A novel dynamic kinetic model of oxygen isotopic exchange on a supported metal catalyst
Résumé
A time-resolved kinetic analysis has been developed for modeling experimental results of (18)O/(16)O isotopic exchange over oxide-supported metal catalysts. Model is based on two very important points: (1) the parallel calculation of surface and bulk diffusion and (2) the implication of certain 0 species such as superoxides. The model includes adsorption-desorption processes on metal clusters and oxygen spillover from the metal to the surface of support and vice versa. Different mechanisms of exchange were also taken into account via mononuclear (O atoms, O(-), OB) or binuclear (superoxides) oxygen species. A refined model taking into account surface diffusion, direct exchange on surface of support by binuclear oxygen species and bulk diffusion was also developed. Kinetic (reaction rates and diffusion coefficients) as well as thermodynamic parameters (activation energies) were derived by fitting theoretical and experimental curves of (18)O(2), (18)O(18)O and (16)O(2) gas phase concentrations versus time. The experimental results of Pt/CeZrO(2) catalyst samples obtained in the 200-450 degreesC range of temperatures are examined. The refined model provides a very good fitting of the kinetic curves recorded with ceria-zirconia-supported catalysts. Moreover, values of diffusion coefficients and activation energies are in good agreement with already published values found by other methods. For a better understanding of all the steps of exchange, the kinetics of 180 and 160 distribution on the surface of metal clusters and on the surface of support are calculated and analyzed. On the basis of this model, a computer code is developed for analysis and calculations of kinetic and thermodynamic parameters of automotive catalysts. (C) 2004 Elsevier B.V. All rights reserved.