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A study of the photocatalytic destruction of propylene using microwave discharge electrodeless lamp
Journal of Industrial and Engineering Chemistry 16 (2010) 947–951
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
A study of the photocatalytic destruction of propylene using microwave discharge electrodeless lamp Yeong-Seon Bae, Sang-Chul Jung * Dept. of Environmental Engineering, Sunchon National University, Jeonnam 540-742, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 18 February 2010 Accepted 4 May 2010
A microwave discharge electrodeless lamp (MDEL) was used as the light source for microwave-assisted TiO2 photo-catalysis to degrade propylene. A MDEL filled with low pressure mercury gas has been developed for the photocatalytic treatment of air pollutants over TiO2 balls. TiO2 balls used were produced by the low pressure metal organic chemical vapor deposition method. With increasing microwave power, the degradation efficiency of propylene increased resulting in increased production of CO2, H2O, and CO. It is proposed that propylene is degraded by MDEL and photocatalysts into CH4 and C2H6, which are then mineralized into CO2, H2O, and CO. C2H2 is suggested to be produced from CH4 or C2H6 by microwave. ß 2010 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Keywords: Propylene Microwave Electrodeless lamp TiO2 Chemical vapor deposition
1. Introduction Propylene is the raw material for a wide variety of products including polypropylene, a versatile polymer used in packaging and other applications. Propylene (C3H6) emission due to increase of chemical plants and corresponding use of various organic solvents has gradually increased. Polluted air and water pose serious global concerns. Such alternative technologies as advanced oxidation processes (AOP) are being actively considered to abate the growing environmental pollution. AOP is an advanced gaseous contaminants treatment process, in which highly disinfective and oxidative special ions (OH radicals) are produced as an intermediate product and are used for oxidizing air pollutants [1]. Application of TiO2 photocatalyst in AOP air pollutants treatment has recently been investigated widely [2]. Because of its excellent advantages (i.e., high activity, chemical stability, low toxicity and low cost) [3], TiO2 has become the most extensively investigated catalyst, finding use in energy conversion, water and air purification, and organic contaminant mineralization [4]. In many photo-decomposition reaction systems, TiO2 powders are often used as a photocatalyst [5]. However, powder catalysts have several problems, such as (1) difficulties in the separation of the catalyst from suspension after the reaction and (2) difficulties in the prevention of aggregation in high concentration suspensions. To avoid such agglomeration, suspension must be diluted.
* Corresponding author. E-mail address: [email protected] (S.-C. Jung).
Then the overall reaction rate tends to be slow. On the one hand, these problems can be solved by the use of immobilized (i.e., coated) catalyst particles. However, the coated catalysts are easily detached from the supports. To avoid these problems, TiO2 thin films have been prepared by the sol–gel method [6], the sputter method [7] and the chemical vapor deposition (CVD) method [8]. Among these, CVD is considered as a promising method to prepare high-quality thin films over large surface area with a wellcontrolled composition and low defect density [9–11].Microwave radiation is widely used domestically in heating for food, and industrially in heating, drying devices, and communication technologies, among others. Microwave technology also finds applications in combinatorial chemistry [12]. There is growing interest in using microwave radiation to drive or otherwise assist chemical reactions. Various types of organic and inorganic reactions, once performed using classical heating methods, are now routinely performed using microwave radiation. Giguere et al. [13] and Gedye et al. [14] were among the first to report microwave-assisted organic syntheses. Microwave-induced catalyzed reactions with metal and metal oxide catalysts have been described for syntheses and for various redox processes [15]. Recently, researches have been conducted actively to improve oxidative degradation performance by adding microwave irradiation as an effort to utilize TiO2 photocatalyst treatment more efficiently [16]. The objective of this study is to evaluate the efficacy of microwave-assisted photocatalytic degradation of propylene gas using TiO2 photocatalyst balls prepared via CVD. Oxygen gas was also added to improve the decomposition efficiency in the microwave/UV/TiO2 process.
1226-086X/$ – see front matter ß 2010 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jiec.2010.05.015
[(Fig._1)TD$IG]
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Fig. 1. X-ray diffraction pattern and SEM image of the TiO2 film prepared by CVD method at 1 Torr and 773 K.
2. Experimental 2.1. TiO2 photocatalyst balls TiO2 photocatalyst balls used were prepared using a low pressure metal organic chemical vapor deposition (LPMOCVD) method. Titanium tetraisopropoxide (Ti(OC3H7)4, TTIP) was used as the precursor to generate a TiO2 film on alumina balls (Nikkto, HD-11) with 8-mm diameter. Details of the apparatus were described in our previous paper [9,10]. The CVD conditions used for the preparation of TiO2 films are as follows; total flow rate of gas fed to the reactor 1500 sccm, oxygen concentration at the reactor inlet 50 mol%, operating pressure 1 Torr, deposition temperature 773 K, and TTIP evaporator temperature 323 K. Fig. 1 shows the Xray diffraction pattern and a SEM image of the prepared TiO2 film. It was confirmed from the diffraction pattern that 1 1 2-face oriented anatase crystal structure TiO2 film was prepared. The SEM image showed that the TiO2 film was prepared on the substrate, the alumina balls, with a thickness of 3–5 mm. 2.2. Microwave/UV/TiO2 system Fig. 2 shows the schematic of the microwave/UV/TiO2 experimental apparatus used in this study. It consists of [(Fig._2)TD$IG]
microwave-irradiation equipment, an MDEL, a pyrex reactor tube (30 mm ID, 250 mm length) in which photocatalytic oxidative degradation of propylene gas takes place, a gas flow control system using mass flow controller, and an ozone generator. Microwave radiation was carried out with a microwave/UV/TiO2 system manufactured by Korea Microwave Instrument Co. Ltd. It consisted of a microwave generator (frequency, 2.45 GHz; maximal power, 1 kW), a three-stub tuner, a power monitor, and a reaction cavity. Microwave radiation (actual power used, 200–600 W) used to irradiate the propylene gas flow (2000 sccm) containing TiO2 photocatalyst balls was delivered through a wave-guide. Microwave irradiation was continuous and the microwave intensity was adjusted by connection to a power monitor. Optimal low reflection of the microwave radiation was achieved using the three-stub tuner. A stirrer was installed on the back side in the reaction cavity (Fig. 2) to enhance the transfer of microwave. 2.3. Double tube type MDEL TiO2 photocatalysts are excited by UV light, producing strong oxidants that can degrade organic compounds. Therefore, provision of UV is essential for a use of TiO2 photocatalysts. Typical UV lamps, however, have metal electrodes, which prevents them from being used in the microwave-irradiation equipment. Therefore,
Fig. 2. Schematic of the microwave/UV/TiO2 experimental apparatus.
[(Fig._3)TD$IG]
Y.-S. Bae, S.-C. Jung / Journal of Industrial and Engineering Chemistry 16 (2010) 947–951
Fig. 3. The electrodeless UV lamp in the microwave oven with (a) microwave off and (b) microwave on.
a double tube type MDEL (170-mm length, 36-mm inner diameter, 55-mm outer diameter) that emits UV upon the irradiation of microwave was developed in this study. It was made of quartz to maximize the reaction efficiency. Small amount of mercury gas was doped between the tubes inside the double tube UV lamp that was kept vacuumed. This is a UV-C type lamp that emits UV light in a plasma phase when it is irradiated by microwave. Fig. 3 shows a photograph of the double tube type MDEL emitting UV light by microwave irradiation in the microwave oven. 2.4. Evaluation of photocatalytic reaction activity The degradation experiment was performed to evaluate the activity of microwave-assisted photocatalytic reaction for propylene gas by using a flow type reactor under atmospheric pressure. The reactant was composed of 1 mol% propylene balanced with nitrogen, at a total flow rate of 2 L/min. The decomposition rate was evaluated from the change of propylene concentration at the reactor outlet as a function of irradiation time. The concentrations of C3H6 in the reactants and products (CO2, H2O, CO, CH4, C2H2, C2H6) were measured using an FTIR spectrophotometer (Gasmet Cx-4000, Temet instruments). 3. Results and discussion
[(Fig._4)TD$IG]
3.1. Photocatalytic degradation of propylene Fig. 4 shows the results of propylene degradation by photocatalytic reaction using MDEL. The experiments were carried out with the total reaction gas flow rate of 2 L/min at different propylene concentrations (250–1000 ppm). Three different microwave powers were used: 0.2, 0.4, and 0.6 kW. Solid line plots (left-hand-side yaxis) shows the amount of propylene that was degraded as a
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function of microwave intensity, whereas histograms (right-handside y-axis) represent the fraction of propylene that was degraded, i.e., the degradation efficiency. Both the propylene degradation efficiency and the amount of propylene degraded increased with increasing microwave power. The degradation efficiency was higher at a lower inlet propylene concentration. Generally, photo-catalysis is known to exert high efficiency at low concentrations [2,10], which agrees with the result of this study. On the other hand, the amount of propylene degraded was larger at a higher inlet concentration. When the microwave power was highest (0.6 kW), the degradation efficiency stayed almost the same at the low concentration range (250–500 ppm), whereas the amount of propylene degraded remained almost constant at the high concentration range (750– 1000 ppm). This result indicates that chemical reaction itself is the rate determining step of the overall propylene degradation reaction at high concentrations, while diffusion is the rate determining step at low concentrations. Microwave has thermal effect and non-thermal effect. The thermal effect means selective, fast, uniform increase of temperature by microwave. The non-thermal effect represents the enhancement of the chemical reaction rate resulting from increased collision frequency. Sometimes, the thermal effect and the non-thermal effect can create a synergy effect. In this study, a short wavelength electromagnetic wave UV-C is emitted by MDEL upon the irradiation of microwave. Therefore, the intensity of UV-C increases with the microwave power. UV-C, which carries intense energy, is used for exciting photocatalyst. It can also contribute to degrading propylene directly. Therefore, the intensity of UV-C emitted by MDEL should be measured to analyze the propylene degradation reaction by microwave accurately. Unfortunately, however, it was not possible in this study to measure the UV intensity quantitatively because a UV sensor could not be installed in the microwave cavity. Therefore, it was not possible to figure out the detailed mechanism how microwave took part in the degradation of propylene. Nevertheless, it can be inferred from the experimental result, which showed a higher degradation efficiency at a higher microwave intensity, that microwave contributed to degradation of propylene indirectly by increasing UV intensity. The thermal and non-thermal effects of microwave are also presumed to have contributed directly to the degradation reaction. The products of the propylene degradation reaction by the microwave/UV/TiO2 system were analyzed. Fig. 5 shows the amounts of CO2, H2O, and CO produced at different propylene inlet concentrations (250–1000 ppm) as a function of microwave intensity. The amount of all these products increased with increasing microwave power and with increasing propylene inlet concentration. The CO2 production was highest, while CO was produced only marginally. The intermediate products of the propylene degradation reaction by the microwave/UV/TiO2 system were analyzed. The amounts of CH4, C2H4, and C2H6 produced by the propylene degradation reaction at the total flow rate of 2 L/min and the propylene inlet concentration of 1000 ppm are shown in Fig. 6. The amounts of all these intermediate products were much smaller than those of the final products CO2 and H2O and were even smaller than that of another intermediate product CO. C2H6 was produced the most among them, whereas the amounts of CH4 and C2H4 were similar. The C2H4 production increased with increasing microwave power throughout the microwave power range tested. The production of CH4 and C2H6, however, decreased at the highest microwave power (0.6 kW). 3.2. Reaction mechanism
Fig. 4. Photocatalytic degradation of propylene on various microwave intensity.
The mechanism for the photocatalytic degradation of propylene using MDEL is proposed in Fig. 7. The mechanism was suggested
[(Fig._5)TD$IG]
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[(Fig._7)TD$IG]
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Fig. 7. Proposed pathway for the photo-degradation of propylene.
based on the tendency of products from the propylene degradation reaction. The double bond of propylene is broken by OH that is produced by MDEL and photocatalysts, resulting in production of CH4 and C2H6. These two intermediate products are mineralized into CO2, H2O, and CO. C2H4 is suggested to be produced from CH4 or C2H6 by microwave because its production increased with increasing microwave power even at highest microwave power, different from the cases of CH4 and C2H6 whose productions were reduced at the highest microwave power. Generation of C2H4 from CH4 by catalytic reaction was previously reported by Kim et al. [17,18]. 3.3. Effect of O2 Fig. 8 illustrates the effect of oxygen gas concentration on the photocatalytic propylene degradation reaction using MDEL. The experiments were carried out with the total flow rate of 2 L/min and the propylene inlet concentration of 500 ppm at different oxygen concentrations. Three different microwave powers were used: 0.2, 0.4, and 0.6 kW. At all oxygen concentrations, the amount of propylene degraded increased with increasing microwave power, whereas the difference in the degradation rate caused by the difference in oxygen concentration was not significant. This result suggests that oxygen does not take an important role in the propylene degradation reaction. Therefore, it is believed that the oxidation reaction in the propylene degradation by the microwave/ UV/TiO2 system is led mainly by OH radicals, which are created by MDEL and photo-catalysis. 3.4. Effect of photocatalyst Fig. 5. Formation of CO2, H2O and CO in the photo-oxidation of propylene.
Fig. 9 compares the conversion of propylene by photocatalytic
[(Fig._8)TD$IG]degradation at various experimental conditions. The experiments [(Fig._6)TD$IG]
Fig. 6. Formation of CH4, C2H2 and C2H6 in the photo-oxidation of propylene.
Fig. 8. Photocatalytic degradation of propylene at various oxygen gas concentrations.
[(Fig._9)TD$IG]
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4. Conclusions The following conclusions were inferred from the results of photocatalytic degradation of propylene using MDEL:
Fig. 9. Photocatalytic degradation of propylene at various experimental conditions.
were carried out with the total flow rate of 2 L/min, the propylene inlet concentration of 500 ppm, and the microwave power of 0.6 kW. The degradation efficiency was lowest (4.2%) when only microwave was used (M). When MDEL was used without photocatalysts (MU), the degradation efficiency was 25.0%. The highest degradation efficiency (30.4%) was obtained when both photocatalysts and MDEL were used (MUP). The difference between the results of MU and MUP was 5.4%, which indicates that the contribution of photo-catalysis accounted for 17.8% of the total degradation. In this study, microwave was additionally irradiated on the conventional photo-catalysis system for pollutant degradation expecting a synergy effect that activates reactants and promotes oxidation reactions. The experimental results showed that a higher microwave intensity led to a higher degradation efficiency. The effect of microwave on the oxidation of reactants and intermediate products, however, was not observed in the experiments. The addition of oxygen was expected to contribute to production of active oxidants when oxygen was irradiated by microwave, but it turned out to be not true. Therefore, additional experiments are needed in the future expecting a synergy effect by adding more powerful oxidants such as ozone. It was not possible in this study to quantitatively measure the intensity of UV emitted by the microwave-assisted MDEL because a UV sensor could not be installed in the microwave cavity, which hindered investigation on the detailed mechanism how microwave took part in the degradation of propylene. Thus, it is required to improve the experimental equipment in the future for more quantitative assessments of each component technology and for more fundamental investigation on various phenomena.
(1) With increasing microwave power, the degradation efficiency of propylene increased resulting in increased production of CO2, H2O, and CO. In particular, the CO2 production was high, while the CO production was marginal. (2) The amount of CH4, C2H2, and C2H6 produced by the reaction were very small. The C2H2 production increased with increasing microwave power throughout the microwave power range tested. The production of CH4 and C2H6, however, decreased at the highest microwave power. (3) It is proposed that propylene is degraded by MDEL and photocatalysts into CH4 and C2H6, which are then mineralized into CO2, H2O, and CO. C2H2 is suggested to be produced from CH4 or C2H6 by microwave. (4) The degradation efficiency was lowest (4.2%) when only microwave was used. When MDEL was used without photocatalysts, the degradation efficiency was 25.0%. The degradation efficiency was highest (30.4%) when both photocatalysts and MDEL were used. Acknowledgements This research was financially supported by the Ministry of Commerce, Industry and Energy (MOCIE) and Korea Industrial Technology Foundation (KOTEF) through the Human Resource Training Project for Regional Innovation. References [1] W.H. Glaze, J.W. Kang, D.H. Chapin, J. AWWA 80 (1988) 57. [2] R. Hoffmann, S.T. Martin, W.Y. Choi, D.W. Bahnemann, Chem. Rev. 95 (1995) 69. [3] M.V.B. Zanoni, J.J. Sene, M.A. Anderson, J. Photochem. Photobiol. A: Chem. 157 (2003) 55. [4] X. Quan, S. Chen, J. Su, J. Chen, G. Chen, Sep. Purif. Technol. 34 (2004) 73. [5] R.W. Matthews, Water Res. 20 (1986) 569. [6] N. Venkatachalam, M. Palanichamy, V. Murugesan, Mater. Chem. Phys. 104 (2007) 454. [7] E. Dorjpalam, M. Takahashi, Y. Tokuda, T. Yoko, Thin Solid Films 483 (2005) 147. [8] M. Hitchman, F. Tian, J. Electroanal. Chem. 165 (2002) 538. [9] S.C. Jung, B.H. Kim, S.J. Kim, N. Maishi, Y.I. Cho, Chem. Vap. Deposition 11 (2005) 137. [10] S.C. Jung, S.J. Kim, N. Imaishi, Y.I. Cho, Appl. Catal. B: Environ. 55 (2005) 253. [11] J.W. Ha, Y.W. Do, J.H. Park, C.-H. Han, J. Ind. Eng. Chem. 15 (2009) 607. [12] A. Stadler, C.O.J. Kappe, Comb. Chem. 3 (2001) 624. [13] R.J. Giguere, T.L. Bray, S.M. Duncan, G. Majetich, Tetrahedron Lett. 27 (1986) 4945. [14] R. Gedye, F. Smith, K. Westaway, H. Ali, L. Baldisera, L. Laberge, J. Rousell, Tetrahedron Lett. 27 (1986) 279. [15] Y. Wada, Y. Hengbo, S. Yanagida, Catal. Surv. Jpn. 5 (2002) 127. [16] S. Horikoshi, H. Hidaka, N. Serpone, J. Photochem. Photobiol. A: Chem. 159 (2003) 289. [17] S.C. Kim, L.E. Erickson, E.Y. Yu, J. Hazard. Mater. 41 (1995) 327. [18] S.C. Kim, S.J. Kim, E.Y. Yu, Appl. Catal. A: Gen. 150 (1997) 63.
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
A study of the photocatalytic destruction of propylene using microwave discharge electrodeless lamp Yeong-Seon Bae, Sang-Chul Jung * Dept. of Environmental Engineering, Sunchon National University, Jeonnam 540-742, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 18 February 2010 Accepted 4 May 2010
A microwave discharge electrodeless lamp (MDEL) was used as the light source for microwave-assisted TiO2 photo-catalysis to degrade propylene. A MDEL filled with low pressure mercury gas has been developed for the photocatalytic treatment of air pollutants over TiO2 balls. TiO2 balls used were produced by the low pressure metal organic chemical vapor deposition method. With increasing microwave power, the degradation efficiency of propylene increased resulting in increased production of CO2, H2O, and CO. It is proposed that propylene is degraded by MDEL and photocatalysts into CH4 and C2H6, which are then mineralized into CO2, H2O, and CO. C2H2 is suggested to be produced from CH4 or C2H6 by microwave. ß 2010 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Keywords: Propylene Microwave Electrodeless lamp TiO2 Chemical vapor deposition
1. Introduction Propylene is the raw material for a wide variety of products including polypropylene, a versatile polymer used in packaging and other applications. Propylene (C3H6) emission due to increase of chemical plants and corresponding use of various organic solvents has gradually increased. Polluted air and water pose serious global concerns. Such alternative technologies as advanced oxidation processes (AOP) are being actively considered to abate the growing environmental pollution. AOP is an advanced gaseous contaminants treatment process, in which highly disinfective and oxidative special ions (OH radicals) are produced as an intermediate product and are used for oxidizing air pollutants [1]. Application of TiO2 photocatalyst in AOP air pollutants treatment has recently been investigated widely [2]. Because of its excellent advantages (i.e., high activity, chemical stability, low toxicity and low cost) [3], TiO2 has become the most extensively investigated catalyst, finding use in energy conversion, water and air purification, and organic contaminant mineralization [4]. In many photo-decomposition reaction systems, TiO2 powders are often used as a photocatalyst [5]. However, powder catalysts have several problems, such as (1) difficulties in the separation of the catalyst from suspension after the reaction and (2) difficulties in the prevention of aggregation in high concentration suspensions. To avoid such agglomeration, suspension must be diluted.
* Corresponding author. E-mail address: [email protected] (S.-C. Jung).
Then the overall reaction rate tends to be slow. On the one hand, these problems can be solved by the use of immobilized (i.e., coated) catalyst particles. However, the coated catalysts are easily detached from the supports. To avoid these problems, TiO2 thin films have been prepared by the sol–gel method [6], the sputter method [7] and the chemical vapor deposition (CVD) method [8]. Among these, CVD is considered as a promising method to prepare high-quality thin films over large surface area with a wellcontrolled composition and low defect density [9–11].Microwave radiation is widely used domestically in heating for food, and industrially in heating, drying devices, and communication technologies, among others. Microwave technology also finds applications in combinatorial chemistry [12]. There is growing interest in using microwave radiation to drive or otherwise assist chemical reactions. Various types of organic and inorganic reactions, once performed using classical heating methods, are now routinely performed using microwave radiation. Giguere et al. [13] and Gedye et al. [14] were among the first to report microwave-assisted organic syntheses. Microwave-induced catalyzed reactions with metal and metal oxide catalysts have been described for syntheses and for various redox processes [15]. Recently, researches have been conducted actively to improve oxidative degradation performance by adding microwave irradiation as an effort to utilize TiO2 photocatalyst treatment more efficiently [16]. The objective of this study is to evaluate the efficacy of microwave-assisted photocatalytic degradation of propylene gas using TiO2 photocatalyst balls prepared via CVD. Oxygen gas was also added to improve the decomposition efficiency in the microwave/UV/TiO2 process.
1226-086X/$ – see front matter ß 2010 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jiec.2010.05.015
[(Fig._1)TD$IG]
Y.-S. Bae, S.-C. Jung / Journal of Industrial and Engineering Chemistry 16 (2010) 947–951
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Fig. 1. X-ray diffraction pattern and SEM image of the TiO2 film prepared by CVD method at 1 Torr and 773 K.
2. Experimental 2.1. TiO2 photocatalyst balls TiO2 photocatalyst balls used were prepared using a low pressure metal organic chemical vapor deposition (LPMOCVD) method. Titanium tetraisopropoxide (Ti(OC3H7)4, TTIP) was used as the precursor to generate a TiO2 film on alumina balls (Nikkto, HD-11) with 8-mm diameter. Details of the apparatus were described in our previous paper [9,10]. The CVD conditions used for the preparation of TiO2 films are as follows; total flow rate of gas fed to the reactor 1500 sccm, oxygen concentration at the reactor inlet 50 mol%, operating pressure 1 Torr, deposition temperature 773 K, and TTIP evaporator temperature 323 K. Fig. 1 shows the Xray diffraction pattern and a SEM image of the prepared TiO2 film. It was confirmed from the diffraction pattern that 1 1 2-face oriented anatase crystal structure TiO2 film was prepared. The SEM image showed that the TiO2 film was prepared on the substrate, the alumina balls, with a thickness of 3–5 mm. 2.2. Microwave/UV/TiO2 system Fig. 2 shows the schematic of the microwave/UV/TiO2 experimental apparatus used in this study. It consists of [(Fig._2)TD$IG]
microwave-irradiation equipment, an MDEL, a pyrex reactor tube (30 mm ID, 250 mm length) in which photocatalytic oxidative degradation of propylene gas takes place, a gas flow control system using mass flow controller, and an ozone generator. Microwave radiation was carried out with a microwave/UV/TiO2 system manufactured by Korea Microwave Instrument Co. Ltd. It consisted of a microwave generator (frequency, 2.45 GHz; maximal power, 1 kW), a three-stub tuner, a power monitor, and a reaction cavity. Microwave radiation (actual power used, 200–600 W) used to irradiate the propylene gas flow (2000 sccm) containing TiO2 photocatalyst balls was delivered through a wave-guide. Microwave irradiation was continuous and the microwave intensity was adjusted by connection to a power monitor. Optimal low reflection of the microwave radiation was achieved using the three-stub tuner. A stirrer was installed on the back side in the reaction cavity (Fig. 2) to enhance the transfer of microwave. 2.3. Double tube type MDEL TiO2 photocatalysts are excited by UV light, producing strong oxidants that can degrade organic compounds. Therefore, provision of UV is essential for a use of TiO2 photocatalysts. Typical UV lamps, however, have metal electrodes, which prevents them from being used in the microwave-irradiation equipment. Therefore,
Fig. 2. Schematic of the microwave/UV/TiO2 experimental apparatus.
[(Fig._3)TD$IG]
Y.-S. Bae, S.-C. Jung / Journal of Industrial and Engineering Chemistry 16 (2010) 947–951
Fig. 3. The electrodeless UV lamp in the microwave oven with (a) microwave off and (b) microwave on.
a double tube type MDEL (170-mm length, 36-mm inner diameter, 55-mm outer diameter) that emits UV upon the irradiation of microwave was developed in this study. It was made of quartz to maximize the reaction efficiency. Small amount of mercury gas was doped between the tubes inside the double tube UV lamp that was kept vacuumed. This is a UV-C type lamp that emits UV light in a plasma phase when it is irradiated by microwave. Fig. 3 shows a photograph of the double tube type MDEL emitting UV light by microwave irradiation in the microwave oven. 2.4. Evaluation of photocatalytic reaction activity The degradation experiment was performed to evaluate the activity of microwave-assisted photocatalytic reaction for propylene gas by using a flow type reactor under atmospheric pressure. The reactant was composed of 1 mol% propylene balanced with nitrogen, at a total flow rate of 2 L/min. The decomposition rate was evaluated from the change of propylene concentration at the reactor outlet as a function of irradiation time. The concentrations of C3H6 in the reactants and products (CO2, H2O, CO, CH4, C2H2, C2H6) were measured using an FTIR spectrophotometer (Gasmet Cx-4000, Temet instruments). 3. Results and discussion
[(Fig._4)TD$IG]
3.1. Photocatalytic degradation of propylene Fig. 4 shows the results of propylene degradation by photocatalytic reaction using MDEL. The experiments were carried out with the total reaction gas flow rate of 2 L/min at different propylene concentrations (250–1000 ppm). Three different microwave powers were used: 0.2, 0.4, and 0.6 kW. Solid line plots (left-hand-side yaxis) shows the amount of propylene that was degraded as a
949
function of microwave intensity, whereas histograms (right-handside y-axis) represent the fraction of propylene that was degraded, i.e., the degradation efficiency. Both the propylene degradation efficiency and the amount of propylene degraded increased with increasing microwave power. The degradation efficiency was higher at a lower inlet propylene concentration. Generally, photo-catalysis is known to exert high efficiency at low concentrations [2,10], which agrees with the result of this study. On the other hand, the amount of propylene degraded was larger at a higher inlet concentration. When the microwave power was highest (0.6 kW), the degradation efficiency stayed almost the same at the low concentration range (250–500 ppm), whereas the amount of propylene degraded remained almost constant at the high concentration range (750– 1000 ppm). This result indicates that chemical reaction itself is the rate determining step of the overall propylene degradation reaction at high concentrations, while diffusion is the rate determining step at low concentrations. Microwave has thermal effect and non-thermal effect. The thermal effect means selective, fast, uniform increase of temperature by microwave. The non-thermal effect represents the enhancement of the chemical reaction rate resulting from increased collision frequency. Sometimes, the thermal effect and the non-thermal effect can create a synergy effect. In this study, a short wavelength electromagnetic wave UV-C is emitted by MDEL upon the irradiation of microwave. Therefore, the intensity of UV-C increases with the microwave power. UV-C, which carries intense energy, is used for exciting photocatalyst. It can also contribute to degrading propylene directly. Therefore, the intensity of UV-C emitted by MDEL should be measured to analyze the propylene degradation reaction by microwave accurately. Unfortunately, however, it was not possible in this study to measure the UV intensity quantitatively because a UV sensor could not be installed in the microwave cavity. Therefore, it was not possible to figure out the detailed mechanism how microwave took part in the degradation of propylene. Nevertheless, it can be inferred from the experimental result, which showed a higher degradation efficiency at a higher microwave intensity, that microwave contributed to degradation of propylene indirectly by increasing UV intensity. The thermal and non-thermal effects of microwave are also presumed to have contributed directly to the degradation reaction. The products of the propylene degradation reaction by the microwave/UV/TiO2 system were analyzed. Fig. 5 shows the amounts of CO2, H2O, and CO produced at different propylene inlet concentrations (250–1000 ppm) as a function of microwave intensity. The amount of all these products increased with increasing microwave power and with increasing propylene inlet concentration. The CO2 production was highest, while CO was produced only marginally. The intermediate products of the propylene degradation reaction by the microwave/UV/TiO2 system were analyzed. The amounts of CH4, C2H4, and C2H6 produced by the propylene degradation reaction at the total flow rate of 2 L/min and the propylene inlet concentration of 1000 ppm are shown in Fig. 6. The amounts of all these intermediate products were much smaller than those of the final products CO2 and H2O and were even smaller than that of another intermediate product CO. C2H6 was produced the most among them, whereas the amounts of CH4 and C2H4 were similar. The C2H4 production increased with increasing microwave power throughout the microwave power range tested. The production of CH4 and C2H6, however, decreased at the highest microwave power (0.6 kW). 3.2. Reaction mechanism
Fig. 4. Photocatalytic degradation of propylene on various microwave intensity.
The mechanism for the photocatalytic degradation of propylene using MDEL is proposed in Fig. 7. The mechanism was suggested
[(Fig._5)TD$IG]
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[(Fig._7)TD$IG]
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Fig. 7. Proposed pathway for the photo-degradation of propylene.
based on the tendency of products from the propylene degradation reaction. The double bond of propylene is broken by OH that is produced by MDEL and photocatalysts, resulting in production of CH4 and C2H6. These two intermediate products are mineralized into CO2, H2O, and CO. C2H4 is suggested to be produced from CH4 or C2H6 by microwave because its production increased with increasing microwave power even at highest microwave power, different from the cases of CH4 and C2H6 whose productions were reduced at the highest microwave power. Generation of C2H4 from CH4 by catalytic reaction was previously reported by Kim et al. [17,18]. 3.3. Effect of O2 Fig. 8 illustrates the effect of oxygen gas concentration on the photocatalytic propylene degradation reaction using MDEL. The experiments were carried out with the total flow rate of 2 L/min and the propylene inlet concentration of 500 ppm at different oxygen concentrations. Three different microwave powers were used: 0.2, 0.4, and 0.6 kW. At all oxygen concentrations, the amount of propylene degraded increased with increasing microwave power, whereas the difference in the degradation rate caused by the difference in oxygen concentration was not significant. This result suggests that oxygen does not take an important role in the propylene degradation reaction. Therefore, it is believed that the oxidation reaction in the propylene degradation by the microwave/ UV/TiO2 system is led mainly by OH radicals, which are created by MDEL and photo-catalysis. 3.4. Effect of photocatalyst Fig. 5. Formation of CO2, H2O and CO in the photo-oxidation of propylene.
Fig. 9 compares the conversion of propylene by photocatalytic
[(Fig._8)TD$IG]degradation at various experimental conditions. The experiments [(Fig._6)TD$IG]
Fig. 6. Formation of CH4, C2H2 and C2H6 in the photo-oxidation of propylene.
Fig. 8. Photocatalytic degradation of propylene at various oxygen gas concentrations.
[(Fig._9)TD$IG]
Y.-S. Bae, S.-C. Jung / Journal of Industrial and Engineering Chemistry 16 (2010) 947–951
951
4. Conclusions The following conclusions were inferred from the results of photocatalytic degradation of propylene using MDEL:
Fig. 9. Photocatalytic degradation of propylene at various experimental conditions.
were carried out with the total flow rate of 2 L/min, the propylene inlet concentration of 500 ppm, and the microwave power of 0.6 kW. The degradation efficiency was lowest (4.2%) when only microwave was used (M). When MDEL was used without photocatalysts (MU), the degradation efficiency was 25.0%. The highest degradation efficiency (30.4%) was obtained when both photocatalysts and MDEL were used (MUP). The difference between the results of MU and MUP was 5.4%, which indicates that the contribution of photo-catalysis accounted for 17.8% of the total degradation. In this study, microwave was additionally irradiated on the conventional photo-catalysis system for pollutant degradation expecting a synergy effect that activates reactants and promotes oxidation reactions. The experimental results showed that a higher microwave intensity led to a higher degradation efficiency. The effect of microwave on the oxidation of reactants and intermediate products, however, was not observed in the experiments. The addition of oxygen was expected to contribute to production of active oxidants when oxygen was irradiated by microwave, but it turned out to be not true. Therefore, additional experiments are needed in the future expecting a synergy effect by adding more powerful oxidants such as ozone. It was not possible in this study to quantitatively measure the intensity of UV emitted by the microwave-assisted MDEL because a UV sensor could not be installed in the microwave cavity, which hindered investigation on the detailed mechanism how microwave took part in the degradation of propylene. Thus, it is required to improve the experimental equipment in the future for more quantitative assessments of each component technology and for more fundamental investigation on various phenomena.
(1) With increasing microwave power, the degradation efficiency of propylene increased resulting in increased production of CO2, H2O, and CO. In particular, the CO2 production was high, while the CO production was marginal. (2) The amount of CH4, C2H2, and C2H6 produced by the reaction were very small. The C2H2 production increased with increasing microwave power throughout the microwave power range tested. The production of CH4 and C2H6, however, decreased at the highest microwave power. (3) It is proposed that propylene is degraded by MDEL and photocatalysts into CH4 and C2H6, which are then mineralized into CO2, H2O, and CO. C2H2 is suggested to be produced from CH4 or C2H6 by microwave. (4) The degradation efficiency was lowest (4.2%) when only microwave was used. When MDEL was used without photocatalysts, the degradation efficiency was 25.0%. The degradation efficiency was highest (30.4%) when both photocatalysts and MDEL were used. Acknowledgements This research was financially supported by the Ministry of Commerce, Industry and Energy (MOCIE) and Korea Industrial Technology Foundation (KOTEF) through the Human Resource Training Project for Regional Innovation. References [1] W.H. Glaze, J.W. Kang, D.H. Chapin, J. AWWA 80 (1988) 57. [2] R. Hoffmann, S.T. Martin, W.Y. Choi, D.W. Bahnemann, Chem. Rev. 95 (1995) 69. [3] M.V.B. Zanoni, J.J. Sene, M.A. Anderson, J. Photochem. Photobiol. A: Chem. 157 (2003) 55. [4] X. Quan, S. Chen, J. Su, J. Chen, G. Chen, Sep. Purif. Technol. 34 (2004) 73. [5] R.W. Matthews, Water Res. 20 (1986) 569. [6] N. Venkatachalam, M. Palanichamy, V. Murugesan, Mater. Chem. Phys. 104 (2007) 454. [7] E. Dorjpalam, M. Takahashi, Y. Tokuda, T. Yoko, Thin Solid Films 483 (2005) 147. [8] M. Hitchman, F. Tian, J. Electroanal. Chem. 165 (2002) 538. [9] S.C. Jung, B.H. Kim, S.J. Kim, N. Maishi, Y.I. Cho, Chem. Vap. Deposition 11 (2005) 137. [10] S.C. Jung, S.J. Kim, N. Imaishi, Y.I. Cho, Appl. Catal. B: Environ. 55 (2005) 253. [11] J.W. Ha, Y.W. Do, J.H. Park, C.-H. Han, J. Ind. Eng. Chem. 15 (2009) 607. [12] A. Stadler, C.O.J. Kappe, Comb. Chem. 3 (2001) 624. [13] R.J. Giguere, T.L. Bray, S.M. Duncan, G. Majetich, Tetrahedron Lett. 27 (1986) 4945. [14] R. Gedye, F. Smith, K. Westaway, H. Ali, L. Baldisera, L. Laberge, J. Rousell, Tetrahedron Lett. 27 (1986) 279. [15] Y. Wada, Y. Hengbo, S. Yanagida, Catal. Surv. Jpn. 5 (2002) 127. [16] S. Horikoshi, H. Hidaka, N. Serpone, J. Photochem. Photobiol. A: Chem. 159 (2003) 289. [17] S.C. Kim, L.E. Erickson, E.Y. Yu, J. Hazard. Mater. 41 (1995) 327. [18] S.C. Kim, S.J. Kim, E.Y. Yu, Appl. Catal. A: Gen. 150 (1997) 63.