Visible-Light-Induced Photocatalytic Oxidation of Polycyclic Aromatic Hydrocarbons over Tantalum Oxynitride Photocatalysts

The photooxidations of five typical polycyclic aromatic hydrocarbons (PAHs) were investigated by using tantalum oxynitride and Pt-tantalum oxynitride as visible light-driven photocatalysts. The electron paramagnetic resonance spin-trap technique and hydrogen peroxide test strip were used to monitor active species formed in these photocatalytic systems. Moreover, the participations of HO center dot, O-2(-center dot) anions, and holes were further examined by adding their scavengers t-butanol, benzoquinone, and iodine anions, respectively. The reaction intermediates were analyzed by gas chromatography-mass spectrometer (GC-MS). The results show that tantalum oxynitride exhibits good photocatalytic activity for the PAHs photodegradation and the activity is greatly promoted by loading cocatalyst Pt. After 6 h visible light irradiation, phenanthrene, anthracene, benzo[a]anthracene, and acenaphthene can be completely oxidized over Pt-tantalum oxynitride. Under UV light irradiation, the photodegradation rate of PHE over Pt-tantalum oxynitride is 8 times faster than that over titanium dioxide (P25, 80% anatase, 20% rutile). Oxygen plays a crucial role on the photooxidations of PAHs. t-Butanol and benzoquinone almost have no effect on PAHs photodegradations, which indicate that HO center dot and O-2(-center dot) anions play a negligible role on the photodegradations of PAHs. However, the presence of iodide anions significantly inhibits these degradation reactions, implying the crucial effect of holes on the photocatalytic systems. The PAHs degradations could therefore be attributed to the formation of holes in these systems. Based on the GC-MS analysis, the possible photooxidation pathways of PAHs were also proposed.