Mechanochemical reduction of manganese dioxide by grinding in organic vapors

Abstract

Mechanochemical redox reactions of manganese dioxide with organic vapors have been studied to control the chemical valence (oxidation number) of manganese ions under grinding. Electrolytic-manganese dioxide (EMD) including double chains of edge-sharing MnO₆-octahedra transforms into β-MnO₂ (Pyrolusite) consisting of the single chains under grinding in Ar. The mechanical dissociation of the double chains generates active sites to adsorb the atmospheric (CH₃)₂CO vapor and the strongly adsorbed (CH₃)₂CO molecules pull out the oxide ions from MnO₂ to reduce Mn⁴⁺ to Mn³⁺ ions, while (CH₃)₂CO is oxidized to CO₂ and H₂O via the intermediate oxidation products, HCOH and CH₃COH. One (CH₃)₂CO molecule can reduce 16 MnO₂ units to trivalent MnO_1.5 (Bixbyite). However, the decomposition products, CO₂ and H₂O, occupying the adsorption sites dominantly to (CH₃)₂CO disturb the redox process. This mechanochemical reduction of MnO₂ can be applied to the synthesis of LiMn^IIIMn^IVO₄ powder from both combinations of the starting mixtures of MnO₂-LiOH and MnO₂-Li₂CO₃. Some alcohols and ketones also provide the similar reduction ability.