22P001-O
Effects of background-gas composition on phenol decomposition
by pulsed-discharge plasma above a water surface
水上パルス放電によるフェノール分解におけるバックグラウンドガスの影響
Haruki Shiota, Hideyuki Itabashi, Kohki Satoh and Hidenori Itoh
塩田晴基, 板橋秀幸，佐藤孝紀，伊藤秀範
Division of Informational and Electronic Engineering, Graduate school of Engineering, Muroran Institute ofTechnology
27-1, Mizumoto, Muroran, Hokkaido 050-8585, Japan
室蘭工業大学大学院工学研究科情報電子工学系専攻， 〒050-8585 室蘭市水元町27-1
By-products from phenol by the exposure of pulsed-discharge plasma above a phenol aqueous solution areinvestigated, and the effects of background-gas composition on phenol decomposition by the pulsed
plasma exposure above a water surface are examined. When Ar is used as a background gas, catechol,hydroquinone and 4-hydroxy-2-cyclohexene-1-on are produced. When O2 is used as a background gas,
CO2, CO, catechol, hydroquinone, formic acid, maleic acid, succinic acid and 4,6-dihydroxy-2,4-hexadienoic acid are produced. When Ar-O2 mixture is used as a background gas, by-products, producedin O2, are produced, and the quantities of the products tend to be proportional to the mixture ratio of O2.

1. Introduction
Water pollution by persistent organic pollutantsand volatile chlorinated organic compounds isserious problem. Species, having high oxidation
potential, such as OH, H2O2 and ozone, can beproduced when pulsed-discharge plasma is
generated above or in water, and those species havepotential to decompose the pollution, water
treatment techniques using a pulsed discharge haveattracted attention. In this work, we minutelyinvestigate the by-products of phenol decomposedby pulsed-discharge plasma generated above a
phenol aqueous solution when Ar, O2 and Ar-O2
mixture are used as a background gas, and deduce
decomposition processes of phenol from theby-products. Further, we investigate effects ofbackgroundgas
compositiononphenoldecomposition.
2. Experimental apparatus and procedure
A needle electrode and a water bath electrode,made of stainless steel, are placed in a cylindricaldischarge chamber made of acrylic resin, having
140 mm in inner diameter and 100 mm in height.
The needle electrode is 1.5 mm in diameter and 19mm in length, and the water bath electrode, whichis earthed, is 119 mm in inner diameter and 12 mm
in depth, having 0.13 L in capacity. 70 g of phenol
aqueous solution with a concentration of 3000 ppm
is poured into the water bath, and the distance
between the needle electrode and the surface of thephenol aqueous solution is fixed at 4 mm. Ar andO2, the purities of which are 99.99% and 99.5 %,respectively, are used as a background gas, and it is

fed into the discharge chamber at a constant flowrate of 1 L/min. The mixture ratios of thebackground gas are Ar:O2=100:0, 95:5, 90:10,50:50 and 10:90.
Liquid samples are taken before and after theplasma exposure, and there are analyzed by a Gas
Chromatograph Mass Spectrometer (Shimadzu,GCMS-QP2010 Plus, column: DB-17ms). Further,
off-gas from the discharge chamber is analyzed by a
Fourier Transform Infrared Spectrophotometer(Shimadzu, FTIR-8900).
3. Results and Discussion
In a previous work [1], it is found that catechol,hydroquinone and 4-hydroxy-2-cyclohexene-1-on
are produced from phenol when Ar is used as a
background gas, and that formic acid, maleic acid,succinic acid, 4,6-dihydroxy-2,4-hexadienoic acid,catechol and hydroquinone are produced fromphenol when O2 is used as a background gas. Also,
CO2 and CO are found in the off-gas from thedischarge chamber only when O2 is used as abackground gas.
Figure 1 shows the decomposition process ofphenol when Ar is used as a background gas. No
ozone production is observed, so that OH radicals
generated in the plasma can initiate thedecomposition of phenol. Also, 4-hydroxy-2-cyclohexene-1-on is detected in this work, it islikely
thatphenolisconverted
into
4-hydroxy-2-cyclohexene-1-on at first by thereaction
withOHradical,
andthat
4-hydroxy-2-cyclohexene-1-on is converted intocatechol and hydroquinone. Since catechol and4x106 10060Ar [%]4020024003002001gas.

0Fig. 1. Decomposition process of phenol when Ar is used
as a background gas.

1000204080

Ar [%]
6040O2 [%]60800100200(a)4x106 100AREA

2hydroquinone can be produced through anotherprocess, which is not clarified, because catechol andhydroquinone are detected when Ar-O2 mixture gas
is used as a background gas, at which no4-hydroxy-2-cyclohexene-1-on is produced. Figure
2 shows the decomposition process of phenol whenpure O2 is used as a background gas. Since ozone isproduced when O2 is used as a background gas,
phenolisprobably
decomposed
into
4,6-dihydroxy-2,4-hexadienoic acid by 1,3-dipolar
additionreaction
withozone.
Further,
4,6-dihydroxy-2,4-hexadienoic acid is decomposed
into two fragments, which contain two and four
carbon atoms, namely, maleic acid or succinic acidand oxalic acid, by 1,3-dipolar addition reaction
with ozone. Oxalic acid is not detected in this work,
but formic acid, CO2 and CO are produced bydecomposition of oxalic acid as shown in reactions(1) and (2)[2,3]. Therefore, oxalic acid can beproduced by phenol decomposition.
HOOCCOOH → HCOOH ＋ CO2(1)
HOOCCOOH → CO2 ＋ CO + H2O(2)
Figure 3 shows the concentration of ozone andby-products as functions of mixture ratio of Ar-O2.
Ozone and by-products produced by cleavage ofbenzene ring in figure 3 (a) decrease with thedecrease of O2 mixture ratio. In Ar, ozone andby-products show in figure 3 (a) are not produced,
so that it is considered that 1,3-dipolar addition
reaction with ozone can cleave a benzene ring of400
300
200

10500

10002040O2 [%]6080

concentration [ppm]catecholhydroquinone
4-hydroxy-2-cyclohexene-1-on

3Fig. 2. Decomposition process of phenol when O2 is used
as a background gas

500
concentration [ppm]5-(2-hydroxyethylidene)-2-(5H)-furanone
formic acidozone
maleic anhydride
succinic anhydride

3AREA

800100(b)Fig. 3. Concentration of O3 and by-products as functionsof mixture ratio of Ar-O2

phenol, but OH radical cannot cleave the benzene
ring. By-products in figure 3 (b) decreases
significantly by the mixture of O2 into Ar, so thatthe decomposition process of phenol can bedramatically changed by the mixture O2 into
background gas.
4. Conclusions
We have investigated the decomposition processof phenol by exposure of pulsed-discharge plasma
above a phenol aqueous solution, and effects ofbackgroundgas
compositiononphenoldecomposition. In Ar, 4-hydroxy-2-cyclohexene-1on can be produced by OH radical. Further,4-hydroxy-2-cyclohexene-1-on can be converted
into catechol and hydroquinone. In O2, phenol isprobably decomposed into 4,6-dihydroxy-2,4hexadienoic acid by 1,3-dipolar addition reaction
with O3, and 4,6-dihydroxy-2,4-hexadienoic acidcan be decomposed into oxalic acid, maleic acidand succinic acid by O3, and oxalic acid is probably
decomposed into formic acid, CO2 and CO.
References[1] K. Satoh et al.: 64th Annual Gaseous Electronics
Conference., 56, FTP1.00031 (2011)[2] G.. Lapidus et al.: J. Phys. Chem., 67 (1964) 1863
[3] T. Kakumoto et al.: J. Phys. Chem., 91 (1987) 2366