Redox reaction induced morphology and microstructure evolution of iron oxide in chemical looping process

Oxygen carrier (OC) plays an important role in the successful operation of chemical looping process. A detailed understanding of the structural evolution of the materials is crucial to design high performance OCs for this technology. In this study, Fe2O3 particles were used as oxygen carrier to investigate the morphology and microstructure changes induced by redox reaction during chemical looping process, which were performed in the single and multiple redox cycles. X-ray diffraction (XRD), Hg intrusion method and scanning electron microscope (SEM) were used to characterize the changes in phase composition, pore size distribution and morphology. Our results indicated that the surface area of Fe2O3 particles decreased during the redox cycles even though more macro-pores ( > 1000 nm) formed for these particles. Sintering was attributed to the surface area decrease of Fe2O3 particles in the redox process. Because of the outward Fe cation diffusion, porous structure in the interior of Fe2O3 particles was formed, which led to the crushing strength of the OC particles decreased significantly. In the reduction process, the aggregation of unstable Fe2O3 could cause the surface sintering of OC particles in the transformation of Fe2O3 into Fe2O3. Such findings can not only explain the performance of iron based OCs in redox reaction, but can also provide theoretical guidance for the synthesis of iron-based OCs.