Collaborative Research: The role of oxide overlayers on adsorbate migration and metal sintering in reactions of CO2

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Combustion of natural gas (chiefly comprised of methane, i.e., CH4) provides a major portion of our nation’s energy needs. Although methane is a relatively “clean” fossil fuel, its combustion produces carbon dioxide (CO2) which constitutes the major component of greenhouse gas (GHG) emissions. Methane can also be reacted with steam (H2O) to produce carbon monoxide (CO) and hydrogen (H2) in a process known as methane steam reforming. The product CO and H2 gases are further reacted to produce a wide range of fuels and chemicals. The project investigates an alternative approach – “dry” reforming of methane (DRM) - which utilizes captured CO2, rather than steam, to generate CO and H2, thus decreasing the overall GHG inventory. Methane reforming, via any technology, is an energy intensive process. Catalysts are utilized to reduce operating temperatures, improve process efficiency, and drive the reactions to desired products. Dry reforming is even more challenging than steam reforming, thus creating a need for research aimed at identifying more active and selective catalysts that are stable under high-temperature reaction conditions. The project addresses those needs by combining theoretical, computational, and experimental methods to identify effective DRM catalysts. In addition, the project will investigate economics of DRM technology, and incorporate educational and outreach activities exposing high-school and undergraduate students to the field of chemical engineering – so important to the fuels, chemicals, and environmental industries. DRM catalysts must operate at high temperatures, which can destroy carefully designed synthetic structures or promote secondary reactions (e.g., reverse water-gas shift reaction (RWGS) and coke formation) that result in lower value products. One mechanism associated with both the primary and secondary processes is the ability of some catalysts to store and release oxygen during different parts of the cycle. Other catalysts can avoid this oxygen-centric route at the expense of higher activation energies. This work develops hybrid catalysts, using both reducible and non-reducible oxides, to combine the best properties of both in generating highly stable and chemically selective methane reforming catalysts which can be used to operate at industrially relevant conditions. The simultaneous methane reforming and RWGS reactions over ceria catalysts occur through mobile oxygen species. Non-reducible catalyst overlayers have the potential to limit hydrogen spillover from the active metal sites, preventing the unwanted secondary reaction and stabilizing the carefully designed catalyst structure without limiting the role of oxygen in the methane reforming. Using a combination of simulation (density functional theory) and experimental work, the project will develop highly active and structurally stable catalysts while limiting the undesired RWGS, which decreases the H2:CO ratio. However, subsequent reactions to make chemicals require higher H2-to-CO ratios than are possible under standard dry reforming conditions. As such, the optimized hierarchical catalysts will be tested under harsh conditions in the presence of low concentrations of water (i.e., a steam/CO2 “bi-reforming” process) to further increase the H2-to-CO ratio.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.