Stability, electronic properties and CO adsorption properties of bimetallic PtAg/Pt(111) surfaces

Aiming at a better fundamental understanding of the chemistry of bimetallic PtAg/Pt(111) surfaces, we have investigated the stability, electronic properties and CO adsorption properties of bimetallic PtAg surfaces, including pseudomorphic Ag film covered Pt(111) surfaces and PtAg_1-/Pt(111) monolayer surface alloys, using periodic density functional theory calculations. The data provide detailed insights into the relative stabilities of different surface configurations, as indicated by their formation enthalpies and surface energies, and changes in their electronic properties, i.e., in the projected local densities of states and shifts in the d-band center. The adsorption properties of different Pt ensembles were systematically tested using CO as a probe molecule. In addition to electronic ligand and strain effects, we were particularly interested in the role of different adsorption sites and of the local CO_ad coverage, given by the number of CO molecules per Pt surface atom in the Pt ensemble. Different from PdAg surfaces, variations in the adsorption energy with adsorption sites and with increasing local coverage are small up to one CO_ad per Pt surface atom. Finally, formation of multicarbonyl species with more than one CO_ad per Pt surface atom was tested for separated Pt₁ monomers and can be excluded at finite temperatures. General trends and aspects are derived by comparison with comparable data for PdAg bimetallic surfaces. Fundamental insights relevant for applications of bimetallic Pt catalysts, specifically PtAg catalysts, are briefly discussed.