The oxidation of carbon monoxide on Pt(111): Kinetic modeling under vacuum conditions

The oxidation of carbon monoxide over Pt(111) surfaces was studied under ultra-high vacuum conditions and at different surface temperatures either by exposing the clean Pt(111) surface to CO+O{sub 2} mixed beams or by exposing oxygen-covered Pt(111) substrates to a molecular beam of CO. Direct measurements of the time evolution of the CO and O{sub 2} sticking probabilities and the CO{sub 2} formation rates were performed by mass spectrometry, by using a variation of a method initially developed by King and Wells. Integration of such curves also provided time-dependent coverage data, allowing for the direct calculation of rate laws for the different steps involved in the overall reaction. The results from this analysis show that in the CO starving regime, namely, at low CO:O{sub 2} ratios and at temperatures above 400 K, the determining step for CO{sub 2} formation rests on the impinging rate of CO on the surface. At lower temperatures, however, the surface recombination of CO with oxygen atoms begins to compete with the adsorption, and the overall dynamic behavior becomes quite complex. Modeling of the reaction kinetics will be presented and discussed.