KINETIC-STUDIES IN HETEROGENEOUS PHOTOCATALYSIS .6. AM1 SIMULATED SUNLIGHT PHOTODEGRADATION OVER TITANIA IN AQUEOUS-MEDIA - A 1ST CASE OF FLUORINATED AROMATICS AND IDENTIFICATION OF INTERMEDIATES

Fluorinated aromatic compounds have been mineralized to CO2 and fluoride by irradiation of aqueous, air-equilibrated suspensions (pH 3, H2SO4) containing TiO2 (Degussa P-25 anatase) and a fluorinated phenol (2-, 3-, or 4-fluorophenol and 2,4- and 3,4-difluorophenol) with AM1 simulated sunlight (lambda greater-than-or-equal-to 310 nm). Decomposition of the fluorophenols is immediately followed by defluorination (less-than-or-equal-to 30 min) as evidenced by the rate of formation of fluoride; only approximately 75-80% of fluoride is recovered in solution, the remaining quantity being adsorbed on the TiO2 particles. This may have certain consequences on the time needed for CO2 evolution as fluoride may block some of the TiO2 catalytic sites. Evolution of CO2 is 3 times slower (approximately 90 to 240 min) than decomposition/defluorination of the starting fluorinated phenols. Apparent kinetic data have been determined by curve-fitting methods on the basis of a simple phenomenological model which implicates consecutive and parallel reactions: A --> B(i) --> C. The major oxidizing species is the .OH radical formed by oxidation of surface hydroxyl groups and/or surface bound water molecules with the photocatalyst's valence band holes. It is argued that the distinction between a surface bound and a free (in bulk solution) .OH radical is in fact a moot point which kinetic considerations alone cannot delineate. A mechanistic route is summarized to rationalize the various intermediate products formed along the temporal course of the photomineralization process: fluorohydroquinone, fluorocatechol, trihydroxybenzene, hydroquinone, catechol, and benzoquinone. The overall order of the total destruction (to CO2) of the fluorophenols examined is 2,4-difluorophenol > 3-fluorophenol approximately 4-fluorophenol > 2-fluorophenol greater-than-or-equal-to 3,4-difluorophenol.