Catalytic reactions over modified surfaces: a Monte Carlo study

This paper deals with a simple model of bidirectional dimer-monomer reactio n over a modified catalyst surface, studied by means of the Monte Carlo sim ulation method. It is assumed that certain active sites of a catalyst surfa ce are blocked by modifier (or poison) atoms and thus unable to catalyze th e reactions. The influence of the modifier species concentration and the to pography of their distribution on the reaction yield and selectivity is als o discussed. Four types of the modifier spatial distributions are used: ran dom, dendritic, compact and also highly ordered patterns corresponding to t he stripes of inactive sites. The assumed model predicts that the reaction occurring between the same partners leads to different products depending o n the state of the reacting molecules, which is directly related to the sta te of the adsorbing sites in the vicinity of the reacting particles. It is demonstrated that the surface concentration of the different types of sites and the topography of their distribution over the surface are both importa nt factors, which influence the behavior of systems considered here. In par ticular, it has been found that the reaction direction, the reaction window location and its width depend strongly on the surface morphology. The most surprising finding is that some topographies of the modifier species relat ed to the stripes of inactive sites can generate an unusual double reaction window. (C) 2000 Elsevier Science S.A. All rights reserved.