Hydrogen Production via Chemical Looping Redox Cycles Using Atomic Layer Deposition-Synthesized Iron Oxide and Cobalt Ferrites

摘要

Iron oxide (γ-Fe2O3) and cobalt ferrite (Co_xFe_(3-x)O4) thin films were synthesized via atomic layer deposition (ALD) on high surface-area (50 m~2 g~(-1)) m-ZrO2 supports. The oxide films were grown by sequentially depositing iron oxide and cobalt oxide, adjusting the number of iron oxide to cobalt oxide cycles to achieve a desired stoichiometry. High resolution transmission electron microscopy and X-ray diffraction indicate that the films are crystalline and have a thickness of ～2.5 nm. Raman spectroscopy was used to confirm the predominance of the spinel phase in the case of cobalt ferrite. Films were chemically reduced at 600 °C using mixtures of H2, CO, and CO2. The evolution of oxide phases as a function of time during this reduction was observed using in situ X-ray diffraction, showing that γ-Fe2O3 are reduced only to FeO, while Co_xFe_(3-x)O4 are reduced all the way to a Co/Fe alloy. Subsequent water splitting measurements in a stagnation flow reactor yielded peak H2 rates exceeding virtually all of those reported in the literature. Co_(0.85)Fe_(2.15)O4 films were successfully cycled without deactivation and produced four times more H2 than γ-Fe2O3 films principally because of the deeper chemical reduction possible. Together, these results suggest a path to robust materials for chemical looping cycles and thermal gas splitting. They also indicate that ALD films can serve as an effective platform for probing the surface chemistry of these processes, since they maintain their reactivity at these temperatures, in contrast with oxide powders that are deactivated by sintering and grain growth.