Cerium LIII-edge EXAFS Investigation of the Structure of Crystalline and Amorphous Cerium Oxides
Cerium oxide solid samples were prepared via precipitation from aqueous solution of hydrous cerium- (III) nitrate in the presence of different percentages of hydrogen peroxide (H2O2) as model corrosion inhibiting coatings materials for aluminum alloys. X-ray absorption spectroscopy at the Ce LIII-edge was applied for the characterization of crystalline anhydrous CeO2, nanocrystalline hydrous CeO2, nanocrystalline CeO2 sample I precipitated in the presence of H2O2, and an amorphous CeO2 sample II precipitated at a higher H2O2 concentration. An analysis by X-ray absorption near-edge structure (XANES) for cerium oxides did not indicate a broad variation in cerium valence state in the precipitated samples as compared to anhydrous CeO2. Furthermore, XANES analysis revealed a decrease in the intensity of the white line peaks for the precipitated samples relative to those in anhydrous CeO2. The EXAFS spectra of the oxides showed that H2O2 reduced the precipitate's particle diameter and bulk crystallinity. Growth in the coordination number of the first, Ce-O, shell was observed with an increased bond distance RCe-O in hydrous and precipitated CeO2 samples. However, the coordination numbers of the second, Ce-Ce, and third, Ce-O, shells were reduced in comparison with anhydrous CeO2. Increasing concentration of H2O2 during alkaline precipitation of cerium oxides caused increased hydration, corresponding to a reduced outer-shell coordination number and reduced bulk crystallinity but no corresponding change in the cerium valence state.
A. M. Shahin et al., "Cerium LIII-edge EXAFS Investigation of the Structure of Crystalline and Amorphous Cerium Oxides," Chemistry of Materials, American Chemical Society (ACS), Dec 2004.
Air Force Research Laboratory (Wright-Patterson Air Force Base, Ohio)
United States. Department of Energy
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© 2004 American Chemical Society (ACS), All rights reserved.