Investigation of thermal and carbothermal reduction of volatile oxides (ZnO, SnO2, GeO2, and MgO) via solar-driven vacuum thermogravimetry for thermochemical production of solar fuels

• Solar thermochemical production of renewable fuels from CO2 and H2O splitting.
• Volatile metal oxide redox cycles involving ZnO, SnO2, GeO2, and MgO are considered.
• Design and operation of a new solar-driven vacuum thermogravimeter in controlled atmosphere.
• Kinetic investigation of thermal and carbo-thermal reduction of volatile oxides.
• Enhanced reduction rates of thermochemical reactions during low pressure operation.

This study addresses the solar thermogravimetry analysis of high-temperature reduction reactions involving volatile metal oxides for application in solar thermochemical fuel production via two-step H2O and CO2 splitting. The redox cycles encompass the thermal or carbo-thermal reduction of a metal oxide using concentrated solar energy followed by its oxidation with CO2 and/or H2O to produce solar-derived fuels (CO and/or H2). A new solar-driven vacuum thermogravimeter has been developed for the investigation of the high-temperature thermochemical reactions. The system is designed for reduced pressure operation down to a few mbar with flowing purge gas for studying low-pressure conditions, while the weight change of the solid reactants is continuously monitored during high-temperature solar heating under non-linear heating profile. The method was applied to investigate the kinetic rates of the thermochemical solid/gas reactions involving ZnO, SnO2, GeO2, and MgO volatile oxides in a controlled atmosphere. The reaction rates were determined as a function of the temperature for different pressures in the system. Metal oxides reduction under low pressure was shown to be of special interest because the reaction rate is greatly enhanced and the required temperature to achieve a given reduction rate is significantly lowered. In contrast to inert gas dilution, a total pressure decrease also reduces the need for a diluent gas, thus simplifying the process and avoiding energy penalties associated with inert gas recycling.

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