Oxygen Vacancy-Assisted Coupling and Enolization of Acetaldehyde on CeO_2(111)

摘要

The temperature-dependent adsorption and reaction of acetaldehyde (CH_3CHO) on a fully oxidized and a highly reduced thin-film CeO_2(111) surface have been investigated using a combination of reflection-absorption infrared spectroscopy (RAIRS) and periodic density functional theory (DFT+U) calculations. On the fully oxidized surface, acetaldehyde adsorbs weakly through its carbonyl O interacting with a lattice Ce~(4+) cation in the η~1-O configuration. This state desorbs at 210 K without reaction. On the highly reduced surface, new vibrational signatures appear below 220 K. They are identified by RAIRS and DFT as a dimer state formed from the coupling of the carbonyl O and the acyl C of two acetaldehyde molecules. This dimer state remains up to 400 K before decomposing to produce another distinct set of vibrational signatures, which are identified as the enolate form of acetaldehyde (CH_2CHO-). Furthermore, the calculated activation barriers for the coupling of acetaldehyde, the decomposition of the dimer state, and the recombinative desorption of enolate and H as acetaldehyde are in good agreement with previously reported TPD results for acetaldehyde adsorbed on reduced CeO_2(111) [Chen et al. J. Phys. Chan. C 2011, 115, 3385]. The present findings demonstrate that surface oxygen vacancies alter the reactivity of the CeO_2(111) surface and play a crucial role in stabilizing and activating acetaldehyde for coupling reactions.