On the effects of adsorbate aggregation on the kinetics of surface reactions

The effects of lateral interactions between chemisorbed reactants on the mechanisms of surface reactions are studied by Monte Carlo simulations for a model system. The model describes a reaction between two adsorbates, A and B, which upon reaction form a rapidly desorbing product. The surface is described by a square lattice with attractive nearest-neighbor interactions between the A atoms; w_AA/kT = -2.5. The B atoms are regarded as "random walkers," unaffected by lateral interactions. (w_AB = w _BB = 0.) The A atoms are allowed to aggregate before B is adsorbed on the surface. Reaction can occur when the much more mobile B's reach the boundary of an "island" of A atoms. The island structures depend on the aggregation period allowed for A and on coverage. Most of the simulations model a reaction beginning after A aggregation has reached a long-lived (yet nonequilibrium) state, characterized by finite and ramified islands with strongly indented boundaries. Typical island characteristics, such as their average boundary length, average size, and specific populations of A atoms with all possible numbers of A neighbors, as well as coverages and reaction rates are followed as a function of time during the aggregation and reaction periods. The rates are much lower than those expected from a random distribution of reactants. The kinetic laws observed are nonstandard. Models assuming arrangement of the aggregating species in regularly shaped islands are shown to be oversimplified. Application of the quasichemical approximation to the calculation of reaction rates is shown to be inadequate.