## 4 Ternary and Multivariate Mn-Based Multi-Metal Oxide Catalysts
### MnO\({}_{\text{x}}\)-TiO\({}_{\text{2}}\)-Y Catalysts  
Many reports have pointed out that doping metal oxides of some other elements into the MnO\({}_{\text{x}}\)-TiO\({}_{2}\) catalyst, known for its excellent denitrification performance, can further enhance its denitrification ability and resistance to H\({}_{2}\)O and SO\({}_{2}\) poisoning. CeO\({}_{2}\) is often used as an additive to modify MnO\({}_{\text{x}}\)-TiO\({}_{2}\) catalysts due to its excellent redox performance and abundant acidic sites. Research shows that the addition of Ce affects many basic characteristics of MnO\({}_{\text{x}}\)-TiO\({}_{2}\) catalysts, mostly in a beneficial manner. These benefits include increased specific surface area, improved MnO\({}_{\text{x}}\) dispersion, enhanced surface acidity, facilitated adsorption and activation of NO and NH\({}_{3}\), boosted redox performance, better N\({}_{2}\) selectivity, and improved resistance to SO\({}_{2}\) and chlorine. Overall, these enhancements contribute to the improved denitrification performance of the MnO\({}_{\text{x}}\)-TiO\({}_{2}\) catalyst. Ce doping can increase the specific surface area of the MnO\({}_{\text{x}}\)-TiO\({}_{2}\) catalyst, improving the dispersion and exposure of MnO\({}_{\text{x}}\). Li et al. (2019) prepared a series of Ce-MnO\({}_{\text{x}}\)-TiO\({}_{2}\) catalysts using the sol-gel template method and analyzed the low-temperature SCR of NO by NH\({}_{3}\) at 353-473 K. The 0.07 Ce-MnO\({}_{\text{x}}\)-TiO\({}_{2}\) catalyst demonstrates the highest activity and best resistance to SO\({}_{2}\) poisoning. This catalyst's remarkable performance is potentially due to Ce doping altering the active component, increasing surface active oxygen and acid sites,reducing crystallinity, and expanding the specific surface area. Additionally, the reduction ability is enhanced. The improved SO2 poisoning resistance of the 0.07 Ce-MnO\({}_{\text{x}}\)-TiO\({}_{2}\) catalyst is attributed to Ce doping effectively reducing the formation of ammonium salt on the catalyst's surface and MnO\({}_{\text{x}}\) sulfation. Liu et al. [60] observed that adding Ce promotes the dispersion of MnO\({}_{\text{x}}\) on the TiO\({}_{2}\) support. Figure 11 shows the proposed scheme of the NO oxidation over the MnO\({}_{2}\) surface. Figure 12 shows the proposed scheme of the NH\({}_{2}\) production pathways as well as the role of NO\({}_{2}\) in the 'fast SCR' reaction. Concurrently, doping Ce into MnO\({}_{\text{x}}\) raises the content of Mn\({}^{4+}\) species, which are crucial in the NO oxidation reaction, as they can trigger the 'fast SCR' reaction and promote NO\({}_{\text{x}}\) conversion. Lee et al. [96] prepared CeO\({}_{2}\)-TiO\({}_{2}\)-supported MnO\({}_{\text{x}}\) with different CeO\({}_{2}\)-TiO\({}_{2}\) ratios using the wet impregnation method and employed them for the low-temperature SCR of NO by NH\({}_{3}\). Their findings indicated that adding a small amount of CeO\({}_{2}\) significantly boosts the activity of the Mn-TiO\({}_{2}\) catalyst within the low-temperature range of 120-160 \({}^{\circ}\)C. The optimal TiO\({}_{2}\)/CeO\({}_{2}\) catalyst loading ratio improves the catalytic activity and Mn dispersion in the SCR reaction. Wang et al. [70] developed MnO\({}_{\text{x}}\)-CeO\({}_{2}\)-TiO\({}_{2}\) catalysts with different Ce doping levels and evaluated their NH\({}_{3}\)-SCR activity between 100 \({}^{\circ}\)C and 400 \({}^{\circ}\)C. The results revealed that a small amount of Ce addition (at a Ce/Ti molar ratio of 0.05) increases the exposure of Mn atoms on the catalyst surface, achieving the highest NH\({}_{3}\)-SCR activity between 100 \({}^{\circ}\)C and 200 \({}^{\circ}\)C (the NO conversion rate exceeds 98% at 175 \({}^{\circ}\)C). Doping with Ce can enhance the surface acidity of MnO\({}_{\text{x}}\)-TiO\({}_{2}\) catalysts, facilitating the adsorption and activation of NO and NH\({}_{3}\), and enhancing their redox performance. Zhu et al.