THE EFFECT OF PHASE-TRANSITIONS, SURFACE-DIFFUSION, AND DEFECTS ON HETEROGENEOUS REACTIONS - MULTIPLICITIES AND FLUCTUATIONS

The influence of attractive absorbate-absorbate interactions and surface diffusion on phase transitions in unimolecular surface reactions is examined by Monte Carlo simulations. Detailed studies in parameter space are performed assuming various forms of desorption and reaction rate expressions. Introduction of rate anomalies can lead either to a kinetic or to an equilibrium phase transition, depending on the reaction kinetics and the rate-determining processes. It is demonstrated that migration affects the reaction rate, nucleation, and phase transitions, and, therefore, detailed study of them at a molecular level is necessary to describe heterogeneous reactions. Hysteresis in reactant coverage and reaction rate are studied and large fluctuations are found to dominate the system dynamics near cusp points. The influence of defects which act as nucleation centers on metastability is also investigated. The Monte Carlo data are compared with the predictions of mean-field theory, and limitations of the latter are discussed.