| Summary: | 博士 === 國立高雄第一科技大學 === 工程科技研究所 === 99 === Abstract
When media of various dielectric constants are placed in a microwave field, the microwave radiation will preferentially couple with the media that have higher dielectric constants. Additionally, under microwave irradiation, polar molecules in a solution will be transformed and shifted vigorously to generate frictional heat that causes the solution temperature to increase rapidly. The microwave irradiation also causes higher activated status for the molecules, and promotes collision among the pollutant molecules to reduce their activation energy so that they are decomposed at higher rates. In this research, 7 different dielectric media, i.e. nano-scale zero-valent iron particles (Feo), cordierite covered with zero-valent iron (cordierite/Feo), TiO2, cordierite covered with TiO2 (cordierite/TiO2), fresh platinum/palladium/rhodium (fresh Pt-Pd-Rh) catalyst, recovered Pt-Pd-Rh catalyst (recovered Pt-Pd-Rh), fresh Pt-Pd-Rh catalyst covered with TiO2 (TiO2/Pt-Pd-Rh), and recovered Pt-Pd-Rh catalyst covered with TiO2 (TiO2/Pt-Pd-Rh), were tested as microwave absorbing media for decomposing chlorobenzene (CB) in solution. Laboratory results show that under similar irradiation conditions (250 W microwave energy applied for 300 sec.), nano-scale zero-valent iron particles reduce the CB solution activation energy by 5.7 kJ/mole, and remove 2.8 (82.6% vs 28.6%) times more CB from 100 mg/L CB solution than without application of zero-valent iron particles. On the other hand, when the solution temperature is maintained at 25 oC for 240 minutes with microwave irradiation, nano-scale zero-valent iron particles have the highest CB removal efficiency. When suspended in solution, the extremely small particles have larger contact surface with the soluble pollutants in the solution to raise the CB removal efficiency. In contrast, the cordierite particles covered with 0.1 g of zero-valent iron have cylindrical shape with limited contact surface, their CB removal efficiency is 26.2%, which is superior to the 12.9% for the zero-valent iron particles covered with cordierite.
When exited by microwave radiation, TiO2 will produce electron-electron hole pair, which will assist in oxidizing/reducing the molecules adsorbed, suppress electron and hole recombination, and prolong the time of photo-catalysis to raise the TiO2 photo-decomposition efficiency. In addition, the Anatase crystal structure has relatively large gap between the valence band and the conductivity band so that electrons have higher transmission rate to lower the electron-electron hole recombination rate, and prolong the life of electron-electron hole pair. Under similar experimental conditions (30 w microwave irradiation for 300 sec.), the CB removal efficiencies are 70.1% for P-25 TiO2, 55.6% for prepared TiO2, 49.1% for TiO2/cordierite, and 14.8% for cordierite. P-25 TiO2 particles have high strength in Anatase and Rutile crystal phases, the Rutile mixed in the Anatase structure can be considered as the surface defect of Anatase crystal structure. The electrons produced can overflow to the Anatase crystal to separate electrons from electron holes thus prolonging the recombination rate of electron-electron hole pairs that raises activities of the photo-catalyst.
When the microwave radiation penetrates the solution to reach the surface of TiO2 particles, the microwave energy absorbed by the TiO2 enhances the formation of more defective sites on the catalyst surface. Hence, the recombination of electron-electron hole pairs on the semi-conductor surface is inhibited. The absorbed microwave energy is converted into heat energy that is evenly distributed in the whole solution in addition to providing more contact between the pollutant and the solution so that the CB decomposition and removal efficiencies are enhanced. Under similar experimental conditions (30W microwave irradiation applied for 300 sec.), fresh Pt-Pd-Rh catalyst has better CB removal efficiency than the recovered Pt-Pd-Rh catalyst (64.4% vs. 58%). Further, fresh Pt-Pd-Rh particles covered with TiO2 also show better CB removal efficiency than the Pt-Pd-Rh particles covered with TiO2 (73.3% vs. 70.3%).
In this experiment, the efficiency of microwave-enhanced technology for removing halogen-containing organic matter is further improved by using the various combinations of various dielectric media. Additionally, using the recovered catalyst in the combined dielectric media as experimented in this study is a new approach to reducing the treatment cost of microwave-enhanced technology in addition to increasing the add-on value of catalyst-containing wastes.
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