1. DECOMPOSITION PROCESS OF BENZENE IN A LOW PRESSURE GLOW DISCHARGE
XV International Conference on Gas Discharges and their Applications (GD2004) in Toulouse
Pierre Baudis Centre de Congres, 5-10 September 2004
DECOMPOSITION PROCESS OF BENZENE
IN A LOW PRESSURE GLOW DISCHARGE
K. Satoh1, T. Sawada1, T. Naitoh1, H. Itoh1,
H. Tagashira1 and M. Shimozuma2
1 Department of Electrical and Electronic Engineering,
Muroran Institute of Technology, Muroran , Japan
2 School of Medicine & Health sciences, Hokkaido University,
Sapporo , Japan
Agenda 1. Background & Objective
2. Experimental apparatus and conditions
3. Results
4. Conclusions

2. Introduction Background
BENZENE --- contained in exhaust gas is one of the toxic substances which have low-degradability
has carcinogenesis
needs to be decomposed at source
Recent approaches to decompose benzene by discharge plasma
McCorkle et al. (J. Phys. D,32, 1999, 46-54, in low pressure DC glow in noble gases)
The dissociative attachment of electrons to benzene excited at high Rydberg state was the dominant decomposition process.
Goto et al. (T. IEE Japan, 123-A, 2003, in Barrier discharge in N2 - O2 at atmospheric pressure)
Nitrogen atom could contribute to benzene decomposition.
Ogata et al. (IEEE Trans. Ind. Applicat., 35, 1999, 753-8, in Packed-bed discharge in N2 - O2 at atmospheric pressure)
Benzene was destroyed by short-lived nitrogen and oxygen species, and electron impact.
In these approaches, the decomposition process and the species which contribute to benzene decomposition are suggested, BUT detailed studies to demonstrate them are not necessarily performed.

3. Introduction
In order to develop a technique for decomposing benzene by discharge plasma
It is important to clarify the decomposition processes of benzene
The species which initiate the decomposition.
The kind and quantity of the fragments and by-products.
In addition, clarifying the decomposition process of benzene has important meaning as a simulation of the decomposition process of toxic low-degradability substances which have benzene rings such as dioxins.
In this work
We minutely investigate the fragments and the by-products in low pressure DC glow discharges in benzene with the background gases(nitrogen, argon, and nitrogen-oxygen mixture) by mass spectrometry, IR spectrometry and emission spectroscopy, and then examine the decomposition process of benzene.
Benzene
Dioxin

4. Experimental apparatus

5. Conditions
The initial partial pressure of benzene is fixed at 13Pa through all experiments.
The initial partial pressure of background gases
Nitrogen : 0, 13, 54 and 120Pa
Argon : 54Pa
Nitrogen-oxygen mixture: N2:O2=54:0, 49:5 and 43:11Pa
(total pressure of the mixture is kept at 54Pa)
These gases are confined in the discharge chamber during discharge.
Discharge current : 2.5mA (constant)
Applied voltage : -300 to -1000V

6. Mass Spectra in N2-C6H6 (N2:C6H6=54:13Pa)
The mass spectra at m/z=2, 25, 26 and 27 increase after the discharge, so that these are found to be the fragments of benzene and/or by-products.
In the consideration of possible products in nitrogen-benzene mixture
The molecules of m/z=2 and 25 are H2 and C2H, respectively.
The mass spectrum at m/z=26 contains C2H2 and/or CN.
The mass spectrum at m/z=27 contains C2H3 and/or HCN.

7. Infrared absorbance spectra in N2-C6H6
(N2:C6H6=54:13Pa)
Variation of the partial pressure of benzene, C2H2 & HCN (Pa)
This result shows that C2H2 and HCN are produced in the discharge.
0s 60s
C6H
C2H
HCN

8. The variations of ion currents vs input energy in N2-C6H6 - effects of N2 -
For benzene
The variations of the signals of QMS (ion currents) agree well in spite that the partial pressure of nitrogen is changed.

9. The variations of ion currents vs input energy in N2-C6H6 - effects of N2 -
For benzene
The variations of the signals of QMS (ion currents) agree well in spite that the partial pressure of nitrogen is changed.
Looking at the emission spectra at this input energy…

10. Emission spectra in N2-C6H6 at 30J
Emission from ions, excited molecules and atoms of nitrogen is clearly observed when nitrogen is added.
However, significant difference is not observed in the variations of the mass signals of benzene.
The nitrogen species do not contribute to benzene decomposition.
Benzene is decomposed by electron collision principally.
C6H6

11. The variations of ion currents vs input energy in N2-C6H6 - effects of N2 -
For m/z=26 (C2H2 and/or CN)
As the mass signals of benzene start to decrease, the mass signals of m/z=26 simultaneously increase.
It is likely that the mass signals are due to the fragment of benzene, namely, C2H2.

12. The variations of ion currents vs input energy in N2-C6H6 - effects of N2 -
For H2
As compared with the mass signals of C2H2, the rising of the mass signals of H2 is delayed.
The mass signals tend to saturate, when the mass signals of C2H2 decrease to zero.
H2 is not a fragment of benzene, but a fragment of C2H2.

13. Comparison of mass spectra in N2-C6H6 & Ar-C6H6
In argon- benzene
Owing to the inactivity of argon, the spectra at m/z=26 and 27 are due to C2H2 and C2H3, respectively.
The mass signal of C2H3 does not increase when the discharge is turned on.
C2H3 is not a fragment produced in a discharge chamber, but in an ionized chamber of QMS.
In nitrogen-benzene (based on the result in argon)
The increase of mass signal of m/z=27 is due to HCN.

14. Comparison of mass spectra of m/z=25 & 26 in N2-C6H6 & Ar-C6H6
For the mass signals of m/z=26
The variations of these mass signals agree very well.
It is confirmed that the mass signals of m/z=26 in N2-C6H6 is due to C2H2.
For the mass signals of C2H (m/z=25 )
The ratio of the mass signal of C2H to that of C2H2 is almost constant, and the value is close to that obtained in pure C2H2.
C2H is a fragment of C2H2 produced in QMS.

15. The variations of ion currents vs input energy in C6H6 -N2-O2 - effects of O2 -
For the mass signals of C2H2 and benzene
The profiles of the mass signals do not change significantly with increasing the oxygen concentration.
Therefore, the decomposition of benzene does not depend on the oxygen concentration.
For the ion currents of m/z=28(CO) and 44(CO2)
The oxygen concentration affects on the production of by-products such as CO and CO2.

16. Conclusions
Decomposition process of benzene in low pressure DC glow discharges in benzene with the background gases of nitrogen, argon and nitrogen-oxygen mixture is investigated in this work.
Benzene is decomposed by electron collision, and C2H2, H2 and HCN(minor) are produced.
C2H2 is a principal fragment of benzene.
H2 is a fragment of C2H2.
Nitrogen and oxygen species do not contribute to the benzene decomposition.
The oxygen concentration affects on the production of by-products such as CO and CO2.