Wavelength-Dependent Solar N(2) Fixation into Ammonia and Nitrate in Pure Water

Solar-driven N(2) fixation using a photocatalyst in water presents a promising alternative to the traditional Haber-Bosch process in terms of both energy efficiency and environmental concern. At present, the product of solar N(2) fixation is either NH(4)(+) or NO(3)(−). Few reports described the simultaneous formation of ammonia (NH(4)(+)) and nitrate (NO(3)(−)) by a photocatalytic reaction and the related mechanism. In this work, we report a strategy to photocatalytically fix nitrogen through simultaneous reduction and oxidation to produce NH(4)(+) and NO(3)(−) by W(18)O(49) nanowires in pure water. The underlying mechanism of wavelength-dependent N(2) fixation in the presence of surface defects is proposed, with an emphasis on oxygen vacancies that not only facilitate the activation and dissociation of N(2) but also improve light absorption and the separation of the photoexcited carriers. Both NH(4)(+) and NO(3)(−) can be produced in pure water under a simulated solar light and even till the wavelength reaching 730 nm. The maximum quantum efficiency reaches 9% at 365 nm. Theoretical calculation reveals that disproportionation reaction of the N(2) molecule is more energetically favorable than either reduction or oxidation alone. It is worth noting that the molar fraction of NH(4)(+) in the total product (NH(4)(+) plus NO(3)(−)) shows an inverted volcano shape from 365 nm to 730 nm. The increased fraction of NO(3)(−) from 365 nm to around 427 nm results from the competition between the oxygen evolution reaction (OER) at W sites without oxygen vacancies and the N(2) oxidation reaction (NOR) at oxygen vacancy sites, which is driven by the intrinsically delocalized photoexcited holes. From 427 nm to 730 nm, NOR is energetically restricted due to its higher equilibrium potential than that of OER, accompanied by the localized photoexcited holes on oxygen vacancies. Full disproportionation of N(2) is achieved within a range of wavelength from ~427 nm to ~515 nm. This work presents a rational strategy to efficiently utilize the photoexcited carriers and optimize the photocatalyst for practical nitrogen fixation.