Electrode Potential-Dependent Stages in OH[sub ads] Formation on the Pt[sub 3]Cr Alloy (111) Surface

Results of a periodic spin-density-functional theory study of the electronic structure and the local adsorption properties of the Pt 3 Cr ( 111 ) alloy surface are presented. A slab composed of four-atom-layer is used to model the (111) surface of the L 1 2 bulk phase with the two topmost layers and adsorbed species allowed to relax. Adsorption energies for OH and H 2 O 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, Δ U ° , for OH ads formation from H 2 O ads on different surface sites of the Pt 3 Cr ( 111 ) alloy relative to the Pt(111) surface. For the first 0.25 monolayer (ML) coverage, H 2 O and OH are more stable on the Cr sites and a Δ U ° = − 0.55 V is calculated. For the next 0.25 ML instalment, forming OH ads on Pt, Δ U ° = − 0.15 V. Shifts in U ° for going to 0.75 and 1 ML OH ads 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 OH ads covered surface and was found to bond more weakly than H 2 O , allowing us to suggest that if the sputtered alloy surface is active toward O 2 reduction, as the sputtered Pt 3 Co and Pt 3 Ni surfaces are, Pt islands may be responsible.