KINETICS OF HYDROGEN OXIDATION TO WATER ON THE RH(111) SURFACE USING MULTIPLE SOURCE MODULATED MOLECULAR-BEAM TECHNIQUES

We have examined the kinetics of the oxidation of hydrogen to water on the Rh(111) surface using modulated molecular beam reactive scattering. For reactant pressures below 10(-4) Torr and temperatures from 450-1250 K we observe serial steps, with apparent activation energies of 2.5 +/- 1 and 10 +/- 1 kcal/mol. Pseudo-first-order preexponential factors are 10(5) and 10(7) s-1, respectively, varying slightly with oxygen coverage. Reaction is inhibited by excess oxygen. Maximum water production occurs around 650 K. At lower temperatures the reaction becomes nonlinear. We use a new three-molecular-beam arrangement. Two continuous, independently adjustable beams establish steady-state surface concentrations, while a weaker modulated third beam induces small concentration perturbations around the selected steady state. With this technique we varied surface oxygen coverages, used isotopic substitution in the three beams to produce H2O, D2O and HDO, and linearized the HDO reaction.