Mixed Metal Oxides for Sustainable Fuel Production via Solar Thermochemical Routes
Creators: name: Riaz, Asim
Publisher: name: The Australian National University
Year: 2021

description: The overall solar-to-fuel efficiency of the synthesis gas production via solar-driven thermochemical splitting of CO2 and H2O reactions is highly dependent on the energy required to break down the strong molecules such as CH4, CO2 and H2O. To maximize the syngas production yields, designing new redox materials and optimizing the reactor designs and receiver models are of great importance. Redox materials mediate the thermochemical process by exchanging oxygen with the reactant gases and their performance is mainly assessed by the oxygen exchange capacity, syngas yields and structural stability. In this thesis, a range of redox materials including LaSrMnO3 perovskites and cerium-vanadium mixed/doped metal oxides are studied for syngas production via cyclic H2O and CO2 splitting coupled with methane partial oxidation and high temperature inert gas reduction. The effects of reducing atmospheres such as Ar and CH4 on the structural features in LaxSr1-xMnO3 perovskites are investigated. The La0.5Sr0.5MnO3 powders composed of nano-crystalline particles are considered as the best performing Perovskites with premium structural stability and a 117% higher initial syngas production rate than that of pure SrMnO3 and LaMnO3 structures. The overall syngas production rates are 9 times faster during the chemical looping reforming of methane when compared to those of inert gas reduction. It is demonstrated that lanthanum incorporation prevents the structural breakdown caused by CH4 and up to 65-100% of the initial perovskite structure is regenerated. Notably, H2 purity of up to 93% is achieved by lanthanum-rich LSM structures during the H2O splitting redox cycles coupled with an efficient methane reforming reaction. These findings provide a robust set of physiochemical properties of LaSrMnO3 systems that can be utilized for enhanced solar fuel production via thermochemical redox cycles. The effects of vanadium (V) and cerium (Ce) concentrations (each varying in the 0-100% range) in CeO2-CeVO4 mixed-phase, Ce4+-doped V2O5 and V5+-doped CeO2 redox materials are explored for synthesis gas production via thermochemical redox cycling of CO2 and H2O splitting coupled to methane partial oxidation reactions. In particular, an optimum mixture of CeO2 and CeVO4 is achieved by 25 wt% of vanadium incorporation in the CeO2 powders, which produce up to 68% higher syngas yields than that of pure ceria. It is observed that V5+ provides more reducing states for the hydrocarbon oxidation, while cerium ions act as an oxygen buffer for the re-oxidation reaction. Notably, doping of vanadium increases the cycle capacity of ceria by 400% and the activation temperature of the methane reforming reaction is lowered by up to 178C, while doping the V2O5 lattice with large cerium cations results in a V2O5-to-V2O3 phase transition and produces up to 100 times higher syngas production rates when compared to the pure V2O5. Finally, these findings suggest that a facile combination of the extraordinary catalytic properties of vanadia and superior oxygen ion mobility of ceria can be a powerful approach for an efficient and effective solar thermochemical fuel production.