Photoelectrocatalytic oxidation of 3-pyridinemethanol to 3-pyridinemethanal and vitamin B3 by TiO2 nanotubes

Abstract

In this paper, the first photoelectrocatalytic (PEC) 3-pyridinemethanol oxidation to 3-pyridinemethanal and vitamin B₃ was investigated. To meet this aim, efficient nanotube structured TiO₂ on a Ti plate as a photoanode was prepared by an anodic oxidation method in ethylene glycol and characterized by XRD, SEM, and photocurrent techniques. The effect of nanotube morphology, applied potential, Na₂SO₄ concentration, stirring speed of solution, and pH on the reaction activity and product selectivities were investigated. The TiO₂ phase of all of the anodes was mainly the anatase one. The PEC activity, the intensity of the XRD peak and the photocurrent increased by increasing the nanotube length. The activity decreased by decreasing both the Na₂SO₄ concentration and the applied potential, whereas 3-pyridinemethanal selectivity increased. By increasing the stirring speed of the solution, both the activity and the 3-pyridinemethanal selectivity increased. A lower or no activity was observed for photocatalytic (PC) and electrocatalytic runs, respectively, which were carried out for the sake of comparison. No PC activity was obtained in the presence of N₂, but PEC reactions in the presence of N₂ were faster than those in the presence of O₂. The produced 3-pyridinemethanal in both N₂ and O₂ atmosphere was reduced at the cathode in the PEC reaction, but its oxidation appeared to be much more favourable. The PC reactions could not be carried out under acidic conditions, whilst the PEC ones could be performed in the pH range of 2–12. Moreover, the results indicate that the PEC method allows higher conversions and selectivities to vitamin B₃ to be obtained at pH 7 with respect to those reported in the literature.