Sonochemical reduction of permanganate to manganese dioxide: The effects of H 2O 2 formed in the sonolysis of water on the rates of reduction

Abstract

Chemical effects of ultrasound have been actively researched in the field of the synthesis of various metal nanoparticles and nanostructured materials. It is very important to understand the reduction mechanism of metal ions, because the reduction processes can be often applied to the synthesis of various materials. In this study, the sonochemical reduction of MnO 4 - to MnO 2 in water under Ar atmosphere was investigated to discuss the reduction mechanism. It has been reported that H, OH, H 2 and H 2O 2 are formed from the sonolysis of water. To understand the roles of H 2O 2 on the reduction, the reaction of MnO 4 - with H 2O 2 without ultrasonic irradiation was investigated. The obtained results suggested the progress of the following reaction: 2 MnO 4 - + 3 H 2 O 2 → 2 MnO 2 + 3 O 2 + 2 OH - + 2 H 2 O . In addition, the rates of the sonochemical reduction of MnO 4 - were investigated in the presence of 1-propanol, where 1-propanol acted as an OH radical scavenger so that the amounts of the sonochemically formed H 2O 2 molecules were able to be controlled. The results clearly indicated that the sonochemically formed H 2O 2 molecules as well as H 2 molecules and H atoms play an important role for MnO 4 - reduction. This mechanism was also supported by the analysis of pH changes during ultrasonic irradiation: the pH value increased as the sonochemical reduction of MnO 4 - proceeded.