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