Highly reusable visible light active hierarchical porous WO3/SiO2 monolith in centimeter length scale for enhanced photocatalytic degradation of toxic pollutants

Abstract

To develop techniques like photocatalytic degradation of toxic-pollutants to reduce water pollution is essential as every human being has the right to have an access to safe water. Metal oxide monoliths have a huge significance regarding this due to their easy-to-separate nature. Porous WO3/SiO2 monoliths were synthesized by vacuum-impregnation of sodium tungstate in silica monoliths and calcination. The XRD spectra confirmed the successful formation of monolith and the morphological studies were done by FESEM exhibiting a connected porous network structure. The elemental-state and uniform elemental-distribution were analyzed by XPS and EDS. BET analysis showed that monoliths displayed multimodal porosity with pore-diameter of 0.70–14.71 nm and high surface area (82 m2/g). DRS revealed that the monolith had a narrow band gap (2.5 eV), which is suitable for visible light photocatalysis. The photocatalytic performance of monolith was estimated by degradation of model-pollutant methylene blue (MB) under visible-light illumination. High photocatalytic efficiency (91%) and rate constant (0.013 min−1) was observed at natural pH of the dye (pH 7.5). The effect of pH and catalyst concentration was explored which revealed that best degradation occurred at pH 3 (97.5%) and catalyst concentration 0.4 g/L (96%). The trapping experiments suggested that the holes were governing reactive-species in the reaction. The photocatalyst was reused for 5 consecutive cycles (80% degradation). The characterization-results of monolith after 2nd cycle of photodegradation explicated its stability. A colorless-pollutant (Imidacloprid) was also degraded to distinguish between direct and indirect photocatalysis. A comparative study was also done with reported catalysts in literature regarding MB degradation.