| Surface processes on titanium dioxide (TiO2) were investigated. This study focused on the adsorption of 2,4,5-trichlorophenol (245TCP), a model highly chlorinated aromatic pollutant, to TiO2. Degussa P25, a commercial preparation of TiO2 consisting of both the anatase and rutile crystalline phases, adsorbs 245TCP to a much greater extent than does either pure anatase or pure rutile. Furthermore, we observed an apparent increase in the extent of adsorption on P25 and anatase under visible illumination from fluorescent lighting. Similar reports indicating “photoenhanced adsorption” for other compounds can be found in the literature. When all bandgap-energy light (λ < 415 nm) was rigorously excluded, no light effects were observed on anatase, but even this sub-bandgap light caused apparently elevated levels of adsorption on P25. No adsorption or reaction was observed on rutile under any conditions. It was found that 245TCP forms a charge-transfer complex (CTC) with P25 that reacts under visible light. Diffuse-reflectance spectroscopy shows that the CTC absorbs light wavelengths as long as 520 nm, with a peak at 415 nm. Excitation of this complex produces trichlorophenoxyl radicals, which combine to form a suite of chlorinated, polymeric molecules with mass ranging as high as 1200 amu.; 245TCP/TiO2 systems were illuminated with narrowband light of 360, 400, 430, 480, and 550 nm. On P25, the amount of reaction at each wavelength correlates well with the spectrum of the CTC. Polymeric products are found mostly at 400, 430, and 480 nm, with a limited amount formed at 550 nm, and none at 360 nm. Furthermore, photoreactions initiated by CTC excitation at 400 nm have a higher quantum efficiency than reactions triggered by anatase bandgap excitation at 360 nm.; A homologous series of phenolic compounds having between zero and three chlorines was studied. On P25, the extent of adsorption, magnitude of the CTC complex, and amount of reaction decrease with decreasing chlorine number. Besides a trace coupling product formed from 2,4-dichlorophenol, none of the other chlorophenols yielded polymeric products. Our results indicate the importance of light wavelength, both ultra- and sub-bandgap, as a parameter for controlling the products produced by photocatalysis. |
