Photocatalytic degradation of persistent organic pollutants under visible irradiation by TiO₂ catalysts sensitized with Zn(II) and Co(II) tetracarboxy-phthalocyanines

Photo-excitation under visible light has been an important step to acquire solar-driven TiO₂ photocatalysts and dye sensitization has been used frequently to extend the optical response of TiO₂ into the visible region. In the present work, new heterogeneous photocatalysts were prepared by anchoring carboxylic acid substituted Zn(II) and Co(II) phthalocyanines onto polycrystalline TiO₂ surface and their photocatalytic activities were investigated. Due to covalent bonding of carboxy-terminated molecules onto TiO2semiconductors, we synthesized symmetric 4-hydroxybenzoic acid-bearing metallophthalocyanines as dye sensitizer molecules. Heterogeneous composites having titanium dioxide and metallophthalocyanines anchored via CO-O-TiO₂ bonds were characterized by using X-ray diffraction (XRD), Fourier-transform infrared spectrometry (FT-IR), and ultraviolet-visible diffuse reflectance spectroscopy. The optimum loading value of the dyes on TiO₂ were 0.98 μmol/g TiO₂ for CoPc and 0.86 μmol/g TiO₂ for ZnPc, nearly independent of the amount of TiO₂ used. These newly obtained heterogeneous photocatalysts were employed in the photocatalytic degradation of 4-chlorophenol(4-CP), chlorobenzene(CB) and 1,2,4-trichlorobenzen(TCB) in aqueous media under visible irradiation. Gas chromatography-mass spectrometry (GC-MS) was used for quantitation. The new photocatalysts showed excellent activities with visible-region irradiation in the photocatalytic degradation of persistent organic pollutants (POPs) as compared to the control experiments used with untreated TiO₂ and the difference was attributed to the cooperation of the two elements, namely TiO₂ and MPc. Experiments show that in two hours nearly complete degradation of POPs were observed.