Incorporation of square-planar Pd2+ in fluorite CeO2: hydrothermal preparation, local structure, redox properties and stability

摘要: The direct hydrothermal crystallisation at 240 °C of Pd2+-containing ceria is investigated to study the extent which precious metal dopants may be introduced into cubic fluorite lattice. Samples composition Ce1−xPdxO2−δ, where 0 ≤ x 0.15 can produced in Pd included within CeO2 structure give a linear lattice expansion. Attempts produce higher Pd2+-substitution result formation PdO as secondary phase. Ce and were determined +4 +2 oxidation states, respectively, by X-ray absorption near edge structure, suggesting oxide deficiency mechanism charge balance. Extended fine (EXAFS) analysis K-edge reveals that Pd2+ has local square-planar coordination, expected, structural model fitted average maintained, but with sitting square faces ions present geometry Ce. This model, consistent previous modelling studies, gives an excellent fit EXAFS spectra, explains observed Transmission electron microscopy shows well dispersed nanocrystalline particles, situ powder XRD upon heating air samples remain stable up 800 °C. H2-TPR Pd-substitution leads low temperature (<200 °C) reduction oxide, increases magnitude increasing Pd-substitution. On prolonged heating, however, lost from metal, inherent instability Pd-doped CeO2.