Fe₂O₃-MgAl₂O₄ for CO Production from CO₂: Mössbauer Spectroscopy and in Situ X-ray Diffraction

Fe₂O₃/MgFeAlO_x materials are promising oxygen storage candidates for chemical looping CO₂ conversion. In this work, the cyclic stability of a 50Fe₂O₃/MgFeAlO_x (containing 50 wt % Fe₂O₃ and 50 wt % MgAl₂O₄) oxygen storage material is investigated. The evolution of its bulk properties over the course of 1000 H₂/CO₂ redox cycles has been studied by means of ⁵⁷Fe Mössbauer spectroscopy and in situ X-ray diffraction. As expected, all iron in the as-prepared oxygen storage material was present as Fe³⁺, 64% of which in iron-rich phases α-Fe₂O₃ and α-FeOOH and 36% in the form of a MgFeAlO_x spinel. In contrast, after 1000 redox cycles, only 19% of iron was present in an iron-rich spinel such as Fe₃O₄, γ-Fe₂O₃, and MgFe₂O₄. The remaining 81% was present in the form of Mg-Fe-Al-O, including Mg_xFe_1-xO. ILEEMS measurements showed surface enrichment of Fe³⁺ in 50Fe₂O₃/MgFeAlO_x after 1000 redox cycles, with 36% of all surface Fe present as Fe³⁺ in iron-rich spinel phases such as γ-Fe₂O₃ and/or MgFe₂O₄.