Hydrogen peroxide decomposition on manganese oxide (pyrolusite): Kinetics, intermediates, and mechanism

The objective of this study is the kinetic interpretation of hydrogen peroxide decomposition on manganese oxide (pyrolusite) and the explanation of the reaction mechanism including the hydroperoxide/superoxide anion. The decomposition of hydrogen peroxide on manganese oxide at pH 7 was represented by a pseudo first-order model. The maximum value of the observed first-order rates constants (kobs) was 0.741 min−1 at 11.8 of [H2O2]/[MnO2] when [H2O2]/[MnO2] were ranged from 58.8 to 3.92. The pseudo first-order rate constants (kMnO2kMnO2) approximated as the average value of 0.025 (min mM)−1 with a standard deviation of 0.003 at [H2O2]/[MnO2] ranged from 39.2 to 11.8. When [H2O2]/[MnO2] was 3.92, the rate constants (kMnO2kMnO2) was 0.061 (min mM)−1 as maximum. Oxygen production showed that the initial rates increased with decreasing [H2O2]/[MnO2] and the total amounts of oxygen was slightly less than the stoichiometric value (0.5) in most experiments. However, oxygen was produced at more than 0.5 in low [H2O2]/[MnO2] (i.e. 3.92 and 9.79). The relative production of hydroperoxide/superoxide anion implied that the production increased with low [H2O2]/[MnO2], and the existence of anions suggested that the mechanism includes propagation reactions with intermediates such as hydroperoxide/superoxide anion in solution. In addition, both [H2O2] decomposition and the production of anion were accelerated in alkaline solution. Manganese ion dissolved into solution was negligible in neutral and alkaline conditions, but it greatly increased in acidic conditions.