STEPS IN THE REACTION H-2 + O-2-REVERSIBLE-ARROW-H2O ON PT - OH DESORPTION AT HIGH-TEMPERATURES

OH radical desorption over a polycrystalline Pt foil exposed to mixtures of H-2, O2, and H2O for surface temperatures between 1000 and 1800 K was measured by laser-induced fluorescence (LIF) and used to determine the elementary steps in this reaction. OH production in H2O decomposition varied nearly linearly with H2O pressure, while addition of O2 enhanced and H-2 reduced OH production. H-2 oxidation yielded the highest OH desorption signals with a sharp maximum OH desorption occurring in lean H-2/O2 mixtures. A 12-step mechanism for reversible H-2 Oxidation over a Pt surface is proposed which assumes the reaction proceeds through H, O, OH, and H2O surface intermediates. Model calculations indicate that the observed results in the H2O, H2O + O2, and H2O + H-2 systems are primarily due to the shifting of the equilibrium: H(a) + OH(a) reversible H2O(a), and the results in the H-2 + O2 system are primarily due relative rates of the reactions H(a) + O(a) --> OH(a) and H(a) + OH(a) --> H2O(a). Pre-exponentials and activation energies for all reactions were determined by fitting the experimental results to model calculations based on the mechanism, and complete potential energy diagrams of reaction pathways are proposed.