Enhanced oxidation kinetics in thermochemical cycling of CeO₂ through templated porosity

Two-step thermochemical cycling was achieved using CeO₂ with sub-micrometer sized macropores, allowing for substantially improved CO production at fast cycle rates when compared to nonporous CeO₂. The effects of porosity, pore order, and packing density were probed by synthesizing ceria materials with different morphologies. Polymeric colloidal spheres were used as templates for the synthesis of three-dimensionally ordered macroporous (3DOM) CeO₂ and nonordered macroporous (NOM) CeO₂. Aggregated CeO₂ nanoparticles with feature sizes similar to those in 3DOM CeO₂ were prepared by fragmenting 3DOM CeO₂ into its building blocks using ultrasonication. The three templated materials and nonporous, commercial CeO₂ were tested in thermochemical cycles using an infrared furnace. CeO₂ was reduced at ∼1200 C, and the reduced CeO_2-δ materials were reoxidized under CO₂ at ∼850 C. The high temperatures required for cycling induced changes in the morphology of the porous materials, which were characterized by electron microscopy, X-ray diffraction, and nitrogen sorption measurements. In spite of sintering, the macroporous materials retained an interconnected pore network during 55 cycles, providing a 10-fold enhancement in CO productivity and production rate when compared to nonporous CeO₂. Additionally, 3DOM CeO₂ provided the fastest rate of CO production of all tested materials and also retained the smallest solid feature sizes. This boost in reaction kinetics allowed for extremely rapid cycling with less than a minute required for complete reduction or oxidation. Characterization of the porous materials also provided some insight into thermal gradients that developed in the sample bed as a result of rapid heating and cooling.