Electrode potential-dependent stages in OHads formation on the Pt3Cr alloy (111) surface

Results of a periodic spin-density-functional theory study of the electronic structure and the local adsorption properties of the Pt3Cr(111) alloy surface are presented. A slab composed of four-atom-layer is used to model the (111) surface of the L1(2) bulk phase with the two topmost layers and adsorbed species allowed to relax. Adsorption energies for OH and H2O have been calculated as functions of initial OH coverage. Using these energies in a model based on reaction energies for acid electrolyte, we have calculated reversible potential shifts, DeltaUdegrees, for OHads formation from H2Oads on different surface sites of the Pt3Cr(111) alloy relative to the Pt(111) surface. For the first 0.25 monolayer (ML) coverage, H2O and OH are more stable on the Cr sites and a DeltaUdegrees = -0.55 V is calculated. For the next 0.25 ML instalment, forming OHads on Pt, DeltaUdegrees = -0.15 V. Shifts in Udegrees for going to 0.75 and 1 ML OHads coverage are both positive. Hydrogen bonding between adsorbed molecules plays a role at higher coverage. O-2 adsorption was studied on the 0.5 ML OHads covered surface and was found to bond more weakly than H2O, allowing us to suggest that if the sputtered alloy surface is active toward O-2 reduction, as the sputtered Pt3Co and Pt3Ni surfaces are, Pt islands may be responsible. (C) 2004 The Electrochemical Society.