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Issue 85, 2017
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Structural changes in equimolar ceria–hafnia materials under solar thermochemical looping conditions: cation ordering, formation and stability of the pyrochlore structure

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Abstract

Equimolar ceria–hafnia oxides form a pyrochlore structure Ce2Hf2O7, which exhibits an ordered arrangement of Ce3+ and Hf4+ cations under the reducing conditions of a solar thermochemical looping reactor for the two-step dissociation of water or carbon dioxide. The ceria–hafnia pyrochlore phase was prepared from oxidized ceria–hafnia powders by chemical reduction in a flow of H2/He and by auto-reduction in a flow of Ar at up to 1825 K. Full conversion of Ce4+ to Ce3+ was confirmed by thermogravimetric analysis and Ce K edge X-ray absorption spectroscopy. X-ray diffraction and Hf K edge X-ray absorption spectroscopy identified the pyrochlore phase. The dynamics of the structural changes were determined by time-resolved in situ Ce K edge X-ray absorption spectroscopy and in situ X-ray diffraction. Under the oxidizing conditions of the regeneration step of isothermal carbon dioxide splitting at 1800 K, the pyrochlore transformed to a mixture of fluorite-type or tetragonal ceria and monoclinic and orthorhombic hafnia phases. A κ-Ce2Hf2O8 phase, an oxidized form of the pyrochlore with an ordered arrangement of cations, was not detected.

Graphical abstract: Structural changes in equimolar ceria–hafnia materials under solar thermochemical looping conditions: cation ordering, formation and stability of the pyrochlore structure

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Article information


Submitted
21 Aug 2017
Accepted
06 Nov 2017
First published
22 Nov 2017

This article is Open Access

RSC Adv., 2017,7, 53797-53809
Article type
Paper

Structural changes in equimolar ceria–hafnia materials under solar thermochemical looping conditions: cation ordering, formation and stability of the pyrochlore structure

M. Rothensteiner, A. Bonk, U. F. Vogt, H. Emerich and J. A. van Bokhoven, RSC Adv., 2017, 7, 53797
DOI: 10.1039/C7RA09261J

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