Comparative study of the reducibility under H2 and CO of two thermally aged Ce0.62Zr0.38O2 mixed oxide samples

The redox behaviour under flowing 5%CO/He and 5%H2/Ar of two Ce0.62Zr0.38O2 mixed oxides is investigated. The samples, hereafter referred to as CZ-MO and CZ-SO, were obtained by application to the same starting material two well established thermal ageing routines. These routines ensured the preparation of oxide samples with significantly different chemical and structural properties. To evaluate their redox behaviour, ultimate oxygen storage capacity (OSC) data were determined from stepwise thermo-gravimetric experiments at temperatures ranging from 473 K to 1173 K. In good agreement with earlier studies from the literature, both CO-OSC and H2-OSC data show that CZ-MO exhibits a superior reducibility throughout the whole range of temperatures. If CO-OSC and H2-OSC data are compared, we may notice that, in qualitative agreement with the thermodynamic predictions, CO-OSC values are significantly larger than those of H2-OSC at T ≤ 773 K. Above this reference temperature CO-OSC and H2-OSC show much closer values. Though ultimate OSC data are generally assumed to be determined by the thermodynamics of the oxide-reductant system, it is shown that the CO-OSC values recorded at the lowest temperatures are not consistent with this assumption. As in the case of H2-OSC, the low-temperature CO-OSC data are actually determined by kinetic factors.