Production of hydrogen peroxide enabled by microdroplets

Geometry and dimensionality of a reaction system are known to play an important role in determining the yield as well as the rate of the reaction, especially in simple bimolecular reactions (1–8). Recently, several results have been reported for reactions in small droplets, which include charged microdroplets (3), microdiameter emulsions (2), inverted micelles (4), and the surfaces of aerosol particles (1). It has been found that chemical reactions can be accelerated in water microdroplets (2, 3, 6, 7), which indicates that the surface of aqueous microdroplets provides a unique reaction environment with different thermodynamics and kinetic properties compared to the bulk phase. In PNAS, Lee et al. (8) report experimental evidence that hydrogen peroxide (H₂O₂) is spontaneously produced from pure water by atomizing bulk water into microdroplets, which does not occur in bulk aqueous solutions. Production of H₂O₂ increases with decreasing microdroplet size, and the generated H₂O₂ concentration is ∼30 µM. Further analysis suggests that hydroxyl radical (OH) (which is generated by loss of an electron from OH⁻ near the surface of the water microdroplet) recombination is the most likely source.

H₂O₂ is a very simple compound in nature but with great importance in clinical, pharmaceutical, textile, environmental, and food manufacturing applications. One of the most important applications of H₂O₂ is its use in pulp and paper bleaching (9). It is also a very attractive oxidant for liquid-phase reactions, since H₂O₂ can oxidize a large variety of inorganic and organic substrates in liquid-phase reactions under very mild reaction conditions. Also, it can be used to improve the environment by oxidative removal of toxic compounds (10). Upon decomposition, H₂O₂ generates oxygen and water, and so it is considered environmentally friendly. Currently, H₂O₂ is produced industrially almost exclusively with the anthraquinone oxidation (AO) process, in which atmospheric oxygen, hydrogen, and an anthraquinone derivative are employed in the reaction cycle (9). The crude H₂O₂ obtained from this process is purified and concentrated. However, the AO process is not considered an environmentally friendly process since organic wastes are generated from inefficient oxidation of the anthraquinone. Some advances in H₂O₂ synthesis have been based on the partial oxidation of primary or secondary alcohols, the oxidation of a methylbenzyl alcohol, and electrolysis of a dilute solution of NaOH in an electrochemical cell (Dow process). However, these synthesis methods have limitations including low yields and high energy consumption (11).

Using a H₂O₂-sensitive water-soluble fluorescent probe, i.e., peroxyfluor-1 (PF-1), Lee et al. (8) have probed the fluorescence intensity of an aqueous solution containing 10 µM PF-1 as a function of microdroplet diameter. Their experiments showed that strong fluorescence emission was observed from microdroplets, suggesting that H₂O₂ was produced in microdroplets, but not in bulk water. By detailed analyses of the relationship between fluorescence intensity and microdroplet size, they showed that the yield of H₂O₂ increased as the microdroplet size decreased when the diameter of microdroplet is below ∼20 µm. The production of H₂O₂ in aqueous microdroplets can be further confirmed by assaying the cleavage of 4-carboxyphenylboronic acid by H₂O₂. Spontaneous generation of H₂O₂ from aqueous microdroplets observed in the absence of applied voltage, catalyst, or any other added chemicals may be used as an alternative method for green production of H₂O₂ (9). Measurement of H₂O₂ produced under different nebulization gases (dry air, N₂, and O₂) suggests that the reactions that generate H₂O₂ in microdroplets do not involve atmospheric oxygen or dissolved oxygen as a reactant. Furthermore, triboelectric effect, asymmetric charge separation during microdroplet fission, contact electrification, and the oxidation of water by the intrinsic surface potential of the water microdroplet surface are excluded for the formation of H₂O₂ by Lee et al. Based on these analysis, they propose that several factors unique to microdroplets may play a key role in the formation of H₂O₂. The strong electric field of the air–water interface of a microdroplet can ionize hydroxide ions to form hydroxyl radicals, which recombine to form H₂O₂ (12).

It is noteworthy that, to investigate the formation of H₂O₂ in microdroplets, microdroplets with diameter ranging from 1 to 250 µm were generated by spraying water (8). Such microdroplets have counterparts in the natural environment. Therefore, understanding chemistry in microdroplets from laboratory model studies can be generalized. Aqueous microdroplets and water–air interfaces are ubiquitous in nature, as manifested in the form of the surfaces of lakes, oceans, and atmospheric cloud/aerosols, as illustrated in Fig. 1. Atmospheric aerosols have typically a few micrometers in diameter, while cloud droplets and raindrops have 10² and 10³ micrometers in diameter, respectively (13). It is well known that clouds play an important role in regulating surface precipitation and the atmospheric radiative balance, and hence influence Earth’s climate system (14). Aerosols are central to the formation of cloud droplets, meaning changes to the chemical properties of aerosols can act to impact cloud properties, precipitation, and cloud radiative effects.

Fig. 1.

Microdroplets in the natural environment. H₂O₂ is spontaneously produced in microdroplets with diameter of less than ∼20 µm, but not in bulk water.

Atmospheric aerosols, which have been found to contain a large number of chemical elements and a high content of organic material, also provide sites for chemical reactions to take place (15–18). Both the air–water interface and the aqueous bulk interior of aerosols contribute to its overall effect. Atmospheric processes at the air–water interface following mechanisms that are quite different from those in the gas phase have recently been reported in the literature. One of the most significant of these reactions is that which leads to the formation of the ozone hole. During winter in the polar regions, aerosols grow to form polar stratospheric clouds, which offer a surface for chemical reactions to take place. These reactions convert HCl and ClONO₂ molecules into the photochemically active chlorine and ultimately lead to the destruction of ozone in the stratosphere (16). Additionally, ab initio molecular dynamics simulation results show that the typical timescale for the reaction of the smallest Criegee intermediate, CH₂OO, with water at the air–water interface is on the order of a few picoseconds, 2–3 orders of magnitude shorter than that in the gas phase. Interestingly, the loop-structure formation between CH₂OO and water molecules is not a prerequisite for the stepwise mechanism observed on the air–water interface, and is not possible in the gas phase (17). Another feature of an aerosol is that it is a polar solvent, which could catalyze the trans to cis isomerization of glyoxal solvation at the air–water interface compared to the gas phase (18).

In PNAS, Lee et al. report experimental evidence that hydrogen peroxide (H₂O₂) is spontaneously produced from pure water by atomizing bulk water into microdroplets, which does not occur in bulk aqueous solutions.

H₂O₂ is an important chemical component involved in maintaining the oxidative capacity of the atmosphere (19). Furthermore, the reactions involving aqueous H₂O₂ play an important role in the chemistry of atmospheric aerosols. They include oxidation of S(IV) to S(VI), which is important in the formation of acid fog and acid rain. Gas-phase H₂O₂ in the atmosphere is mainly produced by the recombination of the HO₂ radical. Alternately, the source of aqueous-phase H₂O₂ formation is still an open question, although simultaneous measurements show that raindrops contain H₂O₂ (20). Lee et al. confirmed that H₂O₂ is spontaneously generated in microdroplets with a diameter of <20 µm, which is the same size as atmospheric aerosols. Thus, their experimental results may help to understand a well-known fact of how nature behaves.

Lee et al. report a very interesting phenomena that gives important insight into the question of sources of H₂O₂. As with all innovations, it opens up some questions. OH recombination is suggested as the most likely source for the formation of H₂O₂ in the microdroplets, in which OH radical is generated by loss of an electron from OH⁻ due to the existence of strong electric field of the air–water interface of a microdroplet. Future studies will need to explore whether OH radicals can spontaneously form in a microdroplet in the absence of any added chemicals. Equally, the fate of the released electrons waiting to be found will need to be investigated if the OH radical is generated by loss of an electron from OH⁻.