Selective Oxidation and Reduction Reactions.
“What happens when you add water?” is possibly the most frequently asked question after presentations in heterogeneous catalysis. This question is indeed paramount, and I will report on our group’s recent studies of the promotional and inhibiting role of water for CO and H2 oxidation at Au/TiO2 interfaces, as well as for the reduction of phenolic alcohols over Ru/TiO2.
Preferential oxidation (PrOx) of CO is a promising energy efficient alternative to CO methanation for purifying H2 streams from steam reforming processes. With high-purity H2 being the desired product, the obvious challenge is to find a catalyst that readily oxidizes CO, but does not burn H2. Through integrated experimental and computational studies we have produced evidence suggesting that O2 and H2 activation over Au/TiO2 catalysts occurs at the metal-support interface (MSI).[1,2] The activation of O2 on Au is assisted by support protons originating from hydroxyl groups or weakly adsorbed water molecules. Meanwhile, H2 dissociation across the MSI occurs heterolytically resulting in a Au-hydride and a proton on the oxide support. Notably, H2 activation is inhibited by reduced charge transfer from Au to the proton acceptor site located on a basic support hydroxyl.[3]
The concept of water-modulated acid/base strength of sites at the MSI is rather general. For example, we show through first principles kinetic Monte Carlo simulations that the selectivity for direct deoxygenation over hydrogenation of phenol and m-cresol during reductive treatment with H2 can be tuned by adjusting the water partial pressure. In this reaction, Brønsted acidic protons co-catalyze C–OH bond cleavage and are recreated by heterolytic activation of H2 across the Ru/TiO2 interface.[4,5] Thus, rather than invoking the traditional redox terminology such as hydrogen spillover, support reduction or vacancy defects, we provide a new interpretation of the support effects in terms of acid/base chemistry.
Overall, these examples demonstrate the sensitivity of oxide chemistry to the presence of various amounts of moisture, which in turn opens up interesting opportunities to improve catalytic activity and selectivity without the need for time-consuming catalyst design.
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Assistant Professor at University of Houston
Prof. Lars Grabow is the Dan Luss Professor in the William A. Brookshire Department of Chemical and Biomolecular Engineering at the University of Houston and holds a courtesy appointment in the Department of Chemistry. He received his PhD in Chemical Engineering from the University of Wisconsin in 2008, followed by postdoctoral appointments at the Technical University of Denmark and Stanford University. His expertise is the application of electronic structure calculations, kinetic modeling, data science and transient kinetic characterization to problems in heterogeneous catalysis, surface science and electrochemical energy storage. His papers have been cited more than 4,000 times and he was elected into the 2018 Class of Influential Researchers by Industrial and Engineering Chemistry (IE&C) Research. Prof. Grabow won the prestigious U.S. Department of Energy (DOE) Early Career Award (2014) and the NSF CAREER Award (2015), the Excellence in Research Award at the assistant professor level from the University of Houston (2017). Currently, he serves as Chair of the Southwest Catalysis Society (SWCS) and just became Editor of Surface Science.
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