Enhanced Oxidation Kinetics in Thermochemical Cycling of CeO2 through Templated Porosity

Abstract

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 CeO2−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.