Monte Carlo simulations of the effect of crystallite size on the activity of a supported catalyst

A dynamic Monte Carlo (MC) simulation is presented for the catalytic reacti on A + 1/2B(2) --> C. The catalyst surface is modeled as an array of crysta llites each one represented as the top projection of a truncated hexagonal pyramid on a flat support. The proportion of corners, edges, base and face atoms or sites varies with crystallite size. The probabilities for the elem entary steps comprising the catalytic cycle are calculated from the rate pr ocesses at which each elementary step occurs and varies in the different ty pe of sites. We first analyze crystallites of the same size and then a cata lyst with a non-uniform distribution of crystallites. The results show that the rate relative to a base case can increase or decrease with crystallite size depending on which elementary step is enhanced in the various types o f sites analyzed. Results are relative to parameters used in the base case, which was taken from experimental results for the CO oxidation reaction. M onte Carlo simulations such as the one presented in this paper provide a me thod of analyzing the effect of crystallite size in the reaction, not avail able in continuos models. This method is today at the frontier of the model ing of chemical reacting systems and they will enhance our understanding of fundamental issues in catalytic reactions. (C) 2001 Elsevier Science B.V. All rights reserved.