CHARACTERIZATION OF COPRECIPITATED FE2O3-AL2O3 POWDERS

Powders belonging to the Fe2O3-Al2O3 system prepared by a coprecipitation method have been characterized from the point of view of their solid-state and surface structure by X-ray diffraction (XRD), Fourier-transform (far) infrared (FTIR/FT-FIR), diffuse reflectance ultraviolet-visible (DR-UV-VIS) Spectroscopy, BET surface area and porosity and adsorption of probe molecules. In samples calcined at 673 K a disordered defective spinel-type phase, gamma-FeAlO3, has been found, containing Fe3+ in octahedral sites and Al3+ in both octahedral and tetrahedral sites, both in the bulk and on the surface. Samples calcined at 1173 K are composed of mixtures of two saturated solid solutions with corundum-haematite-type structures. The surface of these biphasic materials is dominated by the higher reactivity of alpha-Fe2O3 with respect to alpha-Al2O3, although tetrahedral Al3+ is also found at the surface of the saturated solid solution of Fe2O3 in alpha-Al2O3. Al3+ added to Fe2O3 tends to stabilize the spinel-type phase and to hinder sintering, while Fe3+ added to Al2O3 accelerates the gamma --> alpha phase transition and sintering.