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Seminar Series 2018 – David Hibbitts, Ph.D.
October 8, 2018 @ 4:05 pm - 5:00 pm
David Hibbitts, Ph.D.
Department of Chemical Engineering at the University of Florida
“Defining and Counting Reaction Site Requirements on Curved and Crowded Catalyst Surfaces.”
Catalysis by supported metal nanoparticles often occurs at conditions which result in their curved surfaces being covered by one or more intermediate (MASI). Counting the number of total exposed metal atoms on such surfaces is often done by titration experiments (e.g., H2 chemisorption) that assume a specific (and often incorrect) saturation coverage of the titrant (e.g., H*). Defining the number of sites required for a given surface reaction, however, is more ambiguous and estimating site requirements from low- or fixed-coverage calculations is fraught with error.Density functional theory (DFT) calculations are often used to model surface reactions and the effects of coverage therein, typically by modeling metal nanoparticles using flat periodic single-crystal surfaces. Common surfaces (for FCC metals) include the close-packed (111) or (100) in addition to a variety of surfaces that are ‘kinked’ to expose defect sites (under-coordinated metal atoms) such as the (211). Reaction energetics are then calculated on these surfaces independently—on surfaces either essentially bare or at ‘moderate’ (≤ 0.75 ML) coverages—and from these data the effects of coverage are estimated, and the effects of particle size are inferred by contrasting the behavior of close-packed surfaces and defect sites on ‘kinked’ surfaces. These flat periodic surface models, however, cannot allow for the lateral relaxation of the adlayer that occurs when adlayer strain is created by repulsive co-adsorbate interactions or by surface reactions that have a positive activation area, meaning their transition states are larger than their relevant precursors, analogous to activation volume in single-phase reactions. The inability of the adlayer to laterally relax leads to inaccurate coverage estimates and coverage effects.Here, we will demonstrate the necessity for curved catalyst models in estimating the saturation coverage of H* or CO* species, the site-requirements for surface reactions, and the rates and kinetic dependencies of high coverage reactions.
David Hibbitts has been an Assistant Professor in the Department of Chemical Engineering at the University of Florida since August 2015. He received his B.S. degree cum laude in Chemical Engineering from Clemson University in 2007 and his Ph.D. degree in Chemical Engineering from the University of Virginia in 2012. His Ph.D. thesis was a study of the nature and reactivity of chemisorbed oxygen and hydroxide on metal surfaces using density functional theory (DFT) calculations in Prof. Matt Neurock’s group. This work included studies of Au-catalyzed oxidations of biomass-derived ethanol and glycerol published in Science and J. Catal. Additional studies included the examination of bifunctional, bimetallic catalysts for selective C–O hydrogenolysis of biomass-derived oxygenates published in JACS and J. Catal.Following completion of his Ph.D., David Post-Doc’d in Prof. Enrique Iglesia’s group at UC-Berkeley. There, he combined kinetic, isotopic, and theoretical studies to examine Fischer-Tropsch synthesis in work published in Angewandte Chemie. Furthermore, he collaborated with other researchers at UC-Berkeley to publish multiple papers in C–C Hydrogenolysis (JACS, J. Catal., ACS Catal., JPCC), C–O Hydrogenolysis (JACS), and NO Reduction (J. Catal.). His work in the activation of strong bonds (C–O, O–O, N–O, and N–N bonds) was recently featured in Accounts of Chemical Research. His research interests at the University of Florida include the determination of reaction mechanisms and structure-function relationships for the conversion of biomass- and fossil-derived feedstocks into fuels and chemicals through heterogeneous catalysts.
MONDAY, October 8, 2018
4:05 pm| 201 NEB