Photoelectrochemical degradation of naphthol blue black diazo dye on WO{sub}3 film electrode

摘要

Photoelectrochemical degradation of a textile diazo dye, naphthol blue black (NBB), has been investigated using different semiconductor electrodes. A higher photoclectrocatalytic activity has been observed for WO{sub}3 film electrodes, prepared by electrodeposition than for TiO{sub}2 nanoparticulate film electrodes. By annealing of the WO{sub}3 film deposited on a Pt substrate. we were able to increase the photodegradation rate of NBB, owing to the reduced losses due to the electron-hole recombination. This treatment also increased the long-term photostability of the electrode. The effect of the supporting electrolyte was examined using NaCl, NaClO{sub}4, KNO{sub}3, and Na{sub}2SO{sub}4 solutions. The highest NBB degradation rate was found in the acidic Cl{sup}- medium. With increasing solution pH, the degradation rate decreased to a lower level indicating a change in the NBB photodecomposition mechanism. The reaction is of the first order with respect to NBB in the concentration range up to ca. 6×10{sup}(-5) M NBB. The apparent rate constants for the photoelectrocatalytic degradation of NBB in 0.5 M NaCl + 8.8×10{sup}(-5) M NBB solution, on a WO{sub}3 film electrode at E = 1.08 V versus SCE are 1.302×10{sup}(-4) and 3.41×10{sup}(-5) cm{sup}(-2) s{sup}(-1), under illumination with ultraviolet and visible light, respectively. It has been found that the mechanism of the sub-band excitation with the dye self-sensitization is a minor degradation path for NBB, while the main path involves the electron-hole generation and heterogeneous oxidation with participation of valence-band electron holes. The latter reactions involve oxidation of solution components with generation of powerful oxidants such as the OH{sup}·, Cl{sub}2{sup}(·-), and Cl{sup}· radicals, being directly responsible for the fast dye decomposition.