Activation and Reactivity of Iron Oxides as Oxygen Carriers for Hydrogen Production by Chemical Looping

We report the activation principle, the different forms of activated oxygen carriers, and the optimal activation method for iron-based oxygen carriers. The activation of an oxygen carrier is commonly referred to as the enhancement of the oxygen transfer capacity of an oxygen carrier by repeated reduction and oxidation reactions, which is mainly attributed to enhanced intraparticle ionic/gaseous diffusivity over the support material. Accurate knowledge of the activation process for an oxygen carrier is a key issue in the development of particles and optimization of a chemical looping system. In this study, we found that two key factors, the interaction between iron oxide and the support as well as the localized hematite-lower oxides stress, are fundamental reasons for the activation of iron-based oxygen carriers. The dispersion of the iron oxide in the activated particles was determined by the iron oxide–support interaction. The larger the localized stress applied at the interface between the hematite and lower oxides, the more cracks/pores were created, facilitating gas diffusion. We observed the different forms of the activated particles by altering the reduction and oxidation conditions, and the various formations could be explained by the aforementioned key factors. One of the activated particles covered with sintered iron oxide resulting from the outward diffusion of Fe showed enhanced reactivity. However, this particle revealed lower reactivity than the highly dispersed oxygen carrier and the gradual formation of inactive iron due to sintering in repeated redox cycles. In regard to the optimal activation method, the particles subjected to conventional redox cycles (reduction–steam oxidation–air oxidation) were slowly activated as the cycles repeated, while particles subjected to redox cycles without steam oxidation were rapidly activated during the redox cycles due to larger localized stresses at the interface between the hematite and Fe than between the hematite and magnetite. We expect these findings will be useful in developing high-performance oxygen carriers and activating them efficiently