Correlation of reactivity with structural factors in a series of Fe(II) substituted cobalt ferrites

A series of powdered cobalt ferrites, CoxFe3−xO4 with 0.66⩽x<1.00 containing different amounts of FeII, were synthesized by a mild procedure, and their Fe and Co site occupancies and structural characteristics were explored using X-ray anomalous scattering and the Rietveld refinement method. The dissolution kinetics, measured in 0.1 M oxalic acid aqueous solution at 70 °C, indicate in all cases the operation of a contracting volume rate law. The specific rates increased with the FeII content following approximately a second-order polynomial expression. This result suggests that the transfer of FeIII controls the dissolution rate, and that the leaching of a first layer of ions CoII and FeII leaves exposed a surface enriched in slower dissolving octahedral FeIII ions. Within this model, inner vicinal lattice FeII accelerates the rate of FeIII transfer via internal electron hopping. A chain mechanism, involving successive electron transfers, fits the data very well.