Ind. Eng. Chem. Res. 2007, 46, 7611-7614
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Combining Cold Plasma and TiO2 Photocatalysis To Purify Gaseous Effluents: A Preliminary Study Using Methanol-Contaminated Air Je´ roˆ me Taranto,†,‡,§ Didier Frochot,*,† and Pierre Pichat*,‡ EDF-Les Renardie` res, 77818 Moret-sur-Loing, France, and “Photocatalyse et EnVironnement”, CNRS UMR “IFoS”, STMS, Ecole Centrale de Lyon, 69134 Ecully CEDEX, France
Filtered air (relative humidity ) 9-10%) containing methanol was flown through a cold plasma reactor connected to a photocatalytic reactor. The results showed a synergetic effect between the two purifying technologies, which is principally attributed to plasma-generated O3 benefiting the photocatalytic process. Introduction The treatment of gaseous effluents by various advanced oxidation processes (AOPs) is widely investigated in view of, at least in some cases, substituting an AOP to adsorption and biofiltration or complementing these classical techniques by an AOP. Each AOP presents advantages and disadvantages. Consequently, it can be of interest to combine them either simultaneously or successively to improve the treatment efficacy. We report here the results of a preliminary study dealing with the successive applications of a homogeneous AOP, viz. nonthermal (or cold) plasma,1 and of photocatalysis over TiO22 to purify a stream of air containing methanol as a model organic contaminant. Methanol was chosen because it can form only a small number of incompletely oxidized intermediate products, viz. HCHO, HCOOH, HCOOCH3, and CO; accordingly, a stationary-state can be achieved for the removal of the initial contaminant, since the potential accumulation of intermediate products is minimized with respect to organic pollutants containing a greater number of C atoms. To our knowledge, papers on the combination of cold plasma and photocatalysis are rare and concern only the simultaneous use of these two techniques. Although the existence of a synergy has been claimed,3-6 the simultaneous use presents a priori two drawbacks: (i) a higher probability for the plasma-generated active species to be deactivated on easily coming into contact with the photocatalyst and (ii) difficulties in ensuring, in the same reactor, the formation of the plasma and an optimum irradiation of the photocatalyst by UV lamps (the UV light produced by the plasma is very weak). By contrast, it makes sense to first treat an effluent by cold plasma if the concentrations of pollutants are high enough to allow this homogeneous method to be reasonably applied, and then to complete the purification by photocatalysis, which, because it is based on adsorption, should be a more efficient method for pollutants concentrations that have been lowered. Experimental Section Plasma Reactor. This reactor (Figure 1) was designed and fabricated at EDF R&D. Two metallic serrated blades * Corresponding author. Tel.: 33 (0) 4 78 66 05 50. Fax: 33 (0) 4 78 33 11 40. E-mail:
[email protected]. † EDF. ‡ CNRS UMR. § Present address: Biowind, 36, rue Oberlin, 67000 Strausbourg, France. E-mail:
[email protected].
(length ) 423 mm and thickness ) 1.5 mm) were separated by a ceramic dielectric tube (diameter ) 10 mm) surrounding the central cylindrical electrode (diameter ) 7 mm) and placed in the middle of a cylinder (diameter ) 70 mm) made of glass. On connecting the blades to a Cirtem (model P205) electricity supply and a Wavetec frequency generator (20-50 kHz), a cold plasma formed in the 2.5 mm interval between the dielectric and the 1.5 mm deep tips of the saw-edged blades. We used a voltage of 13.5 kV, a frequency of 9 kHz, and a power of 10.8 W. These values were measured using a Yokogawa DL708 oscilloscope and a voltage divider. Photocatalytic Reactor. The parallelepiped reactor (Figure 2) (43 × 23 × 10 cm) had three walls (15 mm thick) made of Teflon; the top wall (same thickness) was made of glass, which both absorbs the 254 nm radiations emitted by the lamp inside the reactor and enables one to check that the lamp was actually on or off because of the weak fraction of visible light emitted. Air-tightness was insured by a flat gasket. The TiO2-coated porous material (170 cm2) was maintained in the middle of the reactor in a 1 cm thick Teflon frame. One PL-L TUV 18 W Philips lamp (U-shaped; 225 mm long) was placed on each side of this material at a distance of 77 mm, so that the irradiance of the material was sufficiently homogeneous, with the mean value being 19.3 W m-2. The methanol-containing air was admitted into the reactor by two ports and exited by two other ports. The tubes connected to the out-ports joined for the analysis of methanol. TiO2-Coated Material. A thin, nonwoven tissue made of synthetic fibers and cellulose was impregnated with an aqueous mixture containing equal amounts of TiO2 (Degussa P25: ca. 50 m2 g-1, nonporous, about 80% anatase-20% rutile) and colloidal SiO2 (ca. 100 m2 g-1; particle size ) 25-30 nm) and allowed to dry at room temperature.7 The TiO2 mass supported was 18.5 g/m2 of tissue. Procedures. Liquid methanol was constantly added into air using a Bronkhorst High Technology vaporization system. The methanol amount was such as to obtain a concentration of about 122 ppmv (163 mg m-3 or 5.09 mmol m-3) with a flow rate of air through the chamber of 0.3 m3 h-1, corresponding to a velocity of ca. 4.9 × 10-3 m s-1 through the photocatalytic material. Outdoor air passed through an oil and dust filter, and an activated carbon filter was used. The relative humidity was 9-10%. In all cases, the methanol-containing air was continuously flown through the two reactors in series, with the plasma reactor being placed first. Each reactor was either in operation
10.1021/ie0700967 CCC: $37.00 © 2007 American Chemical Society Published on Web 07/03/2007
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Figure 1. Scheme of the cold-plasma reactor ((a) front view, (b) side view); see text for the dimensions.
Figure 2. Scheme of the photocatalytic reactor.
Figure 3. Variations in the methanol concentration depending on the operating conditions.
or not. Methanol concentrations were measured by use of a M&A (model Thermo-FID PT) flame ionization detector. Results Figure 3 shows the results. First (curve 1), in the absence of plasma and UV irradiation of the photocatalytic material, the methanol concentration, C, stabilized at ca. 122 ppmv. Second
(curve 2), when the plasma was operated, C dropped rapidly and eventually stabilized at ca. 38 ppmv. Third (curve 3), if, in addition to the plasma, the photocatalytic reactor was working, C further decreased to ca. 11 ppmv. Fourth (curve 4), in the absence of plasma and with the UV lamps on, C decreased from ca. 122 ppmv to ca. 97 ppmv. Fifth (curve 5), an initial C of ca. 38 ppmvsi.e., the value found on starting from ca. 122 ppmv
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when only the plasma reactor was functioning (curve 2)swas decreased to ca. 19 ppmv when the UV lamps were on and the plasma was not in operation. All these results were satisfactorily reproducible. Discussion Our purpose was not to compare the efficacy of cold plasma and photocatalysis regarding the removal of methanol because the reactors and the photocatalytic material were not optimized and the electrical power differed (around 50 W for the cold plasma reactor and 36 W for the photocatalytic reactor). Therefore, we will not directly compare curves 2 and 4 (Figure 3) because they depended on the reactors, the photocatalytic material, and other conditions. By contrast, it is worth comparing curves 3 and 5 (Figure 3). The decrease in C obtained by photocatalysis was higher (from 38 to 11 ppmv vs from 38 to 19 ppmv) when the methanolcontaining air was issued from the functioning plasma reactor. The origin of this difference must be sought in the plasmaproduced changes in the composition of the air. These composition changes include the formation of O, N, •OH, HO2•, O2•-, O3 (H2O2 is significantly formed only for RH higher than the present value), and nitrogen oxides, as well as organic radicals and molecules, viz. HCHO, HCOOH, HCOOCH3, CO, and CO2, issued from CH3OH. Among these chemical species, the atoms and radicals are likely too short-lived in the gas phase to be transferred to the photocatalytic reactor and, hence, to play a significant role in the photocatalytic decrease in C. CO2 is inert, whereas HCHO, HCOOH, HCOOCH3, and CO can compete with CH3OH for the photocatalytically formed active species and, accordingly, negatively affect the removal of CH3OH. Consequently, the difference between curves 3 and 5 (Figure 3) has to be sought in the presence of O3 and nitrogen oxides in the gas issued from the plasma reactor. In the gas phase, the following sequence of reactions can occur,
NO2 f NO + O O + O 2 f O3 NO + O3 f NO2 + O2 with the first reaction being produced by the absorption of 254 nm photons by NO2 and its dimer. The net result of these reactions is merely to change the relative concentrations of the molecules involved. Both NO8 and NO2 have been mentioned to oxidize organic molecules over UV-irradiated TiO2, at least in the absence of competing O2. N2O is photocatalytically inert.9 Gaseous ozone is also decomposed by the absorption of 254 nm photons: O3 f O2 + O (3P) + a very low percentage of O (1P) Atomic oxygen thus-formed can play a role in the removal of methanol in the photocatalytic reactor. However, it must be kept in mind that the concentration of gaseous methanol is very low, as is, presumably, that of gaseous ozone. Accordingly, we suggest that reactions occurring on the TiO2 surfaceswhere both ozone and methanol are concentrated by adsorptionslikely play a greater role. Indeed, even in the absence of photons absorbable by O3, a very important effect of O3 on photocatalytic oxidations has been reported.10,11 For instance, the presence of O3 in O2 not only increases the removal of n-octane but also considerably increases its mineralization,
illustrating the effect of O3 on the transformation of all the intermediate products of the photocatalytic oxidation of noctane.11 This substantial effect has been attributed to the difference in electron affinity between O3 (2.1 eV) and O2 (0.44 eV).11 Consequently, in the presence of ozone, the electrons in the conduction band of UV-irradiated TiO2 can more easily be captured, either directly,
e- + O3 f O•- + O2 or indirectly (given the much higher concentration of O2),
O2•- + O3 f O2 + O3•The radical anion O3•- is still more unstable than O3 and can presumably easily split at the surface of TiO2,
O3•- f O•- + O2 In addition, the increase in the scavenging rate of photoproduced electrons shown above should decrease the recombination rate of electrons and holes and thereby enhance the formation rate of hydroxyl radicals from basic OH surface groups and adsorbed H2O. According to both these mechanisms, very oxidizing species, viz. either O•- or OH•, would, thus, be generated. Therefore, we suggest that O3sboth formed in the plasma and resulting from NO2 photodecomposition (vide supra)sis the main cause of the increase in methanol removal in the photocatalytic reactor when comparing the plasma-treated gas flow (Figure 3, curve 3) to that untreated (Figure 3, curve 5), which was admitted into the photocatalytic reactor at equal methanol concentrations (38 ppmv). Note that consuming O3 is of great interest as O3 is a health hazard and corrodes metals. Conclusion At least for this case study, a synergy has been observed when a cold plasma treatment is followed by a photocatalytic treatment. This synergy has been suggested to arise from the presence of ozone, which is decomposed by 254 nm photons and, above all, is known to enhance the photocatalytic efficacy. On the basis of this interpretation, it might be envisaged to use an ozonizer as a better means than the cold-plasma reactor of supplying O3; however, by doing so, purification due to the plasma-generated active species, especially •OH radicals, would be lost. Obviously, further researchsincluding other case studies, reactor design, and economic evaluationssis needed to better assess the interest of this particular combination of AOPs. Acknowledgment The authors warmly thank Mr. Frederic Tuvache (EDF) for his helpful assistance with the plasma system. J.T. is grateful to ANRT for its contribution to his Ph. D. scholarship. Literature Cited (1) See for example: Helfritch, D. J. Plasma Technologies Applied to Air Pollution Control. IEEE Trans. Ind. Appl. 1993, 29, 882; Hurashima, K.; Chang, J. S. Removal of Volatile Organic Compounds from Air Streams and Industrial Flue Gases by Non-Thermal Plasma Technology. IEEE Trans. Dielectr. Electr. Insul. 2000, 7, 602. (2) See for example: Pichat, P. Photocatalytic degradation of pollutants in water and air: Basic concepts and applications. In Chemical Degradation Methods for Wastes and Pollutants: EnVironmental and Industrial Applications; Tarr, M. A., Ed.; Marcel Dekker: New York/Basel, 2003; pp 77-119; Agrios, A. G.; Pichat, P. An overview of the state of the art and
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perspectives on materials and applications of photocatalysis over TiO2. J. Appl. Electrochem. 2005, 35, 347. (3) Mizuno, A.; Kisanuki, Y.; Noguchi, M.; Katsura, S.; Lee, S. H.; Hong, Y. K.; Shin, S. Y.; Kang, J. H. Indoor Air Cleaning Using a Pulsed Discharge Plasma. IEEE Trans. Ind. Appl. 1999, 35, 1284. (4) Li, D.; Yakushiji, D.; Kanazawa, S.; Ohkubo, T.; Nomoto, Y. Decomposition of Toluene by Streamer Corona Discharge with Catalyst. J. Electrost. 2002, 55, 311. (5) Kang, H.; Choi, B.; Son, G.; Foster, D. E. C2H4 Decomposition Behaviour of a Non-thermal Plasma Discharge-Photocatalyst System for an Air Purifying Device. JSME Int. J., Ser. B 2006, 49, 419. (6) Peka´rek, S.; Pospı´sˇil, M.; Kry´sa, J. Non-Thermal Plasma and TiO2Assisted n-Heptane Decomposition. Plasma Process. Polym. 2006, 3, 308. (7) Navarre, F.-P.; Bossand, B.; Girard, P.; Dussaud, J. Filled Paper for Gas Filtration. U.S. Patent 5,965,091, October 12, 1999. (8) Pichat, P.; Courbon, H.; Disdier, J.; Mozzanega, M.-N.; Herrmann, J.-M. Heterogeneous photocatalysis: NO Decomposition and Oxidation of
Butanols by NO over TiO2 at Room Temperature. In Studies in Surface Science and Catalysis; Seiyama, T., Tanabe, K., Eds.; Elsevier: New York, 1981; 7A, Part B, p 1498. (9) Courbon, H.; Pichat, P. Room-temperature Interactions of N18O with Ultraviolet Illuminated Titanium Dioxide. J. Chem. Soc., Faraday Trans. I 1984, 80, 3175. (10) Alberici, R. M.; Jardim, W. F. Gas-phase destruction of VOCs using TiO2/UV and TiO2/O3/UV. J. AdV. Oxid. Technol. 1998, 3, 182. (11) Pichat, P.; Disdier, J.; Hoang-Van, C.; Mas, D.; Goutailler, G.; Gaysse, C. Purification/Deodorization of Indoor Air and Gaseous Effluents by TiO2 Photocatalysis. Catal. Today 2000, 63, 363.
ReceiVed for reView January 15, 2007 ReVised manuscript receiVed April 12, 2007 Accepted May 8, 2007 IE0700967
