Visible-Light-Induced Photocatalytic Degradation of 4-Chlorophenol and Phenolic Compounds in Aqueous Suspension of Pure Titania:Demonstrating the Existence of a Surface-Complex-Mediated Path

摘要

The visible-light-induced degradation reaction of 4-chlorophenol (4-CP) was investigated in aqueous suspension of pure TiO_2- Contrary to common expectations,4-CP could be degraded under visible illumination (lambda > 420 nm),generating chlorides and CO_2 concomitantly.The observed visible reactivity was not due to the presence of trace UV light since the visible-light-induced reactions exhibited behaviors distinguished from those of UV-induced reactions.Dichloroacetate could not be degraded under visible light,whereas it degraded with a much faster rate than 4-CP under UV irradiation.The addition of tert-butyl alcohol,a common OH radical scavenger,did not affect the visible reactivity of 4-CP,which indicates that OH radicals are not involved.Other phenolic compounds such as phenol and 2,4-dichlorophenol were similarly degraded under visible light.The surface complexation between phenolic compounds and TiO_2 appears to be responsible for the visible light reactivity.Diffuse reflectance UV-vis spectra showed that 4-CP adsorbed on TiO_2 powder induced visible light absorption.The visible light reactivity among several TiO_2 samples was apparently correlated with the surface area of TiO_2.The visible-light-induced photocurrents on a TiO_2 electrode could be obtained only in the presence of 4-CP.It is proposed that a direct electron transfer from surface-complexed phenol to the conduction band of TiO_2 upon absorbing visible light (through ligand-to-metal charge transfer) initiates the oxidative degradation of phenolic compounds.When the surface complex formation was hindered by surface fluorination,surface platinization,and high pH,the visible-light-induced degradation of 4-CP was inhibited.The evidence of visible-light-induced reactions and the experimental conditions affecting the visible reactivity were discussed in detail.