Chemistry of Sulfur Oxides on Transition Metals. II. Thermodynamics of Sulfur Oxides on Platinum(111)

The thermodynamics of S, O, SO, SO 2, SO 3, and SO 4 chemisorption on the Pt(111) surface are studied using first-principles density functional theory (DFT) computations. The adiabatic potential energy surfaces of SO x ( x = 1, 2, 3, and 4) on Pt(111) are probed systematically to yield comprehensive sets of local minima. The most energetically stable surface species are found to include always a tetrahedral S center bound to x O atoms and 4 − x surface Pt atoms. Novel surface reconstructions are observed at the highest coverages and sulfur oxidation states. Calculated vibrational spectra are used to assign observed surface spectra. The SO x adsorbates experience strong lateral repulsion due to dipole−dipole interactions, and a procedure is developed to extrapolate these effects to low coverage. At low coverage, all SO x adsorbates are found to be energetically unstable with respect to dissociation to atoms. Molecular SO x moieties on the Pt(111) surface thus owe their existence to a combination of kinetic barriers to dissociation and lateral interaction effects. At high coverage or in an oxygen-saturated background, SO 4 becomes the preferred SO x species on Pt(111), consistent with observation.