Cationic distribution and oxidation kinetics of trivalent molybdenum ions in submicron molybdenum substituted magnetites

Abstract

Valence states of molybdenum and iron ions and their cationic distribution on both octahedral (B) and tetrahedral (A) sites on the spinel structure of submicron molybdenum-substituted magnetites which are oxidized in cation deficient spinels have been performed by derivative thermogravimetry, IR spectroscopy and XPS. It was demonstrated that Fe 2+ B, Mo 3+ B, Mo 4+ B, Fe 2+ A and Mo 4+ A are successively oxidized into Fe 3+ and Mo 6+ ions below 450°C. A quantitative analysis based on this difference of reactivity of iron and molybdenum in relation to occupied sites (A or B) permits us to propose a cationic distribution for initial and oxidized phases. Kinetic studies of the oxidation process of Mo 3+ ions suggest that oxidation proceeds by way of a diffusion-controlled reaction in a cation deficient phase of variable composition with a chemical diffusion coefficient dependent on the vacancy content.