Sensitization of TiO2 with Aluminum Phthalocyanine: Factors Influencing the Efficiency for Chlorophenol Degradation in Water under Visible Light

Aluminum tetracarboxyphthalocyanine adsorbed on TiO2 has been examined as a sensitizer for degradation of 4-chlorophenol (4-CP) in water under visible light irradiation (lambda >= 450 nm). It was observed that 4-CP was mostly decomposed into CO2 and chloride ions, whereas the reaction rate was greatly influenced by several parameters. The optimum loading of the dye on TiO2 was about 1.0 wt %, nearly independent of TiO2 used. The solution pH which favored 4-CP degradation over the dye loaded TiO2 was about pH 7. An increase in 4-CP concentration below 0.27 mM resulted in increased rate of 4-CP dearadation, the kinetics well fitted into the Langmuir-Hnshelwood equation. In the flow of N-2, the reaction was almost completely inhibited. Experiments through EPR spin trapping, methanol quenching, and silver ions as electron acceptors revealed the involvement of superoxide, hydroxyl and dye cation radicals in the dye sensitized reaction. Evidence is well interpreted in terms of the electron transfer from the excited dye to TiO2, followed by generation of the reactive species. However, among different sensitizers, the observed difference in activity is hardly correlated with their differences in the particle size, Surface area, crystalline structure, and crystallinity of bare TiO2, and in the dye absorbance and aggregation as well, whereas it is well in accordance with the trend in pore volume, measured for these samples through O-2 adsorption-desorption at 77 K. The result demonstrates for the first time the predominant role of O-2 to the dye sensitized degradation of organic pollutants over TiO2 in aqueous suspension. Furthermore, the dye loaded TiO2 under visible light was also active for degradation of other aromatic pollutants such as sulfosalicylic acid, from which the dye cation radical with a redox potential of about 1.25 V vs NHE at pH 6.5 is inferred.