Improving the Reactivity and Stability of Fe₂O₃/Al₂O₃ in Chemical Looping Process by Optimizing the Al₂O₃ Precursor

Oxygen carriers (OCs) with high reactivity and stability are eagerly desired in the chemical looping process to achieve efficient oxygen transfer among different reductants. Al₂O₃ is a widely used support in preparing Fe₂O₃-based OCs due to its low cost and adjustable texture. Also, diverse performances were obtained for the Fe₂O₃–Al₂O₃ synthesized with different Al₂O₃ precursors; however, how the Al₂O₃ type influences the performance of Fe₂O₃-based OCs is still unclear, which confuses the choice of Al₂O₃ in preparing the OCs. In the present work, seven types of Al₂O₃ precursors were adopted to prepare the Fe₂O₃–Al₂O₃ by mechanical mixing, then the reactivity and stability of these OCs were assessed in a chemical looping hydrogen generation process, and the way Al₂O₃ affects the performance of OCs was analyzed via XRD, SEM, BET, TPR, and XPS analysis on the calcined Al₂O₃ precursors and the prepared OCs. Results showed that the high reactivity of Fe₂O₃–Al₂O₃ originated from the improved internal diffusion and high oxygen deficiency and adsorbed oxygen content. Furthermore, structure–activity correlation analysis implied that the microtexture plays a bigger role in the OC reactivity and stability when compared with other physical characteristics, and the Fe₂O₃–Al₂O₃ OCs with high surface area and pore volume achieved nearly 100% fuel conversion, meanwhile produced a higher yield of H₂ (1.6 mmol/g Fe₂O₃), and showed a stable behavior in cycling tests. Furthermore, compared to other properties of the Al₂O₃-supported OCs, we found that the solubility of Al₂O₃ in Fe₂O₃ is the dominant factor that affects the hercynite formation during the Fe₂O₃–Al₂O₃ reduction process.