Photocatalyzed Degradation of Adsorbed Nitrophenolic Compounds on Semiconductor Surfaces

Using semiconductor materials as photocatalysts to degrade hazardous organics has gained much attention in recent years. Colored compounds adsorbed onto semiconducting surfaces such as TiO2 or ZnO have been shown to be excited by visible light and to readily undergo degradation (Gopidas and Kamat, 1989). This same principle can be applied to the photo-induced destruction of colored pollutants such as nitrophenols. Since visible light can be used to initiate the photodegradation of colored pollutants, this type of semiconductor system offers a potentially efficient treatment process. The purpose of this research is to study the mechanisms involved in the photodegradation of nitrophenols by semiconductors. Steady state diffuse reflectance photolysis studies indicate that TiO2, which exhibits semiconducting properties, gave the smallest half-life for photodegradation of 2,5-dinitrophenol. The metal oxides which acted only as support materials produced little photodegradation. Various nitrophenols are compared to study the effect of substituent nitro groups on rate of reaction. For example, 4-nitrophenol degradation has a half-life of 10 minutes while the half-life of 3-nitrophenol is 70 minutes. Also, an increase in the number of nitro groups decreases the rate of reaction. The half-life of mononitrophenols is on the order of an hour and dinitrophenols on the order of two hours while trinitrophenol still does not degrade to its half-life even after 8 hours of irradiation. Flash photolysis experiments with 4-nitrophenol in ethanol indicate the presence of a 420 nm transient species. Experiments in aerated and deaerated conditions show that the transient is an oxidation product and that it is not a triplet. Pulse radiolysis studies indicate that the transient is most likely a hydroxyl adduct of the parent compound. The presence of ZnO quenched the 420 nm transient.