In Situ X-ray Absorption Spectroscopy and X-ray Diffraction of Non-stoichiometric Ceria-based Oxides for Two-step Solar Thermochemical Fuel Production
Embargoed until 2020-09-07
- Doctoral Thesis
The adverse effects of anthropogenic climate change and the detrimental environmental impact of fossil fuels urge a transition to sustainable, environmentally benign primary energy supplies. Solar power has the highest potential of all renewable energy sources for the production of clean energy vectors. However, solar radiation is a rather dilute energy source, that is intermittent and unevenly distributed on the surface of the earth. The energy-efficient conversion and the storage of solar energy in chemical bonds by driving two-step thermochemical cycles with high-temperature heat from concentrated solar radiation requires new high-performance oxygen storage materials. Two-step thermochemical cycles enable the dissociation of water or carbon dioxide and the production of hydrogen, carbon monoxide or synthesis gas, which can be further processed to hydrocarbons. State-of-the-art solar thermochemical reactors require stable, solid, non-volatile materials with a high oxygen storage capacity and favorable thermodynamics. Thermochemical looping of non-stoichiometric ceria-based oxygen storage material is performed at a typical reaction temperature of up to 1773 K, which makes the determination of structure-property relationships under relevant conditions very challenging. The focus of this thesis is the in situ characterization of non-stoichiometric ceria-based materials under relevant reaction conditions using hard X-rays and the experimental capabilities of beamline BM01B at the European Synchrotron Radiation Facility in Grenoble, France. A suitable setup for fast thermochemical cycling, consisting of an automated gas and vapor dosing system, a high-temperature furnace and a quadrupole mass spectrometer for the quantification of oxygen and hydrogen was built and optimized. A cell for in situ X-ray absorption spectroscopy and a cell for quasi-simultaneous in situ X-ray absorption spectroscopy and X-ray diffraction in transmission mode were developed. In situ X-ray absorption spectroscopy at the Ce K edge enables time-resolved determination of the average electronic structure of cerium by measuring the shift in the energy of the absorption edge, which is proportional to the non-stoichiometry of a compound. At relevant process conditions – reduction at 1773 K and oxidation at 1073 K – analysis of the X-ray absorption fine structure is limited to the near edge region due to strong thermal damping of the extended X-ray absorption fine structure. X-ray absorption spectroscopy in transmission mode gives access to the absorption edges of heterocations that are introduced into the material to tune its properties with the aim to improve the overall efficiency of the process. Structural changes in equimolar ceria-zirconia were determined by Ce K and Zr K edge X-ray absorption near edge structure at isothermal carbon dioxide splitting conditions at 1773 K. Features of the Zr K edge X-ray absorption near edge structure indicated changes to a more centrosymmetric oxygen coordination upon reduction of the initial structure of the mixed oxide obtained by sintering at 1873 K in air. Time-resolved in situ X-ray diffraction of equimolar ceria-hafnia indicated the formation of the pyrochlore-type structure, an ordered arrangement of the cations by reducing pressed pellets of powders prepared by polymerized-complex method and calcination. Under isothermal carbon dioxide splitting conditions, cation ordering was not stable, the ordered phase gradually disappeared and a secondary phase, monoclinic hafnia formed in the oxidation step. Hf K edge X-ray near edge absorption spectroscopy of equimolar ceria-hafnia indicated structural changes and Hf K edge extended X-ray absorption fine structure of the reduced material at room temperature agreed with the pyrochlore-type structure. The possibility of direct production of hydrocarbons in a two-step solar driven thermochemical cycle using rhodium-ceria and nickel-ceria was evaluated in durability tests. While rhodium-ceria converted water and carbon dioxide into oxygen, hydrogen, carbon monoxide and traces of methane, nickel-ceria was not stable under thermochemical looping conditions with reduction at 1673 K and oxidation at 773 K. Diffraction is the method of choice for the determination of the bulk properties and crystal structure of oxygen storage materials at high temperature. X-ray absorption spectroscopy provides valuable complementary information of the average oxidation state of cerium and the electronic and local geometric structure of heterocations. New materials for solar-driven thermochemical fuel production and the opportunities of in situ X-ray absorption spectroscopy and X-ray diffraction as well as alternative characterization techniques are discussed Show more
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ContributorsExaminer: van Bokhoven, Jeroen A.
Examiner: Wokaun, Alexander
SubjectThermochemical redox cycle; In situ X-ray absorption spectroscopy; In situ X-ray diffraction; Ceria; Pyrochlores; High-temperature chemistry; SOLAR ENERGY USE (ENERGY TECHNOLOGY); Ceria-zirconia; ceria-hafnia; synchrotron radiation; Concentrated solar power (CSP); Water splitting; CARBON DIOXIDE (INORGANIC CHEMISTRY); WATER AND WATER CHEMISTRY (INORGANIC CHEMISTRY); Hydrogen production; HYDROGEN (FUEL TECHNOLOGY)
Organisational unit01538 - DR Chemieingenieurwissenschaften
03746 - Van Bokhoven, Jeroen A.
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