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 Ce 0.62 Zr 0.38 O 2 mixed oxide two alternative aging routines consisting of a reduction with H 2 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 (T redn e 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 H 2 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 temperatureprogrammed reduction diagrams recorded for the corresponding oxide-supported rhodium samples. By contrast, in the high-temperature reduction range (T redn g 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.

Ce–Zr–Sr ternary mixed oxides structural characteristics and oxygen storage capacity

“…It is inferred that different cation distributions may lead a significant Zr-O coordination orders, indicating that the oxygen migration performance is greatly affected by the random distribution of Zr-O bonds. This relationship between lattice distortion and the improved OSC is to some extent similar to the situation of pyrochlore-type structure of ceria-zirconia by redox treatment [35][36][37][38][39]. It is concluded pyrochlore-type ceria-zirconia is formed by filling of half of oxygen vacancies.…”

Effect of the thermal pre-treatments on ceria–zirconia redox properties: An Eu3+ luminescence study

“…A strong dependence of their redox properties from the pre-treatment of the material has been observed, even in the absence of bulk modification of their structural and/or textural properties [16].…”

“…However, the surface enrichment in the larger cation is not a sufficient requisite to promote the low temperature reduction: in fact, the samples C (after SO treatment) present a single high temperature reduction process. Surface segregation is evidently necessary to the formation of the pyrochlore-like species responsible of the low temperature reduction [16]. Their formation is effective during the subsequent TPR-MO treatment and they remain on the surface of the material until a new SO treatment is applied.…”

Reducing the chemical expansion coefficient in ceria by addition of zirconia

“…The reduced chemical expansion coefficient of PZCO and ZCO solid solutions has important ramications in SOFC electrodes as well as three-way catalysts for automotive applications, where the oxygen storage capacity of ZCO serves as a buffer for redox reactions. 10,20 In both cases, mechanical stability on redox cycling is important, as well as the enhanced oxygen deciency, or storage capacity. For example, with changes in power demand or during initial startup, large changes in oxygen activity at an SOFC anode occur as more or less steam is produced or the cell is exposed to fuel gas for the rst time, respectively, resulting in changes in d and hence expansion.…”