Thermochemical Dissociation of CO2 into Renewable Fuels Via CexZryHfzO2 Based Redox Reactions Using Concentrated Solar Energy

Today the world's energy economy is still based to a large extent on the abundance of fossil fuels. This has led to a rapid depletion of the easily accessible oil reserves resulting in continuously rising oil prices. Furthermore severe environmental problems caused by the CO₂ induced greenhouse effect have begun to become apparent. Thus, there is a pressing need to develop technologies to produce carbon free renewable fuels such as H₂ or renewable precursors for fuels such as syngas (a mixture of H₂ and CO). The latter can be processed to liquid fuels via the Fischer-Tropsch process. Solar radiation is an essentially inexhaustible energy source that delivers about 100000 TW to the earth. To harvest the solar radiation and to convert it effectively into renewable fuels such as H₂ and syngas directly from H₂O and captured CO₂ provides a promising path for a future sustainable energy economy. One of the ways of producing solar fuels is by metal oxide based two-step solar thermochemical cycles. In these cycles, the first step consists of the endothermic reduction of a metal oxide at elevated temperatures releasing O₂. The second step corresponds to the slightly exothermic re-oxidation of the reduced metal oxide at lower temperatures by H₂O, CO₂, or a mixture of the two producing H₂, CO or syngas. Among many metal oxides investigated for solar fuel production, in recent years, research has been focused towards non-volatile mixed metal oxides such as doped ceria.

In this investigation, Zr and Hf doped ceria based redox materials (various doping combinations) were synthesized using a co-precipitation method. The respective metal precursors were dissolved in water. Upon complete dissolution, excess ammonium hydroxide (NH₄OH) was added drop-wise to the mixture under vigorous stirring to precipitate the mixed-metal hydroxides (final pH = ~9). The obtained precipitates were filtered, washed with water until free from anion impurities and oven dried at 100^oC for 8 – 10 h. Subsequently calcination was performed at different temperatures in air. The calcined powders were characterized by powder X-ray diffraction, BET surface area analysis, scanning (SEM) and transmission electron microscopy (TEM), etc. Synthesized Zr and Hf doped ceria redox materials were further tested for thermochemical CO₂-splitting by using a high-temperature thermogravimetric analyzer (TGA). Multiple thermal reduction and oxidation (by CO₂) cycles were performed at various operating conditions by using TGA while the O₂ and CO was quantified by gas chromatography. The results related to synthesis, physiochemical characterization, and thermogravimetric experiments will be presented in detail.