Necessary levels of detail in microkinetic models of catalytic reactions on nonuniform surfaces

Abstract

A quantitative evaluation of approximate models which account for the kinetic effects of nonuniformity of adsorption on heterogeneous catalyst surfaces has been performed. Model adequacy was evaluated by comparing the predictions to a base case model in which the dependence of the reaction rate on the nonuniformity of the surface was rigorously taken 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 adsorption energies under a variety of conditions. Surprisingly, a uniform surface model, which treated the catalyst surface as if it were homogeneous in adsorption energy, provided the most accurate representation. The prediction of the uniform site model was always within a factor of 4–5 for all conditions investigated with adsorption energy distribution breadths ranging up to 20 kcal mol −1. The same general conclusions were drawn without requiring the allowed adsorption energy distribution to be centered about the so-called “optimum” reactive site. In fact, the uniform site model prediction was within a factor of 4 whether or not the optimum reactive site was contained within the allowed range of adsorption energies. It was concluded that if the kinetic effects of nonuniform surfaces are of central importance and must be incorporated in a kinetic model, a computationally intensive, site-by-site averaging of the reaction rate at each unique adsorption site must be performed.