The kinetics of CO₂ reduction over nonstoichimetric ceria, CeO_2−δ, a material of high potential for thermochemical conversion of sunlight to fuel, has been investigated for a wide range of nonstoichiometries (0.02 ≤ δ ≤ 0.25), temperatures (693 ≤ T ≤ 1273 K), and CO₂ concentrations (0.005 ≤ pCO₂ ≤ 0.4 atm). Samples were reduced thermally at 1773 K to probe low nonstoichiometries (δ < 0.05) and chemically at lower temperatures in a H₂ atmosphere to prevent particle sintering and probe the effect of higher nonstoichiometries (δ < 0.25). For extents greater than δ = 0.2, oxidation rates at a given nonstoichiometry are hindered for the duration of the reaction, presumably because of near-order changes, such as lattice compression, as confirmed via Raman Spectroscopy. Importantly, this behavior is reversible and oxidation rates are not affected at lower δ. Following thermal reduction at very low δ, however, oxidation rates are an order of magnitude slower than those of chemically reduced samples, and rates monotonically increase with the initial nonstoichiometry (up to δ = 0.05). This dependence may be attributed to the formation of stable defect complexes formed between oxygen vacancies and polarons. When the same experiments are performed with 10 mol % Gd³⁺ doped ceria, in which defect complexes are less prevalent than in pure ceria, this dependence is not observed.

The thermodynamics and defect chemistry of Zr⁴⁺-doped ceria is investigated and discussed in regards to the efficiency of solar thermochemical redox cycles.

Reticulated porous ceramic (RPC) made of ceria are promising structures used in solar thermochemical redox cycles for splitting CO₂ and H₂O. They feature dual-scale porosity with mm-size pores for effective radiative heat transfer during reduction and µm-size pores within its struts for enhanced kinetics during oxidation. In this work, the detailed 3D digital representation of the complex dual-scale RPC is obtained using synchrotron submicrometer tomography and X-ray microtomography. Total and open porosity, pore size distribution, mean pore diameter, and specific surface area are extracted from the computer tomography (CT) scans. The 3D digital geometry is then applied in direct pore level simulations (DPLS) of Fourier’s law within the solid and the fluid phases for the accurate determination of the effective thermal conductivity at each porosity scale and combined, and for fluid-to-solid thermal conductivity from 10⁻⁵ to 1. Results are compared to predictions by analytical models for structures with a wide range of porosities 0.09–0.9 in both the strut’s µm-scale and bulk’s mm-scale. The morphological properties and effective thermal conductivity determined in this work serve as an input to volume-averaged models for the design and optimization of solar chemical reactors.

We report on the experimental performance of a solar aerosol reactor for carrying out the combined thermochemical reduction of CeO₂ and reforming of CH₄ using concentrated radiation as the source of process heat. The 2 kW_th solar reactor prototype utilizes a cavity receiver enclosing a vertical Al₂O₃ tube which contains a downward gravity-driven particle flow of ceria particles, either co-current or counter-current to a CH₄ flow. Experimentation under a peak radiative flux of 2264 suns yielded methane conversions up to 89% at 1300 °C for residence times under 1 s. The maximum extent of ceria reduction, given by the nonstoichiometry δ (CeO_2−δ), was 0.25. The solar-to-fuel energy conversion efficiency reached 12%. The syngas produced had a H₂:CO molar ratio of 2, and its calorific value was solar-upgraded by 24% over that of the CH₄ reformed.