Spectroscopic Characterization of Heterogeneity and Redox Effects in Zirconium-Cerium (1:1) Mixed Oxides Prepared by Microemulsion Methods

摘要

Two Zr-Ce (1:1 molar ratio) mixed oxide specimens, made by the same microemulsion method from different cerium precursor salts (samples ZC1 and ZC2), have been studied by a combination of physicochemical techniques. After calcination at 773 K (high surface are materials HS: S_(BET) = 96 ± 1 m~2g~(-1) both samples present similar characteristics in XRD, Ramanand TEM (pseudocubic phase t"), and XPS/Ar~+-etching experiments. This latter evidence for those materials gives a similar moderate surface enrichment in cerium and a surface anion vacancy concentration (judged from O(1s) peak shifts) lower than in CeO_2; the latter effect suggests an easy diffusion of vacancies to bulk or subsurface regions. Only EPR of adsorbed superoxide species detects a difference between both samples, evidencing in ZC2-HS the presence of small, more reducible Ce-rich bidimensional patches over a Zr-richer substrate. According to XPS/Ar~+ etching profiles, by calcination at 1173 K (LS materials; S_(BET) < 8 m~2g~(-1)), surface segregation of small (ca. 3 nm) Ce-rich particles, not yet distinguishable by Raman or diffraction data and able to sustain at the surface local anion vacancy concentrations higher than in the other specimens (closer to the behavior of pure CeO_2), occurs on the ZC2 material; the ZC1 specimen, in contrast, actually becomes more homogeneous upon calcination at 1173 K. The short-scale heterogeneity indicated by EPR for sample ZC2-HS, and not for ZC1-HS, is presumed to act as a nucleus favoring formation of this new phase upon calcination. The data obtained evidence the ability of EPR and XPS/Ar~+-etching methods to reveal in these materials heterogeneity differences, not detectable by the other techniques, leading to higher reducibility of the Ce-rich surface domains in the ZC2 materials. The results are discussed in connection with the oxygen storage and buffering properties of these mixed oxides.