## 4 Ternary and Multivariate Mn-Based Multi-Metal Oxide Catalysts
### MnO\({}_{\text{x}}\)-TiO\({}_{\text{2}}\)-Y Catalysts
[108] substituted part of the Ti\({}^{4+}\) in the catalyst with Y\({}^{3+}\) to promote the charge compensation of oxygen vacancies, resulting in the creation of Y-doped TiO\({}_{2}\)-supported MnO\({}_{\rm x}\). Analyses using PL, XPS, and EPR indicate that Y doping increases the quantity of oxygen vacancies and superoxide ions. This doping also boosts the catalytic activity for NO. Nb doping enhances the Zn resistance of the MnO\({}_{\rm x}\)-TiO\({}_{2}\) catalyst. Li et al. [85] discovered that modifying the Mn-TiO\({}_{2}\) catalyst with Nb significantly boosts its Zn resistance. Characterization results indicated that Nb introduction reduces the crystallinity of the Mn-TiO\({}_{2}\) catalyst, thereby increasing the acid site, Mn\({}^{4+}\) concentration, and surface chemisorbed oxygen concentration. Nb doping on the Mn-TiO\({}_{2}\) catalyst can promote the adsorption of NH\({}_{3}\) and NO\({}_{\rm x}\). These characteristics endow the Mn-Nb-TiO\({}_{2}\) catalyst with high SCR activity and excellent Zn resistance. Zr doping improves the N\({}_{2}\) selectivity of the MnO\({}_{x}\)-TiO\({}_{2}\) catalyst. Jia et al. [83] synthesized a series of Zr\({}_{y}\)Ti\({}_{1-y}\)O\({}_{2}\) (y = 0.1, 0.4, 0.5, 0.6, and 0.9) using a co-precipitation method, followed by impregnation with Mn (NO\({}_{3}\))\({}_{2}\) to achieve a 5 wt% Mn loading. The activity test revealed that Zr/Ti ratios affect the NH\({}_{3}\)-SCR activity of MnO\({}_{x}\)-Zr\({}_{y}\)Ti\({}_{1-y}\)O\({}_{2}\), but the catalyst maintains high N\({}_{2}\) selectivity at low temperatures with similar changes over the 80-360 \({}^{\circ}\)C range. The NO removal rate of MnO\({}_{x}\)-Zr\({}_{0.6}\)Ti\({}_{0.4}\)O\({}_{2}\) can reach 80% at 140 \({}^{\circ}\)C and 100% at 200-360 \({}^{\circ}\)C. Moreover, MnO\({}_{x}\)-Zr\({}_{0.6}\)Ti\({}_{0.4}\)O\({}_{2}\) has excellent H\({}_{2}\)O resistance and certain SO\({}_{2}\) resistance at 200 \({}^{\circ}\)C.  
Reports indicate that modifying some composite ores or non-metallic materials can enhance the performance of MnO\({}_{x}\)-TiO\({}_{2}\) catalysts. HeLian et al. [109] found that the tourmaline-MnO\({}_{x}\)-TiO\({}_{2}\) catalyst achieves a DeNO\({}_{x}\) efficiency of up to 97% under conditions of 1000 ppm NO, 1000 ppm NH\({}_{3}\), and 5% O\({}_{2}\) at 200 \({}^{\circ}\)C, maintaining an efficiency above 91% for 10 h. This improvement is mainly attributed to the polarization effect of tourmaline. The doping of tourmaline improves the dispersion of the MnO\({}_{x}\)-TiO\({}_{2}\) denitration catalyst, reduces the band gap width, and facilitates the activation of NH\({}_{3}\) and NO oxidation. Hossein Chitsazi et al. [110] found that among the MnO\({}_{x}\)-TiO\({}_{2}\) catalysts modified with expanded graphite (EG), the catalyst doped with 5 wt% EG displays the best SCR performance. The EG-modified catalyst features an increased presence of Mn\({}^{4+}\) species and a higher oxygen absorption capacity, resulting in more surface acid sites that boost SCR activity. S. Raja [105] observed that adding CNTs to Mn-TiO\({}_{2}\) improves the catalyst's activity. The addition of CNTs increases the specific surface area and total pore volume while reducing the average pore size, contributing to excellent NH\({}_{3}\)-SCR activity and resistance to SO\({}_{2}\)/H\({}_{2}\)O. Jin et al. [111] modified MnO\({}_{x}\)-TiO\({}_{2}\)-based catalysts with Ce, W, and the oxidant H\({}_{2}\)O\({}_{2}\). The high dispersion and specific surface area of oxidant H\({}_{2}\)O\({}_{2}\) increase the number of Mn\({}^{4+}\), Ce\({}^{3+}\), and active oxygen sites on the catalyst surface. The synergistic interaction between Mn and Ce promotes the redox process of the catalyst.