Wood-Templated CeO₂ as Active Material for Thermochemical CO Production

Abstract

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.

Additive‐Manufactured Ordered Porous Structures Made of Ceria for Concentrating Solar Applications

“…We have demonstrated the technical feasibility of the ceria‐based redox cycle using a solar cavity receiver containing porous structures and the entire production chain to solar kerosene from H 2 O and CO 2 . It was shown that the morphology of the porous structure has a significant impact on the cycle's performance, e.g., on the molar conversion and energy efficiency, because the reduction step is heat transfer controlled, while the oxidation step is surface/mass transfer controlled . Therefore, for a given volume of the solar cavity receiver, a desired porous structure should feature high mass loading for maximum fuel output, appropriate optical thickness for volumetric absorption and uniform heating during the endothermic reduction step, and a high specific surface area for rapid reaction kinetics during the exothermic oxidation step with H 2 O and/or CO 2 .…”

“…[16] It was shown that the morphology of the porous structure has a significant impact on the cycle's performance, e.g., on the molar conversion and energy efficiency, because the reduction step is heat transfer controlled, while the oxidation step is surface/mass transfer controlled. [13,17,18] Therefore, for a given volume of the solar cavity receiver, a desired porous structure should feature high mass loading for maximum fuel output, appropriate optical thickness for volumetric absorption and uniform heating during the endothermic reduction step, and a high specific surface area for rapid reaction kinetics during the exothermic oxidation step with H 2 O and/or CO 2 . Reticulated porous ceramic (RPC) foam-type structures with dual-scale interconnected porosity (mm-and μm-sized pores within the struts) fulfill some of these desired characteristics.…”

High-flux optical systems for solar thermochemistry

“…Sometimes the geometrical concentration ratio is defined (Malonzo et al, 2014), which correspond to the ratio between the collector area and the receiver area. However, this definition assumes a perfect energy conversion efficiency during the redirection and concentration steps.…”

“…This field has recently picked up momentum driven by the desire for alternative, renewable and sustainable approaches for fuel processing, and material and chemical commodity production, as well as for direct, energydense, and long-term storage of solar energy. Among the existing solar-driven, non-biological chemistry routes which include solar thermochemistry, photocatalysis and photoelectrochemistry, solar thermochemistry has reached the largest scale demonstrations (up to 100 kW) (Villasmil et al, 2013;Chueh et al, 2010;Säck et al, 2016), demonstrated stability over hundreds of cycles (Malonzo et al, 2014), and enormous versatility in demonstrated chemical reactions (Scheffe and Steinfeld, 2014;Steinfeld, May 2005;Romero and Steinfeld, 2012;Bader and Lipinski, 2017).…”

“…metal oxides which are doped with other metals in order to facilitate the reduction and/or improve the thermal stability. The first of this kind have been doped ferrites ( perovskite-type materials have shown potential in terms of fuel productivity and thermal stability (Chueh et al, 2010;Malonzo et al, 2014), and are currently more intensively researched. These oxides generally present a lower reduction temperature (around 1800 K), but also a smaller productivity (220 mmol CO /g for structured pure ceria (Furler et al, 2014) than stoichiometric oxides.…”