Mechanistic evaluation of heterocyclic aromatic compounds mineralization by a Cu doped ZnO photo-catalyst

Abstract

In this study, Cu doped ZnO photo-catalysts were used for the degradation of the heterocyclic compounds, pyridine and quinoline. Three ZnO based photo-catalysts with different amounts of Cu doping (1%, 5% and 10%) were synthesized by precipitation method. The characterization of the catalyst was done using field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Brunauer Emmett Teller (BET), diffuse reflectance spectroscopy (DRS) and photo-luminescence (PL) techniques. The band gaps of the pure ZnO, and 1%, 5% and 10% Cu doped ZnO photo-catalysts were found to be 3.27 eV, 3.21 eV, 3.17 eV and 2.91 eV, respectively. The effects of pH, photo-catalyst dose and irradiation time were studied. Under optimum conditions (5% Cu doped ZnO, dose of 1.2 g L⁻¹ for pyridine and 1.6 g L⁻¹ for quinoline, pH = 11 and time = 5 h), the maximum pyridine and quinoline mineralization efficiencies were found to be 92.4% and 74.3%, respectively. The mineralization process followed first-order kinetics. The in situ formation of singlet oxygen, hydroxyl radicals and superoxide radicals was confirmed by reactive oxygen species (ROS) scavenger studies. Catalyst reusability studies showed excellent mineralization up to four consecutive cycles. The enhanced photo-catalytic mechanism was studied by comparing the band structure with respect to the potential of highly reactive species (˙OH and O₂˙⁻). A possible mineralization pathway was proposed on the basis of the intermediates detected by gas chromatography coupled with mass spectrometry (GC-MS) analysis.