Scientists achieve "artificial photosynthesis of ammonia"

Ammonia is a chemical essential to many agricultural and industrial processes — particularly due to its use in fertiliser — but its mode of production requires industrial plants need to reach temperatures of more than 400°C and extremely high pressures, making ammonia production a major contributor to global greenhouse gas emissions. With this in mind, researchers around the world are trying to create a cleaner, more efficient means to produce ammonia.

Now, Professor Yoshiaki Nishibayashi and his team at The University of Tokyo have developed a novel catalytic system for efficiently producing ammonia from abundant molecules found on Earth, including atmospheric nitrogen and water. As explained in the journal Nature Communications, the key lies in a combination of two kinds of catalysts — intermediate compounds which enable or speed up chemical reactions without contributing to the final mixture — made especially for ammonia production, and which are driven by sunlight.

“This is the first successful example of photocatalytic ammonia production, using atmospheric dinitrogen as a nitrogen source and water as a proton source, that also uses visible light energy and two kinds of molecular catalysts,” Nishibayashi said.

One of the team’s catalysts was based on the transition metal molybdenum for the activation of dinitrogen, while the other was based on the transition metal iridium for the photoactivation of both tertiary phosphines and water. A third component, called tertiary phosphines, was also key to helping get the protons out of water molecules.

“When the iridium photocatalyst absorbs sunlight, its excited state can oxidise the tertiary phosphines,” Nishibayashi said. “The oxidised tertiary phosphines then activate water molecules via formation of a chemical bond between the phosphine’s phosphorous atom and the water, yielding protons.

“The molybdenum catalyst then enables nitrogen to bond with these protons to become ammonia. The use of water for producing dihydrogen or hydrogen atoms is one of the most important processes for achieving green ammonia production.”

The team’s process, which mirrors natural processes found in plants utilising symbiotic bacteria, resulted in reaction efficiencies that were “higher than expected compared to previous reports of visible light-driven photocatalytic ammonia formation”, Nishibayashi said. The team also managed to produce this reaction at a scale 10 times that of previous experiments, suggesting it is ready for trials at larger scales (although its effectiveness and safety could be improved further).

“In plants, ammonia is formed by biological nitrogen fixation using cyanobacteria and is linked with photosynthesis,” Nishibayashi said. “Here, the electrons for the reaction are supplied by photosynthesis and protons are derived from water. Therefore, the findings of our recent study can be regarded as a successful example of the artificial photosynthesis of ammonia.”

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