A brief catalyst study on direct methane conversion using a dielectric barrier discharge

Article abstract of DOI:10.1002/jccs.200700120

A series of metal catalysts was used for methane conversion to higher hydrocarbons and hydrogen in a dielectric barrier discharge. The main goal of this study is to identify the metal catalyst components which can influence the reactions in room-temperature plasma conditions. The catalysts supported by γ-Al₂O₃ and zeolite (ZSM 5x) were prepared by the incipient wetness method with solutions containing the metal ions of the second component. Among the catalysts tested, only Pt and Fe catalysts showed a unique result of catalytic reaction in a reactor bed packed with glass beads.

Full text of DOI:10.1002/jccs.200700120

Journal of the Chinese Chemical Society, 2007, 54, 823-828
823
Communication
A Brief Catalyst Study on Direct Methane Conversion Using a Dielectric
Barrier Discharge
a,b
Antonius Indarto, * Jae-Wook Choi, Hwaung Lee and Hyung Keun Song
b
b
b
a
Department of Chemical and Biological Engineering, Korea University, Seoul 136-701, Korea
b
Plasma-Catalyst Chemical Process Lab., Korea Institute of Science & Technology,
P.O. Box 131, Cheongryang, Seoul 130-650, Korea
A series of metal catalysts was used for methane conversion to higher hydrocarbons and hydrogen in a
dielectric barrier discharge. The main goal of this study is to identify the metal catalyst components which
2 3
can influence the reactions in room-temperature plasma conditions. The catalysts supported by g-Al O
and zeolite (ZSM 5x) were prepared by the incipient wetness method with solutions containing the metal
ions of the second component. Among the catalysts tested, only Pt and Fe catalysts showed a unique result
of catalytic reaction in a reactor bed packed with glass beads.
Keywords: Plasma; Dielectric barrier discharge; Metal catalyst; Methane conversion.
INTRODUCTION
The identification of catalysts showing high activity
studying its effects on methane conversion and the distribu-
tion of products.
and selectivity for the direct conversion of methane to
higher hydrocarbon or hydrogen is the major purpose of
this research. Many different catalyst systems have been
investigated and varying degrees of success have been
achieved; however, until now none of them have demon-
EXPERIMENTAL SETUP
Fig. 1 shows a schematic diagram of the experimental
setup. Methane was introduced into a cylindrical reactor at
room temperature and atmospheric pressure. The products
were analyzed by gas chromatography. Details of each part
of the system are described in the next sections.
1
,2
strated outstanding performance. Generally, catalysts op-
erate under conditions where temperature of the process
reaches an appropriate point so that a catalyst can be active.
Some references claim that it is possible to activate the
metal catalyst in near-room temperature using some alter-
Reactor
3
-5
native methods, e.g. plasma.
The reactor is a cylindrical Pyrex tube (ID of 7.5 mm)
with 2 parallel-straight wires (0.2-mm diameter, stainless
steel) as the inner metal electrode and silver film coated
around the tube as the outer electrode. A high frequency
alternating current (AC) power supply was connected to
the electrodes. The effective volume and length of the reac-
tor were 8.8 mL and 200 mm, respectively. In order to
maintain a similar reactor configuration, e.g. gap distance,
the reactor capacitance was checked by a RCL meter (Fluke
PM6304) before and after experiments. The reactor capaci-
tances were in the range of 9.5-10.0 pF in air-filled gap con-
ditions.
However, different plasma conditions in each experi-
ment make it difficult to confirm and compare the real ac-
tivity of the catalysts on the reactions. In order to overcome
this problem, a comprehensive study was done by compar-
ing the catalyst activity in the same plasma system. Experi-
ments were done using a dielectric barrier discharge (DBD)
at room temperature and atmospheric pressure. Among
non-thermal plasmas, DBD has a better future to be applied
at an industrial level for chemical synthesis. Flexibility to
6
-7
8-9
be used in liquid or gas environment condition is an ad-
vantage. The activation of a catalyst was investigated by
*
Corresponding author. Tel: +82-10-2296-3748; E-mail: indarto_antonius@yahoo.com

8
24 J. Chin. Chem. Soc., Vol. 54, No. 4, 2007
Indarto et al.
Power Supply
The maximum voltage and frequency of the AC
detector (FID) for measuring CH
bons.
4
and higher hydrocar-
power supply (Auto Electric, model A1831) were 10 kV
and 20 kHz, respectively. To measure the total supplied
power to the reactor, a digital power meter (Metex, model
M-3860M) was inserted into the electric line of the power
supply. The general waveform of voltage and current used
during experiments is shown in Fig. 2.
The evaluation of system performance was done
based on product selectivity and methane conversion which
are formulated as:
(
(
1)
2)
Materials
All experiments were carried out by introducing
methane (CH
4
, purity > 99.99%) to the reactor at room tem-
(3)
perature and atmospheric pressure. The gas flow rate was
controlled by a calibrated mass flow controller (Milipore,
model FC-280SAV). Analysis of the products was carried
out using a gas chromatograph (YoungLin, model M600D)
with a thermal conductivity detector (TCD, Column: Hayesep
2 4
D 80/100) for measuring H and CH , and a flame ionized
Fig. 1. Schematic diagram of experimental set up.
Fig. 3. XRD spectra of (a) Al
2 3
O and (b) zeolite (ZSM
5
x)-supported catalyst.
Fig. 2. Voltage and current profile.

Catalyst Study for Methane Conversion in DBD
J. Chin. Chem. Soc., Vol. 54, No. 4, 2007 825
A series of metal catalysts was prepared by incipient wet-
rates were varied from 5 mL/min to 40 mL/min while the
supplied power was set at 50, 60, and 80 watts. Some parts
of these works have been presented before where we com-
2 3
ness impregnation method with g-Al O (purchased from
Junsei Chemical, Japan) and zeolite 5x (ZSM 5x, Junsei
Chemical, Japan) used as the supports. The aqueous solu-
tions of metal catalyst were made by dissolving precursors
in distillate water. All catalysts were dried at 388 K for 2 h,
8
pared the plasma process with the thermal process. It was
concluded that the plasma process has better results for
higher hydrocarbons production.
followed by calcination at 873 K for 3 h in O
2
-rich gas con-
Fig. 5a shows the effect of flow rate and voltage vari-
ation on the methane conversion. Higher conversion of
methane can be achieved by supplying higher power to the
reactor or lowering the flow rates. Higher supplied power
means an increase in the concentration of higher energetic
species in the plasma zone which are able to decompose the
chemical bond of methane. The maximum point of the con-
version reaches 80% at the flow rate of 5 mL/min and the
supplied power of 80 watts. Fig. 5b presents the product
distribution of methane conversion by non-catalytic plasma
ditions. Reduction of catalyst was done by flowing hydro-
gen at 673 K for 2 h. Amounts of metal loaded were all set-
tled to be 3 wt% of the catalyst.
Fig. 3 shows the XRD spectra of each catalyst. The
XRD characterization was done after the reduction step. In
order to measure the exact amount of metal doped on the
catalyst support, energy dispersive spectroscopy (EDS)
analysis was conducted and the spectra example of 3% Ni
2 3
over Al O is shown in Fig. 4. In these experiments, 0.5
gram of catalyst was packed at the end of the plasma zone
to prevent the decomposition of products by plasma.
process. The highest selectivity of H
lowest input flow rate. It is believed that fragmentation of
methane into smaller molecules, e.g. H and C, is more fa-
vorable than recombinant reactions to produce higher hy-
2
was produced at the
2
8
,10-11
RESULTS AND DISCUSSION
drocarbons.
This condition will lead to re-decomposi-
tion reactions of higher hydrocarbons which were previ-
ously produced during plasma reaction, e.g. coupling reac-
tions of methyl radicals.
Non-catalytic plasma process
In order to investigate the effects of catalyst existence
in the DBD, a non-catalytic plasma process was conducted
as a comparison. All experiment ranges were done at atmo-
spheric pressure and ambient temperature. The total flow
An electron which has an important role in the cold
1
plasma process, e.g. DBD, will initiate the radical reac-
2
tions:
Fig. 4. EDS spectra of 3%Ni¤g-Al2O3.

8
26 J. Chin. Chem. Soc., Vol. 54, No. 4, 2007
Indarto et al.
CH
CH
4
4
+ e ® CH
4
×
(4)
(5)
Instead of H×, re-decomposition of higher hydrocarbon
could be caused by electrons. Electron collision with mole-
cules is faster than molecule collision with ions or radi-
× ® CH × + H× + e
3
1
3
Recombinant reactions of higher hydrocarbons could occur
by coupling reactions of methyl radicals (CH ×). However,
cals. Longer resident time of molecules in the plasma re-
3
action will reduce the concentration of higher hydrocar-
the existence of hydrogen in the reactor will give the oppo-
site effect because the hydrogen radical (H×) has a tendency
to become a reducer species or reactant decomposer for
higher hydrocarbons.
2
bons and produce more H molecules compared to the
shorter resident time reactions. In this experiment, very
small amounts of solid carbon were found and they can be
ignored.
CH
CH
CH
3
3
3
× + CH
× + CH
3
3
× ® C
× ® C
2
2
H
H
6
4
(6)
(7)
(8)
(9)
Catalyst-assisted plasma process
+ H
2
In order to convert methane to the desired products
(such as ethylene and benzene) with high selectivity, the
combination of plasma with catalyst has been explored sep-
× + C
2 3 3 6
H × ® C H
C
x
H
y
x 2
+ H× ® C Hy-1× + H
4
,14-15
arately in many different plasma conditions.
How-
ever, this situation results in difficulty to make a direct
comparison of the activity among those catalysts. Some re-
search papers also show ambiguous arguments, e.g. differ-
ent results. To overcome these problems, we investigated
2 3
the plasma methane conversion over various g-Al O and
zeolite (ZSM 5x)-supported metal catalysts. Table 1 shows
the effects of catalysts in the discharge plasmas on methane
2 3 4 2
conversion and the selectivity of C , C , C , and H . Gen-
erally, catalysts supported by zeolite (ZSM 5x) produce
higher acetylene and ethane compared to non-catalytic pro-
cess. On the other hand, this condition will influence the se-
3 8 4
lectivity of higher hydrocarbons, e.g. C H and C , that is
lower compared to a non-catalytic process. It can be con-
cluded that the existence of zeolite in the plasma reaction
leads to the formation of lighter hydrocarbons rather than
higher hydrocarbons. This idea is supported by previous in-
vestigations which showed the same tendency of increas-
1
5
ing selectivity of acetylene and ethane.
However, from Table 1, we can draw a conclusion
that the activity of the metal catalysts for methane conver-
2 3
sion prepared by incipient wetness of the g-Al O and zeo-
lite (ZSM 5x)-supported catalyst are somewhat disappoint-
ing. Although sometimes the existence of metal catalysts in
the plasma process gives diverse results, the product distri-
bution is relatively the same, and the difference selectivity
values are not significant compared to the non-catalytic
process. In dielectric barrier discharge where the tempera-
ture is equal to the room temperature, most metal catalysts
2 3
supported by g-Al O and zeolite (ZSM 5x) were not acti-
vated only by the existence of energetic species, such as
electrons and ions. It is believed that activation temperature
is still the most important factor to activate the metal cata-
Fig. 5. Effects of flow rates on (a) methane conversion
and (b) products distribution.

Catalyst Study for Methane Conversion in DBD
J. Chin. Chem. Soc., Vol. 54, No. 4, 2007 827
Table 1. Effects of catalysts on methane conversion and products selectivity
1
Note: Run 1 is flow rate: 30 mL/min, power: 35 W; run 2 is flow rate: 30 mL/min, power: 50 W; run 3 is
flow rate: 15 mL/min, power 50 W.
lyst. Further studies of these effects and catalyst character-
ization analysis are underway.
Among metal catalysts, only Pt and Fe show a unique
result on the selectivity of hydrocarbons. Pt and Fe sup-

8
28 J. Chin. Chem. Soc., Vol. 54, No. 4, 2007
Indarto et al.
ported by both g-Al
2
O
3
and zeolite (ZSM 5x) increase the
ACKNOWLEDGEMENTS
selectivity of ethane by a factor of 1.2-1.4 compared to the
non-catalytic process. Increasing ethane selectivity affects
the selectivity of acetylene which is lower than that of the
non-catalytic process. The reverse phenomenon shows that
the hydrogenation process of acetylene to ethane occurs
during plasma reaction when Pt and Fe exist. Low activa-
tion energy (~0 kJ/mol) of acetylene adsorption on the sur-
This study was supported by the global R&D program
of the Korea Foundation for International Cooperation of
Science & Technology (KICOS). The authors are thankful
to the Korea Institute of Science and Technology for sup-
port.
1
6
face of Pt and Fe can be the reason for this case. Acety-
lene will be attached easily on the surface of the catalyst.
Hydrogen atoms which are numerously produced by colli-
Received August 14, 2006.
sion between CH
4
and electrons will initiate the series of
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