Sunlight mediated removal of chlorophenols over tungsten supported ZnO: Electrochemical and photocatalytic studies

Abstract

The photocatalytic performance of W6+ modified ZnO photocatalysts was assessed for the mineralization of chlorophenols (2-chlorophenol, 3-chlorophenol, 4-chlorophenol, 2,3-dichlorophenol, 2,4-dichlorophenol, 2,4,6-trichlorophenol and pentachlorophenol) in natural sunlight exposure. The comparison of cyclic voltammograms (CV), acquired in the dark and under illumination, revealed the vital role of surface W6+ entities in protecting the surface of ZnO against photocorrosion. An enhanced capacity of the W6+ loaded ZnO catalyst for screening the charge and slow discharge was evidenced under illumination, as compared to pure ZnO, by electrochemical impedance spectroscopy (EIS) and chronopotentiometry (CP). The W6+ modified catalysts showed superior activity for the removal of chlorophenols. The experimental evidences revealed the mineralization of chlorophenols as multistep process that proceeds with the formation of intermediates. Additionally, from the correlation of the experimental data, a significant role of the number and position of the chloro groups was unveiled in the degradation of chlorophenol/s. The identification of oxygen inserted degradation intermediates, rapid mineralization and release of chloride ions in the solution divulged the leading role of superoxide anion (O2−) radicals in the degradation process. Efforts were made to correlate the electrochemical properties and the photocatalytic activity to establish the efficacy of the W6+ supported catalysts for chlorophenol/s removal.