The Formation and Some Properties of Hydrous Manganese(IV) Oxide

Abstract

Abstract The properties of two kinds of hydrous manganese(IV) oxides (abbreviated as hydrous MnO2) prepared by the reduction of KMnO4 and by the disproportionation of K2MnO4 in alkaline solutions were investigated by means of chemical analysis, X-ray analysis, IR spectroscopy, wide-line NMR spectroscopy, thermal analysis, magnetic susceptibility measurement, etc. The chemical reactivity of the hydrous MnO2 was also evaluated from the extent of oxidation to K2MnO4 by the reaction with an alkaline KMnO4 solution at 80 °C. The properties and reactivity of the hydrous MnO2 predominantly depended on the pH of the suspensions. At a pH 8 or higher, amorphous ternary compounds of manganese(IV) oxide with an approximate composition of K2O·4MnOx·4H2O (x=1.97–2.00) were formed. The IR spectra and wide-line NMR spectra showed that the chemically bound water was in two different forms. The hydrous MnO2 had the greater chemical reactivity, being easily soluble as manganate(VI) ions by the reaction described above. The contents of the chemically bound water and alkali in the hydrous MnO2 decreased with a decrease in the pH of the suspensions and upon the acid-treatment of the precipitates. This variation in the chemical composition caused a decrease in the reactivity and a marked variation in the thermal behavior. The structure of the hydrous MnO2 was discussed on the basis of findings regarding the magnetic susceptibility and its temperature dependence.