Making ammonia at lower temperatures: the world’s first air-stable surface electrene

This is where electrenes have attracted attention. In these materials, electrons form a layer that appears to float at the surface. Because these surface electrons can be readily transferred to nearby molecules, electrenes are considered promising for catalytic applications.

Until now, however, electrenes have had a major weakness: they are highly sensitive to air and moisture, making them difficult to handle.

A research team led by Hideo Hosono, Honorary and Institute Professor at Institute of Science Tokyo (Science Tokyo), has now demonstrated the world’s first air-stable surface electrene, using barium silicon nitride (BaSiN₂) containing a small amount of oxygen. The material overcomes one of the major limitations of previous electrenes: their instability in air. The researchers confirmed that its crystal structure remained intact even after one week of exposure to air.

What makes the surface electrene particularly intriguing is the way its surface changes during a reaction. When the electrons floating at the surface are transferred to nitrogen molecules, the nitrogen molecules remain on the surface, temporarily stabilizing it. When hydrogen is then introduced, the nitrogen is converted into ammonia and leaves the surface, allowing electrons to reappear.

In other words, nitrogen protects the surface, and once the reaction proceeds, the surface returns to a state in which it can once again supply electrons.

The researchers also combined the material with ruthenium (Ru) and showed that it could efficiently catalyze ammonia production at lower temperatures and pressures than conventional approaches. Its catalytic performance was maintained even after repeated exposure to air.

A closer look at the reaction mechanism revealed that ammonia could be produced even at 30°C. The researchers also confirmed that the floating surface electrons make nitrogen molecules more reactive-a distinctive feature of this material.

Ammonia is attracting attention not only as a raw material for fertilizers, but also as an energy carrier that can be used to transport hydrogen. Further improving catalysts that can operate at low temperatures while remaining stable in air could open the way to distributed ammonia production-making ammonia where it is needed using energy from renewable sources.

I have worked in materials research for many years, and materials often reveal unexpected sides that simple predictions cannot capture. That is one of the great pleasures of materials research.

The mechanism we uncovered in this study-where nitrogen stabilizes the surface, and the surface returns to its original active state once the reaction takes place-was particularly fascinating. Once you understand it, it may seem obvious but in research, the key is being the first to uncover it.

When we find a property like this, it immediately makes us want to look at what else might be possible from a different angle. Ultimately, I hope we can turn the unique properties of this material into technologies that are useful in the real world.

(Hideo Hosono, Honorary and Institute Professor, Institute of Integrated Research, Institute of Science Tokyo)

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