N-Doped Anatase/Rutile Photocatalysts for the Synthesis of Aromatic Aldehydes Under Ultraviolet and Solar Irradiation

N-doped anatase/rutile photocatalysts were prepared by a sol–gel method, using TiCl4 as TiO2 precursor and urea as N-dopant source. The catalysts required a thermal treatment at temperature higher than 300 °C to achieve actual nitrogen doping; this treatment also allowed obtaining crystalline and hydrophobic samples. Moreover, nitrogen doping modified the surface of samples by increasing the anatase to rutile phase ratio and hydrophilicity. The catalysts were characterized by BET specific surface area, XRD, ESEM, TGA, FT-IR, DRS and XPS measurements, with particular attention to the assessment of the N-doping effect. It was found that XPS analysis should be used together with DRS and FT-IR to discriminate whether N had been incorporated in TiO2 lattice or N species had just adsorbed on the TiO2 surface. Moreover, XPS analysis could be also used for determining the crystallinity of TiO2. The photocatalytic activity of the samples was tested for the partial oxidation of 4-methoxybenzyl alcohol to p-anisaldehyde in water under near-UV, visible, UV-visible and simulated solar light, respectively. Commercial (Degussa P25) and undoped home-prepared TiO2 anatase-rutile catalysts were also used for comparison. The results show that N-doping improves absorption of visible light by the catalysts, but the reaction rate does not increase accordingly. Selectivity was higher in the presence of poorly crystallized catalysts, whereas doping seemed to have a slightly improving effect.