Thermodynamics of Environment-Dependent Oxygen Chemisorption on Pt(111)

The reactivity of heterogeneous metal catalysts can be a strong function of the coverage of adsorbates. For example, Pt-catalyzed NO oxidation to NO2 requires high concentrations of chemisorbed (surface-bound) O, but the development of surface oxides is detrimental to reaction kinetics. Quantifying the structures, properties, and especially the conditions that produce various adsorbate coverages is essential to developing qualitatively and quantitatively correct models of surface reactivity. In this work, we examine these ideas in the context of oxidation reactions on Pt(111), the lowest energy face of bulk Pt. We use extensive supercell density functional theory (DFT) calculations to catalog and characterize the stable binding sites and arrangements of chemisorbed O on Pt(111), as a function of O coverage, θ. O atoms are found to uniformly prefer FCC binding sites and to arrange to minimize various destabilizing interactions with neighbor O. These destabilizing interactions are shown to have electronic and strain components that can either reinforce or oppose one another depending upon O−O separation. Because of the nature and magnitudes of these lateral interactions, the thermodynamically stable O orderings partition into four coverage regimes of decreasing adsorption energy: 0 < θ ≤ 1/4 monolayer (ML), 1/4 < θ ≤ 1/2 ML, 1/2 < θ ≤ 2/3 ML, and 2/3 < θ ≤ 1 ML. We use equilibrium models to quantify the oxygen chemical potentials μO necessary to access each of these regimes. These equilibrium models can be used to relate surface coverage to various external environmental conditions and assumptions about relevant reaction equilibria: dissociative equilibrium of the surface with O2 (g) can produce coverages up to 1/2 ML; either NO2 decomposition or “NO-assisted” O2 dissociation can access coverages approaching 2/3 ML, as observed during NO oxidation catalysis, and equilibrium with a solid-oxygen storage material, like ceria-zirconia, can buffer equilibrium coverages at a constant 1/4 ML O. These various oxidation reaction energies can be summarized in a single “Ellingham” free energy diagram, providing a convenient representation of the relationship between surface coverage and reaction thermodynamics, and a useful guide toward relevant coverage regimes for more detailed study of reaction kinetics.