Synthesis and Redox Pathways of MgAl2O4-Stabilized, Fe-Cu-Based Oxygen Carriers for Chemical Looping Water Splitting

Hydrogen may become an important energy carrier in the future. The current technologies for the large scale production of H₂, i.e. steam reforming of natural gas and steam gasification of coal, are very energy intensive owing to the need for various separation steps to obtain pure H₂.¹ However, for H₂ to become a major energy carrier it has to be produced in an efficient and sustainable way. Chemical looping water-splitting (CLWS) is an emerging technology that can inherently produce a pure stream of H₂, thus eliminating the need for energy intensive purification steps.² In CLWS, lattice oxygen of an oxygen carrier, typically a transition metal oxide, is used to combust a hydro-carbonaceous fuel. In the subsequent step, the reduced oxygen carrier is re-oxidized with steam to produce a pure stream of H₂. Iron oxide is an attractive material for the CLWS process owing to its wide availability, low cost, minimal environmental impact and high equilibrium partial pressure of H₂.³ Nevertheless, its reactivity and cyclic redox stability are two major concerns with regards to the CLWS process. Pure Fe₂O₃ that is completely reduced to Fe deactivates after a few cycles, probably due to sintering.⁴ To circumvent this rapid deactivation it has been proposed to stabilize iron oxide with a high Tammann temperature support, such as Al₂O₃, ZrO₂, TiO₂, SiO₂ or MgAl₂O₄. However, it was found that supported Fe₂O₃ has often a high tendency for carbon deposition. An approach to improve the reactivity of iron oxide with CH₄ and higher hydrocarbons is the addition of more reactive transition metal oxides.⁵

Here, we synthesized MgAl₂O₄-stabilized, Fe-Cu-based oxygen carriers containing 70 wt. % Fe₂O₃ for CLWS using CH₄ as a fuel. The reactivity, H₂ yield and carbon deposition characteristics of the oxygen carriers were compared to pure Fe₂O₃ and CuFe₂O₄ and MgAl₂O₄-stabilized Fe₂O₃. The oxygen carriers were characterized using powder X-ray diffraction (XRD), CH₄ and H₂ temperature programmed reduction, N₂ adsorption measurements, scanning electron microscopy, transmission electron microscopy, Raman spectroscopy and X-ray absorption spectroscopy (XAS). The redox and carbon deposition characteristics of the synthesized oxygen carriers were determined at 900 °C using both a thermo-gravimetric analyzer and a packed bed reactor.

Pure CuFe₂O₄ was found to have a low surface area, compact morphology, slow kinetics and high tolerance towards carbon formation. On the other hand MgAl₂O₄-supported Fe₂O₃ possessed a 'grainy' surface texture, high surface area, high reactivity towards CH₄ and a high activity for carbon deposition. The surface morphology of MgAl₂O₄-supported, Fe-Cu-based oxygen carriers were found to be an intermediate of MgAl₂O₄-supported Fe₂O₃ and CuFe₂O₄. Thermo-gravimetric and pack bed measurements revealed that the oxidation kinetics and H₂ yield of MgAl₂O₄-stabilized, Fe-Cu-based oxygen carriers were, respectively, substantially faster and higher than for CuFe₂O₄. For MgAl₂O₄-stabilized, Fe-Cu-based oxygen carriers in-situ XRD and XAS measurements revealed that Fe₂O₃ was stabilized by both a spinel phase (MgAl₂O₄) and a delafossite phase (CuFeO₂). Additionally, MgAl₂O₄-stabilized, Fe-Cu-based oxygen carriers effectively inhibited the decomposition of CH₄ and the formation of iron carbide. Scanning electron micrographs and XAS measurements indicated that the reduced quantity of carbon deposition on the bimetallic oxygen carriers is due to an alteration of the surface morphology, impeding the Fe-based activation of CH₄.

References

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