## 2 Single MnO\({}_{\text{x}}\) Catalyst
Within the temperature range of 200-350 \({}^{\circ}\)C, MnO\({}_{\text{2}}\) nanospheres can achieve a NO conversion rate of 100%. Figure 1 shows the reaction mechanism of NH\({}_{\text{3}}\)-SCR on MnO\({}_{\text{2}}\) with different morphologies. The specific surface area of the catalyst is a crucial factor affecting its denitrification performance. Generally, a larger specific surface area leads to more acidic sites available for the reaction and a higher adsorption of reactive substances, resulting in better denitrification activity for the catalyst. Kang et al. [28] found that the MnO\({}_{\text{X}}\) catalyst, prepared using precipitation technology with sodium carbonate as a precipitant, exhibits high denitration activity at low temperatures. This is likely due to its high surface area, amorphous structure, and residual carbon oxides. Tang et al. [29] found that amorphous MnO\({}_{\text{x}}\) obtained through solid-state reaction (SP) and co-precipitation (CP) shows higher activity than highly crystalline MnO\({}_{\text{x}}\) prepared using citric acid (CA) at low temperatures. The amorphous MnO\({}_{\text{x}}\) catalysts produced by these three methods show excellent activity in the SCR of NO\({}_{\text{x}}\) with NH\({}_{\text{3}}\) at low temperatures in the presence of excess O\({}_{\text{2}}\). The high specific surface area of the amorphous phase and manganese oxide is likely the primary reason for the enhanced denitrification activity of these catalysts at low temperatures. For the NH\({}_{\text{3}}\)-SCR reaction in pure NO simulated flue gas, pure MnO\({}_{\text{x}}\) catalysts typically demonstrate higher catalytic activity and N\({}_{\text{2}}\) selectivity. However, in the real flue gas condition, the presence of H\({}_{\text{2}}\)O and SO\({}_{\text{2}}\) is continuous and inevitable. Experiments with flue gas that add H\({}_{\text{2}}\)O and SO\({}_{\text{2}}\) indicate that these components significantly inhibit the NH\({}_{\text{3}}\)-SCR reaction, resulting in a reduction in NO\({}_{\text{x}}\) conversion to varying extents and  
Figure 1: Reaction mechanism of NH\({}_{\text{3}}\)-SCR on MnO\({}_{\text{2}}\) with different morphologies. (Reproduced with permission from Ref. [27]. Copyright 2016, Royal Society of Chemistry). even causing catalyst deactivation. Studies have shown that among Mn oxides, Mn\({}_{3}\)O\({}_{4}\) has the best SO\({}_{2}\) resistance. An et al. [30] prepared three types of MnO\({}_{\rm x}\) catalysts with different valence distributions (MnO\({}_{2}\), Mn\({}_{2}\)O\({}_{3}\), and Mn\({}_{3}\)O\({}_{4}\)) and conducted an NH\({}_{3}\)-SCR reaction in the presence of SO\({}_{2}\). Their results showed that the three catalysts exhibit valence-related SO\({}_{2}\) resistance (Mn\({}_{3}\)O\({}_{4}\) > Mn\({}_{2}\)O\({}_{3}\) > MnO\({}_{2}\)) within 7 h under SCR conditions, with an SO\({}_{2}\) concentration of 100 ppm and a temperature of 100 \({}^{\circ}\)C. Further characterization revealed that the Mn\({}_{3}\)O\({}_{4}\) catalyst has more basic sites and forms less bulk sulfate, which may account for its slower deactivation over a limited test period. Other studies have shown that adding Al\({}_{2}\)O\({}_{3}\) can enhance the SO\({}_{2}\) resistance of MnO\({}_{\rm x}\) catalysts. To improve the SO\({}_{2}\) resistance of MnO\({}_{\rm x}\) catalysts during low-temperature SCR denitration, Fan et al. [31] carried out thermogravimetric (TG) and temperature-programmed desorption of SO\({}_{2}\) (SO\({}_{2}\)-TPD) on 21 different single metal oxides. The results indicated that Al\({}_{2}\)O\({}_{3}\) significantly reduces the thermal stability of NH\({}_{4}\)HSO\({}_{4}\), and the adsorption of Al\({}_{2}\)O\({}_{3}\) by the material formed after SO\({}_{2}\) adsorption is weak. Given the complex composition of industrial flue gas, which typically contain H\({}_{2}\)O and SO\({}_{2}\), the denitrification activity of single MnO\({}_{\rm x}\) catalysts is often significantly diminished.