Effect of Different Catalysts on the Decomposition of ... - IEEE Xplore

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IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 34, NO. 3, JUNE 2006

Effect of Different Catalysts on the Decomposition of VOCs Using Flow-Type Plasma-Driven Catalysis Hyun-Ha Kim, Atsushi Ogata, and Shigeru Futamura, Member, IEEE

Abstract—This paper presents the effect of different catalysts on the decomposition of benzene and toluene using flow-type plasmadriven catalyst (PDC) system. Three representative materials of titanium dioxide, two types of -alumina and two zeolites were tested. Several types of metal catalysts (Ag, Ni, Pt, Pd) and their loading amount were also investigated for the optimization of the PDC system. Three key factors of energy consumption, carbon balance and safety of products were emphasized in evaluating the performance of different catalysts. The type of catalysts greatly influenced on the carbon balance, 2 selectivity, ozone formation, while no much difference was observed in the degree of enhancement in energy efficiency. 2 3 catalyst was found to be effective in enhancing the 2 selectivity. The 2 selectivity increased as Ag-loading amount on 2 catalyst increased. The 4.0 wt% 2 catalyst was effective in suppressing the formation of 2 and 2 . Zeolites showed comparable decomposition efficiency and good carbon balance, while the 2 selectivity was poor compared to the other catalysts. Mechanical mixing of 2.0 wt% Ag/H-Y zeolite with 2 3 was effective in enhancing the 2 selectivity without changing other performance.

Ag TiO NO NO CO

CO Pt Al O CO TiO Pt

Al O

CO

CO

Index Terms—Byproduct, dielectric-barrier discharge (DBD), nonthermal plasma, plasma-driven catalysis (PDC), volatile organic compounds (VOCs).

I. INTRODUCTION

T

HERE has been extensive research on using nonthermal plasma (NTP) over the past 20 years, especially based on electrical discharges, as an emerging technology for environmental protection. The major advantages of NTP technology over the competing conventional technologies include the moderate operation conditions (room temperature and atmospheric pressure), moderate capital cost, and compact system, etc. In the field of air pollution control, the NTP technology has been tested for the abatement of various types of hazardous air pollu[1], [2], NOx [3]–[5], CFCs [6]–[10], odors tants such as [11]–[14], volatile organic compounds (VOCs) [15]–[18], mercury [19], dioxins [20], [21], etc. For concentrations below several hundreds parts per million by volume (ppmv), except for saturated hydrocarbons, the NTP technology found to be effective for the most of air pollutants in terms of destruction efficiency. Despite the many successful demonstrations of the lab- and pilot-scale tests for various air pollutants, some critical problems have been raised in the course of independent research Manuscript received October 18, 2005; revised December 5, 2005. This work was support by Ministry of Education, Culture, Sports, Science and Technology (MEXT); Grand-in-Aid for Young Scientist (A) (16681007). The authors are with the National Institute of Advanced Industrial Science and Technology (AIST), Ibaraki 305-8569, Japan (e-mail: [email protected]). Digital Object Identifier 10.1109/TPS.2006.875728

and development activities by many groups over the world [22]. Two key factors required for the further development of NTP technology are the reduction of energy consumption and the control of unwanted byproducts. Especially, these two requirements must be satisfied together with the removal efficiency before the industrial applications. If one of these requirements is not satisfied, the NTP process may lose its potential for the commercial use. In this sense, NTP technology is now facing an important turning point of further development and extended use in industries. As similar technologies to the electrical discharge nonthermal plasmas, microwave-induced plasma and radio-frequency (RF) discharge plasma are also studied for the decomposition of volatile organic compounds (VOCs). These two technologies appear to be successful in terms of removal efficiency. However, the operating energy levels are about 1–2 orders magnitude higher than those of electrical discharge plasmas. For example, microwave plasma required about 3 kJ/L for the destruction [23]. Specific of 1700 ppm trichloroethylene (TCE) input energy (SIE) requires for a RF plasma to achieve 99.00% is about 60 kJ/L destruction of 3% methyl chloride at 20 torr of operation pressure [24]. Furthermore, these two methods use expensive inert gases (He, Ar) for dilution, which leads to high operation cost. It is, therefore, believed that both the RF plasma and the microwave (MW) plasma may not compete with electrical discharge plasmas for processing low concentration VOCs. Possible application of the MW plasma and the RF plasma may be the decomposition of persistent toxic chemicals with relatively high concentration [25]–[27]. In recent years, hybrid NTP technology, which combines NTP with catalysts is the subject of increasing attention for the decomposition of various types of air pollutants [12], [28]–[30]. This combination can be subdivided into single-stage [31]–[33] and two-stage [34]–[36] processes depending on the position of catalysts. We have been developing single-stage NTP-catalyst system, which is referred to as plasma-driven catalyst (PDC) system. The PDC system is also operated at atmospheric pressure and near room temperature, where thermal catalysis does not take place without plasma application [37]. The distinctive characteristics of the PDC system are briefly summarized in Table I together with the conventional NTP alone system. The primary advantage of the PDC system over the plasma alone is the high energy efficiency. The PDC – catalysts showed 4 and 7 times system packed with higher energy efficiency than that of the NTP alone methods for the decomposition of 200 ppm benzene at humid and dry conditions, respectively [22]. Both the plasma alone and the

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KIM et al.: EFFECT OF DIFFERENT CATALYSTS ON THE DECOMPOSITION OF VOCS

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TABLE I COMPARISON OF NONTHERMAL PLASMA ALONE AND PLASMA-DRIVEN CATALYSIS

PDC system are not influenced by gas hourly space velocity (i.e., residence time) because the characteristic time of plasma chemical reactions is usually short compared to the gas residence time in the reactors. There have been many publications concerning the energy-dependent characteristics of NTPs for NO removal [38]–[41], ozone formation [42], and decomposition of VOCs [43], [44]. In contrast to the kinetics of gas-phase homogeneous processing of VOCs in the NTP alone (first-order kinetics) [45]–[47], the PDC system shows a zeroth-order, which indicates the important role of the surface reaction in is the the decomposition of VOCs. The energy constant reciprocal of the exponential-folding parameter ( parameter) proposed by Rosocha et al. [48]. A larger indicates higher capability in the decomposition of VOCs. Since the formation of nanometer sized aerosols is effectively suppressed in the PDC system, carbon balance is very good with the PDC system [47]. Supporting data for the good carbon balance in the PDC system was also obtained from the long-term test over 150 h in the decomposition of benzene and toluene [44]. A critical drawback of the NTP processing of VOCs in mixture is the formation of species (NO, , , , , and ) [37], [49]–[51]. Again, a careful attention must be paid to compare the behavior of NOx formation in NTP reactors because the distribution of is strongly affected by reaction conditions (i.e., humidity, pressure, temperature, presence of ozone, etc.). The formation depends on plasma energy, so a NTP reactor must of be operated at proper energy level according not only to the

required destruction efficiency of VOCs but also to the forspecies. This restriction of applicable energy mation of confines the optimum concentration range, where the NTP technology can be applied. The optimum concentration range of flow-type PDC reactor to meet this requirement is below 100 ppm for aromatic VOCs [37], [44]. The other attractive way of PDC system application is the cycled operation of adsorption (without plasma) and the decomposition of adsorbed VOCs using oxygen plasma. The cycled system is a very promising selectivity, and technology in terms of energy efficiency, no formation of nitrogen oxides [52]. The important basis of the cycled system is the highly oxygen content-dependent behavior of the PDC system. As the oxygen content of the gas mixture increases, both the decomposition efficiency and the yield of are greatly increased even at fixed specific input energy [52]. There is still considerable room for further optimization of the PDC system. One of the key points is the screening of the proper catalyst for the desired reactions. Since the mechanism of synergy effect in the PDC system is unclear at this stage, it would be useful to know the effect of different catalysts on the VOCs decomposition in the PDC system. The main objective of this work is quite simple and straightforward. Three representative , , and zeolite) with different types of materials ( metal catalysts were tested for the decomposition of VOCs. The characteristics of the different catalysts were evaluated based on the three key points mentioned above; energy consumption, carbon balance and byproducts.

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Fig. 1. Schematic diagram of the plasma-driven catalyst (PDC) reactor. (Color version available online at http://ieeexplore.ieee.org.)

II. EXPERIMENTAL DESCRIPTION A. Plasma Reactor and Experimental Setup In this work, we have investigated the decomposition of VOCs using PDC systems consisting of nonthermal plasma and catalysts with a single-stage configuration. Fig. 1 shows the details of the PDC reactor. Unless otherwise noted, the inner diameter and the effective length of the quartz tube were 1.3 and 20 cm, respectively. In the case of the H-Y zeolite which has high adsorption capability, the effective length was 15 cm, all else being identical. It should be noted that further increase of energy efficiency is possible because the decomposition efficiency increases as the diameter of plasma reactor decreases [52]. A coil-type electrode made of stainless steel (0.45 mm diameter) was set on the inside wall of the tube as a high-voltage electrode. When there is void between the tube and outer ground electrode, discharges in the void consume energy and resulting in low energy efficiency [53]. To solve this problem, silver paste was painted on the outside of the tube as a ground electrode. Catalysts were placed within the effective length of the reactors. This configuration rendered all the catalysts exposed to the plasma and also allowed to ignore additional adsorption or additional chemical reactions outside the plasma region. For most of the experiments, 200 ppm benzene was used as a model compound of VOCs, but 150 ppm toluene was used for one set of runs. According to our previous results [37], the 200 ppmv of benzene is beyond the optimum concentration for the PDC system, it will be advantageous to compare the catalytic activity of different catalysts. Nitrogen gas was purged the temperature controlled bubbler containing liquid benzene or toluene, and then mixed with synthetic air (20% with balance). Gas flow rate was in the range of 4–10 liter per min (LPM) at normal conditions (293 K, 0.1 MPa). The corresponding gas hourly space velocity , and 75000 (GHSV) at 4 LPM for the PDC was 22500 for the case of H-Y. The gas flow rate was regulated using mass flow controllers (KOFLCO, FCC-3000). The plasma reactor was placed in an oven, where the temperature was set at 373 K. Gas temperature in the PDC reactor while in operation was slightly higher than 373 K due to the heating by the plasma. B. Materials , two types of and Three types of catalysts ( two types of zeolites) with different metal catalysts were investi-

gated in this study. In the conventional thermal catalyst system, as photocatalyst, these have been considered just except for as supporting materials for metal catalysts. However, all these materials packed in the PDC reactor are regarded as catalysts, since they show a catalytic effect under the plasma activation even at room temperature. The amounts of catalysts packed in the PDC reactors were about 24 cm . Table II summarizes the details of the catalysts. The pellet was anatase type, which is well known as a photocatalyst. Each type of the catalyst pellets has different BET surface area. It will be interesting to see how the surface area affects the performance of VOCs decomposition. Except for and catalysts, which were comthe mercial products (NE Chem. CAT Company, Tokyo, Japan), all metal catalysts were supported on the catalysts by an impregnation method. After several steps of metal catalysts loading and drying, the pellet catalysts were calcined at 773 K for 10 h in an air flow. Although there are many different types of zeolites based , structure, pore size, etc., the two zeon the ratio of olites (good one and poor one) were chosen based on the previous work [54]. The pore sizes of the H-Y and the ferrierite were 7.4 and 4.3–5.3 , respectively. Since the molecular size of benzene is 5.9 , it can be easily assimilated into the micropore in the H-Y zeolite but not in the ferrierite. C. Electrical Measurement The information of energy consumption in the NTP reactor is very important both for the comparison purpose and system scaling-up. The discharge power and the specific input energy (SIE) in the PDC reactor were measured using an automated program (Visual Basic), which is operated on a PC with a Windows XP. Fig. 2 shows an example of discharge power measurement using the V–Q Lissajous program. The charge Q was determined by measuring the voltage across the capacitor of 100 nF connected in series to the ground line of the plasma reactors. Alternating current (ac) high-voltage was supplied with an amplifier (Trek, 20/20B) and a function generator (Tektronix, AFG 310). Applied high voltage was measured with a 1000:1 high-voltage probe (Tektronix, P6015A). The waveforms of the charge and the applied voltage were monitored with a digital oscilloscope (Tektronix, TDS3032B). The waveform data and the parameters of the oscilloscope were transformed to the PC via crossover cable. The measurement data (discharge power, and , etc.) and the parameters of the oscilloscope SIE, can be saved as an EXCEL file format. Discharge current was measured using a Rogowiski coil type current probe (Pearson Electronics, Inc. Model 2877). Specific input energy, which is to the gas flow rate , is the ratio of discharge power one of important scaling parameters of NTP processing Specific input energy discharge power gas flow rate L/min) discharge power gas flow rate

J/L (1)

is usually used for a large-scale experiThe unit of ment. The conversion factor from Wh/Nm to J/L is 3.6. There


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TABLE II DETAILS OF THE CATALYSTS

Fig. 2. Automated program of V–Q Lissajous method for the measurement of discharge power and specific input energy. (Color version available online at http:// ieeexplore.ieee.org.)

are a number of different ways in describing the in the literature; (specific energy input, specific deposited energy, specific power input, specific discharge input, adsorbed discharge energy, energy deposition, specific energy density, energy input density, corona energy density, discharge energy density, input is basically identical to energy density, etc.). The term of

the specific corona power, which was used in the electrostatic precipitator (ESP) [55] Specific corona power discharge power watt gas flow rate

(2)

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Both the specific input energy and the specific corona power are useful parameters, especially for scaling-up of the system, because they provide intuitive insights on total energy consumption for given flow rates. D. Gas Measurement For quantitative gas analysis, a Fourier transform infrared (FTIR) spectrometer (Perkin Elmer, spectrum one) equipped with a gas cell of 6.4 m optical path length and a TGS detector was used. FTIR can avoid the interference of ozone, which can cause overestimation of decomposition efficiency in the case of gas chromatography (GC) measurement of plasma treated gas sample. To minimize a baseline fluctuation which is disadvantageous to the quantitative measurement, the space between the main body of the FTIR and the gas cell was purged with high purity nitrogen ( 99.999%). Gas measurements were done at 1 min intervals and 2 scans were averaged at a resolution of 1 cm . The gas cell (0.75 L) was heated to 343 K to prevent any possible condensation during the measurement. Measurements were done for 60–90 min for each SIE, while the time to reach a steady state after the change of SIE was in range of 10–20 min depending on the packing materials. As reported in the previous publications [37], [44], the main decomposition products from the tested VOCs were CO and . Formic acid (HCOOH) was also produced as minor inas SIE intermediate only at low SIE and decomposed to creased. Therefore, the carbon balance was simply calculated , and HCOOH as follows: from the sum of CO,

Carbon balance (3) Here, and indicate inlet concentrations of VOC and the number of carbon atoms in them, respectively. selectivity is defined as follows: The

(4)

s

III. EXPERIMENTAL RESULTS A. Voltage (V)–Current (I) Characteristics The mode of discharge provides important information on the status of electrical discharge plasma, so it would be useful to know the effect of the presence of catalysts on the discharge mode in the PDC reactors. Of course, there are large differences both in physics and chemistry for the discharge plasmas in different modes. For example, the change of discharge mode from glow to streamer greatly enhances the formation of ozone in direct current corona [56]. A simple way to determine the nature (i.e., mode) of the discharge is by measuring the waveforms of applied voltage and discharge current. Fig. 3 shows the waveforms of voltage ( ) and the current ( ) in the plasma reactors with or without catalysts; (a) plasma alone – , (c) 4.0 wt% (without catalysts), (b) with 2.0 wt%

– , (d) 2.0 wt% – , (e) 5.0 wt% , , (g) 0.5 wt% , (h) 2.0 (f) 0.5 wt% wt% Ag/H-Y. The applied ac voltage and frequency were the and 500 Hz, respectively. Current wavesame at 28 forms showed quite similar patterns to that of a normal DBD consisting of many short current pulses, which are attributed to streamer discharges. Regardless of the type of catalysts, the presence of catalyst did not affect the mode of discharge. From the similar V-I characteristics, it is expected that there is no much difference between the plasma alone and the PDC system in the property of plasma. Therefore the enhanced performance with in PDC system can be ascribed to the catalytic reaction rather than the change of discharge characteristics. The shapes of V-Q Lissajous figures also showed quite similar patterns regardless of the type of catalysts. B. Decomposition of Benzene and Carbon Balance 1) Catalyst: Fig. 4 shows the decomposition of (a) benzene and (b) the carbon balance using the PDC reactors catalysts of different Ag-loading amounts. packed with The Ag-loading amount up to 2.0 wt% did not affect the decomposition efficiency of benzene even at 0% Ag-loading (i.e., alone). These findings indicate that the plays an important role in the decomposition of benzene rather than the supported Ag catalysts. However, further increase of the supported Ag amount to 4.0 wt% decreased the decomposition efficiency of benzene. On the other hand, a large difference was observed in the carbon balance data. When the supported Ag amount was smaller than 0.5 wt%, the carbon balance ranged far below or above 100% depending on the SIE. As the Ag-loading amounts became larger than 1.0 wt%, relatively good carbon balance was obtained even at low SIE conditions. The 4.0 wt% catalyst showed the most stable carbon balance. The data in Fig. 4 indicates that the and Ag supported have different roles in the benzene decomposition. The on initial step of benzene decomposition proceeds on the surface of , and some intermediates are built up on the surface. The supported Ag plays important role for the decomposition of the surface intermediates. When the Ag-loading exceeds a certain value (i.e, at 4.0 wt%) loaded-Ag may reduce the surface area of responsible for the initial decomposition of benzene. On the other hand, the higher the Ag-loading amount the better the carbon balance due to its activity for the surface intermediates. also increased with Ag loading amount. The selectivity of selectivity was about 68% with 0.5 wt% Ag For example, and increased to 73% and 88%–92% with 2 wt% Ag and 4 wt% Ag, respectively. Fig. 5 shows the comparison of different metal catalysts (Ag catalyst. The amount of each versus Ni) supported on metal catalyst was the same at 2.0 wt%. Although the showed higher conversion of benzene, a critical problem of catalyst was the poor carbon balance. This poor the carbon balance indicates that some reaction intermediates catalysts. This were deposited on the surface of the has higher catalytic observation also shows that the for the decomposition of surface activity than the intermediates.


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Fig. 3. Waveforms of applied voltage and discharge current; (a) without catalysts, (b) with 2.0 wt% Ag=TiO , (c) with 4.0 wt% Ag=TiO , (d) with 2.0 wt% Ni=TiO , (e) 5.0 wt% Ag= Al O , (f) 0.5 wt% Pt= Al O , (g) 0.5 wt% Pd= Al O , (h) 2.0 wt% Ag/H-Y.

0

2) Catalyst: Fig. 6 shows the decomposition of catalysts supported with Ag, benzene using the two Pt, and Pd. The decomposition efficiency of benzene using the catalyst was similar to the catalyst regardless of the Ag-loading amount. On the contrary to the , ten times difference in the Ag-loading amount did not affect the carbon balance. also showed similar activity in benzene decomposition up to about 150 J/L but became more effective as SIE further increased. However, the Pd catalyst was found to be less effective in benzene decomposition than the other metal catalysts. The reason for this different behavior of Pt catalysts at higher SIE is believed to be the temperature increase. As was reported previously catalysts shows no thermal catalytic activity at [37], temperature below 473 K. However, although data is not shown catalyst here, thermal catalysis of benzene over the started at around 403 K, and reached 100% decomposition at in the PDC reactor was 523 K. The temperature increase

0

0

also showed approximately 70 K at 200 J/L. The and the best carbon balance, and followed by . 3) Zeolite: Fig. 7 shows the decomposition of benzene using two different types of zeolites of 2.0 wt% Ag/H-Y and ferrierite. A mechanically mixed catalyst (2.0 wt% Ag/H-Y and 0.5 wt% ) was also tested. Since the size of micropore in the H-Y zeolite is larger than that of benzene molecule, the H-Y zeolite has large capability toward benzene adsorption. In view of the large adsorption capability of the H-Y zeolite, gas flow rate was set at 10 L/min for the H-Y experiment and the mixed catalyst, while 4 L/min for the ferrierite. The two zeolites and mixed catalysts showed very similar performance in the decomposition of benzene. For carbon balance, 2.0 wt% Ag/H-Y was slightly better than the ferrierite. The mechanically mixed catalysts had quite comparable or slightly better activity in the decomposition of benzene, while the carbon balance was slightly poorer.

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Fig. 4. Effect of Ag-loading amount on the decomposition of benzene (a) and carbon balance (b). (Color version available online at http://ieeexplore.ieee.org.)

C.

Selectivity

The desired final product of carbon in VOCs structure is becauseCOisstill verytoxicandalsohardtooxidizeto using selectivity for the tested catplasma alone. Fig. 8 shows the alysts at two different SIE levels. A common trend in all tested selectivity is less sensitive to catalytic materials is that the the SIE compared to the decomposition efficiency and carbon balance. Regardless of the SIE values, the formations of CO and proceed at almost constant branching ratio. In other words, once CO is produced it is hard to obtain further oxidation to even using the PDC reactor. However, the effect of the type of metal selectivity. For catalysts was most prominent for the catalyst, Ag showed higher selectivity than Ni at the same loading amount of 2.0 wt%. In the case of catalysts, selectivity was in the order of the . In addition to the carbon balance, the Ag catalysts and the enhanced the supported on the selectivity. Ferrierite showed better selectivity than the 2.0 wt% Ag/H-Y, which is opposite to the carbon balance data. In addition to the decomposition efficiency and the carbon balance, catalyst showed the highest selectivity the among the tested catalysts in this study. D. Byproducts It should be noted that the oxides of nitrogen can appear in many different oxidation states (NO, , , , ) depending on the reaction conditions. Fig. 9 shows the

Fig. 5. Comparison of Ag=TiO and Ni=TiO catalysts for the decomposition of benzene (a) and carbon balance (b). (Color version available online at http:// ieeexplore.ieee.org.)

formation of (a) and (b) as a function of SIE for all the tested catalysts. In the experimental conditions using the PDC and negligible reactors, the most of nitrogen oxides was amount of NO was produced. The concentration of was . Except for 4.0 wt% always higher than that of catalyst, and catalysts showed similar behavior formation. The 4.0 wt% catalyst produced the in least amount of and . The outlet concentrations and catwith zeolites were lower than with the alysts due probably to adsorption at the SIE below 200 J/L. As concentration beSIE further increase above 200 J/L, the come quite comparable with the other catalysts. Ozone is very susceptible to the heterogeneous decomposition on the surface of catalyst, so the large difference in outlet ozone concentration is expected depending on the type of catalyst. Fig. 10 shows the outlet ozone concentration as a function of SIE for different catalysts. In the case of plasma alone (i.e., without catalyst), ozone concentration went through a maximum at 383 ppm with 136 J/L and then decreased as SIE further increased. Although the ozone concentration is low due to the temperature condition, the behavior of ozone formation is consistent with the surface discharge and the DBD reactor [57]–[59]. Alcatalyst. In the case most no ozone was found in the case of , the loading amount of Ag greatly affected the of outlet ozone concentration. The two zeolites produced the highest amount of ozone (about 200 ppm at maximum). The high outlet ozone concentration with the zeolites indicates the low capability


0

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Fig. 6. Performance of Al O catalysts for the decomposition of (a) benzene and (b) carbon balance. (Color version available online at http://ieeexplore.ieee. org.)

Fig. 7. Performance of zeolites for the decomposition of benzene (a) and carbon balance (b). Mixed catalysts are the mixture of 2.0 wt% Ag/H-Y zeolite and 0.5 wt% Pt= Al O with the volume ratio of 3:1.

of the zeolites in ozone decomposition. An interesting point is that the large difference in the ozone formation did not influence the decomposition of benzene. For example, despite the large differcataence in the outlet ozone between the zeolites and the lysts, there was no much difference in terms of the decomposition efficiency of benzene.

and the different catalysts are similar or not, the results obtained in this work suggest that the role of plasma is more important than the catalysts in the initial step of benzene decomposition. The important roles of the type of catalysts and the loading amount of metal catalysts were observed in the subsequent decomposition processes, resulting in very different trends in selectivity, and the formation of ozone. carbon balance, Pro and con arguments have been reported on the role of ozone in the VOCs decomposition using NTP-catalyst hybrid systems. Ozone formation in a NTP reactor proceed via two step processes; atomic oxygen formation (R1 and R2), and recombination of atomic oxygen with oxygen molecule (R3, cm molecule s )

IV. DISCUSSIONS Various types of catalysts have been evaluated using benzene as a model compound of VOCs to find a guideline in optimizing the PDC system. Several important features of interaction between nonthermal plasma and the catalysts were derived in this study. First, contrary to our expectation, the types of catalysts did not much influence on the initial step of benzene decomposition. If the catalysts play important roles in the decomposition of benzene, there must be a clear difference in the catalytic effect on decomposition efficiency. However, the degree of initial conversion of benzene was mostly determined by specific input energy regardless of the type of catalysts. These observations also suggested that the large enhancement of energy efficiency may be less possible to achieve by changing catalysts. This conclusion was also supported from the data of 150 ppm toluene decomposition, which is shown in Fig. 11. The tested two catalysts showed the same decomposition efficiencies. It is also indicated that the surface area of catalysts has little influence on the decomposition efficiency. At this stage, it is hard to determine whether the interactions between the plasma

0

electron

(R1) (R2) (R3)

The reaction R3 has negative temperature dependence. Thermal become significant as temperature indecomposition of cm molecule s ). crease (R4; atom (R5; Ozone is also converted to oxygen by cm molecule s ) (R4) (R5)

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Fig. 8. Selectivities of CO for the different catalysts; (a) SIE = 95 ieee.org.)


105 J=L, (b) SIE = 200 210 J=L. (Color version available online at http://ieeexplore.

Fig. 10. Effect of different types of catalysts on the outlet ozone concentration of the PDC reactor.

Considering the low rate constants of -benzene reaction cm molecule s and -toluene reaction cm molecule s at 375 K, ozone-induced gas-phase decomposition of the tested VOCs can be neglected. However, ozone can be decomposed over the surface of catalysts and to form more reactive atomic oxygen (R6) and surface (R7), which can accelerate surface oxidation reactions [60], [61] Fig. 9. Comparison of (a) NO and (b)N O formation for the different catalysts.

Catalyst Catalyst

Catalyst Catalyst

(R6) (R7)


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2)

3)

4) Fig. 11. Decomposition of 150 ppmv toluene using the 0.5 wt% Pt= and 2.0 wt% Ag=TiO catalyst.

0Al O

It has been reported that ozone acts as an oxidant precursor in the decomposition of odor and benzene in a two-stage system [11], [62]. Although experimental condition is slightly different, a similar result was reported for the decomposition of toluene using a surface discharge packed with zeolite [54]. On the other hand, Sekiguchi et al. reported that ozone did not contribute to the decomposition of benzene using a DBD reactor packed catalysts [63]. In the PDC reactors with difwith concentrations highly depended on ferent catalysts, outlet the type of catalyst. As one can see from the data with ze, however, the outlet concentrations have olites and little or nothing to do with the decomposition efficiency of benzene. A similar trend was also observed for the and . The Ag-loading amount on greatly influenced the outlet ozone concentration. However, the decomposition efficiency and the carbon balance were not af, fected by the Ag-loading amount. In the case of was reversed in the the order of ozone suppression decomposition efficiency and the carbon balance. This observation strongly suggests that the ozone and its decomposition intermediates on the catalyst surface may play a minor role at least in the initial step of benzene decomposition. Possible surface chemical species responsible for the VOCs decomposition and because they can be directly produced from may be the oxygen molecules adsorbed on the surface of catalysts. Further studies including in situ spectroscopic analysis of the surface of catalyst under plasma application are required. V. CONCLUSION In this study, we presented the effect of different catalysts on the decomposition of gas-phase benzene and toluene using the PDC reactors. The behavior of the tested catalysts was found to be quite different from each other according to the reactions in question. The main findings of this study can be summarized as follows. catalyst 1) The optimum loading amount of Ag on the seems to be around 2 wt%. Although the decomposition efficiency of benzene decreased as the Ag amount exceeded increased with Ag loading 2 wt%, the selectivity of amount. 4.0 wt% was also found to be effective

5)

and . Despite the in reducing the formation of cathigher conversion efficiency of benzene, the – – due to alyst seems to be less attractive than the its poor carbon balance. catalyst was found to be a very effecThe tive for the flow-type PDC system in terms of good carbon selectivity. For given catalysts, the balance and high selectivity was rather constant regardless of the SIE. – catalysts, all the tested Except for the 4.0 wt% catalysts showed a very similar trend in the formation of and . An important finding in this experimental works is role of catalyst on the decomposition of VOC. In most cases, the initial conversion of benzene was not much influenced by the type of packing materials or metal catalysts. However, the type of catalysts and the loading amount of metal catalyst greatly influenced on the carbon balance and selectivity. No correlation was observed between the outlet ozone concentration and the catalytic performances such as decomselectivity. position efficiency, carbon balance and REFERENCES

[1] A. Mizuno, J. S. Clements, and R. H. Davis, “A method for the removal of sulfur dioxide from exhaust gas utilizing pulsed streamer corona for electron energization,” IEEE Trans. Ind. Appl., vol. 22, 1986, 516-222. [2] M. B. Chang, J. H. Balbach, M. J. Rood, and M. J. Kushner, “Removal of SO from gas streams using a dielectric barrier discharge and combined plasma photolysis,” J. Appl. Phys., vol. 69, pp. 4409–4417, 1991. [3] S. Masuda and K. Nakao, “Control of NOx by positive and negative pulsed corona discharges,” IEEE Trans. Ind. Appl., vol. 26, no. 2, pp. 374–383, Mar./Apr. 1990. [4] G. Dinelli and M. Rea, “Pulse power electrostatic technology for the control of flue gas emissions,” J. Electrostat., vol. 25, pp. 23–40, 1990. [5] T. Ohkubo, S. Kanazawa, Y. Nomoto, J. S. Chang, and T. Adachi, “NOx removal by a pipe with nozzle-plate electrode corona discharge system,” IEEE Trans. Ind. Appl., vol. 30, no. 4, pp. 856–861, Jul/Aug. 1994. [6] T. Yamamoto and B. W. L. Jang, “Aerosol generation and decomposition of CFC-113 by the ferroelectric plasma reactor,” IEEE Trans. Ind. Appl., vol. 35, no. 4, pp. 736–742, Jul./Aug. 1999. [7] J. Y. Park, J. G. Jung, J. S. Kim, G. H. Rim, and K. S. Kim, “Effect of nonthermal plasma reactor for CF decomposition,” IEEE Trans. Plasma Sci., vol. 31, no. 6, pp. 1349–1354, Dec. 2003. [8] T. Oda, T. Takahashi, H. Nakano, and S. Masuda, “Decomposition of fluorocarbon gaseous contaminants by surface discharge-induced plasma chemical processing,” IEEE Trans. Ind. Appl., vol. 29, no. 4, pp. 787–792, Jul./Aug. 1993. [9] C. L. Ricketts, A. E. Wallis, J. C. Whitehead, and K. Zhang, “A mechanism for the destruction of CFC-12 in a nonthermal, atmospheric pressure plasma,” J. Phys. Chem. A, vol. 108, pp. 8341–8345, 2004. [10] A. Gal, A. Ogata, S. Futamura, and K. Mizuno, “Mechanism of the dissociation of chlorofluorocarbons during nonthermal plasma processing in nitrogen at atmospheric pressure,” J. Phys. Chem. A, vol. 107, pp. 8859–8899, 2003. [11] S. Masuda, S. Hosokawa, X. Tu, M. Tsutsumi, T. Ohtani, T. Tsukahara, and N. Matsuda, “The performance of an integrated air purifier for control of aerosol, microbial, and odor,” IEEE Trans. Ind. Appl., vol. 29, no. 4, pp. 774–780, Jul./Aug. 1993. [12] A. Mizuno, Y. Yamazaki, S. Obama, E. Suzuki, and K. Okazaki, “Effect of voltage waveform on partial discharge in ferroelectric pellet layer for gas cleaning,” IEEE Trans. Ind. Appl., vol. 29, no. 2, pp. 262–267, Mar./Apr. 1993. [13] H. Yoshida, Z. Marui, M. Aoyama, J. Sugiura, and A. Mizuno, “Removal of odor gas component utilizing plasma chemical reactions promoted by the partial discharge in a ferroelectric pellet layer,” J. Inst. Electrostat. Jpn., vol. 13, pp. 425–430, 1989. [14] M. B. Chang and T. D. Tseng, “Gas-phase removal of H s and NH with dielectric barrier discharges,” J. Environ. Eng., pp. 41–46, 1996.

994


[15] C. M. Nunez, G. H. Ramsey, W. H. Ponder, J. H. Abbott, L. E. Hamel, and P. H. Kariher, “Corona destruction: an innovative control technology for VOCs and air toxics,” J. Air & Waste Manage. Assoc., vol. 42, pp. 242–247, 1993. [16] T. Yamamoto, K. Ramanathan, P. L. Lawless, D. S. Ensor, J. R. Newsome, N. Plaks, and G. H. Ramsey, “Control of volatile organic compounds by an ac energized ferroelectric pellet reactor and a pulsed corona reactor,” IEEE Trans. Ind. Appl., vol. 28, no. 3, pp. 528–534, May/Jun. 1992. [17] S. Futamura, A. H. Zhang, and T. Yamamoto, “The dependence of nonthermal plasma behavior of VOCs on their chemical structures,” J. Electrostat., vol. 42, pp. 51–62, 1997. [18] R. G. Tonkyn, S. E. Barlow, and T. M. Orlando, “Destruction of carbon tetrachloride in a dielectric barrier/packed-bed corona reactor,” J. Appl. Phys., vol. 80, pp. 4877–4886, 1996. [19] S. Masuda, “Pulse corona induced plasma chemical process: a horizon of new plasma chemical technologies,” Pure Appl. Chem., vol. 60, pp. 727–731, 1988. [20] R. McAdams, “Prospects for non-thermal atmospheric plasmas for pollution abatement,” J. Phys. D, Appl. Phys., vol. 34, pp. 2810–2821, 2001. [21] H. Sun, H. Felix, A. Nasciuti, Y. Herieti, and W. Hoffelner, “Reduction of NO=NO & SO and destruction of VOCs & PCDD/F in industrial flue gas by electrical discharge,” Chemosphere, vol. 37, pp. 2351–2359, 1998. [22] H. H. Kim, “Nonthermal plasma processing for air pollution control: a historical review, current issues, and future prospects,” Plasma Process. Polym., vol. 1, pp. 91–110, 2004. [23] Y. S. Ko, G. S. Yang, D. P. Y. Chang, and I. M. Kennedy, “Microwave plasma conversion of volatile organic compounds,” J. Air Waste Manage. Assoc., vol. 53, pp. 580–585, 2003. [24] L. Hsieh, W. Lee, C. Chen, Y. Wu, S. Chen, and W. Wang, “Decomposition of methyl chloride by using an RF plasma reactor,” J. Hazard. Materials, vol. B63, pp. 69–90, 1998. [25] L. J. Bailin, B. L. Hertzler, and D. A. Oberacker, “Development of microwave plasma detoxification process for hazardous wastes—Part 1,” Environ. Sci. Technol., vol. 12, pp. 673–678, 1978. [26] Y. Kabouzi, M. Moisan, J. C. Rostaing, C. Trassy, D. Guerin, D. Keroack, and Z. Zakrzewski, “Abatement of perfluorinated compounds using microwave plasmas at atmospheric pressure,” J. Appl. Phys., vol. 93, pp. 9483–9496, 2003. [27] Y. F. Wang, W. J. Lee, C. Y. Chen, Y. G. Wu, and C. P. C. Chien, “Reaction mechanisms in both a CCl F =O =Ar and a CCl F =H =Ar RF plasma environment,” Plasma Chem. Plasma Proc., vol. 20, pp. 469–494, 2000. [28] B. M. Penetrante, R. M. Brusasco, B. T. Merritt, and G. E. Vogtlin, “Environ. application of low-temperature plasmas,” Pure Appl. Chem., vol. 71, pp. 1829–1835, 1999. [29] A. Ogata, D. Ito, K. Mizuno, S. Kushiyama, and T. Yamamoto, “Removal of dilute benzene using a zeolite-hybrid plasma reactor,” IEEE Trans. Ind. Appl., vol. 37, no. 4, pp. 959–964, Jul./Aug. 2001. [30] A. Ogata, H. Einaga, H. Kabashima, S. Futamura, S. Kushiyama, and H. H. Kim, “Effective combination of nonthermal plasma and catalysts for decomposition of benzene in air,” Appl. Catal. B: Environ., vol. 46, pp. 87–95, 2003. [31] U. Roland, F. Holzer, and F.-D. Kopinke, “Improved oxidation of air pollutants in a non-thermal plasma,” Catal. Today, vol. 73, pp. 315–323, 2002. [32] Y. H. Song, M. S. Cha, Y. H. Kim, K. T. Kim, S. J. Kim, S. Y. Han, and K. I. Choi, “Synergetic effects of non-thermal plasma and catalysts on VOCs decomposition,” J. Adv. Oxid. Technol., vol. 6, pp. 11–16, 2003. [33] H. Lee and M. B. Chang, “Gas-phase removal of acetaldehyde via packed-bed dielectric barrier discharge reactor,” Plasma Chem. Plasma Proc., vol. 21, pp. 329–343, 2001. [34] K. Sekiguchi, A. Sanada, and K. Sakamoto, “Degradation of toluene with an ozone-decomposition catalyst in the presence of ozone, and the combined effect of TiO addition,” Catal. Commun., vol. 4, pp. 247–252, 2003. [35] M. Magureanu, N. B. Mandache, P. Eloy, E. M. Gaigneaux, and V. I. Parvulescu, “Plasma-assisted catalysis for volatile organic compounds abatement,” Appl. Catal. B: Environ., vol. 61, pp. 12–20, 2005. [36] K. P. Francke, H. Miessner, and R. Rudolph, “Cleaning of air streams from organic pollutants by plasma-catalytic oxidation,” Plasma Chem. Plasma Proc., vol. 20, pp. 393–403, 2000. [37] H. H. Kim, S. M. Oh, A. Ogata, and S. Futamura, “Decomposition of gas-phase benzene using plasma-driven catalyst (PDC) reactor packed with Ag=TiO catalyst,” Appl. Catal. B: Environ., vol. 56, pp. 213–220, 2005.

[38] K. Yan, S. Kanazawa, T. Ohkubo, and Y. Nomoto, “Evaluation of NOx removal by corona induced non-thermal plasmas,” Trans. IEE Jpn., vol. 119, pp. 731–737, 1999. [39] K. Takaki, M. A. Jani, and T. Fujiwara, “Removal of nitric oxide in flue gases by multipoint to plane dielectric barrier discharge,” IEEE Trans. Plasma Sci., vol. 27, pp. 1137–1145, 1999. [40] B. M. Penetrante, M. C. Hsiao, B. T. Merritt, G. E. Vogtlin, P. H. Wallman, M. Neiger, O. Wolf, T. Hammer, and S. Broer, “Pulsed corona and dielectric-barrier discharge processing of NO in N ,” Appl. Phys. Lett, vol. 68, pp. 3719–3721, 1996. [41] C. R. McLarnon and V. K. Mathur, “Nitrogen oxide decomposition by barrier discharge,” Ind. Eng. Chem. Res., vol. 39, pp. 2779–2787, 2000. [42] M. Simek and M. Clupek, “Efficiency of ozone production by pulsed positive corona discharge in synthetic air,” J. Phys. D, Appl. Phys., vol. 35, pp. 1171–1175, 2002. [43] L. A. Rosocha and R. A. Korzekwa, “Advanced oxidation and reduction processes in the gas phase using non-thermal plasmas,” J. Adv. Oxid. Technol., vol. 4, pp. 247–264, 1999. [44] H. H. Kim, A. Ogata, and S. Futamura, “Atmospheric plasma-driven catalysis for the low temperature decomposition of dilute aromatic compounds,” J. Phys. D, Appl. Phys., vol. 38, pp. 1292–1300, 2005. [45] O. P. Korobeinichev, A. A. Chernov, V. V. Sokolov, and L. N. Krasnoperov, “Kinetics of destruction of diisopropyl methylphosphonate in corona discharge,” Int. J. Chem. Kinet., vol. 34, pp. 331–337, 2002. [46] B. M. Penetrante, M. C. Hsiao, J. N. Bardsley, B. T. Merritt, G. E. Vogtlin, A. Kuthi, C. P. Burkhart, and J. R. Bayless, “Identification of mechanisms for decomposition of air pollutants by non-thermal plasma processing,” Plasma Sources Sci. Technol., vol. 6, pp. 251–259, 1997. [47] H. H. Kim, H. Kobara, A. Ogata, and S. Futamura, “Comparative assessment of different non-thermal plasma reactors on energy efficiency and aerosol formation from decomposition of gas-phase benzene,” IEEE Trans. Ind. Appl., vol. 41, no. 1, pp. 206–214, Jan./Feb. 2005. [48] L. A. Rosocha, G. K. Anderson, L. A. Bechtold, J. J. Coogan, H. G. Heck, M. Kang, W. H. McCulla, R. A. Tennant, and P. J. Wantuck, Book “Treatment of Hazardous Organic Waste Using Silent Discharge Plasmas”. New York: Springer-Verlag, 1993, pt. B. [49] A. Rousseau, A. Dantier, L. Gatilova, Y. Ionikh, J. Ropcke, and Y. Tolmachev, “On NOx production and volatile organic compound removal in a pulsed microwave discharge in air,” Plasma Sources Sci. Technol., vol. 14, pp. 70–75, 2005. [50] T. Yamamoto, “Optimization of nonthermal plasma for the treatment of gas streams,” J. Hazard. Materials, vol. B67, pp. 165–181, 1999. [51] A. Ogata, K. Miyamae, K. Mizuno, S. Kushiyama, and T. Yamamoto, “Decomposition of benzene in air in a plasma reactor: effect of reactor type and operating conditions,” Plasma Chem. Plasma Proc., vol. 22, pp. 537–552, 2002. [52] H. H. Kim, S. M. Oh, A. Ogata, and S. Futamura, “Decomposition of gas-phase benzene using plasma-driven catalyst reactor: complete oxidation of adsorbed benzene using oxygen plasma,” J. Adv. Oxid. Technol., vol. 8, pp. 226–233, 2005. [53] ——, “Decomposition of benzene using Ag=TiO packed plasma-driven catalyst reactor: influence of electrode configuration and Ag-loading amount,” Catal. Lett., vol. 96, pp. 189–194, 2004. [54] S. M. Oh, H. H. Kim, A. Ogata, H. Einaga, S. Futamura, and D. W. Park, “Effect of zeolite in surface discharge plasma on the decomposition of toluene,” Catal. Lett., vol. 99, pp. 101–104, 2005. [55] H. J. White, “Electrostatic precipitation of fly ash. Precipitator equipment, precipitator case histories,” J. Air Pollution Control Asso., vol. 27, pp. 308–318, 1977. [56] J. A. Dorsey and J. H. Davidson, “Ozone production in electrostatic air cleaners with contaminated electrodes,” IEEE Trans. Ind. Applicat., vol. 30, no. 2, pp. 370–376, Mar./Apr. 1994. [57] S. Masuda, K. Akutsu, M. Kuroda, Y. Awatsu, and Y. Shibuya, “A ceramic-based ozonizer using high-frequency discharge,” IEEE Trans. Ind. Appl., vol. 24, no. 2, pp. 223–231, Mar./Apr. 1988. [58] T. Oda, “Non-thermal plasma processing for environmental protection: decomposition of dilute VOCs in air,” J. Electrostat., vol. 57, pp. 293–311, 2003. [59] D. Braun, U. Kuchler, and G. Pietsch, “Behaviour of NOx in air-fed ozonizers,” Pure Appl. Chem., vol. 60, pp. 741–746, 1988. [60] S. Dimitrova, G. Ivanov, and D. Mehandjiev, “Metallurgical slag as a support of catalysts for complete oxidation in the presence of ozone,” Appl. Catal. A: General, vol. 266, pp. 81–87, 2004. [61] H. Einaga and S. Futamura, “Catalytic oxidation of benzene with ozone over alumina-supported manganese oxides,” J. Catal., vol. 227, pp. 304–312, 2004.


[62] H. Einaga, T. Ibusuki, and S. Futamura, “Performance evaluation of a hybrid system comprising silent discharge plasma and manganese oxide catalysts for benzene decomposition,” IEEE Trans. Ind. Applicat., vol. 37, no. 5, pp. 1476–1482, Sep./Oct. 2001. [63] H. Sekiguchi, “Catalysis assisted plasma decomposition of benzene using dielectric barrier discharge,” Can. J. Chem. Eng., vol. 79, pp. 512–516, 2001.

Hyun-Ha Kim received the B.E. and M.E. degrees from Dong-A University, Pusan, Korea, both in environmental engineering, in 1994 and 1996, respectively. He received the Ph.D. degree from the Department of Ecological Engineering, Toyohashi University of Technology, Aichi, Japan, in 2000. He was with the Japan Atomic Energy Research Institute (JAERI) as a postdoctral researcher from 2000 to 2002, where he worked on the decomposition of dioxins from municipal waste incinerator flue gas using electron-beam technology. He joined the National Institute of Advanced Industrial Science and Technology (AIST) in 2002, where he has worked on nanosized aerosol formation in the gas-phase plasma chemical reaction and the abatement of hazardous air pollutants using nonthermal plasma and catalysts. His research interests include the chemical reactions involved with charged particles, atmospheric chemistry, and application of nonthermal plasma for air and water pollution control. Dr. Kim is a member of the Institute of Electrostatics of Japan, the Institute of Electrical Engineers of Japan, Japan Association of Aerosol Science and Technology, the Society of Chemical Engineering Japan, the Korean Society of Atmospheric Environment, and Korean Society of Environmental Engineering.

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Atsushi Ogata received the B.S. degree in applied chemistry from Muroran Institute of Technology, Muroran, Japan, in 1984, and the M.S. and Ph.D. degrees in chemistry from Hokkaido University, Sapporo, Japan, in 1986 and 1989, respectively. He is a Senior Researcher with the National Institute of Advanced Industrial Science and Technology (AIST), Ibaraki, Japan. His research interests are chemical processing in nonthermal plasma and environmental catalysis for the removal of air pollutants. Dr. Ogata is a member of the Institute of Electrostatics of Japan, Catalysis Society of Japan and the Chemical Society of Japan.

Shigeru Futamura (M’01) received the Ph.D. degree in organic photochemistry from The University of Tokyo, Tokyo, Japan. He was an Associate Professor in the Research Center for Advanced Science and Technology, The University of Tokyo. He is currently the Leader of Excited State Chemistry Group, Research Institute for Environmental Management Technology, National Institute of Advanced Industrial Science and Technology (AIST), Ibaraki, Japan. His research interest is application of nonthermal plasma and catalysts/photocatalysts to HAPs control and energy conversion. He has been engaged in several projects for HAPs control and fuel reforming. He is the author of over 120 published works. Dr. Futamura is a member of the Chemical Society of Japan, American Chemical Society, and the Institute of Electrostatics Japan.