Photoelectrocatalytic synthesis of commercially valuable molecules by nanotube-structured TiO2 electrodes

This thesis consists of two main studies. In both studies, nanotube-structured TiO2s were prepared on Ti plates by anodic oxidation method for different periods, and their crystallinity was increased by calcining at 500 °C. Then, selective photoelectrocatalytic oxidation of 3-methylpyridine to vitamin B3 with prepared electrodes was carried out under suitable light sources in an aqueous medium. In other words, photoelectrocatalytic reactions are environmentally friendly as they are carried out at room temperature, ambient pressure, and neutral pH using water as the solvent and air or water as the oxidant. Electrocatalytic and photocatalytic experiments were also carried out for comparison purposes. In order to determine the kinetic parameters and the reaction mechanism, the concentrations of the substrate and products in the samples taken from the reactor at certain times were analyzed by the HPLC, and the amount of mineralized CO2 at the end of the reaction was determined by the TOC. In the first part of the thesis, nanotube/nanowire structured TiO2 samples on Ti plates were prepared by anodic oxidation method in aqueous, glycerol, or ethylene glycol medium under different experimental conditions and characterized by XRD, SEM-EDX and electrochemical methods (photocurrent profiles and electrochemical impedance spectroscopy). The nanostructured TiO2 was prepared on relatively large Ti plates. Consequently, in some preparation conditions, different morphologic properties were observed in the underside and upside zones of the electrodes, especially in ethylene glycol medium. The electrodes were tested for selective photoelectrocatalytic oxidation of 3-methylpyridine to 3-pyridinemethanol, 3-pyridinemethanal, and vitamin B3 in water under UVA irradiation. The best performance was found with the electrode prepared in ethylene glycol/water/NH4F (98/2/0.3 w/w) medium at 36 V potential for 3 h, while the worst was that prepared in aqueous medium, which showed the shortest nanostructure length. Unlike photoelectrocatalysis and photocatalysis, no carbonyl products were obtained in electrocatalytic and photolytic experiments in which the reaction mechanism is different. The selectivity values of the products were similar for the various electrodes, while the morphology of the nanotubes/nanowires and their quantity on the electrode surface which affects the total active surface area were essential for the substrate conversion. The results also show that the photocurrent intensity, XRD peak intensity, and electrical conductivity are correlated with photoelectrocatalytic activity of the Ti/TiO2 electrodes. In the second part of the thesis, nanotube-structured TiO2s on Ti plates were prepared in ethylene glycol for different periods (15 minutes-5 hours), calcined at 500 °C to increase its crystallinity, and finally decorated with WO3 in different amounts by the cyclic voltammetry. The prepared electrodes were characterized by XRD, XPS, SEM-EDX and electrochemical methods (photocurrent profiles). Selective photoelectrocatalytic oxidation of 3-methylpyridine to vitamin B3 with prepared electrodes was carried out under UVA, UV-visible, and visible lights in an aqueous medium. Electrocatalytic and photocatalytic experiments were also carried out for comparison purposes. Electrodes and working conditions (TiO2 nanotube length, amount of WO3 decorated to TiO2, applied potential, and electrolyte concentration) are optimized. In addition, the effects of these parameters on the reaction rate and product selectivity/dispersion were investigated. Kinetic parameters were calculated, and a reaction mechanism was proposed. The vitamin B3 yield was increased significantly by WO3 decorating (ca. 17 fold) under UVA with respect to undecorated one, while under visible irradiation, low activity and no 3-pyridinemethanol, 3-pyridinemethanal, or vitamin B3 product selectivity were obtained. This unexpected result was clarified for the first time in the literature; under visible irradiation, only unselective photolytic reactions occurred, while under UVA, selective photoelectrocatalytic oxidation to vitamin B3 and unselective photolytic reactions occurred in the presence of catalysts.