Determining site requirements for reactive species in multi-site catalysis on metal surfaces using excluded areas

Abstract

Many metal-catalyzed reactions (e.g., hydrogenolysis, (de)hydrogenation, and hydro-deoxygenation) involve reactive species with molecular volumes that extend beyond the cross-sectional areas of exposed metal atoms. The kinetic behaviors of such reactions are well described by lattice-based models that account for adsorbates occupying multiple adjacent sites (i.e., exposed metal atoms). Site requirements are often inferred from the number of metal atoms an adsorbate coordinates to, which can underpredict the number of contiguous sites that are inaccessible to co-adsorbates by lateral repulsion. Here, we instead determine adsorbate site requirements from the surface areas they exclude from co-adsorbates. These areas are determined from adsorbate structures, optimized previously using density functional theory (DFT), by projecting their molecular volumes onto the surface plane (A_S) or by tracing their areas with a circular probe that represents co-adsorbates (A_enc). These excluded areas agree with those inferred from the experimentally measured saturation coverages of eight polyatomic adsorbates on Pt(1 1 1) and Ni(1 1 1). They predict a number of sites needed for ethane hydrogenolysis on Ir nanoparticles (two to three exposed Ir atoms) that is consistent with previous kinetic measurements and DFT calculations. The areas further estimate site requirements for benzene hydrogenation on Pt nanoparticles (six exposed Pt atoms) that accurately describe rate dependences on benzene pressure, under physically realistic benzene and H-adatom coverages and with adsorption enthalpies consistent with experimental benchmarks. Excluded areas therefore offer a practical and accurate way to determine site requirements in multi-site kinetic models, facilitating mechanistic studies and guiding ab initio catalyst design for reactions of bulky molecules that inevitably cover multiple contiguous surface atoms.