CO oxidation on Pt(111) -: Scanning tunneling microscopy experiments and Monte Carlo simulations

The catalytic reaction between adsorbed oxygen atoms and CO molecules on Pt(111) was investigated by scanning tunneling microscopy and modeled by Monte Carlo simulations. Experiments were performed by dosing preadsorbed O-ad layers with CO between 237 and 274 K. Two stages were observed during dosing with CO, an initial reordering and compression of (2x2)O-ad islands, and a subsequent shrinking of the islands by the reaction of O-ad to give CO2. The reaction occurs exclusively at boundaries between (2x2)O-ad and c(4x2)COad domains. The reaction order with respect to the oxygen coverage is 0.5; the reactivity of the boundary increases during the reaction. The Monte Carlo simulations included surface diffusion of O-ad atoms, attractive interactions between O-ad atoms, the O-ad-COad reaction probability (with parameters from quantitative scanning tunneling microscopy measurements), adsorption/desorption of CO, and a high mobility of COad. The experimentally observed domain shapes, the reaction order of 0.5, and the increasing boundary reactivity could only be reproduced by additionally including an O-ad coordination-dependent activation energy DeltaE(react)* of 25 meV per O-ad neighbor that accounts for the attractive O-ad-O-ad interactions. The initial ordering stage could be modeled by incorporating an additional repulsive interaction between O-ad and COad. The fact that no reaction occurs in the interior of the (2x2)O-ad domains, although they are covered by a layer of interstitial COad molecules, is attributed to the crucial role of reactive O-ad-COad configurations that only exist at the domain boundaries. (C) 2001 American Institute of Physics.