Infrared study of oxygen adsorption and activation on cerium-zirconium mixed oxides
Résumé
Oxygen adsorption, activation, and exchange over a wide range of CexZr((1-x))O-2 (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 (O-2(-)) that are web 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 nu (o-o) vibration band, appeared to correlate with the oxygen storage capacity (OSC) of the oxide measured at 400 degreesC. Furthermore,O-16(2)- superoxide species adsorbed on ceria-zirconia samples were shown to be very reactive and selectively exchangeable to O-18(2)- 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, O-16/O-18 isotopic exchange experiments, and the present FT-IR study results. (C) 2000 Academic Press.