Effects of thermal pretreatment on the redox behaviour of Ce0.5Zr0.5O2: isotopic and spectroscopic studies

Abstract

The temperature programmed reduction (TPR) of a single phase Ce_0.5Zr_0.5O₂ solid solution is investigated by using ¹H₂/Ar and ²H₂/Ar mixtures as reducing agents. The effects of mild (700 K) or severe (1273 K) pre-oxidation (MO, SO) on the TPR profile are analysed. It is observed that after SO pretreatment hydrogen activation/scrambling occurs at about 600 K and the reduction profile is characterised by a single reduction peak at 920 K. In contrast, a TPR/MO pretreatment of such SO sample leads to a facile hydrogen activation/scrambling and low temperature reduction profiles, the main reduction peak being observed at ca. 650 K. Using ¹H₂ and ²H₂ as reducing agents, an isotopic effect is observed for the reduction processes with a peak occurring above 700 K, while equal activation energies are measured for those occurring below 700 K. This suggests that under the TPR conditions the surface reduction steps (activation of the reducing agent at the surface, surface reduction steps and water evolution) are not rate limiting for the low temperature reduction process, which is controlled by bulk diffusion, while some surface step (probably water formation) is limiting for the high temperature reduction. Studies by EPR on the nature of the oxygen species adsorbed on variously pretreated specimens and by FTIR of the methoxy species formed upon methanol adsorption on the same materials reveal that the main changes induced by high temperature calcination are related to segregation of ceria- and zirconia-type islands, with some surface enrichment in cerium probably occurring as well. Only subtle differences between SO and MO pretreated samples can be detected at either surface or bulk (XRD experiments) levels, in contrast with the significant modifications produced by the different pretreatments on the reduction characteristics of the sample.