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 Mont e Carlo simulations. Experiments were performed by dosing preadsorbed O-ad layers with CO between 237 and 274 K. Two stages were observed during dosin g 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)C Oad domains. The reaction order with respect to the oxygen coverage is 0.5; the reactivity of the boundary increases during the reaction. The Monte Ca rlo simulations included surface diffusion of O-ad atoms, attractive intera ctions between O-ad atoms, the O-ad-COad reaction probability (with paramet ers from quantitative scanning tunneling microscopy measurements), adsorpti on/desorption of CO, and a high mobility of COad. The experimentally observ ed domain shapes, the reaction order of 0.5, and the increasing boundary re activity could only be reproduced by additionally including an O-ad coordin ation-dependent activation energy DeltaE(react)* of 25 meV per O-ad neighbo r that accounts for the attractive O-ad-O-ad interactions. The initial orde ring stage could be modeled by incorporating an additional repulsive intera ction between O-ad and COad. The fact that no reaction occurs in the interi or of the (2x2)O-ad domains, although they are covered by a layer of inters titial COad molecules, is attributed to the crucial role of reactive O-ad-C Oad configurations that only exist at the domain boundaries. (C) 2001 Ameri can Institute of Physics.