Photocatalytic transformation of organic compounds in the presence of inorganic anions. 1. Hydroxyl-mediated and direct electron-transfer reactions of phenol on a titanium dioxide-fluoride system

The effect of fluoride ions on the photocatalytic degradation of phenol in an aqueous suspension of TiO2 has been investigated. Fluoride ions displace surficial hydroxyl groups and coordinate surface-bound titanium atoms dire ctly. For 0.01 M fluoride concentration and 0.10 g L-1 of TiO2 in the range pH 2-6, the degradation rate of phenol is up to 3 times that in the absenc e of fluoride ions. This behavior has been correlated with the computed sur face speciation. The decrease in the degradation rate of phenol as a functi on of the substrate concentration observed in naked TiO2 at a high concentr ation of phenol (over 0.01 M) is largely diminished in the presence of fluo ride ions. A photocatalytic model which takes into account the primary even ts and recombination reactions is able to account for these experimental re sults. The competition between OH-radical-mediated reaction versus direct e lectron transfer is discussed. Finally, under a helium atmosphere and in th e presence of fluoride ions, phenol is slowly but significantly degraded, a lthough total organic carbon does not decrease, suggesting the occurrence o f a photocatalytically induced hydrolysis.