Hiley, Craig I., Fisher, Janet M., Thompsett, David, Kashtiban, Reza J., Sloan, Jeremy and Walton, Richard I. (2015) Incorporation of square-planar Pd2+ in fluorite CeO2 : hydrothermal preparation, local structure, redox properties and stability. Journal of Materials Chemistry A, Volume 3 (Number 24). pp. 13072-13079. doi:10.1039/c5ta02007g ISSN 2050-7488.

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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.

Item Type:	Journal Article
Subjects:	Q Science > QD Chemistry
Divisions:	Faculty of Science, Engineering and Medicine > Science > Chemistry
Library of Congress Subject Headings (LCSH):	Cerium oxides, Oxidation-reduction reaction
Journal or Publication Title:	Journal of Materials Chemistry A
Publisher:	Royal Society of Chemistry Publications
ISSN:	2050-7488
Official Date:	28 June 2015
Dates:	Date Event 28 June 2015 Published 26 May 2015 Available 21 May 2015 Accepted 18 March 2015 Submitted
Volume:	Volume 3
Number:	Number 24
Number of Pages:	8
Page Range:	pp. 13072-13079
DOI:	10.1039/c5ta02007g
Status:	Peer Reviewed
Publication Status:	Published
Access rights to Published version:	Open Access (Creative Commons)
Date of first compliant deposit:	30 December 2015
Date of first compliant Open Access:	30 December 2015
Funder:	Johnson Matthey Plc., Advantage West Midlands (AWM), European Regional Development Fund (ERDF)

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