A Kinetic Study on the Photoelectrocatalytic Decolorization of Methylene Blue with a TiO2 Thin Film

• Main Authors: Wei, Kuo-Hsiu, 魏國修
• Other Authors: Kuo-Chuan Ho
• Format: Others
• Language: zh-TW
• Published: 1998
• Online Access: http://ndltd.ncl.edu.tw/handle/43277590859954263815

碩士 === 國立臺灣大學 === 化學工程學系 === 86 === This thesis is a kinetic study on the photoelectrocatalytic decolorization of methylene blue solution with a TiO2 thin film. The TiO2 film absorbs photons of incident UV light thus generating electron-hole pairs (e-h+). In order to enhance the decolorization rate of methylene blue solution, the rate of recombination between photogenerated charge carriers is suppressed by applying an external anodic bias. TiO2 particulate thin films were prepared based on three reported methods. Namely, sintering method of TiO2 powders, sol- gel method with alcoholysis, and sol-gel method with hydrolysis. After comparisons of the film/substrate adhesion, the reproducibility of each process, and the stability of sol, the sol-gel method with hydrolysis was chosen throughout this study to prepare TiO2 films. Under the same light intensity, the photocurrent density does not increase when the number of dip coating is greater than three. In addition, when the TiO2 film is dip coated for more than five times, the decolorization rate decreases due to an increased electric resistance. It''s concluded that the most suitable number of coating is maintained at five. A mechanism on the photoelectrocatalytic decolorization of methylene blue with a TiO2 thin film is proposed, which is consistent with the results reported by related literatures. Factors affecting the decolorization rate were analyzed. It was found that the rate is a function of the concentration of methylene blue, the applied external bias, the pH value, the thickness of TiO2 film, and the light intensity absorbed by TiO2 film. The relationship between the light intensity absorbed by a TiO2 film and the OHo concentration is a quadratic function. The decolorization rate of methylene blue (r) can be expressed as: r={Keq[MB+(aq)]/(1+Keq[MB+(aq)])}*{[-b+(b^2+4afI)^.5]/2a} And the reaction time (t) can also be expressed as: t={2a/[-b+(b^2+4afI)^.5]}*{[MB+(aq)]0X-1/Keq*ln(1-X)} where Keq is the absorption equilibrium constant, [MB+(aq)] is the concentration of methylene blue (M), f is the effective factor, I is the light intensity absorbed by TiO2 film (W/cm2), X is the conversion of methylene blue, and a, b are constants (a = 3.25*10^13 Wmin2/M2cm2, b = 8.94*10^4 Wmin/Mcm2). Experiment data indicate that the equilibrium constant of methylene blue absorbs onto TiO2 film is 2.03*10^6 M-1 at +0.6 V (vs. Ag/AgCl). When the concentrations of methylene blue decrease from 10.0 mM to 1.0 mM and 0.1 mM, the amounts of methylene blue absorbed onto TiO2 film decrease, rendering the proportions of recombination of OHo increase. The ratios of effective factor decay to 0.171 and 0.139, respectively. Under the external anodic bias, methylene blue cation is somewhat difficult to absorb onto TiO2 films. The absorption equilibrium constant decays to a steady value when the bias is more positive than +0.3 V. On the other hand, when the external bias is more positive than +0.6 V, the concentration of OHo on TiO2 film is larger than that without any bias. It is found that the decolorization rate is enhanced when the applying external bias is more positive than +0.6 V.