Oxidation of Pt (111) by ozone (O₃) under UHV conditions

Ozone (O₃) is a reactive oxidant, creating high effective oxygen pressures under ultra-high vacuum (UHV) conditions. Large oxygen concentrations up to θ_o=2.4 ML were produced by ozone exposure on Pt(111) at 300 K and analyzed using temperature-programmed desorption (TPD), Auger electron spectroscopy (AES), and low-energy electron diffraction (LEED). O₂ desorption occurs in broad TPD peaks that shift from 814 to 558 K as the oxygen coverage increases from 0.03 to 0.95 ML. For coverages of θ_o≥1.2 ML, "undercutting" of the desorption curves occurs, along with a narrowing and shift to higher temperature, such that O₂ desorption occurs in a sharp (23 K wide) peak at 708 K at θ_o=2.4 ML. LEED shows a (2 × 2) pattern for θ_o near 0.25 ML, but then only a (1 × 1) pattern between 0.3 and 1.2 ML. For oxygen coverages larger than 1.2 ML, the Pt(111) surface is disordered. Using the leading edge method, we estimate that the desorption activation energy decreases from 45 to 20 kcal mol^-1 with increased oxygen concentration for 0<θ_o<1.2 ML, and thereafter increases with increasing concentration to 38 kcal mol^-1 at θ_o=2.4 ML. We interpret the O₂ desorption curves for θ_o>1.2 ML as arising from decomposition of particles of platinum oxide of increasing size near the surface. A very weakly bound state of oxygen also exists at high θ_o and desorbs at 390 K. Oxidized platinum is reduced upon heating to Pt metal in the temperature range of 674-708 K in UHV.