Ammonia (NH₃) is an indispensable chemical for synthesis of fertilizers and fibers. It has also received much attention as a potential hydrogen carrier due to its high hydrogen density (17.6 wt %) and low liquefying pressure (∼8 atm). Traditionally, NH₃ has been manufactured by the Haber−Bosch process using H₂ and N₂ for over 100 years. This process, however, needs extremely high pressures (>200 bar) and high temperatures (>673 K), with large amounts of H₂ produced by steam reforming of fossil fuels with a large concomitant emission of CO₂. Catalytic processes that produce NH₃ using N₂ and earth-abundant reducing reagents at atmospheric pressure and room temperature are desired for a clean, safe, and sustainable NH₃ synthesis.

—Hirakawa et al.

Photocatalytic nitrogen reduction is an attractive pathway because it can use light energy, the team pointed out. The basic concept uses photoformed valence band holes (VB h⁺) to oxidize water; N₂ reduction by the conduction band electrons (CB e⁻) produces NH₃. As a result of this, NH₃ is produced from water and N₂ under ambient conditions by using sunlight as energy source. The large free energy gain of this reaction (ΔG° = 339 kJ mol⁻¹)13 makes this a potential new artificial photosynthesis.

A number of inorganic or organic semiconductors have been tried for NH₃ production, but these suffer from low activity or stability.

The rate-determining step of the N₂ reduction cycle is the cleavage of the N≡N bond. This bond has an extremely high dissociation energy (941 kJ mol⁻¹). Creation of active sites that efficiently promote N≡N cleavage is therefore necessary. It is well-known that transition-metal complexes with Mo, W, Fe, or Ru cations efficiently promote the N≡N cleavage by strong coordination, where trivalent titanium (Ti³⁺) is one possible cation.

Here we report that the Ti³⁺ species are inherently created on the surface defects of a commercially available TiO₂ and behave as very active sites for N₂ reduction. The TiO₂ with a large number of surface defects, therefore, successfully produces NH from water and N under sunlight irradiation.

—Hirakawa et al.

In addition, the cheap, robust, and noble-metal-free TiO₂ successfully produces NH₃ very efficiently (180 μM) under sunlight.

Resources

• Hiroaki Hirakawa, Masaki Hashimoto, Yasuhiro Shiraishi, and Takayuki Hirai (2017) “Photocatalytic Conversion of Nitrogen to Ammonia with Water on Surface Oxygen Vacancies of Titanium Dioxide” Journal of the American Chemical Society doi: 10.1021/jacs.7b06634