Photoemission electron microscopy (PEEM) of the NO + NH3 reaction on a Pt(100) single crystal surface under UHV conditions reveals complicated dynamical behavior. As a rationalization for the observed spatio-temporal evolution of the surface a so-called 'island growth mechanism' was proposed. Here, we present the results of 1D simulations with a realistic 7-variable model that takes explicitly into account macroscopic surface defects on the catalyst. Depending on whether or not we consider site-blocking effects of coadsorbates on diffusing species, we obtain different patterns. The experimental PEEM data can be qualitatively reproduced in this way. (c) 2006 Elsevier B.V. All rights reserved.