Journal ol
ELECTROSTATICS ELSEVIER
Journal of Electrostatics 42 (1997) 51-62
The d e p e n d e n c e of nonthermal plasma b e h a v i o r of VOCs on their chemical structures S. Futamu raa, A. H. Zhang a and T. Yamamoto b aNational Institute for Resources and Environment, 16-3, Onogawa, Tsukuba, Ibaraki, 305 Japan bBeltran, Inc., 1133 East 35 St. Brooklyn, NY 11210, USA
The dependence of nonthermal plasma behavior of volatile organic compounds (VOCs) on their chemical structures was investigated, using methan e, ethane, ethylene, butane, 1,1,2-trichloroethane, trichloroethylen e, and tetrachloroethylene as target VOCs. It has been shown that the above VOCs decompose homolytically via their excited states, irrespective of their electron affinities. Efficiencies of energy transfer from hot electrons to the VOCs could be important in the initial steps of their decomposition. Active oxygen species can be involved in the decomposition of nonchlorinated hydrocarbons under humid conditions. Byproduct distributions have been affected by residence time, carrie r gas, and humidification, depending on VOC structures.
1. INTROD UCTION
Plasma chemical VOC decomposition has attracted much attention due to its wide and convenient applicability to decomposition of volatile detergents emitted from semiconductor-making factories, indoor air cleaning, on-site decomposition of fungicides, fumigants, etc. The physical characterization and general chemistry of nonthermal plasma have already been discussed [1], and gas discharge technology has been investigated by many groups with different types of reactors [2]. However, optimization of reactor geometry and operating conditions is still one of our largest challenges since mechanisms for VOC decomposition and byproduct formation have not yet been fully understood. This paper presents decomposition mechanisms and structure-reactivity relationships for nonchlorinated and chlorinated hydrocarbons, and plasma chemical roles of oxygen sources such as BaTiO 3, water, and triplet oxygen molecules (3%), based on byprod uct analyses and thermochemical data. 0304-3886/97/$17.00 © Elsevier Science B.V. All rights reserved. P l l S0304-3886(97)00142-3
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S. Futamura et al./Journal of Electrostatics 42 (1997) 51 62
2. EXPERIMENTAL
DESCRIPTION
2.1. Characteristics of plasma reactors and experimental systems Experiments were carried out in a ferroelectric pellet packed-bed reactor. The packed-bed reactor operated with a relatively small volume fraction of plasma and a large surface area of material that could be catalytically activated by free radicals or UV irradiation from the plasma [3]. The ferroelectric packed-bed reactor employed an ac power supply at 50 Hz in conjunction with a ferroelectric (high-dielectric ceramic) pellet layer. The packed-bed reactor was a coaxial type: the inner cylindrical electrode was 16.6 mm and the outer electrode was 47.3 mm in diameter, resulting in a gap distance of 15.4 mm. Average field strengths, which will be described later, can be given as ratios of applied voltages and the gap distance between the concentric electrodes. The BaTiO3 pellets of 1 mm in diameter, were packed between the two concentric electrodes with the high ac voltage applied in the radial direction. The pellets were held by a perforated Teflon plate at both ends. The effective reactor length was 127 mm. The gas streams passed through the entry tube (6.4 mm in diameter) and dispersed into the plasma zone as shown in Fig. 1. When external ac voltage was applied across the high dielectric layer in a radial direction, the pellets were polarized, and an intense electric field was formed around each pellet contact point, resulting in partial discharge. The reactor was energized with 50 Hz ac at up to 8 kV rms. AC Power Supply BaTiO3 Pellets
w Gas Flow
Teflon Figure 1. Schematic of ac packed-bed reactor
2.2. Plasma chemical decomposition of VOCs About 1,000 ppm o f a VOC balanced with air or N 2 in a standard gas cylinder was introduced to the reactor through a Teflon tube. The flow rate was adjusted with a flowmeter. The residence time for the above reactor varied from 3.0 to
S. Futamura et aL /Journal o f Electrostatics 42 (1997) 51-62
53
8.9 s in the flow rate range of 0.5 to 1.5 L/min. Moisture was added to the gas by bubbling the sample gas through a small volume water bath. The flowing gas lowered the water temperature by a few degrees Celsius, but the estimated water vapor concentration remained near 2% by volume for all the experiments. Since the rest of the gas system remained at or above room temperature, there was no possibility of condensation. None of the VOCs except 1,1,2-trichloroethane were absorbed by water in the bubbler. As for 1,1,2-trichloroethane reactions under wet conditions, the voltage was applied after passing the reactant gas through the reactor for several minutes. With this method, 1,1,2-trichloroethane concentration in water was saturated, and that on the upstream of the reactor could be maintained at around 1,000 ppm. The VOC concentrations did not change on the upstream and the downstream of the reactor with no voltage applied. The concentrations of CO and CO 2 were overestimated for the reactions under aerated conditions when they were carried out immediately after runs under deaerated conditions. Their unexpectedly higher concentrations could be ascribed to the oxidative decomposition of carbonaceous materials deposited on the BaTiO 3 surface. To avoid this problem, after the runs under deaerated conditions, air was reacted at 7 kV for 2 min to oxidatively remove the above carbon deposits from the inside of the reactor.
2.3. Analys is The volatile byproducts were identified by GC-MS (JEOL JMS-SX102A (El, 70 eV)-HP5890 with a capillary column of DB-1 (i.d. 0.53 mm¢, length 30 m, width 3 #m)). The VOC decomposition efficiencies and organic byproduct concentrations were determined by GC (GC 353 (GL Sciences) with a capillary column of TC-1 (i.d. 0.53 mm e, length 30 m, width 5 p.m)). The concentrations of CO, CO 2, N20, methane, ethane, ethylene, and acetylene were determined by GC (GC-9A (Shimadzu) with combined columns of Porapak Q+N and Molecular Sieve 13X). 3. RESULTS AND DISCUSSION 3.1. Chemical structure-dependent VOC decomposition efficiencies Table 1 summarizes some VOC decomposition efficiencies obtained at 1.0 L/min of dry N2 at a field strength of 4.5 kV/cm. It is said that lifetimes of hot electrons generated in nonthermal plasma could range from 1 to 100 ns. Energetically, a mean energy of 4 ~ 5 eV can be achieved for hot electrons in nonthe rmal plasma with a maximum energy up to 10 eV [4], which is theoretically large enough to break any bonds in the VOCs in Table 1 (Table 2). Thus, these data suggest that energy transfer from hot electrons is not necessarily efficient in nonthe rmal plasma media containing around 1,000 ppm of VOC. Moreover, ethylene is much more reactive than ethane. This difference cannot be rationalized by a thermal reaction mechanism since the strengths of
54
S. Futamura et al./Journal o f Electrostatics 42 (1997) 51-62
the C-H bonds in ethane and ethylene are comparable and the C=C bond is much stronger than the C-C bond (Table 2). Thus, it is most likely that they decompose via their excited states since the excitation energy for ethylene (=,~*) is much lower than that for ethane ((~,(~*). A similar phenomenon was observed for 1,1,2-trichloroethane and trichloroethylen e (TCE) although the decomposition efficiency difference was much smaller than for ethane and ethylene.
Table 1 Chemical structures and decomposition effici encies of VOCs VOC Decomposition efficiency, mol% CH 4 29 CH3CH ~ 51 CH2=CH 2 96 CH3CH2CH2CH ~ 58 CI2CH-CH2CI 90 CI2C=CHCI 99 CI2C=CCI2 99 VOC concentration 1,000 ppm, 1.0 L/min of dry N2, average field strength 4.5 kV/cm
Table 2 Bond dissociation ener£1ies of selected compounds Compound Bond BDE, eV Compound OH 4 C-H 4.47 N2 CH3CH 3 C-H 4.29 02 CH3CH 3 C-C 3.80 CO CH2=CH 2 C-H 4.33 CO2 CH2=CH 2 C-C 7.45 H20 CH2=CHCI C-CI 3.63 • OH CH3CI C-H 4.38 CI2 CH3CI C-CI 3.54 HCI
Bond N-N O-O C-O C-O O-H O-H CI-CI H-CI
BDE, eV 9.75 5.11 11.10 5.45 5.11 4.40 2.48 4.43
3.2. Background effect on VOC decomposition efficiency Figures 2 and 3 show the background effect on the decomposition of TCE [5] and butane [6]. Higher TCE decomposition efficiencies were obtained in nitrogen than in air (Fig. 2). Humidification decreased the TCE decomposition efficiencies both in nitrogen and in air although their decrements were smaller in nitrogen than in air. These facts suggestthat hot electrons are quenched by 302 to suppress TCE excitation and that the superoxide anion is not reactive enough to promote TCE decomposition. It is also unlikely that active oxygen species generated from water and/or 302 are responsible for the initial step of TCE decomposition. A similar carrier gas effect observed in the decomposition of
S. Futamura et al./Journal o f Electrostatics 42 (1997) 51-62
55
butane (Fig. 3), which is much less electron-withdrawing than TCE, strongly suggests that energy transfer from hot electrons to VOCs is essential in facilitating their decomposition. Therefore, electron transfer from hot electrons in the initial step of VOC decomposition is less likely for chlorinated and nonchl orinated hydrocarbons. At lower field strengths, butane decomposition efficiencies were slightly higher under wet conditions both in nitrogen and in air. This trend, which has not been observed in TCE decomposition, can be ascribed to the direct butane oxygenation promoted by water (see below). 100 t-
,'-o~ .o_
~E 0 0..
90 80
~ ~
70
~
_
,
Wet nitrogen ~ ~
Dryair
~
Wet air
60
E
0 0
50
(3
40
, 3.5
, , , , 4.0 4.5 5.0 5.5 Field strength, kV/cm TCE concentration 1000 ppm, flow rate 1.0 L/min
3.0
Figure 2. Background effect on TCE decomposition effici ency 100 0
E 0t-
C
0
.°°
60
~
g
40
~, ~ , " ' "
e
o~o
n
.o__ Z Wet nitrogen - - -o- - - Dry air
0 0..
E
0 0 a
(3" 20
I 3.0
I
~ ~
Wet air I
,
3.5
4.0 4.5 5.0 5.5 Field strength, kV/cm Butane concentration 1000 ppm, flow rate 1.0 L/min Figure 3. Background effect on butane decomposition efficiency
S. Futamura et al./Journal of Electrostatics 42 (1997) 51-62
56
3.3. Radical mechanism governing VOC decomposition in nonthermal plasma In plasma chemical VOC decomposition, various kinds of reactive species can be generated in situfrom background gases, water, and VOC itself: radicals, radical ions, and ions. The point here is the identification of the key intermediates affecting VOC decomposition efficiency and product distribution. The product distribution obtained in TCE decomposition with 1.0 L/min of wet nitrogen at 3.2 kV/cm (Fig. 4) shows that 1,1,2,2- and 1,1,1,2tetrachloroethanes were obtained at a 3:1 molar ratio. If apparent HCI addition to TCE proceeds by an ionic mechanism, only the 1,1,2,2-isomer should be formed due to the different stabilities of the carbenium ions as intermediates (Fig. 5). Normally, ionic reactions are expected to occur more favorably under wet conditions than under dry conditions, but the above isomer distribution under wet conditions supports the hypothesis that TCE decomposition proceeds mainly by radical mechan ism.
CH2CI2
Cl3C-CHCI 2 CI2C=CHCI
3e I
~e
1,2,3~
CHCl 3
1: N ~ C - C ~ N 2: H-C m N 3: CI-C~C-H 4: Cl-CmN 5: Cl-C~C-Cl 6: Cl2C=CH 2 7: (E)-CICH=CHCl 8: (Z)-CICH=CHCl 9: Cl2CH-C~ N CI2CH-CHCI 2
c,2c=cc,, c,3c ..c, I
--* RT (min) Figure 4. Byproducts obtain ed in TCE decomposition in wet nitrogen Flow rate 1.0 L/min, average field strength 3.2 kV/cm In the decomposition of 1,1,2-trichloroethane in nitrogen, various chloro hydrocarbo ns were obtain ed as byproducts: dichlo romethane, chloro form, carbon tetrachloride, 1,1,1,2and 1,1,2,2-tetrachl oroethanes, 1,2dichloroethylenes, and TCE. These findings suggest that C-C cleavage, substitution from H to CI and from CI to H, dehydrogenation, and HCI elimination can occur in plasma. Occurrence of substitution from CI to H strongly supports a homolytic mechanism for 1,1,2-trichloroethane decomposition because nucleophilic substitution is most unlikely due to energetically unfavorable
S. Futamura et al. /Journal o f Electrostatics 42 (1997) 51-62
57
hydride formation in situ. In this reaction, the stabilities of the intermediate ethyl radicals decrease in the order: 1,1,2-trichloro-l-ethyl > 1,2-dichloroethyl ~ 1,2,2-trichloro-l-ethyl > 2,2-dichloro-l-ethyl. Interestingly, only the byproducts derived from the first three radicals were detected by GC-MS (Fig. 6). These facts indicate that chemical reaction-limiting conditions are achieved in nonthermal plasma. Ionic mechanism
+
,. CI2CH.CHCI CI2C=CHC! + H + _ _
+
/~/ : CI2C-CHzCI +
CI2CH-CHCI + CI"
- CIzCH-CHCI 2
Radical mechanism
CI2C=CHCI CI2C=CHC !
a
•
CI •
• 0 2C-CHCI 2
H.
CI2(~.CH2C I CI •
,. CI2CH-CHC! 2 CI3C.CH2C!
Figure 5. Altern alive pathways for tetrachloroethan e formation from TCE
CI2CH-CH2CI*
C I CI3C.CH2CI -I-I CI2C.CHzCI _.~ . H CI2C=CHC I H im - CI CICH.CH2CI.~ . H CICH2"CH2CI CICH=CHCI , C I CI2CH.CHCI 2 - H CI2CH.CHCI ~ CIzC=CHCI I - cL CICH=CHCI
-/CI CI2CH'CH2"-"~
H CI2CH-CH3 - H: CI2C=CH2
Figure 6. Pathways for 1,1,2-trichloroethane decomposition via polychloroethyl radicals 3.4. Roles of oxygen sources in plasma chemical decomposition of VOCs
Figure 7 shows the plots of combined concentration of CO and CO 2 against reacted carbon concentration, using the data for the VOCs listed in Table 1. Reacted carbon concentration is given by initial VOC concentration x decomposition efficiency x VOC carbon number. In dry nitrogen (Fig. 7(a)), the data points were far below the line of slope=unity except for methane. These facts can be interpreted by the oxygen deficiency in nitrogen. Oxygen could be afforded from lattice oxygens in BaTiO 3. In wet nitrogen, higher concentrations
S. Futamura et al. /Journal o f Electrostatics 42 (1997) 51-62
58
were obtained for CO and CO2 although the reacted carbon concentrations decreased• These data show that water can be activated as an oxygen source in nitrogen. In air (Fig. 7(b)), much better carbon recoveries were obtained, irrespective of humidification. These findings can be ascribed to promoted autoxidation processes in air. The above data clearly show that 302 is essential in diminishing organic byproduct formation and carbon deposit on the BaTiO 3 surface• In fact, formation of acetonitrile derived from butane decomposition was almost completely suppressed in air (Fig. 8). This was the case also for the other VOCs, viz., 302 suppressed incorporation of nitrogen and/or chlorine into byproducts. Fragment radicals from VOCs were rapidly trapped by 302to give hydrop eroxy radicals which were susceptible to further oxidative decomposition. Thus, the role of 302 is essential in cleaning gases emitted from plasma reactors.
o" o 2000
0 0 2500 (a)
E
0 2000 o .9
.•• O
1500
•
Wet N 2
.......
Butane
o
o 500
.•.'
~ 0
•
CH4
• O
o•.6 '° O
Dry air
•
Wet air
E
o
....•'•' .•.," •••.'••'•
(b)
E
3 1500
•
•-'
1000
o
.,....'"
.•"•'
0
E o
Dry N 2
...-"'•
•,,•'•
1000
O
0 0
O
0 0
t-
I I ~ p 0 500 1000 1500 2000 2500 OE (,3 Reacted carbon concentration, ppm
.•" . . i ".." • ~utane
•.•.•"
t-" 0 •
O'" •.-"O O
..m' •
500
•••.•'" ~"CH4
0 0
r 500
• Cl2CH-CH2CI
I 1000
i 1500
2000
Reacted carbon concentration, ppm
Figure 7. RelatJ onship between concentrations of reacted carbon and CO + CO2 Flow rate 1.0 L/min, average field strength 4.5 kV/cm (a) N 2 (b) air Another point regarding organic byproduct formation is the residence time effect• Acetonitrile was obtained in higher concentrations at shorter residence times, and similar trends were observed for the chlorinated byproducts. Figure 9 shows the plots of [CO2] / { [ C O ] + [CO2] } against combined concentration of CO and CO2 for the same VOCs as in Fig. 7. In dry nitrogen (Fig. 9(a)), CO 2 selectivities were less than 25%, but they were increased by humidification although its effect was less pronounced for the chlorohydrocarbons. Higher concentrations of CO and CO 2 were obtained in dry air than in dry nitrogen. Humidification also increased CO 2 selectivi~es in air although combined concentrations of CO and CO 2 were lower than in dry air. The above data show that water promotes the oxidation of CO to CO 2 although it depresses VOC decomposition effici encies in nonthe rmal plasma•
59
S. F u t a m u r a et a l . / J o u r n a l o f Electrostatics 42 (1997) 5 1 - 6 2
7.0 E
6.0 c. BO
5.0 4.0
e0 o Z
3.0 2.0
•
o
CO
"1-
1.0
0
~,
,
0.0
3.0
3.5
,~
4.0
,
4.5
~
5.0
,
5.5
Field strength, kV/cm oN2
•Air
Figure 8. Carrie r gas effect on aceton itrile formation in butane decomposition
0.7 0.6 O o +
O O O
•
0.7
(a)
CH3CH3• CH2=CH2
(b) • Butane
0.6
0.5
CH2=CH2 • • CH3CH3
O o 0.5
• Cl2CH-CH2Cl
0.4
+
Cl2C=CHCI •
0.3
• 0120=0012
O 0120=0012
0.2
O
0.1
O O
0
i I i 500 1000 1500 2000 Combined concentration of CO and CO2, ppm O
CH4
O O 0.4
O • •O CH3CH3 O O
O 0.3
qb O
0
Butane O
Dry N 2
•
Wet N 2
0.2 0
O
i i 500 1000 1500 2000 Combined concentration of CO and CO2, ppm O
Dry air
•
Wet air
Figure 9. Relationship between combined concentration of CO and CO 2 and
[co2] / { [co] + [co2] } Flow rate 1.0 L/min, average field strength 4.5 kV/cm (a) N 2 (b) air Some interesting findings were obtained for N20 formation accompanied by VOC decomposition in nonthermal plasma (Fig. 10). Under dry conditions, N20
S. Futamura et al./Journal o f Electrostatics 42 (1997) 51-62
60
concentrations were much higher in air than in nitrogen. Homogeneous gaseous reactions of excited N2 and oxygen molecules [7] could be predominant compared to the reaction of excited N2 and lattice oxygens in BaTiO 3. The data in Fig. 10 also support the hypothesis that water promotes monooxygenation of N2 in plasma as in the oxidation of CO to CO2. In air, humidification depressed N20 formation at similar levels of reacted carbon concentrations. Interpretation of this phenomenon is difficult at this stage, but deactivation of excited N2 by water could predominate over its oxygen donation in air. 500
550
(a) CI2C=CCl2
E Q. 400 •
o
E tO
300
O
t-
200
CI2C=CCI20
O CH4
Z
0
I
0
I
o I
Reacted carbon concentration, ppm Dry N2
•
Wet N2
CI2C=CCI2 •
O
CI2C=CHCI •
O
-CH4
•
Z O
500 1000 1500 2000 2500 O
350
8 250 O
O P
O O OH4
O ¢-
CI2CH-CH2CI O
100
Cl2CH-CH2Cl O
& 450
•
CH4 O tO
Cl2C=CHCl Cl2C=CCI20
(b)
150
I
I
I
0 500 1 0 0 0 1 5 0 0 2000 Reacted carbon concentration, ppm O
Dry air
•
Wet air
Figure 10. Relationship between concentrations of reacted carbon and N20 Flow rate 1.0 L/min, average field strength 4.5 kV/cm (a) N2 (b) air In the butane reactions in wet nitrogen, trace amounts of butanols and methanol were detected by GC-MS along with the epoxides such as 2,3dimethyl- and 2-ethyloxiranes. These byproducts can be formed by the direct insertion of an oxygen atom into the C-H bonds in paraffins and epoxidation of the intermediate olefins such as 1- and 2-butenes (Fig. 11). Slightly higher butane decomposition efficiencies at lower field strengths can be ascribed to butanol formation. Formation of epoxides suggests the intermediacy of oxygen atoms in situ because they cannot be formed in the reactions of OH radical with olefins. Methyl ethyl ketone and butyraldehyde are isomers of the epoxides, and higher concentrations were obtained for the epoxides and the carbonyl compounds in humidified media (Fig. 12). In the reactions of the chlorohydrocarbons, epoxides, ketone s and aldehydes were not detected by GC-MS analyses, which can be ascribed to lower reactivities of these compounds towards active oxygen species generated in plasma and also to instabilities of their oxygen-containing intermediates.
S. Futamura et al. /Journal o f Electrostatics 42 (1997) 51-62
CH4 (O)= (CH3OH)
61
. CH3OCH3
CHCH,CH=CH (0). (CHCHCH, \
c.,=c.,
OH
I
CH3CH=CHCH 3
(O).
CH2=CHCH2CH3
"-'. ~ O
- CH3-~-CH2CH 3 /
.c.,c.o = CH3.~-CH2CH 3 - CH3-~-CH2CH 3 - - H-~)-CH2CH2CH3
Figure 11. Pathways for monooxygenation of paraffins and olefins
20 o ¢..
._o E
16 12 8
c-uj
4 E 0 0
0
i
20
40 60 80 Butane decomposition efficiency, mol%
o Dry N2
• Wet N2
[] Dry air
100
• Wet air
Figure 12. Background effect on formation of methyl ethyl ketone (MEK) and butyraldehyde (BA) Flow rate 1.0 L/min
4. CONCLU SIONS
Plasma chemical reactions of nonchlorinated and chlorinated hydrocarbons were carried out to obtain insight into the dependence of nonthermal plasma behavi or of VOCs on their chemical structures. (1) VOC decomposition proceeds homolytically via its excited state, irrespective of its chemical structure.
62
S. Futamura et aL /Journal o f Electrostatics 42 (1997) 51-62
(2)302 depresses VOC decomposition efficiencies due to quenching of hot electrons caused by superoxide anion formation, but it promotes autoxidation of intermediate radicals, resulting in higher carbon recoveries and suppressed formation of organic byprod ucts. (3)Water affects VOC decomposition efficiencies and product distributions, depending on VOC chemical structures and reaction conditions although its action mechanism is complicated. Depression of VOC decomposition efficiencies caused by humidification can be ascribed to quenching of hot electrons and the excited states of VOCs and the background gases. Some oxenoid species generated from water in plasma promote decomposition of only the nonchlorinated hydrocarbons at lower field strengths by inserting oxygen atoms into the C-H bonds in them. Epoxidation of intermediate olefins and oxidation of CO to CO 2 are also promoted, but intermediates from chlorohydrocarbons are not as reactive with the above oxenoid species as those from nonchlorinated hydrocarbons. (4) Lattice oxygens in BaTiO 3 are involved in the oxidative decomposition of VOCs and nitrogen oxide formation, but their contribution is much smaller, compared to 302 and water. REFERE NCES
1. B. Eliasson, IEEE Trans. Plasma Sci., 19 (1991) 1063. 2. T. Yamamoto, Proc., Inst. Electrostatics Jpn., 19 (1995) 301. 3. U. Kogelschatz, in: B. M. Penetrante and S. E. Schultheis (Eds.), Nonthermal Plasma Techniques for Pollution Control, Springer-Verlag, NATO ASI Series, vol. 34, Part B, 1993, p. 339. 4. T. G. Beuthe and J. S. Chang, in: J. S. Chang, A. J. Kelly and J. M. Crowley (Eds.), Handbook of Electrostatic Processes, Marcel Dekker, New York, 1995, p. 161. 5. S. Futamura, T. Yamamoto and P. A. Lawless, Proc. IEEE-IAS Annual Meeting, Orlando,1995, p. 1453. 6. A. H. Zhang, S. Futamura, G. Prieto and T. Yamamoto, Proc. Workshop on Nonthermal Plasma Processing, Tokyo, 1996, p. 37. 7. B. Eliasson and U. Kogelschatz, J. Chim. Phys., 83 (1986) 279.