(456a) Surface Atomic Distribution and Water/Atomic Oxygen Adsorption On Pt-Co Alloys

Pt-based alloy surfaces are used to catalyze the molecular oxygen and its subsequent reduction to water on fuel cell electrodes. Due to surface segregation phenomena, alloy surface composition may be very different from the overall composition. Also, the adsorption of intermediates or products species on the surface may modify the properties of both surfaces and species. Here we address the problem of determining surface atomic distribution using density functional theory (DFT) calculations on slabs of PtCo and Pt₃Co overall compositions, as well as a water molecule adsorption on PtCo(111) and Pt-skin structures. Pt-rich surfaces are energetically favored under vacuum in the PtCo and Pt₃Co alloys. The adsorption trend on the studied structures agrees with the d band model, with stronger adsorption at higher surface Co composition. The bonding between water and surfaces of PtCo and Pt-skin monolayers are investigated in term of energetic and electron orbitals. We also found that water adsorption on Pt-skin shows larger effect on atomic oxygen binding energy than that on PtCo. On both surfaces, the surface reconstruction occurs at high oxygen coverage (~0.5 ML) but the presence of water at low oxygen coverage of 0.125 ML can present a comparable magnitude of surface reconstruction. The strength of O adsorption, as well as surface reconstruction effects due to the adsorbate are strongly influenced by the presence of the oxyphilic transition metal on the surface or subsurface.