ROLE OF ADSORPTION IN PHOTOCATALYZED REACTIONS OF ORGANIC-MOLECULES IN AQUEOUS TIO2 SUSPENSIONS

The photocatalyzed transformation of chemical compounds strongly adsorbed on a particle surface has been investigated in the presence of different photoactive and "inert" supports. For several compounds, such as dioctylquinol and crysene, the rate of degradation is only slightly affected by the initial adsorption onto nonphotocatalytic materials (SiO2, Al2O3) when irradiated in a slurry with added micrometer size TiO2 particles. A rapid exchange of the substrate between the different inorganic supports was experimentally observed and explains the photocatalytic results. Decafluorobiphenyl (DFBP), which adsorbs tenaciously on Al2O3, degrades slowly when irradiated in the presence of TiO2 particles. Measurements confirm that DFPB is poorly exchanged from alumina to TiO2. Comparison with the results obtained using colloidal TiO2 or silica particles, and with the behavior of pentafluorophenol, under otherwise identical conditions, suggests that the photogenerated oxidizing species does not migrate far from the photogenerated active centers and that the degradation process occurs at the surface or within a few monolayers around the photocatalytic particles.