Equilibrium-reduction of ceria-based materials for solar fuel production

Abstract

very broad variety of chemical substances like gaseous and liquid synthetic fuels, polymers, ammonia, etc. Employing concentrated solar energy may be a viable alternative to the state-of-the-art methods in order to cover the required heat demand for the generation of carbon monoxide (CO) solely from carbon dioxide (CO2). This emission-free CO production may be carried out in a two-step thermochemical cycle using a metal oxide as the reactive material. Due to preliminary theoretical studies as well as reported in the literature, ceria-based materials are viable candidate materials for such cycles. In screening campaigns, the performance of ceria was enhanced via doping with zirconium and trivalent lanthanides. The most promising compositions were subjected to equilibrium reduction experiments under varying conditions. Within a physical reaction model, the function of the reduction extend on temperature and partial pressure of oxygen was described. Based on the derived relations, Ellingham diagrams were established that convey the reduction enthalpy and entropy as a function of the reduction extend. Employing these functions in a process model, which considers the major losses, maximum solar-to-fuel efficiencies of pure ceria as well as of the doped materials were estimated. A parametric study, including partial pressure of oxygen, temperature, heat recuperation and concentrating system, revealed solar-tofuel efficiencies of the doped materials to be twice as high compared to pure ceria. Suitable Doping improves the efficiency, in particular, at a temperature as low as 1200°C, which should facilitate the technical realization of the process considerably.