In situ investigation of the catalytic reaction H2+< ![IGNORE[O2→H2 O with second-harmonic generation

2 +O₂→H₂ O in the pressure range 0.2 Torr≤p_tot≤10 Torr on Pt(111) surface. At a catalyst temperature of T=700 K the equilibrium oxygen coverage θ_o is determined as a function of hydrogen partial pressure α. The experimentally obtained θ_o is modelled in a two step process considering the mass transport in the gas phase as well as the catalytic reaction on the surface. In this pressure range the mass transport in the gas phase changes from molecular flow conditions to laminar flow, inducing a strong modification of the gas phase present at the catalyst through different diffusivities of the reactants as well as through desorbing reaction products from the catalyst. It is shown that these gas phase alterations have to be taken into account for a proper modelling of the surface mechanism. Simulation calculations allow one to identify the sequential hydrogen addition reaction as the main reaction path for water production in this parameter range. Excellent agreement with previous investigations is obtained for the determined activation energies of the water-producing reaction steps equal to E^f _H2O≥0.7 eV. Received: 20 September 1998 / Revised version: 15 December 1998