Transition Metal Oxides for Sustainable Fuels Production via Solar Chemical Looping Reforming

Abstract

Energy storage by chemical looping steam/dry reforming is a promising alternative for the utilization of solar energy in the industrial and transport sectors. Efficient oxygen carriers with a facile and scalable synthesis method are crucial to achieve economic competitiveness for this solar thermochemical process. In this thesis, a comprehensive overview of solar chemical looping reforming is provided and the state of the art research of its associated oxygen carriers is discussed. Improvement in syngas yields and production rates in solar chemical looping reforming were then explored via morphological and structural enhancements of the oxygen carriers. Firstly, the impact of ceria structural features on its syngas production performance during two-step isothermal redox cycles for four different nano and micro morphologies was investigated. Highly porous flame-made agglomerates composed of small crystalline particles were determined as the best performing morphology with initial production rates of H2 and CO up to 167% higher than that of commercial sub-micro ceria. Upon 10 isothermal redox cycles at 1173 K, these flame-made structures still maintained at up to 57% faster production rates. It was shown that the high porosity of the flame-made agglomerates was important in inhibiting sintering and grain growth. Notably, higher specific surface area flower-like morphologies collapsed and densified rapidly, and exhibited the slowest kinetics. These findings provide a robust set of structural properties to engineer efficient materials for enhanced solar fuel production by high temperature thermochemical cycles. Secondly, a first-time investigation of using an earth-abundant manganese-based oxygen carrier in solar chemical looping dry methane reforming was demonstrated. It revealed a manganese carbide/oxide redox cycle that resulted in high mass-specific syngas yields and production rates when the oxygen carrier's matrix was incorporated with fractional amount of cerium ions. In particular, 15 times higher CO2 splitting rates than the undoped manganese oxide, and also 8 times higher CO yields than cerium oxide was achieved. The long-term performance with 100 cycles revealed that this is not a short-lived enhancement and that the synergetic contribution by cerium ions were highlighted. A thorough investigation of this manganese carbide/oxide redox mechanism was experimentally pursued further with a series of custom-synthesized Ce-Mn oxygen carriers via solar chemical looping steam methane reforming cycles. Interesting discoveries of 3% Ce suggest the intense surface distribution of ceria-rich nanoparticles that efficiently dissociate the chemisorbed species into H2 and CO during methane reforming and water splitting. Additionally, the abundance of Ce ions in the bulk lattice effectively unlock the oxygen carrier’s reversible diffusion of oxygen and carbon in this vacancy-based redox mechanism.