On the mechanism of fast oxygen storage and release in ceria-zirconia model catalysts

CO oxidation over fresh and aged CexZr1-xO2 (0.15 <= x <= 1) mixed oxides was carried out under oscillating feed-stream composition by feeding the reactor with CO, He and O2 at a frequency of 0.2 Hz. The activity of fresh, high surface area (HSA) catalysts of different compositions is similar and depends mainly on exposed surface redox centers; on the contrary, aged, low surface area (LSA) samples of ceria-zirconia show larger differences of activity with the composition. The sequence and the shape of the product peaks suggest that the kinetics of oxygen storage/release is influenced by surface phenomena that are not taken into account when the reaction is carried out under stationary condition. In particular, in HSA samples the CO2 peak splits into two components. The first component is observed when CO is pulsed over the catalyst, while the second component appears when O2 is pulsed. Temperature and composition of the catalyst determine the intensity ratio of the two peaks. The first peak increases with the amount of zirconia and is the result of direct reaction of CO with the oxygen from the oxide. The second contribution to CO2 production is highly influenced by surface-area and temperature and may be related to a more complex surface dynamic. In situ diffuse reflectance infrared (DRIFT) spectra show that chemisorbed carbonate-like species are involved as intermediates. On the basis of these results, a mechanism and a model for CO oxidation is proposed.