A two-step reaction mechanism of oxygen release from Pd/CeO2: Mathematical modelling based on step gas concentration experiments

Abstract

A mathematical model was developed to study the transient release of oxygen from a 1 wt % Pd/CeO2 catalyst in the 450°-550°C range based on alternate step concentration switches between CO and O2. The proposed mathematical model allows the calculation of the transient rates of CO oxidation and back-spillover of oxygen during the transient period. A comparison of the two rates showed that as the reaction temperature increases from 450° to 550°C, the relative ratio of the rate of CO oxidation to that of back-spillover of oxygen decreases by a factor of 2.6. It also allows the estimation of the transient rates of the CO oxidation reaction and or the back-spillover of oxygen process. It also allows the calculation of the intrinsic rate constant k1 (s-1) of the Eley-Rideal step for the reaction of gaseous CO with surface oxygen species of PdO to form CO2. An activation energy of 10.1 kJ/mol was estimated for this elementary reaction step. An apparent rate constant k2/app (S-1) was estimated for the process of back-spillover of oxygen. The presented two-step kinetic scheme and the mathematical model derived could be extended to cover the case of two oxygen storage pools in the support material, the case in commercial three-way catalysts (TWC), e.g., CeO2 and CexZr1x O2), and also on the metal surface (e.g., Pd-Rh TWC). In addition, further chemical reaction steps might be needed to describe the transient kinetics of oxygen release in a much wider reaction temperature range (e.g., 200°-800°C).