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      Splitting CO 2 with a ceria‐based redox cycle in a solar‐driven thermogravimetric analyzer

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          Abstract

          Thermochemical splitting of CO 2 via a ceria‐based redox cycle was performed in a solar‐driven thermogravimetric analyzer. Overall reaction rates, including heat and mass transport, were determined under concentrated irradiation mimicking realistic operation of solar reactors. Reticulated porous ceramic (RPC) structures and fibers made of undoped and Zr 4+‐doped CeO 2, were endothermally reduced under radiative fluxes of 1280 suns in the temperature range 1200–1950 K and subsequently re‐oxidized with CO 2 at 950–1400 K. Rapid and uniform heating was observed for 8 ppi ceria RPC with mm‐sized porosity due to its low optical thickness and volumetric radiative absorption, while ceria fibers with μm‐sized porosity performed poorly due to its opacity to incident irradiation. The 10 ppi RPC exhibited higher fuel yield because of its higher sample density. Zr 4+‐doped ceria showed increasing reduction extents with dopant concentration but decreasing specific CO yield due to unfavorable oxidation thermodynamics and slower kinetics. © 2016 American Institute of Chemical Engineers AIChE J, 63: 1263–1271, 2017

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          High-flux solar-driven thermochemical dissociation of CO2 and H2O using nonstoichiometric ceria.

          Because solar energy is available in large excess relative to current rates of energy consumption, effective conversion of this renewable yet intermittent resource into a transportable and dispatchable chemical fuel may ensure the goal of a sustainable energy future. However, low conversion efficiencies, particularly with CO(2) reduction, as well as utilization of precious materials have limited the practical generation of solar fuels. By using a solar cavity-receiver reactor, we combined the oxygen uptake and release capacity of cerium oxide and facile catalysis at elevated temperatures to thermochemically dissociate CO(2) and H(2)O, yielding CO and H(2), respectively. Stable and rapid generation of fuel was demonstrated over 500 cycles. Solar-to-fuel efficiencies of 0.7 to 0.8% were achieved and shown to be largely limited by the system scale and design rather than by chemistry.
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            Concentrating solar thermal power and thermochemical fuels

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              A thermochemical study of ceria: exploiting an old material for new modes of energy conversion and CO2 mitigation

              We present a comprehensive thermodynamic and kinetic analysis of the suitability of cerium oxide (ceria) for thermochemical fuel production. Both portions of the two-step cycle, (i) oxygen release from the oxide at 1773 and 1873 K under inert atmosphere, and (ii) hydrogen release upon hydrolysis at 1073 K, are examined theoretically as well as experimentally. We observe gravimetric fuel productivity that is in quantitative agreement with equilibrium, thermogravimetric studies of ceria. Despite the non-stoichiometric nature of the redox cycle, in which only a portion of the cerium atoms change their oxidation state, the fuel productivity of 8.5-11.8 ml of H(2) per gram of ceria is competitive with that of other solid-state thermochemical cycles currently under investigation. The fuel production rate, which is also highly attractive, at a rate of 4.6-6.2 ml of H(2) per minute per gram of ceria, is found to be limited by a surface-reaction step rather than by ambipolar bulk diffusion of oxygen through the solid ceria. An evaluation of the thermodynamic efficiency of the ceria-based thermochemical cycle suggests that, even in the absence of heat recovery, solar-to-fuel conversion efficiencies of 16 to 19 per cent can be achieved, assuming a suitable method for obtaining an inert atmosphere for the oxygen release step.
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                Author and article information

                Contributors
                aldo.steinfeld@ethz.ch
                Journal
                AIChE J
                AIChE J
                10.1002/(ISSN)1547-5905
                AIC
                Aiche Journal. American Institute of Chemical Engineers
                John Wiley and Sons Inc. (Hoboken )
                0001-1541
                1547-5905
                05 October 2016
                April 2017
                : 63
                : 4 ( doiID: 10.1002/aic.v63.4 )
                : 1263-1271
                Affiliations
                [ 1 ] Dept. of Mechanical and Process EngineeringETH Zurich Zurich 8092Switzerland
                [ 2 ] Laboratory of Materials for Energy ConversionEMPA Dübendorf 8600Switzerland
                [ 3 ]Institute for Geo‐ and Life Sciences, Crystallography, Albert‐Ludwigs‐Universität Freiburg Freiburg 79085Germany
                [ 4 ]Institute of Solar Research, German Aerospace Center Linder Höhe Köln 51147Germany
                [ 5 ] Dept. of Mechanical and Aerospace EngineeringUniversity of Florida Gainesville FL 32611
                Author notes
                [*] [* ]Correspondence concerning this article should be addressed to A. Steinfeld at aldo.steinfeld@ 123456ethz.ch .
                Article
                AIC15501
                10.1002/aic.15501
                5367271
                e9d1d8e3-f205-487a-aced-c423571701b3
                © 2016 The Authors AIChE Journal published by Wiley Periodicals, Inc. on behalf of American Institute of Chemical Engineers

                This is an open access article under the terms of the Creative Commons Attribution‐NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes.

                History
                : 27 June 2016
                : 26 August 2016
                Page count
                Figures: 7, Tables: 1, Pages: 9, Words: 5522
                Funding
                Funded by: Swiss Federal Office of Energy
                Award ID: SI/501213‐01
                Funded by: Helmholtz–Gemeinschaft Deutscher Forschungszentren (Virtuelles Institut SolarSyngas)
                Funded by: European Research Council
                Award ID: 320541
                Categories
                Reaction Engineering, Kinetics and Catalysis
                Reaction Engineering, Kinetics and Catalysis
                Custom metadata
                2.0
                aic15501
                April 2017
                Converter:WILEY_ML3GV2_TO_NLMPMC version:5.0.9 mode:remove_FC converted:23.03.2017

                solar fuels,doped ceria,thermochemical,kinetics,heat transfer

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