Effects of Surface Modification on the Photocatalytic Activity of TiO2

The photocatalytic activity of titanium dioxide modified by surface fluorination (F-TiO2) and by differently deposited gold nanoparticles (Au/TiO2) was tested in the degradation of organic substrates, i.e. mainly formic acid and the azo dye Acid Red 1 (AR1), in comparison to that of unmodified titania. The effect of different noble metals deposition on TiO2 was also investigated in the photocatalytic production of hydrogen from water solutions. Surface fluorination increases the rate of photocatalytic oxidation proceeding through a hydroxyl radical-mediated path. These radicals accumulated in higher amount on F-TiO2, because of the shielding effect of adsorbed fluoride. By contrast, the degradation of substrates implying direct interaction with the photogenerated holes occurred at a lower rate on F-TiO2, because of their hampered absorption on the fluorinated surface. The method employed to reduce Au(III) to metallic gold in the preparation of Au/TiO2 photocatalysts was found to affect their photoactivity, also by modifying the surface properties of TiO2. The presence of gold on TiO2 facilitates both the electron transfer to O-2 and the mineralization of formic acid, proceeding mainly through direct interaction with photoproduced valence band holes. For substrates undergoing mainly oxidation through a hydroxyl radical mediated mechanism, the photogenerated holes may partly oxidize gold nanoparticles, which consequently act as recombination centers of photoproduced charge carriers. Photocatalytic hydrogen production tests, either from water or from methanol reforming, performed in a special set up, closed recirculation laboratory scale photoreactor employing titanium dioxide and noble-metals-modified titanium dioxide samples prepared by different techniques, confirmed the main role of noble metals nanoparticles on TiO2 in facilitating electron transfer reactions also under anaerobic conditions. Higher hydrogen production rates were attained with the photocatalysts in contact with the vapor phase, i.e. in the absence of the liquid phase mass transfer limitations of aqueous suspensions. The activity of metal nanoparticles-containing photocatalysts prepared by one step flame spray pyrolysis was always higher than that of photocatalysts prepared by deposition on pre-formed gold nanoparticles on TiO2, most probably because of the better noble metal dispersion on the photocatalyst surface.