Visible Light Absorbed NH2 Species Derived from NH3 Adsorbed on TiO2 for Photoassisted Selective Catalytic Reduction

The formation of a NH 2 radical on TiO 2 under photoirradiation was confirmed by electron paramagnetic resonance spectroscopy. Photogenerated holes and electrons on TiO 2 are trapped by adsorbed NH 3 and Ti 4+ species, respectively, to form the NH 2 radical and Ti 3+ species. The half-life of the NH 2 radical determined by the Arrhenius equation was found to be 1.4 min for the photoassisted selective catalytic reduction (photo-SCR) of NO with NH 3 . The NH 2 radical was quenched just after the introduction of NO leaving behind the Ti 3+ species. Additionally, the action spectrum showed that the photo-SCR takes place under not only UV ray (<400 nm) but also visible light (400−450 nm), which TiO 2 cannot absorb. This result indicates the presence of another path of the NH 2 radical formation except for the photoexcitation of TiO 2 (O 2p valence band → Ti 3d conduction band). Density functional theory calculations were employed in order to investigate the formation of a new energy level laid between the HOMO and LUMO levels. The result revealed that the N 2p orbital is located between O 2p and Ti 3d when NH 2 species is adsorbed on Ti site of the Ti 7 O 27 H 27 cluster model which was used as TiO 2 (101) surface model and that the excitation energy from N 2p to Ti 3d is lower than that from O 2p to Ti 3d. We concluded that this N 2p electron donor level formation lying between O 2p and Ti 3d promotes the visible light induced SCR.