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 (O-2) at Po-2 = 2 x 10(-2) Torr. Under these conditions, the surface oxygen concentration increased to a maximum oxygen uptake of Theta (O) = 1.2 ML (1 ML = 1.505 x 10(15) atom/cm(2)) for the (2 x 2)-Sn/Pt(111) alloy (with Theta (Sn) = 0.25) and Theta (O) = 1.4 ML for the (root3 x root3)R30 degrees -Sn/Pt(111) alloy (with Theta (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 "quasi-metallic" 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 x 2)-Sn/Pt(111) alloy, oxygen uptake to Theta (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 Theta (O) = 0.5 ML were distinguished, with apparent activation energies E-app of 14 and 20 kcal/mol for oxygen concentrations of Theta (O) = 0.5-0.8 and 0.8-1.0 ML respectively. The oxygen uptake curve for the (root3 x root3)R30 degrees -Sn/Pt(111) alloy also displayed two distinct regions. In the first region, with Theta (O) < 0.4 ML, E-app was 9 kcal/mol. In the second region, with Theta (O) greater than or equal to 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. (C) 2001 Elsevier Science B.V. All rights reserved.