Oxygen-Vacancy Assisted Formation of Enolate Species on Reduced CeO2(111) Surfaces

CALAZA, FLORENCIA C.; XU, YE; MULLINS, DAVID R.; OVERBURY, STEVEN H.

Nashville, TN

Simposio; AVS 58th Annual International Symposium and Exhibition; 2011

American Vacuum Society

Enolate species are key intermediates proposed in a number of important organic reactions heterogeneously catalyzed by metals and metal oxides, but enolate has been difficult to identify on active catalytic surfaces due to difficulties of isolating it in the keto-enol equilibrium. Reflection absorption infrared spectroscopy (RAIRS) was coupled with density functional theory (DFT) to study the adsorption of acetaldehyde, a simple C2 aldehyde, on CeO2-x(111) surfaces of different extent of oxidation(where x= 0 - 0.5). It is found experimentally that the molecule adsorbs weakly on the fully oxidized surface (x=0) at low temperatures and desorbs without further reaction near 215 K. The molecule bonds to c.u.s. Ce4+ cations through the oxygen lone pair electrons in the carbonyl group with its C-C bond perpendicular to the surface plane and the acyl hydrogen tilted slightly towards one of the lattice oxygen anions of the first layer. On the reduced surfaces (x=0.1 ¨C 0.4), acetaldehyde interacts more strongly with the surface upon adsorption at low temperatures by losing its carbonyl bond character and adsorbing as 1,1-dioxyethane and forming dimers and polymers. Heating the surface to 400 K leads to desorption of some amount of these strongly adsorbed species as acetaldehyde and the appearance of hydroxyl and yet a different organic species. The identities and structures of the different intermediates on the CeO2 and CeO2-x surfaces have been determined by their characteristic signatures in RAIRS and by DFT calculations. Our observations for the CeO2-x surfaces are consistent with the vacancy-promoted dehydrogenation in the original methyl position of acetaldehyde and the formation of enolate (CH2=CHO-Ce). Experiments with isotopically labeled acetaldehyde have verified the vibrational assignments for the enolate species and are in excellent agreement with DFT results. The assignment of the enolate species is furthermore consistent with C 1s XPS and C k-edge NEXAFS results.