(222g) Mixed Oxide Based Redox Catalysts for Hydrogen and Liquid Fuel Co-Generation Via a Hybrid Solar-Redox Scheme

Hydrogen is an attractive fuel due to its zero-emission combustion and high weight-based energy density. Among the various approaches for sustainable hydrogen generation, solar thermochemical water-splitting based on redox cycles of metal oxides represents an attractive approach. A key challenge for this process is the high temperature required for thermal decomposition of the metal oxide (>1200 °C). In addition, the need to balance the oxygen release and water-splitting properties of metal oxides often leads to low steam conversion. The current study investigates a hybrid solar-redox process for co-generation of hydrogen and liquid fuels using methane and solar energy at significantly lower reaction temperatures and an order of magnitude higher steam conversions. In such a process, a reduced metal-oxide-based redox catalyst is used to split water, producing concentrated hydrogen. The resulting (oxidized) redox catalyst is subsequently used to partially oxidize methane, forming syngas for Fischer-Tropsch synthesis. A rationalized strategy to optimize transition metal oxide-based redox catalysts for the scheme is proposed and validated. While monometallic transition metal oxides do not possess desirable properties, Density Functional Theory (DFT) calculations indicate that redox properties of mixed metal oxides such as equilibrium P_O2 and vacancy concentration relationships can be adjusted via doping of heteroatoms. Based on such principles, a rationally developed, Ba and Fe-containing perovskite based redox catalyst was experimentally found to be able to achieve 90% steam-to-hydrogen conversion during water-splitting and over 90% syngas yield in the methane partial oxidation step under repeated redox cycles. Results from these experimental studies are then used in an ASPEN Plus® simulation model to evaluate the performance of the proposed hybrid solar-redox scheme