Can water droplets spontaneously produce hydrogen peroxide? Richard n. Zare, former dean of the Department of chemistry at Stanford University, proved this magical phenomenon in a recently published paper.

Zare is a big player. At the age of 29, he became a professor of Columbia University. At the age of 37, he was elected an academician of the American Academy of Sciences and the American Academy of Arts and Sciences. Since then, he has also won the titles of foreign academician of Chinese Academy of Sciences, the Royal Society of England, the Royal Swedish Academy of Engineering Sciences, the Indian Academy of Sciences and other academies of Sciences, as well as communication academician of the Academy of Sciences of developing countries.

In fact, as early as 2019, Zare discovered that hydrogen peroxide can be produced at the junction of water and air (its aqueous solution is commonly known as hydrogen peroxide). However, a scholar who had just been evaluated as an associate professor for a few years stood up and directly issued a paper to overturn Zare's research results.

In the face of the doubt, Zare didn't immediately make a sound counterattack, but carried out follow-up research. Finally, on April 22, 2022, 82 year old Zare, as the corresponding author, published a paper on the Journal of the American Chemical Society (JACS), and answered all the questions with data.

Richard n. Zare source: Stanford University

First thought

The most common use of hydrogen peroxide is sterilization.

From the molecular formula, the difference between hydrogen peroxide (H2O2) and water (H2O) is not large, but it is not easy to break the covalent bond of H2O and add an oxygen atom. If water is changed into hydrogen peroxide in a simple way, wouldn't it be possible to disinfect it by spraying water?

After the idea came into being, Zare began to experiment with it.

On August 26, 2019, Zare team reported the experimental results for the first time through the proceedings of the National Academy of Sciences (PNAs).

The results show that the diameter of water mist is 1 μ m ~20 μ M, pure water will naturally generate hydrogen peroxide. In micron sized water droplets, water molecules will spontaneously oxidize near the water air interface to form hydrogen peroxide. This process does not require any chemical reagent, catalyst, applied potential or radiation. This is the cleanest hydrogen peroxide manufacturing method in history.

More interestingly, the smaller the droplet, the more hydrogen peroxide will be produced proportionally.

Zare believes that hydroxyl radical (· oh) recombination is the most likely inducement, in which (· oh) is generated by oh- losing electrons on the surface of water droplets.

After obtaining the preliminary experimental results, Zare is curious about how hydrogen peroxide is produced and how hydroxyl radicals are recombined, so he will further study it.

On november23,2020, Zare team published a paper on PNAs again, proposing that the formation of hydrogen peroxide is caused by the strong electric field at the air-water interface of droplets, so no catalyst is required.

They also found that the production of hydrogen peroxide was closely related to environmental conditions such as humidity and temperature; At the same time, with the increase of the size of the small water droplets, the hydrogen peroxide produced proportionally becomes less. These results show that hydrogen peroxide is produced not only in water droplets, but also in the condensation process of water vapor, which indicates that spontaneous oxidation of water droplets to hydrogen peroxide is a common phenomenon.

The successive publication of papers not only made the media exaggerate this result, but also attracted extensive attention of the scientific community.

Challenge attack

Himanshu Mishra, associate professor of the seawater desalination and reuse center (WDRC) of King Abdullah University of science and technology, published articles on November 18, 2021 and January 14, 2022 respectively, questioning the experimental results in Zare's paper.

Mishra mentioned in the article published in the Physical Chemistry Letters (jpcl) that in order to deeply understand the phenomenon that hydrogen peroxide can naturally form at the air-water interface of water droplets, they conducted condensation experiments.

The concentration of hydrogen peroxide in water droplets was observed by heating and ultrasonic humidification in the range of 50-70 ℃. The results showed that the concentration of hydrogen peroxide in the water droplets condensed by heating was lower than the detection limit (0.25 µ m) regardless of the droplet size or wettability. In contrast, the water droplets condensed by ultrasonic humidification have significantly higher hydrogen peroxide concentration, but the highest concentration is 3 µ m, which is far lower than the 115 µ m experimental result of Zare team. Therefore, Mishra believes that ultrasonic humidifier can contribute to the production of hydrogen peroxide.

Later, Mishra pointed out in a paper published in chemical science that ozone in the air is also a necessary factor in the generation of hydrogen peroxide.

In the absence of ozone, hydrogen peroxide in water droplets cannot be detected. When the water droplets are in a gas with the same ozone concentration (10-100 ppb) as the air, hydrogen peroxide will naturally form, and its concentration will increase with the extension of the gas-liquid surface contact time.

Mishra's view is that the strong electric field at the air-water interface of water droplets turns itself into a micro battery, and a large number of unique oxidation-reduction reactions occur all the time, but these reactions do not occur in larger water droplets.

Respond to queries

In the face of doubts, Zare team carried out follow-up experiments and finally published several papers in 2022 to comprehensively respond to Professor Mishra's doubts.

On February 2, 2022, Zare team published a paper proving that water droplets can be used as electrochemical micro cells.

　　On February 14, Zare published a paper again in PNAS, proving that NMR can be used to test trace hydrogen peroxide.

　　On April 22, Zare issued a document on JACS to confirm that hydrogen peroxide was really produced in his experiment.

　　They designed a device to inject ultra pure nitrogen into the water for bubbling. At the same time, the gas containing almost no ozone is injected into a closed chamber through the humidifier. In the closed chamber, the water is sprayed through the silica (SiO2) capillary to form droplets.

NMR spectrum analysis shows that the difference of flow conditions keeps the concentration of hydrogen peroxide in the droplets between 0.3-1.5 µ M. Moreover, this result was also verified by fluorescence spectrometry. This finding confirmed that when water is sprayed to form droplets, it has its own ability to produce hydrogen peroxide. When compressed air or oxygen is used to replace nitrogen, the concentration of hydrogen peroxide will increase, which indicates that the interaction between water droplets and oxygen can promote the formation of hydrogen peroxide.

Finally, they believe that the spontaneous generation of hydrogen peroxide in ultra pure water may be related to the charge separation on the surface of droplets caused by atomization and the generation of hydroxyl radicals. As for ozone, tiny drops of water, with or without it, can produce hydrogen peroxide. However, the concentration is not as high as 115 µ m previously reported.

It is worth mentioning that at the beginning of this year, the research group of zhangxinxing, a researcher from the school of chemistry of Nankai University, published two papers in succession, and the research results support the mechanism proposed by Zare team.