Comparative visible-light driven selective oxidation to aldehydes of phenylmethanol (benzyl alcohol) and 4-pyridinylmethanol (4-pyridinecarbinol) on N-TiO2 and some commercial TiO2 samples

Visible light (λ > 420 nm) selective photooxidation of phenylmethanol and 4-pyridinylmethanol in CH3CN to the corresponding aldehydes on N-TiO2 is compared with homemade undoped TiO2 (U-TiO2) and commercial undoped anatase specimens (such as PC105, PC500). Significant differences observed between N-TiO2 and undoped TiO2 are neither directly related to the surface area nor to the adsorbed amount of alcohol in the dark by surface area unit. FTIR and EPR spectroscopies are used to study the surface of TiO2 samples and to deeply understand the phenomena intervening in the visible-light photocatalytic activation of the doped vs the undoped oxides. In particular, it is shown that on N-TiO2 (and also on undoped PC105) strong Lewis acid sites (LAS) exist. The favorable role of LAS on the photocatalytic activity is illustrated by the higher photooxidation of 4-pyridinylmethanol vs phenylmethanol over N-TiO2 and PC105 in contrast to the other undoped samples, whose visible light sensitivity originates from a charge transfer between the alcohol and the solid. EPR spectra of N-TiO2 point out the presence of paramagnetic centers related to nitrogen that disappear when the photocatalyst is irradiated with visible light in the presence of alcohol, which starts its oxidative process. On the basis of presented results, we propose that doping with N introduces new intraband gap states that not only contribute to LAS and adsorption of alcohol but also are directly involved in the photochemical process occurring under visible light irradiation.