Characterization of coprecipitated Fe2O3–Al2O3 powders

Abstract

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, γ-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 α-Fe₂O₃ with respect to α-Al₂O₃, although tetrahedral Al³⁺ is also found at the surface of the saturated solid solution of Fe₂O₃ in α-Al₂O₃. Al³⁺ added to Fe₂O₃ tends to stabilize the spinel-type phase and to hinder sintering, while Fe³⁺ added to Al₂O₃ accelerates the γ→α phase transition and sintering.