## 3 Results and Discussion
### Reaction Mechanism for Thermocatalytic Decomposition of DMMP  
12 McCo2 nr and 12 McCo2 nc with superior catalytic performances and 2 McCo2 np were selected for the study on the mechanism of thermocatalytic decomposition of DMMP. After the protection tests, the severely deactivated catalyst collected at the front end of the bed was analyzed using XPS in Figure 8. Based on Figure 8a-c, distinct P2p peaks were observed for all samples, which suggests that the surface of CeO2 is covered by phosphorus species. The OIs spectra presented in Figure 8d-f could be deconvoluted into four peaks, i.e., surface lattice oxygen (O\({}_{\text{Lat}}\)), phosphorus oxygen, adsorbed oxygen (O\({}_{\text{Ads}}\)), and adsorbed water/hydroxyl oxygen [50]. The presence of phosphorus oxygen species further indicates the accumulation of P-containing byproducts on the surface of CeO2, which essentially suppress the decomposition of DMMP. Meanwhile, the Ce3+ content in the sample after the reaction was also calculated from the fitted XPS spectra (Figure 8g-i). Variation of Ce3+ content in clean and deactivated catalysts are summarized in Table 4. Ce3+ content in the spent sample as compared to that in the unused sample (\(\Delta\)Ce3+) increased obviously (Table 4). Combined with the mechanism analysis of CeO2 in the reaction pathway inference (Scheme 1), the surface oxygen of CeO2 participates in the catalytic DMMP decomposition reaction, and the lattice oxygen migrates onto the surface, causing the formation of oxygen vacancies, \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Catalyst** & **Reaction Condition** &
\begin{tabular}{c} **DMMP** \\ **Concentration** \\ \end{tabular} & **Protection Time** & **Reference** \\ \hline
0.5\% Pt-Al2O3 & 299 \({}^{\circ}\)C, flow rate 8.85 L/min & 3.5 g/m3 & 8 h & [PERSON] et al. [46] \\ \hline Cu2-HA & 400 \({}^{\circ}\)C, flow rate 100 mL/min & 3.58 g/m3 & 7.5 h & \\ \hline
1.6\% Pt-TiO2 & 300 \({}^{\circ}\)C, flow rate 100 mL/min & 3.58 g/m3 & 8 h & \\ \hline
10\% V/Al2O3 & & & 12.5 h & \\ \hline
1\% Pt/Al2O3 & & & 8.5 h & \\ \hline
10\% Cu/Al2O3 & & & 7.5 h & \\ \hline Al2O3 & 400 \({}^{\circ}\)C, flow rate 50 mL/min & 1300 ppm & 4.0 h & [PERSON] et al. [13] \\ \hline
10\% Fe/Al2O3 & & & 3.5 h & \\ \hline
10\% Ni/Al2O3 & & & 1.5 h & \\ \hline
10\% V/SiO2 & & & 25 h & \\ \hline CuO/\(\gamma\)-Al2O3 & & & 1.8 h & \\ \hline CuO-1\% CeO2/\(\gamma\)-Al2O3 & & & 2.1 h & \\ \hline CuO-5\% CeO2/\(\gamma\)-Al2O3 & 350 \({}^{\circ}\)C, flow rate 100 mL/min & 4.0 g/m3 & 3.9 h & [PERSON] et al. [48] \\ \hline CuO-10\% CeO2/\(\gamma\)-Al2O3 & & & 1.8 h & \\ \hline CeO2 & & & 2.33 h & \\ \hline
10\% Cu/Ce & & & 4.2 h & \\ \hline
20\% Cu/Ce & & & 4.43 h & \\ \hline
50\% Cu/Ce & 400 \({}^{\circ}\)C, flow rate 100 mL/min & 8.46 g/m3 & 5.36 h & [PERSON] et al. [49] \\ \hline
80\% Cu/Ce & & & 2.33 h & \\ \hline CuO & & & 0.93 h & \\ \hline
2 McCo2 np & & & 5.8 h & \\ \hline
6 McCo2 nr & & & 7.0 h & \\ \hline
12 McCo2 nr & 300 \({}^{\circ}\)C, flow rate 50 mL/min & 5.32 g/m3 & 8.1 h & This work \\ \hline
6 McCo2 nc & & & 3.5 h & \\ \hline
12 McCo2 nc & & & 6.3 h & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Protection time on various catalysts. and extra electrons are transferred to Ce\({}^{4+}\), eventually, Ce\({}^{4+}\) cations are reduced to Ce\({}^{3+}\). However, the accumulation of the byproducts on the surface of the catalyst can inhibit the replenishment of external oxygen into the CeO\({}_{2}\) lattice, and Ce\({}^{3+}\) could not be oxidized to Ce\({}^{4+}\), this will increase the Ce\({}^{3+}\) content. Therefore, it is speculated that the change in Ce\({}^{3+}\) content may be related with the life of the catalysts (protection time). As such, the variation of Ce\({}^{3+}\) content before and after catalytic decomposition reaction per unit protection time (\(\Delta\)Ce\({}^{3+}\)/Time) of three catalyst materials was calculated (Table 4).