Spontaneous generation of hydrogen peroxide from aqueous microdroplets

We show H2O2 is spontaneously produced from pure water by atomizing bulk water into microdroplets (1 mu m to 20 mu m in diameter). Production of H2O2, as assayed by H2O2-sensitve fluorescence dye peroxyfluor-1, increased with decreasing microdroplet size. Cleavage of 4-carboxyphenylboronic acid and conversion of phenylboronic acid to phenols in microdroplets further confirmed the generation of H2O2. The generated H2O2 concentration was similar to 30 mu M (similar to 1 part per million) as determined by titration with potassium titanium oxalate. Changing the spray gas to O-2 or bubbling O-2 decreased the yield of H2O2 in microdroplets, indicating that pure water microdroplets directly generate H2O2 without help from O-2 either in air surrounding the droplet or dissolved in water. We consider various possible mechanisms for H2O2 formation and report a number of different experiments exploring this issue. We suggest that hydroxyl radical (OH) recombination is the most likely source, in which OH is generated by loss of an electron from OH- at or near the surface of the water microdroplet. This catalyst-free and voltage-free H2O2 production method provides innovative opportunities for green production of hydrogen peroxide.