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

Abstract 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. CO 2 was used for reoxidation, resulting in the production of CO. The structural characteristics, CO formation and stability of a range of x CoFe 2 O 4 /NiFe 2 O 4 (x = 20 − 100 wt%) modified with CeZrO 2 were investigated through STEM and in situ XRD. Crystallographic changes during two consecutive H 2 -TPR and CO 2 -TPO cycles were followed using in situ XRD. Further, the long term stability was assessed during one hundred isothermal chemical looping redox cycles using H 2 for reduction and CO 2 for reoxidation. The 20 wt% CoFe 2 O 4 material could be reduced and reoxidized to the as prepared state without loss in oxygen storage capacity. In 80 wt% CoFe 2 O 4 however, deactivation due to phase segregation into Co and Fe 3 O 4 along with material sintering occurred. On the other hand, all NiFe 2 O 4 materials modified with CeZrO 2 suffered from sintering as well as phase segregation into separate Ni and Fe 3 O 4 phases, which could not be restored into the original spinel (NiFe 2 O 4 ) phase. During chemical looping with methanol, carbon formation was observed on CoFe 2 O 4 /NiFe 2 O 4 ferrites modified with CeZrO 2 . 20 wt% CoFe 2 O 4 was the best performing material with a CO yield of ∼40 mol CO k g C o F e 2 O 4 − 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 k g C o F e 2 O 4 − 1 ) because of incomplete conversion of CH 4 and water. A pre-catalyst bed configuration for complete conversion of CH 4 and water is proposed.