Mg–Fe–Al–O for advanced CO 2 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% Fe 2 O 3 ) is carried out in order to investigate the relationship between material transformation, stability and CO yield from CO 2 conversion. In situ XRD during H 2 -TPR, CO 2 -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 Fe 2 O 3 phases and show deactivation in cycling as a result of Fe 2 O 3 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 MgFe 3+ AlO x starts at 400 °C and proceeds via partial reduction to MgFe 2+ 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 CO 2 conversion are found in Mg–Fe–Al–O materials with 10 wt% Fe 2 O 3 , i.e. the lowest oxygen storage capacity among the tested samples. Compared to 10 wt% Fe 2 O 3 supported on Al 2 O 3 or MgO, the CO yield of the 10 wt% Fe 2 O 3 –MgFeAlO x spinel is ten times higher.