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 fixedbed 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 D 2 O 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).
The size of ceria particles influenced the activity of Au/CeO2 catalysts for ethanol oxidation, demonstrating a linear correlation between oxygen storage capacity and catalytic activity.
Mixed metal oxides have been investigated for a host of applications, yet many questions regarding the relationship between their material properties and their functional behavior remain unanswered. Mixed metal oxides are frequently used as support materials for catalytic processes, and the structure/composition of the oxide can significantly influence the behavior of a mixed oxide supported catalyst. In this article, we study the material properties of binary mixed cerium oxide supports synthesized with various metal additives and their corresponding influences on the behavior of gold catalysts for selective oxidation of ethanol. We have found that, although all of these materials exhibited X-ray diffraction patterns consistent with a pure cerium oxide phase, compositional heterogeneities were present in many of the samples, as indicated by time-of-flight secondary ion mass spectrometry and temperature-programmed-reduction techniques. Furthermore, the mixed cerium oxide supported gold catalysts demonstrated differences in both activity and selectivity for ethanol oxidation, with the materials that exhibited heterogeneous phases promoting the esterification reaction with higher selectivities than the homogeneous mixtures. Our results show that the composition of mixed metal oxide supports can have an important impact on catalytic behavior, providing a means of influencing activity and tuning selectivity.
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