PUB - Publikationen an der Universität Bielefeld

The reaction of NO with a CO adsorbate has been studied in a supersonic molecular beam experiment where gas-phase oriented NO molecules strike a CO-precovered Pt(100) surface. By seeding NO in Ar and H-2, the translational energy of the NO molecules in the supersonic beam was varied continuously between 70 and 400 meV. Two quadrupole mass analyzers mounted behind the platinum target recorded the yield of scattered and desorbed NO as well as the reaction product CO2 as a function of time. The CO, signal strongly depends on the translational energy of the incident NO molecules and on the initial NO orientation, i.e. preferential N-end or O-end collisions at normal incidence to the surface. A comparison of the observed steric effects for CO2 and NO leads to the conclusion that the CO2 reaction asymmetry induces a NO trapping asymmetry. Angle-resolved time-of-flight measurements of the CO2 reaction product have been performed to investigate the dynamics of the reaction, especially the reaction mechanism. A cosine-like angular distribution of the CO2 desorption has been found, indicating that a direct or precursor-mediated reaction mechanism is improbable. A Monte Carlo simulation of the NO + CO reaction models the course of the reaction quite well, and shows that NO dissociation dominates the course of the reaction. The observed shift of the CO2 peak for different translational energies of the NO molecules can be reproduced by an energy-dependent NO dissociation. A large steric effect in the CO2 production is found at high translational energies in the experimental data. This cannot be explained solely by an orientation-dependent trapping or sticking of the NO molecules. Consequently, another step in the reaction path, presumably dissociative NO adsorption, is orientation-dependent. (C) 1997 Elsevier Science B.V.