Shulou(Shulou.com)11/24 Report--

The original title: "this substance feeds half of the world's population, and now it has been knocked out by scientists with an iron ball."

Webb's chemical factory, Diglis, Worcestershire, c1860. Photo: OXFORD SCIENCE ARCHIVE / HERITAGE IMAGES / SCIENCE PHOTO LIBRARY synthetic ammonia seems to have been dealing with life and death-whether it ended the bird poop war, fed at least 40 per cent of the world's population, or later became a weapon of war.

Under the influence of multiple factors, hunger is still a common issue to be faced and solved by the whole world. In the long history of mankind trying to eradicate hunger, synthetic ammonia has left a strong and colorful brushstroke in it. Moreover, in more than 100 years, scientists have never given up the innovation of traditional ammonia synthesis technology, or completely overturned it. But either way, the goal of scientists is the same: to get the best technology.

Seen from afar, the Peruvian the Chincha Islands shines brightly in the sun. This is not the color of rocks or buildings, but the white droppings of seabirds. These bird droppings contain 15% nitrogen, 30 times the amount of nitrogen in cow feces. In fact, as early as the 19th century, when French chemist Jean-Baptiste Busengo (Jean-Baptiste Boussingault) conducted experiments on his farm, he found that nitrogen played a key role in increasing food production. In other words, bird manure, which is high in nitrogen, is an excellent fertilizer.

Bird droppings from Peru. Photo source: Tom á s Munita, tomasmunita.com. As a result, these pungent resources became valuable commodities in the 19th century and even sparked several "bird poop wars". Unfortunately, as a result of uncontrolled human mining, almost all the bird droppings dozens of meters deep in the Qincha Islands were dug up in just a few decades, and even seabirds were slaughtered in the end-only because humans believed that the island birds prevented them from digging.

Fortunately, however, we no longer needed seabirds to contribute their droppings all the time, because chemists were very interested in fixing nitrogen directly from the air and made a major breakthrough.

The "king" nitrogen element of synthetic ammonia castle is an important part of biological macromolecules such as protein and DNA, which is extremely important for the survival and growth of organisms. Although nitrogen (N2) accounts for the vast majority of the earth's atmosphere (about 78%), because the two nitrogen atoms in nitrogen are connected by three covalent bonds, the chemical properties of nitrogen are very stable and it is difficult for nitrogen molecules to be "broken".

For nature, nitrogen fixation is one of the basic reaction processes: after hundreds of millions of years of evolution, some organisms have learned to fix nitrogen from the air, and the nitrogenase produced by cells converts nitrogen into nitrogen-containing compounds, which can be used by organisms. But for human beings, the efficient dissociation of nitrogen molecules is a "mountain" that is difficult to cross easily so far.

Plants use nitrogenase produced by parasitic bacteria to convert nitrogen in the air into nitrogenous compounds that plant cells can use. (photo Source: DOI:10.1021 / acs.est.8b03853) at the beginning of the 20th century, humans achieved the first industrial production of ammonia: in 1908, German chemist Fritz Haber used metal catalysts to directly convert nitrogen and hydrogen into ammonia under high temperature and high pressure. When Hubble showed the ammonia synthesis process he designed to the German company BASF, the director of the BASF lab was shocked by the reaction conditions at 100 standard atmospheric pressure, because it was no small thing, and if something went wrong, it would be a matter of life and death.

The synthetic ammonia plant designed by Hubble is used to synthesize a large amount of ammonia in the laboratory. (photo source: DOI:10.1007 / s12045011-0130-0) however, Carl Bosch of BASF Lab believes: "synthetic ammonia is worth the risk." Finally, he solved the technical problem of high pressure ammonia synthesis and designed an industrial device capable of cyclic reaction. In 1913, BASF built the world's first plant for the synthesis of ammonia and produced the first batch of ammonia. This method of synthesizing ammonia (artificial nitrogen fixation) is called the Harper-Bosch method, for which Hubble and Bosch won the Nobel Prize in chemistry in 1918 and 1931, respectively.

In the following more than 100 years, with the development of technology, the continuous increase of ammonia gradually increased food production and fed more and more people. After data analysis and estimation, William Jan Willem Erisman of the Free University of Amsterdam in the Netherlands and Vaclav Smil of the University of Manitoba in Canada agree that nitrogen fertilizer produced by artificial nitrogen fixation feeds at least 40 per cent of the world's population. In addition to being used in the manufacture of fertilizers (accounting for about 80% of the total industrial synthetic ammonia), about 20% of the ammonia is the raw materials for the synthesis of nitrogenous drugs, nylon and other polymer compounds.

The impact of ammonia synthesized by the Harper-Bosch method on the global population (20th century). (photo source: DOI:10.1038 / ngeo325) some scientists will vividly compare the ammonia plant to a castle, and Harper Bosch is the king of the castle for more than 100 years, and no technology has yet shaken its position. Even so, however, there are many problems with Harper-Boshi. Among them, the biggest disadvantage lies in the extremely stringent reaction conditions-high temperature of 450550℃ and more than 100 standard atmospheric pressure.

Possible methods in fact, the reaction between nitrogen and hydrogen is an exothermic reaction. The advantage of the exothermic reaction is that it is thermodynamically feasible, and the lower temperature is beneficial to the exothermic process (remember the Le Chatelier principle of high school chemistry). So why does Hubble-Bosch still set such a high temperature? the reason is the extremely inert nitrogen we mentioned earlier.

Stable nitrogen molecules forced chemists to raise the temperature to around 500 ℃, which was achieved with the help of a catalyst. But the Le Chatelier principle tells us that high temperatures inhibit the exothermic process. As a result, the chemist compromised again and had to raise the pressure to 100 standard atmospheric pressure, when we got just "passed" the ammonia synthesis process. It can be said that the Harper-Bosch method is the result of a compromise between thermodynamic and kinetic limitations. For more than 100 years, chemists have been looking for alternative technologies to solve the problem of extreme reaction conditions.

The possible process of conversion from nitrogen to nitrogen-containing compounds. (photo source: K. HOLOSKI / DOI:10.1126 / science.aar6611) in a study published in Natural Nanotechnology (Nature Nanotechnology), an international team of research units took a different approach. In a ball mill equipped with iron balls, they used a mechanochemical strategy to design ammonia synthesis under mild conditions-- using iron powder as a catalyst. Up to 82.5% ammonia (the ratio of the concentration of ammonia to all gases after the reaction, including ammonia, nitrogen and hydrogen) was obtained under an initial reaction condition of 45 ℃ at 1 standard atmospheric pressure.

In order to synthesize ammonia under mild conditions, chemists have tried many methods. Among them, electrochemical ammonia synthesis has attracted the interest of a large number of scientists. But even a well-designed experiment ends up with very little ammonia-even before the experiment, researchers may have to worry about how to detect such a small amount of ammonia. And even if a small amount of ammonia is detected, it is likely that it is not converted from nitrogen and hydrogen. Therefore, the results of electrochemical synthesis of ammonia are always questioned.

Mechanochemical method is another synthesis strategy, the energy that drives the reaction process comes from mechanical energy, not the thermal energy provided by high temperature. A commonly used mechanochemical method is ball milling: colliding iron balls, reaction vessel walls and catalyst particles, generating local heat, creating highly active defect sites, or exposing new active surfaces, thereby reducing the harsh conditions required for the reaction.

"Mechanization is more suitable for small-scale ammonia synthesis than the traditional Harper-Bosch method," said Professor Jong-Beom Baek of the Ulsan Institute of Science and Technology in South Korea, one of the study's correspondent authors. "mild reaction conditions make simple production devices possible, which may mean that we no longer need reaction devices as large and complex as cities."

The collision of ball and powder in this new study, compared with the one-step reaction of Harper-Bosch method, the researchers divided the whole synthesis process into two steps: the dissociation of nitrogen and the preparation of ammonia.

The new study divides the whole synthesis process into two steps. (photo Source: original paper) scientists have known that mechanical collisions can produce high-density defects, and the defect sites help activate nitrogen molecules, making it easier to dissociate into nitrogen atoms. Therefore, the first step of the design is to use the collision of iron ball and iron powder to produce the defect site of iron catalyst, so as to accelerate the dissociation of nitrogen molecules and form iron nitride particles.

The researchers then introduced hydrogen into the ball mill to carry out the second step of hydrogenation, that is, the reaction of iron nitride with hydrogen. In the process of forming ammonia (NH3), nitrogen and hydrogen atoms take the lead in forming a variety of intermediate states (NHx,x=0-2). These intermediate products would be strongly adsorbed on the defect sites, so it is not easy to leave these active sites and inhibit the hydrogenation reaction.

However, in this study, the strong extrusion process between the iron ball and the catalyst particles will provide a certain amount of energy to promote the strong adsorption intermediate state to "fall off" from the defect site, thus "reviving" the active site and promoting the production of ammonia. In this way, the researchers no longer need high temperature to "fall off" the intermediate state, nor do they need to make high-pressure compromises, thus greatly reducing the reaction conditions.

Stuart James, a mechanochemistry expert at Queen's University in Belfast (not involved in the study), said the results of the study were striking-it was not easy to reduce the original high temperature and pressure conditions to lower temperatures and pressures. However, James points out that other mechanochemical methods (such as extruding screws) may be easier to achieve industrial production of mechanochemical ammonia than ball milling.

Although mechanochemical methods have made considerable progress in other fields, there are only a handful of cases of mechanochemical nitrogen fixation so far-perhaps only 5. For future research, Professor Bai Zhongfan said that they are looking for more efficient catalysts and are also designing devices that can continuously synthesize ammonia to meet industrial requirements.

Will the technology of synthetic ammonia change? In fact, after years of research, Harper-Bosch ammonia synthesis process has been quite mature. Today, a factory can produce 3000 tons of ammonia a day, adding up to more than 100 million tons of global annual output. Some scientists say that industry is unlikely to make much change to this important response in the coming decades, because it is already a very optimized process-70% energy efficiency is almost unmatched by "people". There may not be any way to turn the Hubble Bosch around.

However, chemists will not stop their research, because no one knows what will happen in the further future. What's more, every new discovery is exciting, which may be the beauty of basic research.

At present, the "war" between mankind and hunger has not stopped. However, in recent years, nitrogen pollution in agriculture has attracted the attention of climatologists, so the question of whether to use nitrogen fertilizer and how to use it reasonably remains to be solved. There will be a protracted war between human beings and climate change, in which science and technology are also facing the dilemma of transformation.

Original paper:

Https://www.nature.com/articles/s41565-020-00809-9

Reference link:

Https://www.chemistryworld.com/features/a-fixation-with-nitrogen/6076.article

Https://www.nature.com/articles/22672/

Https://www.nature.com/articles/ngeo325

Https://www.chemistryworld.com/opinion/the-seabirds-saved-by-synthetic-chemistry/4014400.article

Https://www.advancedsciencenews.com/playing-ball-with-the-haber-bosch-process/

Https://www.chemistryworld.com/news/mechanochemistry-makes-ammonia-under-mild-conditions/4012945.article

Https://link.springer.com/article/10.1007/s12045-011-0130-0

Https://doi.org/10.1016/j.joule.2021.01.009

Https://www.nature.com/articles/ngeo325

Https://pubs.acs.org/doi/10.1021/acsenergylett.0c01895

Https://www.nature.com/articles/s41586-020-2464-9

Https://doi.org/10.1002/anie.202112095

This article comes from the official account of Wechat: global Science (ID:huanqiukexue), written by Wang Yibo, revision: 27

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