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 (k(obs)) was 0.741 min(-1) at 11.8 of [H(2)O(2)]/[= MnO(2)] when [H(2)O(2)]/[= MnO(2)] were ranged from 58.8 to 3.92. The pseudo first-order rate constants (k(MnO2)) approximated as the average value of 0.025 (min mM)(-1) with a standard deviation of 0.003 at [H(2)O(2)]/[= MnO(2)] ranged from 39.2 to 11.8. When [H(2)O(2)]/[= MnO(2)] was 3.92, the rate constants (k(MnO2)) was 0.061 (min mM)(-1) as maximum. Oxygen production showed that the initial rates increased with decreasing [H(2)O(2)]/[= MnO(2)] 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 [H(2)O(2)]/[= MnO(2)] (i.e. 3.92 and 9.79). The relative production of hydroperoxide/superoxide anion implied that the production increased with low [H(2)O(2)]/[= MnO(2)], and the existence of anions Suggested that the mechanism includes propagation reactions with intermediates such as hydroperoxide/Superoxide anion in solution. In addition, both [H(2)O(2)] 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. (C) 2008 Elsevier Ltd. All rights reserved