Nanocrystalline Cerium−Bismuth Oxides: Synthesis, Structural Characterization, and Redox Properties

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• 作者：Kripasindhu Sardar ; Helen Y. Playford ; Richard J. Darton ; Emma R. Barney ; Alex C. Hannon ; David Tompsett ; Janet Fisher ; Reza J. Kashtiban ; Jeremy Sloan ; Silvia Ramos ; Giannantonio Cibin ; Richard I. Wal
• 刊名：Chemistry of Materials
• 出版年：2010
• 出版时间：November 23, 2010
• 年：2010
• 卷：22
• 期：22
• 页码：6191-6201
• 全文大小：1157K
• 年卷期：v.22,no.22(November 23, 2010)
• ISSN：1520-5002

文摘

The hydrothermal oxidation of Ce³⁺ in basic aqueous solution by NaBi^VO₃ at 240 °C produces fine powders of cerium−bismuth oxides with crystallite sizes of less than 10 nm, as observed by transmission electron microscopy (TEM). Analysis of X-ray absorption near-edge structure at the Bi L_III and Ce L_III edges confirms the reduction of Bi⁵⁺ to Bi³⁺ and the oxidation of Ce³⁺ to Ce⁴⁺, consistent with a general chemical composition Ce^IV_1-xBi^III_xO_2-(x/2) (x ≤ 0.6) for the mixed oxides. The refined cubic lattice parameters from powder neutron diffraction indicate a fluorite-like solid solution with volume that increases with Bi content. Pair distribution functions derived from total neutron scattering yield estimated values for refined particle diameters that agree with TEM, showing a decreasing crystallite diameter with increasing bismuth content, and a full analysis reveals that despite average long-range structure being described well as a fluorite solid-solution, the local structure is distorted. Thus, the short-range structure can be simulated as a mixture of symmetric eight-coordinate Ce sites, as seen in CeO₂, and distorted Bi sites, as seen in β-Bi₂O₃. Temperature programmed reduction studies reveal a large hydrogen uptake in the mixed oxides, which is reproducible on a second subsequent cycle. In situ powder XRD under hydrogen flow, however, reveals that this is due to partial phase separation to give bismuth metal. After six cycles, a material of initial composition Ce_0.5Bi_0.5O_1.75 is completely phase-separated and its hydrogen uptake is reduced with high-resolution TEM analysis confirming the permanent reduction of the sample.