## 3 Binary Mn-Based Metal Oxide Catalysts
### MnO\({}_{x}\)-TiO\({}_{2}\) Catalysts  
Many reports have highlighted that the MnO\({}_{\rm x}\) catalyst, when affixed to a carrier, can improve its surface acidity and optimize the catalyst's redox performance to some extent. An appropriate support can extensively disperse the surface active components, boost electron transfer between these components, and provide sufficient reaction space for the denitrification catalytic reaction. Due to its high specific surface area, the active component can be highly dispersed on the surface of TiO\({}_{2}\), significantly increasing the number of active centers on the catalyst, thereby increasing the NH\({}_{3}\) adsorption capacity. TiO\({}_{2}\) itself possesses strong Lewis acidity and a robust resistance to SO\({}_{2}\) poisoning, making it one of the most commonly used carriers in SCR catalysts. Many scholars have uniformly loaded \begin{table}
\begin{tabular}{c c c} \hline \hline
**Catalyst** & **Preferred Raw Materials/Process Conditions** & **Ref.** \\ \hline \multirow{6}{*}{MnO\({}_{\rm x}\)-CeO\({}_{\rm x}\)} & Surfactant-template method \textgreater{} Co-precipitation method & [50] \\  & Citric acid method \textgreater{} Co-precipitation method and & [51] \\  & impregnation method & \\ \cline{1-1}  & Hydrothermal treatment \textgreater{} Sol-gel \textgreater{} Co-precipitation \textgreater{} & [52] \\ \cline{1-1}  & Impregnation \textgreater{} Mechanical mixing & [53] \\ \cline{1-1}  & MnO\({}_{\rm x}\): MnO\({}_{2}\)-CeO\({}_{2}\)\textgreater{} Mn\({}_{2}\)O\({}_{3}\)-CeO\({}_{2}\)\textgreater{} Mn\({}_{3}\)O\({}_{4}\)-CeO\({}_{2}\) & [49] \\ \cline{1-1}  & Solvent: Oxalic acid \textgreater{} Acetic acid, anhydrous ethanol, and & [53] \\ \cline{1-1}  & deionized water & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Preferred raw materials/process conditions of MnO\({}_{\rm x}\)-CeO\({}_{\rm x}\) catalyst. Figure 3: SCR-deNO\({}_{\rm x}\) activity (**a**), N\({}_{2}\) selectivity (**b**), BET surface (**c**), and (**d**). (Reproduced with permission from Ref. [49]. Copyright 2023, Elsevier). MnO\({}_{\rm x}\) onto TiO\({}_{2}\) using a particular method to synthesize the MnO\({}_{\rm x}\)-TiO\({}_{2}\) catalyst. This catalyst has garnered significant attention due to its large specific surface area and excellent redox performance. Table 3 presents some preferred raw materials and process conditions for the MnO\({}_{\rm x}\)-TiO\({}_{\rm x}\) catalyst. Liu et al. [54] prepared a series of Mn-Ti oxide catalysts using the sol-gel method and found that these catalysts exhibit high low-temperature SCR activity. The outstanding low-temperature NH\({}_{3}\)-SCR activity can be attributed to the appropriate structural properties, amorphous manganese oxides with an equal Mn\({}^{3+}\)/Mn\({}^{4+}\) ratio, excellent low-temperature reducibility, and abundant surface acid sites. The determination of raw materials and the selection of their proportions are the first step in the preparation of the catalyst. It was found that the type of TiO\({}_{2}\), the carrier's particle size, and the MnO\({}_{\rm x}\) to TiO\({}_{2}\) ratio greatly influence the denitrification performance of the MnO\({}_{\rm x}\)-TiO\({}_{2}\) catalyst. Wu et al. [55] discovered that the anatase TiO\({}_{2}\) crystal phase, compared to amorphous TiO\({}_{2}\), better enhances the electronic interaction between MnO\({}_{\rm x}\) and the carrier, represented by Mn\({}^{4+}\) + Ti\({}^{3+}\leftrightarrow\) Mn\({}^{3+}\) + Ti\({}^{4+}\). NH\({}_{3}\) is more likely to be activated on the surface of the sample with lower lattice oxygen binding energy during the SCR of NO by NH\({}_{3}\) (Figure 4). Additionally, the calcination temperature has an essential influence on the valence state of MnO\({}_{\rm x}\), the dispersion degree of the active phase, and the crystal structure of the carrier. The sample calcined at 400 \({}^{\circ}\)C demonstrates excellent low-temperature activity and medium-temperature N\({}_{2}\) selectivity. Yang et al.