Ceramics-Silikáty 45 (2) 55-61 (2001)

Electrochemical dissolution of mixed oxides of Mn and Fe: the relationship between phase composition and reactivity

T. Grygar, S. Bakardjieva, P. Bezdička, P. Vorm

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 a-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. Mn-doped hematite a-(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.