RUI:Collaborative Research: Understanding and exploiting proton mobility in Au catalyzed selective oxidation reactions

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The US produces more than nine billion tons of hydrogen (H2) gas per year. Much of the hydrogen must undergo clean-up reactions prior to end use. One such reaction used widely for fuel cells involves catalytic removal of small amounts of unwanted carbon monoxide (CO) in a reaction known as preferential oxidation of carbon monoxide (CO-PROX). The study will focus on the important role of water in the CO PROX reaction and other related catalytic reactions. This project will build on previous studies by the investigators to provide more detailed understanding of how water promotes catalytic CO-PROX while exploring the extent to which water, or other molecules with similar characteristics, can be used to promote additional industrial reactions such as those involving hydrocarbons. The collaboration between Trinity University - a small undergraduate institution - and Pennsylvania State University will provide opportunities for undergraduates from Trinity (many of whom come from Hispanic background) to conduct research in collaboration with graduate students from Penn States. Via their previous joint research projects, the investigators have developed an excellent track record of inspiring their undergraduate students to continue their education in both chemistry and engineering graduate programs. CO oxidation and CO-PROX have been of considerable scientific and commercial interest for the past 30 years. Despite this attention, progress in developing PROX catalysts has been hindered by lack of fundamental understanding of the reaction mechanisms at work. Based on experimental observations and density functional theory (DFT) calculations, the investigators have recently proposed a new CO oxidation reaction mechanism that explains most of the disparate results in the supported catalyst, surface science, and computational literature. The key feature of this new mechanism is the role of water as a weak acid-base co-catalyst. Water first acts as a proton donor, facilitating O2 binding to Au as Au-OOH. The reactive Au-OOH intermediate oxidizes CO to Au-COOH; water then acts as a weak base, accepting a proton as Au-COOH decomposes to release CO2. This project will further characterize the Au-OOH intermediate and expand the scope of this new oxidation chemistry, in which water acts as a "proton shuttle", carrying protons to and from reactive intermediates on the metal. A variety of spectroscopic techniques will be employed to identify and characterize Au-OOH on the catalyst surface as a function of water and oxygen pressures. Au-OOH is a potentially strong yet selective oxidant, offering opportunities to use Au catalysts in new oxidation reactions. However, it relies on the weak acid-base chemistry associated with weakly adsorbed water, which limits the reaction conditions under which Au-OOH is accessible. To this end, a new approach termed "collaborative bifunctional catalysis" will be pursued in which the metal particle and weak acid/base chemistry will work in concert at the active site. Via this approach, new proton shuttles will be developed to replace water as the primary proton carrier on metal oxide surfaces, thus allowing for collaborative bifunctional catalysis to take place at higher temperatures. Generation of Au-OOH at elevated temperatures will be confirmed by reaction kinetics and kinetic isotope effect measurements. The novel chemistry will also be extended to alkyne oxidation/hydration reactions. Alkynes are well known to adsorb onto Au, making this an excellent test reaction to develop new reaction chemistries.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.