To predict the effect of pore structure on the performance of heterogeneous catalysts, a realistic model of the catalyst particle is required Lattice-based models in which the diffusion and reaction phenomena are restricted to sites and bonds within a regular or irregular lattice are widely used. However, for the realistic simulation of diffusion and reaction in amorphous catalyst supports, such as alumina or silica, a continuum model which does not artificially restrict the domain in which the reactants are allowed to diffuse, is required. An efficient method based on a "first passage time" approach is developed for the simulation of diffusion and reaction in a supported catalyst. The model catalyst is composed of spheres representing the support and active sites. By varying the algorithm used to generate the model catalyst, a range of structures can be created The effect of the structure, and the size and distribution of active sites on the reaction rate is studied.