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 (H 2 O 2 ) as model corrosion inhibiting coatings materials for aluminum alloys. X-ray absorption spectroscopy at the Ce L III -edge was applied for the characterization of crystalline anhydrous CeO 2, nanocrystalline hydrous CeO 2, nanocrystalline CeO 2 sample I precipitated in the presence of H 2 O 2, and an amorphous CeO 2 sample II precipitated at a higher H 2 O 2 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 CeO 2 . Furthermore, XANES analysis revealed a decrease in the intensity of the white line peaks for the precipitated samples relative to those in anhydrous CeO 2 . The EXAFS spectra of the oxides showed that H 2 O 2 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 R Ce - O in hydrous and precipitated CeO 2 samples. However, the coordination numbers of the second, Ce−Ce, and third, Ce−O, shells were reduced in comparison with anhydrous CeO 2 . Increasing concentration of H 2 O 2 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.
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Shahin et al. (2004) studied this question.
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