Preparation and characterization of Fe 2–x Cr x O ₃ mixed oxide powders

Mixed oxides of the general formula Fe2–xCrxO3 have been prepared by a coprecipitation–calcination procedure. The precursors contain a crystalline goethite-like phase and an amorphous trihydroxide phase if Cr is present. Calcnnation for 5 h at 673 K resulted in crystalline powders whose surface areas decrease from 102 to near 20 m2 g–1 with increasing Cr content. IR spectra suggest that the structure of these materials consists of a superstructure of the corundum-type lattice of haematite. In parallel with the decrease of surface area, the smallest slit-shaped pores arising from goethite-to-haematite topotactic decomposition progressively disappear. Cr is observed to form chromate species, some of which are exposed on the surface of the calcined materials. These species are thought to have a role in the catalytic combustion activity of these materials. The optical behaviour of the powders in the IR, visible and UV regions is discussed.