Pt promotion and spill-over processes during deposition and desorption of upd-H_ad and OH_ad on Pt_xRu_1-x/Ru(0001) surface alloys

The electrochemical adsorption of underpotential deposited hydrogen (upd-H_ad) and OH_ad on structurally well-defined Pt_xRu_1-x/Ru(0001) surface alloys was investigated by cyclic voltammetry and density functional theory (DFT) calculations. The adsorption energies of both upd-H_ad and OH_ad decrease with increasing Pt content in the adsorption ensemble, shifting the onset of upd-H_ad and OH_ad formation to increasingly cathodic and anodic potentials, respectively. For bare Ru(0001) and for Ru₃ sites in the surface alloy, the stability regions of these two species overlap or almost overlap, respectively. Similar to previous findings for upd-H_ad adsorption/desorption on partly Pt monolayer island covered Ru(0001) surfaces (J. Phys. Chem. B 2004, 108, 14780), we find a sharp peak at ∼100 mV vs. RHE in each scan direction, which is attributed to a Pt catalyzed OH_ad↔ upd-H_ad replacement on Ru₃ sites, via adsorption on Pt rich sites and spill-over to Ru₃ sites. The decrease of the integrated charge in these peaks with the third power of the Ru surface concentration, which for a random distribution of surface atoms reflects the availability of Ru₃ sites, supports the above assignment.