Generation of hydrogen from methanol in a dielectric-barrier discharge-plasma system

Article abstract of DOI:10.1246/cl.2000.1116

The decomposition of CH₃OH was investigated in a dielectric-barrier discharge-plasma induced by low power from 0.27 to 6.40 Wh(NCm³)^-1. The maximum CH₃OH conversion of 80% into H₂ was achieved with CO or CO₂ being the other major product, respectively, in the absence or presence of water.

Full text of DOI:10.1246/cl.2000.1116

1116
Chemistry Letters 2000
Generation of Hydrogen from Methanol in a Dielectric-Barrier Discharge-Plasma System
Shuji Tanabe, Hiroshi Matsuguma, Kenji Okitsu, and Hiroshige Matsumoto*
Department of Materials Science and Engineering, Faculty of Engineering, Nagasaki University,
1-14 Bunkyo-machi, Nagasaki 852-8521
(Received June 16, 2000; CL-000591)
The decomposition of CH3OH was investigated in a dielec-
spectra, which increased rapidly with an increase in input volt-
age.7 Input powers of 0.27, 0.38, 1.58, 3.87, and 6.40
Wh(Ncm3)–1 was observed at input voltages of 2, 3, 4, 5, and 6
kV, respectively. All of these spectra for the Ar atom excitation
could be extrapolated to one point at about 1 kV on the input
voltage axis, suggesting that the break-down voltage of the Ar
discharge was 1 kV in the present system.
tric-barrier discharge-plasma induced by low power from 0.27
to 6.40 Wh(Ncm3)–1. The maximum CH3OH conversion of
80% into H2 was achieved with CO or CO2 being the other
major product, respectively, in the absence or presence of
water.
All of the emission spectra due to the Ar excitation almost
completely disappeared upon introduction of such reactant as
CH3OH (and/or H2O) into the Ar stream, indicating efficient
energy transfers from excited Ar species to reactant molecules.
Major gaseous products of the CH3OH decomposition were H2,
CO and carbon deposit in the absence of water, and H2 and CO2
in the presence of water. In the absence of water, the H2 yield
increased with increasing input voltage above 1 kV and attained
the maximum 80% at 6 kV, as shown by the curve (a) in Figure
2. Thus, the voltage dependence of the CH3OH conversion
well coincided with that of the emission intensity of Ar dis-
charge (Figure 1), indicating additional evidence of the energy
transfer7 from excited Ar species to CH3OH molecules.
With a reactant mixture containing water (curve b), on the
other hand, the H2 yield based on supplied CH3OH
([H2]generated/2[CH3OH]supplied) was higher in comparison with
the curve (a) and similarly increased with input voltage. At
input voltages above 5 kV, the H2 yield was observed to be
more than 100%, indicating a contribution of H2 formation
from H2O molecules. The decomposition of H2O in the
absence of CH3OH was carried out in order to understand the
reactivity of H2O molecules, as shown by the curve (c) in
Figure 2. Only small portions of H2O below 10% were inde-
pendently decomposed at above 1 kV of input voltage. The dif-
ference of the H2 yields between the curves (a) and (b) was
We have recently been investigating performances in ac
silent-discharge-plasma systems in decompositions of such sta-
ble compounds as NO,1 H2O,2 and CO2,3 in addition to reform-
ing4 of CH4 with CO2 and with N2O. In the present report, the
CH3OH decomposition, which has recently been considered to
be one of the attractive and practical reactions to generate
hydrogen as an on-board process for vehicles with fuel cells,5
was studied in a discharge-plasma system.2–4 Numerical inves-
tigations in conventional catalytic system using Pd-based cata-
lysts have been reported on this subject.6
The dielectric-barrier discharge-plasma reactor used in this
work has been described elswhere.2–4 Basically, the inner elec-
trode of copper rod was supported at the center of a quartz tube,
which was tightly surrounded by a cylindrical aluminum jacket
as the outer electrode. The decomposition was carried out with
a gas mixture of 1% CH3OH (and/or 1% H2O) in Ar at atmos-
pheric pressure by using a conventional flow reaction sys-
tem.2–4 The reactant gas was fed at a flow rate of 50 mL min–1
into the plasma zone of 1.9 cm3 in this work. The operating
condition was 1–6 kV and 1 kHz in the present work. The reac-
tant and products were analyzed by GC with Polapak Q and
Molecular Sieves 5A. The diagnostics of the plasma zone have
been also described in the previous papers.2–4
When a power with higher voltage above 2 kV was sup-
plied to the reactor in the pure Ar flowing, a homogeneous
emission was observed and an appreciable current passed
between the inner electrode and quartz wall of the reactor tube.
During the Ar discharge several sharp and strong lines of the
emission spectra were observed at wavelengths of 696, 706,
750, and 811 nm. Figure 1 shows intensities of these emission
Copyright © 2000 The Chemical Society of Japan

Chemistry Letters 2000
1117
ously proceed in the presence of water.
Figure 4 shows profiles in the formation of the other major
products, CO2 and CO. Ratio of the CO2 and CO yield ([CO2]f
/ [CO]f) in the absence of water was very small, while that in
the presence of water rapidly increased with increasing input
voltage. This result in the presence of water might be attributa-
ble to water-gas shift reaction, i.e., CO formed by the decompo-
sition of CH3OH reacts with H2O to form H2 and CO2. Further
detailed investigations are, of course, required to gain insight
into plasma excitation mechanism.
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much larger than that in the direct H2O decomposition (c). It is
speculated, therefore, that a reaction between CH3OH and H2O
to produce H2 takes place in the plasma zone.
The H2 formation in the present system can be postulated
to occur by the two kinds of process (1) and (2), as follows
CH3OH = 2H2 + CO
CH3OH + H2O = 3H2 + CO2
(1)
(2)
7
Figure 3 shows ratio of amount of H2 formed ([H2]f) and
that of CH3OH consumed ([CH3OH]c). The average ratios
were estimated to be 2.2 in the absence of water and 2.7 in the
presence of water. It can be roughly considered from this point
of view that the process (1) is the major reaction in the absence
of water and that both of these processes (1) and (2) simultane-