Adsorbate islanding in surface reactions: A combined Monte Carlo-lattice gas approach

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Abstract
The effects of reactant aggregation and lateral interactions on bimolecular surface reactions A+B→AB are studied using a theoretical model combining Monte Carlo simulations with (modified) lattice gas approximations, with special reference to temperature programmed reactions. A and B are chemisorbed species yielding a rapidly desorbing product AB. We are particularly interested in systems where one reactant (A) tends to organize in ordered domains, as a result of strong A–A attractive forces. Our modeling scheme consists of two stages. First, A is randomly adsorbed at a temperature lower than the critical temperature of the gas–solid transition, and phase separation (i.e., A aggregation) starts taking place. This process is modeled by (conserved) Monte Carlo dynamics. As in previous studies, we find that after a while the A atoms form finite, metastable, often highly ramified islands, whose further growth is a very slow process. At this stage, or earlier, B is adsorbed on the A-free lattice sites, T is raised (linearly) and reaction begins. It is assumed that A is stationary while B is highly mobile and that B diffusion is much faster than A+B reaction. Thus, the distribution of B on A-free sites is always in equilibrium, but it depends on A–B and B–B interactions, as well as on coverage, temperature, and, in particular, the given distribution of A atoms. To calculate the B distribution we formulate and employ two lattice gas models, which are appropriately modified versions of the mean-field and the quasichemical approximations. To follow the time-space evolution of the system we use Monte Carlo methods to simulate the slow processes, A aggregation and AB formation, and (one of the) lattice gas models for B distribution. Calculations of thermal desorption spectra are performed for A and B adsorbed on a 500×500 square lattice (with periodic boundary conditions) assuming nearest-neighbor lateral interactions wAA, wBB, and wAB. The results reveal nontrivial kinetic behaviors, which depend strongly on initial conditions (coverages, dosing sequence) and interaction parameters. These are reflected in the temperature programmed spectra and the apparent (coverage dependent) activation energies derived from them. It is shown, for example, that the major effect of A–B interactions on the rate is via their influence on local concentration of B’s near A’s. B–B interactions affect directly the reaction activation barrier. A–A interactions show both ‘‘topological’’ and ‘‘energetic’’ effects.