st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
Methanol decomposition using a catalytic reactor with electric discharge
T. Kim1, D. Lee2
1
Department of Aerospace Engineering, Chosun University, Gwangju, Republic of Korea
Plasma Laboratory, Korea Institute of Machinery and Materials, Daejeon, Republic of Korea
2
Abstract: Methanol decomposition using a catalytic reactor with electric discharges was investigated in the present study. The methanol conversion at 190 °C was 58.0%; however, the
methanol conversion increased by 65.9% under the electric discharge. An electric discharge
produces high energy electrons with sufficient energy to break a C-OH bond.
Keywords: Methanol, Decomposition, Catalytic reactor, Electric discharge
1. Introduction
Hydrogen is a promising energy carrier for future society that is not dependent on fossil fuel energy. There are
various hydrogen sources such as hydrocarbon species
and oxygenate hydrocarbons including alcohol. Methanol
has merits of being liquid phase and having high hydrogen to carbon ratio.
Heterogeneous catalytic processes at high temperature
have been used to produce hydrogen from hydrocarbons
and alcohols such as methanol. High reforming temperature caused serious catalyst deteriorations; thus, the periodic catalyst replacement was required. In order to improve the stability and durability of the reforming catalyst
[1], plasma-catalyst hybrid process was used. Plasma-catalyst hybrid process is expected to improve the
catalytic performance at low temperature.
Recently, plasma-catalyst hybrid process has been attempted to reduce the reforming temperature [2-6]. Yu et
al. [4] performed CO2 reforming of propane in combination of non-thermal plasma and Ni/γ-Al2O3 catalyst. They
showed that the temperature for activating the reaction
was reduced, and the propane conversion was improved
compared with using the plasma or the catalyst only.
Sekine et al. [5] reported that at low temperature condition, where no reactivity was observed using catalyst only,
methane was converted by introducing discharge in the
volume of catalyst. They also disclosed that ZrO2, which
is not active on a catalytic reaction, improved the reactivity on methane reforming under the electric discharge
because the reducibility of lattice oxygen on a catalyst
support was improved. The electric discharge was also
used to initiate ethanol-steam reforming at low temperature [6]. The ethanol under the electric discharge was initiated to be decomposed at low temperature where the
catalyst was not working. The effect of the electric discharge on water-gas shift reaction was higher than that of
ethanol decomposition.
In our previous study [3], we used an electric discharge
on the catalyst for methanol steam reforming. The electric
discharge improved the methanol conversion. The effect
of the discharge condition such as the discharge voltage
and frequency on the reaction was investigated at the various temperatures and feed rates. However, the methanol
steam reforming is a combination of methanol decomposition and water-gas shift reaction. Thus, the detailed reaction mechanism can be disclosed by investigating the
methanol decomposition under the electric discharge.
In the present study, methanol decomposition under the
electric discharge was investigated. The reaction mechanism of methanol decomposition under the electric discharge was discussed.
2. Experiments
Cu/ZnO was used as the catalyst for methanol-steam
reforming, considering its proven reactivity and selectivity [7-9]. A spherical γ-Al2O3 pellet (Alfar Aesar) with a
diameter of 2 mm was used as the support. The Cu/ZnO
was loaded in the γ-Al2O3 pellet using the wet impregnation method. A mixture containing a 0.7 M aqueous solution of Cu(NO3)2 and a 0.3 M aqueous solution of
Zn(NO3)2 was prepared. The catalyst support, γ-Al2O3
pellet, was immersed in the mixture. The moisture was
removed by drying the catalyst-loaded support in a convection oven at 70 ˚C for 12 hours followed by calcination in a furnace at 350°C for 3 hours. After the coating,
the catalyst surface was activated by reduction in a hot
hydrogen flow. The Cu/ZnO loading was 5.0 wt.% of the
total weight of the catalyst support. The catalysts were
coated uniformly on the surface of the spherical pellet and
Fig.1 Catalytic reactor with the annular-shaped electrodes.
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
Fig.2 Schematic diagram of the experimental apparatus.
the particle size of the catalyst was approximately 20 nm
[3].
A quartz tube was used as the reactor, in which the catalyst pellet was packed as shown in Fig. 1. Two electrodes
were used; one was inserted in the middle of reactor and
the other was wrapped around the outer wall of the reactor.
A flange, which is not electrically conducting, was installed at the lower part of reactor to fix the catalyst pellet.
The flange was also used to fix the electrode precisely at
the middle of the reactor because the electric discharge
can be distorted if the gap between the two electrodes is
not uniform. The gap between the two electrodes was
4.925 mm, between which 10 g catalyst pellets were
packed. During the reaction, the reaction temperature was
monitored with a K-type thermocouple, which was placed
1 cm away from the catalyst to avoid an electric discharge
to the thermocouple. The electric discharge was generated
only between the two electrodes due to the larger space
between the catalyst and thermocouple than between the
two electrodes [3].
Fig. 2 shows a schematic diagram of the experimental
apparatus for methanol decomposition using an electric
discharge. The reaction temperature was controlled using
a programmable electric furnace. The methanol solution
was fed through a syringe pump (KDS200). High-purity
nitrogen gas was used as a carrier gas of the reactants
vaporized before being supplied to the reactor. The methanol decomposition as the temperature from 190 to
250 °C was carried out. The electric discharge was generated by sine wave with 5 kV at 1 kHz [3].
A high-voltage amplifier (Trek 20/20C) was used to
generate a high-voltage electric discharge on the catalyst.
The voltage, frequency and waveform were controlled
using a function generator (Agilent 33220A). The discharge voltage, frequency and waveform were monitored
using an oscilloscope (WaveSurfer 424, LeCroy).
The reformate gas production was condensed prior to
Fig.3 XRD patterns of CuO/ZnO/γ-Al2O3 and
Cu/ZnO/γ-Al2O3 catalysts.
gas analysis. The liquid product, such as unreacted methanol, was separated through a liquid trap. The uncondensed gas products were analyzed by gas chromatography (YL6100, YONGLIN) equipped with a thermal
conductivity detector (TCD) and a flame ionized detector
(FID). The gas species measured included H2, O2, N2,
CH4, CO, CO2 and C2-C3 hydrocarbons.
The discharge power was measured using the
Lissajous method [10]. A 1,000 pF capacitor was connected between the reactor electrode and ground junction.
The discharge energy charged the capacitor when the
electric discharge was initiated. The discharge power can
be calculated indirectly from the level of capacitor charging by integrating the area of the Q-V plot. The electric
discharge was generated by sine wave with 5 kV at 1 kHz.
3. Results and discussion
Fig. 3 shows XRD patterns of CuO/ZnO/γ-Al2O3 and
Cu/ZnO/γ-Al2O3 catalysts. The metal existed as an oxide
state (CuO) after the calcination. The CuO was perfectly
reduced to Cu as shown in Fig. 3. Fig. 4 shows the
Lissajous plot of the Cu/ZnO/γ-Al2O3 catalyst synthesized
by wet impregnation method. The specific energy density
was calculated to 12.7 J/L.
Fig. 5 shows the methanol conversion as a function of
reaction temperature at the discharge voltage of 5.0 kV.
The zero voltage means the reforming reaction using the
catalyst only. The reaction temperature varied from 190 to
250°C. The feed rate and discharge frequency were fixed
to 0.5 ml/h and 1 kHz, respectively. The methanol conversion increased with increasing the reaction temperature.
Normally, a catalytic reaction depends strongly on the
temperature of the catalyst. The catalytic reaction under
the electric discharge has also the same dependency to the
catalytic reaction using the catalyst only. The methanol
conversion at 190°C was 58.0% when using the catalyst
only. However, the methanol conversion under the electric
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
Fig.5 Methanol conversion as a function of reaction temperature at the discharge voltage of 5.0 kV.
Fig.4 Lissajous plot of the synthesized Cu/ZnO/γ-Al2O3
catalyst.
discharge increased by 65.9%. The electric discharge provided the sufficient energy to break chemical bonds of
methanol and steam. The C-O bond in methanol has bond
dissociation energy of ~380 kJ/mol [10]. The electric discharge produces electrons having a higher energy than the
bond dissociation energy. Moreover, the electric discharge
increased the absorption intensities of reactants on the
active sites of the catalyst surface. The electrons generated by the electric discharge can excite vibrationally the
reactants. Halonen et al. [11] reported that the
vibrationally-exited species are absorbed with a lower
activation energy than that of ground-state species.
The methanol is decomposed to produce CO and H2,
which is an overall reaction. We need to see the elementary steps to understand the effect of the electric discharge
on the catalytic reaction. First, the methanol is absorbed
dissociatively on the Cu sites to form methoxy species as
expressed in Eq. (1) [12].
S1 S1a CH3OH ( g )
CH3O(1)
H (1a )
(1)
where S is the active sites that have two types of sites
such as S1 and S1a. The S1 is the active site for the methanol decomposition and the S1a is the H2 absorbing site.
The methoxy species are dehydrogenated to form
oxymethylene, which is subsequently converted to a
formate as expressed in Eq. (2)-(3).
CH 3O(1)
S1a
OH (1) CO(1)
CH 2O(1)
HCOO(1)
H (1a )
(2)
S1
(3)
The reactions in Eq. (5) and (6) are the rate-determining
step (RDS) in the methanol decomposition. The strong
electric discharge on the surface of catalyst can accelerate
the reaction rate of the RDS. If the RDS can be faster under the electric discharge, the hydrogen production also
increases at the low temperature. Moreover, the activation
energy can be decreased by the electric discharge; thus,
the methanol decomposition and RDS can be thermodynamically more favorable at the low temperature [3].
The electric discharge can break the strong bonds of
CH3OH into CH3 and OH radicals. It is expected that the
electric discharge will make the other radicals such as
CH3, OH, H, and O, which are more reactive on the active
sites of the catalyst at low temperature. The excited radicals such as CH3* and OH* played an important role to
break the chemical bond of CH3OH molecules. Especially,
the highly excited CH3* at the high voltage seems to enhance the surface reaction of radicals on the catalyst [3].
We estimated that the reaction mechanism could be
progressed through another reaction path way under the
electric discharge. An electric discharge produces high
energy electrons with sufficient energy to break a C-OH
bond. Although a homogeneous reaction under an electric
discharge synergistically contributes to the total conversion of methanol, not all the homogeneous reactions result
in the production of H2 and CO. The selectivity of species
produced by plasma chemistry did not follow the thermal
equilibrium conditions. This can explain the formation of
hydrocarbon species, such as CH4 and C2H6, as shown in
Fig. 6. A discrete reaction path will be suggested to be the
most likely mechanism for the synergetic effect between
plasma and catalyst. These results are expected to provide
a guide for understanding the plasma-catalyst hybrid reaction.
4. Conclusion
The temperature for activating the process needs to improve the stability and durability of the reforming catalyst.
Methanol decomposition using a catalytic reactor with
electric discharges was investigated in the present study
The methanol conversion at 190 °C was 58.0%; however,
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
93 (2006).
[9] T. Kim, S. Kwon, Journal of Micromechanics and
Microengineering, 16, 1752 (2006).
[10] X. Tu, J. Whitehead, Appl. Catalysis B: Environmental, 125, 439 (2012).
[11] L. Halonen, S. Bernasek, D. Nesbitt, The Journal of
Chemical Physics, 115, 5611 (2011).
[12] B. Peppley, J. Amphlett, L. Kearns, R. Mann, Applied
Catalysis A: General, 179, 21, (1999).
Fig.6 Formation of hydrocarbon species at the different
temperatures at the discharge voltage of 5.0 kV.
under the electric discharge, the methanol conversion increased by 65.9%. The hydrocarbon species, such as CH 4
and C2H6, were formed under the electric discharge, while
those were not observed when the catalyst was used only.
From the above results, the methanol decomposition was
more improved on the catalyst at low temperature using
the electric discharge. The detailed mechanism of the
methanol decomposition under the electric discharge will
be studied in order to understand the synergic effect of the
plasma-catalyst hybrid process.
5. Acknowledgement
The authors appreciate the financial support from "Hybrid technology of nano catalyst-plasma for low carbon/emission" of MKE (Ministry of Knowledge Economy) and ISTK (Korea Research Council for Industrial
Science and Technology) of Republic of Korea.
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[4] Q. Yu, M. Kong, T. Liu, J. Fei, X. Zheng, Catalysis
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[5] Y. Sekine, M. Haraguchi, M. Matsukata, E. Kikuchi,
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[6] Y. Sekine, M. Haraguchi, M. Tomioka, M. Matsukata,
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