## 6 Preparation Method  
Generally, the high catalytic ability and stability of composite metal oxide catalysts depend on the interaction between metal ions and the dispersion of active components. The method of preparation plays a decisive role in the catalyst's structure, phase composition, and distribution of the active component. Hence, selecting the appropriate preparation method is essential to optimize the catalytic performance of the catalyst. Table 1, Table 4, and Table 5 reveal that the majority of Mn-based catalysts are synthesized using the sol-gel, co-precipitation, or impregnation methods. According to Jiang et al. [128], the SCR activity order of MnO\({}_{\text{x}}\)/TiO\({}_{2}\) catalysts prepared by these methods is sol-gel \(>\) impregnation \(>\) co-precipitation. This is because the sol-gel method results in MnO\({}_{\text{x}}\)/TiO\({}_{2}\) catalysts with stronger interaction between active components, excellent dispersion, and low crystallinity. Liu et al. [54] studied the effects of key synthesis parameters on the low-temperature SCR activity of Mn-Ti oxide catalysts prepared via the sol-gel method. Catalysts with a higher citric acid to metal ion ratio (6:1) and a lower Mn: Ti molar ratio (2:1) exhibit superior catalytic activity. The effects of calcination time and pH value are minimal, whereas calcination temperature and metal sources (Mn and Ti) have a significant effect on the structure and SCR activity. Lower calcination temperatures favor low-temperature denitrification activity due to a higher BET-specific surface area, smaller nanoparticle size, amorphous oxide form, good low-temperature reducibility, and abundant surface Bronsted acid sites. Catalysts using manganese nitrate or manganese acetate as a Mn source and tetrabutyl titanate as a Ti source demonstrate better SCR activity. Crystalline Mn\({}_{2}\)O\({}_{3}\) forms when TiO\({}_{2}\) is used as a Ti source. The Mn oxide catalyst with MnO\({}_{4}\) as a Mn source mainly exists in an amorphous state but exhibits poor reduction and unsatisfactory SCR activity. Chao et al. [129] proposed that Mn-Ce-Ox/TiO\({}_{2}\) catalysts prepared by the co-precipitation method have better low-temperature denitration activity and SO\({}_{2}\) resistance compared to those prepared by the sol-gel method. The co-precipitation method enhances the Mn-Ce-Ox/TiO\({}_{2}\) catalyst's adsorption capacity, specific surface area, surface Mn\({}^{4+}\) content, and surface adsorbed oxygen. Zappi et al. [130] proposed that Mn-Fe-TiO\({}_{2}\) catalysts prepared by the deposition-precipitation method exhibit superior low-temperature SCR activity compared to those prepared by the impregnation method. The deposition-precipitation method facilitates the formation of amorphous MnOx and FeOx, increases specific surface area, chemisorbed oxygen, acid site number and strength, and improves the reducibility of MnOx and FeOx. In addition to the three traditional preparation methods mentioned above, new techniques have been developed for preparing Mn-based catalysts, achieving advancements in process simplification and catalyst performance enhancement. Zhang et al. [131] developed an in situdeposition technique for Mn-based catalysts via a rapid redox reaction between Mn\({}^{2+}\) and MnO\({}_{4}\) in aqueous solution. This method, compared to the impregnation and co-precipitation methods, greatly simplifies catalyst preparation through one-step oxidation, resulting in a higher specific surface area, sponge-like pore structures, more dispersed columnar MnOx species, and increased acid site distribution on the catalyst surface. The impregnation method leads to MnOx aggregation, poor dispersion, and suboptimal denitrification performance due to the CEC limitations of Mn\({}^{2+}\) on the cations between the montmorillonite layers. The catalyst prepared by in situ deposition not only introduces more MnOx and more protons into the interlayer space of montmorillonite but also increases the Bronsted and Lewis acid sites. Yu et al.