DETERMINATION OF THE RATE-LIMITING STEP IN THE OXIDATION OF CO ON PT(111) SURFACES

The kinetic behavior of the oxidation of carbon monoxide on Pt(111) wa s studied at different temperatures by exposing both oxygen-covered su rfaces to molecular CO beams and clean surfaces to CO + O-2 mixed beam s under ultra-high vacuum (UHV) conditions. Results from the first typ e of experiments show that on oxygen-covered surfaces the rate for CO2 production above 400 K is determined by the impinging rate of the car bon monoxide molecules. On clean surfaces the oxidation rate is equall y determined by the CO incoming flux as long as the CO : O-2 ratio in the CO + O-2 mixed beams is low. For high CO : O-2 ratios, however, th e CO steady-state coverage increases, poisoning the adsorption of oxyg en, and slowing down the overall CO2 production. The rate of surface r ecombination of CO with oxygen also competes with that of CO adsorptio n at low temperatures, giving rise to a fairly complex overall dynamic behavior.