figure 15). An optimal amount of Ho-doped Mn-Ce-TiO\({}_{2}\) catalyst can effectively improve the catalyst's low-temperature denitrification performance and SO\({}_{2}\) resistance. Li et al. [85] prepared a Ho-doped Mn-Ce-TiO\({}_{2}\) catalyst using the impregnation method and investigated the low-temperature SCR of NO by NH\({}_{3}\). The results revealed that the catalyst with a Ho/Ti ratio of 0.1 exhibits excellent catalytic activity, achieving a conversion rate of over 90% within the temperature range of 140 to 220 \({}^{\circ}\)C. Characterization results indicated that the increase in SCR activity of the Mn-Ce-TiO\({}_{2}\) catalyst caused by Ho doping is attributed to the rise in specific surface area, the increase in chemisorbed oxygen concentration, the higher surface Mn\({}^{4+}\)/Mn\({}^{3+}\) ratio, and increased acidity. The main reason for the deactivation of the catalyst is that sulfate is formed on the surface of the catalyst, and Ho doping can inhibit sulfation to a certain extent. Therefore, an appropriate amount of Ho can effectively improve the low-temperature denitrification performance and SO\({}_{2}\) resistance of the Mn-Ce-TiO\({}_{2}\) catalyst. Ca doping can improve the N\({}_{2}\) selectivity of the MnO\({}_{\rm x}\)-CeO\({}_{2}\)-TiO\({}_{2}\) catalyst. Liu et al. [112] used a series of Ca-modified Ce\({}_{0.02}\)Mn\({}_{0.4}\)TiO\({}_{2}\) SCR catalysts to study this mechanism. Their analysis revealed that Ca addition significantly reduces the formation of NH on the catalyst surface and restricts its reaction with NO to form N\({}_{2}\)O. Moreover, the addition of Ca also slows down NO\({}_{2}\) formation, reduces the response of NO\({}_{2}\) and NH\({}_{3}\) to form N\({}_{2}\)O, and further improves the catalyst's N\({}_{2}\) selectivity. Sm- and Ce-modified MnO\({}_{\rm x}\)-TiO\({}_{2}\) catalysts improve the resistance to SO\({}_{2}\). Kim et al. [86] discovered that introducing Sm into the MnO\({}_{\rm x}\)-TiO\({}_{2}\) catalyst enhances the redox performance of the MnO\({}_{\rm x}\) catalyst and the ratio of Mn\({}^{4+}\). The addition of Ce further boosts the redox performance of the Sm-modified catalyst.