Because of the low cost and easy availability, iron oxides have been widely used in chemical looping combustion. However, pure iron oxides often suffered from a loss of oxygen carrying capacity in redox cycles due to thermal sintering. The redox performance of iron oxides can be improved by the addition of Al 2 O 3 as a support. In this study, the investigation was carried out regarding the activation behavior of a Fe 2 O 3 -Al 2 O 3 oxygen carrier in initial cycles. The combined use of different textural and morphological characterization techniques (X-ray diffraction (XRD), X-ray photoelectron spectroscopy XPS, Hg intrusion, and scanning electron microscopy−energy-dispersive spectrometry (SEM-EDS)) was conducted to reveal how microscopic changes triggered the activation of the Fe 2 O 3 -Al 2 O 3 sample. It indicated that the activation process of the Fe 2 O 3 -Al 2 O 3 sample was caused by the outward migration of Fe cations to the sample surface. A fresh sample presented a surface with a dense and nonporous structure. However, once the sample was activated after the cyclic reactions, Fe cations were enriched on the sample surface in the form of small grains. This leads to enhanced redox activity and increased oxygen carrying capacity.

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Summary NiFe2O4 presents high redox activity and good promoted application prospect in chemical looping processes. However, phase segregation caused by outward diffusion of Fe cations often lead to low cycling stability for NiFe2O4. In this study, the inherent mechanism of phase segregation for NiFe2O4 was investigated deeply. The results indicated that reduction degree exhibited significant influence on the phase segregation of NiFe2O4 in successive redox cycles. When NiFe2O4 was reduced to Ni and Fe3O4 in redox cycles, NiFe2O4 displayed stable redox activity without phase segregation. As the reduction degree reached to FeO and FeNi, a needle‐like structure with Fe enrichment was formed on the surface of the first cycled NiFe2O4. In the subsequent redox cycles with deep reduction degree, both severe phase segregation and deactivation were occurred for spent NiFe2O4 sample. It can be seen that phase segregation is the main reason for the deactivation of NiFe2O4 instead of surface sintering in redox cycles. The results of this work provide guidance for the development of NiFe2O4‐based oxygen carriers with high redox performance.

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