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It was previously shown that hydrogen peroxide (H2O2) is spontaneously produced in micrometer-sized water droplets (microdroplets), which are generated by atomizing bulk water using nebulization without the application of an external electric field. Here we report that H2O2 is spontaneously produced in water microdroplets formed by dropwise condensation of water vapor on low-temperature substrates. Because peroxide formation is induced by a strong electric field formed at the water-air interface of microdroplets, no catalysts or external electrical bias, as well as precursor chemicals, are necessary. Time-course observations of the H2O2 production in condensate microdroplets showed that H2O2 was generated from microdroplets with sizes typically less than ∼10 μm. The spontaneous production of H2O2 was commonly observed on various different substrates, including silicon, plastic, glass, and metal. Studies with substrates with different surface conditions showed that the nucleation and the growth processes of condensate water microdroplets govern H2O2 generation. We also found that the H2O2 production yield strongly depends on environmental conditions, including relative humidity and substrate temperature. These results show that the production of H2O2 occurs in water microdroplets formed by not only atomizing bulk water but also condensing water vapor, suggesting that spontaneous water oxidation to form H2O2 from water microdroplets is a general phenomenon. These findings provide innovative opportunities for green chemistry at heterogeneous interfaces, self-cleaning of surfaces, and safe and effective disinfection. They also may have important implications for prebiotic chemistry. [ABSTRACT FROM AUTHOR]

We have shown that, unlike bulk water, tiny water droplets (microdroplets) cause reduction of gold ions (1) as well as a number of organic compounds (2). Evidence has been presented that the source of electrons arises from hydroxyl anions (OH-) at or near the surface of the microdroplet (2). We report the formation of hydrogen peroxide (H2O2) in aqueous microdroplets and suggest that the observed H2O2 results from the recombination of hydroxyl radicals (OH) at or near the air- water interface of aqueous microdroplets sprayed into roomtemperature air. Hydrogen peroxide is a commodity chemical that has many different applications, such as chemical synthesis or as a disinfectant, in mining and metal processing, as well as pulp and textile bleaching (3). H2O2 has often been touted as a green oxidant because, upon decomposition, it generates oxygen and water (4). However, the most common industrial method (~95% worldwide) for 2O2 synthesis (5), the 2-step anthraquinone process, cannot be considered green (6) because organic wastes are generated from inefficient oxidation of the anthraquinone. Some advances in H2O2 synthesis have focused on catalytically combining 2 and 2 (7, 8). Other methods electrochemically generate H2O2 by electrolysis of O2 at the anode (9, 10), or photocatalytically generate reactive superoxo radicals (11). Recently, H2O2 was formed from a reaction between plasma and a water surface (12). However, these direct synthesis methods of H2O2 have limitations, including the use of precious metal catalysts, low yields, required 2 supply, and high energy consumption (13, 14). In what follows, we report the direct, spontaneous generation of H22 from aqueous microdroplets in the absence of applied voltage, catalyst, or any other added chemicals. We alsospeculate about the nature of the mechanism responsible for these observations. [ABSTRACT FROM AUTHOR]

It was previously shown [J. K. Lee et al. , Proc. Natl. Acad. Sci. U.S.A , 116, 19294-19298 (2019)] that hydrogen peroxide (H 2 O 2 ) is spontaneously produced in micrometer-sized water droplets (microdroplets), which are generated by atomizing bulk water using nebulization without the application of an external electric field. Here we report that H 2 O 2 is spontaneously produced in water microdroplets formed by dropwise condensation of water vapor on low-temperature substrates. Because peroxide formation is induced by a strong electric field formed at the water-air interface of microdroplets, no catalysts or external electrical bias, as well as precursor chemicals, are necessary. Time-course observations of the H 2 O 2 production in condensate microdroplets showed that H 2 O 2 was generated from microdroplets with sizes typically less than ∼10 µm. The spontaneous production of H 2 O 2 was commonly observed on various different substrates, including silicon, plastic, glass, and metal. Studies with substrates with different surface conditions showed that the nucleation and the growth processes of condensate water microdroplets govern H 2 O 2 generation. We also found that the H 2 O 2 production yield strongly depends on environmental conditions, including relative humidity and substrate temperature. These results show that the production of H 2 O 2 occurs in water microdroplets formed by not only atomizing bulk water but also condensing water vapor, suggesting that spontaneous water oxidation to form H 2 O 2 from water microdroplets is a general phenomenon. These findings provide innovative opportunities for green chemistry at heterogeneous interfaces, self-cleaning of surfaces, and safe and effective disinfection. They also may have important implications for prebiotic chemistry., Competing Interests: The authors declare no competing interest.