KINETIC-STUDY OF THE CO OXIDATION OVER PT GAMMA-AL2O3 AND PT/RH/CEO2/GAMMA-AL2O3 IN THE PRESENCE OF H2O AND CO2/

The oxidation of CO by O-2 was Studied for a Pt/gamma-Al2O3 catalyst a nd for a commercially available Pt/Rh/CeO2/gamma-Al2O3 three-way catal yst. Kinetic experiments were carried out in an isothermal fixed-bed m icroreactor under intrinsic conditions, i.e., in the absence of mass a nd heat transfer limitations, in the temperature range from 436 to 503 K, with CO and O-2 inlet partial pressures between 0.12 and 8.3 kPa a nd H2O and CO2 inlet partial pressures between 0 and 10 kPa, For the P t/gamma-Al2O3 catalyst, the CO2 production rate was found to be essent ially proportional to the oxygen and inversely proportional to the car bon monoxide partial pressures, although at large CO and small O-2 par tial pressures deviations occur, A kinetic model, based on elementary reaction steps, was constructed, It was concluded that for the experim ental conditions considered, the noble metal surface is almost complet ely covered with CO, the CO adsorption being in quasi-equilibrium, and that irreversible molecular adsorption of oxygen is the rate-determin ing step, followed by potentially instantaneous dissociation. The pres ence of steam was found to enhance the reaction rate, For the experime nts carried out over Pt/Rh/CeO2/gamma-Al2O3 in the presence 10 kPa H2O and 10 kPa CO2, it was found that the CO2 production rate becomes zer o order in CO at high CO partial pressures. The partial reaction order in O-2 is approximately 0.5, The experimental observations were expla ined by the existence of a second bifunctional reaction path next to t he reaction path catalyzed by the noble metal only, The bifunctional r eaction path involves a reaction between CO adsorbed on the noble meta l and oxygen from ceria at the noble metal/ceria interface, The experi ments could be described adequately over the investigated range of con ditions by a kinetic model incorporating the mono- and bifunctional re action paths, For the quantification and understanding of the changes in the partial reaction orders in CO and O-2 as a function of the expe rimental conditions, a kinetic model based on elementary reaction step s is necessary. (C) 1997 Academic Press.