Surface Structure Dependence of Selective Oxidation of Ethanol on Faceted CeO2 Nanocrystals
Significance and Impact:
Synthetic control of the shapes of oxide catalysts has been shown to be a route to vary selectivity in ethanol partial oxidation reactions.
We have outlined the pathways for ethanol selective oxidation as a function of temperature and under conditions of sub-stoichiometric amounts of O2, as catalyzed by CeO2 nanoshapes with different morphologies. Different nanoshapes present different crystallographic faces and therefore comparing these shaped particles provides a method of probing the effects of site geometry and surface structure upon catalytic behavior. We find many differences in the surface interactions and reactions leading to different catalytic product selectivity. These include differences in types and stability of ethoxide intermediates, temperature of reaction onset, dehydrogenation: dehydration selectivity as determined by competition between a-CH and ß-CH scission, branching between two pathways for ethylene formation, and ratio of methane: (CO + CO2) in C1 decomposition product. For all shapes, the dominant surface species during both TPD and under reaction conditions (TPSR) conditions are ethoxide below 300 C which is eventually reacted away and displaced by acetate and carbonates at higher temperature. Furthermore, significant differences between TPD in He and TPSR in reaction mixture are observed including inhibition of ethylene formation under selective oxidation reaction conditions compared to TPD, yielding pronounced differences in acetaldehyde: ethylene: CO2 selectivity. Factors that may lead to the observed structural differences are the oxygen storage and availability especially as it relates to H2O/H2 selectivity, base strength, and mobility of oxygen anions on different surface terminations, variation in acid strength of the Ce cation sites caused by variation in their oxygen coordination, geometry of the adsorption site relative to the molecules and intermediates, and the relative numbers of vacancy or active sites created from oxygen removal during the reaction.
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