CO 2 Splitting via Two-Step Solar Thermochemical Cycles with Zn/ZnO and FeO/Fe 3 O 4 Redox Reactions II: Kinetic Analysis P. G. Loutzenhiser, † M. E. Ga ´lvez, ‡ I. Hischier, ‡ A. Stamatiou, † A. Frei, † and A. Steinfeld* ,†,‡ Solar Technology Laboratory, Paul Scherrer Institute, 5232 Villigen PSI, Switzerland, and Department of Mechanical and Process Engineering, ETH Zurich, 8092 Zurich, Switzerland ReceiVed December 30, 2008. ReVised Manuscript ReceiVed March 7, 2009 Two-step thermochemical cycles for splitting CO 2 with Zn/ZnO and FeO/Fe 3 O 4 redox pairs using concentrated solar energy are considered. Thermogravimetric-based kinetic analyses were performed for the reduction of CO 2 to CO with Zn and FeO. Both reactions are characterized by an initial fast interface-controlled regime followed by a slow diffusion-controlled regime, which are described using a shell-core kinetic model. In the interface-controlled regime, a power rate law is applied with apparent activation energies 113.7 and 73.4 kJ mol -1 , and corresponding reaction orders 0.339 and 0.792, for the Zn/CO 2 and FeO/CO 2 systems, respectively. In the diffusion-controlled regime, limited by the ion mobility through the oxide shells, the apparent activation energies are 162.3 kJ mol -1 for Zn/CO 2 and 106.4 kJ mol -1 for FeO/CO 2 . Additional reaction mechanisms above the Zn melting point for Zn/CO 2 reactions are postulated. 1. Introduction In a previous work, 1 thermodynamic analyses were performed for two-step thermochemical cycles based on the Zn/ZnO and FeO/Fe 3 O 4 redox reactions that chemically reduce CO 2 to CO or C(s) using concentrated solar energy. The first step of the cycle is the solar-driven endothermic reduction of the metal oxide, represented by: ZnO f Zn + 0.5O 2 ΔH 298K ° ) 350 kJ mol -1 (1) or Fe 3 O 4 f 3FeO + 0.5O 2 ΔH 298K ° ) 317 kJ mol -1 (2) The second step of the cycle is the nonsolar exothermic oxidation of the metal or lower-valence metal oxide with CO 2 , represented by: Zn + CO 2 f ZnO + CO ΔH 298K ° )-68 kJ mol -1 (3) or 3FeO + CO 2 f Fe 3 O 4 + CO ΔH 298K ° )-34 kJ mol -1 (4) A second-law (exergy) analysis for the net reaction CO 2 ) CO + 0.5O 2 indicates maximum solar-to-chemical energy conversion efficiencies of 39 and 29% for the Zn/ZnO and FeO/ Fe 3 O 4 cycles, respectively. 1 Previous investigations dealt with the thermodynamics and kinetics of the thermal dissociations of ZnO 2-4 (eq 1) and Fe 3 O 4 5,6 (eq 2). Both reactions proceeded at reasonable rates at above about 2000 K. Recently, the solar reactor technology for ZnO dissociation was experimentally demonstrated using a 10 kW solar receiver-reactor prototype. 4 Stoichiometric and nonstoichiometic forms of wuestite (Fe 1-y O) were produced from Fe 2 O 3 under an inert gas flow, with Fe 3 O 4 being an important intermediary. 7 As for the second step of the cycle, most of the previous experimental studies dealt with the kinetics of Zn vapor reoxidation within the imperial smelting process. 8-11 Activation energies of ∼170 kJ mol -1 were obtained for 1073-1173 K. 9 Although kinetically hindered, some traces of carbon formation were observed. 10,11 CO 2 reduction was experimentally demonstrated in exploratory studieswithFeO, 12,13 oxygen-defectiveironoxides, 14-17 nanocrystalline Fe 2 O 3 , 18 and Ni-, Co-, and Cu-doped ferrites. 19-24 Kinetic * To whom correspondence should be addressed. E-mail: aldo.steinfeld@ eth.ch. † Paul Scherrer Institute. ‡ ETH Zurich. (1) Ga ´lvez, M. E.; Loutzenhiser, P. 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