ISOTHERMAL STUDY OF THE KINETICS OF CARBON-MONOXIDE OXIDATION ON PT(111) - RATE DEPENDENCE ON SURFACE COVERAGES

The kinetics of the oxidation of carbon monoxide on Pt(111) surfaces w as studied isothermally by using an effusive directional molecular bea m in an arrangement based on a variation of the dynamic method origina lly devised by King and Wells. Three temperature regimes were identifi ed for this reaction on surfaces precovered with atomic oxygen. Below 300 K no reaction is observed, and the presence of preadsorbed atomic oxygen on the surface does not significantly affect the initial sticki ng coefficient of CO but only reduces its saturation coverage by less than half, which it does by preferentially blocking the bridge sites. Above 400 K, on the other hand, the desorption of CO2 from oxygen-cove red surfaces is controlled by the impinging frequency of the incoming CO. The most interesting temperature range is that between 300 and 400 K, where the rate of surface recombination of CO with oxygen competes with that of CO adsorption; under those conditions the overall dynami c behavior is fairly complex, and not all the surface oxygen is reacti ve. Furthermore, the reaction rates in this regime not only depend on the coverages of the reactants, but also on how the surface is prepare d. Two kinetically distinct types of oxygen atoms develop during the c ourse of reaction in spite of the fact that they all sit on identical sites at the start of the kinetic runs, suggesting that the reactivity of chemisorbed CO depends on the local oxygen coverage of neighboring sites. We propose that such local arrangements modify the adsorption energy for atomic oxygen, and that this in turn changes the activation energy for the oxidation reaction. Previous reported molecular beam e xperiments were also extended to cover a wider range of surface covera ges in order to better determine the dependence of the rate constant f or the surface oxidation step on the coverages of CO and oxygen. It wa s found that while the presence of oxygen on the surface helps the pro duction of CO2, increasing CO coverages augment the activation barrier for this reaction, an observation that is in direct contrast with pre vious reports. Finally, the adsorption sites for CO during the surface CO+O recombinatory reaction were characterized by reflection-absorpti on infrared spectroscopy. The data reported here is analyzed and discu ssed in terms of possible kinetic models. (C) 1997 American Institute of Physics.