Improving thermochemical splitting performance of ceria based materials: novel preparation routes, doping and co-doping

Solar thermochemical splitting cycles have been gaining scientific and technological relevance as an environmentally sustainable route to produce synthetic fuels from H2O and CO2. Actually, they allow direct solar energy harvesting and conversion to synthesis gas, that can be further converted into gaseous or liquid fuels. Metal oxides based two-step thermochemical redox splitting cycles hold huge promise, ensuring lower complexity as well as higher theoretical solar-to-fuel energy efficiency as a result of the use of the whole solar spectrum. Due to good thermal stability, high oxygen storage capacity (OSC) without any structural changes, faster kinetics of both reduction and splitting reactions, ceria is considered the most attractive material for solar thermochemical splitting cycles. However, some critical issues, including high reduction temperature and low thermal stability, still prevent technological feasibility of CeO2 based thermochemical cycles. Two main routes have been proposed to improve splitting performance of ceria-based materials: i) the adoption of advanced preparation routes and ii) ceria doping with both reducible and non-reducible elements. In this work the most interesting results obrtained by our research group are reviewed. In particular, the reactivity of ceria-based materials prepared by both co-precipitation and hydrothermal synthesis towards both CO2 and H2O splitting was assessed. The effect of doping was studied by adding transition metals (Cu, Mn, Fe), while on the most promising materials the effect of the K addition on both self-reducibility and splitting activity was investigated. Results from activity tests were integrated with an in-depth physico-chemical characterization by EPR and XPS spectroscopy to assess both surface and bulk properties accounting for red-ox behaviour, so as to identify strategies for designing catalytic systems with tuned structure and composition.