1. Introduction Motivation Objective
Benzene : air pollutants contained in exhaust gases of automobile, chemical factories, petrol station etc.
: a carcinogenic substance
Benzene is needed to be removed or decomposed.
Recent report for decomposition of dioxins (H.Yasui et al, Proc. of GD2000, vol.2,pp.692-5)
Dioxins are decomposed by dissociation of C-O
bond and destruction of benzene ring.
The data obtained by decomposition of benzene
can make contribution to decomposition of dioxins.
Objective
To decompose benzene by a low pressure glow discharge in nitrogen, to investigate the decomposition characteristics and to estimate the decomposition process.
DC glow discharge is generated in nitrogen-benzene mixture gas and the temporal variation of the emission from the discharge is measured by Photonic Multi-Channel Analyzer. Also, concentration of benzene is measured by GASTEC and decomposition rate is deduced.

2. Experimental apparatus

3. Experimental conditions
total gas pressure
p (Torr)
discharge current
I (mA)
gap length
d (mm)
partial pressure of benzene
k (%)
discharge time
T (s)
0.5
5.0 10
1 ~ 100
1500
3000 15
10 ~ 40
7.5
10 ~ 30
1.0
10 ~ 70
1.5
6 ~ 60
1800
2.0
5 ~ 50
2100

4. The temporal variations of emission intensity #01
p=0.5Torr, k=20%, d=10mm, I=5mA
1n(0,0)band
2p(0,0)band Ha
1p(2,0)band CN C2
1p(1,0)band
N2:2p(0,0)band
N2:1n(0,0)band
N2:1n(0,1)band
N2:2p(0,1)band
N2:2p(1,0)band
Ha:656.28nm
CN:694.75nm
CN:708.04nm
CN:787.27nm
CN:828.55nm
CN:918.95nm
C2:809.51nm
benzene
C2:516.52nm
C2:486.02nm
CH:431.42nm
: after s
: after 200s
: after 600s
: after 1000s

5. The temporal variations of emission intensity #02
p=0.5Torr, k=90%, d=10mm, I=5mA
Benzene
2p(0,0)band
2p(0,1)band
1n(0,0)band
1n(0,1)band C2 CH Ha
N2:2p(0,0)band
N2:1n(0,0)band
N2:1n(0,1)band
N2:2p(0,1)band
N2:2p(1,0)band
Ha:656.28nm
C2:516.52nm
C2:486.02nm
CH:431.42nm
: after 0s
: after 20s
: after 60s
: after 100s
benzene

6. The temporal variations of gas pressure, Vg and emission intensities
gas pressure,p
CH:431.42nm
applied voltage,Vg
Ha:656.28nm
CN:918.95nm
N2:2nd positive(0,0)band,2p
2p/1n
N2:1st negative(0,0)band,1n

7. The temporal variations of gas pressure, Vg and emission intensities
CH:431.42nm
Energy dissipation between time 0 and T’
pressure
partial pressure
of benzene
: 1%
:10%
:20%
:30%
:40%
:50%
:60%
Ha:656.28nm
CN:918.95nm
T’ for pressure and emission of Ha
The time when the temporal variations become constant.
for emissions of CH & CN :
The time when the temporal variations
reach their maxima.

8. The temporal variation of decomposition rate of benzene
p=0.5Torr, k=20%, d=10mm, I=5mA ① ② ③ ④ ⑤ ⑥ ⑦
voltage
sampled time (s)
residual benzene(mg/l)
decom-position rate (%) ①
0.459 ② 30
0.428
6.8 ③ 50
0.252
45.1 ④ 70
0.189
58.8 ⑤ 85
0.126
72.5 ⑥
155
0.041
91.1 ⑦
220
0.038
91.7
pressure
CH:431.42nm
CN:918.95nm

9. Decomposition rate of benzene
p=0.5Torr, d=10mm, I=5mA, T=1500s
partial pressure of benzene k(%) [mTorr]
quantity of benzene(mg/l)
decom-position rate(%)
initially
after discharge
1% [ 5]
0.021
0.015
28.6
10% [ 50]
0.207
0.025
87.9
20% [100]
0.413
0.033
92.0
30% [150]
0.621
0.028
95.5
40% [200]
0.829
0.035
95.8
50% [250]
1.043
0.022
97.9
60% [300]
1.247
0.047
96.2

10. Decomposition rate of benzene as a function of the partial pressure

11. Residual benzene after discharge & energy dissipation for decomposition

12. Decomposition process of benzeneCH
C6H eintermediate C2H eslow
(6~11eV) C4H4
decomposition
Benzene is decomposed by electron collision (the process is shown above).
Assuming that the dissociation cross section of benzene is much larger than the cross sections of nitrogen, electrons collide mainly with benzene.
Decomposition of benzene has almost finished.
The cross section of benzene is larger, but the number density of benzene decreases. Therefore, electrons can collide with nitrogen molecules.
electron energy
Electrons have enough energy (6~11eV) to decompose benzene, but the energy is lower than the threshold of N2*(C3Pu)
Electron has enough energy to create N2*(C3Pu) and N2+(ei=15.5eV)
by-product
CH and (CH)n (?) are immediately deposited on the electrodes and wall.
CN, H etc. are created by the interaction between CH, (CH)n and electron, N2* and N2+

13. Dennis L McCorkle et al XYZ : VOC molecules e : electron RG : rear gas
pressure
CH & Ha
2p(0,0)band & 1n(0,0)band
decomposition rate
2p/1n
intensity (a.u.)
decomposition
rate (%)
gas pressure
(mTorr)
Dennis L McCorkle et al
XYZ : VOC molecules
e : electron
RG : rear gas
* : excitation state
m : metastable state
HR : high Rydberg states
(J.Phys.D:April.Phys., Vol.32, 1999, pp.46-54)

14. Conclusions
The spectra of CN, C2, Ha, CH etc. which are regarded as the fragments of benzene, are measured in DC glow discharge in nitrogen-benzene mixture.
Benzene is decomposed by the low pressure glow discharge in nitrogen.
When benzene is being decomposed, the gas pressure and the emission intensities of CH, Ha, CN etc. change temporally, and when benzene is almost decomposed, the temporal variations of the gas pressure and the intensities tend to be constant.
The removal of benzene can be judged by monitoring the variations of the gas pressure and the intensities.
For K > 20%, decomposition rate is more than 90%,
K < 20%, decomposition rate decreases swiftly.
Electron collision can be dominant process for the decomposition of benzene in the experiment.