Structure–reactivity relationship for bimetallic electrodes: Pt overlayers and PtAu surface alloys on Au(1 1 1)

We report calculations based on density functional theory of the adsorption properties of CO on pseudomorphic Pt overlayers and PtAu surface alloys on the Au(1 1 1) surface. Similar to the Pd/Au system, electrodes consisting of pseudomorphic Pt overlayers on Au show a higher reactivity than pure Pt electrodes due to a combination of geometric strain and electronic interaction effects. Furthermore, we find a significant second-layer effect for CO adsorption at the ontop site of the Pt/Au(1 1 1) overlayers which is much smaller for the fcc-hollow site. We have clarified that this second-layer effect for the ontop-site adsorption essentially comes from the highly-directional chemical bonding of C and Pt atoms in the direction perpendicular to the surface, where bonding orbitals have significant overlap with the second-layer atoms. In the case of monolayer PtAu surface alloys, the binding energy on the most stable adsorption sites becomes remarkably larger as the number of Pt atom in the topmost layer increases.