MICROWAVE-ASSISTED HETEROGENEOUS PHOTOCATALYTIC OXIDATION

Transcription

1 MICROWAVE-ASSISTED HETEROGENEOUS PHOTOCATALYTIC OXIDATION S. Kataoka, D.T. Tompkins, M.A. Anderson, M.E. Zorn, W.A. Zeltner Civil and Environmental Engineering Department University of Wisconsin - Madison 66 N. Park St. Madison, WI 5376 J.H. Booske Electrical and Computer Engineering Department University of Wisconsin - Madison 1415 Engineering Dr. Madison, WI 5376 ABSTRACT Photocatalytic oxidation makes use of light of appropriate frequency to activate a catalyst. In the presence of water on the surface of the catalyst, rates of some photocatalytic reactions are slowed. Microwaves are known for an ability to couple with water. Herein a photocatalytic reactor system was integrated with a waveguide allowing for concurrent application of microwaves and photocatalysis. Data from initial experiments are reported on the ability of this system to oxidize ethylene gas in an air stream at two levels of temperature and water vapor. INTRODUCTION Photocatalysis is the use of photons from UV-rich light to generate electronhole pairs near the surface of a metal oxide material. In our laboratory, photocatalytic reactions are studied in feed streams that contain high molar flow rates of molecular oxygen that, when adsorbed to the surface of the material, act as electron acceptors. If the remaining holes can reach the surface of the material before recombining with an electron, the holes become available to react with adsorbed water and other adsorbed inorganic and organic molecules. It is known that the presence of water vapor can deleteriously alter the kinetics of some photocatalytic reactions that occur in the gas-phase. While some reactions require certain amounts of water as a stoichiometric ingredient, many other compounds

2 suffer reduced degradation kinetics in humid conditions. It has been hypothesized that water competes for reaction sites on the surface of the semiconducting photocatalysts and therefore depresses the rates of these reactions as the concentration of water vapor is increased [1]. Since microwaves preferentially heat molecules that have a high dielectric, one might expect microwaves to couple directly with water molecules adsorbed at the semiconductor interface. It has been demonstrated that concurrent application of UV light and microwaves has a beneficial effect on organic reactions [2] and photodegradation of pollutants and drugs [3]. Herein, the application of microwaves (MW) and photocatalytic oxidation (PCO) is examined to determine if concurrent MW-PCO enhances conversion (or removal) of a test gas ethylene above that achieved if PCO is employed alone. EXPERIMENTAL Apparatus Schematics are provided of the entire MW-PCO reactor system (Fig. 1), and in particular, the PCO reactor (Fig. 2). The reactor system is a closed-cycle system consisting of a microwave generator (2.45 GHz) containing a variablepower magnetron, a rectangular waveguide, a sampling port and the PCO reactor. In order to measure the temperature and the microwave power, optical probes are inserted into the photocatalytic reactor. Also, cooling air is supplied in order to control the temperature. The catalyst is located in the middle of the reactor. A fluorescent light source or bulb is inserted along the axial centerline of the reactor. Sampling Port Photocatalytic Reactor MW Generator 2.45 GHz Pump Reservoir Waveguide Figure 1. Entire MW-PCO system Figure 2. PCO reactor

3 Experimental Description The probe reaction is the oxidation of ethylene as given by the following equation: C 2 H 4 + 3O 2 2CO 2 + 2H 2 O (1) This reaction is studied at two temperatures (6 C and 9 C) and with two amounts of water vapor (3.3x1-6 L and 3.3x1-5 L liquid water injection) in the gas mixture. The gas mixture is treated with PCO only and concurrent MW-PCO. These experiments are conducted with a mixed-metal oxide (TiO 2 /ZrO 2 ) thin-film (~2-5 nm) photocatalyst supported on borosilicate glass spheres (4 mm diameter). The (desired test) temperature in the packed bed PCO reactor is monitored by an optical temperature sensor and is used to adjust the power to the magnetron and to control the externally operated, cooling air flow rate. At the beginning of each experiment, the entire reactor contains an initial concentration (C o ) of ethylene (95 ppm) in ultra-pure air. During the course of each experiment and typically at ten-minute intervals, 25x1-6 L samples of the gaseous contents of the reactor are obtained with a gas-tight syringe. The samples are analyzed for ethylene concentration (C) using a Hewlett Packard 589 II gas chromatograph with a flame ionization detector. In this manner, the conversion or removal of ethylene from the reaction vessel is determined by C C f = (2) C RESULTS AND DISCUSSION The reaction conversions achieved with PCO and MW-PCO treatments are plotted against reaction time for all reaction conditions (Figures 3, 4, 5, and 6). At 6 C, the reaction conversions of MWPCO and PCO are nearly the same at both low humidity (Figure 3) and high humidity (Figure 4). With only one test at each test condition, the data suggest that there is not a significant advantage of MW- PCO over PCO. At 9 C, three replicates are conducted and the average of the replicate data sets are provided in Figures 5 and 6. The conversions achieved in studies with MW-PCO are higher than those with PCO at both low and high humidity. At low humidity, the final reaction conversion of MW-PCO is almost twice as high as that of PCO. These data suggest that there is significant advantage with concurrent MW-PCO than just PCO at the higher reaction temperature; the advantage appears to be better in experiments at lower humidity than at higher humidity.

4 Figure 3. Conversion data (6 C, 3.3µl-water) Figure 4. Conversion data (6 C, 33µl-water) Figure 5. Conversion data (9 C, 3.3µl-water) Figure 6. Conversion data (9 C, 33µl-water) CONCLUSION It is demonstrated that conversion of a target gas in a photocatalytic reactor system simultaneously irradiated with microwaves can be significantly higher than that achieved with photocatalytic oxidation only. ACKNOWLEDGEMENT Support for this work was provided by a (UW-Madison) University Industry Relations Industrial and Economic Development Research grant and by the Research Products Corp. (Madison, WI).

5 REFERENCES [1] Fu, X., Clark, L.A., Zeltner, W.A., and Anderson, M.A. Effects of reaction temperature and water vapor content on the heterogeneous photocatalytic oxidation of ethylene. J. Photochem. Photobiol. A: Chem. 97, 1996, [2] Chemat, S., Bartels, P., Esveld, E., and Chemat, F. Effect of simultaneous microwave and ultraviolet irradiations on organic reactions. In Proceedings from the International Conference on Microwave and High Frequency Heating, Valencia, Spain, September 13-17, 1999, pp [3] Campanella, L., Cresti, R., Sammartino, M.P., and Visco, G., Microwaves assisted photodegradation of pollutants and drugs. In Proceedings from the International Conference on Microwave and High Frequency Heating, Valencia, Spain, September 13-17, 1999, pp