Detailed modeling of the oxidation of CO on platinum: A Monte-Carlo model

A Monte Carlo (MC) model for the simulation of chemical reactions of a surface coupled to a reactive gas phase with correct real-time dependence is presented. To avoid the shortcomings of the mean-field (MF) approximation, a lattice-gas model is used to describe the kinetics on the reactive surface. The local environment of an adsorbate molecule is included explicitly. The corresponding master equation is solved by simulation of stochastic processes with a Monte Carlo method. The model includes a description for the adsorption and desorption equilibria of CO and molecular oxygen on platinum as well as the surface reaction of CO with O atoms. The focus is on the influence of adsorbate-adsorbate interactions, which cannot be accounted for correctly in the mean-field model. The transport processes in the gas phase are described by a molecular multicomponent transport model. The governing equations are formulated for the geometry of a stagnation-point flow onto the catalytically active plate. Gas phase and reactive surface are treated separately. The coupling between both parts of the system is realized by choosing time steps small enough to limit the change of the mass fractions at the heterogeneous boundary within a defined range. Under this assumption, the mass fraction in the gas phase at the first grid point above the surface can be regarded as constant for the Monto Carlo simulation. After each time step, the mass fractions at the boundary are updated according to the fluxes resulting from the events during the Monte Carlo step. This is followed by a time step for the gas phase generating new initial values for the subsequent Monte Carlo step. Between each Monte Carlo step, the spatial distribution of the adsorbed species is preserved.