The hydrogen reduction of cobalt-tungsten mixed oxides

Abstract

The hydrogen reduction of two non-stoichiometric samples of cobalt tungstate (one cobalt-rich, the other heavily tungsten-rich) has been studied over the temperature range 600 to 1100° C using thermogravimetry, X-ray diffraction analysis and scanning electron microscopy. The products are shown to be non-equilibrium at most reduction temperatures. In order to explain the experimentally observed X-ray diffraction data it is postulated that the reduction process occurs via the formation of an amorphous phase which contains cobalt, tungsten and oxygen. The amorphous phase becomes unstable at low oxygen potentials and precipitates either, or both, Co₃W and Co₇W₆ depending upon the degree of cobalt enrichment of the amorphous phase. These are the only two cobalt-containing crystalline phases in the products of reduction and are not detected before at least 53% reduction has occurred. During the early stages of reduction either tungsten (for near stoichiometric, cobalt-rich oxide) or WO₂ (for tungsten-rich oxide-CoWO₄ plus WO₃) are the only crystalline products of reduction.