oxidative decomposition of chlorinated hydrocarbons by glow discharge in pact (plasma and catalyst...

Applied Catalysis A: General 219 (2001) 25–31

Oxidative decomposition of chlorinated hydrocarbonsby glow discharge in PACT (plasma and catalyst

integrated technologies) reactors

Xiao Chen a, Jeff Rozak b, Jung-Chou Lin b, Steven L. Suib a,b,c,∗,Yuji Hayashi d, Hiroshige Matsumoto e

a Department of Chemistry, University of Connecticut, Storrs, CT 06269, USAb Department of Chemical Engineering, University of Connecticut, Storrs, CT 06269, USA

c Department of Chemistry and Chemical Engineering, Institute of Material Science, University of Connecticut,215 Glenbrook Road, Storrs, CT 06269 3060, USA

d Fujitsu Laboratories Ltd., 1015 Kamikodanaka, Nakahara 211, Japane Department of Chemistry, Nagasaki University, Bunkyomachi 1-14, Nagasaki 852, Japan

Received 30 October 2000; received in revised form 1 May 2001; accepted 4 May 2001

Abstract

The oxidative decomposition of methyl chloride (CH3Cl) and methylene chloride (CH2Cl2) by glow discharge has beencarried out in a new type of reactor using plasma and catalyst integrated technologies (PACT) in our laboratory. The PACTreactors with metal (Fe, Pt, or Au) coated electrodes were used in our experiments at room temperature and atmosphericpressure. Feed compositions were 5000 ppm of either CH3Cl or CH2Cl2 and 5% of oxygen with a balance of helium and atotal flow rate of around 20 ml/min. Significant conversions (as high as 90%) of CH3Cl or CH2Cl2 have been achieved for theoxidative decomposition of CH3Cl or CH2Cl2 using PACT reactors. © 2001 Elsevier Science B.V. All rights reserved.

Keywords: Plasma; Chlorinated hydrocarbons; Decomposition; Methyl chloride; Methylene chloride

The increased production and application of chlori-nated hydrocarbons as solvents have led to consider-able research on catalytic oxidative decomposition ofchlorinated hydrocarbons [1–7] to prevent pollutionproblems caused by chlorinated hydrocarbons. Somenon-thermal plasma-assisted decompositions of halo-genated hydrocarbons have been reported in recentliterature [8–12]. Either high-energy electron beamirradiation or electrical discharge processing methodswere applied to generate these non-thermal plasmas,which have been considered as highly efficient media

∗ Corresponding author. Fax: +1-860-486-2981.E-mail address: [email protected] (S.L. Suib).

for use of electrical energy to excite gaseous reac-tants into highly ionized excited states. Heating in theorder of 10,000 K is required to reach a comparablelevel of gas excitation by common thermal means.Rosocha et al. have demonstrated that the atmosphericdielectric barrier discharge is a feasible method fordestroying halogenated hydrocarbon wastes [13].

Recently, the oxidative decomposition of methylchloride (CH3Cl) and methylene chloride (CH2Cl2) byglow discharge has been carried out in a new type ofreactor [14] using plasma and catalyst integrated tech-nologies (PACT). The structure of the reactor is shownin Fig. 1 . The basic principle of the reactor involvesmetal coated electrodes as catalysts for activation of

0926-860X/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved.PII: S0926 -860X(01 )00644 -5

26 X. Chen et al. / Applied Catalysis A: General 219 (2001) 25–31

Fig. 1. Cross-section of a PACT reactor.

reactants. The glow discharge between the stator androtor electrodes where a high alternating current (ac)voltage is applied produces a plasma for excitation ofmolecules. The synergistic effects of catalytic activa-tion and plasma excitation are expected to exist in thereactor. The volume of the plasma zone in the reactoris about 0.26 cm3.

PACT reactors with metal (Fe, Pt, or Au) coatedelectrodes (M-PACT reactors) were used in our exper-iments at room temperature and atmospheric pressure.Feed compositions were 5000 ppm of either CH3Cl

Table 1Results for oxidative decomposition of CH3Cl in PACT reactors with Fe or Au coated electrodesa

Run no. Voltage dialsetting on UHV-10b

Vsc (V) Vg

d (V) tge (�s) ConversionCH3Clf (%)

Specific energyg

(kJ/mol)

1b,h 20 750 500 14 71 5.27 × 104

2b,h 30 750 500 25 90 7.82 × 104

3b,h 35 750 500 29 90 9.65 × 104

4h,i n.a.j 180 70 1.80 × 104 45 n.a.j

5b,k 20 1000 500 12 79 4.74 × 104

a Feed conditions — CH3Cl: 5000 ppm; O2: 5%; balance gas: He. Total flow rate: 20 ml/min. Detected products: CO, CO2, H2O, HCl.b With a UHV-10 high frequency (8.1 kHz) ac power supply.c Breakdown voltage.d Normal glow voltage.e Time length of normal glow range.f Conversion of CH3Cl.g Energy consumed in the reactor.h With a PACT reactor with Fe coated electrodes.i With variac and transformer as ac power supply (frequency: 60 Hz).j n.a.: not applicable.k With a PACT reactor with Au coated electrodes.

or CH2Cl2 and 5% of oxygen with a balance of he-lium and a total flow rate of 20 ml/min. The reactionwas started by applying a high ac voltage, which isproduced by an ac power supply, between the statorand rotor electrodes of a PACT reactor. The voltagewaveforms between the stator and rotor electrodes inthe reactor were recorded with a Yokogawa (Yoko-gawa Electric Corporation, Japan) DL1540 digitaloscilloscope during reaction. The total reaction timeis around 2–3 h. The outlet gases were analyzed witha Perkin-Elmer Sigma 3 gas chromatograph (GC)

X. Chen et al. / Applied Catalysis A: General 219 (2001) 25–31 27

equipped with a Porapak Q (100/120 mesh) columnand a thermal conductivity detector. The formation ofcarbon monoxide in the products was observed withIR spectroscopy.

Table 1 shows the experimental results for oxidativedecomposition of CH3Cl. The formation of carbondioxide and water in the outlet gas after reactionwere detected by GC. The IR spectrum of the outletgas is shown in Fig. 2. Absorption bands at 2170and 2120 cm−1 indicate the formation of carbonmonoxide in the products and are much weaker thanabsorption bands at 2340 and 2360 cm−1 which aredue to CO2. This suggests that CO formed in the reac-tion is much less than CO2. Experimental results withcommercially available Drager gas detector tubes forhydrochloride (HCl) and chlorine gas (Cl2) showed

Fig. 2. The IR spectrum of outlet gas mixture from CH3Cl oxidation.

that most of the chlorine (Cl) in CH3Cl or CH2Cl2was converted into HCl and no Cl2 was detected. Thisis consistent with stoichiometric expectations andexperimental results in [6,7,15,16].

A UHV-10 type high frequency (8.1 kHz) powersupply (made by Nihon Inter Electronics Corporation,Japan) was used to produce high frequency (8.1 kHz)ac high voltage across the Fe coated electrodes in theFe-PACT reactor during plasma runs 1–3. The outputvoltage of the power supply can be increased directlyby increasing the voltage dial setting.

Fig. 3 shows the recorded waveforms of voltageacross the electrodes for experiments before plasmaignition and during plasma runs 1–3. When the volt-age dial setting on the UHV-10 power supply was 15,the voltage across the electrodes was not high enough


Fig. 3. The voltage waveforms for a Fe-PACT reactor. (I) Voltagedial setting on UHV-10: 15; Vmax: 690 V. (II) Voltage dial settingon UHV-10: 20; Vs: 750 V; Vg: 500 V. (III) Voltage dial settingon UHV-10: 30; Vs: 750 V; Vg: 500 V. (IV) Voltage dial settingon UHV-10: 35; Vs: 750 V; Vg: 500 V.

to produce a glow discharge between the electrodesand the voltage waveform was roughly in sin form, asshown by trace I in Fig. 3. On increasing the voltagedial setting to 20, the voltage across the electrodeswas high enough to produce a glow discharge betweenthe electrodes and its voltage waveform is shown bytrace II in Fig. 3.

After the voltage across the electrodes was increa-sed from 0 to 750 V at point A on voltage waveformII, there was a voltage collapse across the electrodesand then the voltage during the period between B andC was relatively stable compared to the same periodfor voltage waveform I where there was no glow dis-charge. The voltage at point A on voltage waveform

II is referred to as the breakdown voltage Vs (the volt-age at which a glow discharge starts to form acrossthe electrodes when the voltage potential betweenthe electrodes is high enough). The relatively stableperiod between B and C is referred to as the normalglow range. The average voltage and the length oftime during the normal glow range are defined as thenormal glow voltage (Vg) and the normal glow rangetime (tg), respectively.

When the voltage dial setting on the UHV-10 powersupply was increased from 20 to 35, tg increased from14 to 29 �s, although Vs and Vg were the same forruns 1–3, as shown in Fig. 3 and Table 1. Interestingly,the conversion of CH3Cl also increased as shown inTable 1. The results for oxidative decomposition ofCH2Cl2 in the PACT reactor with Pt coated elec-trodes show the same trend as shown in runs 6–8in Table 2. The conversion of CH3Cl or CH2Cl2 in-creases with increasing tg when other conditions aresimilar.

A variac and transformer were combined and usedas a power supply to produce low frequency (60 Hz) achigh voltage across the Fe electrodes in the Fe-PACTreactor in run 4 in Table 1. A normal glow range withlower Vg and much longer tg was formed at lower Vs(180 V) when using the low frequency power supplyin run 4 compared to the results for runs 1–3 wherethe UHV-10 high frequency (8.1 kHz) power wasused. The conversion of CH3Cl is much lower inrun 4 than in runs 1–3. This is probably because theglow discharge that was formed at lower Vg in run4 produced less excitation of the reactants comparedto the glow discharge formed in runs 1–3. Note thata longer normal glow discharge can be achieved atmuch lower Vs by using the combination of variacand transformer than using the UHV-10 type highfrequency power supply.

A PACT reactor with Au coated electrodes was usedin run 5 in Table 1. A higher Vs was needed to forma glow discharge with almost the same Vg and tg inthe reactor in run 5 in comparison to experiments ofrun 1. The conversion of CH3Cl was higher in run 5than in run 1. These data suggest that different metalcoated electrodes of the PACT reactors have differentactivities for oxidative decomposition of halogenatedhydrocarbons. The Au coated electrodes used for run5 show a larger catalytic effect than the Fe coatedelectrodes used for run 1.


Table 2Results for oxidative decomposition of CH2Cl2 in the PACT reactor with Pt coated electrodesa

Run no. Feed flowrate

Voltage dial settingon UHV-10b

Vsc (V) Vg

d (V) tge (�s) ConversionCH2Cl2f (%)

Specific energyg

(kJ/mol)

6 20 20 600 500 20 45 8.31 × 104

7 20 30 600 500 26 60 1.17 × 105

8 20 35 600 500 30 60 1.45 × 105

9 13 35 600 500 30 80 1.67 × 105

10 30 35 600 500 30 60 9.65 × 104

a Feed conditions — CH2Cl2: 5000 ppm; O2: 5%; balance gas: He. Total flow rate: 20 ml/min. Detected products: CO, CO2, H2O, HCl.b With a UHV-10 high frequency (8.1 kHz) ac power supply.c Break down voltage.d Normal glow voltage.e Time length of normal glow range.f Conversion of CH2Cl2.g Energy consumed in the reactor.

Similar metal effects have been observed whenglow discharges in the PACT reactors are employedfor the activation of CO2, NO, and H2O in the pres-ence of metal catalysts coated onto the electrodesurfaces [17,18]. The electron temperatures of theplasmas of various metal electrodes are different andaffect the activation of CO2, NO, and H2O. Similarstudies indicate increased reactivity for metal coatedtubular PACT reactors relative to glass coated reactors[19]. The catalytic effects of metals have also beenreported in [20,21]. These effects have been attributedto the synergistic effects of both catalytic activationand plasma excitation.

Helium is the main component of the feed toPACT reactors and is probably excited initially viacollisions with electrons, which are introduced bythe high ac voltage, in the plasma zone. The excitedhelium species will likely transfer their energy, viacollision processes, to the introduced O2 and CH3Cl(or CH2Cl2) molecules, which are sequentially acti-vated into their excited states. The probability thatO2 and CH3Cl (or CH2Cl2) molecules are exciteddirectly in the plasma zone is much smaller dueto their much lower populations when compared tohelium molecules. The excited O2 and CH3Cl (orCH2Cl2) molecules can either react with each otherto form CO2 and HCl, or be adsorbed on the surfaceof metal coated electrodes and involved in catalyticoxidation reactions. Detailed diagnostic optical emis-sion spectroscopic (OES) studies of plasma speciesand their interaction with metal surface are neededand currently in progress. In order to understand

the mechanism of the oxidative decomposition ofCH3Cl or CH2Cl2 in PACT reactors and observedmetal effects.

The possible energy transfer processes of excitedHe to O2 and CH3Cl (or CH2Cl2) molecules are sim-ilar to the case of CO2 activation in PACT reactorsinvestigated with OES studies [17,18]. The OES stud-ies reveal that emission intensities of all excited Hespecies (such as He+ and He2

+) in the plasma zone de-crease in different proportions after CO2 is introducedinto the system. This suggests that excited He specieswill selectively transfer their energy to the introducedCO2 molecules and activate them into excited states,such as CO+ and CO2

+, as detected by OES.He has higher energy transfer efficiency in plasma

activation processes in PACT reactors and yieldshigher reaction activity, compared to other diluentgases, such as Ar and N2 [18]. Therefore, He waschosen as the diluent gas in this preliminary research.Other diluent gases, especially N2, will be investi-gated in order to apply PACT reactors to the oxidativedecomposition of halogenated hydrocarbons underreal environmental conditions.

When the total feed flow rate was increased from13 ml/min in run 9 to 30 ml/min in run 10 in Table 2,although Vs, Vg, and tg are still the same, the conver-sion of CH2Cl2 decreased from 80 to 60%. This isprobably due to a shorter residence time of reactantsat the higher flow rate of the feed.

Specific energies, in kJ/mol of converted reactant,consumed in the reactors for the various conditions andgases tested are shown in Tables 1 and 2. The specific


Table 3Comparison of specific energy requirements for plasma dehalogenation processes

Author Gas Concentration (ppm) Flow rate (ml/min) Conversion (%) Specific energy range (kJ/mol)

Chen CH3Cl 5000 20 71–90 4.74 × 104 to 9.65 × 104

Chen CH2Cl2 5000 20 45–80 8.31 × 104 to 1.67 × 105

Oda CF2ClCFCl2 100–1000 471–196 99 5.29 × 105 to 2.21 × 106

Krause CCl3CH3 1310 1590 75–99 2.81 × 105 to 4.25 × 105

Hsiao CHClCCl2 160 20000 50–84 3.41 × 104 to 8.71 × 104

Nakagawa CCl2F2 1572 Batch 5–20 2.00 × 104 to 3.33 × 105

energy consumed for CH3Cl ranges from 4.74 × 104

to 9.65 × 104 kJ/mol. The specific energy consumedfor CH2Cl2 is ranging from 8.31 × 104 to 1.67 ×105 kJ/mol, which is higher than the specific energyrequirement for CH3Cl.

The additional chlorine in CH2Cl2 increases theenergy requirement and decreases the conversion,which is consistent with the thermodynamic data. Thecarbon chlorine bond strength is 397 kJ/mol whilethe carbon hydrogen bond strength is 338 kJ/mol[22]. Also, the enthalpy of dissociation of CH3Clis 81.9 kJ/mol while the enthalpy of dissociation forCH2Cl2 is 95.6 kJ/mol [22]. Using the same reac-tor, gas concentrations, flow rate, and power settingsthe CH2Cl2 will have a lower conversion and higherspecific energy requirement than CH3Cl due to theadditional chlorine.

The specific energy requirements for this reactorsystem are comparable to those of Krause [23], Oda[24], Hsiao [12], and Nakagawa [11]. Table 3 lists therange of specific energy requirements, conversions,and gases tested of Krause [23], Oda [24], Hsiao [12],and Nakagawa [11]. Their work included decom-position of chlorocarbons and chlorofluorocarbons(CFC) in an atmospheric pressure plasma [11,12,23].Nakagawa’s work used high energy electron beamsfor the decomposition of CFC’s [11]. Kraus’s plasmawas initiated in a vacuum before increasing thepressure to 1 atm [23].

In summary, significant conversion of CH3Cl orCH2Cl2 has been achieved for the oxidative decom-position of CH3Cl or CH2Cl2 using PACT reactors.Ultimately our goal is to apply novel plasma andcatalysis integrated technologies to the oxidativedecomposition of a mixture of halogenated hydro-carbons for pollution prevention under environmentalconditions.

Acknowledgements

We acknowledge the University of Connecticut,Fujitsu Limited, Hokushin Corporation, and HondaR&D Co. Ltd., for support of this research. We alsothank Prof. K. Itoyama for helpful discussions.

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oxidative decomposition of chlorinated hydrocarbons by glow discharge in pact (plasma and catalyst...

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