It has been reported that addition of polyoxometalates (POM) or fluoride anions into the TiO(2) dispersions can significantly enhance the photocatalytic degradation (PCD) of weakly adsorptive organic pollutants in water such as chlorophenol. In this work, however, contradictory effects of POM and fluoride were observed on the PCD of highly adsorptive substrate X3B, an anionic organic dye, under similar conditions. The total rate of X3B PCD, determined by total loss of X3B both in solution and on the catalyst surface, was increased in the presence of fluoride, but the rate was decreased in the presence of POM. In both cases, the dark adsorption of X3B on TiO(2) was greatly decreased, ascribed to competitive adsorption of POM or fluoride that reduces the positive charges on the catalyst surface. The spectral analysis and the kinetic study using tert-butyl alcohol as hydroxyl radical scavenger revealed that the PCD of X3B on naked TiO(2) was predominately initiated by direct hole transfer, whereas addition of POM or fluoride into the TiO(2) dispersions enhanced the degradation of X3B via hydroxyl radical pathway. It is proposed that the surface occupation of POM on TiO(2) accelerates the production of surface-bound hydroxyl radicals, due to enhanced charge separation, whereas the fluoride replacement of surface hydroxyl groups of TiO(2) increases the production of free hydroxyl radicals in solution, due to enhanced hole availability for water oxidation. Assume that the relative reactivity among various active follows the order of free hydroxyl radicals > subsurface holes > surface-bound hydroxyl radicals, the proposal could account for the observed effects of POM and fluoride on the PCD of both weakly and highly adsorptive organic substrates over TiO(2) such as chlorophenol and X3B.

Several studies have shown that addition of NaF into the aqueous dispersion of TiO 2 (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 TiO 2 in aqueous suspension under UV light irradiation (λ g 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 AgNO 3 , 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 TiO 2 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 hydroquinone 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 TiO 2 , by fluoride ions present in the Helmholtz layers, through a fluorine hydrogen bond.

Various factors that influence the photocatalytic activity of TiO2 for organic degradation in aerated aqueous solution have been reported. For instance, anatase is considered to be much more active than rutile. However, no attention has been paid to the difference in their sorption capacities toward O2 in water, which might be critical to the activity determination. In this work, Ag+ has been used as an electron scavenger, for the photocatalytic degradation of 4-chlorophenol in the N2-purged aqueous suspension of TiO2 under UV light. Three different TiO2 samples in the crystal forms of anatase and rutile, containing micro- and mesopores, were prepared, followed by sintering at different temperatures (T s). The initial rate of 4-chlorophenol photodegradation, per the initial amount of Ag+ adsorbed, increased exponentially with T s. Such T s-dependent normalized rates were observed with three differently prepared TiO2 samples, and three curves were almost overlapped. Comparatively, in the aerated aqueous suspension of TiO2, the initial rate of 4-chlorophenol photodegradation, per surface area of the catalyst, first increased and then decreased with T s, the trend similar to those widely reported. Moreover, from the literature data of water photosplitting over TiO2, the initial rate of O2 evolution, per the initial amount of Ag+ adsorbed, increased exponentially with T s. Evidence clearly shows that with the same amount of electron scavenger on the catalyst surfaces, anatase and rutile actually have a similar photocatalytic activity at a given T s, for organic degradation or water oxidation. It is recommended that to evaluate the photocatalytic activities of different TiO2 samples in an aerated aqueous solution, the difference in O2 adsorption needs to be taken into account.

The photocatalytic activity of titanium oxide supported on ZSMS, zeolite A, silica, and alumina was investigated using the photooxidation of 4-chlorophenol and acetophenone as test reactions. It was found that the photocatalytic activity of titanium oxide on ZSMS and zeolite A was higher than that of a Ti02 powder prepared in parallel or Ti02 formed on silica and alumina, respectively. Ti02/ZSMS, which showed the highest photoactivity at low Ti content, was observed to have large adsorptivity for the organic substrates. It was also found that a decrease in zeolite crystallinity decreased the photoactivity of TiOz/zeolite A. These observations suggest that adsorption and zeolite structure are factors which may be responsible for enhancement of photocatalytic activity of supported Ti02. The sample characterization by XRD, BET, FTIR, and Raman spectroscopy indicates that titanium oxide is formed as small particles of anatase on all the supports and that there are also noncrystalline titanium oxide species formed on zeolite supports. Maximal photocatalytic activity is observed at low (<2%) titanium oxide loading on ZSMS.