Reaction Mechanism of Selective Photooxidation of Amines over Niobium Oxide: Visible-Light-Induced Electron Transfer between Adsorbed Amine and Nb2O5

Mechanistic studies were performed using several spectroscopic techniques (ultraviolet–visible, Fourier transform infrared, and electron spin resonance spectroscpies) and quantum chemical calculations to elucidate the photoactivation process and catalytic cycle of photooxidation of amines over niobium oxide (Nb2O5) under visible light. The reaction mechanism was found to involve oxidative dehydrogenation of amines to imines via photoactivation of a Nb2O5–amide surface complex derived from dissociatively adsorbed amine on Nb2O5. Formation of the surface complex generates a donor level consisting of a N 2p orbital localized on the amide nitrogen atom within the forbidden band of Nb2O5. Direct excitation from this new donor level to the conduction band of Nb2O5 enables excitation at lower energy than the bandgap. Although Nb2O5 itself is transparent to visible light (λ > 390 nm), amine oxidation proceeds even under visible irradiation because of photoactivation of the Nb2O5–amide surface complex. When a primary amine is used as a substrate, the produced dehydrogenated imine is immediately hydrolyzed to aldehyde, followed by condensation with an amine to form an dimerized imine.