Some recent results on the correlation of nano-structural and redox properties in ceria-zirconia mixed oxides

Some recent results on the redox behaviour of thermally aged ceria-zirconia mixed oxides with Ce/Zr molar ratios typically ranging from 50150 to 70/30 are briefly reviewed. In accordance with them, a tentative model allowing us to correlate ageing conditions, surface and bulk nano-structural properties of the oxides, and changes occurred in their redox behaviour is proposed. As revealed by the analysis of appropriate chemical studies and the nano-structural information provided with High Resolution Transmission (HREM) and High Angle Annular Dark Field-Scanning Transmission (HAADF-STEM) electron microscopies, the presence/absence of a pyrochlore-related K-phase in the aged oxides plays a key role in determining their redox response. In the low-temperature region (T-red <= 773 K), the enhanced reducibility exhibited by the oxide resulting from a high-temperature reduction/mild re-oxidation ageing cycle (SR-MO sample) is interpreted as due to kinetic reasons, the occurrence of the K-like phase in its surface being responsible for a faster H-2 chemisorption, the rate controlling step of the overall reduction process. By contrast, in the high-temperature range (T-red >= 973 K), the observed differences of reducibility would have a thermodynamic origin, which may be correlated with the total amount Of K-like phase present in the aged sample. (C) 2007 Elsevier B.V. All rights reserved.