Simulated Temperature Programmed Desorption of Acetaldehyde on CeO₂(111): Evidence for the Role of Oxygen Vacancy and Hydrogen Transfer

The temperature programmed desorption of acetaldehyde adsorbed on partially reduced CeO₂(111) has been studied in detail using microkinetic modeling based on self-consistent, periodic density functional theory calculations at the GGA-PW91+U level. Previous experimental studies (Chen et al. J. Phys. Chem. C 115: 3385, 2011; Calaza et al. J. Am. Chem. Soc. 134: 18034, 2012) have shown that, whereas on fully oxidized CeO₂(111) acetaldehyde desorbs molecularly with a peak temperature of 210 K, the polymerization and enolization of acetaldehyde dominate the surface reactivity on partially reduced CeO₂(111), resulting in acetaldehyde desorption at higher temperatures. Here we propose a comprehensive reaction mechanism that is consistent with the spectroscopic evidence of the identities of the surface intermediates and with the observed desorption activities, including the formation of ethylene and acetylene. Besides acetaldehyde (CH₃CHO) and its enolate (CH₂CHO), several other C₂H _x O species are proposed as key intermediates which are not seen spectroscopically, including ethoxy (CH₃CH₂O), ethyleneoxy (CH₂CH₂O), and formylmethylene (CHCHO). Our study suggests that oxygen vacancies play the critical role of activating the carbonyl bond and thereby facilitating β C–H bond scissions in acetaldehyde, leading to enolization, intermolecular hydrogen transfer, deoxygenation, and potentially C–C coupling reactions.