Cerium L III -Edge XAS Investigation of the Structure of Crystalline and Amorphous Cerium Oxides Ahmed M. Shahin, † Fernande Grandjean, †,‡ Gary J. Long, † and Thomas P. Schuman* ,† Department of Chemistry and Graduate Center for Materials Research, UniVersity of MissourisRolla, Rolla, Missouri 65409-0010 and Department of Physics, B5, UniVersity of Lie` ge, B-4000 Sart-Tilman, Belgium ReceiVed May 13, 2004. ReVised Manuscript ReceiVed October 7, 2004 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 , nano- crystalline 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. Introduction As a consequence of changing properties, e.g., mechanical, magnetic, or electrical, the nanocrystalline phase of rare earth metal oxides has become of immense interest. 1-4 Of these, cerium oxides have attracted considerable research interest due to a diversity of applications. For instance, the basicity function of cerium oxide when combined with the hydro- genation property of a metal such as Pt or Pd denotes cerium oxide as a promising conversion catalyst. Thus, cerium oxide can lead to selective hydrogenation catalysis of unsaturated compounds. 5,6 Of modern interest is the ability of the cerium oxide to store and transport oxygen. The phenomenon is associated with a fast valence change in the solid, i.e., Ce IV T Ce III , and also with anionic vacancies, CeO 2 f CeO 2-x + (x/2)O 2 . 7,8 Hinton and co-workers 9 examined the inhibition of cor- rosion of high tensile strength aluminum alloys by rare earth metal salts. Immersion of aluminum alloy in CeCl 3 aqueous solution for several days formed a cerium-rich film and provided significant corrosion inhibition upon exposure to a corrosive NaCl solution. 10-13 Surface analysis revealed incorporation of cerium into compact protective surface films. Adding several weight percent of hydrogen peroxide to a CeCl 3 solution resulted in improved corrosion-resistant films. 14 Corrosion protection is attributed to the formation of cerium oxide or hydroxide films containing Ce III and Ce IV . 15,16 Cerium(IV) oxides detected in the coating were believed to arise from oxidation of Ce III in oxygenated * To whom correspondence should be addressed. E-mail: tschuman@umr.edu. † University of Missouri-Rolla. ‡ University of Lie`ge. (1) Winterer, M.; Nitsche, R.; Hahn, H. J. Phys. IV Fr. 1997, 7, C2- 1211. (2) Nitsche, R.; Winterer, M.; Groft, M.; Hahn, H. Nucl. Instrum. Methodol. B 1995, 97, 127. (3) Luca, V.; Djajanti, S.; Howe, R. F. J. Phys. Chem. B 1998, 102, 10650. (4) Chen, L. X.; Rajh, R.; Wang, Z.; Thurnauer, M. C. J. Phys. Chem. B 1997, 101, 10688. (5) Fierro, J. L. G.; Soria, J.; Sanz, J.; Rojo, M. J. J. Solid State Chem. 1987, 66, 154. (6) Sim, K. S.; Hilaire, L.; Le Normand, F.; Touroude, R.; Paul-Boncour, V.; Percheron-Guegan, A. J. Chem. Soc., Faraday Trans. 1991, 87, 1453. (7) Bunluesin, T.; Gorte, R. J.; Graham, G. W. Appl Catal. B 1997, 14, 984. (8) Sun, Y.; Sermon, P. A. J. Mater. Chem. 1996, 6, 1025. (9) Hinton, B. R. W.; Arnott, D. R.; Ryan, N. E. Met. Forum 1984, 7, 211. (10) Groshart, E. A. Designing for Finishing, Part IIsConversion Coating. Met. Finish. 1984, 82 (6), 69-70. (11) Seon, J. Less-Common Met. 1989, 148, 73-78. (12) Schuman, T. P. Protective Coatings for Aluminum Alloys. In Handbook of EnVironmental Degradation of Materials; Kutz, M., Ed.; William Andrew: New York, in press. (13) Schuman, T. P.; Shahin, A.; Stoffer, J. O. Proceedings of the International Waterborne, High Solids, and Powder Coating Sympo- sium 2002, 29, 371-382. (14) Wilson, L.; Hinton, B. R. W. A Method of Forming a Corrosion Resistant Coating. Patent WO 88/06639. (15) Hayes, S. A.; Yu, P.; O’Keefe, T. J.; O’Keefe, M. J.; Stoffer, J. O. J. Electrochem. Soc. 2002, 148, C253-C256. (16) Hughes, A. E.; Taylor, R. J.; Hinton, B. R. W.; Wilson, L. Surf. Interface Anal. 1995, 23, 540-550. 315 Chem. Mater. 2005, 17, 315-321 10.1021/cm0492437 CCC: $30.25 © 2005 American Chemical Society Published on Web 12/16/2004 Downloaded via MISSOURI UNIV SCIENCE & TECHNOLOGY on August 12, 2024 at 13:52:19 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.