Ceramics-Silikáty 45, (2) 55 - 61 (2001)

Reductive dissolution of mixed oxides of Mn and Fe was studied by voltammetry of microparticles in acetate buffer at pH 4.4. The following four series were synthesized: (1) C-Mn₂O₃ to α-Fe₂O₃, (2) LiMn₂O₄ to LiFe₅O₈, (3) CaMnO₃ through Ca₃(Mn,Fe)₃O_8+x to CaFe₂O₅, and (4) almost amorphous MnO_x to FeOOH. The ranges of isostructural solid solutions were identified by XRD analysis. The following solid solutions with continuous change of both structure and dissolution reactivity were found: the bixbyite C-(Fe,Mn)₂O₃ part in series (1), the whole spinel series (2), and O-deficient perovskite Ca₃(Mn,Fe)₃O_8+x in the middle of (3). The stability range of Ca₃(Mn,Fe)₃O_8+x depends on the calcination temperature. Mndoped hematite α-(Fe,Mn)₂O₃ with Fe/(Fe+Mn)=0.9 is not reductively dissolved before hydrogen evolution. The reductive dissolution of CaMnO₃ part of (3) significantly depends on the calcination temperature. Mn(IV) in the series (4) is most easily reductively dissolved, and only the series (4) behaves like a physical mixture of two phases with two separate reaction steps corresponding to reductive dissolution of Mn(IV) and Fe(III). Voltammetric peak potentials of C-(Fe,Mn)₂O₃ and LiMn₂O₄ - LiFe₅O₈ are very sensitive to Fe content, whereas the lattice parameters are negligibly affected by Fe amount in the former case.