Oxidation of ordered Pt-Sn surface alloys by O₂

We have studied the oxidation of two, ordered Pt-Sn surface alloys at 380-425 K using moderately high pressures of oxygen (O₂) at Po₂=2×10^-2 Torr. Under these conditions, the surface oxygen concentration increased to a maximum oxygen uptake of Θ_O=1.2 ML (1ML=1.505×10¹⁵ atom/cm²) for the (2×2)-Sn/Pt(1 1 1) alloy (with Θ_Sn=0.25) and Θ_O=1.4 ML for the (√3×√3)R30°-Sn/Pt(1 1 1) alloy (with Θ_Sn=0.33). Oxygen accumulation was accompanied by a shift in the Sn(3d_5/2) XPS peak from 484.9 to 485.5 eV, with most of the pre-alloyed tin oxidized to a "quasimetallic" state (a form more reduced than SnO). In addition, an oxidic state of Sn (with composition SnO or SnO_x, where x<2) is formed. No change occurred in the Pt(4f) peaks, suggesting that no "Pt oxide" phase was formed under these conditions. On the (2×2)-Sn/Pt(1 1 1) alloy, oxygen uptake to Θ_O=0.5 ML was achieved instantly (in less than 10 s) and then occurred more slowly until a saturation uptake was reached. Two kinetic regions for oxygen uptake exceeding Θ_O=0.5 ML were distinguished, with apparent activation energies E_app of 14 and 20 kcal/mol for oxygen concentrations of Θ_O=0.5-0.8 and 0.8-1.0 ML, respectively. The oxygen uptake curve for the (√3×√3)R30°-Sn/Pt(1 1 1) alloy also displayed two distinct regions. In the first region, with Θ_O≥0.4 ML, E_app was 9 kcal/mol. In the second region, with Θ_O≥0.4 ML, oxidation proceeded with E_app=17 kcal/mol. Overall, these results are consistent with previous studies on bulk Pt-Sn alloys, but new information is obtained on the role of alloy surface structure in controlling the initial stages of oxidation kinetics.