INSIGHTS INTO CERIA REDUCTION AND THE ROLES OF OXYGEN-VACANCIES IN REDOX AND ACID-CATALYZED REACTIONS OVER CERIA SURFACES

Cerium oxide is a versatile catalyst which is well known for its propensity to undergo rapid reduction-oxidation cycles in response to exposure to sub and over-stoichiometric amounts of oxygen respectively. We’ve investigated different classes of surface reactions to assess the origin of catalytic functionality of the ceria surfaces based on qualitative and quantitative analyses to identify the active catalytic domains. Bulk cerium oxide is found to catalyze anisole hydrodeoxygenation at 698K and ambient pressures of hydrogen providing high selectivity towards benzene which shows its utility to reduce oxygen contents of biomass derived oxygenates with high hydrogen efficiency. Quantitative analyses of four sets of aerobic/anaerobic ethanol conversion transients point to the evolution of a native high surface area cerium oxide surface that effects the reduction half of the ethanol oxidation turnover to catalyzing, exclusively, non-oxidative ethanol dehydrogenation upon complete surface reduction. High-temperature hydrogen pretreatments and phenol-an alpha hydrogen-free titrants have been found to selectively titrate sites contributing to stoichiometric oxidative and catalytic non-oxidative dehydrogenation without altering other one respectively. Utility of in-situ quantification of oxidized and reduced ceria domains is very valuable as it can enable the clear interpretation of steady state and transient kinetic data. We quantify and exploit the existence of a well-defined peak near 2150 cm-1 corresponding to the 2F5/2 to 2F7/2 electronic transition of reduced cerium, and demonstrate its utility in the quantification of oxygen vacancy concentrations during the occurrence of catalytic turnovers of H2-D2 exchange and tert-butanol dehydration over ceria surfaces. H2 activation and hydrogenation of unsaturated hydrocarbons over inexpensive metal oxides have significant industrial impact. We investigated kinetics, mechanism and active site requirements for ethene hydrogenation over H2-treated ceria surfaces, which suggests the sequential H-addition mechanism involving dual site models as well as the optimum densities of active sites required for the maximum turnovers. Finally, the in-situ site titration by CO reveals the limitation of breakthrough experiments which may overestimate the site density due to unselective adsorption. We show different methods to interpret the titration data which can provide more meaningful information. All of these techniques and methods in general can be utilized to interpret the catalytic functionality.