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 C2HxO 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 beta C-H bond scissions in acetaldehyde, leading to enolization, intermolecular hydrogen transfer, deoxygenation, and potentially C-C coupling reactions.