Reaction of NO and CO on a Rh(100) surface studied with gas-phase oriented NO

The reaction of NO with CO adsorbates has been studied in a supersonic molecular beam experiment with gas-phase oriented NO molecules incident on a CO-precovered Rh(100) surface. Two quadrupole mass analyzers mounted behind the rhodium target record the yield of scattered and desorbed NO as well as the reaction product CO2 as a function of time. The CO2 signal strongly depends on the initial NO orientation, that is to say, preferential N-end or O-end collisions at normal incidence on the surface. The corresponding CO2 reaction asymmetry shows a very high value of 0.35 at the beginning of the CO2 production and cannot be explained solely by an orientation dependent trapping or sticking of the NO molecules on the CO precovered surface. This points to a transition in the reaction mechanism from a direct to an indirect surface reaction channel.