Dec 22, 2008· * Manganese dioxide also catalyses the decomposition of hydrogen peroxide to oxygen and water: 2 H2O2(aq) → O2(g) + 2 H2O(l) * Eighteenthcentury British chemists referred to MnO2 simply as manganese. Elemental manganese was known as regulus of manganese. * Ancient cave painters used MnO2 as a black or brown pigment.