Pickering Emulsions of Fluorinated TiO2: A New Route for Intensification of Photocatalytic Degradation of Nitrobenzene

Fluorination of the TiO2 surface has been often reported as a tool to increase the photocatalytic efficiency due to the beneficial effects in terms of production of oxidizing radicals. Moreover, it is shown that the unique amphiphilic properties of the fluorinated TiO2 (TiO2-F) surface allow one to use this material as a stabilizer for the formulation of Pickering emulsions of poorly soluble pollutants such as nitrobenzene (NB) in water. The emulsions have been characterized in terms of size of the droplets, type of emulsion, possibility of phase inversion, contact angle measurements, and optical microscopy. The emulsified system presents micrometer-sized droplets of pollutant surrounded by the TiO2-F photocatalyst. Consequently, the system can be considered to be composed of microreactors for the degradation of the pollutant, which maximize the contact area between the photocatalyst and substrate. The enhanced photocatalytic activity of TiO2-F was confirmed in the present paper as the apparent rate constants of NB photodegradation were 16 × 10-3 and 12 × 10-3 min-1 for fluorinated and bare TiO2, respectively. At NB concentrations largely exceeding its solubility, the rate constant was 0.04 × 10-3 min-1 in the presence of both TiO2 and TiO2-F. However, unlike TiO2, TiO2-F stabilized NB/water emulsions and, under these conditions, the efficiency of NB photocatalytic degradation in the emulsified system was ca. 18 times higher than in the nonemulsified one. This result is relevant also in terms of practical applications because it opens the route to one-pot treatments of biphasic polluted streams without the need of preliminary physical separation treatments.