Researchers in Japan report that a commercially available TiO2 with a large number of surface oxygen vacancies, when photo-irradiated by UV light in pure water with nitrogen—successfully produces ammonia (NH3). The solar-to-chemical energy conversion efficiency is 0.02%, which is the highest efficiency among the early reported photocatalytic systems. This is, however, lower than that of natural photosynthesis (0.1%) and artificial photosynthesis such as overall water splitting and H2O2 production (0.2%).
Although improved catalytic activity is necessary, the noble-metal-free TiO2 system therefore shows a potential as a new artificial photosynthesis for green NH3 production, the team suggests in a paper published in the Journal of the American Chemical Society.
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.
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