In this paper a simple lattice gas model for the catalytic CO + NO --> CO2 + 1/2 N-2 surface reaction is studied by means of Monte Carlo simulations. In the simulations we take into consideration the diffusion of all particles and the desorption of CO. The square and triangular lattices are used to model the surface of an ideal catalyst, whereas percolation clusters, diffusion limited aggregates (DLA), the Sierpinski carpet, and the Sierpinski gasket are introduced as better models for supported catalysts. In many simulations an interval of the gas phase concentration y(CO) is an element of [y(1), y(2)] exists, where the simulation remains in a reactive state. The kinetic phase transitions of second (y(1)) and first-order (y(2)) which appear in this reaction system are studied as a function of the system parameters. The above interval is broadened with increasing diffusion of the adsorbed particles. Desorption of CO results in a shift of y(2) to y(CO) = 1. On the Sierpinski lattices only very small reactive intervals appear, whereas on DLA and percolation clusters no reaction occurs. Estimates for the order parameter beta of the second order phase transition from an active into one (out of infinitely many) absorbing states show that the model belongs to the universality class of directed percolation. The very strong influence of the lattice structure on the reaction system is dicussed in detail.