Ferrites as redox catalysts for chemical looping processes

Spinel ferrites can accommodate oxygen vacancies in their structure and reversibly exchange oxygen with the environment at high temperatures, depending on the partial pressure of oxygen. Therefore their use in Chemical Looping Combustion (CLC) is being studied intensively. In the present work the use of ferrites with the general formula of MeFe2O4 (Me = Mn, Ni, Zn, Co, Cu) as oxygen carriers or potentially reactive supports for oxygen carriers in Chemical Looping Combustion is explored. Polycrystalline ferrite samples are prepared using the conventional ceramic technology of "solid-state reaction". The performance of the prepared materials is evaluated at a fixed-bed laboratory scale pulse reaction unit. During the fuel oxidation step, methane pulses are injected over the ferrite powder while subsequent oxidation of the solid is performed with gaseous oxygen. The performance of the candidate materials is ranked by comparing the amount of oxygen per mole solid (d) that can be delivered reversibly to the fuel which corresponds to the Oxygen Transfer Capacity of the different ferrite materials as well as the methane conversion during the fuel oxidation step and the gaseous product distribution. The stability of the materials during multiple reduction-oxidation cycles is evaluated during 5-10 subsequent cycles, each of which is comprised of an activation-regeneration and a solid oxidation step. All the examined ferrites have the ability to deliver their lattice oxygen to a fuel and to some extend regain it in the presence of air. Cu ferrites are very active in CH4 oxidation with maximum conversion higher than 95%, while Zn and Ni ferrites are also highly ranked with methane conversion reaching up to 90% and 78% respectively, however they deactivate very fast after redox cycles. Mn ferrites present relatively low activity (similar to 30%), while Co ferrites present moderate methane conversion, approximately 68% but remarkable stability during subsequent redox cycles. Cu-, Co-and Ni-ferrites can deliver higher amounts of lattice O, compared to Mn ferrite. The structural and morphological stability of the materials after the multiple redox cycles is examined by X-ray diffraction and SEM-EDS analysis. (C) 2015 Elsevier Ltd. All rights reserved.