Ammonia, which is widely used in industrial sites, is synthesized at a high temperature of 400℃ to 500℃ and high pressure at a depth of about 2,000m. A new technique has emerged to synthesize ammonia by rolling small iron balls. The findings were released on December 14 in “Nature Nanotechnology”, the leading journal in nanoscience.
Ammonia (NH3) is one of the most important industrial feedstock for fertilizers, explosives, plastics, and other chemicals. Around 140 million tons were produced worldwide in 2019, and ammonia is one of the world’s top 10 chemicals. The most commonly used ammonia synthesis method is the Harbor-Bosch process, which is to synthesize ammonia from nitrogen (N2) and hydrogen (H2) at 400-500 degrees and above 100 bar. However, this process requires high temperature and high pressure at the same time and generates carbon dioxide.
The existing ammonia generation process, the Harbour-Bosh process, uses high temperatures (400-500 degrees) and harsh conditions of more than 100 bar, but no replacement process has been developed for 100 years. The reaction rate is also very slow, as the synthesis of ammonia requires very high activation energy. However, in order to increase the efficiency of ammonia synthesis, the reaction rate must be fast, so if the temperature is increased and the reaction speed is accelerated, the yield will be lost due to loss of balance. Therefore, the compromised temperature is between 400 and 500 degrees, and high-pressure conditions are used to increase yield.
The researchers presented a ball milling method that can replace these conditions, mitigating the conditions using the kinetic energy in which iron balls rotate.
The researchers opened the possibility of mass production of ammonia with ball-milling, which causes mechanical and chemical reactions using the physical force of the iron balls. Ball milling is a method of sealing a container with metal balls and reactants together, and then giving it a fast rotational speed to create a shock. In this study, ammonia was produced at a higher yield rate (82.5%) at lower temperatures and lower pressures than conventional methods by adding iron balls and iron catalysts, followed by nitrogen gas and hydrogen gas.
This study is expected to replace the existing ammonia synthesis method, the Harbour-Bosh process, to apply a new process for mass production of ammonia useful to the industry. In addition, as the ball milling method is a method that can be used on-site production where actual ammonia is needed, it can play a major role in reducing costs since transportation and storage processes may be omitted.
Reference: Han, GF., Li, F., Chen, ZW. et al. Mechanochemistry for ammonia synthesis under mild conditions. Nat. Nanotechnol. (2020). https://doi.org/10.1038/s41565-020-00809-9