Oxidation of Pt(111) by ozone (O-3) under UHV conditions

Ozone (O-3) is a reactive oxidant, creating high effective oxygen pressures under ultra-high vacuum (UHV) conditions. Large oxygen concentrations up t o Theta(o) = 2.4 ML were produced by ozone exposure on Pr(111) at 300 K and analyzed using temperature-programmed desorption (TPD), Auger electron spe ctroscopy (AES), and low-energy electron diffraction (LEED). O-2 desorption occurs in broad TPD peaks that shift from 814 to 558 K as the oxygen cover age increases from 0.03 to 0.95 ML. For coverages of Theta(o) greater than or equal to 1.2 ML, "undercutting" of the desorption curves occurs, along w ith a narrowing and shift to higher temperature, such that O-2 desorption o ccurs in a sharp (23 K wide) peak at 708 K at Theta(o) = 2.4 ML. LEED shows a (2 x 2) pattern for Theta(o) near 0.25 ML, but then only all (1 x 1) pat tern between 0.3 and 1.2 ML. For oxygen coverages larger than 1.2 ML, the P t(111) surface is disordered. Using the leading edge method, we estimate th at the desorption activation energy decreases from 45 to 20 kcal mol(-1) wi th increased oxygen concentration for 0 < Theta(o) < 1.2 ML, and thereafter increases with increasing concentration to 38 kcal mol(-1) at Theta(o) = 2 .4 ML. We interpret the O-2 desorption curves for Theta(o)>1.2 ML as arisin g from decomposition of particles of platinum oxide of increasing size near the surface. A very weakly bound stare of oxygen also exists at high Theta (o) and desorbs at 390 K. Oxidized platinum is reduced upon heating to Pt m etal in the temperature range of 674-708 K in UHV. (C) 1999 Elsevier Scienc e B.V. All rights reserved.