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CHBE 565 Seminar, Prof. David Hibbitts, University of Florida, "Defining and Counting Reaction Site Requirements on Curved and Crowded Catalyst Surfaces""

Event Type
Chemical & Biomolecular Engineering and International Paper Company
116 Roger Adams Laboratory
Apr 12, 2022   2:00 pm  
Christy Bowser
Originating Calendar
Chemical & Biomolecular Engineering - Seminars and Events

Catalysis by supported metal nanoparticles often occurs at conditions which result in their 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*, CO*, NO*, and S* species, the site-requirements for surface reactions, and the rates and kinetic dependencies of high coverage reactions.

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