A molecular beam scattering investigation of the oxidation of CO on Rh(111). II. Angular and velocity distributions of the CO2 product

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Abstract
Molecular beam and time of flight methods have been used to examine the angular distributions and velocity distributions of the CO2 product molecules formed in the catalytic oxidation of CO on a Rh(111) single crystal in the surface temperature range 700–1000 K. The angular distribution was sharply peaked about the surface normal, and cannot be described by a simple cosn θ expression. No temperature dependence was observed in the angular distribution over the range of temperatures studied here. Observed velocity distributions were clearly non‐Maxwellian and had average translational energies in excess of those expected at the surface temperatures. Furthermore, the average velocity depended strongly on the desorption angle. Molecules desorbing along the surface normal had an average translational energy of ∼8 kcal/mol. The average energy decreased with increasing angle, reaching a value of ∼4 kcal/mol at an angle of 60°. All of the observed velocity distributions were narrower than Maxwellian distributions with the same average energies. Product velocity distributions did not appear to vary with surface temperature. The observed excess energies are believed to arise from the crossing of the activation barrier to reaction, with a fraction of the reaction energy being carried away from the surface by the product molecules.