Titania versus zinc oxide nanoparticles on mesoporous silica supports as photocatalysts for removal of dyes from wastewater at neutral pH

Abstract

For the photocatalytic wastewater treatment containing lower concentrations of dyes as pollutants, a suitable photocatalyst may be selected for anionic or cationic dye removal according to the photocatalyst surface charging and desired adsorption/degradation ratio. Herein, for the first time, we evaluate mesoporous silica supported TiO2 versus mesoporous silica supported ZnO photocatalysts at neutral pH and at equal conditions. The evaluation shows that model anionic dye Reactive Blue 19 (RB19) was adsorbed onto silica supported ZnO nanoparticles at neutral pH in a much higher rate than on silica supported TiO2 nanoparticles, even though the surfaces of ZnO and TiO2–silica composite catalysts were negatively charged in both cases (ca. −20mV of zeta potential). This was ascribed to the strong positive zeta potential of pure ZnO nanoparticles at neutral pH value (ca. +37mV) and consequently to more mixed oxide (spinel-like) ZnO/silica composite behaviour if compared to the physical mixture of oxide TiO2/silica composites. The highest dye adsorption (50%) and the fastest dye degradation (kapp=0.0544min−1 at 25°C) was observed on disordered ZnO/silica with Zn:Si molar ratio 1:1. Pure TiO2 or silica supported TiO2 nanoparticles were stable under UV irradiation. On the other hand, the interaction between SiO2 and ZnO resulted in increased resistance of ZnO to photocorrosion, which was proven to be an issue in pure ZnO nanoparticles.