| Summary: | 博士 === 國立臺灣科技大學 === 化學工程系 === 94 === The purpose of this research was to study the effects of solution pH on the photoelectrochemical properties of TiO2 photoelectrode and the photocatalytic reactions of dyestuffs in aqueous solution using photocatalyst membranes. The open circuit voltage and the short circuit photocurrent of TiO2 photoelectrode increased as solution pH decreased and indicated more photoelectrons were generated in acidic solutions. The response times reaching half maximum photovoltage and photocurrent were determined to be faster in acidic solutions than those in alkaline solutions, indicating higher electron diffusion occurred in acidic solutions. The effect of electrolyte solution pH, fluid pressure and UV irradiation on the tangential streaming potential (TSP) and filtration streaming potential (FSP) of Degussa P-25 titanium dioxide membrane were examined. Both TSP and FSP measuring systems for TiO2 membrane were developed. Experimental results depicted TSP and FSP measurements were sensitive to solution pH, temperature, fluid pressure and UV irradiation. Zeta potentials of TiO2 membranes calculated with TSP data were different from those of TiO2 suspensions measured by electrophoretic light-scattering technique, but the isoelectric point (pHiep) was measured to be almost the same value of 6.8.
The pH drifts (difference of initial and final pH) were observed during the TSP measurements of TiO2 membranes. A modified model was developed to calculate the change in Fermi energy of the electrolyte/TiO2 membranes system presented at different solution pH levels. A hysteresis phenomenon was found that two zeta potentials were observed at the same pH through different pH adjustment paths. Slight decreases of TSP (<15mV) were observed in acidic solutions under UV irradiation; however, more considerable decreases of FSP (20~60mV) were examined in both acidic and alkaline solutions. The FSP increased with increasing filtration pressures and exhibited an exponential decay with increasing light intensity.
The ultraviolet light emitting diode (UV-LED) was used as the UV light source for the photocatalytic decomposition of Reactive Red 22 (RR 22). The temporal behavior of the photocatalytic decomposition of RR 22 in aqueous solution by the UV-LED/TiO2 with a rectangular planar fixed-m reactor operated in a recirculation mode was studied under various conditions including initial dye concentration, periodic illumination, light intensity, and arrangements of TiO2 coating. The decomposition of RR 22 in aqueous solution by TiO2 photocatalytic processes with the UV-LED was found to be technically feasible with a high TiO2 coated weight (1.135g) and low pH value (pH 2). A Langmuir-Hinshelwood type kinetic equation was adequate for modeling the photocatalytic decomposition of RR 22 by the UV-LED/TiO2 photocatalytic processes. The experimental results indicated that the photonic efficiency with periodic illumination was much higher than those with continuous illumination. The photonic efficiencies with the QLC (quartz-liquid-catalyst) arrangement were higher than those with the QCL (quartz-catalyst-liquid) arrangement for experiments conducted at lower applied light intensity; however the photonic efficiencies for these two arrangements were nearly identical for experiments conducted at higher light intensities. The coating of 0.5 gram of Nafion per gram of TiO2 was enough to reduce the zeta potential of Nf/TiO2 to less than -20mV in aqueous solution and exhibited a hydrophobic surface that might decrease the adsorption and photocatalytic decomposition of dye. Experimental results on the adsorption and photocatalytic decomposition of RR22 and BR2 indicating that the charges of TiO2 surface and reactant dye markedly influence the reaction rate. The photocatalytic decompositions of RR22 using TiO2 were favored to occur in acidic conditions and exhibited a minimum decomposition rate near the isoelectric point of TiO2. Nevertheless, no obvious RR22 decompositions were found in experiments conducted using Nf/TiO2. Decompositions of BR2 using both TiO2 and Nf/TiO2 were more favorable in alkaline conditions; however, decompositions of BR2 were found to be decreased in experiments conducted using Nf/TiO2.
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