Structural Changes in Ce0.5Zr0.5O2-δ under Temperature-Swing and Isothermal Solar Thermochemical Looping Conditions Determined by in Situ Ce K and Zr K Edge X-ray Absorption Spectroscopy

Structural changes in an equimolar ceria-zirconia solid solution were determined from room temperature to 1773 K under typical reducing and oxidizing conditions in a reactor for the solar thermochemical dissociation of water or carbon dioxide in a two-step redox cycle. We report the first in situ X-ray absorption spectroscopy (XAS) measurements at both the Ce K and Zr K edges under the conditions of thermochemical carbon dioxide splitting and a temperature swing of 1773 to 1073 K and isothermal conditions at 1773 K. The shift in the Ce K absorption edge reveals quantitative information about changes in the electronic structure of cerium. The maximum extent of reduction at 1773 K was 53% +/- 5%; about 9% +/- 5% of the cerium atoms changed valence from 4+ to 3+ and vice versa during isothermal looping in a flow of argon and carbon dioxide, respectively. Zr K edge XANES indicated a transformation of the oxygen coordination of zirconia to a more centrosymmetric cubic geometry upon reduction. During isothermal cycling at 1773 K, there were no visible changes in the zirconium-oxygen coordination geometry.