Title: Fundamental Studies of the Reforming of Oxygenated Compounds over Supported Metal Catalysts
Abstract: The main objective of our research has been to elucidate fundamental concepts associated with controlling the activity, selectivity, and stability of bifunctional, metal-based heterogeneous catalysts for tandem reactions, such as liquid-phase conversion of oxygenated hydrocarbons derived from biomass. We have shown that bimetallic catalysts that combine a highly-reducible metal (e.g., platinum) with an oxygen-containing metal promoter (e.g., molybdenum) are promising materials for conversion of oxygenated hydrocarbons because of their high activity for selective cleavage for carbon-oxygen bonds. We have developed methods to stabilize metal nanoparticles against leaching and sintering under liquid-phase reaction conditions by using atomic layer deposition (ALD) to apply oxide overcoat layers. We have used controlled surface reactions to produce bimetallic catalysts with controlled particle size and controlled composition, with an important application being the selective conversion of biomass-derived molecules. The synthesis of catalysts by traditional methods may produce a wide distribution of metal particle sizes and compositions; and thus, results from spectroscopic and reactions kinetics measurements have contributions from a distribution of active sites, making it difficult to assess how the size and composition of the metal particles affect the nature of the surface, the active sites, and the catalytic behavior. Thus, we have developed methods to synthesize bimetallic nanoparticles with controlled particle size and controlled composition to achieve an effective link between characterization and reactivity, and between theory and experiment. We have also used ALD to modify supported metal catalysts by addition of promoters with atomic-level precision, to produce new bifunctional sites for selective catalytic transformations. We have used a variety of techniques to characterize the metal nanoparticles in our catalysts, including scanning transmission electron microcopy (STEM) to measure size and structure, energy dispersive X-ray spectroscopy (EDS) to measure atomic composition, X-ray absorption spectroscopy (XAS) to measure oxidation state and metal coordination, Fourier transform infrared spectroscopy (FTIR) to study adsorbed species, laser Raman spectroscopy to probe metal oxide promoters, and temperature programmed reaction/desorption to study the energetics of adsorption and desorption processes. We have studied our bimetallic catalysts for the selective cleavage of carbon-oxygen bonds, and we have studied the effects of adding metal oxide promoters to supported platinum and gold catalysts for water-gas shift (i.e., the production of hydrogen by reaction of carbon monoxide with water). We anticipate that the knowledge obtained from our studies will allow us to identify promising directions for new catalysts that show high activity, selectivity, and stability for important reactions, such as the conversion of biomass-derived oxygenated hydrocarbons to fuels and chemicals.