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Selective Nitrate-to-Ammonia Transformation on Surface Defects of Titanium Dioxide Photocatalysts
journal contribution
posted on 2017-04-19, 00:00 authored by Hiroaki Hirakawa, Masaki Hashimoto, Yasuhiro Shiraishi, Takayuki HiraiAmmonia
(NH3) is an essential chemical in modern society,
currently manufactured via the Haber–Bosch process with H2 and N2 under extremely high pressure (>200
bar)
and high-temperature conditions (>673 K). Toxic nitrate anion (NO3–) contained in wastewater is one potential
nitrogen source. Selective NO3–-to-NH3 transformation via eight-electron reduction, if promoted
at atmospheric pressure and room temperature, may become a powerful
recycling process for NH3 production. Several photocatalytic
systems have been proposed, but many of them produce nitrogen gas
(N2) via five-electron reduction of NO3–. Here, we report that unmodified TiO2,
when photoexcited by ultraviolet (UV) light (λ > 300 nm)
with
formic acid (HCOOH) as an electron donor, promotes selective NO3–-to-NH3 reduction with 97% selectivity.
Surface defects and Lewis acid sites of TiO2 behave as
reduction sites for NO3–. The surface
defect selectively promotes eight-electron reduction (NH3 formation), while the Lewis acid site promotes nonselective reduction
(N2 and NH3 formation). Therefore, the TiO2 with a large number of surface defects and a small number
of Lewis acid sites produces NH3 with very high selectivity.