## 3 Binary Mn-Based Metal Oxide Catalysts
### Other Binary Mn-Based Metal Oxide Catalysts
Appropriate Ni loading can effectively adjust the surface lattice oxygen activity and acidity of the catalyst, ensure the NH\({}_{3}\) activation ability at low temperatures, and inhibit the direct oxidation of NH\({}_{3}\) at higher temperatures. Wan et al. [44] prepared a series of Ni-Mn bimetallic oxide catalysts with different Ni/Mn ratios via the co-precipitation method for SCR of NO by NH\({}_{3}\) at low temperatures in excess O\({}_{2}\). The Ni (0.4)-MnO\({}_{\mathrm{x}}\) catalyst exhibits the highest catalytic activity, with an 85% NO conversion at 95 \({}^{\circ}\)C and a 100% NO conversion at 120-240 \({}^{\circ}\)C. Characterization results indicated that Mn\({}^{4+}\) is the primary active substance in the SCR reaction, and adding Ni increases the surface concentration and acidity of Lewis acid sites. Nb doping improves the acidity and N\({}_{2}\) selectivity of the MnO\({}_{\mathrm{x}}\) catalyst. Lian et al. [45] prepared a series of Mn-Nb mixed oxide (Mn-Nb) catalysts using the homogeneous precipitation method and conducted low-temperature SCR of NOx with NH\({}_{3}\). Among the Mn-Nb series catalysts, Mn\({}_{2}\)Nb\({}_{1}\)O\({}_{\mathrm{x}}\), calcined at 500 \({}^{\circ}\)C, exhibits the highest catalytic activity. MnO\({}_{\mathrm{x}}\) possesses a stronger redox performance than Mn\({}_{2}\)Nb\({}_{1}\)O\({}_{\mathrm{x}}\), but its N\({}_{2}\) selectivity is low due to the non-selective oxidation of NH\({}_{3}\) at elevated temperatures. The addition of Nb to MnO\({}_{\mathrm{x}}\) increases the acidity, especially Bronsted acidity, which contributes to the catalyst's excellent low-temperature SCR activity. The combination of MnO\({}_{\mathrm{x}}\) with strong redox performance and NbO\({}_{\mathrm{x}}\) with high acidity confers the Mn\({}_{2}\)Nb\({}_{1}\)O\({}_{\mathrm{x}}\) catalyst with a higher NOx conversion and N\({}_{2}\) selectivity. The V-doped MnO\({}_{\mathrm{x}}\) catalyst enhances its Lewis acid density and N\({}_{2}\) selectivity. Xin et al. [46] developed a bifunctional V\({}_{\mathrm{a}}\)-MnO\({}_{\mathrm{x}}\) catalyst, composed of Mn\({}_{2}\)O\({}_{3}\) and Mn\({}_{2}\)V\({}_{2}\)O\({}_{7}\), which separates NH\({}_{3}\) activation and N\({}_{2}\) formation via the transfer of the active intermediate NH\({}_{2}\). In comparison to Mn\({}_{2}\)O\({}_{3}\), the NOx conversion and N\({}_{2}\) selectivity are significantly enhanced at low temperatures with the combination of Mn\({}_{2}\)O\({}_{3}\) and Mn\({}_{2}\)V\({}_{2}\)O\({}_{7}\). This improvement is due to the synergistic interaction between Mn\({}_{2}\)O\({}_{3}\) and Mn\({}_{2}\)V\({}_{2}\)O\({}_{7}\), boosting the Lewis acid density of V\({}_{\text{a}}\)-MnO\({}_{\text{x}}\). The maximum activity of V\({}_{0.05}\)-MnO\({}_{\text{x}}\) is attributed to the optimal contact between the two components, highlighting the importance of intermediate transfer proximity. The doping of Si notably enhances the catalyst's specific surface area and surface acidity, thereby boosting its SO\({}_{2}\) resistance. Liang et al. [65] designed and prepared a MnO\({}_{\text{x}}\)-SiO\({}_{2}\) mixed oxide catalyst using the co-precipitation method. The incorporation of SiO\({}_{2}\) considerably increases the catalyst's specific surface area and surface acidity, which facilitates NH\({}_{3}\) adsorption. Concurrently, the catalyst's redox performance decreases, and its N\({}_{2}\) selectivity improves. The sulfur resistance mechanism study revealed that SiO\({}_{2}\) doping reduces SO\({}_{2}\) adsorption on the catalyst surface, protects the active component, avoids sulfate accumulation, and obtains substantial sulfur resistance. Figure 10 shows the possible anti-poisoning mechanism of the MnO\({}_{\text{x}}\)-SiO\({}_{2}\) catalyst. Additionally, Si doping can improve the lead resistance of MnO\({}_{\text{x}}\) catalysts. Wu et al. [66] discovered that traditional Mn-based catalysts suffer from lead toxicity, necessitating additional transition metal oxides to enhance their resistance to the NH\({}_{3}\)-SCR of NOx.