| Summary: | 博士 === 國立成功大學 === 化學工程學系 === 87 === The usage of photoenergy is a significant subject of study in photocatalytic reaction. Using solar energy as a light source to excite the activity of photocatalyst, the photocatalytic reaction can be promoted and carried out. The solar energy can be converted to chemical energy by photocatalytic reaction. Nitrite ion is a toxic material produced from denitrification in nature. Nitrite ion exist in natural water, and poses an immediate poison to aquatic species, when the concentration exceeds the 0.1ppm limiting level. Therefore, it is a practical subject to oxidize the nitrite ion to less poison nitrate ion using photocatalytic reaction and solar energy. The purpose of this study is to develop a high efficiency photoelectrochemical process to treat the nitrite ion.
The heterogeneous photocatalytic oxidation of nitrite ion using suspension TiO2 photocatalyst is investigated. The experimental results indicate that bubbling air through the solution promotes the photocatalytic reaction. The adsorption of reactant on photocatalyst is a significant reaction step in photocatalytic oxidation of nitrite ion. A semitheoretical kinetic rate equation,
Ri=2.36E-4[TiO2]//0.12[NO2-][OH-]//-0.30（I-1）
was obtained in dilute basic NO2- solution. However, the photocatalytic oxidation rate of NO2- could not be measured or be determined exactly in acidic solution. In a 1~3% dilute NaCl solution, Cl- ion only slightly affects the photocatalytic reaction. Moreover, the Cl- ion can not promote the photocatalytic oxidation of nitrite ion. Furthermore, the percentage of nitrite ion removal in the presence of 0.51 M of Cl- solution by photooxidation is only 85 % of the system without Cl- ion.
It is very convenient to use TiO2 powder as photocatalyst in fundamental study, but to recover the TiO2 powder is a practical problem. To Support the TiO2 powder on a substrate can solve this problem. When TiO2 powder supported on a conductive substrate, the electrochemically assisted photocatalytic reaction can proceed. In this study, immobilized the Degussa p25 TiO2 powder on Ti plate as a coated film using modified sol-gel method, and then the TiO2/Ti electrode with TiO2 coated film was prepared. When the calcination temperature is higher than 650 °C, the TiO2/Ti electrode with excellent adhesion can be obtained. Based on XRD data, all the crystal of TiO2 on the TiO2/Ti photoelectrode transfers from anatase form to rutile form during the calcination at 650 °C Experimental results indicate that the rutile form of TiO2/Ti film electrode has excellent photoactivity by applying a bias potential and irradiation simultaneously. The photocurrent efficiency (IPCE) is a function of the applied bias potential. Even applying high bias potential, the prepared TiO2/Ti film electrode still has excellent photoactivity and adhesion.
The photocatalytic oxidation of nitrite ion in a NaCl aqueous solution using the rutile form of TiO2/Ti as the working electrode was obtained, and also the photoelectrochemical oxidation of nitrite ion in seawater was carried out. Experimental results indicate that the photocurrent efficiency of nitrite ion oxidation was 33~40% at pH 7. The oxidation rate of the nitrite ion in brine wastewater using the rutile form of TiO2/Ti electrode can be estimated by photocurrent measurements. The applying bias potential, light power and pH value were the major factors affecting the oxidation rate and the photocurrent efficiency of nitrite ion oxidation, while the concentrations of nitrite ion was minor.
A reaction rate equation with the variance of applied bias potential was obtained by kinetic study. Theoretical analysis results correlate well with experimental ones, indicating that the reaction is of zero order with respect to the concentrations of nitrite ion and the hydroxyl ion. The reaction is of 0.30 and 0.80 order with respect to the applying bias potential and the output power of light. The rate equation is,
R = K'' P//0.80 E//0.30。（I-2）
P and E indicated light power and applied bias potential, respectively..
The photoelectrochemical oxidation of nitrite ion was proceeded under the applied of both photoenergy and electric energy. When the solar energy is limited, the electric energy is an alternative choice. The kinetics of anodic oxidation of nitrite ion in a 1 to 3% NaCl aqueous solution using in situ electrogenerated HClO oxidant was obtained. The oxidation of NO2- was found to be a chemical reaction of NO2- with the active mediator oxidant produced from the anodic oxidation of Cl-. The experimental results indicate that the nitrite ion oxidation is mainly affected by both the concentrations of NaCl and NO2-. When the NaCl concentration is in the range of 0.17 to 0.51 M, and the NO2- concentration is lower than 20 mg/L, the Langmuir-Hinshelwood model can be employed to describe the behavior of the nitrite and Cl- ions.
The integration of electrolysis and photoelectrolysis for oxidizing nitrite ion in a 1 to 3 % NaCl aqueous solution was developed. The effects of potential, concentrations of NaCl and NO2- on the reaction rate and current efficiency of this integrated process were obtained. The potential at about 1.50 V vs. Ag/AgCl is the optimal operating condition in electrolysis. A potential higher than -0.22 vs. Ag/AgCl will induce the photoelectrolysis. Assembling these two types of oxidation processes in a reactor, which has a switch to change the working electrode, will be an economical and effective method to treat the nitrite ion in the water.
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