Kinetics and multiple rate states of CO oxidation on Pt I. Model development and multiplicity analysis

Abstract

This first of a two-part study focuses on the kinetics and rate multiplicity features of CO oxidation on Pt. The objective is to formulate models that predict the kinetics and rate multiplicity features observed by experimentalists over a wide range of catalyst temperature, gas composition, and total pressure. It is demonstrated that the commonly used three-step sequence (model I), consisting of reversible CO adsorption, dissociative oxygen adsorption, and a Langmuir reaction step, capably predicts multiplicity features observed under UHV conditions, such as the shape of the multiplicity region (bifurcation map) in the catalyst temperature-CO pressure plane. A method is presented in which parameter space is divided into regions in which different shapes of temperature-CO pressure bifurcation maps are encountered. This scheme provides a useful means of checking if a model can predict the qualitative multiplicity features, and of bounding parameter values based on the shape of the bifurcation map. It is shown that model I cannot predict the correct CO reaction order in the limits of low and high CO pressure, or the apparent activation energy in the CO inhibition regime. Thus, prediction of the multiplicity features alone is insufficient. Four new kinetic models built upon the model I framework are developed and analyzed. Model II contains three additional steps involving molecularly adsorbed oxygen and reaction steps between gas phase CO and adsorbed oxygen species. Oxygen may adsorb by a direct dissociative route or by a two-step molecular precursor route. Model III consists of the three model I steps, the three model II molecular oxygen steps, and an oxygen site exclusion feature. Model IV incorporates site exclusion for CO and oxygen into the three model I steps. Model V has a structure similar to model II, the main difference being that oxygen adsorption proceeds only by an equilibrated molecular adsorption step and irreversible dissociation. Our analyses show that the three steps involving molecular oxygen (models II, III, and V) or CO site exclusion (model IV) remedy the incorrect model I predictions in the CO inhibition regime. Moreover, the Eley-Rideal type reactions (models II and V) or oxygen site exclusion (models II and IV) remedy the incorrect model I predictions of the CO reaction order for the oxygen covered surface. Bifurcation map constructions also demonstrated at each model can predict the correct qualitative multiplicity features.