KINETICS OF THE HYDROGEN OXYGEN REACTION ON PLATINUM

A kinetic model has been constructed primarily to describe the rates of water and hydroxyl desorption during the hydrogen-oxygen reaction on Pt at high temperatures, ∼ 1000 K, and pressures in the range 1-1000 m Torr. The calculated results are compared with experimental observations. The model is based on dissociative sticking of H2 and O2 and hydrogen addition to oxygen to form OH. For water formation the two alternative routes OH + H → H2O and OH + OH → H2O + O are considered. OH decomposition, OH → O + H, is found to be an important reaction step. Using available literature data and results from the model calculations, an enthalpy diagram for the reaction is constructed. It is concluded that a unique enthalpy diagram for the H2 + O2 reaction on Pt is still lacking, even at nearly zero coverage. Adsorbate-adsorbate interactions are expected to modify the enthalpy diagram at high coverages. The kinetic equations for various reaction steps have been formulated assuming random distribution of adsorbed species on a uniform surface. At fixed temperature, both routes for H2O formation mentioned above can give a reasonably good quantitative description of the OH and H2O desorption rates as functions of gas mixture and pressure in the regimes where experimental data are available. However, a closer analysis shows that the relative importance of the two water formation routes could depend sensitively on temperature and gas-phase H2/O2 ratio. High temperature and hydrogen excess favors the OH + H → H2O route, while oxygen excess and low temperatures may favor the OH disproportionation reaction. The model has also been used to predict the reaction kinetics at high pressures up to 105 Torr. The latter results may be useful as guides to high-pressure experiments and in calculations of catalytic combustor performance. © 1991.