## 5 Nano-Mn-Based Catalyst
### Composite MnO\({}_{x}\) Nanocatalysts
The TiO\({}_{2}\) shell not only avoids inhibiting the catalytic activity of MnO\({}_{x}\)-CeO\({}_{2}\)@TiO\({}_{2}\) but also improves the catalyst's physical and chemical properties. Its unique core-shell structure reduces the possibility of surface active sites being poisoned by SO2 and also prevents the formation of active sites covered by ammonium sulfate species. Li et al. [116] synthesized core-shell FMTs catalysts with excellent low-temperature denitration activity, high N2 selectivity, and good sulfur resistance by introducing nitrogen trioxide and iron oxide through the hydrothermal method and electro-coupling reaction. Characterization and experimental results indicated that TNTs' confinement and the appropriate oxidation of the interaction between MnOx and Fe2O3 enhance N2 selectivity, with the Fe2O3 shell inhibiting manganese sulfate formation and promoting sulfur resistance. Yang et al. [123] synthesized MnOx-CeO2 nanorods and MnOx-CeO2 nanofacet catalysts using the hydrothermal method for the SCR of NO by NH3. Catalytic performance tests revealed that loading MnOx significantly improves the NO removal rate of the CeO2 catalyst, with the Mn-Ce catalyst outperforming the CeO2 catalyst. SEM and TEM results demonstrate that the Mn-Ce catalyst possesses a uniform core-shell structure. The NH3-SCR reaction mechanism of the Mn-Ce catalyst was proposed: MnOx is the main active component, which catalyzes NO to generate N2. CeO2, as a co-catalyst, improves the catalytic reaction by oxidizing MnOx and supplementing chemisorbed oxygen. The establishment of a TiO2 shell on MnFeOx nanocatalysts can effectively improve the adsorption and activation performance of the catalyst. Cai et al. [124] prepared MnFeOx@TiO2 double-walled nanocages featuring hollow porous structures. Compared to MnFeOx single-walled nanocages without a TiO2 shell, these MnFeOx@TiO2 double-walled nanocages show a superior catalytic performance in low-temperature NH3-SCR, achieving a NO removal rate of up to 80% at 145-260 \({}^{\circ}\)C. The establishment of the TiO2 shell markedly boosts the catalyst's adsorption and activation performance. A series of characterizations confirmed that MnFeOx@TiO2 double-walled nanocages possess a large specific surface area, abundant structural defects, and oxygen vacancies. Moreover, the MnFeOx@TiO2 catalyst contains a higher concentration of Mn4 species, Fe3+ species, and surface-adsorbed oxygen (Oa) species, with a strong interaction between MnOx, FeOx, and the TiO2 shell, resulting in improved catalytic activity. Rao et al. [125] discovered that the oxidation state of MnOx significantly affects the performance of Mn-Ce nanocatalysts. They prepared CeO2 nanorod support using the hydrothermal method and then impregnated different manganese oxides (MnO, MnO2, and Mn2O3) on the support to form MnO-CeO2-NR, MnO-CeO2-NR, and Mn2O3-CeO2-NR. Among these, the MnO2-CeO2-NR catalyst has the highest NO conversion rate (65%) and N2 selectivity (89%). Within the temperature range of 250 to 300 \({}^{\circ}\)C, both NO conversion and N2 selectivity exceed 90%. Zhu et al. [117] developed a novel low-temperature nano-SCR catalyst with Cu-doped MnOx supported on activated carbon and investigated its performance for the SCR of NO. The experimental findings revealed that at a calcination temperature of 350 \({}^{\circ}\)C, with MnOx loading at 5%, Cu loading at 3%, and a reaction temperature of 200 \({}^{\circ}\)C, the SCR catalyst achieves peak catalytic activity for NO conversion. Under these conditions, the NO conversion reaches 96.82%, and the N2 selectivity is close to 99%. Sulfate modification can improve the high-temperature activity of Mn-Ti nanocatalysts. Kang et al. [126] introduced TiO2 nanoparticles and sulfate into the MnOx catalyst successively to adjust the NH3-SCR activity. Compared to the original MnOx catalyst (about T80: 100-268 \({}^{\circ}\)C), the active temperature window (NO conversion higher than 80%, T80) is significantly expanded to 100-350 \({}^{\circ}\)C (SO4\({}^{2-}\)-TiO2@MnOx).