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

The hydrothermal oxidation of Ce3+ in basic aqueous solution by (NaBiO3)-O-V at 240 degrees 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 (Ce1-xBixO2-(x/2))-Bi-IV-O-III (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 beta-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 Ce0.5Bi0.5O1.75 is completely phase-separated and its hydrogen uptake is reduced with high-resolution TEM analysis confirming the permanent reduction of the sample.