Oxidation of ordered Sn/Pt(111) surface alloys and thermal stability of the oxides formed

Two Pt-Sn surface alloys were oxidized at 300 K by ozone (O-3) exposure in UHV. Both alloys were less reactive than Pt(lll), and the p(2 x 2) alloy (T heta(Sn) = 0.25) was more reactive than the (root 3x root 3)R30 degrees all oy (Theta(Sn) = 0.33). The relative O-3 dissociative sticking coefficients on these surfaces at 300 K were 1.0:0.79:0.33, respectively. Ozone dissocia tion was inhibited more easily on the alloys than on Pt(111), and large O-3 doses on the p(2 x 2) and (root 3x root 3)R30 degrees surface alloys produ ced oxygen coverages of 1.2 and 0.87 monolayers, respectively, compared to 2.4 monolayers on Pt(lll). Both chemisorbed and "oxidic" oxygen states were characterized by using Auger electron spectroscopy (AES), temperature-prog rammed desorption (TPD), and low-energy electron diffraction (LEED). At 300 K, chemisorbed oxygen adatoms are formed at low exposures, but oxidation o f Sn occurs at large oxygen coverages, as evidenced by a 1.6 eV downshift o f the Sn(MNN) AES peak. Heating during TPD causes SnOx formation even at lo w coverages, and this decomposes to liberate O-2 in desorption peaks at 101 5 and 1078 K on the p(2 x 2) and (root 3x root 3)R30 degrees surfaces, resp ectively. After oxidation of Sn, TPD indicates desorption of oxygen from ch emisorbed adatoms bound at Pt sites and eventually formation of platinum ox ide particles. SnOx particles formed in intimate contact with Pt by oxidati on of these Pt-Sn alloys and high-temperature heating are easier (100 K) to reduce by heating in a vacuum than a corresponding thick SnOx film. We als o find additional stability (130 K) imparted to PtOx particles by the prese nce of oxidized Sn following oxidation of these alloys. Heating these oxidi zed alloys to 1000 K produces a (4 x 1) LEED pattern that we have assigned to the formation of large domains of an SnO2 overlayer on both of the surfa ce alloys.