## 2 Single MnO\({}_{\text{x}}\) Catalyst  
Many studies have indicated that MnO\({}_{\text{x}}\) exhibits effective denitrification activity at low temperatures (<200 \({}^{\circ}\)C). Research has demonstrated that the denitrification activity of standalone MnO\({}_{\text{x}}\) catalysts is mainly associated with factors such as the type of Mn oxides, their surface structure, surface acidity, redox performance, specific surface area, and N\({}_{2}\) selectivity. Among these, the redox performance is crucial as it facilitates the activation of ammonia and NOx on the catalyst and has a significant effect on the catalytic performance of NOx reduction. The redox performance of the catalyst is typically evaluated using the temperature-programmed reduction (H\({}_{2}\)-TPR) test. Surface acid sites (Lewis, Bronsted) play an important role in the NH\({}_{3}\)-SCR reaction because they affect the NH\({}_{3}\) adsorption process on the catalyst. The surface acidity of the catalyst is usually assessed through the ammonia temperature-programmed desorption (NH\({}_{3}\)-TPD) test. All factors influencing the denitrification activity of the single MnO\({}_{\text{x}}\) catalyst are interconnected and collectively affect the catalyst's denitrification performance. The catalytic performance of a single MnO\({}_{\text{x}}\) catalyst is influenced by its crystal and surface structures. MnO\({}_{\text{x}}\) with different oxidation states often exhibit different crystal structures, which in turn affects their NH\({}_{3}\)-SCR reactivity. In general, high-oxidized MnO\({}_{\text{x}}\) is generally more favorable for NH\({}_{3}\)-SCR reaction. According to Kapteijn et al. [24], who studied the NH\({}_{3}\)-SCR reaction on pure MnO\({}_{\text{x}}\), it was found that under the same surface area of various MnO\({}_{\text{x}}\) catalysts, the NO conversion rate follows the order: MnO\({}_{2}\) > Mn\({}_{5}\)O\({}_{8}\) > Mn\({}_{2}\)O\({}_{3}\) > Mn\({}_{3}\)O\({}_{4}\) > MnO. Thisindicates that the denitrification ability of the MnO\({}_{\text{x}}\) catalyst increases with the oxidation state of Mn, with MnO\({}_{\text{2}}\) showing the highest NO conversion rate. The higher NO conversion rate of highly oxidized manganese oxide is attributed to its higher surface active oxygen content, larger specific surface area, and stronger redox performance. Even if the oxidation state of Mn remains constant, differences in the crystal or surface structure can greatly impact the performance of the NH\({}_{\text{3}}\)-SCR reaction. Kapteijn et al. [24] prepared MnO\({}_{\text{2}}\) catalysts using different methods and discovered that both the specific surface area and the surface regularity significantly affect NH\({}_{\text{3}}\)-SCR activity: a higher specific surface area leads to a higher NO\({}_{\text{x}}\) conversion rate, while a more irregular surface structure increases N\({}_{\text{2}}\) selectivity, possibly due to the abundance of defect sites. Zhang et al. [25] observed that non-loaded amorphous MnO\({}_{\text{x}}\) exhibits a superior denitrification performance compared to the crystalline MnO\({}_{\text{x}}\) phase. This heightened activity is attributed to the unsupported amorphous MnO\({}_{\text{X}}\) possessing a better redox performance, numerous weak acid sites, improved MnO\({}_{\text{x}}\) dispersion, and Mn\({}^{4+}\) content. Additionally, the morphology of nano-MnO\({}_{\text{x}}\) also affects catalytic performance. Tian et al. [26] compared the low-temperature denitration efficiency of MnO\({}_{\text{2}}\) nanotubes, nanorods, and nanoparticles, finding that MnO\({}_{\text{2}}\) nanorods achieve the highest efficiency (more than 90%) at 250-300 \({}^{\circ}\)C, due to their excellent NH\({}_{\text{3}}\) adsorption performance. To explore the link between catalytic efficiency and structure, Li et al. [27] successfully prepared MnO\({}_{\text{2}}\) nanomaterials (nanospheres, nanosheets, nanorods) and found that their SCR activity follows the trend of MnO\({}_{\text{2}}\) nanospheres > nanosheets > nanorods.