Rate enhancement and rate inhibition of phenol degradation over irradiated anatase and rutile TiO2 on the addition of NaF:: New insight into the mechanism

Several studies have shown that addition of NaF into the aqueous dispersion of TiO2 (Degussa P25) can result in significant enhancement in the photocatalytic degradation (PCD) of organic pollutants, ascribed to the enhanced production of free OH radicals in solution as a result of fluoride displacement of surface hydroxyl groups. In this work, we have observed different results of NaF addition for the PCD of phenol over synthetic TiO2 in aqueous suspension under UV light irradiation (lambda >= 320 nm). Upon the addition of NaF, the rate of phenol PCD was only increased with anatase, but it was decreased with rutile under similar conditions. In the presence of AgNO3, however, the fluoride-induced rate enhancement of phenol PCD could be observed with both anatase and rutile, ascribed to the increased rate of scavenging the conduction band electrons. As the catalyst sintering temperature was increased, the amount of fluoride adsorption on TiO2 was decreased, but the degree of PCD rate enhancement due to NaF addition as observed with anatase was first increased and then decreased, the trend of which was similar to that in the absence of NaF. The result reveals that the excess fluoride ions present in the suspension play some positive role to the phenol PCD, which is hardly interpreted by previous mechanism of surface fluorination. Moreover, as initial concentration of fluoride and initial pH of suspension were increased, the degree of rate enhancement was increased and decreased, respectively, which also could not be ascribed solely to the change in fluoride adsorption. Possible interference from catechol and hydroquirione intermediates and the fluoride-induced enhancement in the production of OH radicals in solution are analyzed. A new mechanistic model is proposed, involving enhanced desorption of surface bound OH radicals from irradiated TiO2, by fluoride ions present in the Helmholtz layers, through a fluorine hydrogen bond.