Roles of oxygen vacancies in the reactivity of acetaldehyde on CeO2(111) surfaces

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

Indianapolis

Conferencia; 246th ACS National Meeting; 2013

AMERICAN CHEMICAL SOCIETY

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 surface reduction. We use acetaldehyde as a probe molecule to further explore this phenomenon and to elucidate the roles of oxygen vacancies in redox reactions on ceria surfaces. A combination of multiple surface characterization techniques and theoretical DFT calculations have been applied to demonstrate that oxygen vacancies on CeO2(111) thin-film surfaces are key to activating acetaldehyde and stabilizing it for further reactions. In TPD, acetaldehyde desorbs without reaction from the stoichiometric CeO2(111) surface around 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 to be the enolate form of acetaldehyde (CH2CHO) on the basis of IR, X-ray spectroscopies, and DFT calculations. A reaction energy profile is constructed based on the identified surface intermediates on CeOx(111) and DFT-calculated energetics, and finds close agreement with the TPD results. This work also has relevance to the conversion of biomass-derived feedstock because enolate species are key intermediates in organic C-C coupling reactions including aldol condensation and because ceria is a relatively abundant and available oxide.