Coverage-Dependent Adsorption at a Low Symmetry Surface: DFT and Statistical Analysis of Oxygen Chemistry on Kinked Pt(321)

Abstract

We explore the influence of adsorbate interactions on the thermodynamic and spectroscopic properties of oxygen on the stepped, kinked Pt(321) surface. The ground state arrangements of atomic oxygen are identified with the aid of a cluster expansion and analyzed for coverages up to one oxygen per surface Pt (1 ML). We find that oxygen prefers to bind in bridge sites at the step edge at coverages up to 0.2 ML, but at higher coverages oxygen atoms actually experience mild, localized attractions such that both bridge and threefold hollow sites are occupied to form square planar, fourfold-coordinated PtO4-like structures. These structures progressively dominate the surface with increasing coverage up to 0.8 ML, at which point every kink Pt is saturated with four oxygens. We compute stability regions for these ground states with respect to gas-phase O2 and to NO/NO2 mixtures. The ground state structures at 0.2, 0.6, and 0.8 ML dominate over a wide range of conditions, with the 0.6 ML structure being most prominent. We also explore site preferences for molecular O2 adsorbed on key O ground state structures. Calculations of vibrational modes and core electron binding energy shifts allow us to relate both ground state and non-equilibrium structures to experimental HREELS and XPS results. We show that adsorption sites are primarily characterized by their surface coordination, such that O in atop, bridge, and threefold hollow sites possess distinct and identifiable vibrational modes and core level shifts. However, within these broad categories, we find variability due to interactions with proximal adsorbates. Adsorption energies and vibrational modes of O2 are found to be particularly sensitive to the local adsorption environment. Lastly, we develop a one-dimensional adsorption model to understand and rationalize experimentally observed non-equilibrium behavior at low coverages.