Unique approaches for controlling selectivity in heterogeneous catalysis
Department of Chemical and Environmental Engineering, University of California Riverside
Friday, September 26, 2014
Physical Sciences H-Wing (PSH) 151, Tempe campus [map]
The use of heterogeneous catalysts for important chemical conversions often relies on the design of metallic catalytic active sites supported on oxide materials. Key to the design of these systems is identification of active site geometries and compositions that facilitate desired selective chemical pathways. Furthermore, inherent relationships between the energetics of adsorption energies and activation barriers of surface bound intermediates limit the ultimate control of reaction selectivity attainable for catalytic processes on metal surfaces operating on electronic ground state potential energy surfaces. In this talk, I will highlight two examples from our lab where we identify unique phenomena that allow us to address these issues to control reaction selectivity in oxide supported metal catalysis.
In the first example, Phillip Christopher will discuss thermally driven CO2 reduction by H2 on TiO2 supported Rh catalysts. Typical approaches to identify active sites on heterogeneous catalysts have involved crystal facet dependent studies in ultra high vacuum conditions or metal particle size dependent reactivity studies at more relevant conditions. However, these approaches often ignore the potential role of isolated metal atoms exiting on oxide supports in controlling the catalytic processes. He will show that quantitative FTIR spectroscopy can be used to identify isolated Rh sites on TiO2 supports and relate their concentration to reaction selectivity in the competing pathways of CO and CH4 production. The results strongly suggest that relating electron microscopy based characterization of catalysts to performance may hide important details regarding the nature of active sites in heterogeneous catalysis.
In the second example, Christopher will show that strong chemisorption bonds formed between CO and Pt metal surfaces can be activated with visible photons to drive catalysis through direct, resonant photoexcitation of hybridized Pt-CO states. This is enabled as the dominant photoexcitation mechanism driving catalysis by using sub-5-nanometer Pt nanoparticle catalysts. It is also demonstrated that targeted photoexcitation of Pt-CO bonds on sub-5-nm Pt nanoparticles enhances selectivity towards CO2, over H2O production, in the selective oxidation of CO by O2 in an H2 rich stream. The approach is analogous to coherently controlled molecular photochemistry in the gas phase, although by confining the direct bond excitation event to a catalyst surface the required energy input to drive chemistry is reduced significantly while maintaining control of selective bond breaking. These results open new avenues to control catalytic reaction selectivity on sub 5-nm catalytic particles by resonant photoexcitation of adsorbate-specific electronic transitions involving hybridized metal and adsorbate states.