An isothermal thermochemical cycle to split CO 2 based on nonstoichiometric reduction and oxidation of ceria is demonstrated. Carbon monoxide is produced via an oxygen partial pressure swing by alternating inert sweep gas and CO 2 flows over the ceria. The rates of reduction and oxidation at 1500 °C in a porous ceria particle bed are measured for sweep gas and CO 2 flow rates from 50 to 600 mL min −1 g −1 and analyzed to identify cycle operating conditions (gas flow rates and reduction and oxidation durations) that maximize process efficiency. For a solar reactor assumed to operate at 3000 suns concentration and with 90% of the sensible heat of the gases recovered, the optimal cycle uses 150 mL min −1 g −1 sweep gas and 50 mL min −1 g −1 CO 2 at reduction and oxidation periods of 100 and 155 s, respectively. This cycle is demonstrated in an IR imaging furnace over 102 cycles, yielding a stable average rate of CO production of 0.079 μmol s −1 g −1 and a projected reactor efficiency of 4%. The optimal conditions apply at large scale if the flow rates are scaled in proportion to the ceria mass.

Zirconium-doped ceria (Ce(1-x)Zr(x)O2) was synthesized through a controlled electrospinning process as a promising approach to cost-effective, sinter-resistant material structures for high-temperature, solar-driven thermochemical redox cycles. To approximate a two-step redox cycle for solar fuel production, fibrous Ce(1-x)Zr(x)O2 with relatively low levels of Zr-doping (0 < x < 0.1) were cycled in an infrared-imaging furnace with high-temperature (up to 1500 °C) partial reduction and lower-temperature (∼800 °C) reoxidation via CO2 splitting to produce CO. Increases in Zr content improve reducibility and sintering resistance, and, for x≤ 0.05, do not significantly slow reoxidation kinetics for CO production. Cycle stability of the fibrous Ce(1-x)Zr(x)O2 (with x = 0.025) was assessed for a range of conditions by measuring rates of O2 release during reduction and CO production during reoxidation and by assessing post-cycling fiber crystallite sizes and surface areas. Sintering increases with reduction temperature but occurs primarily along the fiber axes. Even after 108 redox cycles with reduction at 1400 °C and oxidation with CO2 at 800 °C, the fibers maintain their structure with surface areas of ∼0.3 m(2) g(-1), higher than those observed in the literature for other ceria-based structures operating at similarly high temperature conditions. Total CO production and peak production rate stabilize above 3.0 mL g(-1) and 13.0 mL min(-1) g(-1), respectively. The results show the potential for electrospun oxides as sinter-resistant material structures with adequate surface area to support rapid CO2 splitting in solar thermochemical redox cycles.

The design procedure for a 3 kWth prototype solar thermochemical reactor to implement isothermal redox cycling of ceria for CO2 splitting is presented. The reactor uses beds of mm-sized porous ceria particles contained in the annulus of concentric alumina tube assemblies that line the cylindrical wall of a solar cavity receiver. The porous particle beds provide high surface area for the heterogeneous reactions, rapid heat and mass transfer, and low pressure drop. Redox cycling is accomplished by alternating flows of inert sweep gas and CO2 through the bed. The gas flow rates and cycle step durations are selected by scaling the results from small-scale experiments. Thermal and thermo-mechanical models of the reactor and reactive element tubes are developed to predict the steady-state temperature and stress distributions for nominal operating conditions. The simulation results indicate that the target temperature of 1773 K will be reached in the prototype reactor and that the Mohr–Coulomb static factor of safety is above two everywhere in the tubes, indicating that thermo-mechanical stresses in the tubes remain acceptably low.

Production of hydrogen and synthesis gas via solar thermochemical partial redox cycles is one route to renewable fuels and storage of solar energy. The efficiency at which these cycles produce fuel for candidate non-stoichiometric metal oxides and reactor concepts is normally evaluated from thermodynamic models that implicitly assume that the transport processes and reaction kinetics are rapid enough that the gas and solid attain chemical equilibrium. In this paper, we develop an equilibrium model of a fixed-bed reactor and demonstrate its applicability for reduction and oxidation of porous ceria (CeO 2 ) particles with a volume-specific surface area of ∼10 6 m 2 m −3 over a wide range of gas flow rates and reaction temperatures. The model predicts the measured rate of O 2 production during reduction for mass-specific flow rates up to 900 mL min −1 g ceria −1 and for temperatures from 740 to 1500 °C, and it predicts the measured rate of CO production during oxidation for flow rates up to 50 mL min −1 g ceria −1 at 1500 °C. It does not apply for oxidation below 930 °C. We compare the equilibrium model developed for the fixed-bed reactor to the models for the mixed flow and countercurrent flow reactors. In comparison to the mixed flow reactor, the fixed-bed reactor reduces the sweep gas and excess oxidizer required for fuel production, an important step toward increasing efficiency closer to the theoretical limit established by the countercurrent flow reactor.

Heterogeneous reactions benefit from active materials with accessible pores and high surface areas. Such materials can be synthesized, for example, by templating methods. Particularly high surface areas are obtained in nanoporous solids. However, the solid walls surrounding nanopores also have typical dimensions in the nanometer range, and these walls are prone to sintering at high temperatures, reducing the active surface area. Here we demonstrate wood templating as an approach that balances accessible porosity with higher thermal stability, using cerium oxide as a representative active material in a high-temperature process: thermochemical CO production. Wood-templated CeO 2 (WT CeO 2 ) was synthesized via the Pechini method and annealed at 1200−1500 °C, temperatures suitable for the thermochemical reduction of CeO 2 . The pore structure consists of interconnected channels with pores tens of micrometers in diameter and micrometer-thick walls. The WT CeO 2 maintained its interconnected pore structure with surface areas of ∼0.1 m 2 g −1 at temperatures up to 1400 °C, despite significant grain growth. The cycling performance of the annealed WT CeO 2 was tested by conducting 21 cycles in an infrared imaging furnace. WT CeO 2 samples were reduced at temperatures from 1200 to 1500 °C and reoxidized with CO 2 at 800 °C. The presintered WT CeO 2 samples retained their structure after cycling. With a reduction temperature of 1400 °C, the WT CeO 2 achieved CO production rates 6 times higher than nonporous CeO 2 compared at the same nonstoichiometry δ (for CeO 2−δ ). CO production rates are comparable to those obtained with electrospun Zr-doped CeO 2 fibers under similar conditions.