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

Abstract

The effect of fluoride ions on the photocatalytic degradation of phenol in an aqueous suspension of TiO 2 has been investigated. Fluoride ions displace surficial hydroxyl groups and coordinate surface-bound titanium atoms directly. For 0.01 M fluoride concentration and 0.10 g L - 1 of TiO 2 in the range pH 2−6, the degradation rate of phenol is up to 3 times that in the absence of fluoride ions. This behavior has been correlated with the computed surface speciation. The decrease in the degradation rate of phenol as a function of the substrate concentration observed in naked TiO 2 at a high concentration of phenol (over 0.01 M) is largely diminished in the presence of fluoride ions. A photocatalytic model which takes into account the primary events and recombination reactions is able to account for these experimental results. The competition between OH-radical-mediated reaction versus direct electron transfer is discussed. Finally, under a helium atmosphere and in the presence of fluoride ions, phenol is slowly but significantly degraded, although total organic carbon does not decrease, suggesting the occurrence of a photocatalytically induced hydrolysis.