Article,
Fast reaction products from the oxidation of CO on Pt(111): Angular and velocity distributions of the COsub 2 product molecules
J. Chem. Phys., 100 (5): 3985-3998 (1994)
DOI: 10.1063/1.466332
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 (alpha, beta3, beta2, beta1) 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 alpha-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, beta1-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 beta3 and beta2
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 alpha-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 alpha, beta2, and beta3 are far from thermal
equilibrium with the surface as indicated by average kinetic energies
between 220 and 360 meV, corresponding to approximately-equal-to10
(for beta3 and beta2) and approximately-equal-to30 kTs (for alpha),
normalized speed ratios between 0.6 and 0.8, and strongly peaked
angular distributions (~cosn theta, n=8 for alpha, n>~10 for beta3
and beta2). For beta1 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>approximately-equal-to330
meV) resulting in a strongly peaked angular distribution with napproximately-equal-to9.
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 beta1 is estimated
to about 40%. A discussion of the various possible mechanisms is
given.
Tags
- angular
- carbon
- dioxide;
- distribution;
- monoxide;
- oxidation;
- platinum;
- reactions
- reactions;surface
- surface
- velocity
- yields;
