Production of Hydrogen and Carbon Monoxide from Water and Carbon Dioxide through Metal Oxide Thermochemical Cycles

Abstract

Two-step thermochemical cycles using ferrite-based materials to split water and carbon dioxide are promising routes for the production of H2 and CO (syngas). To aid in the design of highly efficient materials for H2 and CO production, this work aims to identify the metal oxide phases present during thermochemical cycling and how they change as a function of temperature and gas composition. Hightemperature X-ray diffraction (HT-XRD) was used to monitor the structure of iron oxides supported on YSZ (10 wt.-% Fe2O3 basis) and cobalt-substituted ferrites during thermochemical cycling. HT-XRD showed dynamic behavior as iron migrated into and out of YSZ at elevated temperatures, monitored by the lattice parameter of the YSZ. Iron oxides were seen to thermally reduce stepwise from Fe2O3 to Fe3O4 and finally FeO as the temperature increased from ambient to 1400 °C under He with a low background of O2. Between 800 and 1100 °C no iron species were detected, indicating that all iron was in solid solution with YSZ. Similar cycles were performed with a cobalt-substituted ferrite which exhibited similar phase evolution. Exposure of FexCo1-xO to CO2 or air resulted in re-oxidation to Fe3xCo3-3xO4. Thermogravimetric analysis corroborated the reduction/oxidation behavior of the materials during thermal reduction and subsequent re-oxidation by H2O or CO2. A complimentary study on diffusion of iron oxide into YSZ revealed a steep increase in diffusion rate once temperatures exceeded 1475 °C. Fusion and vaporization of iron species at these high temperatures occurs. INTRODUCTION The primary goal of this work is to lay the foundation to enable the synthesis of hydrocarbon fuels from carbon dioxide and water 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, Figure 1 as illustrated in simplified form in B. 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 wustite, 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) However, hydrogen production using Fe3O4, originally proposed by Nakamura, is not practical since the TR requires 1800 °C resulting in sintering or fusion that must be undone by, e.g., mechanical crushing or milling in order to activate the material for successive cycles; supporting Fe3O4 on zirconia, or yttria-stabilized zirconia (YSZ) reduces this problem. Alternative redox systems using AxFe3-xO4 where A ≠ Fe enable reduced temperature TR, e.g., A = Mn, Co, Ni, Zn, and are now receiving considerable attention. The TR can be driven as low as 1100 °C, although kinetics usually dictate that temperatures above 1300 °C are used, which are readily achievable using concentrated