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Decomposition Of Benzene By A Low Pressure Glow Discharge
K.Satoh†, T.Sawada†, T.Naitoh†, H.Itoh† , M.Shimozuma‡ and H.Tagashira†
†
Department of Electrical and Electronic Engineering, Muroran Institute of Technology, Muroran 050-8585, Japan
‡ College of Medical Technology, Hokkaido University, Sapporo 060-0812, Japan
[email protected]
Experimental Apparatus & Procedure
Introduction
 Motivation


Discharge Chamber (stainless steel)
155mm in diameter
300mm in height
BENZENE contained in exhaust fumes of automobiles, coke furnaces, chemical
factories, etc. is a toxic substance which causes cancer and other disease, so that it
needs to be removed or decomposed before it is released in the air.
Yasui et al. (Proc. of GD2000, vol.2, pp.692-5, 2000) reported that the destruction
of benzene ring is one of the dominant reduction processes of dioxins in pulse
corona discharge, so that the information about the decomposition characteristics of
benzene in glow discharges would contribute to decomposition of dioxins using
discharge plasma.
GASTEC
measurement range :
2.5-120ppm
ANELVA M-430HG
measurement range :
13.2-0.01 10 -7Pa


ALCATEL M2005I
ultimate pressure :
1.0 10 -2Pa
pumping speed :
80/min
Photonic
Multi-Channel
Analyzer
Orifice
0.1mm
HAMAMATSU PMA-11
range : 200-950nm
resolution : <2nm
MKS 622A11TAE
3
full scale range : 1.33 10 Pa
resolution : 1.0 10 -4 F.S.
precision : 0.25%
Baratron
Manometer
Diffusion
Pump
Rotary
Pump
N2
ALCATEL M2015SD
ultimate pressure :
1.0 10 -2 Pa
pumping speed :
208/min
Rotary
Pump
Varian HS-2
ultimate pressure : 1.3 10-6 Pa
pumping speed : 285/sec
C6H6
AIR WATER
KANTO KAGAKU
purity : 99.999% purity : 99%
heated at 60
SHIMADZU FTIR-8900
range : 7800-350cm -1
resolution : 0.5,1,2,4,8,16cm
Fourier Transform
Infrared
Spectrophotometer
 C6H6(10%)-N2(90%) mixture
 Gas pressure: p=133Pa
 Gap length: d=1.0cm
 Discharge current: I=5mA
 The temporal variations of
 emission spectra is measured by Photonic
Multi-Channel Analyzer.
 benzene concentration is measured by
GASTEC.
 infrared absorption spectra of the film
deposited on a silicon wafer on the lower
electrode is measured by FTIR.
-1
 Gas-Flow Experiment
★ C6H6(8%)-N2(92%) mixture ★ Gas pressure: 8.7Pa ★ Gap length: 2.5cm ★ Discharge current: 1mA
★ The temporal variations of
 emission spectra is measured by Photonic Multi-Channel Analyzer.
 mass spectra is measured by Quadrupole Mass Spectrometer.
 Objective

B-A Gauge
Quadrupole
Mass
Spectrometer
McCorkle et al. (J. Phys. D:Appl. Phys., vol.32, pp.46-54, 1999) reported that
benzene was decomposed by DC discharges in noble gases(Ne and Ar). They
suggested that the dissociative attachment by slow electrons to the benzene
molecules in high Rydberg state could be a dominant process for benzene
decomposition.
Morris(Bulletin of The American Physical Society, vol.43, pp.1412-3, 1998)
suggested that nitrogen ion could contribute to destruction of benzene ring.
Decomposition characteristics, however, are not investigated in detail in these papers.
 Gas-Confined Experiment
KEPCO BOP 1000M
DC output range:Vmax= 1kV
Imax= 40mA
DC Power
Supply
ANELVA M200QA-M
mass range : 1-200amu
resolution : M/ M 2M
ALCATEL ATP80
ultimate pressure : Turbo
5 10 -7 Pa
Molecular
pumping speed :
Pump
N2 : 80 /sec
He : 50/sec
H2 : 40/sec
 Previous work

Electrodes (stainless steel)
60mm in diameter
parallel plate
GASTEC
Investigating the decomposition process of diluted benzene with nitrogen in a low
pressure DC glow discharge.
Results (1) – Gas-Confined Experiment
Emission spectra
Emissions-CH, H & CN
Voltage & pressure
350
400
450
wave length (nm)
500
550
50 sec
2000
CH:431.42nm
130
125
200
100
200
300
time (sec)
400
500
100
110
0
0
600
90 sec
pressure
120
115
0
600
300
infrared absorbance (a.u.)
emission of CH, Ha (a.u.)
N2:1N(0,1) 427.81nm
600
The absorption spectrum of CH is
observed from 50sec, however, the
absorption spectra of CN and NH, which
are regarded as by-products by the
interaction between benzene or its
fragments and excited nitrogen
molecules (N2*) and/or nitrogen ions
(N2+), are observed from 190sec, namely,
the time when most of the benzene is
decomposed.
400
voltage
CN:918.95nm
0
300
H 2O
CN
900
300
0
250
CH
10 sec
135
pressure (Pa)
N2:1N(0,0) 391.44nm
N2:2P(0,0) 337.13nm
N2:2P(0,1) 357.69nm
N2:2P(1,0) 315.93nm
10000
NH
500
voltage (V)
20000
4000
Ha:656.28nm
10 sec
90 sec
190 sec
600 sec
emission of CN (a.u.)
emission intensity (a.u.)
30000
Deposition
140
1200
100
200
300
time (sec)
400
500
600
Decomposition rate
Emission-nitrogen
190 sec
400 sec
CN:918.95nm
120
1N00
1.0
20000
1N00 / 2P00
15000
decomposition rate (%)
emission of 1N00, 2P00 (a.u.)
N2:1st Positive band
C2:809.51nm
NH3:791.90nm
N2:1st Positive band
N2:1st Positive band
Ha:656.28 nm
CN:694.75nm
2000
100
25000
1N00 / 2P00
Benzene is chiefly decomposed by the
collision with electrons. N2* and/or N2+
do not make a large contribution to
benzene destruction directly.
80
60
40
1800 sec
0.8
20
3500
0
600
650
700
750
800
wave length (nm)
850
900
3000
2500
2000
1500
1000
0
10000
950
0
100
200
Decomposition rate of benzene increases almost linearly
against time first, and then it tends to be constant around
200s. The profile of the decomposition rate is similar to
that of the ratio of 1N00/2P00* from which electron
mean energy can be inferred.
Electrons collide selectively with benzene molecules
first because of its large collision cross section, and
that the energy of the electrons is transferred to
benzene through inelastic collision.
*1N00:First negative(0,0) band (391.44nm)
300
time (sec)
400
500
0
600
100
200
300
400
500
-1
600
wave number (cm )
time (sec)
When benzene concentration decreases, the emissions of
1N00 and 2P00 and the ratio 1N00/2P00, namely, electron
mean energy increase.
Electrons escaping a collision with benzene molecule can
increase their energy and have excitation and ionization
collisions with nitrogen molecules.
It is estimated that the energy to decompose benzene is
lower than the threshold energies of nitrogen electronic
excitations.
The emissions of CH reaches its
maximum first, then the emissions of
Ha and CN reach their maxima, and the
gas pressure decreases when the
decomposition rate increases.
Benzene is decomposed into CH and
other molecules, and that these
fragments of benzene immediately
deposit on the electrodes or the wall.
The emission peak of Ha appears behind
that of CH.
Destruction of benzene ring is more
dominant process than dissociation of
hydrogen atom from benzene ring.
This is similar to the result obtained
by Yasui et al* for decomposition of
dioxins
*Proceedings of the XIII International Conference on Gas Discharges
and their Applications, Vol.2, 2000, pp.692-695
2P00:Second positive(0,0) band (337.13nm)
Results (2) – Gas-flow Experiment
Mass spectra
10
NH
CH3 NH3
OH
N
H
-11
H3
CH 4
120x10
discharge OFF
discharge ON
HCN
C 2H3
H2
C2H
C 2H5
H2O
C2
C 3H1
CH
C
C 3H4
C 4H1
-13
0
10
20
30
40
mass (a.m.u.)
50
60
70
80
discharge
ON
discharge
OFF
discharge
ON
discharge
OFF
-12
480
4x10
3
80
2amu
2
3amu
24amu
26amu
40
360
-12
1
CH
27amu
360
240
300
240
180
180
120
Ha
CN
Similar tendency is seen in the temporal variations
of mass spectra of 12, 13, 16 and 17amu, although
the intensities of those spectra are low and
fluctuated.
120
H
NH 3
1amu
0
420
300
60
0
Many mass spectra are observed in addition to those of nitrogen
(28amu) and benzene (78amu) molecules between 1 and 80amu,
when the discharge is not generated.
Mass spectra of 2, 3, 24, 25 and 26amu vary in the
opposite manner to that of benzene against time.
Emission spectra
discharge
OFF
25amu
C 6H6
CN
C 2H2
-12
discharge
ON
emission of H, NH3 (a.u.)
10
N2
C 2H4
-9
-10
discharge
OFF
ion currents of 3, 24 and 25amu (A)
10
discharge
ON
emission of Ha, CH and CN (a.u.)
10
10
Mass spectra
-8
ion currents of 1, 2, 26 and 27amu (A)
10
ion current (A)
H2, H3, C, CH, CH4, NH3, C2, C2H, C2H2 and
CN are produced in the glow discharge.
60
0
0
Voltage & pressure
520x10
28amu
-12
voltage
The spectrum of 28amu increases fractionally
when the discharge is generated.
800
500
18amu
2.0
480
460
13amu
16amu
1.0
15amu
440
600
pressure
400
C2H4 is produced.
8.6
200
17amu
0.5
8.7
420
12amu
0.0
100
200
300
time (sec)
400
500
400
600
voltage (V)
1.5
ion current of 28amu (A)
The benzene and nitrogen molecules effusing through the orifice
are decomposed by the electron beam (acceleration voltage:40V)
to ionize the neutral molecules in the mass spectrometer. It is,
therefore, obvious that electrons play an important role on
benzene decomposition.
8.8
-12
2.5
pressure (Pa)
3.0x10
ion currents of 12,13,15,16,17 and 18amu (A)
emission intensity (a.u.)
4000
1.2
2P00
8.5
0
100
200
300
time (sec)
400
500
600
From the emission spectra, H (1amu) is judged
a fragment of benzene
Conclusions
Decomposition process of benzene in the glow discharge is investigated in this work.
In the gas-confined experiment
 It is likely that benzene is decomposed into CH and other fragments chiefly by electrons the energy of which is lower than the threshold energies of
nitrogen electronic excitations, and then some of CH are deposited on the electrodes or wall.
 It is also likely that N2* and/ or N2+ do not make a large contribution to benzene destruction directly.
In the gas-flow experiment
 It is found that H, H2, H3, C, CH, CH4, NH3, C2, C2H, CN, C2H2, and C2H4 are produced in the glow discharge in the benzene-nitrogen mixture gas.
Acknowledgement
This work was supported by Grant-in-Aid
(No.13750236) of Japan Society for the
Promotion of Science.
Dept. of Electrical & Electronic Eng.
Muroran Institute of Technology