## 4 Ternary and Multivariate Mn-Based Multi-Metal Oxide Catalysts
### MnO\({}_{\text{x}}\)-TiO\({}_{\text{2}}\)-Y Catalysts
[97] synthesized a series of Ce\({}_{\text{x}}\)-Mn-Ti\({}_{\text{y}}\) catalysts via the co-precipitation method. The Ce\({}_{0.1}\)-Mn-Ti\({}_{0.1}\) catalyst exhibits an excellent catalytic performance due to its superior redox performance, abundant acid sites, high Mn\({}^{4+}\) and Ce\({}^{3+}\) content, and surface-adsorbed oxygen (O\({}_{\text{s}}\)). It achieves over 90% NO\({}_{\text{x}}\) conversion in the 75-225 \({}^{\circ}\)C range. Lin et al. [98] synthesized TiO\({}_{2}\)-supported manganese oxides and cerium oxides (MnO\({}_{\text{x}}\)-TiO\({}_{2}\), CeO\({}_{2}\)-TiO\({}_{2}\), and MnO\({}_{\text{x}}\)-CeO\({}_{2}\)-TiO\({}_{2}\)) using the sol-gel method. The MnO\({}_{\text{x}}\)-CeO\({}_{2}\)-TiO\({}_{2}\) catalyst exhibits the best low-temperature performance, reducing nearly 98% of NO below 200 \({}^{\circ}\)C. This improvement is attributed to the enhanced redox performance and surface acidity from the combination of MnO\({}_{\text{x}}\)-TiO\({}_{2}\) and CeO\({}_{2}\), primarily because the Mn atoms integrate into the bulk CeO\({}_{2}\) to form the active Mn-O-Ce structure. Liu et al. [99] studied the performance  
Figure 11: Proposed scheme of the NO oxidation over the MnO\({}_{2}\) surface. (Reproduced with permission from Ref. [60]. Copyright 2020, Elsevier). of the Mn-Ce-Ti mixed oxide catalyst, prepared via the hydrothermal method, for the SCR of NO\({}_{\rm X}\) by NH\({}_{3}\) in the presence of oxygen. The results showed that the Mn-Ce-Ti catalyst possesses an excellent NH\({}_{3}\)-SCR activity, strong resistance to H\({}_{2}\)O and SO\({}_{2}\), and a broad operating temperature range. Compared to Mn-Ti and Ce-Ti catalysts, the Mn-Ce-Ti catalyst promotes the adsorption and activation of NO and NH\({}_{3}\), thereby improving the NH\({}_{3}\)-SCR performance. Ce doping can improve the N\({}_{2}\) selectivity, SO\({}_{2}\) resistance, and chlorine resistance of the MnO\({}_{\rm X}\)-TiO\({}_{2}\) catalyst. Zhang et al. [100] synthesized a CeMnO\({}_{\rm X}\)-Ti catalyst using a wet impregnation method. In the SCR of NO by NH\({}_{3}\), the CeMnO\({}_{\rm X}\)-Ti catalyst demonstrates higher N\({}_{2}\) selectivity. Wang et al. [101] discovered that the MnO\({}_{\rm X}\)-CeO\({}_{2}\)-TiO\({}_{2}\) catalyst exhibits high resistance to SO\({}_{2}\) poisoning, mainly because the Ce dopant on the surface preferentially forms sulfate, thus protecting the main active component (MnO\({}_{\rm X}\)). Figure 13 shows the NH\({}_{3}\)-SCR activities of Mn/Ti, Ce/Ti, and MnCe/Ti catalysts with or without SO\({}_{2}\) at 150 \({}^{\circ}\)C, 175 \({}^{\circ}\)C, and 200 \({}^{\circ}\)C. Liu et al. [99] studied the performance of the Mn-Ce-Ti mixed oxide catalyst prepared by hydrothermal method for the SCR of NO\({}_{\rm X}\) by NH\({}_{3}\) in the presence of oxygen. The results indicated that the Mn-Ce-Ti catalyst possesses excellent NH\({}_{3}\)-SCR activity, robust resistance to H\({}_{2}\)O and SO\({}_{2}\), and a broad operating temperature range. Based on catalyst characterization, the double redox cycle (Mn\({}^{4+}\) + Ce\({}^{3+}\), Mn\({}^{4+}\) + Ti\({}^{3+}\)\(\leftrightarrow\) Mn\({}^{3+}\) + Ce\({}^{4+}\), Mn\({}^{4+}\) + Ti\({}^{3+}\)) and amorphous structure play a crucial role in NO\({}_{\rm X}\) catalytic  
Figure 12: Proposed scheme of the NH\({}_{2}\) production pathways as well as the role of NO\({}_{2}\) in the ‘fast SCR’ reaction. (Reproduced with permission from Ref. [60]. Copyright 2020, Elsevier). performance. Wang et al. [102] found that adding CeO\({}_{2}\) to the MnO\({}_{x}\)-TiO\({}_{2}\) catalyst can enhance the chlorine (HCl) resistance of the catalyst. The addition of Ce causes Cl atoms to preferentially anchor to the non-coordinated Ce atoms, improving chlorine resistance, preventing the chlorination of active MnO\({}_{x}\) species, and ensuring the regeneration of active oxygen species on the catalyst surface.