(72b) Characterization and Thermal Stability of Oxidized Sn/Pt(100) Alloy Surfaces

Pt-Sn surface alloys formed on a Pt(100) substrate are remarkably stable to oxidation - no thermal dissociative adsorption of O₂ occurs under UHV conditions.  Nonetheless oxidation of these alloys can be carried out clearly and controllably in UHV by using O₃ (ozone).  Ozone is a reactive oxidant, creating high “effective” oxygen pressure in UHV such that large concentrations (up to Q_O = 2.2 and 2.1 ML) of chemisorbed and “oxidic” oxygen were produced on the two ordered c(2x2) and (3(rt2xrt2))R45 degrees Sn/Pt(100) surface alloys, respectively.  AES, LEED, HR-XPS, and TPD were used to characterize the oxidized alloy surface, measure the oxygen uptake kinetics, and probe the thermal stability of the oxidized surface.  Ozone exposed on the alloy surfaces at 300 K oxidized Sn.  Annealing the alloy surfaces during TPD revealed two types of O₂ desorption peaks on each alloy surface: peaks observed at 910 and 1100 K on the c(2x2) Sn/Pt(100) surface alloy are attributed to Pt-O-Sn and Sn-O interactions respectively, and peaks at 750 and 1100 K observed on the (3(rt2xrt2))R45degrees Sn/Pt(100) surface alloy are attributed to Pt-O and Sn-O interactions.  At both surfaces Sn-O interactions lead to the formation of SnO_x at high temperature (> 800 K), which decomposed around 1100 K.  SnO_x formed on these surfaces is destabilized by at least 100 K due to the presence of Pt as compared to the oxide formed on a thick layer of Sn on the Pt(100) surface.  In contrast, PtO_x on the (3(rt2xrt2))R45degrees surface is 90 K more stable in the presence of SnO_x on the surface as compared to the oxide formed on the clean Pt(100) surface.  Both Sn/Pt(100) surface alloys oxidized more readily than Pt(100), and a c(2x2) Sn overlayer was even more reactive for dissociating ozone.