Transcript Slide 1
Photocatalytic Degradation of Polychlorobiphenyls Using Anodic Bias on Nano Tubular TiO2 Tal Zaid, N. Baram, D. Starosvetsky, Y. Ein-Eli Department of Materials Engineering, Technion-Israel Institute of Technology, Haifa 32000, Israel Introduction • Different aspects of water treatment are considered the most urgent topics at the present and will influence our future life. Photocatalytic oxidation of organic compounds is an advanced method for removal of impurities from water. • Polychlorobiphenyls (PCBs) are a group of man-made organic compounds with 1 to 10 chlorine atoms that are attached to a biphenyl molecule. There are 209 congeners of PCB, which all have low water solubility, but also have high solubilities in most organic solvents. The chronic toxic effects of PCBs include failure in reproduction, birth defects and brain damage, in addition to being carcinogenic. The Principle of Photocatalysis Anodization in aqueous solutions Under UV illumination electrons and holes are produced3,4: •Electrolyte – 1M Na2SO4 + 0.5%wt NaF TiO2 h e h •2hr, constant potential The following reactions occur: H2O h H OH 2 Eg=3.1 eV E0 2.74VSHE 0 Photocatalysis of PCBs O H HO2 •A Petri dish with working, counter and reference electrodes. HO2 e H H2O2 • Titanium dioxide is close to being the ideal photocatalyst in several ways: relatively inexpensive, chemically stable, the light required to activate the catalyst may be long-wavelength UV such as the natural UV component of the sunlight and the produced oxidant is powerful with elimination potential of most types of microorganisms1. The main problem of this process is the low efficiency due to high electron/hole recombination rate2. The efficiency of the photocatalysis process depends on the amount of generated holes, which is typically low, due to the high electron-hole recombination rate. In order to enhance the efficiency of the process, an anodic bias was applied on nanotubular TiO2 which was grown by anodization in aqueous solution with addition of fluoride ions. Comparison of the degradation rate of two PCB congeners of 20V, room temp. E 0.28VSHE O2 e O 2 Experimental •PCB – 100ml of 10 mg/L aqueous solution. schematic diagram showing the potentials for various RedOx processes occurring on the TiO2 surface at pH 7 •Anodic bias – 0-5V UV lamp – l=360nm Hydroxyl radicals have high oxidation potential: OH H e H2O E 2.74VSHE Analysis E 1.78VSHE •The absorption of the samples were determined by spectrophotometer. 0 H2O2 2H 2e 4H2O 0 Photocatalysis results Effect of Anodic Bias on the current density Effect of Ti/TiO2 catalyst size on the degradation rate 0.07 2 1.8 cm 2 50 J [mA/cm ] 2 Intensity 0.020 0.05 Intensity 40 0.025 0.06 OCP 1V 2V 3V 4V 0.030 0.015 30 20 0.04 0.03 0.02 0.010 0.01 10 0.005 2,2',3,4,4',5'-Hexachlorobiphenyl 2-Chlorobiphenyl 0 0.000 0 50 100 150 200 250 300 350 The PCB with less Chlorine substitutes had an higher degradation rate. 0 20 40 60 80 100 60 90 120 150 180 210 Time [min] 120 Time [min] Higher anodic bias resulted in higher current density. Increasing the photocatalyst size enhances the degradation rate. Effect of Anodic Bias on the degradation rate Effect of the UV lamp intensity on the current density 0.30 30W 15W 35 30 Jmcm 0.20 Intensity 40 OCP 1V 2V 3V 4V 0.25 2,2',3,4,4',5'-hexachlorobiphenyl 30 0 400 Time [min] 2-Chlorobiphenyl 25 cm 0.15 0.10 25 20 15 10 0.05 5 0 0.00 0 Characterization 50 100 150 200 250 300 350 0 50 100 Time [min] 150 200 250 300 350 400 Time [min] An increase in the applied anodic bias until potential of 4 VSHE has led to an increase in degradation rate. higher current density is achieved with the more intense UV lamp. Summary • Anodic polarization is capable of growing thick, crystalline, nanotubular oxide layer with high surface area • Anodic bias is also capable of reducing electron/hole pair recombination process i.e. increasing the efficiency of the photocatalytic process and the degradation rate. Top and cross section HRSEM micrographs of TiO2 growth via anodization in 1M Na2SO4 + 0.5%wt NaF solution • The combination of immobilize, electrochemically grown titania with an application of extremely high anodic bias and UV illumination led to a dramatic improvement in measured photocurrent. • The size of the photocatalyst is an important factor on the degradation rate of PCBs. • The intensity of the UV illumination determined the current density received in the experiment, and hence, can have a major influence on the degradation rate. • Different PCB congeners will produce different degradation rate, depending on the number of chlorine substitutes. References Only Ti! The oxide is Amorphous The oxide is crystalline: Anatase 1. Serpone, N., Pelizzetti, E., Photocatalysis Fundamentals and Applications, A. Wiley, USA p. 126-157, 1989. 2. Hoffmann, M.R., Scot, T.M., Wonyong, C.H., Bahnemann, D.W., Chem. Rev., 95, 69-96 (1995). 3. Fujishima. A., Rao, T.N., Tryk, D.A., J. Photochem. & Photobio. C, 1, 1-21, 2000. 4. Sunada, K., Kikuchi, Y., Hashimoto, K., Fujishima, A., Enviro. Sci. &Tech., 32, 5 (1998).