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

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.