Kinetic models of oxygen supply from CeOx to active nanometer particles of three-way catalysts

Nearly complete conversion of the reducing pollutants on the three-way catalysts (TWC), used for catalytic treatment of motor vehicle gas, is achieved if these species are exactly matched by oxides of nitrogen and oxygen. In practice, however, the air/fuel mixture oscillates near the balanced composition. These oscillations are partly compensated by introducing ceria into TWC. This function of ceria is based on its ability to store oxygen. We have derived kinetic equations describing different scenarios of oxygen transport from ceria to the Pt catalyst particles and used the surface-science based data to choose the kinetic parameters for the most appropriate schemes. Our analysis shows that the energy of oxygen atoms on Pt is nearly the same or only slightly above (from 0 up to 5 kcal/mol) than in the surface layer of ceria. The lowest possible value of the activation barrier, E-a, for jumps of oxygen atoms from ceria to Pt over the Pt-CeOx boundary is 10 kcal/mol. The rate of oxygen transport is strongly suppressed by the site-blocking effect on Pt. The effective rate constant calculated for this process is of realistic magnitude if E-a is about or higher than 25 kcal/mol. (C) 1998 Elsevier Science B.V. All rights reserved.