FAST REACTION-PRODUCTS FROM THE OXIDATION OF CO ON PT(111) - ANGULAR AND VELOCITY DISTRIBUTIONS OF THE CO2 PRODUCT MOLECULES

Angular and velocity distributions of CO2 desorbing as reaction produc t of CO;oxidation on Pt(lll) were measured during heating of layers of initially molecular oxygen and CO adsorbed at a surface temperature o f 100 K. In the velocity integrated desorption spectra of the reaction product CO2 four different peaks (alpha, beta(3), beta(2), beta(1)) c an be discriminated which, for linear heating rates of 5 K/s, appear a t 145, 210, 250, and 330 K, respectively. They can be attributed to di fferent reaction mechanisms which depend on the binding conditions of oxygen and the geometric arrangement and coverages of both species. Wh ereas alpha-CO2 coincides with the O-2 desorption from and the dissoci ation of pure chemisorbed molecular oxygen, and thus indicates a react ion channel coupled with desorption and dissociation of O-2, beta(1)-C O2 corresponds to the reaction path investigated before by many resear chers and is most likely due to the reaction at the boundaries of orde red CO and oxygen islands. The structural conditions for beta(3) and b eta(2) are less clear, but we believe them to stem from reactions in m ixed and/or partly mixed layers at high coverages of O and CO. The alp ha-CO2 species is most likely due to reaction of CO with O atoms stemm ing from O-2 dissociation which react before becoming accommodated. Th e velocity distributions of alpha, beta(2), and beta 3( )are far from thermal equilibrium with the Surface as indicated by average kinetic e nergies between 220 and 360 meV, corresponding to similar or equal to 10 (for beta(3) and beta(2)) and similar or equal to 30 kT(s) (for alp ha), normalized speed ratios between 0.6 and 0.8, and strongly peaked angular distributions (similar to cos(n) l, n=8 for a, n > 10 for beta (3) and beta(2)) For beta(1) both the angular and velocity distributio ns show bimodal behavior with one channel fully accommodated to the su rface whereas the other contains again an appreciable amount of reacti on energy as kinetic energy ([E] similar or equal to 330 meV) resultin g in a strongly peaked angular distribution with n similar or equal to 9. Some TOF results for steady state reaction at high temperatures (4 20-800 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 beta(1) is es timated to about 40%. A discussion of the various possible mechanisms is given.