Paper SS-MoA11
Reactivity of Acetaldehyde on CeO₂(111) Surfaces and the Roles of Oxygen Vacancies

Monday, October 28, 2013, 5:20 pm, Room 201 A

Ceria is a widely used catalytic and functional catalyst support material, well known for its ability to store oxygen and change oxidation state. There is a growing body of evidence that the surface reactivity of ceria can vary significantly with the extent of reduction. We use acetaldehyde as a probe molecule to explore this phenomenon and to elucidate the roles of oxygen vacancies in redox reactions on ceria surfaces. Multiple surface characterization techniques and theoretical density functional theory (DFT) calculations have been applied in combination to elucidate the mechanism of the temperature-program desorption (TPD) of acetaldehyde on CeO₂(111) thin-film surfaces. In TPD, acetaldehyde desorbs without reaction from the stoichiometric CeO₂(111) surface at 210 K. When the surface is partially reduced, acetaldehyde loses its carbonyl bond character at low temperatures. Annealing to 400 K leads to the desorption of some of this strongly adsorbed species as acetaldehyde and the appearance of another species, conclusively identified by RAIRS and DFT to be the enolate form of acetaldehyde (CH₂CHO), which has not been captured previously on ceria surfaces. A microkinetic model based on the identified surface intermediates on CeO_x(111) and DFT energetics has been constructed to simulate the TPD, and finds close agreement with the experimental results. Our findings demonstrate that surface oxygen vacancies are key to activating acetaldehyde and stabilizing it for further reactions, and that the dominant surface reaction pathway changes as a function of vacancy concentration. This work has relevance to the conversion of biomass-derived oxygenates because enolate species are key intermediates in C-C coupling reactions including aldol condensation.