## 3 Binary Mn-Based Metal Oxide Catalysts
### Other Binary Mn-Based Metal Oxide Catalysts
Characterization results revealed that the Sm in the Sm-MnO\({}_{\mathrm{x}}\) catalyst prevents MnO\({}_{\mathrm{x}}\) crystallization, increases its surface area, and elevates the relative content of Mn\({}^{4+}\) and surface oxygen (OS) on the catalyst. Gd doping can effectively inhibit the crystallization of MnO\({}_{\mathrm{x}}\), expand the specific surface area, elevate the concentration of surface Mn\({}^{4+}\) and chemisorbed oxygen, and enhance the number and strength of surface acid sites. At the same time, it curbs the conversion of MnO\({}_{2}\) to Mn\({}_{2}\)O\({}_{3}\) and the formation of MnSO\({}_{4}\). In the context of SCR of NO\({}_{\mathrm{x}}\) by NH\({}_{3}\), Fan et al. [39] were the first to utilize gadolinium (Gd) as a dopant to modify MnO\({}_{\mathrm{x}}\), thereby improving the catalytic performance and sulfur resistance of MnO\({}_{\mathrm{x}}\). The results indicated that introducing an appropriate amount of Gd effectively inhibits the crystallization of MnO\({}_{\mathrm{x}}\), increases the specific surface area, boosts the concentration of surface Mn\({}^{4+}\) and chemisorbed oxygen, and enhances the number and strength of surface acid sites. In situ drift spectroscopy results revealed that Gd doping facilitates NH\({}_{3}\) adsorption on the catalyst pre-adsorbed with NOx components, aiding the involvement of active NH\({}_{4}\) components in the SCR reaction. Notably, compared to the pure MnO\({}_{\mathrm{x}}\) catalyst, the MnGdO\({}_{\mathrm{x}}\) catalyst exhibits greater resistance to water vapor and sulfur poisoning, attributed to Gd modification inhibiting the conversion of MnO\({}_{2}\) to Mn\({}_{2}\)O\({}_{3}\) and the formation of MnSO\({}_{4}\). This modification prevents the decrease in Lewis acid sites and the increase in Bronsted acid sites, and alleviates competitive adsorption between NO and SO\({}_{2}\). The Eu-modified MnO\({}_{\mathrm{x}}\) catalyst can inhibit the crystallization of MnO\({}_{\mathrm{x}}\), resulting in a larger surface area and enhanced reducibility. Sun et al. [40] prepared a series of MnEuO\({}_{\mathrm{x}}\) catalysts using the co-precipitation method and applied them in the SCR of NO\({}_{\mathrm{x}}\) with NH\({}_{3}\). The results indicated that when the Eu/Mn molar ratio is 0.1, the SCR activity of the MnO\({}_{\mathrm{x}}\) catalyst is notably enhanced, the operating temperature window is widened, and the catalyst exhibits strong resistance to SO\({}_{2}\) and H\({}_{2}\)O. Characterization results revealed that the high SCR activity of the MnEuOx-0.1 catalyst is attributed to several factors: the increased surface area resulting from inhibited MnO\({}_{\mathrm{x}}\) crystallization, the enriched surface Mn\({}^{4+}\) and chemisorbed oxygen, and the improved reducibility. The Cr-modified MnO\({}_{\mathrm{x}}\) catalyst possesses a greater number of active components and facilitates efficient electron transfer between Cr and Mn. Chen et al. [41] developed Cr-Mn mixed oxide-based catalysts for the low-temperature SCR of NO\({}_{\mathrm{x}}\) using ammonia in excess oxygen. Their results revealed that Cr (0.4)-MnO\({}_{\mathrm{x}}\) exhibits the highest activity at 120 \({}^{\circ}\)C, achieving a NOx conversion rate of 98.5%. Characterization showed a crystalline phase of CrMn\({}_{1.5}\)O\({}_{4}\) in the Cr-MnO\({}_{\mathrm{x}}\) catalyst, which contains active substances. XPS analysis of fresh, used, and regenerated Cr (0.4)-MnO\({}_{\mathrm{x}}\) catalysts indicated the existence of Mn\({}^{2+}\), Mn\({}^{3+}\), and Mn\({}^{4+}\) and Cr\({}^{2+}\), Cr\({}^{3+}\), and Cr\({}^{5+}\) oxidation states. The effective electron transfer between Cr and Mn in the CrMn\({}_{1.5}\)O\({}_{4}\) crystal structure is attributed to the high activity and longevity of the Cr (0.4)-MnO\({}_{\mathrm{x}}\) catalyst. Upon doping the MnO\({}_{\mathrm{x}}\) catalyst with Zr, MnO\({}_{\mathrm{x}}\) exhibits high dispersion and a strong interaction between zirconia and manganese oxide, enhancing the reduction capability of MnO\({}_{\mathrm{x}}\). Zuo et al.