## 5 Nano-Mn-Based Catalyst
### Composite MnO\({}_{x}\) Nanocatalysts  
Compared to the traditional MnO\({}_{x}\) catalyst, the composite MnO\({}_{x}\) nanocatalyst exhibits a notable increase in specific surface area, more dispersed active material, and the formation of amorphous MnO\({}_{x}\), CeO\({}_{x}\), and other oxides. Li et al. [113] developed a new type of MnO\({}_{x}\)-CeO\({}_{2}\) nanospheres catalyst with outstanding low-temperature activity and SO\({}_{2}\) resistance, significantly superior to the non-structured MnO\({}_{x}\)-CeO\({}_{2}\) catalyst. Additionally, the MnO\({}_{x}\)-CeO\({}_{2}\) nanospheres NH\({}_{3}\)-SCR catalyst demonstrates excellent long-term operational stability at 150 \({}^{\circ}\)C. The improved redox performance, smaller grain size, and higher surface concentrations of Ce\({}^{3+}\), Mn\({}^{4+}\), and oxygen, as well as the types of adsorbed NH\({}_{3}\) and NOx, are the primary reasons for its higher denitrification efficiency compared to the non-structured MnO\({}_{x}\)-CeO\({}_{2}\) catalyst. Yan et al. [114] observed that the MnO\({}_{x}\)-ZnO-CC catalyst shows higher activity than TiO\({}_{2}\) nanorod arrays (MnO\({}_{x}\)-TiO\({}_{2}\)-CC) at 100-250 \({}^{\circ}\)C, with the NOx conversion rate of the 0.06-MnO\({}_{x}\)-ZnO-CC sample reaching 96.8% at 200 \({}^{\circ}\)C. The introduction of ZnO nanorod arrays not only enhances the specific surface area but also leads to the formation of amorphous MnO\({}_{x}\). The MnO\({}_{x}\)-ZnO-CC catalyst, compared to the MnO\({}_{x}\)-TiO\({}_{2}\)-CC, has a higher Mn\({}^{4+}\) content, an appropriate surface adsorbed oxygen (O\({}_{\alpha}\)) ratio, and more Lewis acid sites. Yao et al. [121] found that the addition of Zr\({}^{4+}\), Al\({}^{3+}\), and Si\({}^{4+}\) ions into the lattice of CeO\({}_{2}\) nanorods (CeO\({}_{2}\)-NR) increases the specific surface area of the catalyst. Using undoped nanorods and Zr\({}^{4+}\)-, Al\({}^{3+}\)-, and Si\({}^{4+}\)-doped nanorods as supports, they prepared MnO\({}_{x}\)-CeO\({}_{2}\)-NR, MnO\({}_{x}\)-CZ-NR, MnO\({}_{x}\)-CA-NR, and MnO\({}_{x}\)-CS-NR catalysts, respectively. The MnO\({}_{x}\)-CS-NR catalyst achieved the highest NOx conversion and the lowest N\({}_{2}\)O concentration due to the highest oxygen vacancies and acid sites, the highest Mn\({}^{4+}\) content, and the lowest redox performance. This catalyst also demonstrated excellent resistance to H\({}_{2}\)O and SO\({}_{2}\). Liu et al. [122] prepared a series of Mn-TiNR catalysts by loading MnO\({}_{x}\) onto TiO\({}_{x}\) nanorod arrays using a solvent impregnation method. These catalysts have an excellent low-temperature denitrification effect in the temperature range of 100-300 \({}^{\circ}\)C. Characterization of their physical properties and chemical composition revealed that TiO\({}_{x}\) nanorod arrays provide a high specific surface area for active site loading and improve the interaction between Ti and Mn elements. Compared to the Mn-Ti catalyst, the Mn-TiNR catalyst had apparent advantages in surface chemical oxygen content, promoting the 'fast SCR' reaction. Many studies have shown that preparation of binary or multi-component nano-Mn-based catalysts can result in a unique core-shell structure. During catalytic reactions, Ce, Fe, and Ti ions in the shell preferentially react with SO\({}_{2}\) in the flue gas, protecting the Mn\({}^{n+}\) active component and significantly alleviating catalyst poisoning in the presence of SO\({}_{2}\). Ma et al. [115] successfully prepared MnO\({}_{x}\)-CeO\({}_{2}\)@TiO\({}_{2}\) with a core-shell structure as a high-performance denitration catalyst. Compared to MnO\({}_{x}\)-CeO\({}_{2}\)-TiO\({}_{2}\), MnO\({}_{x}\)-CeO\({}_{2}\)@TiO\({}_{2}\) exhibits a larger specific surface area and pore volume, providing more active sites and channels for the reaction gas to fully contact the active sites. Additionally, increased oxygen vacancies and acidic sites in MnO\({}_{x}\)-CeO\({}_{2}\)@TiO\({}_{2}\) facilitate the oxidation of NO.