Fundamental study on preferential reduction of Ni in NiFe₂O₄

The effect of thermodynamic variables on the preferential reduction of nickel in trevorite was investigated using thermogravimetric analysis (TGA). Experimental results of X-ray diffraction (XRD), backscattered electron-scanning electron microscopy (BSE-SEM), and inductively coupled plasma-optical emission spectroscopy (ICP-OES) indicated that the reduction behaviour of trevorite depends significantly on the thermodynamic stability determined by the temperature and oxygen potential. The nickel content in the metal phase increased as the temperature increased and CO partial pressure decreased, and the composition of the metal phase approached the stoichiometric nickel concentration of trevorite. The oxide phases that equilibrate with the metal phase during the reduction process change from spinel to monoxide. The stability diagram of trevorite represented the non-stoichiometric characteristics of wustite. The preferential reduction of nickel strongly depends on the stability diagram of trevorite, and the relationship between the stability diagram of trevorite and preferential reduction of nickel is discussed in detail.