Plasma assisted oxidative coupling of methane (OCM) over Ag/SiO2 and subsequent regeneration at low temperature

Article abstract of DOI:10.1016/j.apcata.2018.03.007

In this study, oxidative coupling of methane (OCM) was carried out to produce C₂ and C₃ hydrocarbons directly from methane in a plasma-catalyst hybrid system. Since catalyst only reaction requires high temperature above 700 °C, dielectric barrier discharge (DBD) plasma was applied as plasma source to lower the reaction temperature. Among various supports, only SiO₂ showed the higher yield than plasma only reaction when combined with DBD plasma, which led us to focus on the SiO₂-based catalyst. It was found that Ag/SiO₂ demonstrated the highest C₂₊ hydrocarbon yield of about 9.7% at 385 °C. In this process, oxygen was proved to play an essential role in the coupling of methane to C₂₊ hydrocarbons over Ag/SiO₂ catalyst. However, Ag/SiO₂ catalyst under plasma condition became deactivated with time-on-stream because of coking. It was demonstrated that plasma regeneration at 378 °C gave rise to the full recovery of activity while thermal regeneration did not due to the partial removal of coke and the sintering of Ag.

Full text of DOI:10.1016/j.apcata.2018.03.007

Accepted Manuscript
Title: Plasma assisted oxidative coupling of methane (OCM)
over Ag/SiO2 and subsequent regeneration at low temperature
Authors: Heesoo Lee, Dae-Hoon Lee, Jeong Myeong Ha, Do
Heui Kim
PII:
S0926-860X(18)30117-0
DOI:
Reference:
https://doi.org/10.1016/j.apcata.2018.03.007
APCATA 16581
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
10-1-2018
28-2-2018
13-3-2018
Please cite this article as: Lee H, Lee D-H, Ha JM, Kim DH, Plasma
assisted oxidative coupling of methane (OCM) over Ag/SiO2 and subsequent
regeneration at low temperature, Applied Catalysis A, General (2010),
https://doi.org/10.1016/j.apcata.2018.03.007
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Plasma assisted oxidative coupling of methane (OCM) over Ag/SiO2 and
subsequent regeneration at low temperature
Heesoo Leea, Dae-Hoon Leeb, Jeong Myeong Hac, Do Heui Kima,*
a School of Chemical and Biological Engineering, Institute of Chemical Processes,
Seoul National University,
1 Gwanak-ro, Gwanak-gu, Seoul, 151-742, Republic of Korea
b Korea Institute of Machinery and Materials,
156 Gajeongbuk-ro, Yuseong-gu, Daejeon, 305-343, Republic of Korea
c Clean Energy Research Center, Korea Institute of Science and Technology,
Seoul, 136-791, Republic of Korea
* Corresponding author.
E-mail address: dohkim@snu.ac.kr
Tel.: +82 2 880 1633
Postal address: School of Chemical and Biological Engineering, Institute of Chemical Processes, Seoul National
University, 1 Gwanak-ro, Gwanak-gu, Seoul, 151-742, Republic of Korea
1

Graphical abstract
Highlights





Oxidative coupling of methane (OCM) was done in plasma-catalyst hybrid system.
Ag/SiO2 showed the highest C2+ hydrocarbon yield (9.7%) with plasma at 385 ºC.
Oxygen played an important role in OCM using plasma-catalyst hybrid system.
Ag/SiO2 was deactivated in the hybrid system because of coke formation.
Plasma regeneration at 378 ºC fully recovered the OCM activity of Ag/SiO2.
Abstract
In this study, oxidative coupling of methane (OCM) was carried out to produce C2 and C3
hydrocarbons directly from methane in a plasma-catalyst hybrid system. Since catalyst only reaction
requires high temperature above 700 °C, dielectric barrier discharge (DBD) plasma was applied as
plasma source to lower the reaction temperature. Among various supports, only SiO2 showed the higher
yield than plasma only reaction when combined with DBD plasma, which led us to focus on the SiO2-
based catalyst. It was found that Ag/SiO2 demonstrated the highest C2+ hydrocarbon yield of about 9.7%
at 385 °C. In this process, oxygen was proved to play an essential role in the coupling of methane to
C2+ hydrocarbons over Ag/SiO2 catalyst. However, Ag/SiO2 catalyst under plasma condition became
deactivated with time-on-stream because of coking. It was demonstrated that plasma regeneration at
378 °C gave rise to the full recovery of activity while thermal regeneration did not due to the partial
removal of coke and the sintering of Ag.
2

Keywords: Oxidative coupling of methane; Dielectric barrier discharge; Plasma-catalyst hybrid system;
Ag/SiO2; Regeneration
1. Introduction
Methane is the major component of natural gas, which is one of the abundant fossil fuels on
earth [1]. However, the utilization of methane has been limited to the route of synthesis gases to produce
liquid hydrocarbons and other chemical products, which is regarded as indirect methods [2, 3]. Such
indirect process has weaknesses such as high operating cost and low thermodynamic efficiency. Hence,
if methane is directly utilized as alternative feedstock to petroleum, it will be highly desirable from the
economic point of view. Therefore, many efforts have been made to directly convert methane into more
useful products like olefins, aromatics, and alcohols by using various catalysts for decades [4-9].
Among them, direct synthesis of C2 and C3 hydrocarbons which are suitable for chemical feedstocks
has been extensively investigated [10-13]. There are two ways to produce these hydrocarbons directly
from methane with one step. One is oxidative coupling of methane (OCM) and the other is non-
oxidative coupling of methane (non-OCM). Non-OCM uses only methane as reactant gas and thus the
coking problem occurs seriously [14]. On the other hand, OCM utilizes oxygen as well as methane so
that it can mitigate the deactivation of catalyst arising from coking. Hence, many OCM catalysts such
as Li/MgO, Mn-Na2WO4/SiO2, and Ag catalysts have been commonly studied so far [15-20]. However,
high temperature above at least 700 °C is required for these catalysts to demonstrate significant activity
because methane is hardly activated due to its stable C-H bond [21-24]. In other words, it is a difficult
target to achieve only with catalyst from the practical point of view because the reaction requires high
temperature which causes deactivation of the catalyst [25]. An alternative way to activate methane at
lower temperature would be to use plasma [26, 27]. There are various plasma sources such as dielectric
barrier discharge (DBD), corona, microwave, arc, spark, and radio frequency [28-30], which can be
divided into thermal plasma and non-thermal plasma [31, 32]. In case of thermal plasma condition, the
3

gas temperature increases significantly to above 1000 °C since electron and bulk gas are in thermal
equilibrium state [33]. Thus, such plasma is not desirable for catalytic reaction because the catalyst
deactivation such as sintering can occur more severely under the reaction condition. On the other hand,
the non-thermal plasma condition provides non-equilibrium condition in which temperatures of bulk
gas and electron are different [34]. The non-thermal plasma can activate electrons without activating
the rest of bulk gas. High-energy electrons (1–20 eV) which can initiate the formation of various
reactive radicals are produced [35]. Since electron mass is very light, non-thermal plasma causes the
gas temperature to increase by only a few degrees [36]. Therefore, partially ionized gas can be created
in a simple reactor configuration with relatively inexpensive power source, while reaction condition
would be maintained almost isothermally. By virtue of active species such as electron and ionized gas
generated by plasma the reaction can be initiated without applying high temperature. Among various
non-thermal plasma sources, dielectric barrier discharge (DBD) plasma has been commonly utilized by
the researchers so far since it is easier to set up than other non-thermal plasma sources [37-42].
Therefore, DBD plasma was used in this work as a plasma source.
In this paper, OCM has been carried out to produce C2 and C3 hydrocarbons directly from
methane using plasma-catalyst hybrid system with Ag/SiO2 catalyst. Since catalyst only reaction
required high temperature above 700 °C, dielectric barrier discharge (DBD) plasma, one of non-thermal
plasma, was applied to lower the reaction temperature. We aimed at finding the optimum OCM catalyst
to have the highest C2+ hydrocarbon yield at low temperature (< 400 °C) in the plasma-catalyst hybrid
system, which was found to be Ag/SiO2. In addition, the time-on-stream (TOS) activity of Ag/SiO2
under the plasma-catalyst hybrid system was investigated. The plasma regeneration and the thermal
regeneration in oxygen-rich condition were introduced to the deactivated catalyst in order to provide
the appropriate regeneration strategy in the OCM reaction in plasma-catalyst hybrid system.
2. Experimental
4

2.1. Plasma-catalyst hybrid system
A schematic view of plasma-catalyst hybrid system is presented in Fig. 1. It consisted of a
1000:1 high voltage probe (Tektronics P6015A), a current probe (Pearson electronics 6585), and a
capacitor (2000 pF) for measuring voltage (V), current (A), and transferred electric charge (Q),
respectively. All output signals were transmitted to a 100 MHz digital oscilloscope (Tektronics DPO
2014) which was utilized to calculate discharge power by the V-Q Lissajous figure method. The system
is described well in the previous paper [43].
The catalysts in bead form (30–40 mesh) were located on the quartz wool in the middle of
reactor inside the plasma discharge zone. The height of catalyst bed varied from 5.0 mm to 15.0 mm
because of the difference in the density of catalyst where weight of the catalyst was fixed to 0.5 g except
for the case of BaTiO3. In order to fill the discharge gap where the plasma was generated, the weight of
BaTiO3 which had high density increased to 1.0 g. In the OCM reaction, detection of reaction
temperature is important. However, thermocouple also could be a ground electrode. Since it could be
interfered by electric discharge of plasma, it is hard to measure the catalyst temperature directly.
Therefore, the catalyst bed temperature was measured and recorded using K type thermocouple which
was protected by another quartz tube just below the quartz wool in the reactor. Another thermocouple
in the furnace was used to control the setting temperature.
Configuration of the plasma-catalyst hybrid reactor is shown also in Fig. 1. The DBD reactor
was made of α-Al2O3 tube with an inner diameter of 12.0 mm, a thickness of 2.0 mm and a length of
300.0 mm. The quartz wool was located at the middle of reactor and its thickness was about 5.0 mm.
In this reactor, catalyst zone and plasma zone were overlapped at the same position by loading catalyst
into the plasma zone. Electrical discharge source of the reactor was DBD. The reactor consisted of a
stainless steel rod of which thickness was 4.0 mm. The rod was located at the center of α-Al2O3 tube
reactor. The outer surface of the reactor was surrounded by stainless steel plate with a length of 20.0
mm and a thickness of 0.2 mm. Therefore, the length of discharge zone was 20.0 mm, and the discharge
5

gap was 4.0 mm, resulting in a reaction volume of about 2.0 cm3 in plasma only condition. However,
in plasma-catalyst hybrid reaction, this reaction volume varied depending on the packing material since
plasma could not be generated at the space occupied by the packing material. In order to create the
plasma, an AC voltage was generated by an arbitrary function generator (GW INSTEK AFG-2012) and
amplified by a high voltage amplifier (TREK 20/20C-HS) with a maximum of 7.5 kVP-P. Then,
dielectric barrier (i.e. α-Al2O3 tube) was located between high voltage electrode and ground electrode
to stabilize plasma formation. All experiments were carried out under the identical conditions of
sinusoidal waveform with the driving frequency of 4 kHz.
2.2. Preparation of catalyst
Commercial supports such as BaTiO3 (Sigma Aldrich), TiO2 (Millennium Inorganic
Chemicals), γ-Al2O3 (Sasol), fumed SiO2 (Sigma Aldrich), and glass bead (Sigma Aldrich, 30–40 mesh)
were utilized to investigate the effect of support under DBD plasma condition. Then, Ag were loaded
on SiO2 by using wet impregnation method. AgNO3 (≥99.0%, Sigma Aldrich) were used as metal
precursor. The loading amount of metal was fixed to 2 wt% of the catalyst. The catalysts were dried and
subsequently calcined in static air at 500 °C for 2 h. Then, the catalysts were sieved to the particle size
of 30–40 mesh. Solid-state phases were identified by using X-Ray Diffraction (RIGAKU SMARTLAB)
with a Cu Kα radiation operated at 40 kV and 50 mA. Surface area of the catalysts was also obtained
by using the N2 adsorption/desorption method in an ASAP 2010 instrument (Micromeritics Co.).
2.3. Activity measurement
All of the reaction results including catalyst only reaction, plasma only reaction, and plasma-
catalyst hybrid reaction were measured at atmospheric pressure in the same α-Al2O3 reactor. The
reactants were 24% CH4, 6% O2, balanced in Ar. The total gas flow rate was 100 ml/min. Each gas was
controlled by using mass flow controllers (SIERRA). Effluent gases passing through the discharge
6

region went through the water trap and were analyzed by a gas chromatography (AGILENT GC 6890N)
equipped with a thermal conductivity detector (TCD) and CP-7429 capillary column. The column was
composed of two parallel columns (CP-Molsieve 5Å and PoraBOND Q) which allowed us to detect H2,
O2, CO, CH4, CO2, C2H2, C2H4, C2H6, C3H6, and C3H8. Among the reaction products, C2+ hydrocarbon
includes C2H2,C2H4, C2H6, C3H6, and C3H8 in this experiment. Methane consumption except the amount
used for hydrocarbon production was regarded as being consumed for coke formation. The activity of
the hybrid system was measured at 20 min after the OCM reaction started except the case of time-on-
stream reaction. Methane conversion and C2+ hydrocarbon yield were obtained while temperature was
varied from room temperature to about 700 ºC in the presence and in the absence of catalyst.
Conversion of methane, and selectivity and yield of CO, CO2, and hydrocarbons are obtained in the
following:
(
)
푚표푙푒 푐표푛푠푢푚푒푑 퐶퐻4
(
)
[ ]
× 100 %
Conversion 퐶퐻4
=
(1)
(2)
(3)
(
)
푚표푙푒 ꢀ푛푡푟표푑푢푐푒푑 퐶퐻4
푚표푙푒 (푝푟표푑푢푐푒푑 퐶푂)
푚표푙푒 (푐표푛푣푒푟푡푒푑 퐶퐻4)
(
)
[ ]
× 100 %
Selectivity CO =
푚표푙푒 (푝푟표푑푢푐푒푑 퐶푂2)
푚표푙푒 (푐표푛푣푒푟푡푒푑 퐶퐻4)
(
)
[ ]
× 100 %
Selectivity 퐶푂2
=
(
)
(
)
푚표푙푒 푝푟표푑푢푐푒푑 퐻퐶 × 퐶푁 푝푟표푑푢푐푒푑 퐻퐶
(
)
[ ]
× 100 %
Selectivity HC =
(4)
(
)
푚표푙푒 푐표푛푣푒푟푡푒푑 퐶퐻4
(
)
Yield (HC) = 퐶표푛푣푒푟푠ꢀ표푛 퐶퐻4 × 푆푒푙푒푐푡ꢀ푣ꢀ푡푦 (퐻퐶)
Where CN and HC stands for carbon number and 퐶2+ ℎ푦푑푟표푐푎푟푏표푛, 푟푒푠푝푒푐푡ꢀ푣푒푙푦.
(5)
In addition to the gas chromatography, quadrupole mass spectrometer (QMS, Pfeiffer Vacuum,
PrismaTM QMS 200) was utilized to detect the product in real time. Especially, QMS was applied for
the O2 temperature-programmed desorption (O2 TPD) and the detection of coke oxidation to CO and
CO2. Before O2 TPD, oxygen plasma treatment was applied at the same condition as OCM reaction
except the gas ratio (30% O2, balanced in Ar) for 30 min.
Long term activity test was conducted at 385 ºC for 180 min in plasma-catalyst hybrid system.
7

After the reaction for 180 min, two regeneration processes were carried out: plasma regeneration and
thermal regeneration. The plasma regeneration proceeded in the condition of 30% O2 balanced in Ar at
378 ºC for 30 min in order not to alter the total flow rate and the proportion of Ar, discharge gas. Thermal
regeneration was applied at 600 ºC for 30 min after ramping from room temperature to 600 ºC at 5
ºC/min.
3. Result and discussion
3.1. Oxidative coupling of methane under plasma only condition
The first set of experiment was conducted under plasma only condition in order to establish a
reference for plasma-catalyst hybrid reaction. Plasma can activate methane and oxygen even at room
temperature [44]. By activating the methane, methyl radical is formed which leads to the production of
ethane or propane. Besides, CO, CO2, and other unsaturated hydrocarbons can be generated by activated
oxygen species. In case of plasma only reaction, methane conversion and C2+ hydrocarbon selectivity
were measured from room temperature to 397 °C at 7.5 kVP-P. The results are summarized in Table 1.
At about 400 °C, plasma only reaction reached the methane conversion of 21.3% and the C2+
hydrocarbon yield of 4.7%. Depending on the temperature, DBD showed the plasma power from 5.0 W
to 8.6 W. As described in Table 1, the growth in C2+ hydrocarbon yield with increasing temperature
could be ascribed to the fact that more reactive species like electrons were generated because of the
higher plasma power. Methane conversion and C2+ hydrocarbon selectivity in DBD increased as the
reaction temperature went up as described in Table 1. At 397 °C, the majority of the product was CO
(60.4%) and most abundant hydrocarbon product was C2H6 (17.1%), which corresponded well with
previous reports about the coupling of methane under plasma condition [45-47]. Other products like
C2H4 (2.3%), C3H8 (2.3%), and CO2 (17.0%) were also produced. Such product distribution was
attributed to the fact that the OCM reaction at relatively low plasma power started commonly from the
dissociation of methane molecules to produce methyl radicals which were readily combined to form
8

C2H6. As the temperature went up, both methane conversion and C2+ hydrocarbon selectivity increased
as shown in Table 1 at the same input voltage and frequency under DBD plasma condition. In Fig. 2,
Lissajous plots of DBD plasma are demonstrated in addition to the plasma power calculated from the
plots at each temperature. The plasma discharge power can be calculated by the multiplication of
capacitor value, plasma frequency, and area of Lissajous figure [43]. The lines (A-B) and (C-D) in Fig.
2 indicate the discharge transitions, and their slope is equal to dielectric capacitance. Line (B-C) and
(D-A) are capacitive transitions of which slope are equal to total capacitance [48]. According to Fig. 2,
it was identified that the dielectric capacitance increased as the reaction temperature went up, resulting
in the increased plasma power.
3.2. Oxidative coupling of methane with various supports under plasma condition at low
temperature
OCM reaction over the conventional catalysts under the catalytic system requires high reaction
temperature as mentioned above. It is hard to control the selectivity of products though activation of
methane can be achieved through plasma. Thus, a new approach for lowering reaction temperature
while maintaining high selectivity using plasma-catalyst hybrid system was devised in this research. At
first, various supports such as γ-Al2O3, TiO2, and BaTiO3 as well as SiO2 were applied under DBD
plasma condition to investigate which support was appropriate for OCM under the plasma condition at
low temperature (< 400 °C). As displayed in Fig. 3, OCM under DBD plasma condition with various
supports was carried out from ambient temperature to about 400 °C. Among the support materials, SiO2
showed higher C2+ hydrocarbon yield than plasma only reaction in a plasma-catalyst hybrid system.
According to Table 2 showing dielectric constant and surface area of the supports, hydrocarbon yield
of the supports under plasma condition was inversely proportional to the dielectric constant of the
supports. There are some controversial opinions about the relationship between dielectric constant and
plasma-catalytic performance [49-52]. However, according to Wang, et al. [53], the net electric field
9

decreases due to the accumulation of electrons on the solid surface, especially for the supports with
large dielectric constant, resulting in the decreased activity. Besides the dielectric constant, large surface
area of the support could have a beneficial effect on the charge accumulation over the surface, which
can lead to the formation of enhanced discharge. For this reason, the hydrocarbon yield might be
proportional to the surface area of the supports. Hence, glass bead, which has same dielectric constant
with SiO2 but much smaller surface area, was tested to examine the effect of surface area on the activity
compared with SiO2. As shown in Fig. 3, the activity result confirmed that glass bead showed similar
synergistic effect to SiO2. Therefore, dielectric constant seemed to play a crucial role in the OCM
performance when using various support materials with plasma.
3.3. Ag/SiO2 catalyst under plasma-catalyst hybrid system
3.3.1. Oxidative coupling of methane over Ag/SiO2 catalyst with plasma
It was proposed that the performance of catalysts was proportional to that of supports in
plasma-catalyst hybrid system [53]. Because SiO2 showed the best performance among the supports
under DBD plasma condition, Ag was loaded on SiO2. The amount of metal was fixed to 2 wt%. XRD
patterns of the catalysts (not shown) demonstrated that only the peaks assigned to the amorphous SiO2
existed without any metal-related peaks, indicating that the metal species were highly dispersed over
SiO2 surface. Ag revealed the highest hydrocarbon yield at 385 °C as shown in Fig. S1. The Ag on SiO2
catalyst showed the hydrocarbon yield of 9.7% at the reaction temperature of 385 °C. In order to
evaluate the synergistic effect of the reaction, the synergistic effect factor was introduced. The
synergistic effect factor was defined as the yield of C2+ hydrocarbon in plasma-catalyst hybrid reaction,
divided by the sum of C2+ hydrocarbon yield in plasma only and catalyst only reaction. The synergistic
effect factor was calculated by using Eq. (6):
푌
퐻퐶(푃−퐶)
( )
6
Synergistic effect factor =
× 100 [%]
푌
퐻퐶(푃) + 푌
퐻퐶(퐶)
10

Where YHC(P-C), YHC(P), and YHC(C) indicates the C2+ hydrocarbon yield under plasma-catalyst hybrid
system, the C2+ hydrocarbon yield with plasma only, and the C2+ hydrocarbon yield with catalyst only,
respectively.
Surface area and synergistic effect factor of the catalysts are 308 m2/g and 209%, respectively.
The value of synergistic effect factor was calculated based on the activity result at 385 °C. In addition,
the catalyst only reaction did not present any C2+ hydrocarbon yield at reaction temperature below
400 °C and thus YHC(P) + YHC(C) was equal to YHC(P). The synergistic effect factor shows the correlation
among plasma-catalyst hybrid reaction, plasma only reaction, and catalyst only reaction. If the yield of
hybrid reaction is equal to the sum of plasma only and catalyst only reaction yields, the synergistic
effect will be 100%. Therefore, the value of synergistic effect factor above 100% indicates that a
synergistic effect existed on OCM under plasma-catalyst hybrid system. Ag/SiO2 demonstrates high
value of synergistic effect factor (209%) in the plasma-catalyst hybrid reaction at 385 °C. In addition,
the maximum plasma power in this study was about 8.4 W (5.1 kJ/L, Fig. S2), which was much lower
than that of previous works (17.7 kJ/L–45.0 kJ/L) [26, 54-56]. Therefore, it can be mentioned that the
synergistic effect between plasma and Ag/SiO2 is remarkable. Since the addition of Ag/SiO2 catalyst
did not lead to change of plasma power in spite of the increased total capacitance, it can be claimed that
the synergistic effect did not arise from plasma configuration but from interaction between plasma and
catalyst. According to Bao, et al. [57], Ag catalysts were verified to be active for OCM when it was
applied under oxygen-limited condition at atmospheric pressure, which is consistent with our result.
3.3.2. Role of oxygen in oxidative coupling of methane under plasma-catalyst hybrid system
Methane conversion, C2+ hydrocarbon yield, and carbon product selectivity under the reaction
condition of OCM at about 400 °C are displayed in Fig. 4. As shown in Fig. 4, 2 wt% Ag/SiO2
demonstrated synergistic effect in the presence of oxygen (from 4.7% to 9.7% with regard to C2+
hydrocarbon yield). Furthermore, OCM reaction in plasma-catalyst hybrid system with Ag/SiO2
11

showed carbon balance above 95% without considering coke formation in 20 min of reaction, which
meant that little amount of coke was formed. In addition, compared with OCM reaction under plasma
only condition, selectivity of olefin products such as C2H4 and C3H6 increased in the plasma-catalyst
hybrid system, representing that methane was dissociated more abundantly over Ag/SiO2 in the
presence of oxygen. This led to the result that Ag/SiO2 could alter the reaction pathway of OCM.
According to Rocha, et al. [58], different oxygen species could be created on and in the Ag catalyst.
Among the oxygen species, Oγ were assigned to strongly bound, intercalated oxygen which diffused via
interstitialcy diffusion into the uppermost layers of the reconstructed Ag surface at high temperature
above 600 °C, where it might react with gaseous reactants [59]. Such subsurface oxygen (Oγ) was
suggested to react with methane to give the dehydrogenation of methane to methyl radical as shown in
Fig. 5 [60]. Ag metal played a role as the active catalyst for oxi-dehydrogenation while Oγ worked for
pure dehydrogenation. In the plasma condition, Oγ might be generated at relatively low temperature
since the specific input energy by plasma (273.7 kJ/mol at 1 atm, 385 °C, assuming ideal gas) was
sufficiently higher than the activation energy of oxygen diffusion in Ag (140.0 kJ/mol) which was
regarded as the rate determining step of the reaction [61]. Hence, the generation of Oγ led to H
abstraction from methane molecule, which resulted in the coupling of methane. In order to identify the
existence of Oγ, O2 temperature-programmed desorption (O2 TPD) was conducted by using QMS after
the oxygen plasma treatment for 30 min. As shown in Fig. 6, the oxygen peak originated from Oγ after
the oxygen plasma treatment was observed at about 800 °C, consistent with the result of Nagy, et al.
[60]. Note that 0.5 g of 2 wt% Ag/SiO2 (i.e. Ag 0.01 g) was applied in this study whereas 1.0 g of Ag
was used in the reference. Therefore, the peak size was far smaller than the reference one. Nevertheless,
it can be said that Oγ which is active for the dehydrogenation of methane was generated by the plasma
treatment at much lower temperature (380 °C) compared to catalyst only reaction that requires high
temperature above 600 °C. This is in good agreement with the conclusion of Xu, et al. [62] who
mentioned that subsurface oxygen increased the reactivity of the Ag surface and enhanced the kinetics
of H2 dissociation substantially based on density functional theory calculations. Furthermore, the
12

selectivity of C2H4 and C3H6 that were formed by H abstraction from C2H6 and C3H8, respectively,
increased in the presence of oxygen. This confirmed that the role of oxygen (Oγ) was important for the
dehydrogenation of saturated hydrocarbons as well as methane, which resulted in the production of
unsaturated hydrocarbons.
3.4. Long term activity of plasma-catalyst hybrid system for oxidative coupling of methane
Long term activity of the catalyst was explored in order to utilize in practical application. Fig.
7 shows the methane conversion, C2+ hydrocarbon selectivity, and C2+ hydrocarbon yield of Ag/SiO2 in
the plasma-catalyst hybrid system with time-on-stream (TOS). As shown in Fig. 7, both methane
conversion and C2+ hydrocarbon selectivity decreased as the reaction proceeded. At first, fresh Ag/SiO2
revealed 9.7% of C2+ hydrocarbon yield. However, after 180-min TOS in the plasma-catalyst hybrid
system, the yield dropped to 5.9%. At the same time, the color of the catalyst turned to dark brown in
addition to the deposition of black carbon on the surface of high voltage electrode and reactor wall.
Under plasma condition, methane can decompose into atomic carbon [63]. Such carbon deposition is
expected to have the detrimental effect on both catalyst and plasma.
Therefore, the regeneration process was applied in order to recover the reactivity of plasma-
catalyst hybrid system by eliminating the carbon deposition. Two regeneration methods were designed:
plasma regeneration and thermal regeneration. The plasma regeneration was carried out for 30 min at
378 °C. The specific input energy in the plasma regeneration was calculated to 290.1 kJ/mol (Fig. S3,
at 1 atm, 378 °C, assuming ideal gas). The evolution of CO and CO2 during plasma regeneration on
aged Ag/SiO2 measured by QMS is demonstrated in Fig. 8(a). As shown in Fig. 8(a), oxidation of the
carbon deposition to CO and CO2 (mostly to CO2) started immediately as plasma was turned on. This
was attributed to the formation of reactive oxygen species, which could easily interact with the carbon
deposition to produce CO and CO2. To ensure the effect of oxygen plasma regeneration, thermal
regeneration was applied to the post-reaction catalyst at same gas ratio with the plasma regeneration.
13

Fig. 8(b) demonstrates the evolution of CO and CO2 during thermal regeneration on aged Ag/SiO2
observed by QMS. In case of thermal regeneration, most of the coke did not react until temperature
reached about 500 °C while the plasma regeneration started instantly at 378 °C. This can be attributed
to the deficiency of reactive bulk oxygen species that promotes the oxidation of carbon deposition in
the case of thermal regeneration. When the temperature reached about 500 °C, the MS signals of CO
and CO2 began to increase. The thermal regeneration reaction was carried out until MS signals of CO
and CO2 became stabilized and flattened. After each regeneration processes, OCM reaction over the
regenerated catalysts proceeded in the plasma-catalyst hybrid system. The OCM reaction result of two
regenerated catalysts compared with fresh one in the plasma-catalyst hybrid system is displayed in Fig.
9. Plasma regenerated Ag/SiO2 catalyst showed completely recovered activity, while the thermally
regenerated one partially did (7.3% of C2+ hydrocarbon yield). In addition, as OCM reaction went on
over the latter sample, the deactivation got much worse. In order to identify the influence of plasma
regeneration and thermal regeneration on catalyst, X-Ray Diffraction (XRD) was taken. As displayed
in Fig. 10, XRD pattern of the fresh Ag/SiO2 shows that only the peaks assigned to the amorphous SiO2
existed without any metal-related peaks, indicating that the Ag was highly dispersed over the SiO2
surface. Crystalline phase of Ag is hardly seen after plasma regeneration although the catalyst exhibited
highly crystalline Ag (JCPDS 65-2871) phase after the thermal regeneration. Therefore, it can be
claimed that sintering of Ag took place significantly during the thermal regeneration. Since the catalyst
was initially calcined at 500 °C, thermal regeneration above 500 °C can give rise to Ag sintering, which
resulted in the deactivation of the catalyst. Furthermore, aged catalyst after the plasma regeneration
even showed the better C2+ hydrocarbon yield (10.5%) than fresh catalyst at the beginning of the
reaction. This can be explained by the fact that additional Oγ formed during plasma regeneration
improved the dehydrogenation of methane without sintering of Ag.
Besides, additional regeneration by plasma at about 378 °C was conducted on the thermally
regenerated Ag/SiO2 to verify whether the imperfect thermal regeneration was caused by sintering of
the catalyst. CO and CO2 peaks were detected again by QMS during plasma regeneration, which implies
14

that residual carbon deposition still existed after thermal regeneration (Fig. S4). Furthermore, it was
confirmed that after the additional regeneration by plasma, the reactivity of plasma-catalyst hybrid
system was restored to 9.0%. However, the reactivity was not fully recovered even after the additional
regeneration by plasma because of Ag sintering. From the results, it was confirmed that both sintering
of the catalyst and residual carbon deposition on the catalyst contributed to the partial recovery after
thermal regeneration. In addition to this regeneration process, method of enhancing the coke resistance
of catalyst by adding ZrO2 promoter can be suggested [64]. However, we focused on the regeneration
process in this study.
4. Conclusion
Combined reaction system of plasma and catalyst was applied to OCM reaction in order to
lower the reaction temperature of OCM. Under plasma only condition, methane started to be activated
to produce C2+ hydrocarbon at low temperature. SiO2 showed the best hydrocarbon yield among the
various supports like BaTiO3, TiO2, γ-Al2O3, and SiO2. Hence, Ag was loaded on SiO2. The Ag/SiO2
catalyst presented suitable performance with DBD plasma (9.7%) at 385 °C where common OCM
reaction using catalytic process did not take place. In this process, it was confirmed that oxygen
adsorbed on the catalyst played a key role in promoting OCM reaction.
The OCM reaction was carried out for 180 min to investigate the long term activity of plasma-
catalyst hybrid system, resulting in the continuous deactivation due to the formation of coke deposition.
Therefore, plasma and thermal regeneration were applied to regenerate the aged catalyst for OCM
reaction. The plasma regeneration could recover the activity of the plasma-catalyst hybrid system
completely at low temperature whereas the thermal regeneration could not.
15

Acknowledgement
This work was supported by C1 Gas Refinery Program through the National Research
Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning
(2016M3D3A1A01913252).
Appendix A. Supplementary data
Supplementary data (Fig.S1-S4) associated with this article can be found in the online version.
16

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Figure captions
Fig. 1. Schematic view of plasma-catalyst hybrid system and reactor (bottom right corner).
Fig. 2. Plasma power with increasing temperature measured by Lissajous method in the condition of
DBD plasma only.
Fig. 3. C2+ hydrocarbon yield under dielectric barrier discharge plasma condition using various supports.
Fig. 4. Methane conversion, hydrocarbon yield and product selectivity in the condition of oxidative
coupling of methane at about 400 ºC after reaction for 20 min; Hybrid=2 wt% Ag/SiO2 + Plasma.
Fig. 5. Proposed reaction pathway of OCM which is initiated by the dehydrogenation of methane to
methyl radical via Oγ.
Fig. 6. Oxygen temperature-programmed desorption (O2 TPD) of Ag/SiO2 pretreated in oxygen plasma
from room temperature to 900 °C with the ramping rate of 2 °C/min.
Fig. 7. Methane conversion, C2+ hydrocarbon selectivity, and C2+ hydrocarbon yield of plasma-catalyst
hybrid system with time-on-stream.
Fig. 8. The evolution of CO and CO2 during plasma regeneration (a) and thermal regeneration (b)
measured by quadrupole mass spectrometry (QMS).
Fig. 9. Oxidative coupling of methane (OCM) reaction results of fresh Ag/SiO2, aged Ag/SiO2 after
thermal regeneration, and aged Ag/SiO2 after plasma regeneration.
Fig. 10. X-Ray Diffraction (XRD) patterns of fresh Ag/SiO2, aged Ag/SiO2 after plasma regeneration,
and aged Ag/SiO2 after thermal regeneration.
21

Fig. 1. Schematic view of plasma-catalyst hybrid system and reactor (bottom right corner).
22

397℃,8.60W
303℃,6.61W
216℃,6.02W
141℃,5.20W
104℃,5.02W
400
300
200
100
0
A
D
-100
-200
-300
-400
C
B
-4000 -3000 -2000 -1000
0
1000 2000 3000 4000
Input voltage(V)
Fig. 2. Plasma power with increasing temperature measured by Lissajous method in the condition
of DBD plasma only.
23

6
4
2
0
Plasma
TiO2 + Plasma
SiO2 + Plasma
-Al2O3 + Plasma
BaTiO3 + Plasma
Glass bead + Plasma
50
100
150
200
250
300
350
400
450
Temperature(℃)
Fig. 3. C2+ hydrocarbon yield under dielectric barrier discharge plasma condition using various
supports.
24

Coke selectivity
CO2 selectivity
CO selectivity
C3H6 selectivity
C3H8 selectivity
C2H4 selectivity
C2H6 selectivity
Methane conversion
C2+ hydrocarbon yield
30
25
20
15
10
5
100
90
80
70
60
50
40
30
20
10
0
0
Plasma
Hybrid
Plasma
Hybrid
Fig. 4. Methane conversion, hydrocarbon yield and product selectivity in the condition of
oxidative coupling of methane at about 400 ºC after reaction for 20 min; Hybrid=2 wt% Ag/SiO2
+ Plasma.
25

Fig. 5. Proposed reaction pathway of OCM which is initiated by the dehydrogenation of methane
to methyl radical via Oγ.
26

2.50E-011
2.00E-011
1.50E-011
1.00E-011
5.00E-012
after plasma treatment
before plasma treatment
0
100 200 300 400 500 600 700 800 900
Temperature(℃)
Fig. 6. Oxygen temperature-programmed desorption (O2 TPD) of Ag/SiO2 pretreated in oxygen
plasma from room temperature to 900 °C with the ramping rate of 2 °C/min.
27

CH4 conversion
39
36
33
30
27
24
21
10
9
C2+ hydrocarbon selectivity
C2+ hydrocarbon yield
8
7
6
20
40
60
80
100
120
140
160
180
200
Time(min)
Fig. 7. Methane conversion, C2+ hydrocarbon selectivity, and C2+ hydrocarbon yield of plasma-
catalyst hybrid system with time-on-stream.
28

Fig. 8. The evolution of CO and CO2 during plasma regeneration (a) and thermal regeneration
(b) measured by quadrupole mass spectrometry (QMS).
29