The Reducing Agents in Sonochemical Reactions without Any Additives

It has been experimentally reported that not only oxidation reactions but also reduction reactions occur in aqueous solutions under ultrasound without any additives. According to the numerical simulations of chemical reactions inside an air or argon bubble in water without any additives under ultrasound, reducing agents produced from the bubbles are H, H-2, HO2 (which becomes superoxide anion (O-2(-)) in liquid water), NO, and HNO2 (which becomes NO(2)(- )in liquid water). In addition, H2O2 sometimes works as a reducing agent. As the reduction potentials of H and H-2 (in strongly alkaline solutions for H2) are higher than those of RCHOH radicals, which are usually used to reduce metal ions, H and H-2 generated from cavitation bubbles are expected to reduce metal ions to produce metal nanoparticles (in strongly alkaline solutions for H-2 to work). It is possible that the superoxide anion (O-2(-)) also plays some role in the sonochemical reduction of some solutes. In strongly alkaline solutions, hydrated electrons (e(-)aq) formed from H atoms in liquid water may play an important role in the sonochemical reduction of solutes because the reduction potential is extremely high. The influence of ultrasonic frequency on the amount of H atoms produced from a cavitation bubble is also discussed.

Automated summary

It has been experimentally observed that oxidation and reduction reactions occur in ultrasound-assisted aqueous solutions without any additives. Numerical simulations show that reducing agents produced from air or argon bubbles in water under ultrasound include H, H-2, HO2, NO, HNO2, and sometimes H2O2. These agents can reduce metal ions to form metal nanoparticles in strongly alkaline solutions. Superoxide anion (O-2(-)) and hydrated electrons (e(-)aq) may also play a role in the sonochemical reduction of solutes, especially in strongly alkaline solutions. The influence of ultrasonic frequency on the production of H atoms from cavitation bubbles is also discussed.