## 4 Ternary and Multivariate Mn-Based Multi-Metal Oxide Catalysts
### MnO\({}_{\text{x}}\)-TiO\({}_{\text{2}}\)-Y Catalysts
In addition to CeO\({}_{2}\), many other metal oxides such as Fe, Co, Ni, Sm, Nd, and various redox agents have been utilized to modify MnO\({}_{x}\)-TiO\({}_{2}\) catalysts, aiming to further optimize their denitrification performance and improve their resistance to H\({}_{2}\)O and SO\({}_{2}\) poisoning. Studies have shown that doping metal oxides such as Nd, Sm, Fe, Cu, tourmaline, and carbon nanotubes (CNTs) into MnO\({}_{x}\)-TiO\({}_{2}\) catalysts can increase the specific surface area and improve MnO\({}_{x}\) dispersion. Doping metal oxides such as W, Cu, Ru, Ni, Sn, Y, and expanded graphite can increase the acidic sites and oxygen vacancies on the catalyst's surface, thereby boosting its redox performance. Adding metal oxides like Sm, Zr, and others can markedly improve the N\({}_{2}\) selectivity of the catalyst. Incorporating metal oxides such as Sn, Pr, Eu, Sm, and others can inhibit the SO\({}_{2}\) poisoning of MnO\({}_{2}\), promote sulfate decomposition to a certain extent, and enhance sulfur resistance. Under some special conditions, improving the catalyst's resistance to alkali metals and Zn is also necessary. It has been observed that doping Co and Nb in MnO\({}_{x}\)-TiO\({}_{2}\) catalysts can improve resistance to alkali metals and Zn, respectively. Nd doping can increase the specific surface area of the MnO\({}_{x}\)-TiO\({}_{2}\) catalyst and ensure MnO\({}_{x}\) is well dispersed. Huang et al. [75] synthesized a series of rare earth metal-doped 30% Mn-3% RE-TiO\({}_{2}\) (RE = Ce, Sm, Nd, Er, Y) catalysts using a two-step incipient wetness impregnation method, investigating their effects on the low-temperature NH\({}_{3}\)-SCR performance of the MnO\({}_{x}\)-TiO\({}_{2}\) catalyst. Their findings indicated that adding 3% rare earth elements (Nd, Er, Y) into the 30% Mn-TiO\({}_{2}\) catalyst boosts its low-temperature catalytic performance, in addition to the commonly used Ce and Sm. Among these elements, Nd exhibits the most prominent promoting effect. The addition of Nd increases the catalyst's specific surface area, reduces the average pore size, and ensures a high dispersion of MnO\({}_{x}\), which are the primary reasons for the catalyst's enhanced activity. Figure 13: 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. (**a**) Mn/Ti, (**b**) Ce/Ti, (**c**) MnCe/Ti (Reproduced with permission from Ref. [101]. Copyright 2022, Elsevier). Sm doping enhances the specific surface area of the MnO\({}_{\rm x}\)-TiO\({}_{2}\) catalyst, inhibits the crystallization of MnO\({}_{\rm x}\) and TiO\({}_{2}\), and improves its redox performance and N\({}_{2}\) selectivity. Gao et al. [76] developed a series of Sm-modified MnO\({}_{\rm x}\)-TiO\({}_{2}\) (SMT) catalysts. Their findings indicated that the introduction of Sm effectively inhibits the crystallization of MnO\({}_{\rm x}\) and TiO\({}_{2}\), expands the specific surface area, raises the surface concentration of chemisorbed oxygen, and increases the number of strong acid sites on the surface, thereby optimizing the catalyst's performance. Kim et al. [86] discovered that incorporating Sm into MnO\({}_{\rm x}\)-TiO\({}_{2}\) catalysts improves the redox performance of MnO\({}_{\rm x}\) catalysts and increases the Mn\({}^{4+}\) ratio. The presence of Ce further enhances the redox performance of Sm-modified catalysts. Sm- and Ce-modified MnO\({}_{\rm x}\) increases N\({}_{2}\) selectivity and suppresses the formation of N\({}_{2}\)O by inhibiting the formation of N\({}_{2}\)O\({}_{4}\) intermediates, thus preventing SO\({}_{2}\) adsorption and significantly improving SO\({}_{2}\) resistance. Sun et al. [103] prepared a Sm-doped MnO\({}_{\rm x}\)-TiO\({}_{2}\) catalyst that exhibits superior N\({}_{2}\) selectivity and SO\({}_{2}\) resistance compared to an undoped MnO\({}_{\rm x}\)-TiO\({}_{2}\) catalyst in the NH\({}_{3}\)-SCR of NO.