Ammonia (NH₃) is widely used for raw materials such as nitrogen fertilizers, nylon fibers for clothes, and medicines. In addition, since ammonia does not generate carbon dioxide (CO₂), which is a greenhouse gas even when burned, easily becomes liquid, and is easy to store and transport, it is expected to be used as an energy source for thermal power generation.

At present, ammonia is synthesized by a chemical reaction between nitrogen (N₂) and hydrogen (H₂) at factories by the “Haber-Bosch method.” In the Haber-Bosch method, however, high temperatures (400-600℃) and high pressures (100-200 atm) are required for the chemical reaction between nitrogen and hydrogen. Furthermore, a large amount of energy is required and a large amount of CO₂ is generated through the process, because hydrogen is now produced mainly from fossil fuels such as oil, coal and natural gas. Therefore, a new method of ammonia synthesis that replaces the Haber-Bosch method has been strongly expected to be developed.

Under such circumstances, Professor Nishibayashi Yoshiaki, Research Director of CREST, succeeded in developing an innovative method to simply and rapidly synthesize large amounts of ammonia from nitrogen gas and water at room temperature and atmospheric pressure for the first time in the world. The research result, published in Nature (online) in April 2019, is now attracted worldwide attention.

In the Haber-Bosch method, fossil fuels are used for producing hydrogen that is the raw material of ammonia. In nature, however, it is known that there is an enzyme that synthesizes ammonia at room temperature and atmospheric pressure. It is “nitrogenase” in bacteria named “rhizobia” that grows symbiotically with leguminous plants possesses. When nitrogenase synthesizes ammonia from nitrogen, it uses molybdenum (Mo) existing in the nitrogenase molecule. Focusing on this nitrogenase, Professor Nishibayashi succeeded in synthesizing ammonia at room temperature and atmospheric pressure at a speed comparable to that by nitrogenase, by using samarium diiodide (SmI₂).

The more detailed reaction process is as follows. First, a nitrogen molecule (N₂) is bonded to a catalyst containing molybdenum. Next, another catalyst approaches, and then breaks the chemical bond between two nitrogen atoms (N) in the nitrogen molecule. As a result, the nitrogen atom (N) is bonded to the catalyst containing molybdenum. Then, samarium diiodide approaches there. Samarium diiodide plays a role as a reducing reagent that produces hydrogen (H), a raw material of an ammonia molecule (NH₃), from a water molecule (H₂O). Three hydrogen atoms produced by the samarium diiodide and water are bonded to the nitrogen atom, forming ammonia. The produced ammonia is separated from the catalyst, then the catalyst returns to its original state. By repeating this cycle, a large amount of ammonia is synthesized.

It can be said that this synthesis process is “an artificial reproduction of the enzyme mechanism of nitrogenase.”

The advantage of this synthesis method is that is excellent both in cost and energy. Hydrogen can be produced at room temperature and atmospheric pressure. In addition, the method scarcely needs energy to synthesize ammonia. Ammonia can be simply synthesized by mixing it in a flask.

One of the important points that this synthesis method made possible was that Professor Nishibayashi discovered a phenomenon wherein ammonia synthesis proceeds at room temperature and atmospheric pressure at a speed comparable to nitrogenase by using samarium diiodide. The combination of samarium diiodide and water has been used in organic synthesis for a long time as a reducing reagent for the carbon-oxygen double bond (C=O). The great breakthrough was that Professor Nishibayashi noticed that this reagent could be used also for the reduction of the nitrogen-nitrogen triple bond (N≡N).

Although samarium (Sm) is inexpensive, it has to be reused when it is practically applied, because it is one of the rare-earth elements. Focusing on the practical application of samarium, the research group of Professor Nishibayashi plans to work on the industrialization of collection and reuse cycle of samarium in collaboration with industries.

Furthermore, the present research results not only invented the innovative method of ammonia synthesis, but are also expected to greatly contribute to the future resolution of environmental and energy problems. Hydrogen is currently attracting much attention as a next-generation energy source. However, there remains the problem that a large amount of CO₂ is generated to produce hydrogen. Also, the method to store and transport hydrogen is another problem to be solved. As to these problems, ammonia has advantages, because it easily becomes liquid and the storage/transport of ammonia is easy. In addition, since ammonia does not generate CO₂ even when burnt like hydrogen, it can be used for fuels of thermal power generation as a clean energy source. This synthesis method has the potential to cause a paradigm shift in the energy resources of society.