## 3 Results and Discussion
### Reaction Mechanism for Thermocatalytic Decomposition of DMMP
The variations in the surface species were identified by employing time-resolved in situ DRIFTS over 2 MCeO\({}_{2}\)np, 12 MCeO\({}_{2}\)nr, and 12 MCeO\({}_{2}\)nc at 300 \({}^{\circ}\)C (Figure 10). The corresponding functional groups in accordance with the literature [51,52] are shown in Tables 6 and 7. According to Figure 10, the peaks from 3500 cm\({}^{-1}\) to 3800 cm\({}^{-1}\), could be  
Figure 9: Mass spectrometry of the tail gas produced during the thermocatalytic decomposition of DMMP on (**a**) 2 MCeO\({}_{2}\)np, (**b**) 12 MCeO\({}_{2}\)nr, and (**c**) 12 MCeO\({}_{2}\)nc. (The insets show the high magnification picture of 25 times). associated with the vibration mode of the surface hydroxyl on CeO\({}_{2}\)[53; 54]. With longer reaction durations, the vibration intensity of partial surface hydroxyl increased negatively, which indicates the participation of surface hydroxyl during the decomposition of DMMP. The disappearance of \(\
u\)(P\(-\)O) vibrational modes, i.e., 821 cm\({}^{-1}\) and 788 cm\({}^{-1}\), and \(\
u\)(P\(-\)C) vibrational mode, i.e., 713 cm\({}^{-1}\), implies the removal of \(-\)OCH\({}_{3}\) and \(-\)CH\({}_{3}\) groups from the DMMP molecule. The \(\
u_{\rm a}\)(O\(-\)CH\({}_{3}\)), \(\delta_{\rm a}\)(O\(-\)CH\({}_{3}\)), \(\rho\)\(\parallel\)(O\(-\)CH\({}_{3}\)), \(\
u_{\rm a}\)(C\(-\)O), \(\
u_{\rm s}\)(C\(-\)O), \(\
u_{\rm a}\)(P\(-\)CH\({}_{3}\)), and \(\rho\)\(\parallel\)(P\(-\)CH\({}_{3}\)) vibration modes that are ascribed to \(-\)OCH\({}_{3}\) and P\(-\)CH\({}_{3}\) groups disappeared. However, with prolonged testing duration, the peaks belonging to \(\
u_{\rm s}\)(P\(-\)CH\({}_{3}\)), \(\
u_{\rm s}\)(O\(-\)CH\({}_{3}\)), and \(\delta_{\rm s}\)(P\(-\)CH\({}_{3}\)) still exist and are inferred to be the peaks of intermediate products. The peak at 2813 cm\({}^{-1}\) could be assigned to \(\
u\)(Ce\(-\)O\(-\)CH\({}_{3}\)). As [PERSON] et al. reported, due to the P\(-\)OCH\({}_{3}\) cleavage, \(-\)OCH\({}_{3}\) combined with Ti ion to form surface methoxy. Compared to DMMP, the vibrational peak of surface methoxy will shift to a lower frequency [55]. It is worth noting that the intense peak belonging to \(\
u\)(P=O) at 1257 cm\({}^{-1}\) vibrational mode shifted to 1262 cm\({}^{-1}\), 1279 cm\({}^{-1}\), and 1272 cm\({}^{-1}\) for CeO\({}_{2}\)np, CeO\({}_{2}\)nr, and CeO\({}_{2}\)nc, respectively. This observation indicates that the DMMP chemisorbed to the Lewis acid sites of CeO\({}_{2}\), and/or bonded with the surface hydroxyl group of CeO\({}_{2}\) via H bond [32; 52]. New distinct peaks located at 1208-1050 could be detected, which corresponds to P\(-\)O stretching in PO\({}_{\rm x}\)[32; 55]; 2716 cm\({}^{-1}\) and 1365 cm\({}^{-1}\) belong to 2\(\delta\)(C\(-\)H) and \(\delta\)(C\(-\)H) of bidentate formate [54]; 1615 cm\({}^{-1}\) and 1620 cm\({}^{-1}\) belong to \(\
u_{\rm a}\)(OCO) in monodentate formate bonded with Ce\({}^{3+}\); 1535 cm\({}^{-1}\) belongs to \(\
u_{\rm a}\)(OCO) of bidentate formate bonded with Ce\({}^{4+}\)[56] (Figure 10a,b). Moreover, 1396 cm\({}^{-1}\) and 1294 cm\({}^{-1}\) that belong to \(\
u\)(CO\({}_{3}\)) in monodentate and bidentate carbonate could be detected, respectively (Figure 10a) [57]. In Figure 10b,c, \(\delta_{\rm s}\)(P\(-\)CH\({}_{3}\)) vibration mode of products coincided with that of \(\
u\)(CO\({}_{3}\)) in bridged carbonate at 1318 cm\({}^{-1}\)[57], indicating that bridged carbonate may exist on the surface of CeO\({}_{2}\)nr and CeO\({}_{2}\)nc. In addition, 1236 cm\({}^{-1}\) that belongs to \(\
u\)(CO\({}_{3}\)) of bridged carbonate could be detected (Figure 10c) [57]. According to the above in situ DRIFTS analysis, the difference of surface products between the three morphologies lies in the carbon containing byproducts. Monodentate formate, bidentate formate, monodentate carbonate, and bidentate carbonate were observed on 2 MCeO\({}_{2}\)np. Monodentate formate and possibly bridged carbonate are considered to be the main carbon-containing byproducts of 12 MCeO\({}_{2}\)nr.