Photocatalytic performance of ZnO nanorod coupled photocatalysts under fluorescent light and sunlight irradiation for phenol and 2, 4-dichlorophenoxyacetic acid degradation
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Date
2014-08
Authors
Lam, Sze Mun
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Abstract
Over the years, the surge of industrial activities that inevitably resulted in an increasing flux of pollutants in the environment still remains an intricate challenge for nations. Heterogeneous photocatalysis, particularly on metal oxide coupled on one dimensional (1D) ZnO nanorods (MOx/ZnO) as promising heterostructured photocatalysts were employed to treat two endocrine disrupting chemical (EDC) pollutants, namely phenol and 2,4-dichlorophenoxyacetic acid (2,4-D). A series of MOx/ZnO (MOx = CuO, WO3 and Nb2O5) photocatalysts with different relative energy band positions between ZnO and MOx were developed. The photocatalysts were prepared by a hydrothermal–deposition method and adopted in the photocatalytic degradation of the above mentioned pollutants. The WO3/ZnO photocatalysts exhibited the best performance in the fluorescent light degradation of phenol among the three photocatalysts. It was also found that the photoactivities of developed photocatalysts were much higher than those of pure ZnO, commercial ZnO and commercial TiO2 under the similar conditions. Additionally, the developed photocatalysts showed favourable recycle use potential because they could settle out of solution in less than 2 h after irradiation and their photocatalytic activities were still maintained >80% after four cycles of reaction. The developed photocatalysts were analyzed by various characterization techniques to obtain their physico-chemical properties. The coupling of studied metal oxides revealed in a reduction in the band gap of ZnO nanorod from 3.24 eV to 3.07–3.22 eV and hence, the
photocatalytic reaction was pushed into the visible region. The photocatalysts had high surface areas available for photocatalysis and were of N2 adsorption-desorption isotherms of type IV with type H3 hysteresis loops. In contrast with the pure ZnO, the photoluminescence (PL) spectra of MOx/ZnO photocatalysts indicated the dramatic decreased in their intensities. As for WO3/ZnO photocatalysts, the PL intensity was the lowest. The enhancement in the WO3/ZnO photocatalytic activity can be attributed to the high e– and h+ pair separation and the suitability of redox potential of the two semiconductors to produce highly active •OH radicals. This implication was proven by the radical scavenger and terephthalic acid–PL (TA–PL) experiments. Various operational parameters such as initial pollutant concentration, solution pH, calcination temperature and coupled metal oxide loading were investigated. The extent of degradation of EDCs and their mineralization were also verified further by HPLC and TOC analyses. The kinetics study revealed that the reaction order that best described the fluorescent light degradation of EDCs was first-order kinetic based on the Langmuir-Hinshelwood model. The electrical energy consumption per order of magnitude for degradation of EDCs was lower via the developed photocatalysts than that of the commercial TiO2. Furthermore, the developed photocatalysts could be effectively utilize sunlight to degrade EDCs and their photocatalytic activities were again much superior to that of pure ZnO and commercial TiO2 under the same conditions.
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An increasing flux of pollutants in the environment , still remains an intricate challenge for nations