Deciphering the mechanism of hydrogen peroxide formation in ultrasound-mediated water-in-oil microdroplets

Microdroplet chemistry has emerged as a fascinating field, demonstrating remarkable reaction acceleration and enabling thermodynamically unfavorable processes. The spontaneous generation of hydrogen peroxide (H₂O₂) in water microdroplets presents a particularly intriguing phenomenon with significant implications for green chemistry and prebiotic processes. However, the transient nature of conventional microdroplets has hindered in-depth mechanistic investigations. This study employs ultrasound-mediated water-in-oil microdroplets to elucidate the underlying mechanism of H₂O₂ generation. Under ultrasound irradiation, the H₂O₂ concentration increases linearly with a production rate of 0.24 mM min^-1, reaching 14.37 mM after one hour. Notably, 99% of this production occurs at the water-oil interface, corresponding to approximately 0.10 mM m^-2 min^-1. Quantification of key intermediates reveals that superoxide radical (·O₂ ^-) concentrations are approximately tenfold higher than those of H₂O₂ and thousandfold higher than those of hydroxyl radicals (·OH). Through radical scavenging and isotope labeling experiments, we identify dissolved oxygen as the primary source and ·O₂ ^- as the main intermediate in H₂O₂ formation, following the pathway: O₂ → ·O₂ ^- → H₂O₂. We validate the critical role of the water-oil interface in initiating H₂O₂ production via charge separation reactions and demonstrate the significance of proton availability and surface propensity in facilitating efficient H₂O₂ generation. These findings not only advance our understanding of microdroplet interfacial chemistry but also offer potential applications in atmospheric chemistry, green disinfection, and origins of life research.