Photocatalytic transformation of 2,4,5-trichlorophenol on TiO2 under sub-band-gap illumination

The adsorptive behavior of TiO(2) under various illumination conditions was investigated. In the dark, Degussa P25, which contains both the anatase and rutile phases of TiO(2), adsorbed significantly greater amounts of 2,4,5-trichlorophenol (TCP) than either pure anatase or pure rutile. Results in the literature and in our laboratory that appeared to indicate "photoenhanced adsorption" under ambient fluorescent lighting on P25 and anatase are in fact due to the photoreaction of TCP. On pure-phase anatase, all reactions are due to trace ultra-band-gap energy light present in the fluorescent lighting. On P25, however, reaction also occurs at sub-band-gap energies. TCP forms a charge-transfer complex with P25 that is activated by light wavelengths as long as 520 nm. The trichlorophenoxyl radicals resulting from charge-transfer couple with each other to form a suite of polyaromatic chlorinated products with detected masses as high as 1200 D. These products are not formed under IN irradiation and in fact are destroyed by conventional UV photocatalysis. No reaction occurs on pure-phase anatase or rutile as a consequence of irradiation with sub-band-gap light. Carbon mass balance was closed for all catalysts under all lighting conditions. Our results show that the wavelength of light is an important factor in determining products on P25. This knowledge can be used to avoid charge-transfer complex activation when the resulting products are undesirable. Alternatively, charge-transfer complexes on P25 may be exploitable for polymer syntheses. The differences between surface reactions on P25 and those on pure-phase TiO(2) may be explained in terms of the morphology of Degussa P25, wherein anatase-rutile interfaces give rise to active sites.