(49e) Reactivity of Acetaldehyde On CeO2(111) Surfaces and the Roles of Oxygen Vacancies

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 toward organic molecules can vary significantly with the extent of reduction.  In this study we use acetaldehyde as a probe molecule.  First-principles density functional theory (DFT) calculations have been performed in close collaboration with surface characterization techniques to elucidate the roles of oxygen vacancies in redox reactions on the CeO₂(111) surface.  A microkinetic model based on proposed surface intermediates and their calculated energetics has been constructed and yields close agreement with experimental temperature-program desorption results.  Acetaldehyde desorbs without reaction from the stoichiometric CeO₂(111) surface near 200 K.  When the surface is partially reduced, acetaldehyde loses its carbonyl bond character and forms C-O coupled dimers at very low temperatures.  Increasing temperature to 360 K leads to the decomposition of this dimer species into acetaldehyde, which desorbs immediately, and another surface species, which has been conclusively identified to be the enolate form of acetaldehyde (CH₂CHO) and has not been captured on ceria surfaces previously.  This enolate species can recombinatively desorb, again as acetaldehyde, at 550 K.  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 can change 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.