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(111), and the p(2 × 2) alloy (ϑ Sn = 0.25) was more reactive than the (√3x√3)R30° alloy (ϑ 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 dissociation was inhibited more easily on the alloys than on Pt(111), and large O 3 doses on the p(2 × 2) and (√3x√3)R30° surface alloys produced oxygen coverages of 1.2 and 0.87 monolayers, respectively, compared to 2.4 monolayers on Pt(111). Both chemisorbed and “oxidic” oxygen states were characterized by using Auger electron spectroscopy (AES), temperature-programmed desorption (TPD), and low-energy electron diffraction (LEED). At 300 K, chemisorbed oxygen adatoms are formed at low exposures, but oxidation of Sn occurs at large oxygen coverages, as evidenced by a 1.6 eV downshift of the Sn(MNN) AES peak. Heating during TPD causes SnO x formation even at low coverages, and this decomposes to liberate O 2 in desorption peaks at 1015 and 1078 K on the p(2 × 2) and (√3x√3)R30° surfaces, respectively. After oxidation of Sn, TPD indicates desorption of oxygen from chemisorbed adatoms bound at Pt sites and eventually formation of platinum oxide particles. SnO x particles formed in intimate contact with Pt by oxidation of these Pt−Sn alloys and high-temperature heating are easier (100 K) to reduce by heating in a vacuum than a corresponding thick SnO x film. We also find additional stability (130 K) imparted to PtO x particles by the presence of oxidized Sn following oxidation of these alloys. Heating these oxidized alloys to 1000 K produces a (4 × 1) LEED pattern that we have assigned to the formation of large domains of an SnO 2 overlayer on both of the surface alloys.