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

- Redox properties of the ferrites improved by modification with CeZrO2.
- CoFe2O4 material could be reduced and reoxidized to the as prepared state.
- Reduction and reoxidation of NiFe2O4 lead into separate Ni and Fe3O4 phases.
- Carbon formation enhanced CO yield.

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 °C and 750 °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 H2-TPR and CO2-TPO cycles were followed using in situ XRD. Further, the long term stability was assessed during one hundred isothermal chemical looping redox cycles using H2 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 ∼40 mol CO kgCoFe2O4−1, 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 (∼13 mol CO kgCoFe2O4−1) because of incomplete conversion of CH4 and water. A pre-catalyst bed configuration for complete conversion of CH4 and water is proposed.

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