Metal/non-metal doped TiO2 for the degradation of chlorine/sulfur-containing VOCs under a fluorescent lamp

• Main Authors: Yi-HsingLin, 林依杏
• Other Authors: Hsin Chu
• Format: Others
• Language: en_US
• Published: 2016
• Online Access: http://ndltd.ncl.edu.tw/handle/34q4fd

博士 === 國立成功大學 === 環境工程學系 === 104 === Volatile organic compounds (VOCs) are important air pollutants with regard to their health implications and frequent occurrence. Chlorinated volatile organic compounds (Cl-VOCs) are widely used in industrial manufacture and pharmaceutical industries as solvents, degreasing agents, cleaning agents, and a variety of commercial products. 1,2-Dichloroethane (1,2-DCE), a widely used as one of organic solvents in the industry, can damage the human when it releases. Besides, Volatile organic sulfur compounds (VOSCs) have caused great concern due to their offensive odor, low odor thresholds value (OTV). Dimethyl sulfide (DMS) and dimethyl disulfide (DMDS) have an offensive smells with a very low odor threshold value (OTV). The anthropogenic source can easily result in local concentrations strongly exceeding OTV. Repeated exposure to odorous pollution can cause chronic respiratory and cardiovascular diseases. Eliminating pollutant emissions is achievable to improve the quality of life for people living. In this study, photocatalysts were prepared by a sol-gel method. The photocatalytic performance of several nonmetal-doped TiO2 was evaluated for the degradation of 1, 2-DCE under visible light irradiation. Based on the result of 1, 2-DCE photodegradation, sulfur doped TiO2 for photocatalytic degradation of gaseous DMS was attempted. Soon afterwards, the photocatalysis performance of S,M (M = iron, vanadium, and zinc) co-doped TiO2 for photocatalytic degradation of gaseous DMDS under visible light irradiation was further conducted. The physical and chemical characteristics of photocatalysts were analyzed by thermo-gravimetric/differential-thermal analysis, X-ray diffraction, Fourier transform-infrared spectroscopy, UV-Visible spectroscopy, and XPS, respectively. The photocatalytic degradation of VOCs, kinetics of the reaction, and pathway of SVOCs photooxidation has been investigated under visible irradiation. The results show the degradation rate of 1,2-DCE by S0.15/TiO2 is faster than that by N0.15/TiO2 because of its narrower band gap and the larger specific surface area. Therefore, the photocatalytic decomposition rate of dimethyl sulfide under visible light was expected to increase by doping sulfur. The result shows S-doped can reduce the crystalline size of TiO2 and all photocatalysts are anatase phase structure. The XPS results of S-doped TiO2 indicate that S exists as S6+ on the surface crystal lattices and leads to the formation of Ti–O–S in the TiO2 lattice. The activity of S-doped TiO2 was determined by the measurement of DMS degradation under visible light. According to the result of activity test, S0.05/TiO2 was chosen for further parameter studies. The conversion of DMS increases with the decreasing DMS concentration. The decomposition efficiency of DMS increases with decreasing relative humidity, and with increasing temperature. The presence of water vapor in air with DMS significantly impacts the photocatalytic activity. The phenomenon is ascribed to the competitive adsorption of water vapor and DMS on the active sites of the photocatalysts. By fitting the Langmuir-Hinshelwood model, the result shows that KA is larger than KW and it also represents that adsorption ability of DMS is greater than H2O. The value of k rises with the increasing temperature. The photocatalytic activation energy of the degradation of DMS by S0.05/TiO2 is 13.3 kJ mol–1. The main oxidation products of DMS photodegradation are SO2, CO2, DMDS, DMSO, DMSO2, CO, and MSA. There are two main reactions for photocatalytic degradation of DMS in dry condition: S oxidation by superoxide radicals and C-S bond cleavage. In the case of the wet condition, there are two potential reaction pathways for photocatalytic degradation of DMS: S oxidation by •OH radical and C-S bond cleavage. Compared to S/TiO2, sulfur and transition metal co-doped TiO2 photocatalysts have smaller crystal sizes and shift further to visible-light absorption region. The XPS spectra confirm that V4+, Fe3+, and S6+ are successfully doped into the lattice of TiO2, but Zn2+ is adsorbed on the surface of TiO2. S0.05Zn0.001/TiO2 shows the best photocatalytic activity and S-VOCs tolerance for the degradation of DMDS under visible irradiation among all co-doped TiO2 photocatalysts. The conversion of DMDS decreases with increasing relative humidity. Besides, photocatalytic efficiency of DMDS increases with increasing temperature, which can be described by the rate law and explained by collision theory. The Langmuir-Hinshelwood model 4 is a feasible way to describe the photocatalytic degradation of DMDS by S0.05Zn0.001/TiO2 in this study. Based on the FT-IR and GC-MS characterization results, SO2, C2H6S3 (DMTS), C2H6O2S2 (MMTS), and MSA are the major sulfur-containing products of the photocatalytic degradation of gaseous DMDS, and CO2, CO, HCHO, CH3OOH, and H2O are also presented in the gas phase. Several organic products have been detected in the degradation of DMDS, which show the presence of different types of reaction: (1) oxidation of sulfur, (2) oxidation of carbon, and (3) cleavage of S–S bond. Superoxide radicals and •OH radical are considered the key species for the dry and wet reaction process, respectively.