Oxidation of Pt Surfaces with Gaseous Oxygen Atoms
Jason F. Weaver, University of Florida, Department of Chemical Engineering, Gainesville, FL 32611
High-concentration surface phases of oxygen play a central role in determining the catalytic behavior of platinum at commercially relevant pressures, yet such phases have not been widely explored in ultrahigh vacuum. In this talk, I will discuss our recent investigations of oxygen phases grown on single crystal Pt surfaces in UHV using oxygen atom beams. Our results show that the oxidation of both Pt(111) and Pt(100) can be generally classified as occurring in two stages, namely, the development of a chemisorbed layer, followed by Pt oxide growth above an oxygen coverage of about 1 ML (monolayer). Chemisorbed oxygen atoms undergo several structural rearrangements, depending on the metal substrate, and experience a substantial decrease in their binding strength to the surface as the coverage increases. On both surfaces, platinum oxide grows as three-dimensional particles that decompose explosively during TPD. We find that the acceleratory decomposition kinetics is well described by a model wherein oxygen atoms migrate from oxide to metallic surface domains from which the oxygen atoms recombine and desorb. The key implications of this model are that Pt oxide is more stable than are high concentrations of chemisorbed oxygen atoms, and that oxide growth is kinetically limited on Pt surfaces. Finally, we find from molecular beam experiments that Pt oxide is less active than the metal toward CO oxidation. The measured CO oxidation kinetics is consistent with CO adsorption and reaction occurring selectively on metallic domains, with the oxide acting as an oxygen source that replenishes metal domains with oxygen during reaction. These results show that species interchange between surface phases are key steps in rate processes that occur on oxidized Pt surfaces, a finding that has been consistently demonstrated by the Madix group over the past decade.