Structural changes and surface properties of Cox Fe3—x O4 spinels

The cobalt ferrite spinel oxide series, CoxFe3-xO4 (0 less-than-or-equal-to x less-than-or-equal-to 3), has been prepared by coprecipitation. The adsorption-desorption isotherms of all the compositions calcined between 200 and 600-degrees-C have been measured using nitrogen gas at -196-degrees-C. The structural and the phase changes were characterized by TGA and XRD techniques. The results obtained revealed that the transformation of gamma- to alpha-Fe2O3 was accompanied by a sharp decrease in the S(BET) values. The addition of Co2+ ions into Fe2O3 up to x = 0.6 led to an observable increase in the S(BET) value. This behaviour was attributed to the incorporation of Co2+ ions into the Fe2O3 lattice and the retardation of the phase transition of gamma- to alpha-Fe2O3. The minimum S(BET) values obtained at a lattice composition of x = 1.0 corresponded to the formation of a cobalt ferrite normal spinel which is associated with the existence of narrow pores. The increase in S(BET) values in the cobalt-rich region, with a maximum at x = 2.6 is explained on the basis of the cationic replacement of Fe3+ ions in the Co3O4 lattice. Finally, calculation of pore volume distribution was carried out, in addition to V(a)-t plots, in order to study the nature of the surface porosity, which was found to be mesoporous.