Method inspired by lightning makes ammonia without catalyst | Search

US researchers have invented a way to produce ammonia, which is vital in producing fertilizers to feed 7.7 billion humans on Earth, without relying on catalysts as the current industrial methods. The team, led by Mohan Sankaran and Julie Renner at Case Western Reserve University in Cleveland, produces ammonia electrolytically from water and nitrogen.

This is normally not possible without a catalyst because the nitrogen in its molecular form, N₂, is extremely stable – but nature had already shown how this could be done. 'When you have plasma you can break N₂ without the catalyst, "says Sankaran World of Chemistry. "It is done every day when there are lightning." Free electrons in ray plasmas can break down the nitrogen molecules, but form nitrates and nitrites, not ammonia.

Today we make the ammonia we use through the Haber-Bosch process, which reduces nitrogen gas by using hydrogen gas on the surface of a catalyst at high pressures and high temperatures. This energy intensive process uses about 2% of the world's energy and requires hydrogen from fossil fuels, emitting greenhouse gases that alter the climate.

Renner specializes in electrochemical ammonia synthesis, which involves the passage of an electric current through a liquid containing a catalyst. Shortly after joining Case Western Reserve in 2016, she met Sankaran, who studies what plasmas do on liquid surfaces in a hallway, and the pair combines interests. Sankaran had shown that in such systems plasmas can form solvated electrons on liquid surfaces, which he calls "one of the strongest reducing species known in chemistry".

Its electrochemical method passes current through the water, using a nitrogen plasma as one of the two electrodes of the system. Other electrochemical methods of ammonia synthesis produce hydrogen and ammonia at the same time, but at low currents the plasma-based approach produces only ammonia.

Although this is a great benefit, the electricity consumption of the lightning method, which the researchers say is in terms of "considerably higher" energy cost than the Haber-Bosch process, is a major disadvantage. However, the approach can still be useful as it is to make ammonia on small scales, says Renner. "We are also working on ideas to reduce energy consumption," she adds.

Daren Caruana, of University College London, UK, says the study is a prime example of how plasma can be used for chemical transformation. "The chemistry described is rudimentary, but the use of free electrons from a plasma as a reagent can be a very powerful chemical tool for conducting theoretical reactions either kinetically or thermodynamically," he adds.

Similarly, Annemie Bogaerts of the University of Antwerp in Belgium, who researches to produce ammonia from nitrogen using plasmas in the gas phase, describes the study as very interesting. "In my opinion, plasma has a great potential for N₂ fixation because it operates on the basis of electricity, can be easily switched on and off, and is therefore very promising in combination with renewable electricity.