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.