| Heterogeneous photocatalysis is one of the most promising techniques for water treatment. Titanium dioxide (TiO2) is the most used catalyst in photo-induced reactions, mainly due to its high photocatalytic activity, chemical stability and inexpensiveness. In this thesis is described the synthesis, spectroscopy and characterization of TiO2 based photocatalysts, and their application to the detoxification of contaminated water containing organic pollutants.;The first part of this work is focused on the preparation and characterization of nanocrystalline TiO2, and its use in the photocatalytic degradation of a model compound - phenol - by ultraviolet (UV) irradiation. Materials were synthesized by means of an acid catalyzed sol-gel technique using an alkoxide precursor (titanium isopropoxide). By controlling the temperature of the calcination step, materials with different crystalline, spectroscopic and morphologic properties are obtained. TiO2 obtained at 673K, almost pure anatase is the most active catalyst, while pure rutile, obtained at 973K, showed poor photocatalytic efficiency. The catalytic materials are extensively characterized by several microscopic, spectroscopic and calorimetric techniques. The photocatalytic degradation process is characterized in terms of several operational parameters, namely pH of the medium, catalyst loading, phenol initial concentration, presence of oxidant species, oxygen partial pressure and photon flow. A modified Langmuir-Hinshelwood model with a pseudo-steady state approach is used for kinetic analysis. The dependence of both, kinetic and adsorption equilibrium constants, on light intensity is demonstrated.;In order to increase the TiO2 photo-efficiency, in particular in the visible spectrum, composite photocatalysts using different supports are synthesized by means of a modified sol-gel technique. Materials such as zeolites, mesoporous molecular sieves, activated carbon, activated carbon fibers and multi-walled carbon nanotubes are used as supports. The prepared catalysts are tested in the photocatalytic degradation of phenol by visible irradiation.;The introduction of carbon based materials results in a positive effect in the photodegradation of phenol. This effect, quantified by means of a synergy factor, is more pronounced when carbon nanotube (CNT)-TiO2 composite catalysts are used. The beneficial effect is explained by the action of carbon nanotubes as photosensitizers, injecting electrons into the conduction band of TiO2.;Due to the improved performance of CNT-TiO2 composite, a more detailed study on its properties is described. Catalysts with different CNT content are tested in the photocatalytic degradation of phenol under UV, UV-Visible and Visible irradiation wavelengths. The effect of the introduction of carbon nanotubes into the titania matrix is more obvious when irradiating in the Visible range, confirming the photosensitizing nature of their action. An optimal CNT to TiO2 mass ratio of 1:5 is observed. Also, carbon nanotubes surface chemistry was found to be important in the photo-efficiency of the obtained composite catalyst. CNT with a moderate amount of surface oxygenated groups results in more efficient catalytic activity. Carboxylic and phenol groups are believed to be anchoring points for titania particles. An excess of surface groups leaded to the agglomeration of TiO2 particles and, therefore, to a decrease in the photo-efficiency of the resulting composite catalyst. Besides phenol, the photocatalytic oxidation of other monosubstituted benzene derivatives such as aniline, nitrobenzene and benzoic acid, is investigated using visible irradiation with both TiO2 and CNT-TiO2 catalysts. The efficiency of CNT-TiO2 catalysts in the photocatalytic oxidation of mono-substituted aromatic compounds appears to depend on aromatic ring activating/deactivating properties of the substituents.;The photo-efficiency of the CNT-TiO2 catalyst is studied using 4-chlorophenol as model compound. The effect of several operation parameters, such as catalyst load, pH of the medium, concentration of hydrogen peroxide and concentration of substrate is described. CNT-TiO2 catalyst is also tested in the photodegradation of other para-substituted phenols, such as 4-aminophenol, 4-hydroxybenzoic acid and 4-nitrophenol. A relationship between the Hammet constant of each compound and its degradability by TiO 2 and CNT-TiO2 photocatalysts is demonstrated. These results appeared to be in line with the electron donor/withdrawing properties of the substituent groups in the phenol molecule.;Finally, an exploratory study on the photocatalytic oxidation of clofibric acid, a lipid regulator drug, under visible irradiation using nanocrystalline TiO2 is described as a possible initiator for future work. |
