Photocatalytic degradation of organic molecules in water: photoactivity and reaction paths in relation to TiO2 particles features

Anatase–rutile and anatase–brookite–rutile composite nanocrystals were synthesized by a controlled sol–gel reaction followed by hydrothermal growth and by mechanically mixing pure crystalline phase precursors, respectively. Their physicochemical and photocatalytic properties were investigated in comparison with those of several home-made and commercial pure anatase, rutile and mixed phase TiO2 samples. In particular, the phase composition and the crystallite size were determined by X-ray diffraction measurements. The photocatalytic degradation of the azo dye Acid Red 1 (AR1) and of formic acid (FA) in aqueous suspension were employed as test reactions. Hydrogen peroxide evolution, the main reductive process occurring in parallel, was also monitored during the runs. The investigated photocatalysts exhibit different photoactivity scales towards the two organic substrates, depending on the prevailing photodegradation path. High surface area anatase samples were particularly active in FA degradation, principally occurring through direct interaction with photoproduced valence band holes, whereas rutile samples, even possessing high surface areas, but exhibiting poor wettability, were scarcely active. A good linear correlation was found between H2O2 evolution and the rate of AR1 photodegradation on the investigated series of photocatalysts, whereas no H2O2 was detected during FA mineralization, apart in the case of a low surface area anatase sample, whose peculiar photocatalytic properties were ascribed to the presence of Ti3+ ions in its structure