Using In-situ Techniques to Probe High Temperature Reactions: Thermochemical Cycles for the Production of Synthetic Fuels from Co2 Andwater

Ferrites are promising materials for enabling solar-thermochemical cycles. Such cycles utilize solar-thermal energy to reduce the metal oxide, which is then re-oxidized by H2O or CO2, producing H2 or CO, respectively. Mixing ferrites with zirconia or yttria-stabilized zirconia (YSZ) greatly improves their cyclability. In order to understand this system, we have studied the behavior of iron oxide/8YSZ (8 mol-% Y2O3 in ZrO2) using in situ X-ray diffraction and thermogravimetric analysis at temperatures up to 1500 °C and under controlled atmosphere. The solubility of iron oxide in 8YSZ measured by XRD at room temperature was 9.4 mol-% Fe. The solubility increased to at least 10.4 mol-% Fe when heated between 800 and 1000 °C under inert atmosphere. Furthermore iron was found to migrate in and out of the 8YSZ phase as the temperature and oxidation state of the iron changed. In samples containing > 9.4 mol-% Fe, stepwise heating to 1400 °C under helium caused reduction of Fe2O3 to Fe3O4 to FeO. Exposure of the FeO-containing material to CO2 at 1100 °C re-oxidized FeO to Fe3O4 with evolution of CO. Thermogravimetric analysis during thermochemical cycling of materials with a range of iron contents showed that samples with mostly dissolved iron utilized a greater proportion of the iron atoms present than did samples possessing a greater fraction of un-dissolved iron oxides. INTRODUCTION The primary goal of this work is to lay the foundation to enable the synthesis of hydrocarbon fuels from CO2 and H2O using concentrated solar power as a heat source to drive a two-step solar-thermochemical cycle. This process can be described as a way to “re-energize” CO2 and H2O, which are the thermodynamically stable products of hydrocarbon combustion (Figure 1A). Once CO2 and H2O have been re-energized (reduced) to CO and H2, traditional syngas chemistry can be applied to convert these products into hydrocarbon fuels. Solar-driven two-step ferrite (e.g., Fe3O4) thermochemical cycles are promising as a method for producing H2 and CO via H2Oand CO2-splitting, (Steinfeld, 2005; Kodama, 2003; Miller, 2007; Kodama et al., 2007; Miller et al., 2007), as illustrated in simplified form in Figure 1B. The basic cycles consist of a thermal reduction step (TR; reaction (1)) in which solar thermal energy reduces Fe to Fe, i.e., spinel transforms to wüstite, followed by a water-splitting step (WS; reaction (2)), or carbon dioxide-splitting step (CDS; reaction 3) wherein the ferrite spinel is regenerated: Fe3O4 → 3FeO + 0.5 O2 (1) 3FeO + H2O → Fe3O4 + H2 (2) 3FeO + CO2 → Fe3O4 + CO (3) Nakamura (1977) first proposed a two-step thermochemical cycle for hydrogen production from water using un-supported iron oxide, however, hydrogen production using the bulk iron oxides is not practical since the TR requires temperatures in excess of the melting point for significant 36