Fast reaction products from the oxidation of CO on Pt(111): Angular and velocity distributions of the CO2 product molecules

Abstract

Angular and velocity distributions of CO2 desorbing as reaction product of CO oxidation on Pt(111) were measured during heating of layers of initially molecular oxygen and CO adsorbed at a surface temperature of 100 K. In the velocity integrated desorption spectra of the reaction product CO2 four different peaks (α, β3, β2, β1) can be discriminated which, for linear heating rates of 5 K/s, appear at 145, 210, 250, and 330 K, respectively. They can be attributed to different reaction mechanisms which depend on the binding conditions of oxygen and the geometric arrangement and coverages of both species. Whereas α‐CO2 coincides with the O2 desorption from and the dissociation of pure chemisorbed molecular oxygen, and thus indicates a reaction channel coupled with desorption and dissociation of O2, β1‐CO2 corresponds to the reaction path investigated before by many researchers and is most likely due to the reaction at the boundaries of ordered CO and oxygen islands. The structural conditions for β3 and β2 are less clear, but we believe them to stem from reactions in mixed and/or partly mixed layers at high coverages of O and CO. The α‐CO2 species is most likely due to reaction of CO with O atoms stemming from O2 dissociation which react before becoming accommodated. The velocity distributions of α, β2, and β3 are far from thermal equilibrium with the surface as indicated by average kinetic energies between 220 and 360 meV, corresponding to ≂10 (for β3 and β2) and ≂30 kTs (for α), normalized speed ratios between 0.6 and 0.8, and strongly peaked angular distributions (∼cosn ϑ, n=8 for α, n≳10 for β3 and β2). For β1 both the angular and velocity distributions show bimodal behavior with one channel fully accommodated to the surface whereas the other contains again an appreciable amount of reaction energy as kinetic energy (〈E〉≂330 meV) resulting in a strongly peaked angular distribution with n≂9. Some TOF results for steady state reaction at high temperatures (420–800 K) obtained in the same apparatus are given for comparison. The fraction of reaction energy channelled into the translational degree of freedom for the nonequilibrated part of reaction peak β1 is estimated to about 40%. A discussion of the various possible mechanisms is given.