Angew. Chem. Int. Ed. http://doi.org/f2s5zm (2014)

Nitrogen fixation — the catalytic transformation of nitrogen from the air into ammonia — is one of the most important reactions in the chemical industry. The process, which was developed by Fritz Haber and Carl Bosch, requires high temperature and high pressure. This makes the process expensive and means that it consumes large amounts of energy. Researchers are therefore working to develop nitrogen fixation reactions that can work under ambient conditions. Hiroaki Misawa and co-workers at Hokkaido University have now shown that it is possible to carry out nitrogen fixation at room temperature using visible light irradiation in an artificial ammonia photosynthesis system.

The researchers use a two-chamber reactor: one chamber for the oxidation reaction and one chamber for the reduction reaction. In the oxidation reaction chamber, visible light hits gold nanoparticles supported on a semiconductor material that also acts as a separator for the two chambers. Visible light excites plasmons in gold that decay to produce hot electrons and hot holes. The holes go on to oxidize ethanol (used here as a sacrificial reducing agent). In turn, hot electrons, which can migrate through the membrane into the reduction reaction chamber, and hydrogen ions derived from a hydrochloric acid solution combine with dissolved nitrogen gas to produce ammonia.

The approach will require further work before it would be viable for large-scale applications. But the fact that the quantum efficiency of ammonia formation follows the plasmon resonance of gold, illustrates that plasmon-mediated photochemistry is potentially a useful alternative to the Haber–Bosch process.