Mg-Fe-Al-O for advanced CO₂ to CO conversion: Carbon monoxide yield vs. oxygen storage capacity

A detailed study of new oxygen carrier materials, Mg-Fe-Al-O, with various loadings of iron oxide (10-100 wt% Fe₂O₃) is carried out in order to investigate the relationship between material transformation, stability and CO yield from CO₂ conversion. In situ XRD during H₂-TPR, CO₂-TPO and isothermal chemical looping cycles as well as Mössbauer spectroscopy are employed. All samples show the formation of a spinel phase, MgFeAlO_x. High loadings of iron oxide (50-90 wt%) lead to both spinel and Fe₂O₃ phases and show deactivation in cycling as a result of Fe₂O₃ particle sintering. During the reduction, reoxidation and cycling of the spinel MgFeAlO_x phase, only limited sintering occurs. This is evidenced by the stable spinel crystallite sizes (∼15-20 nm) during isothermal cycling. The reduction of MgFe³⁺AlO_x starts at 400°C and proceeds via partial reduction to MgFe²⁺AlO_x. Prolonged cycling and higher temperatures (>750°C) lead to deeper reduction and segregation of Fe from the spinel structure. Very high stability and CO yield from CO₂ conversion are found in Mg-Fe-Al-O materials with 10 wt% Fe₂O₃, i.e. the lowest oxygen storage capacity among the tested samples. Compared to 10 wt% Fe₂O₃ supported on Al₂O₃ or MgO, the CO yield of the 10 wt% Fe₂O₃-MgFeAlO_x spinel is ten times higher.