Direct conversion of methane over oxide-type catalysts supported on mesoporous silica under electric discharge

Article abstract of DOI:10.1166/jnn.2017.13354

Direct conversion of methane over oxide-type catalysts supported on mesoporous material under the dielectric barrier discharge plasma was investigated in the present study. The oxide catalysts (MgO and NiO) supported on SBA-15 was prepared by a hydrothermal and wet-impregnation method. The low-angle XRD patterns and TEM indicate that the ordered mesoporous structure was well maintained during the catalyst preparation and reaction processes. We also conducted direct conversion of methane using the catalysts under the plasma. As a result, the specific surface area of bare SBA-15 was much higher than that of the NiO/SBA-15, followed by MgO/SBA-15. Overall, the CH₄ conversion increased with increasing the specific surface area. And C₂ selectivity decreased with increasing the specific surface area.

Full text of DOI:10.1166/jnn.2017.13354

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Vol. 17, 2567–2570, 2017
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Direct Conversion of Methane Over Oxide-Type
Catalysts Supported on Mesoporous Silica
Under Electric Discharge
1
2ꢀ∗
Taegyu Kim and Daeil Park
1
Department of Aerospace Engineering, Chosun University, Gwang-Ju 501-759, Republic of Korea
2
Department of Aerospace Engineering, Nagoya University, Nagoya 464-8603, Japan
Direct conversion of methane over oxide-type catalysts supported on mesoporous material under
the dielectric barrier discharge plasma was investigated in the present study. The oxide catalysts
(
MgO and NiO) supported on SBA-15 was prepared by a hydrothermal and wet-impregnation
method. The low-angle XRD patterns and TEM indicate that the ordered mesoporous structure was
well maintained during the catalyst preparation and reaction processes. We also conducted direct
conversion of methane using the catalysts under the plasma. As a result, the specific surface area of
bare SBA-15 was much higher than that of the NiO/SBA-15, followed by MgO/SBA-15. Overall, the
CH conversion increased with increasing the specific surface area. And C selectivity decreased
4
2
with increasing the specific surface area.
IP: 5.188.219.76 On: Sat, 02 Feb 2019 04:11:39
Keywords: Mesoporou sC So ipl i cy ar i, g Oh xt :i dAe m- T ey pr iec aC na tSa cl yi se t n, tDi f ii rce cP t uCb ol ins vhe er sr si on of Methane.
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1
. INTRODUCTION
radicals by electron-impact ionization under the plasma,
Natural gas is discharged as a by-product of oil field or
deposited in a stranded and associated gas field that is dif-
ficult in practical use due to a local restriction. Recently,
a technology converting natural gas into clean petroleum
fuels has received a great attention. In addition, natural
gas conversion technologies are being commercialized by
world major oil companies due to technical and political
merits such as the use of medium and small gas resources
and the countermeasure against depletion of fossil fuels.
In general, methane that is a major ingredient of natural
gas can be synthesized into a clean fuel through a vari-
and then methyl radicals can be dimerized easily into C
as expressed in Eqs. (1) and (2).
2 6
H
CH +e → CH +H+e
(1)
(2)
4
3
CH +CH +e → C H +e
3
3
2
6
Among non-thermal plasmas, DBD (dielectric barrier
discharge) plasma is suitable to the plasma catalysis
because the plasma distribution is more uniform than
6
others. The DBD plasma has a dielectric barrier between
high voltage and ground electrodes, which play a role to
prevent it from being developed into the arc. In DBD, non-
equilibrium discharge takes place and high-temperature
electrons are formed by micro discharges. Thus, the chem-
ical reaction can be initiated by the plasma below the
temperature required for the reaction to be activated. The
selection of catalyst is limited to reduce the activation
energy for the reaction to be selective to desirable species.
Therefore, the hybrid reaction of plasma and catalyst real-
izes interactive synergetic effects with high reactivity at
1
ꢀ2
ety of processes. Recently, direct conversion of methane
into hydrocarbons higher than C has been actively studied
as an alternative to the above complex processes.
2
However, the direct conversion of methane is highly
endothermic reaction so that the temperature higher than
ꢀ
6
00 C is required.
When methane is thermally activated, however, the cat-
alyst should be prevented from sintering and degrada-
tion at the high temperature. Most of all, non-thermal
plasma has been attracted as an alternative process for the
3
ꢀ4
7
low temperature. In the present study, the direct conver-
5
methane activation. Methane can be ionized into methyl
sion of methane on the mesoporous silica under the plasma
was carried out. Catalyst/SBA-15 was characterized by
∗
Author to whom correspondence should be addressed.
XRD, TEM, and N adsorption–desorption.
2
J. Nanosci. Nanotechnol. 2017, Vol. 17, No. 4
1533-4880/2017/17/2567/004
doi:10.1166/jnn.2017.13354
2567

Direct Conversion of Methane Over Oxide-Type Catalysts Supported on Mesoporous Silica Under Electric Discharge Kim and Park
2
. EXPERIMENTAL DETAILS
7 kV with sine waveform and the discharge frequency was
fixed 1.5 kHz.
2
.1. Preparation of Mesoporous Silica (SBA-15)
The mesoporous silica (SBA-15) was synthesized by
hydrothermal method using the guideline reported
in the literature regarding SBA-15 molecular sieve.
In a typical synthesis, triblock-poly(ethylene glycol)-
block-poly(propylene glycol)-block-poly(ethylene glycol)
3
. RESULTS AND DISCUSSION
8
3
.1. Characterization of the Catalyst/SBA-15
Figure 2 shows the low-angle and the wide-angle XRD
pattern of catalyst/SBA-15. Figure 2(a) shows the three
distinct peaks was observed at the Bragg angle ꢂ2ꢃꢁ =
(
EO PO EO , Pluronic P123) of 4 g was added to
20 70 20
ꢀ
deionized water of 90 ml heated by 40 C, and the mixture
was stirred at the stirring speed of 250 rpm for 30 minutes.
After 4M HCl (60 mL) and tetraethyl orthosilicate (TEOS,
ꢀ ꢀ ꢀ
ꢄ87 , 1.52 , and 1.74 that are associated with (1 0 0),
0
(
1 1 0) and (2 0 0) reflection planes. These peaks represent
the characteristic diffraction patterns of the well-ordered
D hexagonal structure with the P6 mm symmetry of
9
.8 mL) were added to the prepared aqueous solution,
2
and was aged under the stirring at 300 rpm for 24 hours.
After the prepared aqueous solution was separated and fil-
SBA-15. And, Figure 2(b) shows the wide-angle XRD pat-
terns of catalyst/SBA-15. A broad peak centered at 22.2
of 2ꢃ was observed for all the samples. In the XRD pat-
ꢀ
ter through D.I water, the aqueous solution were dried at
ꢀ
1
00 C for 20 hours.
ꢀ
ꢀ
ꢀ
tern of NiO/SBA-15, the peak at ꢂ2ꢃꢁ = 37 , 43 , 62 , 75
The powder obtained after drying was calcined in air
ꢀ
and 79 could be assigned to NiO (PDF#44-1159).
ꢀ
at 550 C for 6 hours. The calcination temperature was
Also, no MgO crystallization is detected in the wide-
angle XRD patterns of MgO/SBA-15, implying the good
dispersion of MgO/SBA-15.
The tendency similar with the XRD results could be
predicted by TEM image of catalyst/SBA-15 as shown
in Figure 3. The textural characteristics of catalyst/
SBA-15 were characterized by the N2 adsorption–
desorption method. The specific surface area (SBET), pore
ꢀ
raised with the rate of 1.5 C/min. Finally, the MgO and
NiO catalyst was coated on the prepared SBA-15 using
a wet-impregnation method. The precursor solution was
prepared by dissolving Ni(NO ꢁ and Mg(NO ꢁ in D.I
3
2
3 2
water. SBA-15 was immersed into the precursor solution.
ꢀ
After impregnation, the catalysts were dried at 100 C for
2
4 h and calcined in air condition.
volume (Vtotal), mean pore diameter (d ), pore size distri-
p
2
.2. Plasma System
IP: 5.188.219.76 On: Sat, 0b u2 t iFo en bo 2f 0c a1 t9a l 0y s4 t :s1 a1 r: e3 9s ummarized in Figure 4.
Figure 1 shows the schematic of plasma system and DBD
Copyright: American Scientific Publishers
reactor. A quartz tube was used as the reactor, in wD he i lci vh et rh ee d by Ingenta
3
.2. Catalytic Performance of Oxide/SBA-15
catalyst/SBA-15 sample was packed. The reactor wall also
plays a role as the dielectric barrier of the DBD reactor.
Two electrodes were used; one was inserted in the mid-
dle of the reactor and the other was a 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 non-uniform. The gap between the
two electrodes was 6 mm, between which a 0.15 g cata-
lyst pellet was packed. The discharge voltage was fixed to
The effect of the presence of catalyst species on the
direct conversion of methane under the plasma is inves-
tigated. As a result, the contrast tendency with the
methane conversion could be predicted by C selectivity
2
Figure 1. Schematic diagram of the experimental apparatus for the
Figure 2. XRD patterns catalyst/SBA-15 samples: (a) low angle XRD
direct conversion of methane under the electric discharge.
pattern and (b) wide-angle XRD patterns.
2568
J. Nanosci. Nanotechnol. 17, 2567–2570, 2017

Kim and Park Direct Conversion of Methane Over Oxide-Type Catalysts Supported on Mesoporous Silica Under Electric Discharge
Figure 5. Catalytic performance of the direct conversion of methane
under the plasma at different oxide/SBA-15.
when MgO/SBA-15 was used, followed by NiO/SBA-15
and bare SBA-15. Overall, the CH conversion increased
4
with increasing the specific surface area. And C selectiv-
2
ity decreased with increasing the specific surface area.
The formation of C hydrocarbons is explained by the
2
free-radical mechanism of CH precursors that precede
Figure 3. TEM images of catalyst/SBA-15: (a) bare/SBA-15,
b) NiO/SBA-15 and (c) MgO/SBA-15.
4
(
in stepwise dehydrogenation reactions: CH → CH →
4
3
C H → C H → C H . Plasma can enhance the adsorp-
2
6
2
4
2
2
of SBA-15 according with the cata Il Py s: t s5 .. 1T 8h 8e . m2 1e 9t h. 7a n6 e Oc on n: -S at, 02 Feb 2019 04:11:39
tion intensity on the surface of SBA-15 by producing elec-
version of bare SBA-15 was much Ch i og ph ey rr i gt hh at :n At mh a et r ioc fa n Scientific Publishers
trons with wide energy distribution expressed typically
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the NiO/SBA-15, followed by MgO/SBA-15, as shown
9
by the Maxwellian or Druvestyne distribution. In DBD,
the mean electron energy is approximately 1∼3 eV and
only a small portion of the electrons have energy strong
enough to break atomic bonds. Most of the electrons
with smaller energy (<1 eV) are used in vibrational
Figure 5. However, maximal C selectivity was achieved
2
1
0
excitation. Vibrationally-excited species are more prone
to react and have lower activation energy for adsorption
and reaction than that of ground state species. This effect
can enhance the overall reaction rate considerably result-
ing in a higher conversion.
1
1
4
. CONCLUSION
The synergetic effect between plasma and catalyst/
SBA-15 for the direct conversion of methane was investi-
gated. The methane conversion of bare SBA-15 was much
higher than that of the catalyst/SBA-15. In contrast, max-
imal C selectivity was achieved when MgO/SBA-15 was
2
used, followed by NiO/SBA-15, bare SBA-15. Overall, the
CH conversion increased with increasing the specific sur-
4
face area. And C selectivity decreased with increasing the
2
specific surface area. Consequently, the direct conversion
of methane can be more improved by the plasma catalysis
on a mesoporous material.
Acknowledgment: This research was supported by
Figure 4. Textural characteristics and pore size distributions of
catalyst/SBA-15.
Basic Science Research Program through the National
J. Nanosci. Nanotechnol. 17, 2567–2570, 2017
2569

Direct Conversion of Methane Over Oxide-Type Catalysts Supported on Mesoporous Silica Under Electric Discharge Kim and Park
Research Foundation of Korea (NRF) funded by
the Ministry of Science, ICT and future Planning
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Received: 31 December 2015. Accepted: 27 April 2016.
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