Photocatalytic Degradation of Malachite Green by Titanium Dioxide/Covalent Organic Framework Composite: Characterization, Performance and Mechanism

Abstract In this paper, a titanium dioxide/covalent organic framework (TiO2/COF) composite was prepared and its photocatalytic removal of dye was investigated. Using tetrabutyl titanate as a titanium source, TiO2 nanomaterial was prepared by sol‐gel method. In the presence of TiO2, TiO2/COF core‐shell composite was prepared by solvothermal synthesis using melamine and 1,4‐phthalaldehyde as ligands. The prepared materials are characterized by SEM, TEM, XPS, XRD, TG, FTIR, BET, EPR, PL, and UV‐Vis‐DRS techniques. Using malachite green as a model of dye wastewater, the photocatalytic degradation performance of TiO2/COF composites was investigated under the irradiation of ultraviolet light. The results show that the modification of COF significantly improves the photocatalytic efficiency of TiO2, the degradation rate increases from 69.77 % to 93.64 %, and the reaction rate constant of the first‐order kinetic equation is increased from 0.0078 min−1 to 0.0192 min−1. Based on the free radical capture experiment, the photocatalytic degradation mechanism of TiO2/COF was discussed, and the feasibility of its photocatalytic degradation of malachite green was theoretically clarified. Accordingly, a simple and practical method for photocatalytic degradation of malachite green was constructed, which has potential application value in the degradation of dye wastewater.


Preparation of TiO2
In a clean beaker A, sequentially add 10 mL of tetrabutyl titanate and 35 mL of anhydrous ethanol, shake well, and stir at room temperature for 1 h.In another beaker B, add 2 mL of glacial acetic acid, 35 mL of anhydrous ethanol, and 10 mL of H2O in sequence, shake well, and adjust the pH of the solution to ≤3 with glacial acetic acid.The solutions in the two beakers are mixed, shaken, and stirred in a water bath at 60 °C for 1 h to obtain a milky white gel. [1]After suction filtration and drying, light yellow crystals are obtained.After grinding, it is transferred to a crucible for the high-temperature reaction for 3 h, then cooled and ground to obtain nano-TiO2.

Preparation of TiO2/COF
Weigh 0.40 g melamine and 0.10 g 1,4-phthalaldehyde, add 10 mL of dimethyl sulfoxide, dissolve by ultrasonic, then add 10 mL of H2O and 0.1 g TiO2, and mix well.Then, the solution is transferred to a crucible, evaporated to dryness at 100 °C, and then transferred to a muffle furnace for heating at 180 °C for 3 h.After cooling to room temperature, the product is washed with H2O and anhydrous ethanol until neutral.Finally, the TiO2/COF composites are obtained by vacuum freeze-drying for 48 h.

Photocatalytic Degradation Experiment
Malachite green is used as a simulated dye wastewater to evaluate the photocatalytic degradation activity of TiO2/COF.The photocatalytic reaction is carried out in a self-made photocatalytic reaction box with an illumination wavelength of 254 nm.Add 50 mg of TiO2/COF composite (catalyst dosage is 1.67g/L) to a quartz round-bottomed flask containing 30 mL of 10 mg/L (10 ppm) malachite green solution and shake well.After the reaction reaches adsorption equilibrium in the dark for 20 minutes, turn on the UV l amp, take about 2 mL of the supernatant every 20 min, filter, and then transfer it to a quartz cuvette to measure its ab sorbance with a UV-Vis spectrophotometer.
The relevant calculations for the experiment are as follows: (1) Degradation rate (η) Where C0 is the initial concentration, Ct is the concentration at time t, A0 is the initial absorbance, and At is the absorbance at time t.
(2) First-order kinetic equation Where k is the reaction rate constant, t is the time, C0 is the initial concentration, and Ct is the concentration at time t.