Decomposition of methanol in a low-pressure DC glow discharge
in nitrogen-oxygen mixture
Ayako
1
Katsumata ,
Kohki
1,2
Satoh
and Hidenori
1
Itoh
1Department
of electrical and Electric Engineering, Muroran Institute of Technology, 27-1 Mizumoto, Muroran 050-8585, Japan
2Center of Environmental Science and Disaster Mitigation for Advanced Research, Muroran Institute of Technology, 27-1 Mizumoto, Muroran 050-8585, Japan
1.Introduction
2.Experimental apparatus & conditions
Background
Experimental apparatus
 Methanol is one of the most commonly used volatile organic compounds (VOCs) for adhesive,
paint, degreasing, etc.
 Due to the low toxicity of methanol, only the concentration has been controlled; however, the
quantity also has been limited under the new regulation[1].
 Gas-cleanup using discharge plasmas has recently attracted attentions because chemically active
species and UV radiation, which can initiate or assist the decomposition of the hazardous gases
like VOCs, are generated in the discharge plasmas.
 Electrodes
• Parallel-plate electrodes of 60mm diameter and 20mm separation.
• The lower electrode is earthed and a negative DC voltage is applied to the
upper electrode to generated a glow discharge.
 Discharge chamber
• 155mm inner diameter and 300mm height. Earthed.
 Mass spectrometry
• Gas samples are extracted from the glow discharge through a 0.1mm
diameter orifice fitted at the center of the lower electrode, and the mass
spectra of the samples are obtained using a Quadrupole mass spectrometer
(QMS).
 Emission spectroscopy
Objective
 To investigate gaseous products in a low-pressure DC glow discharge in nitrogen-oxygenmethanol mixture, and examined the influence of oxygen additive on the decomposition
characteristics of methanol, to develop the plasma cleaning of exhaust gases containing methanol.
• The optical emission of the glow discharge is measured by a Photonic MultiChannel Analyzer (PMA).
 Infrared spectroscopy
• A gas sample is extracted into a long-path optical cell immediately after the
glow discharge is switched off, and an infrared absorption spectrum is
obtained using a Fourier Transform Infrared Spectrophotometer (FTIR).
 Electrical-energy
 It is straightforward to examine the decomposition process of methanol in an atmospheric
pressure discharge.
 However, we selected the low-pressure DC glow discharge in order to clarify the products from
methanol in detail because the low-pressure glow discharge is stable and the diagnostics of the
discharge can be done easily, compared with that of the atmospheric-pressure discharge.
[1] Government Amends Laws, Latest Amendment by Law, No. 56 of 2004 (Ministry of the Environment)
• Input electrical-energy (discharge current × applied voltage) to the glow
discharge is measured every second.
Partial gas pressure [Pa]
gas mixture ratios
(N2：O2)
Conditions
 Applied voltage ：DC (-295～-323V)
 Discharge current ：-2.5mA
 The initial total pressure：66.7Pa
 The initial partial pressure of methanol ：13.3Pa
 The initial gas mixture ratios of nitrogen to oxygen are
changed from discharge to discharge as shown in a table.
total pressure [Pa]
methanol
nitrogen
oxygen
100:0
53.4
0
95: 5
50.7
2.7
90:10
48.1
5.3
13.3
66.7
85:15
45.4
8.0
80:20
42.7
10.7
75:25
40.1
13.3
3.Results & discussion
Partial pressure variation of CH3OH, H2, CO, CO2, HCN, CH4 and C2H2
& ion current variation of H2O
4
Methanol decomposition is independent of
oxygen additive.
0
25
10
6
CO
4
51.1%
CH3OH
0
H2
C2H2
HCN
CO
8
0
20
14
12
10
8
6
400
600
800
63.6%
4
2
0
200
CO
1000
CH3OH
0
14
12
10
8
400
600
800
CO
6
77.5%
4
2
0
200
input energy [J]
0
1000
CO2
CH3OH
200
400
600
800
1000
input energy [J]
input energy [J]
The number of C atoms (in Pa)
15
10
Principle products
5
CO
12
H2
• The partial pressure of H2 decreases when the
oxygen concentration increases.
8
CO
• CO concentration peaks when the oxygen
concentration is near 10 and 15%.
4
0
CO2
• The partial pressure of CO2 increases when
the oxygen concentration increases.
5
CO2
4
H2O
• The QMS ion-current of H2O increases when
the oxygen concentration increases.
3
2
Concentration
The number of
×
of methanol
C atoms in a methanol molecule(1)
In a gaseous
The number of
Concentration
=
×
product
C atoms in a gaseous product(1 & 2)
of a gaseous
In methanol
The variations of the gaseous products are
considerably influenced by the oxygen concentration
of the background gas.
0
1
=
Oxygen concentration
Namely, decomposition process can be shown
as follows,
• At low oxygen concentration (N2 : O2 = 95 : 5), there is a
tendency that methanol is chiefly inverted to CO and CO2 via
intermediate products.
Low O2
Concentration
• At high oxygen concentration (N2 : O2 = 80 : 20), methanol is
directly inverted to CO and CO2.
C2H2
HCN
CH4
CO
CO2
CH3OH
• CO and CO2 increase with the oxygen concentration.
High O2
Concentration
The yield and selectivity of COX (CO or CO2)[2]as a function of O2
concentration
 The yield of COX
0
[CO] or [CO2 ]
YCO or YCO 2 
100 (%)
[CH3OH]0 - [CH3OH]
8
6
• HCN, CH4 and C2H2 are also detected as
minor products.
• HCN, CH4 and C2H2 are found to be
intermediate products, which are decomposed
in a glow discharge.
-12
2
Minor products
A]
0
0
200
400
600
input energy [J]
Where
1.5
1.0
0.5
0.0
partial pressure [Pa]
partial pressure [Pa]
partial pressure [Pa]
2.0
CH4
1.5
1.0
0.5
200
400
600
input energy [J]
800
0
200
400
600
input energy [J]
800
:The partial pressure of CO
[CO2]
:The partial pressure of CO2
[CH3OH]
:The partial pressure of methanol
YCO
YCO
2
SCO
2
80
60
60
40
40
20
20
0
0
0
5
10
15
20
25
O2 concentration [%]
• The YCO peaks at the oxygen concentration of 15%.
• The YCO2 increases monotonously and tends to saturate at the oxygen concentration of approximately 20%.
0.4
0.2
• Further, the SCO decreases linearly and the SCO2 increases linearly when the oxygen concentration increases.
0
200
400
600
input energy [J]
800
[2] Hyun-Ha Kim, et al., Appl. Catal. B: Environ. 56 (2005) 213-220.
4.Conclusions
We investigated gaseous products in a low-pressure DC glow discharge in nitrogen-oxygen-methanol mixture, and examined the influence of oxygen additive on the decomposition characteristics of methanol.





SCO
80
C2H2
0.6
0.0
0.0
0
[CO]
[CH3OH]0 :The initial partial pressure of methanol
0.8
2.0
HCN
[CO] or [CO2 ]
SCO or SCO2 
100 (%)
[CO]  [CO2 ]
800
3.0
2.5
 The selectivity of COX
100
H2, CO, CO2, HCN, CH4, C2H2 and H2O are gaseous products in a low-pressure DC glow discharge in nitrogen-oxygen-methanol mixture.
H2, CO, CO2 and H2O are principle products and HCN, CH4 and C2H2 are minor products.
Methanol is chiefly inverted to CO and CO2 via intermediate products at low oxygen concentration (5%) and directly to CO and CO2 at relatively high oxygen concentration (20%).
The YCO peaks at the oxygen concentration of 15%, but the YCO2 increases monotonously and tends to saturate at that of approximately 20%.
The SCO decreases linearly and the SCO2 increases linearly when the oxygen concentration increases.
COX selectivity [%]
4
ion current [x10
H 2O
100
COX yield [%]
partial pressure [Pa]
12
2
16
amount of C atoms [Pa]
8
• No significant difference is shown in the
variations of methanol for the difference
oxygen concentrations.
CH3OH
CH4
CO2
14
amount of C atoms [Pa]
12
N2:O2=80:20
16
16
CH3OH
O 2 concentration
0%
5%
10%
15%
20%
25%
N2:O2=95:5
N2:O2=100:0
amount of C atoms [Pa]
CH3OH
Mass balance for C atoms