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

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-Mn 2 O 3 to α-Fe 2 O 3 , (2) LiMn 2 O 4 to LiFe 5 O 8 , (3) CaMnO 3 through Ca 3 (Mn,Fe) 3 O 8+x to CaFe 2 O 5 , 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) 2 O 3 part in series (1), the whole spinel series (2), and O-deficient perovskite Ca 3 (Mn,Fe) 3 O 8+x in the middle of (3). The stability range of Ca 3 (Mn,Fe) 3 O 8+x depends on the calcination temperature, Mn-doped hematite α-(Fe,Mn) 2 O 3 with Fe/(Fe+Mn)=0.9 is not reductively dissolved before hydrogen evolution. The reductive dissolution of CaMnO 3 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) 2 O 3 and LiMn 2 O 4 -LiFe 5 O 8 are very sensitive to Fe content, whereas the lattice parameters are negligibly affected by Fe amount in the former case.