Water Influences the Activity and Selectivity of Ceria-Supported Gold Catalysts for Oxidative Dehydrogenation and Esterification of Ethanol

In this study, we show that water influences the activity and selectivity of a ceria-supported gold catalyst for the oxidative dehydrogenation and esterification of ethanol in a fixed-bed flow reactor, resulting in the production of acetaldehyde and ethyl acetate, respectively. The rate of acetaldehyde formation was enhanced upon incorporation of water in the feed stream at low pressures; beyond this, the water had little effect on the kinetics of the primary oxidation reaction. However, water induced significant changes to ethyl acetate production, decreasing the reaction order in ethanol and slowing the reaction rate. Furthermore, a large kinetic isotope effect for esterification was observed upon incorporation of either D2O or d-ethanol in the system, suggesting that breaking an OH bond is associated with the rate-determining step for this reaction. Our observations are consistent with the esterification process occurring through secondary oxidation of acetaldehyde to acetate species on the ceria surface followed by the acid-catalyzed esterification of surface acetates with ethanol. These results highlight the importance of water in gold-catalyzed processes and show that water can direct reaction selectivity in addition to influencing activity (as many previous studies have shown for CO oxidation).