Local reactivity of ultrathin platinum overlayers and surface alloys on a gold surface

The geometric and electronic structure, the stability, and CO adsorption properties of pseudomorphic Pt overlayers and PtAu surface alloys on a Au(1 1 1) substrate have been addressed on the basis of first-principles calculations. We have found that two-monolayer thick surface alloys are more stable than one-monolayer thick alloys. The CO binding energies at the top sites of two-monolayer thick surface alloys increase gradually with the Pt concentration, while the energies are almost independent of the concentration for one-monolayer thick surface alloys. This difference is caused by the lower symmetry of the two-monolayer thick surface alloys, which makes the effect of neighboring atoms in the first layer more important.