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 H-2-TPR, CO2-TPO and isothermal chemical looping cycles as well as Mossbauer spectroscopy are employed. All samples show the formation of a spinel phase, MgFeAlOx. 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 MgFeAlOx phase, only limited sintering occurs. This is evidenced by the stable spinel crystallite sizes (similar to 15-20 nm) during isothermal cycling. The reduction of MgFe3+AlOx starts at 400 degrees C and proceeds via partial reduction to MgFe2+AlOx. Prolonged cycling and higher temperatures (>750 degrees 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-MgFeAlOx spinel is ten times higher.