Adsorbate Coverage Effects On Catalytic Reactivity At a Low-Symmetry, Kinked Surface

Adsorbate coverage effects on catalytic reactivity at a low-symmetry, kinked surface

Jason M. Bray, William F. Schneider

Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, IN, USA

Adsorbate coverage can have a large impact on the number and types of sites that are available for reactions at a surface.  Such coverage effects are critical, for instance, in understanding the catalytic oxidation of NO to NO₂ over Pt.  While computational approaches to modeling these coverage effects on high symmetry surfaces, such as (111) facets, are fairly well developed, lower symmetry surfaces remain a challenge.  In this work, we use DFT simulations and cluster expansions to understand oxygen coverage effects during catalytic NO oxidation at the (321) surface of Pt, a system that has been shown to have surprisingly little structure sensitivity.  The chiral (321) facet exposes kinks and step edges that offer a variety of potential adsorption and reaction sites.  In the low coverage limit we characterize all potential O and O₂ adsorption sites as well as a number of pathways for O₂ dissociation and O diffusion, and we use these results to explain the surprisingly facile dissociation of O₂ at low coverage observed experimentally as well as the related vibrational spectrum of adsorbed O and O₂.  While as on the (111) surface, O adsorbates interact repulsively at low coverage, at higher coverages they exhibit complex interactions that cause O atoms to congregate around single Pt kink sites in distinctive 4-fold configurations.  To quantify these O interactions, we use DFT simulations to parameterize an Ising-type cluster expansion of O on Pt(321) for coverages up to 1 ML.  The model allows us to predict the ground state configurations of adsorbed O, determine coverage-dependent O binding energies, and, with GCMC simulations, predict equilibrium adsorbate coverages as a function of reaction environment, in agreement with experimental observation.  Further, GCMC provides information about the number and types of reaction sites that are available at operando coverages, allowing us to rationalize the similarities in (111) and (321) activity.