decomposition of methanol in a low-pressure dc glow discharge in nitrogen-oxygen mixture ayako...
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Decomposition of methanol in a low-pressure DC glow discharge in nitrogen-oxygen mixture
Ayako Katsumata1, Kohki Satoh1,2 and Hidenori Itoh1
1Department of electrical and Electric Engineering, Muroran Institute of Technology, 27-1 Mizumoto, Muroran 050-8585, Japan2Center of Environmental Science and Disaster Mitigation for Advanced Research, Muroran Institute of Technology, 27-1 Mizumoto, Muroran 050-8585, Japan
Applied voltage : DC (-295~ -323V)Discharge current : -2.5mAThe initial total pressure: 66.7Pa The initial partial pressure of methanol : 13.3PaThe initial gas mixture ratios of nitrogen to oxygen are
changed from discharge to discharge as shown in a table.
1.Introduction 2.Experimental apparatus & conditionsBackground
Objective
Experimental apparatusElectrodes
• Parallel-plate electrodes of 60mm diameter and 20mm separation.• The lower electrode is earthed and a negative DC voltage is applied to the up
per 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• The optical emission of the glow discharge is measured by a Photonic Multi-
Channel 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• Input electrical-energy (discharge current × applied voltage) to the glow disc
harge is measured every second.
Conditions
gas mixture ratios(N2: O2)
Partial gas pressure [Pa]
total pressure [Pa]methanol nitrogen oxygen
100:0
13.3
53.4 0
66.7
95: 5 50.7 2.7
90:10 48.1 5.3
85:15 45.4 8.0
80:20 42.7 10.7
75:25 40.1 13.3
3.Results & discussionPartial pressure variation of CH3OH, H2, CO, CO2, HCN, CH4 and C2H2
& ion current variation of H2O
The yield and selectivity of COX (CO or CO2)[2]as a function of O2 concentration
CH3OH• No significant difference is shown in the
variations of methanol for the difference oxygen concentrations.
Methanol decomposition is independent of oxygen additive.
Principle products
The variations of the gaseous products are considerably influenced by the oxygen concentration of the background gas.
H2
• The partial pressure of H2 decreases when the
oxygen concentration increases.
CO• CO concentration peaks when the oxygen
concentration is near 10 and 15%.
CO2
• The partial pressure of CO2 increases when the
oxygen concentration increases.
H2O
• The QMS ion-current of H2O increases when
the oxygen concentration increases.
Minor products• 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.
4.Conclusions
3.0
2.5
2.0
1.5
1.0
0.5
0.0
pa
rtia
l pre
ssu
re [
Pa
]
8006004002000
input energy [J]
0.8
0.6
0.4
0.2
0.0
pa
rtia
l pre
ssu
re [
Pa
]
8006004002000
input energy [J]
HCN CH4 C2H2
2.0
1.5
1.0
0.5
0.0
pa
rtia
l pre
ssu
re [
Pa
]
8006004002000
input energy [J]
The yield of COX
The selectivity of COX
Where
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.
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.
To investigate 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, to develop the plasma cleaning of exhaust gases containing methanol.
[1] Government Amends Laws, Latest Amendment by Law, No. 56 of 2004 (Ministry of the Environment)
Mass balance for C atoms
CH3OH
C2H2
HCNCH4 CO
CO2
Low O2
Concentration
High O2
Concentration
N2:O2=100:0 N2:O2=95:5 N2:O2=80:20
The number of C atoms (in Pa)
Concentration of methanol
The number ofC atoms in a methanol molecule(1)
In methanol = ×
In a gaseousproduct
=Concentration of a gaseous
×The number ofC atoms in a gaseous product(1 & 2)
16
14
12
10
8
6
4
2
0
am
ount
of C
ato
ms
[Pa]
10008006004002000
input energy [J]
CH3OH C2H2 CH4 HCN CO2 CO
51.1%
CO
CH3OH
16
14
12
10
8
6
4
2
0
am
ount
of C
ato
ms
[Pa]
10008006004002000
input energy [J]
CH3OH
77.5%
CO
CO2
16
14
12
10
8
6
4
2
0
am
ount
of C
ato
ms
[Pa]
10008006004002000
input energy [J]
63.6%
CH3OH
CO
Oxygen concentration • At low oxygen concentration (N2 : O2 = 95 : 5), there is a
tendency that methanol is chiefly inverted to CO and CO2 via intermediate products.
• At high oxygen concentration (N2 : O2 = 80 : 20), methanol is directly inverted to CO and CO2.
• CO and CO2 increase with the oxygen concentration.
Namely, decomposition process can be shown as follows,
(%)100OH]CH[-OH]CH[
]CO[or CO][
303
2COorCO 2 YY
(%)100]CO[CO][
]CO[or CO][
2
2COorCO 2
SS
[CO] :The partial pressure of CO
[CO2] :The partial pressure of CO2
[CH3OH] :The partial pressure of methanol
[CH3OH]0 :The initial partial pressure of methanol
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.
• The YCO peaks at the oxygen concentration of 15%.
• The YCO2 increases monotonously and tends to saturate at the oxygen concentration of approximately 20%.
• Further, the SCO decreases linearly and the SCO2 increases linearly when the oxygen concentration increases.
[2] Hyun-Ha Kim, et al., Appl. Catal. B: Environ. 56 (2005) 213-220.
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.
100
80
60
40
20
0
CO
X y
ield
[%
]
2520151050
O2 concentration [%]
100
80
60
40
20
0
CO
X selectivity [%]
YCO YCO2 SCO SCO2
12
8
4
0
pa
rtia
l pre
ssu
re [P
a]
8006004002000input energy [J]
25
20
15
10
5
0
12
8
4
0
5
4
3
2
1
0
8
6
4
2
0
ion
curre
nt [x1
0-1
2 A]
O2 concentration 0% 5% 10% 15% 20% 25%
CH3OH
H2
CO
CO2
H2O