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 ΘO=1.2 ML (1 ML=1.505×1015 atom/cm2) for the (2×2)-Sn/Pt(1 1 1) alloy (with ΘSn=0.25) and ΘO=1.4 ML for the (√3×√3)R30°-Sn/Pt(1 1 1) alloy (with ΘSn=0.33). Oxygen accumulation was accompanied by a shift in the Sn(3d5/2) 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 SnOx, 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 ΘO=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 ΘO=0.5 ML were distinguished, with apparent activation energies Eapp of 14 and 20 kcal/mol for oxygen concentrations of ΘO=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 ΘO<0.4 ML, Eapp was 9 kcal/mol. In the second region, with ΘO⩾0.4 ML, oxidation proceeded with Eapp=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.