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

Reductive dissolution of mixed oxides of Mn and Fe was studied by voltammet ry of microparticles in acetate buffer at pH 4.4. The following four series were synthesized: (1) C-Mn2O3 to alpha -Fe2O3, (2) LiMn2O4 to LiF5O8. (3) CaMnO3 through Ca-3(Mn,Fe)(3)O8+x to CaFe2O5, and (4) almost amorphous MnOx to FeOOH. The ranges of isostructural solid solutions were identified by X RD analysis. The following solid solutions with continuous change of both s tructure 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 perovski te Ca-3(Mn,Fe)(3)O8+x in the middle of (3). The stability range of Ca-3(Mn, Fe)(3)O8+x depends on the calcination temperature. Mn-doped hematite alpha- (Fe,Mn)(3)O-3 with Fe/(Fe+Ms)=0.9 is not reductively dissolved before hydro gen evolution. The reductive dissolution of CaMnO3 part of (3) significantl y depends on the calcination temperature. Mn(IV) in the series (4) is most easily reductively dissolved, and only the series (4) behaves like a physic al mixture of two phases with phases with two separate reaction steps corre sponding to reductivc dissolution of Mn(IV) and Fe(III). Voltammetric peak potentials of C-(Fe,Mn)(2)O-3 and LiMn2O4-LiFe5O8 are very sensitive to Fe content, whereas the lattice parameters ore negligibly affected by Fe amoun t in the former case.