Na-β-Al₂O₃ stabilized Fe₂O₃ oxygen carriers for chemical looping water splitting: correlating structure with redox...

Chemical looping is an emerging technology to produce high purity hydrogen from fossil fuels or biomass with the simultaneous capture of the CO₂ produced at the distributed scale. This process requires the availability of stable Fe₂O₃-based oxygen carriers. Fe₂O₃-Al₂O₃ based oxygen carriers exhibit a decay in the H₂ yield with cycle number, due to the formation of FeAl₂O₄ that possesses a very low capacity for water splitting at typical operating conditions of conventional chemical looping schemes (700-1000 °C). In this study, the addition of sodium (via a sodium salt) in the synthesis of Fe₂O₃-Al₂O₃ oxygen carriers was assessed as a means to counteract the cyclic deactivation of the oxygen carrier. Detailed insight into the oxygen carrier's structure was gained by combined X-ray powder diffraction (XRD), X-ray absorption spectroscopy (XAS) at the Al, Na and Fe K-edges and scanning transmission electron microscopy/energy-dispersive X-ray spectroscopy (STEM/EDX) analyses. The addition of sodium prevented the formation of FeAl₂O₄ and stabilized the oxygen carrier via the formation of a layered structure, Na-β-Al₂O₃ phase. The material, i.e. Na-β-Al₂O₃ stabilized Fe₂O₃, showed a stable H₂ yield of ca. 13.3 mmol g^-1 over 15 cycles.