The development of the production method for Green Ammonia, which is produced only from ubiquitous and clean small molecules using renewable energy, has been desired for a next-generation carbon-free energy carrier to build a carbon-neutral society and solve global warming. From the viewpoint of sustainability and atom economy, green ammonia should be produced from dinitrogen and water (ideal reaction formula: N2 + 3 H2O → 2 NH3 + 3/2 O2). Moreover, the procedure of green ammonia production should proceed under ambient mild reaction conditions to reduce energy consumption, and the reaction energy should be supplied from renewable energy, ideally solar energy. However, dinitrogen and water are stable compounds; thus, the simultaneous activation of dinitrogen and water (i.e., the cleavage of the stable N≡N bond to produce nitrides and the drawing out of protons and/or electrons from water to the hydrogenation of the nitrides) under ambient reaction conditions using solar energy has been difficult to be achieved.
As a pioneering photocatalytic system, Prof. Nishibayashi and his team from the University of Tokyo’s School of Engineering have envisaged visible-light-driven catalytic ammonia formation from dinitrogen and water under an atmospheric pressure of dinitrogen gas at ambient temperature by combining the catalytic cycle by molybdenum complexes for ammonia formation from dinitrogen via the efficient N≡N bond cleavage and the production of the nitride complex as a key intermediate and photosensitizing cycle by iridium complexes for catalytic phosphine-mediated water activation to hydrogenate the nitride complexes. In this reaction system, tertiary phosphines (R3P), which are common and widely used organic compounds, function as electron donors, and visible light energy enables iridium photosensitizers to trigger electron relay from R3P as a weak reductant to molybdenum catalysts. The produced radical cation R3P•+ activates water molecules to donate protons for ammonia formation to molybdenum catalysts via the production of a phosphine-water adducted radical cation (R3P•+-OH2). This is the first successful example of a visible-light-driven catalytic ammonia formation from dinitrogen and water using a combination of molecular catalysts under ambient reaction conditions.
In the near future, the combination of the aforementioned photocatalytic reaction with the direct electroreduction of phosphine oxides, which were obtained as oxidized products of the phosphines in the photoreaction system, to reproduce phosphines using water as an electron donor and solar cells will lead to achieving the above-mentioned ideal reaction (i.e., N2 + 3 H2O → 2 NH3 + 3/2 O2). Therefore, the present study represents a milestone with respect to the eco-friendly methods for green ammonia production from the most ubiquitous and clean small molecules, i.e., dinitrogen and water, utilizing solar energy, which will be one of the desirable and essential scientific techniques as an alternative or in parallel with the Haber-Bosch process.
Papers
Journal: Nature Communications
Title: Catalytic ammonia formation from dinitrogen, water, and visible light energy
Authors: Yasuomi Yamazaki, Yoshiki Endo, Yoshiaki Nishibayashi*
DOI: 10.1038/s41467-025-59727-w
UTokyoFOCUS | Press Release: https://www.u-tokyo.ac.jp/focus/en/press/z0508_00411.html