## 3 Results and Discussion
### Reaction Mechanism for Thermocatalytic Decomposition of DMMP
Based on the result, the \(\Delta\)Ce\({}^{3+}\)/Time value was inversely proportional to the protection time, which further confirms our previous speculation. Among all samples, 12 MCeO\({}_{2}\)nr possessed the smallest \(\Delta\)Ce\({}^{3+}\)/Time and the longest protection time, which indicates its superior catalytic performance. Ion chromatography is used to qualitative analysis of P-containing byproducts on the surface of the spent CeO\({}_{2}\) catalysts and the colorless gelatrous species at the end of the catalytic reaction tube. The results of ion chromatograph indicate the presence of PO\({}_{4}\)\({}^{3-}\) at both sites as shown in Table 5. Therefore, the byproduct on the surface of CeO\({}_{2}\) is CePO\({}_{4}\). The colorless gelatinous PO\({}_{4}\)\({}^{3-}\)-containing species is soluble in water, so it is not CePO\({}_{4}\). We speculate the colorless gelatinous species is H\({}_{3}\)PO\({}_{4}\). For the reason that the boiling point of H\({}_{3}\)PO\({}_{4}\) is lower than the catalytic reaction temperature, the weak adsorption of H\({}_{3}\)PO\({}_{4}\) on the catalyst results in the desorption of H\({}_{3}\)PO\({}_{4}\) from the catalyst bed by air flow. H\({}_{3}\)PO\({}_{4}\) should form steam getting into the air; on the contrary, it was found on the wall at the end of the reaction tube. This phenomenon occurs because the catalytic reaction tube has a constant temperature zone, where the catalyst bed is in, and the temperature at the end of the reaction tube is lower than that in the constant temperature zone, so H\({}_{3}\)PO\({}_{4}\) steam was condensed on the tube wall. Figure 8: XPS spectra of deactivation CeO\({}_{2}\) nanomaterials (**a**) P2p of 2 MCeO\({}_{2}\)np, (**b**) P2p of 12 MCeO\({}_{2}\)nr, (**c**) P2p of 12 MCeO\({}_{2}\)nc, (**d**) O1s of 2 MCeO\({}_{2}\)np, (**e**) O1s of 12 MCeO\({}_{2}\)nr, (**f**) O1s of 12 MCeO\({}_{2}\)nc, (**g**) Ce3d of 2 MCeO\({}_{2}\)np, (**h**) Ce3d of 12 MCeO\({}_{2}\)nr, and (**i**) Ce3d of 12 MCeO\({}_{2}\)nc. The tail gas produced during the catalytic decomposition of DMMP on 2 MCeO\({}_{2}\)np, 12 MeO\({}_{2}\)nr, and 12 MCeO\({}_{2}\)nc were qualitatively analyzed by mass spectrometry to determine its components. The mass spectra of the tail gas produced by 2 MCeO\({}_{2}\)np, 12 MCeO\({}_{2}\)nr, and 12 MCeO\({}_{2}\)nc are displayed in Figure 9. According to the results, signals belonging to methanol, CO\({}_{2}\), H\({}_{2}\)O, and H\({}_{2}\) were observed. Moreover, the mass-to-charge ratio of CO was also 28 in addition to the fragment signals of methanol and CO\({}_{2}\). As such, gas chromatography was employed to confirm the presence of CO in the produced tail gas. \begin{table}
\begin{tabular}{c c c c c} \hline \multirow{2}{*}{**Catalyst**} & **Before Decomposition** & **After Decomposition** & \multirow{2}{*}{**\(\Delta\)Ce\({}^{3+}\)**} & \multirow{2}{*}{**\(\Delta\)Ce\({}^{3+}\)/Time**} \\  & **Reaction** & **Reaction** & & \\ \cline{2-4}  & **Ce\({}^{3+}\)/(Ce\({}^{3+}\)+ Ce\({}^{4+}\))** & **Ce\({}^{3+}\)/(Ce\({}^{3+}\)+ Ce\({}^{4+}\))** & & \\ \hline
2 MCeO\({}_{2}\)np & 19.64 & 34.99 & 15.35 & 0.044 \\
12 MCeO\({}_{2}\)nr & 25.15 & 35.01 & 9.86 & 0.020 \\
12 MCeO\({}_{2}\)nc & 20.82 & 30.57 & 9.75 & 0.026 \\ \hline \end{tabular}
\end{table} 
Table 4: Variation of Ce\({}^{3+}\) content in clean and deactivated catalysts. \begin{table}
\begin{tabular}{c c} \hline \multicolumn{1}{c}{**Samples**} & **PO\({}_{4}\)\({}^{3-}\)** \\ \hline
2 MCeO\({}_{2}\)np & 3.508 (g/kg) \\
12 MCeO\({}_{2}\)nr & 1.450 (g/kg) \\
12 MCeO\({}_{2}\)nc & 1.720 (g/kg) \\
2 MCeO\({}_{2}\)np reaction tube tail residues & 0.2943 (mg/L) \\
12 MCeO\({}_{2}\)nr reaction tube tail residues & 0.0722 (mg/L) \\
12 MCeO\({}_{2}\)nc reaction tube tail residues & 0.1654 (mg/L) \\ \hline \end{tabular}
\end{table} 
Table 5: Content of PO\({}_{4}\)\({}^{3-}\) on various CeO\({}_{2}\) materials and at the end of the reaction tube after catalytic decomposition of DMMP.