Transient and steady-state microkinetic models of catalytic reactions on nonuniform surfaces

Discerning the extent to which thermodynamic and kinetic surface nonuniformity affect observable phenomena in analyses of heterogeneous catalytic reactions is complicated by the complexity of the reactions and interference from other effects. To eliminate these additional phenomena from clouding the interpretation of the kinetics data, a computational approach was developed and adopted in which transient and steady-state data to be investigated were numerically generated from appropriate models. A representative mechanism was used with defined kinetic and thermodynamic parameters, and temperature programmed desorption and reaction and steady-state kinetics data were generated. The transient data were analyzed assuming the reaction was carried out over a uniform surface, ignoring the known surface heterogeneity, and an optimal set of parameters to describe the transient data were obtained. These parameters were then used in a uniform surface model to predict the steady-state rate that would be observed over a range of reaction conditions. The predicted uniform surface rate was in general a factor of 30 higher than the known nonuniform surface rate, indicating that kinetic and thermodynamic parameters obtained from transient studies alone are insufficient when surface nonuniformity is present. Furthermore, this lack of agreement between the predicted and experimental steady-state data may be one "kinetic fingerprint" of reactions on nonuniform catalytic surfaces.