Oxidation of synthetic hausmannite (Mn3O4) to manganite (MnOOH)

The oxidation of aqueous solutions of manganese (II) chloride by hydrogen peroxide leads to the mixture of hausmannite (Mn3O4) with manganite (MnOOH). Being air dried, this mixture undergoes slow oxidation at room temperature during which the content of manganite is rising, and an equilibrium molar ratio Mn3O4:MnOOH reaches 1:3 in approx. 1 year of storage. In terms of reversible first order reactions, rate constants for the forward and reverse reactions are 2.62 x 10(-1) and 8.75 x 10(-2) month(-1), respectively. In the course of oxidation, additional IR adsorption bands emerge in the region of vibrations of physisorbed water molecules at 3160 and 3300 cm(-1). These bands are absent in IR spectra of equilibrium mixtures and disappear after treatment of samples with Sr2+ ions as a result of ion exchange. This finding enables one to suggest that adsorbed water forms cation exchangeable surface groups -Mn=(OH)(2) whose transformation into MnOOH is a step determining the oxidation rate. On the other hand, these data may be considered as the first spectroscopic evidence of sorption of spherical cations onto the surface of hydrous oxides. (C) 2009 Elsevier B.V. All rights reserved.