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

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Abstract
The catalyticreaction 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 (2×2) O ad islands, and a subsequent shrinking of the islands by the reaction of O ad to give CO 2 . The reaction occurs exclusively at boundaries between (2×2) O ad and c (4×2) CO ad 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 –CO ad reaction probability (with parameters from quantitative scanning tunneling microscopy measurements), adsorption/desorption of CO, and a high mobility of CO ad . 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 ΔE 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 CO ad . The fact that no reaction occurs in the interior of the (2×2) O ad domains, although they are covered by a layer of interstitial CO ad molecules, is attributed to the crucial role of reactive O ad –CO ad configurations that only exist at the domain boundaries.