Low-Temperature Reducibility of M_xCe_1–xO₂ (M = Zr, Hf) under Hydrogen Atmosphere

Abstract

Redox cycles utilize the reversible oxygen release/uptake of cerium oxide in a variety of renewable energy applications such as fuel cells, water gas shift reactions, and solar thermochemical fuel production. For all applications the degree of reduction/oxidation determines the overall performance. In this study we report on the redox behavior of M x Ce1–x O2−δ (M = Zr, Hf; x = 0, 0.15, 0.2) solid solutions monitored by high-temperature in situ X-ray diffraction. During reduction in H2 at 600 °C and the successive formation of Ce3+ and oxygen vacancies, the lattice of ceria expands up to 0.3%. The lattice expansion of hafnium-doped ceria samples is 4 times larger than in zirconium-doped or undoped ceria, indicating drastically higher extents of oxygen vacancy formation. The same trends are validated using temperature-programmed reduction measurements. Complete reoxidation of the M x Ce1–x O2−δ solid solutions in air at 600 °C is reflecting the reversibility of the redox process. Scanning electron microscopy and X-ray diffraction analysis before and after redox cycling indicate phase and microstructural stability of all compositions during reduction.

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

“…Ceria forms solid solutions of the fluorite-type structure and tetragonal and monoclinic phases with zirconia and hafnia. 22,51,59,60 There were no observable reflections indicating secondary phases. In contrast to the diffraction patterns recorded with an unfocused beam, the diffraction patterns recorded with a focused beam (Fig.…”

“…Thermal expansion as well as the release of oxygen and the formation of oxygen vacancies led to an expansion of the fluorite-type crystal lattice. 22…”

“…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.…”

“…The sample, a pellet of typically 5 mm diameter and less than 200 µm thick, was introduced and the second alumina tube kept the pellet in vertical position. Pellets of 5 mm in diameter were made with a pellet press (1 ton) (msscientific Chromatographie-Handel GmbH) and consisted of 30 mg Ce 0.5 Hf 0.5 O 2−δ (Hf50) powder and 17 mg ceria prepared by Pechini-type synthesis and calcination at 973 K. 22,51 At the beamline, the in situ cell and the IR furnace were mounted to an aluminum profile on an xyz sample stage. The inner tube of the cell was aligned and was placed in the center of the hot zone of the furnace.…”

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

“…Considerable effort has been made to tune the oxygenstorage properties of ceria by introducing heterocations into the uorite-type structure to increase the energy conversion efficiency of this process. [9][10][11][12][13][14][15][16][17] The fuel yields of ceria-zirconia and ceria-hafnia solid solutions are very similar [18][19][20] and higher than the fuel yield of pure ceria. [21][22][23][24][25][26][27][28] Zirconium and hafnium have a similar chemistry and their oxides are isomorphs.…”