Synthesis and thermochemical redox cycling of porous ceria microspheres for renewable fuels production from solar-aided water-splitting and CO2 utilization

Abstract

Porous ceria-based architected materials offer high potential for solar fuels production via thermochemical H2O and CO2-splitting cycles. Novel porous morphologies and micro-scale architectures of redox materials are desired to provide suitable thermochemical activities and long-term stability. Considering particle-based solar reactors, porous ceria microspheres are promising because of their excellent flowability and large surface area. In this work, such porous microspheres with perfect spherical shape, high density, and interconnected pore network were fabricated by a chemical route involving ion-exchange resins. The method involved the cationic loading of the resin in an aqueous medium followed by thermal treatment for oxide formation and porous microstructure stabilization. The utilization of these microspheres (∼150–350 μm in size) as redox materials for solar fuel production was investigated in packed-bed solar reactors (directly and indirectly irradiated). Superior redox performance was obtained for the pure ceria microspheres in comparison with other morphologies (powders and reticulated foams). Low pO2 values thermodynamically favored the reduction extent and associated fuel yield, whereas high pCO2 kinetically promoted the oxidation rate. The highest fuel production rate reached 1.8 mL/min/g with reduction step at 1400 °C, low total pressure (∼0.1 bar), and oxidation step below 1050 °C under pure CO2. Low pressure during reduction both improved reduction extent (oxygen under-stoichiometry δ up to 0.052) and associated fuel production yield (331 μmol/g CO). After 19 redox cycles (∼32 h under high-flux solar irradiation), the porous microspheres maintained their individual integrity (no agglomeration), spherical shape, and internal porosity, with great potential for stable fuel production capacity in particle-based solar reactors.