KINETICS AND MULTIPLE RATE STATES OF CO OXIDATION ON PT .2. LINKING UHV AND ATMOSPHERIC-PRESSURE KINETIC-BEHAVIOR

In this second part of a two-part study, two new kinetic models developed in the first part (Harold and Garske (1)) are used to simulate Pt-catalyzed CO oxidation over a wide range of catalyst temperatures, gas compositions, and total pressures. The models are modifications of the commonly used three-step sequence (model 1). Model II contains steps involving adsorbed molecular oxygen, and reaction steps between gas phase CO and adsorbed oxygen species. In model III an oxygen site exclusion feature replaces the two Eley-Rideal steps. A comparison of model predictions and experimental data obtained at pressures spanning UHV and atmospheric provides for sensitive discrimination between the models. The models are used to simulate multivalued UHV Pt(110)-catalyzed CO oxidation rate data (Sung (2)). Kinetic parameters are estimated using a novel nonlinear regression scheme involving both singular and nonsingular data. Models 11 and III simulate the data equally well. Agreement is good between model 11 and III predictions and the intermediate pressure data for Pt(100) (Berlowitz et al. (3)), and for polycrystalline Pt (Garske and Harold (4, 5)) using the estimated parameters from the UHV data simulation. Intrinsic rate multiplicity is predicted to persist at these pressures over a narrow range of temperature and gas composition. External mass transport resistance is incorporated into model 11 to simulate atmospheric pressure CO oxidation experiments. The predicted range of conditions giving multiplicity expands considerably. Several atmospheric pressure experimental trends are predicted, such as the upright cusp shape of the temperature-CO pressure bifurcation map for low CO/Oar2 observed in many studies, and the sharp transition in the extinction branch at the stoichiometric ratio observed by Kaul et al. (6). The latter trend is shown to occur for a class of much simpler single-valued kinetic models. The simulations of CO oxidation over a wide pressure range are shown to provide insight about bridging the gap between surface science and realistic operating conditions. © 1991.