## 3 Results and Discussion
### Reaction Mechanism for Thermocatalytic Decomposition of DMMP
The main carbon-containing byproduct on 12 MCeO\({}_{2}\)nc was bridged carbonate. The accumulation of phosphate, formate, and carbonate species causes the masking of the active site, which ultimately deactivates the catalyst. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Vibrational Mode**} & \multicolumn{3}{c}{**IR Frequencies (cm\({}^{-1}\))**} \\ \cline{2-4}  & **DMMP**[51] & **DMMP Gas Phase**[21; 52] & **DMMP Liquid Phase** \\ \hline \(\
u_{\rm a}\)(P\(-\)CH\({}_{3}\)) & 2992 & 3014 & 2996 \\ \(\
u_{\rm a}\)(O\(-\)CH\({}_{3}\)) & 2957 & 2962 & 2958 \\ \(\
u_{\rm s}\)(P\(-\)CH\({}_{3}\)) & 2926 & 2924 & 2927 \\ \(\
u_{\rm s}\)(O\(-\)CH\({}_{3}\)) & 2852 & 2860 & 2854 \\ \(\delta_{\rm a}\)(O\(-\)CH\({}_{3}\)) & 1465 & 1467 & 1465 \\ \(\delta_{\rm s}\)(O\(-\)CH\({}_{3}\)) & 1450 & / & / \\ \(\delta_{\rm s}\)(P\(-\)CH\({}_{3}\)) & 1317 &Figure 10: In situ DRIFTS spectra of CeO\({}_{2}\) nanomaterials (**a**) 2 MCeO\({}_{2}\)np, (**b**) 12 MCeO\({}_{2}\)nr, and (**c**) 12 MCeO\({}_{2}\)nc. Based on the mass spectroscopy, ion chromatography, and in situ DRIFTS studies, the catalytic decomposition of DMMP pathways based on 2 MCeO\({}_{2}\)np, 12 MCeO\({}_{2}\)nr, and 12 MCeO\({}_{2}\)nc are proposed. The 3D model schematics of CeO\({}_{2}\)np, CeO\({}_{2}\)nr, and CeO\({}_{2}\)np were depicted based on different morphologically exposed crystal plane [58]. From the in situ DRIFTS analysis, the main reaction pathways based on the CeO\({}_{2}\) samples with three different morphologies are the same (Scheme 1). Firstly, the electron-rich P=O in DMMP adsorbs onto the Lewis acid active site of CeO\({}_{2}\), and/or bonded with the surface hydroxyl group of CeO\({}_{2}\) via H bond [33; 59]. Then, the surface hydroxyl group provides the nucleophilic attack to the phosphorus, and the methy group combines with the surface hydrogen to produce gaseous methanol. Cerium methyl phosphate is then formed by further losing the methoxy group (Scheme 1a) [15; 17; 22; 32]. The scission of the P\(-\)CH\({}_{3}\) bond through the surface lattice oxygen of the CeO\({}_{2}\) nucleophilic attack leads to the formation of CeO\({}_{4}\) and a transient methoxy intermediate that contains lattice oxygen (Scheme 1b). After sacrificing the C\(-\)H bond of the transient methoxy intermediate, CO and H\({}_{2}\) are generated on the oxygen-deficient surface of CeO\({}_{2}\)[60]. However, based on the tail gas analysis, the formation of CO, H\({}_{2}\), CO\({}_{2}\), and H\({}_{2}\)O were confirmed (Figure 9), which implies that a part of CO and H\({}_{2}\) were oxidized to CO\({}_{2}\) and H\({}_{2}\)O under an oxygenated environment. The methanol generated via the main reaction can continue to react with the surface lattice oxygen in CeO\({}_{2}\), which produces a variety of products (Scheme 1-IV). Such a process is complex, and it is similar to the reports by [PERSON] and [PERSON] group [54; 56]. Combined with the different types of carbon containing byproducts via further methanol reaction, the side reaction pathways of 2 MCeO\({}_{2}\)np, 12 MCeO\({}_{2}\)nr, and 12 MCeO\({}_{2}\)nc are different. According to Scheme 1 II, The lattice oxygen and methanol initially interact to produce an adsorbed methoxy and a hydroxyl group adsorbed on the surface (Scheme 1d) [54]. Then, the methoxyl group reacts with hydroxyl or surface lattice oxygen, whereby CO, H\({}_{2}\), CO\({}_{2}\), and H\({}_{2}\)O are formed during a complete dehydrogenation (Scheme 1e), while monodentate formates are formed during partial dehydrogenation (Scheme 1f) [54]. In addition, the methoxyl group reacts with hydroxyl and it dehydates to form a bidentate coordinated methoxyl group (Scheme 1g) [56]. This is then further dehydrogenated by reacting with two adjacent hydroxyl groups to form a bidentate coordinated formate and