REACTION OF FAST CO MOLECULES WITH CO, NO OR O2 PREADSORBED ON PT(111) SURFACE - A SEARCH FOR ELEY-RIDEAL REACTION-MECHANISM

A beam of CO molecules with energy up to 2.2 eV (obtained by seeding with He) was directed onto a Pt(111) surface, held at 100 K, and covered with: (i) a (4 x 2) structure of CO, (ii) a (2 x 2) structure of NO, and (iii) a disordered monolayer of molecular oxygen. The usual Langmuir-Hinshelwood reaction channel was frozen out by keeping the surface temperature low. A search for reaction products, in particular CO2 formed via the Eley-Rideal reaction mechanism, was carried out in real time by varying the CO normal energy, incoming angle and CO exposure. In addition after each CO exposure a TPD run was performed, tuned to the possible reaction products which remained on the surface. We have not been able to observe any production of CO2 via the Eley-Rideal mechanism in these reactions within the sensitivity limit of our system. The upper limits for the reaction probabilities are calculated, based on the calibrations of the detection system, beam intensity and adsorbate coverage. Their values are < 4 x 10(-5) for the CO(gas) + CO(ads), --> CO2 + C(ads), < 7 x 10(-5) for the CO(gas) + NO(ads) --> CO2 + N(ads) and < 2 x 10(-6) for the CO(gas) + O2ads --> CO2 + O(ads) reaction respectively. Possible reasons for such low values are discussed based on the position and symmetry properties of the HOMO and LUMO orbitals involved in each reaction.