Electrosynthesis of zinc oxide-copper oxide supported on mesostructured silica nanoparticles for photocatalytic decolorization of methyl orange

Abstract

Photodecolorization of dyes using heterogeneous catalyst is an important process in wastewater treatment. In this study, an electrochemical method was used to load zinc oxide and copper oxide onto mesostructured silica nanoparticles (CuO-ZnO/MSN). The catalysts were characterized using X-ray diffraction (XRD), field emission scanning electron microscopy-energy dispersive X-ray (FESEM-EDX), transmission electron microscopy (TEM), nitrogen (N2) adsorption-desorption, Fourier transform infrared (FTIR), 29Si magic angle spin nuclear magnetic resonance (29Si MAS NMR), ultraviolet-visible diffuse reflectance spectroscopy (UV-Vis/DRS), X-ray photoelectron spectroscopy (XPS), electron spin resonance (ESR), photoluminescence (PL) and cyclic voltammetry (CV). The effects of Zinc (Zn) loading, alkaline treatment of MSN and Copper (Cu) loading were investigated for photocatalytic decolorization of methyl orange (MO) dye. Then, the optimization study was performed by response surface methodology (RSM) and the potential of the best synthesized catalyst was tested on decolorization of simulated dye wastewater. From the result, 5 wt% ZnO loaded on MSN (ZnO/MSN) showed the highest photodecolorization rate (9.93 × 10-2 h-1) compared to 1 and 10 wt% ZnO/MSN, due to good dispersion of ZnO on the MSN surface. The alkaline treatment of MSN to load 5 wt% of ZnO catalyst (ZM) of using ammonium hydroxide (NH4OH) (0.5-2.0 M) demonstrated that ZM prepared under 1.0 M NH4OH was the best catalyst that improved the photodecolorization rate up to 3.87 × 10-1 h-1. The formation of silicon-oxygen-zinc (Si-O-Zn) with new silicon-oxygen-silicon (Si-O-Si) bonds and creation of oxygen vacancies became the main factors that enhanced the photocatalytic performance. The introduction of Cu as a second metal (1, 3 and 5 wt%) onto ZM catalyst (C-ZM) showed the existence of a synergistic effect between both metal oxides by increasing the formation of Si-O-metal bonds, oxygen vacancies and lowering band gap energy. The high decolorization rate of MO was achieved (1.282 h-1), which resulted in 99.5 % of photodecolorization when using 1.0 g L-1 of 3 wt% CuO (3C-ZM) at pH 2 of MO solution. From the RSM experiments, a complete decolorization of MO was predicted (99.99 %) at the optimum conditions of pH 3.5 using 1.6 g L-1 of 4 wt% CuO (4C-ZM) catalyst. Lastly, the high decolorization of simulated dye wastewater (>75 %) using CuO-ZnO/MSN catalyst proved that the modifications of ZnO would have great significance in the synthesis and developing the various catalysts for wastewater treatment as well as for other applications.