Hiroaki Hirakawa

Photocatalytic Conversion of Nitrogen to Ammonia with Water on Surface Oxygen Vacancies of Titanium Dioxide

Ammonia (NH3) is an essential chemical in modern society. It is currently manufactured by the Haber-Bosch process using H2 and N2 under extremely high-pressure (>200 bar) and high-temperature (>673 K) conditions. Photocatalytic NH3 production from water and N2 at atmospheric pressure and room temperature is ideal. Several semiconductor photocatalysts have been proposed, but all suffer from low efficiency. Here we report that a commercially available TiO2 with a large number of surface oxygen vacancies, when photoirradiated by UV light in pure water with N2, successfully produces NH3. The active sites for N2 reduction are the Ti3+ species on the oxygen vacancies. These species act as adsorption sites for N2 and trapping sites for the photoformed conduction band electrons. These properties therefore promote efficient reduction of N2 to NH3. The solar-to-chemical energy conversion efficiency is 0.02%, which is the highest efficiency among the early reported photocatalytic systems. This noble-metal-free TiO2 system therefore shows a potential as a new artificial photosynthesis for green NH3 production.

One-Pot Synthesis of Imines from Nitroaromatics and Alcohols by Tandem Photocatalytic and Catalytic Reactions on Degussa (Evonik) P25 Titanium Dioxide

Photoirradiation (λ > 300 nm) of Degussa (Evonik) P25 TiO2, a mixture of anatase and rutile particles, in alcohols containing nitroaromatics at room temperature produces the corresponding imines with very high yields (80-96%). Other commercially available anatase or rutile TiO2 particles, however, exhibit very low yields (<30%). The imine formation involves two step reactions on the TiO2 surface: (i) photocatalytic oxidation of alcohols (aldehyde formation) and reduction of nitrobenzene (aniline formation) and (ii) condensation of the formed aldehyde and aniline on the Lewis acid sites (imine formation). The respective anatase and rutile particles were isolated from P25 TiO2 by the H2O2/NH3 and HF treatments to clarify the activity of these two step reactions. Photocatalysis experiments revealed that the active sites for photocatalytic reactions on P25 TiO2 are the rutile particles, promoting efficient reduction of nitrobenzene on the surface defects. In contrast, catalysis experiments showed that the anatase particles isolated from P25 TiO2 exhibit very high activity for condensation of aldehyde and aniline, because the number of Lewis acid sites on the particles (73 μmol g(-1)) is much higher than that of other commercially available anatase or rutile particles (<15 μmol g(-1)). The P25 TiO2 particles therefore successfully promote tandem photocatalytic and catalytic reactions on the respective rutile and anatase particles, thus producing imines with very high yields.

Correction to “Photocatalytic Conversion of Nitrogen to Ammonia with Water on Surface Oxygen Vacancies of Titanium Dioxide”

miscalculations during the conversion of NH3 amounts (μmol) to NH3 concentrations (μM). The corrected Figure 1d is shown as above. The corresponding solar-to-chemical conversion (SCC) efficiencies for the NH3 formation (Zaxis) were determined by the correct data and are not affected by the corrections. The corrections do not affect any of the results or conclusions as presented in the original publication. Figure 1. (d) Change in the amount of NH3 formed and the SCC efficiency under simulated AM1.5G sunlight irradiation (1-sun). Reaction conditions: water (50 mL), catalyst (200 mg, 2.5 mmol), N2 (1.0 L min−1). SCC efficiency (%) = ([ΔG° for NH3 generation (J mol−1)] × [NH3 formed (mol)])/([total input energy (W)] × [time (s)]) × 100. Total light intensity at 300−2500 nm is 1000 W m−2, and the intensity at 300−420 nm is 71 W m−2 (Figure S7). Addition/Correction