## 3 Binary Mn-Based Metal Oxide Catalysts
### Other Binary Mn-Based Metal Oxide Catalysts
An appropriate amount of modification can significantly affect the MnO\({}_{\text{x}}\) catalyst, such as increasing the specific surface area, enhancing the concentration of surface Mn\({}^{4+}\) and surface oxygen, improving the surface acid strength, and boosting the low-temperature activity of the MnO\({}_{\text{x}}\) catalyst. The Mn\({}_{6}\)In\({}_{1}\) catalyst (Mn:In = 6:1) demonstrates the highest activity, maintaining NOx conversion at approximately 100% between 75 and 200 \({}^{\circ}\)C.  
Cu doping enhances the MnO\({}_{\text{x}}\) catalyst by increasing its specific surface area. Kang et al. [36] prepared Cu-Mn mixed oxides using the co-precipitation method and reduced NO with NH\({}_{3}\) at low temperatures in excess oxygen. A Cu-Mn oxide catalyst with a small copper content can achieve the complete conversion of NOx within the temperature range of 323-473 K. However, the catalyst exhibits reversible deactivation due to water vapor and SO\({}_{2}\). The superior catalytic activity of Cu-Mn mixed oxides is mainly attributed to their enlarged surface area and reduced crystallinity. The Fe-doped MnO\({}_{\text{x}}\) catalyst notably enhances its surface area and acidity. Concurrently, there is a robust interaction between iron oxide and manganese oxide, leading to a significant boost in catalytic activity. Chen et al. [37] developed a novel Fe-Mn mixed oxide catalyst for the low-temperature SCR of NOx using ammonia in the presence of excess oxygen. Their results showed that the Fe (0.4) MnO\({}_{\text{x}}\) catalyst exhibits the highest catalytic activity. Figure 9 shows the mechanism for the SCR process of MnO\({}_{\text{x}}\)-FeO\({}_{\text{x}}\) catalyst. At 120 \({}^{\circ}\)C with a space velocity of 30,000 h\({}^{-1}\), the NOx conversion reaches 98.8%, and the N\({}_{2}\) selectivity is 100%. The XRD results revealed the formation of a new crystalline phase, Fe\({}_{3}\)Mn\({}_{3}\)O\({}_{8}\), within the Fe-MnO\({}_{\text{x}}\) catalyst. TPR and Raman data indicated a strong interaction between iron oxide and manganese oxide, which results in the formation of Fe\({}_{3}\)Mn\({}_{3}\)O\({}_{8}\). X-ray photoelectron spectroscopy (XPS) analysis of fresh, used, and regenerated catalysts showed that the electron transfer between Fe\({}^{\text{n+}}\) and Mn\({}^{\text{n+}}\) ions in Fe\({}_{3}\)Mn\({}_{3}\)O\({}_{8}\) may explain the long lifespan of the Fe (0.4) MnO\({}_{\text{x}}\) catalyst. Zhang et al. [62] prepared Fe-Mn oxides by thermally activating manganese-rich limonite at different temperatures and developed them as low-temperature NH\({}_{3}\)-SCR catalysts. Characterization results demonstrated that the NO conversion rate of the Fe-Mn oxide catalyst (H300) treated at 300 \({}^{\circ}\)C exceeds 90% within the temperature range of 130 to 300 \({}^{\circ}\)C. The large specific surface area (95.9 m\({}^{2}\)/g), numerous acid sites, appropriate reduction behavior, and abundant surface hydroxyl groups greatly contribute to the exceptional performance of H300 for NH\({}_{3}\)-SCR. The Sm-doped MnO\({}_{\text{x}}\) catalyst inhibits MnO\({}_{\text{x}}\) crystallization, enlarges its surface area, and enhances the relative content of Mn\({}^{4+}\) and surface oxygen (OS) on the catalyst, thereby  
Figure 9: Mechanism for the SCR process of MnO\({}_{\text{x}}\)-FeO\({}_{\text{x}}\) catalyst. (Reproduced with permission from Ref. [37]. Copyright 2012, American Chemical Society, Washington, DC, USA). increasing NH\({}_{3}\) and NO absorption. Meng et al. [38] developed a Sm-Mn mixed oxide catalyst via the co-precipitation method and evaluated its catalytic activity for SCR of NO with ammonia at low temperatures. Their findings indicated that the addition of Sm notably influences the activity of the Mn oxide SCR catalyst, displaying a volcanic trend with rising Sm content and identifying an optimal Sm to Mn molar ratio of 0.1. Moreover, the presence of Sm in MnO\({}_{\mathrm{x}}\) catalysts markedly boosts their resistance to H\({}_{2}\)O and SO\({}_{2}\).