Photocatalytic oxidation of cyanide on TiO₂: An electrochemical approach

Photocatalytic oxidation of cyanide on TiO₂ was studied by means of voltammetry, linear polarization resistance and Tafel polarization using TiO₂-carbon paste electrodes (TiO₂-CPEs). In all cases, a positive effect with UV-vis illumination on the oxidation current (i.e. oxidation rate) was observed, showing the convenience of this type of electrodes for the electrochemical evaluation of powdered semiconductor photocatalysts. Voltammetric results indicated that the oxidation of cyanide (CN^-) occurs at lower potentials than the oxidation of hydroxyl ions (OH^-); cyanide is oxidized to cyanate (CNO^-) and at high potentials it can further be oxidized together with OH^-. Linear polarization resistance curves showed that the process is controlled by electron transfer. Tafel slopes from the anodic and cathodic polarization curves enabled the determination of the intrinsic kinetic parameters of the reaction (i.e. rate constant, reaction orders). UV-vis illumination led to an increase in the reaction rate, related to the increment in the rate constant, and a decrease in the reaction orders for both CN^- and OH^- ions.