IMPROVING THE PHOTOCATALYTIC ACTIVITY OF TIO2 FOR ENVIRONMENTAL APPLICATIONS: EFFECTS OF DOPING AND OF SURFACE MODIFICATION

Different routes were explored for the solution of the two limiting aspects of TiO2 use in photocatalytic processes, i.e. its large band gap, allowing the exploitation of only a small portion of sunlight, and the fast recombination of photoproduced electron-hole pairs. The attention was first focused on the consequences of noble metal nanoparticles deposition on TiO2, with a systematic study on the photocatalytic performance of commercial TiO2 bearing gold nanoparticles deposited either by so-called deposition-precipitation, with particular attention to the method, either thermal or chemical, employed to reduce the Au(III) precursor into metallic gold, or by photodeposition. Two organic substrates, i.e. the azo dye Acid Red 1 and formic acid, were mainly employed as substrates in photocatalytic oxidative degradation kinetic tests. In order to minimize the major drawback of TiO2 as a photocatalytic material, i.e. its high-energy band gap, sulphur, fluorine and boron were tested as dopants of TiO2. Doped materials prepared by the sol-gel method, containing different nominal dopant amounts and calcined at different temperatures, were systematically investigated in both liquid and gas phase photocatalytic reactions under polychromatic irradiation. Furthermore, the photo-oxidation of acetic acid was also investigated systematically as a function of irradiation wavelength, by collecting so-called action spectra, which represent the most powerful photocatalytic characterization tool to determine the effective wavelength-dependent response and activity of a photocatalyst. The comparison between the shapes of the absorption and the action spectra of the materials allowed one to distinguish between absorption features which are active or inactive in photocatalysis. XPS and EPR analyses gave an essential contribution in order to correlate extra absorption peaks to the chemical composition of doped-TiO2. Finally, the effect of noble metal (Pt and Au) nanoparticles photodeposition on the activity of the best performing doped TiO2 photocatalysts calcined at 700°C was investigated in both energetically down-hill and up-hill reactions, i.e. in formic acid and acetic acid degradation in aqueous suspension and in hydrogen production from methanol/water vapor mixtures. Intriguing synergistic effects of TiO2 doping and of noble metal nanoparticles deposition were observed in both types of reaction.