Surface properties and dissolution trends of Pt3M alloys in the presence of adsorbates
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abstract
The adsorption of atomic oxygen on Pt-segregated and nonsegregated surfaces of Pt3M(111) (M = Fe, Co, and Ni) has been systematically investigated using periodic density functional theory calculations. It is found that the surface adsorption properties can be considerably modified by introducing the 3d metals. For each alloy system, the binding strength of oxygen follows the order of Pt-skin < Pt(111) < Pt3M. Among the three alloys, Pt3Fe( 111) is the most favorable for O adsorption on the nonsegregated surface, whereas Pt3Ni(111) is preferred on the Pt-segregated surface. Our calculations show that the magnitude of surface adsorption energies is determined by both the surface geometry and the electronic structure. Remarkably, the electronic structure of the surface Pt atoms only differs very slightly for the segregated and nonsegregated alloys. The discrepant adsorption properties are therefore mainly attributed to the direct involvement of 3d metals either on the surface or in the subsurface. In addition to reactivity, we analyze electrochemical stability of the studied alloys in the presence of adsorbed oxygen, evaluating the electrode potential shift with respect to that found in pure Pt surfaces, which is related to the chemical potential change of surface Pt atoms upon metal dissolution. A simplified slab model is employed to evaluate the chemical potentials for pure metals and alloys using density functional theory calculations. Our results are consistent with previous experimental and theoretical studies, indicating an enhanced electrochemical stability of Pt-skin surfaces produced from Pt 3M alloys. Furthermore, the potential shift of Pt-skin surfaces under 0.25 monolayer of adsorbed atomic oxygen is examined. Oxygen adsorption is found to destabilize both the Pt-skin and pure Pt surfaces. However, the Pt-skin surfaces under adsorbed oxygen are still more stable against dissolution than the pure Pt surfaces under the same oxygen coverage. 2008 American Chemical Society.
