Microkinetic Models of Catalytic Reactions on Nonuniform Surfaces:  Application to Model and Real Systems

Nonuniformity, the variation in heats of adsorption and kinetic parameters with changes in coverage or adsorption site, is present in many heterogeneous catalytic reaction systems. Unfortunately, there have been few investigations addressing the incorporation of nonuniformity into kinetic models of catalytic systems. The main reason for this disparity is the difficulty encountered in establishing a quantitative assessment of the effect of nonuniformity on reaction kinetics. Indeed, many systems in which nonuniformity is known to be present are well described by uniform kinetic models. To begin to get a better understanding of the effect nonuniformity has on reaction kinetics, both model and real systems in which nonuniformity is known to be present have been studied using microkinetic modeling. The model system was a simple condensation mechanism involving only one surface reaction. Two types of nonuniformity, adsorbate−adsorbate interactions and biographic heterogeneity, were incorporated into the kinetic parameters of the model, and two sets of nonuniform data were generated. Uniform models were fit to transient nonuniform data, and their accuracy was assessed. The uniform models were then used to predict steady-state reaction rates. While the uniform models performed fairly well in fitting the transient nonuniform data, they overpredicted steady-state reaction rates by roughly a factor of 20 regardless of the type of nonuniformity used to generate the data. The insights gained from the model system were used to model the reaction of carbon monoxide and hydrogen to methane and water (methanation). The nonuniform methanation model included adsorbate−adsorbate repulsive interactions and sites created by the adsorption of carbon monoxide upon which only hydrogen could adsorb. The nonuniform model did an excellent job of fitting and predicting transient methanation data. While it could not predict batch reactor methanation rates within an order of magnitude, it did capture the trends in rate with changing temperature and pressure which had eluded other microkinetic models of methanation reported in the literature.