([n-C4H9)4 N]3PMo2W9(Sn4+.xH2O)O39/TiO2): a new visible photocatalyst for photodegradation of DR16 characterization and optimization process by RSM

Optimization of the photodegradation of direct red 16 (DR16) under visible light irradiation in the presence of new photocatalyst [n-C 4 H 9 ) 4 N] 3 PMo 2 W 9 (Sn 4+ .xH 2 O)O 39 /TiO 2 (PMoWSn/TiO 2 ) was investigated. PMoWSn/TiO 2 with different ratios of PMoWSn to TiO 2 (2, 11, and 20%) were synthesized by modified sol–gel hydrothermal method. Characterization of the prepared nanocatalysts was carried out by photoluminescence spectroscopy, X-ray diffraction, Fourier transform infrared, field emission scanning electron microscopy, Brunauer Emmett–Teller, and diffuse reflectance spectra. Factorial experimental design was applied to study the interaction effects of five operational variables including DR16 concentration, polyoxometalate content, catalyst loading, reaction time, and pH on the photodegradation process performance. By increasing the amount of PMoWSn, the recombination rate of electrons and holes was decreased and redshift to visible range was observed. Bandgap of the photocatalysts was evaluated from Tauc and Mott–Schottky plots. The efficient and suitable photocatalytic performance of photocatalyst can be attributed to the efficient separation of photo-generated electron–hole pairs and polarization resistance of the catalyst evaluated by Nyquist and Bode plots extracted from electrochemical impedance spectroscopy. The results showed that the photocatalytic activity of 20-PMoWSn/TiO 2 was better than the other nanocatalysts. A central composite design based on response surface methodology was successfully used in optimization of the photodegradation of DR16. The optimum conditions were achieved at acidic pH (3), DR16 concentration of 20 mg/L, and polyoxometalate loading of 20 wt.%. Also this catalyst showed excellent reusability at least after four runs.