Electro–synthesis of copper oxide supported on multi–wall carbon nanotubes catalyst for photodegradation of p–chloroaniline

Abstract

Concern due to the toxicity and danger that PCA poses to the aquatic and human life. Photocatalytic degradation is one of the promising techniques to degrade organic pollutants as it is safe and economical for solving environmental problems. In this study, an electrochemical method was used to load copper oxide (CuO) nanoparticles (1-90 wt%) onto multi-wall carbon nanotubes (MWCNT). The catalysts were characterized by X-ray diffraction, nitrogen adsorption-desorption, electron spin resonance, Raman spectroscopy, transmission electron microscopy, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. The effect of CuO loading on the photodegradation of PCA under ultraviolet (UV) and visible (VIS) light irradiation system was investigated. Under UV light, a low amount of CuO was sufficient to provide a synergistic effect with MWCNT in the system. However, a higher loading of CuO was required to shift the adsorption spectrum toward the VIS light region. The degradation of PCA over the CuO/MWCNT catalysts under UV light was in the following order: 3 wt% CuO/MWCNT (96%) > 1 wt% CuO/MWCNT (82%) > 5 wt% CuO/MWCNT (76%), while under VIS light was 50 wt% CuO/MWCNT (97%) > 10 wt% CuO/MWCNT (92%) 90 wt% CuO/MWCNT (82%). It is presumed that the C–N moieties of PCA were chemisorbed on the catalyst prior to photodegradation. Studies on the effect of scavengers showed that hole (h+) was the main active species under the UV system, and electron (e-) for the VIS system. Under the UV system, based on the highest occupied molecular orbital (HUMO) and the lowest unoccupied molecular orbital (LUMO) potentials of both CuO and MWCNT, the electron (e-)?hole (h+) transfer occurred between their conduction band (CB) and valence band (VB) that reduced the e-?h+ recombination and enhanced the degradation as compared to bare CuO photocatalyst. On the other hand, surface defects and oxygen vacancies lowered the band gap energy of the catalyst and allowed for more excitation of e- under VIS light to produce hydroxyl radicals for enhanced degradation of PCA. The Langmuir–Hinshelwood model verified the transformation of first to zero order kinetics model under the UV system upon the increasing initial concentration of PCA, and vice versa for the VIS system. This supported the fact that the higher energy of UV light urged the h+ to directly react with the PCA at VB and resulted in the transition from kinetic control to mass transfer limitation by increasing PCA molecules while the opposite shift occurred under the lesser energy of VIS light. Optimization using response surface methodology gave the highest degradation of PCA at the optimum condition of 11.02 mg L-1 using 0.45 g L-1 50 wt% CuO/MWCNT at pH 7.26. The obtained condition was reasonably close to the predicted value with 0.26% error. Remarkable mineralization results of PCA were attained by total organic carbon (89.1%) and biological oxygen demand (50.7%). Reusability studies showed that the catalysts were still stable even after five cycles. It is believed that the CuO/MWCNT catalyst has a great potential to degrade various types of organic pollutants for wastewater treatment.