Redox Behavior of Thermally Aged Ceria−Zirconia Mixed Oxides. Role of Their Surface and Bulk Structural Properties

Abstract

The relationship existing between aging conditions, redox behavior, and surface/bulk structural properties of two thermally aged ceria−zirconia mixed oxides, CZ-MO and CZ-SO, is analyzed. The samples were prepared by applying to a fresh Ce0.62Zr0.38O2 mixed oxide two alternative aging routines consisting of a reduction with H2 at 1223 K (5 h), followed by either a mild, CZ-MO, or severe, CZ-SO, re-oxidation treatment. By combining high-resolution electron microscopy and a number of chemical charcterization techniques, it is shown that the nanostructure of the aged oxides, specifically the total amount and surface presence of the phase exhibiting an ordered cationic sublattice (κ-like phase), is a key factor in determining their redox response. In the low-temperature reduction range (Tredn ≤ 773 K), the enhanced reducibility of the CZ-MO sample is proposed to be kinetically controlled by its surface structure mainly consisting of the κ-like phase. In accordance with the reported results, the surface activation of the H2 molecule, much faster on the CZ-MO sample, is proposed to be the rate controlling step of the overall reduction process. This proposal was further confirmed by the dramatic downward shift observed in the temperature-programmed reduction diagrams recorded for the corresponding oxide-supported rhodium samples. By contrast, in the high-temperature reduction range (Tredn ≥ 973 K), the observed difference of reducibility, higher in the case of the CZ-MO sample, is interpreted as due to thermodynamic factors related to the nature of the predominant cationic sublattice structure, ordered for CZ-MO and disordered in the case of the CZ-SO sample.