(609d) Continuous Splitting of CO2 into CO and O2 Using an Isothermal Redox Membrane Reactor

Renewable liquid hydrocarbon fuels can be produced with two-step thermochemical redox cycles that convert CO₂ and H₂O with concentrated solar energy into renewable CO, H₂ (syngas) and O₂. Syngas can be further processed into liquid hydrocarbon fuels using commercially available Fischer-Tropsch technology. Typically, solar-driven splitting of CO₂ and H₂O are temperature-swing processes with cyclic production of syngas at lower temperatures and O₂ at higher temperatures. Alternatively, this work develops an isothermal redox membrane reactor that continuously produces O₂ and oxygen vacancies at the outside of a metal oxide membrane. The formed oxygen vacancies migrate along an oxygen vacancy gradient across the membrane. At the inside of the membrane the oxygen vacancies are consumed via oxidation of the membrane with CO₂ or H₂O, yielding continuously CO or H₂, respectively. This work employs thermochemical equilibrium computations to determine the thermodynamic limitations of this concept as a function of process parameters and membrane material. The concept is demonstrated experimentally for CO₂ splitting into CO and O₂ using tubular ceria redox membranes and inert alumina membranes as reference. Steady-state gas production rates are analyzed in-process using gas-chromatography. The membranes are characterized via scanning electron microscopy, X-ray diffraction, and energy dispersive X-ray spectroscopy. Thermodynamic limitations are confirmed experimentally via parametric investigation of the effect of operating temperature, partial pressure of O₂, and metal doping of the ceria membranes on the CO and O₂ production rates.