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 directly. 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 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 TiO2 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.

The photocatalytic transformation of phenol has been investigated on naked TiO2 and on TiO2/F (0.01 M F -) at pH 3.6 in the presence of different alcohols (tert-butyl alcohol, 2-propanol, and furfuryl alcohol). On the basis of a detailed kinetic analysis and the time evolution of the intermediates, it is suggested that on naked TiO2 the oxidation of phenol proceeds for 90% through the reaction with surficial bound hydroxyl radical, the remaining 10% via a direct interaction with the holes. On TiO2/F the reaction proceeds almost entirely via homogeneous hydroxyl radicals because of the unavailability of surface-bound hydroxyl in the presence of fluoride ions. The use of alcohols as a diagnostic tool for the analysis of the photocatalytic mechanism is discussed.

UV irradiation of fluorinated TiO(2) suspensions in water, in the presence of oxygen and a hole scavenger, leads to the production of H(2)O(2) with steady state concentration levels up to 1.3 millimolar; the H(2)O(2) formation rate follows the TiO(2) surface speciation, being maximum when the surface is completely covered by [triple bond]Ti-F groups; these results outline the importance of surface speciation on the photocatalytic process.

In the comparison of formamide and urea photocatalytic degradation, despite their similar structures, the final fate of bound nitrogen under illumination with TiO2 has shown a different behaviour; both the rate and the ratio of NH4+ and NO3- ion evolution seem not to be linked to the initial nitrogen oxidation state, but to the carbon oxidation state.

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