Bifunctional Co- and Ni- ferrites for catalyst-assisted chemical looping with alcohols

Ferrite materials were applied for catalyst-assisted chemical looping with two different alcohol fuels, methanol and an ethanol-water mixture (1:1 mol ratio), at 550 degrees C and 750 degrees C respectively. CO2 was used for reoxidation, resulting in the production of CO. The structural characteristics, CO formation and stability of a range of x CoFe2O4/NiFe2O4 (X = 20 - 100 wt%) modified with CeZrO2 were investigated through STEM and in situ XRD. Crystallographic changes during two consecutive H-2-TPR and CO2-TPO cycles were followed using in situ XRD. Further, the long term stability was assessed during one hunched isothermal chemical looping redox cycles using H-2 for reduction and CO2 for reoxidation. The 20 wt% CoFe2O4 material could be reduced and reoxidized to the as prepared state without loss in oxygen storage capacity. In 80 wt% CoFe2O4 however, deactivation due to phase segregation into Co and Fe3O4 along with material sintering occurred. On the other hand, all NiFe2O4 materials modified with CeZrO2 suffered from sintering as well as phase segregation into separate Ni and Fe3O4 phases, which could not be restored into the original spinel (NiFe2O4) phase. During chemical looping with methanol, carbon formation was observed on CoFe2O4/NiFe2O4 ferrites modified with CeZrO2. 20 wt% CoFe2O4 was the best performing material with a CO yield of similar to 40 mol CO kg(Co2O4)(-1), i.e. almost twice the theoretical amount. This high CO yield was ascribed to the oxidation of carbon formed upon material reduction. During chemical looping with the ethanol-water mixture however, the CO yield remained low (similar to 13 mol CO kg(Co2O4)(-1))because of incomplete conversion of CH4 and water. A pre-catalyst bed configuration for complete conversion of CH4 and water is proposed.