NECESSARY LEVELS OF DETAIL IN MICROKINETIC MODELS OF CATALYTIC REACTIONS ON NONUNIFORM SURFACES

A quantitative evaluation of approximate models which account for the kinetic effects of nonuniformity of adsorption on heterogeneous cataly st surfaces has been performed. Model adequacy was evaluated by compar ing the predictions to a base case model in which the dependence of th e reaction rate on the nonuniformity of the surface was rigorously tak en into account. None of the approximate models were found to provide predictions sufficiently similar to the kinetics observed on a surface possessing active sites with a known heterogeneous distribution of ad sorption energies under a variety of conditions. Surprisingly, a unifo rm surface model, which treated the catalyst surface as if it were hom ogeneous in adsorption energy, provided the most accurate representati on. The prediction of the uniform site model was always within a facto r of 4-5 for all conditions investigated with adsorption energy distri bution breadths ranging up to 20 kcal mol(-1). The same general conclu sions were drawn without requiring the allowed adsorption energy distr ibution to be centered about the so-called ''optimum'' reactive site. In fact, the uniform site model prediction was within a factor of 4 wh ether or not the optimum reactive site was contained within the allowe d range of adsorption energies. It was concluded that if the kinetic e ffects of nonuniform surfaces are of central importance and must be in corporated in a kinetic model, a computationally intensive, site-by-si te averaging of the reaction rate at each unique adsorption site must be performed. Copyright (C) 1996 Elsevier Science Ltd.