Semiconductor-catalyzed degradation of phenols with sunlight

With natural sunlight TiO2 (anatase), Fe2O3, CuO, ZnO, ZnS, ZrO2, CdO, HgO, PbO, PbO2 and Al2O3 catalyze the degradation of phenol but MoO3, Co3O4, CdS, SnO2, Sb2O3, La2O3, CeO2, Sm2O3, Eu2O3, Pb2O3 and Bi2O3 do not. The degradation rates increase linearly with phenol-concentration and area of the catalyst bed but decrease linearly with pH. Air is essential for degradation and the catalysts show sustainable photocatalysis. Pre-sonication fails to enhance the photocatalysis. The system does not respond to application of a potential bias for enhancement of the photocatalysis. o-Cresol, m-cresol, p-cresol, o-chlorophenol, m-chlorophenol, p-chlorophenol, o-nitrophenol, p-nitrophenol, o-aminophenol, m-aminophenol, p-aminophenol, catechol, resorcinol and quinol degrade much slower than phenol. The ease of degradation correlates neither with the electron-releasing or withdrawing capacity of the substituent nor with its position in the phenyl ring. Also, the photocatalytic efficiency of the semiconductor is not in accordance with its band-gap energy.