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

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

Asheville, NC

Simposio; South Eastern Catalysis Society 10th Annual Fall Symposium; 2011

North American Catalysis Society

(Poster - Presenter) Enolate species are key intermediates proposed in a number of important organic reactions heterogeneously catalyzed y 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, the simplest C2 aldehyde, on CeO2-x(111) surfaces of different extents 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 - 0.4), acetaldehyde interacts more strongly with the surface upon adsorption at low temperatures by losing some of its carbonyl bond character and adsorbing as diacetal species by dimerization of two acetaldehyde molecules forming an ether linkage between the two. 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 enolate species (CH2=CHO-Ce). The reaction scheme suggested by DFT calculations involves reaction-limited acetaldehyde desorption at 400 K, consistent with TPD [1] experiments and with the weak adsorption energy of AcH. 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. Experiments with isotopically labeled acetaldehyde have verified the vibrational assignments for the enolate species and are again in excellent agreement with DFT results. The assignment of the enolate species is furthermore consistent with C1s XPS and C k-edge NEXAFS results [1]. [1] Adsorption and Reaction of Acetaldehyde over CeOX(111) Thin Films, Chen et al., J. Phys. Chem. C, 2011, 115 (8), 3385 (Poster - Presenter) Enolate species are key intermediates proposed in a number of important organic reactions heterogeneously catalyzed y 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, the simplest C2 aldehyde, on CeO2-x(111) surfaces of different extents 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 - 0.4), acetaldehyde interacts more strongly with the surface upon adsorption at low temperatures by losing some of its carbonyl bond character and adsorbing as diacetal species by dimerization of two acetaldehyde molecules forming an ether linkage between the two. 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 enolate species (CH2=CHO-Ce). The reaction scheme suggested by DFT calculations involves reaction-limited acetaldehyde desorption at 400 K, consistent with TPD [1] experiments and with the weak adsorption energy of AcH. 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. Experiments with isotopically labeled acetaldehyde have verified the vibrational assignments for the enolate species and are again in excellent agreement with DFT results. The assignment of the enolate species is furthermore consistent with C1s XPS and C k-edge NEXAFS results [1]. [1] Adsorption and Reaction of Acetaldehyde over CeOX(111) Thin Films, Chen et al., J. Phys. Chem. C, 2011, 115 (8), 3385