Dissolution kinetics of manganese oxides. Effects of preparation conditions, pH and oxidation/reduction from solution

Abstract

The dissolution kinetics of the p-type semiconducting oxide MnO in dilute acid solutions have been investigated. For pure MnO, prepared by decomposition of the carbonate above 900 °C in a H₂(5%)/N₂ atmosphere, the specific dissolution rate at pH 2 and 30 °C is ca. 4 × 10^–5 mol s^–1 m^–2. This is similar to the rate for ionic MgO, and two orders of magnitude faster than CoO. Like MgO and CoO, MnO exhibits an initial increase in rate with pH. Beyond this initial regime, the rate dependence on pH is consistent with rate control through the second protonation. The higher oxides, Mn₃O₄, Mn₂O₃ and MnO₂, have been shown to dissolve about three orders of magnitude more slowly than MnO. The presence of these higher oxides as impurities in many preparations of MnO explains the apparently slower dissolution kinetics of such impure monoxide samples. Reduction from solution increases the dissolution rate of the higher oxides. It is concluded that the presence of Mn³⁺ associated with surface or bulk defects in MnO can be rate-controlling for the non-stoichiometric oxide.