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.

The electron exchange between octahedral FeII and FeIII ions has important consequences on the specific dissolution rates.Figure optionsDownload as PowerPoint slide