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RESEARCH NOTES Degradation of Phenol by Simultaneous Use of Gas-Phase Corona Discharge and Catalyst-Supported Mesoporous Carbon Gels Noriaki Sano,*,† Takuji Yamamoto,‡ Isamu Takemori,† Seong-Ick Kim,‡ Apiluck Eiad-ua,‡ Daisuke Yamamoto,† and Masaru Nakaiwa‡ Department of Mechanical and System Engineering, Himeji Institute of Technology, UniVersity of Hyogo, 2167 Shosha, Himeji, Hyogo 671-2201, Japan, and National Institute of AdVanced Industrial Science and Technology (AIST), Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
This note reports an enhancement in the degradation of phenol in aqueous solution by simultaneous use of gas-phase corona discharge and mesoporous carbon gels supporting Ni or Co as catalyst. In this study, a direct-current corona-discharge reactor that consisted of a needle cathode placed above a water film was used. Disk-shaped carbon gel was submerged in the water film under the corona-discharge zone. The degradation rates of phenol by corona-discharge reactor without carbon gels, with plain carbon gels, and with metal-supported (Ni or Co) carbon gels were compared. When Ni or Co was supported on the carbon gel, the phenol degradation was enhanced in this reactor. The degradation of TOC was clearly enhanced by supporting Ni on the carbon gel. Such an effect cannot be realized in the use of TiO2-supported silica gel. Introduction There are several techniques using high-voltage electric discharge for the degradation of stable organic compounds in water.1-10 In such processes, strong reactive species, such as hydroxyl radical OH, are produced by a series of plasma chemistry, and organic compounds are finally mineralized to CO2 by oxidation by such species. Among these techniques, a method using gas-phase corona discharge that is contacted with treated water has been developed, and information related with this method has been accumulated in the past several years.3-10 For serious demands for efficient water purification that degrades stable compounds, we still need to continue improving the efficiency of the relevant methods. To improve the efficiency of the advanced oxidation process (AOP), it is common to combine some effects to enhance the reactivity of the process. For example, ozone (O3) oxidation can be conjugated with a TiO2 photocatalyst to improve the reactivity,11,12 and then a synergetic effect was realized. For another example, porous solid media was added to O3 oxidation, and also a synergetic effect was realized.13,14 Like these examples, a combination of some effects may cause preferable enhancement on reactivity in the water purification process. Although many types of microporous solid, i.e., activated carbon or zeolite, have been extensively applied to water purification as adsorbents, mesoporosity is more desirable than microporosity in order to apply porous solids to supporting materials for catalysts. Carbon gels, which possess developed and controlled mesoporous texture,15 are considered to be suitable for this purpose. Furthermore, since carbon gels can be formed into various shapes, e.g., rod, bead, or disk, they are * Corresponding author. Tel.: +81-792-67-4845. Fax: +81-79267-4845. E-mail:
[email protected]. † Himeji Institute of Technology, University of Hyogo. ‡ National Institute of Advanced Industrial Science and Technology.
Figure 1. Apparatus to degrade aqueous phenol by the corona-discharge reactor with carbon gel disk.
applicable to electrode materials. In the present study, we investigated the combination of catalyst-supported mesoporous carbon gels with the corona-discharge reactor as a novel application of mesoporous carbon gels. Experimental Section Corona-Discharge Reactor. The apparatus used in this study is schematically described in Figure 1. A needle cathode was placed above the water surface to generate the corona discharge. The distance between the cathode tip and the water surface was adjusted to 8 mm. The gas component above the water surface was ambient air, and its pressure was atmospheric. To generate the corona discharge from the cathode tip, the treated water was
10.1021/ie050406q CCC: $33.50 © 2006 American Chemical Society Published on Web 03/10/2006
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earthed. To raise the discharge current to 0.1 mA, 10 kV was required. The treated water was circulated by a tube pump through the corona-discharge reactor and a heat-exchanger unit to keep the water temperature at 5 °C. The water was introduced to the reactor with flow rate ) 10 cm3 min-1 and passed the reaction zone under the corona-discharge region as a liquid film (thickness ) 3 mm). When carbon gel was used, disk-shaped carbon gel (diameter ) 10 mm, thickness ) 1 mm) was submerged in the water film under the corona-discharge zone. The total volume of the treated water in one batch was 20 cm3. A high performance liquid chromatograph (HPLC) with a UVvis detector (Shimadzu, SPD-10AVP) and a total organic carbon (TOC) analyzer (Shimadzu, TOC-5000) were used to analyze the treated water at passing times. Mesoporous Carbon Gels. Carbon gels were synthesized by the sol-gel polycondensation of resorcinol (R) with formaldehyde (F), followed by freeze-drying and pyrolysis in an inert atmosphere. At first, resorcinol-formaldehyde (RF) solutions were prepared according to the method as previously reported.15 Here, sodium carbonate (C) and pure water (W) were respectively used as the basic catalyst and the diluent for the sol-gel polycondensation. The molar ratios of resorcinol to formaldehyde (R/F) and resorcinol to catalyst (R/C) and the ratio of resorcinol to water (R/W) are respectively fixed to 0.5 mol/ mol, 200 mol/mol, and 0.25 g/cm3. The ratios of R/C and R/W are often used to control the mesoporosity of carbon gels.15 The prepared RF solutions were poured into a circular vessel and kept at 25 °C until the solutions were gelled. The thus-obtained RF hydrogel disk was subsequently immersed into tert-butyl alcohol in order to exchange the water in the hydrogel with tert-butyl alcohol. The disk was kept at -30 °C for 6 h and then dried under vacuum at -10 °C for 24 h to obtain the organic RF gel. Finally, disk-shaped carbon gel was prepared by pyrolyzing the organic RF gel at 1000 °C for 4 h. Co or Ni catalyst-supported carbon gels were prepared by incipient wetness impregnation using Co(NO3)2 or Ni(NO3)2 aqueous solution. The amount of metal Co or Ni supported was 5 wt %. The catalysts were obtained after drying at 110 °C overnight followed by calcination at 500 °C for 5 h under a constant gas flow of Ar. The porous properties of the obtained raw carbon gels and catalyst-supported carbon gels were determined by the nitrogen gas adsorption experiments at -196°C using an adsorption apparatus (BEL Japan, Inc.; Belsorp28-SA). BET surface area, SBET, and micropore volume, Vmic, of the samples were respectively evaluated by applying the BET equation and the t-plot to their nitrogen adsorption isotherms. The pore size distribution and mesopore volume, Vmes, were determined by applying the Dollimore-Heal method to their nitrogen desorption isotherms. In this article, the IUPAC (International Union of Pure and Applied Chemistry) definitions of mesopore (Rp ) 1-25 nm) and micropore (Rp < 1 nm) were used. A transmission electron microscope (TEM) (JEOL, JEM2010) was used to analyze the particle sizes of the loaded metals. Results and Discussion The pore size distributions and some related information of the prepared carbon gels are shown in Figure 2 and Table 1. It is possible to confirm that the mesoporosity of the raw carbon gel was almost maintained even after the impregnation of the catalysts. The size of Ni particles supported on the carbon gel was in the range of 5-10 nm, as can be confirmed from TEM observation as shown in Figure 2c. Figure 3 shows the degradation of phenol concentration at four conditions: (1) using
Figure 2. Pore size distribution of (a) carbon gels and (b) silica gel used in this work, and (c) TEM image of Ni-supported carbon gel, in which dark spots correspond to Ni particles.
Figure 3. Degradation of phenol concentration in water by the coronadischarge reactor with plain carbon gel, Ni-supported carbon gel, Cosupported carbon gel, and without the carbon gels. Table 1. Properties of Porous Materials Used in This Work
SBET (m2 g-1) Vmes (cm3 g-1) Vmic (cm3 g-1) a
carbon gel support
Ni-supported carbon gel
740
652
633
304
0.98
0.68
0.61
0.95
0.17
0.16
0.16
NDa
Co-supported carbon gel
TiO2-supported silica gel
Not detected.
only corona discharge, (2) using corona discharge with plain carbon gel disk, (3) using corona discharge with Ni-supported carbon gel disk, and (4) using corona discharge with Cosupported carbon gel disk. One can see that the degradation of phenol concentration was not enhanced significantly by only
Ind. Eng. Chem. Res., Vol. 45, No. 8, 2006 2899 Table 2. Reaction Rate Constants for the Decrease of TOC in the Phenol Degradation by Use of Corona Discharge with Carbon Gels
method
reaction rate const. forTOC degradation, kT (min-1)
corona discharge only corona discharge & plain carbon gel corona discharge & Ni-supported carbon gel corona discharge & Co-supported carbon gel
0.08 0.20 0.56 0.24
obtained for TOC degradation by eq 1 with some assumptions below. Figure 4. Degradation of TOC in aqueous phenol solution by the coronadischarge reactor with plain carbon gel, Ni-supported carbon gel, Cosupported carbon gel, and without the carbon gels. The curves were obtained by eq 1 with the parameters in Table 2.
Figure 5. Degradation of phenol concentration and TOC by the coronadischarge reactor with TiO2-supported silica gel.
using plain carbon gel, when the phenol concentration reached ∼15 mg (kg of water)-1 at 120 min. On the other hand, when Ni or Co was supported on the carbon gel, the phenol degradation rate was significantly enhanced, so that the phenol concentration became negligible at this time. One may be concerned about the adsorption of phenol on the surface of the carbon gels. Nevertheless, the decrease of phenol concentration by adsorption without corona discharge was 2 mg (kg of water)-1 for the blank test, which was not significant. Thus, we insist that the combination of the catalyzed carbon gels to the corona-discharge process has an enhancement effect on the phenol degradation. It was reported by previous observation that pyrocatechol, resorcinol, hydroquinone, 1,4-benzoquinone, and acetic acid were detected as byproducts from the degradation of phenol in the corona-discharge process.8-10 It was also reported that, when the reaction condition is suitable for the degradation process, these byproducts will be finally mineralized to CO2 and TOC will decrease. Figure 4 shows the degradation of TOC at four conditions. When carbon gel was not used, TOC was hardly degraded, even with corona discharge at the present condition. Even when plain carbon gel and Co-supported carbon gel were added to the reaction system, the TOC degradation was not drastically enhanced, although the enhancement effect is observable. Nevertheless, when Ni-supported carbon gel was used, its enhancement effect on the degradation of TOC became significant. According to the previous report,8 the mechanism of the phenol degradation can be divided into two main schemes: (1) reactions by O3 and (2) reactions by OH radicals. In the reactor used in this work, 2500 ppm of O3 was detected in the gas phase above the water. It is known that the conversion of phenol to stable byproducts can be caused by O3 oxidation. Nevertheless, the degradation to decrease TOC cannot be induced by O3 oxidation.8 Then, the OH reaction becomes important for the mineralization of contaminated water. To compare the reaction rates with catalytic carbon gels in the corona-discharge reactor to enhance the reaction of OH radical with organic compounds more quantitatively, simplified first-order reaction rates were
Vt
dCT ) -VrkTCT dt
(1)
where Vr, Vt, CT, kT, and t are, respectively, the volume of the reaction zone, the total volume of the treated water, the TOC concentration, the apparent reaction rate constant, and time. Vr is assumed to be the volume of the water film above the carbon gel disks (0.16 cm3). kT were determined by fitting the curves obtained by eq 1 with experimental plots, and these constants are shown in Table 2. The rate constants shown here describe that the degradation rate with Ni-supported carbon gel becomes 7.0 times higher than that without such catalytic carbon gel. According to our previous work,8 we think that the OH radical produced in the treated water may be the main reactive species to degrade the organic impurities in the corona-discharge reactor, and the reactivity of O3 is much smaller than this. Therefore, the reason for the enhancement effect by adding carbon gels to corona discharge can be considered by enhancement of the reactivity between OH radical and organic compound molecules at the surface of the Ni catalyst. If this mechanism is applicable, OH radicals should diffuse to the surface of the carbon gels. For their efficient diffusion to the surface inside the carbon gel, the pore size distribution should be the critical factor to achieve the relevant enhancement effect. Because the pore size distribution of the carbon gel can be controllable,15 the influence of the pore size should be studied in the future. It should be remembered that the reactor used in the present work is not in the optimized structure,9 and the work described in this note aims to qualitatively show the enhancement effect of the combination of catalyst-supported carbon gel with the corona discharge by use of a preliminary simple reactor. There is literature reporting that the water purification efficiency by a cylindrical water-film corona-discharge reactor is significantly higher than that by the flat water-film corona-discharge reactor. Nevertheless, we employed the flat water-film type because flatdisk carbon gel can be prepared much easier than the cylindrical shape. To realize high efficiency, the corona-discharge reactor using a cylindrical anode made of carbon gel that has wellcontrolled pore size distribution will be worthy of investigation. For reference, we used TiO2-supported silica gel instead of carbon gels in the corona-discharge reactor in the same manner as above. The pore size distribution and the related information of this silica gel are shown in Figure 2 and Table 1. The result from TiO2-supported silica gel is shown in Figure 5. Since the degradation curves of phenol and TOC shown in Figure 5 almost coincide with those shown in Figure 3 or Figure 4 for the case of plain carbon gel, it can be considered that the effect of TiO2supported silica gel on the degradation is not as significant as that of the metal-supported carbon gels. As one of the reasons for this result, the adsorption capacity of the TiO2-supported silica gel to organic compounds from aqueous solution is not large because the silica gel possesses fewer micropores than the carbon gel and its surface is hydrophilic.
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Conclusions A needle-cathode corona-discharge reactor combined with disk-shaped carbon gel was used for the degradation of phenol in water to investigate the new application of mesoporous carbon gels. As a result, the degradation rate of phenol concentration was enhanced by the use of carbon gels which supported Ni and Co catalysts. When Ni-supported carbon gel was adopted, a significant enhancement in TOC degradation was observed. To optimize the process to degrade organic impurities in water by the simultaneous use of corona discharge and carbon gels, a cylindrical anode made of carbon gel that has well-controlled pore size distribution should be used. Acknowledgment Financial support by New Energy and Industrial Technology Development Organization (NEDO) is acknowledged. Nomenclature Cp ) phenol concentration, mg dm-1 kT ) apparent reaction rate constant, min-1 Rp) pore radius, nm SBET ) BET surface area, m2 g-1 t ) time, min Vmes ) volume of mesopore per unit mass, cm3 g-1 Vmic ) volume of micropore per unit mass, cm3 g-1 Vp ) volume of pore per unit mass, cm3 g-1 Vr ) volume of reaction zone, cm3 Vt ) total volume of treated water, cm3 Literature Cited (1) Sharma, A. K.; Locke, B. R.; Arce, P.; Finney, W. C. A Preliminary Study of Pulsed Streamer Corona Discharge for the Degradation of Phenol in Aqueous Solution. Hazard. Waste Hazard. Mater. 1993, 10, 209-219. (2) Sun, B., Sato; M. Harano; A. Clements, J. S. Nonuniform Pulse Discharge-Induced Radical Production in Distilled Water. J. Electrost. 1998, 43 115-126. (3) Hoeben, W. F. L. M.; van Veldhuizen; E. M.; Kroesen G. M. W. Gas-Phase Corona Discharge for Oxidation of Phenol in an Aqueous Solution. J. Phys. D.: Appl. Phys. 1999, 32, L133-L137.
(4) Hoeben, W. F. L. M. Pulsed Corona-Induced Degradation of Organic Materials in Water; Technische Universiteit: Eindhoven, The Netherlands, 2000. (5) Hayashi, D.; Hoeben, W. F. L. M.; Dooms, G.; van Veldhuizen, E. M.; Rutgers, W. R.; Kroesen, G. M. W. Influence of Gaseous Atmosphere on Corona-Discharge-Induced Degradation of Aqueous Phenol. J. Phys. D: Appl. Phys. 2000, 33, 2769-2774. (6) Sharma, A. K.; Josephson, G. B.; Camaioni, D. M.; Goheen, S. C. Destruction of Pentachlorophenol Using Glow Discharge Plasma Process. EnViron. Sci. Technol. 2000, 34, 2267-2272. (7) Sano, N.; Fujimoto, T.; Kawashima, T.;, Kanki, T.; Toyoda, A. Possibility of Utilization of Radicals and Ions Produced by Gaseous Corona Discharge to Degradation of Organic Compounds in Water. In 5th Asian Pacific Conference of Sustainable Energy and EnVironmental Technologies, HongKong, 1999; pp 89-93. (8) Sano, N.; Kawashima, T.; Fujikawa, J.; Fujimoto, T.; Takaaki, K.; Kanki, T.; Toyoda, A. Decomposition of Organic Compounds in Water by Direct Contact of Gas Corona Discharge: Influence of Discharge Conditions. Ind. Eng. Chem. Res. 2002, 41, 5906-5911. (9) Sano, N.; Yamamoto, D.; Kanki, T.; Toyoda, A. Decomposition of Phenol in Water by Cylindrical Wetted-Wall Reactor Using Direct Contact of Gas Corona Discharge. Ind. Eng. Chem. Res. 2003, 42, 5423-5428. (10) Sano, N.; Yamamoto, D. A Simulation Model of Decomposition Process of Phenol in Water by Direct Contact of Gas Corona Discharge in Cylindrical Reactor. Ind. Eng. Chem. Res. 2005, 44, 2982-2989. (11) Wang, S.; Shiraishi F.; Nakano K. A Synergistic Effect of Photocatalysis and Ozonation on Decomposition of Formic Acid in an Aqueous Solution. Chem. Eng. J. 2002, 87, 261-271. (12) Li, L.; Zhu, W.; Zhang, P.; Chen, Z.; Han, W. Photocatalytic oxidation and ozonation of catechol over carbon-black-modified nano-TiO2 thin films supported on Al sheet. Water Res. 2003, 37, 3646-3651. (13) Beltran, F. J.; Rivas, F. J.; Montero-de-Espinosa, R. Mineralization Improvement of Phenol Aqueous Solutions through Heterogeneous Catalytic Ozonation. J. Chem. Technol. Biotechnol. 2003, 78, 1225-1233. (14) Ernst, M.; Lurot, F.; Schrotter, J.-C. Catalytic Ozonation of Refractory Organic Model Compounds in Aqueous Solution by Aluminum Oxide. Appl. Catal. 2004, 47, 15-25. (15) Yamamoto, T.; Nishimura, T.; Suzuki, T.; Tamon, H. Control of Mesoporosity of Carbon Gels Prepared by Sol-gel Polycondensation and Freeze Drying. J. Non-Cryst. Solids 2001, 288, 46-55.
ReceiVed for reView April 1, 2005 ReVised manuscript receiVed January 12, 2006 Accepted March 1, 2006 IE050406Q
