Isomorphous replacement in Fe2O3–NiO/Al2O3 particles and its effect on oxygen carrier performance

• Isomorphous replacement is found in Fe2O3/Al2O3 and Fe2O3–NiO/Al2O3.
• Isomorphous replacement of Al2O3 will inhibit the further reduction of Fe-OCs.
• Addition of NiO can effectively inhibit the Isomorphous replacement of Al2O3.
• Isomorphous replacement of NiO can promote the further reduction of Fe-oxides.
• Isomorphous replacement of NiO decreases the oxygen capacity of the Fe-OCs.

Using Fe2O3, Fe2O3/Al2O3, and Fe2O3–NiO/Al2O3 as oxygen carriers (OCs) on chemical looping combustion with CO was investigated in this study. All the modified iron-based OCs were prepared by sol–gel method, and the samples were characterized by X-ray diffraction (XRD), N2 adsorption/desorption, and scanning electron microscope/energy dispersive X-ray spectroscopy. The temperature-programmed reduction showed that loading on Al2O3 will promote the reduction of Fe2O3 to Fe3O4, but further reduction will be inhibited, and the addition of NiO can effectively reduce this inhibition. Thermogravimetric analysis also revealed that NiO addition can improve the maximum reaction rate of iron-based OCs in the oxidation stage. Mixing with NiO will also decrease the oxygen capacity of iron-based OCs. The XRD data of the reduced samples showed that both Al2O3 and NiO have an isomorphous replacement effect on Fe3O4. The isomorphous replacement effect of NiO is much stronger than that of Al2O3. Thus, NiO introduction can effectively inhibit the formation of FeAl2O4 and promote the further reduction of Fe-oxides to Fe. However, Ni2 + will replace part of the Fe2 + in Fe3O4 to form NiFe2O4 and slightly decrease the oxygen capacity of iron-based OCs.