Structural Changes in Ce_0.5Zr_0.5O_2−δ under Temperature-Swing and Isothermal Solar Thermochemical Looping Conditions Determined by in Situ Ce K and Zr K Edge X-ray Absorption Spectroscopy

Abstract

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.

Tuning the Pt/CeO2 Interface by in Situ Variation of the Pt Particle Size

“…Time-resolved XAS experiments such as quick-scanning X-ray absorption fine structure (QEXAFS) spectroscopy (in the millisecond range) are therefore especially difficult . Only a few experiments have been reported, which investigate the dynamic structural changes in ceria or ceria-based materials: e.g., using dispersive XAS to monitor the oxygen storage and release in a Pt/CeO 2 -ZrO 2 catalyst or Ce K-edge (40433 eV) XAS during temperature cycles of CeO 0.5 Zr 0.5 O 2– x under realistic solar thermochemical reaction conditions for two-step CO 2 /H 2 O splitting as well as during periodic lean/rich cycles of a Cu/CeO 2 catalyst …”

“…Timeresolved XAS experiments such as quick-scanning X-ray absorption fine structure (QEXAFS) spectroscopy (in the millisecond range) are therefore especially difficult. 27 Only a few experiments have been reported, which investigate the dynamic structural changes in ceria or ceria-based materials: e.g., using dispersive XAS to monitor the oxygen storage and release in a Pt/CeO 2 -ZrO 2 catalyst 28 or Ce K-edge (40433 eV) XAS during temperature cycles of CeO 0.5 Zr 0.5 O 2−x under realistic solar thermochemical reaction conditions for twostep CO 2 /H 2 O splitting 29 as well as during periodic lean/rich cycles of a Cu/CeO 2 catalyst. 30 In the present study, we applied QEXAFS to acquire timeresolved data at the Pt L 3 -edge and Ce L 3 -edge under transient conditions (reducing/oxidizing cycles, which are denoted redox cycles in the following) to understand the interplay between the structure and oxidation state of both components.…”

Effect of oxygen storage materials on the performance of Pt-based three-way catalysts

“…The XRD peaks shifted to higher 2 θ values with increasing zirconia content, revealing the contraction of the CeO 2 crystal lattice. 63 The absence of peaks attributed to the ZrO 2 and CeO 2 phases and of asymmetric reflections appearing in the fresh and aged samples with increasing zirconia content indicates a homogeneous solid-solution. No peaks attributable to Pt species were observed in the fresh specimens, owing to the small crystallite size and uniform dispersion of Pt on the support.…”

In situ flow cell for combined X-ray absorption spectroscopy, X-ray diffraction, and mass spectrometry at high photon energies under solar thermochemical looping conditions

“…Element-specific information about the changes in the local structure, such as the electronic structure and oxygen coordination of the elements involved in the redox reactions, is obtained by means of X-ray absorption spectroscopy (XAS). 20,21 The crystal structure of the phases and phase changes in oxygen-storage materials can be determined under reaction conditions by X-ray diffraction (XRD). 17,22,23 Here, we present a reaction cell suitable for XAS and XRD measurements under relevant solar-reactor conditions which combines both techniques in a single experiment under identical conditions.…”

“…The design of the cell was related to our in situ cell for XAS in transmission mode. 20,21 The temperatures required for solar-driven thermochemical looping present a complex engineering problem. High temperature is a significant challenge for in situ structural characterization.…”