(766b) 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.