Preparation and properties of Co-Fe mixed oxides obtained by calcination of layered double hydroxides

Solids containing Co(II) and Fe(III) with molar ratios of 211, 312, 1/1, 213 and 112 have been synthetised by coprecipitation at constant pH. All they displayed a hydrotalcite-like structure with interlayer carbonate, which crystallinity decreases as the iron content was increased. No other crystalline phase was identified, even in the Fe-rich samples. They have been characterised by powder X-ray diffraction, FT-IR spectroscopy, thermal analysis (differential thermal analysis, thermogravimetric analysis and temperature-programmed reduction), in addition to specific surface area assessment by nitrogen adsorption at -196degreesC. A minor oxidation of Co(II) to Co(III) is observed in the Co-rich samples, although it reverses again to Co(II) upon calcination in oxygen at ca. 850degreesC. Thermal decomposition takes place in a single step up to ca. 350degreesC, and the specific surface area increases with the iron content, probably because of the presence of hydrated amorphous iron oxides. The solids calcined at 1200degreesC in air contain crystalline CoO, Co3O4 and CoFe2O4 (spinel), this one being the dominant phase, and only phase detected for large Fe contents. Metallic species are more easily reduced in the original solids than in the calcined ones, and in all cases iron seems to be reduced at a higher temperature than cobalt.