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Incorporation of square-planar Pd

DOI: 10.1039/C5TA02007G DOI Help

Authors: Craig Hiley (University of Warwick) , Janet M. Fisher (Johnson Matthey Technology Centre) , David Thompsett (Johnson Matthey Technology Centre) , Reza J. Kashtiban (University of Warwick) , Jeremy Sloan (University of Warwick) , Richard Walton (University of Warwick)
Co-authored by industrial partner: Yes

Type: Journal Paper
Journal: Journal Of Materials Chemistry A , VOL 3 (24) , PAGES 13072 - 13079

State: Published (Approved)
Published: May 2015
Diamond Proposal Number(s): 8708

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

Subject Areas: Materials, Chemistry

Instruments: B18-Core EXAFS

Added On: 05/08/2015 09:12

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