## 3 Binary Mn-Based Metal Oxide Catalysts
### MnO\({}_{x}\)-TiO\({}_{2}\) Catalysts
[56] synthesized a series of Mn-based catalysts supported on anatase TiO\({}_{2}\) with particle sizes of 5-10 nm, 10-25 nm, 40 nm, and 60 nm using the impregnation method, examining the effect of the initial support particle size on the SCR of NO by NH\({}_{3}\). The change in the initial particle size of TiO\({}_{2}\) support can alter the surface molar ratio of Mn\({}^{4+}\)/Mn, thus affecting NOx adsorption and SCR catalytic performance. The order of Mn\({}^{4+}\)/Mn molar ratio and acid amount on the surface of MnO\({}_{\rm x}\)-TiO\({}_{2}\) catalyst is as follows: MnO\({}_{\rm x}\)-TiO\({}_{2}\) (10-25) > MnO\({}_{\rm x}\)-TiO\({}_{2}\) (40) > MnO\({}_{\rm x}\)-TiO\({}_{2}\) (60) > MnO\({}_{\rm x}\)-TiO\({}_{2}\) (5-10), which is consistent with the order of SCR performance. Shu et al. [57] found that MnO\({}_{\rm x}\) and Ti-doped MnO\({}_{\rm x}\) catalysts completely remove NO at 200-300 \({}^{\circ}\)C. Figure 5 shows the HRTEM images of MnO\({}_{\rm x}\) and Mn\({}_{1}\)Ti\({}_{\rm O}\)\({}_{\rm x}\). Figure 6 shows the mechanism of the steady-state SCR reaction. Below 200 \({}^{\circ}\)C, the order of NO removal efficiency from highest to lowest is Mn\({}_{1}\)Ti\({}_{1}\)O\({}_{\rm x}\) > Mn\({}_{2}\)Ti\({}_{\rm O}\)\({}_{\rm x}\) > Mn\({}_{1}\)Ti\({}_{2}\)O\({}_{\rm x}\) > MnO\({}_{\rm x}\) > TiO\({}_{\rm x}\), with Mn\({}_{1}\)Ti\({}_{2}\)O\({}_{\rm x}\) showing the best NO removal effect above 300 \({}^{\circ}\)C.  
Figure 4: Schematic diagram of the possible existing state of active ingredients on the surface of the prepared samples. (**a**) FMT-400, (**b**) FMT(S)-400, (**c**) FMT(S)-400 with anatase TiO\({}_{2}\), (**d**) FMT(S)-500 with anatase and rutile TiO\({}_{2}\), (**e**) FMT(S)-600 with anatase and rutile TiO\({}_{2}\) (Reproduced with permission from Ref. [55]. Copyright 2015, Elsevier). The crystallinity of the MnO\({}_{x}\)-TiO\({}_{2}\) catalyst is a crucial factor affecting its denitrification performance. It was observed that the catalytic activity of the MnO\({}_{x}\)-TiO\({}_{2}\) catalyst decreases considerably with an increase in its crystallinity. Wei et al. [58] found that the crystallinity of the MnO\({}_{x}\)-TiO\({}_{2}\) catalyst plays a significant role in its denitrification activity. Higher crystallinity lowers the dispersion, specific surface area, and the number of acidic sites of the active components in the catalyst. Characterization results indicated that with increased crystallinity, the Mn\({}^{3+}\) content and surface chemisorbed oxygen ratio decrease, which inhibits NH\({}_{3}\) adsorption and NOx removal, resulting in reduced NO conversion. According to the NH\({}_{3}\)-SCR reaction principle, N\({}_{2}\)O formation during the low-temperature NH\({}_{3}\)-SCR process significantly impacts the denitrification reaction efficiency and introduces secondary pollution. Therefore, some researchers have studied the origin and function of N\({}_{2}\)O on MnO\({}_{x}\)-TiO\({}_{2}\) catalysts and methods to inhibit its formation. Wang et al. [59] found that NH\({}_{3}\) oxidation and the NSCR reaction between NO and NH\({}_{3}\) are the primary causes of N\({}_{2}\)O formation. In the NH\({}_{3}\)-SCR process on MnO\({}_{x}\)-TiO\({}_{2}\), the nitrogen in N\({}_{2}\)O originates not only from NH\({}_{3}\) involved in the oxidation reaction but also from NH\({}_{3}\) and NO participating in the NSCR reaction. Gaseous O\({}_{2}\) serves as an essential source of oxygen in N\({}_{2}\)O formation. Liu et al. [60] conducted an in-depth study of the typical MnO\({}_{x}\)-TiO\({}_{2}\) catalyst. Based on  
Figure 5: HRTEM images of MnO\({}_{x}\) and Mn\({}_{1}\)Ti\({}_{1}\)O\({}_{x}\). (Reproduced with permission from Ref. [57]. Copyright 2019, Elsevier). Figure 6: Mechanism of the steady-state SCR reaction. (Reproduced with permission from Ref. [57]. Copyright 2019, Elsevier). the results of in situ drift, TPD-MS, and density functional theory (DFT) calculations, they proposed a NO oxidation mechanism according to the 'nitro-assisted oxygen activation' concept.