Infrared Study of Oxygen Adsorption and Activation on Cerium–Zirconium Mixed Oxides

Oxygen adsorption, activation, and exchange over a wide range of CexZr(1−x)O2 (x=0, 0.15, 0.5, 0.63, and 1) mixed oxides were monitored by FT-IR spectroscopy. Oxygen adsorption at room temperature on preoxidized samples results in the formation of surface superoxides (O2−) that are well characterized by a sharp band at 1126 cm−1. The variation along the oxide composition of the relative amount of superoxide species present at the surface, deduced from the integrated area of this νO–O vibration band, appeared to correlate with the oxygen storage capacity (OSC) of the oxide measured at 400°C. Furthermore, 16O2− superoxide species adsorbed on ceria–zirconia samples were shown to be very reactive and selectively exchangeable to 18O2− species via the so-called multiple exchange mechanism. The key role of superoxides as “initiators” and/or “intermediates” in the activation of oxygen prior to further diffusion on ceria-containing solids is proposed to explain the similarities observed between previous OSC measurements, 16O/18O isotopic exchange experiments, and the present FT-IR study results.