## 3 Binary Mn-Based Metal Oxide Catalysts
### 3.1. \(MnO_{x}\)-CeO\({}_{2}\) Catalysts  
In recent years, significant attention has been given to SCR catalysts using CeO\({}_{2}\) as an active material or catalytic promoter. CeO\({}_{2}\) is an acid-based material characterized by numerous Lewis acid sites and a limited number of Bronsted acid sites. Additionally, the mutual transformation of CeO\({}_{x}\) between different valence states endows it with a strong redox performance and high SCR activity at elevated temperatures. The synthesis of the MnO\({}_{x}\)-CeO\({}_{2}\) catalyst, combining MnO\({}_{x}\) and CeO\({}_{2}\), results in an expanded active temperature range, enhanced N\({}_{2}\) selectivity, and significantly improved thermal stability and resistance to H\({}_{2}\)O/SO\({}_{2}\) poisoning. Figure 2 shows the proposed mechanism of NH\({}_{3}\)-SCR reaction on the surface of the MnCeO\({}_{x}\) catalyst. Table 2 lists several preferred raw materials/process conditions for the MnO\({}_{x}\)-CeO\({}_{x}\) catalyst. Xu et al. [48] prepared a MnO\({}_{x}\)-CeO\({}_{2}\) catalyst using the deposition-precipitation (DP) method for the SCR of NO by NH\({}_{3}\) at low temperatures. Within a short reaction time, nearly 100% NO conversion is achieved at a space velocity of 50,000 h\({}^{-1}\) over the entire evaluation temperature range of 80-150 \({}^{\circ}\)C. The catalyst maintains stable activity at 120 \({}^{\circ}\)C and a space velocity of 120,000 h\({}^{-1}\) during a 36 h test. \begin{table}
\begin{tabular}{c c c c c} \hline
**Catalyst** & **Preparation Method** & **Reaction Condition** & **Denitrification Activity** & **Refs.** \\ \hline Mn-Eu & co-precipitation & (NO) = (NH\({}_{3}\)) = 600 ppm, 5\% O\({}_{2}\), & (150–400 \({}^{\circ}\)C) \textgreater{}95\% & [40] \\ Mn-Cr & co-precipitation & (NO) = (NH\({}_{3}\)) = 1000 ppm, 3\% O\({}_{2}\), & (120–220 \({}^{\circ}\)C) \textgreater{}100\% & [41] \\ Mn-Zr & co-precipitation & (NO) = (NH\({}_{3}\)) = 500 ppm, 5\% O\({}_{2}\), & (125–250 \({}^{\circ}\)C) \textgreater{}90\% & [42] \\ Mn-Er & co-precipitation & (NO) = (NH\({}_{3}\)) = 500 ppm, 5\% O\({}_{2}\), & (52–250 \({}^{\circ}\)C) \textgreater{}80\% & [43] \\ Mn-Ni & solvent-free & (NO) = (NH\({}_{3}\)) = 500 ppm, 5\% O\({}_{2}\), & (120–240 \({}^{\circ}\)C) \textgreater{}100\% & [44] \\ Mn-Nb & homogeneous precipitation & (NO) = (NH\({}_{3}\)) = 500 ppm, 5\% O\({}_{2}\), & (125–200 \({}^{\circ}\)C) \textgreater{}90\% & [45] \\ Mn-V & sol\(-\)gel & (NO) = (NH\({}_{3}\)) = 500 ppm, 5.3\% O\({}_{2}\), & (120–240 \({}^{\circ}\)C) \textgreater{}90\% & [46] \\ Mn-Si & wet impregnation & (NO) = (NH\({}_{3}\)) = 500 ppm, 6\% O\({}_{2}\), & (160–280 \({}^{\circ}\)C) \textgreater{}80\% & [33] \\ Mn-Al & wet impregnation & (NO) = (NH\({}_{3}\)) = 500 ppm, 6\% O\({}_{2}\), & (180–280 \({}^{\circ}\)C) \textgreater{}80\% & [33] \\ Mn-Co & one-step synthesis & (NO) = (NH\({}_{3}\)) = 500 ppm, 5\% O\({}_{2}\), & (150–225 \({}^{\circ}\)C) \textgreater{}80\% & [47] \\ Mn-Sn & one-step synthesis & (NO) = (NH\({}_{3}\)) = 500 ppm, 5\% O\({}_{2}\), & (100–175 \({}^{\circ}\)C) \textgreater{}80\% & [47] \\  & & GHVS = 32,000 h\({}^{-1}\) & & \\ \hline \end{tabular}
\end{table} 
Table 1: _Cont._  
Figure 2: Proposed mechanism of NH\({}_{3}\)-SCR reaction on the surface of the MnCeO\({}_{x}\) catalyst. (Reproduced with permission from Ref. [32]. Copyright 2023, Elsevier, Amsterdam, The Netherlands). The results showed that the phase effect of MnO\({}_{\rm x}\) plays a crucial role in the low-temperature denitrification performance of the MnO\({}_{\rm x}\)-CeO\({}_{2}\) catalyst. Liu et al. [49] synthesized MnO\({}_{\rm 3}\)-CeCO\({}_{\rm 3}\)OH and decomposed it into MnO\({}_{\rm 2}\)-CeO\({}_{\rm 2}\), Mn\({}_{\rm 2}\)O\({}_{\rm 3}\)-CeO\({}_{\rm 2}\), and Mn\({}_{\rm 3}\)O\({}_{\rm 4}\)-CeO\({}_{\rm 2}\), all with similar sizes and morphologies. The denitrification performance test revealed that MnO\({}_{\rm 2}\)-CeO\({}_{\rm 2}\) achieves the highest NO conversion rate at a low temperature of 150 \({}^{\circ}\)C, reaching nearly 96.5% (Figure 3).