Mechanism of sonochemical reduction of permanganate to manganese dioxide in aqueous alcohol solutions: Reactivities of reducing species formed by alcohol sonolysis

• Reactivity of reducing species formed by alcohol sonolysis was discussed.
• Reduction rate of MnO4− to MnO2 and MnO2 to Mn2+ was controlled by alcohol additives.
• Type of reducing species for MnO4− reduction switched at an alcohol concentration.
• Optical property and size of MnO2 were affected by the rate of MnO4− reduction.
• Presence of alcohol suppressed the reduction of MnO2 to Mn2+.

The sonochemical reduction of MnO4− to MnO2 in aqueous solutions was investigated as a function of alcohol concentration under Ar. The rate of MnO4− reduction initially decreased with increasing alcohol concentration, and then increased when the alcohol concentration was increased further. The concentrations at which the reduction rates were minimum depended on the hydrophobic properties of the added alcohols under ultrasonic irradiation. At low concentrations, the alcohols acted as OH radical scavengers; at high concentrations, they acted as reductant precursors: Rab, formed by abstraction reactions of the alcohols with sonochemically formed OH radicals or H atoms, and Rpy, formed by alcohol pyrolysis under ultrasonic irradiation. The results suggest that the reactivity order of the sonochemically formed reducing species with MnO4− at pH 7–9 is the sum of H2O2 and H > Rpy > Rab. The peak wavelengths of MnO2 colloidal solutions formed at high 1-butanol concentrations shifted to shorter wavelengths, suggesting the formation of small particles at high 1-butanol concentrations. The rates of sonochemical reduction of MnO2 to Mn2+ in the presence of 1-butanol were slower than that in the absence of 1-butanol, because the sonochemical formation of H2O2 and H, which act as reductants, was suppressed by 1-butanol in aqueous solutions.