THE ROLE OF COBALT IN THE OXIDATION MECHANISM MODIFICATION OF THE MO3-SUBSTITUTED MAGNETITES( IONS OF MOLYBDENUM)

A kinetic study of the oxidation of Mo3+ ions has been performed on tw o submicrometer Mo and Mo-Co ferrites of spinel structure with a nearl y identical molybdenum content. We show that for the Co-free sample, o xidation proceeds by cationic diffusion in a cation-deficient spinel w ith a variable chemical diffusion coefficient. For the Co sample, whic h showed a pronounced stability towards oxygen, the oxidation process was divided into three steps. In step I, oxidation declines with time in an initial period (degree of oxidation alpha < 0.1). In step II (0. 1 < alpha < 0.25), oxidation is accelerated and in step III (alpha > 0 .25) oxidation is best described by the diffusion-controlled Jander eq uation, Comparing the vibrational spectra of these two spinels after t otal oxidation for various thermal treatments, we conclude that the co ordination around Mo6+ ions changes from tetrahedral to octahedral wit h Co substitution and that the location of cobalt and molybdenum ions in octahedral sites changes with the annealing process.