Synthesis of Ni-CeO2 catalyst for the partial oxidation of methane using RF thermal plasma

Article abstract of DOI:10.1016/S1872-2067(15)61071-3

Ni-CeO₂ catalysts with a nickel content of 50 mol% were prepared using RF thermal plasma, and their catalytic activities for methane partial oxidation were characterized. For the synthesis of Ni-CeO₂ catalysts, a precursor containing

Full text of DOI:10.1016/S1872-2067(15)61071-3

Chinese Journal of Catalysis 37 (2016) 743–749
a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c h n j c
Article
Synthesis of Ni‐CeO2 catalyst for the partial oxidation of methane
using RF thermal plasma
M. Y. Leea, J. S. Namb, J. H. Seoa,b,
*
a Department ofApplied Plasma Engineering, Chonbuk National University, Jeonbuk 54896, Korea
b Department of Quantum System Engineering, Chonbuk National University, Jeonbuk 54896, Korea
A R T I C L E I N F O
A B S T R A C T
Article history:
Ni‐CeO2 catalysts with a nickel content of 50 mol% were prepared using RF thermal plasma, and
their catalytic activities for methane partial oxidation were characterized. For the synthesis of
Ni‐CeO2 catalysts, a precursor containing Ni (~5‐μm diameter) and CeO2 (~200‐nm diameter)
powders were heated simultaneously using an RF plasma at a power level of ~52 kVA and a powder
feeding rate of ~120 g/h. From the X‐ray diffraction data and transmission electron microscopy
images, the precursor formed into high crystalline CeO2 supports with nanosized Ni particles
(< 50‐nm diameter) on their surfaces. The catalytic performance was evaluated under atmospheric
pressure at > 500 °C and a CH4:O2 molar ratio of 2:1 with Ar diluent. Although the Ni content was
high (~50 mol%), the experimental results reveal a methane conversion rate of > 70%, selectivities
of CO and H2 greater than 90% and slight carbon coking during an on‐stream test at 550 °C for 24 h.
However, at 750 °C, the on‐stream test revealed the formation of filament‐like carbons with an
increased methane conversion rate over 90%.
Received 4 January 2016
Accepted 23 February 2016
Published 5 May 2016
Keywords:
RF thermal plasma
Methane
Partial oxidation
Nickel catalyst
Ceria
Carbon deposition
© 2016, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
1. Introduction
ses of supported metal catalysts have been presented in the
studies Zea et al. [9] and Vissokov [10], who reported the
Thermal plasmas have attracted attention as a heat source
for the preparation of nanosized powders [1–5]. As an ionized
flow with temperatures of > 3000 °C [6,7], the massive en‐
thalpies of thermal plasmas enable the microsized metal pre‐
cursor to evaporate during its flight in the thermal plasma
flame [1,5]. By quenching the resultant vapor species, na‐
nosized metal particles are produced. However, refractory ma‐
terials, such as Al2O3 and ZrO2, show incomplete evaporation
because of their high boiling points coupled with poor thermal
conductivity [1,8]. If the mixture of precursors containing metal
and refractory materials is injected into the thermal plasma
flame, the preferential evaporation of metal powder can occur,
and the metal vapors condense into nanosized metal catalysts
dispersed on the refractory supports. Thermal plasma synthe‐
preparation processes of PdAg on carbon and Ni metal on Al2O3
catalysts, respectively, using a non‐transferred DC arc plasma
torch. They showed that nanosized metal particles were
formed on the support materials. However, the surface area of
the carbon support decreased because of the volatilization of
oxygen groups caused by the strong arc plasma [9]. Moreover,
the DC arc plasma treatment of Ni metal and Al2O3 oxides em‐
bedded Ni metal into Al2O3 droplets, which caused metal loss in
the catalysts [10].
The DC arc torch produces a hot ionized jet with high veloci‐
ties up to hundreds of m/s, and RF plasma torches provide a
flame of a large size and mild velocities less than several tens of
m/s [1,2,5]. In addition, the absence of electrodes allows solid
precursors to be injected along the centerline of the RF torch.
* Corresponding author. Tel: +82‐63‐270‐4295, E‐mail: jhseo@jbnu.ac.kr
DOI: 10.1016/S1872‐2067(15)61071‐3 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 37, No. 5, May 2016

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M. Y. Lee et al. / Chinese Journal of Catalysis 37 (2016) 743–749
Because the trajectories along the centerline of the RF torch are
heated continuously by the plasma flame, the RF plasma torch
is stable and has a controllable heating route to inject the solid
precursor along the centerline of the torches [1,11,12]. The
evaporation process of the mixed solid precursors during the
passage of the RF plasma torch can be controlled easily, de‐
pending on the physical properties of the mixture constituents,
such as thermal conductivity and the melting and boiling
points.
In this work, we explore the synthesis route of a nanosized
Ni metal catalyst highly dispersed on a ceria support by the
coinjection of Ni and ceria powders into an RF thermal plasma.
As an oxide support of Ni catalysts for the catalytic partial oxi‐
dation of methane, ceria is widely used because of a high oxy‐
gen storage capacity and unique redox behavior, depending on
the partial pressure of ambient oxygen [13–17]. The melting
point of ceria is high at 2400 °C [18], which can help the ceria
precursor travel the thermal plasma flame in the solid state.
Therefore, the RF thermal plasma treatment of Ni and ceria
mixtures allows the preferential evaporation of Ni metal. The
Ni vapors can nucleate on CeO2 surface by a quenching process.
Because the preferential evaporation and nucleation processes
of Ni powder occur during the in‐flight treatment, the Ni‐CeO2
composite can form highly dispersed Ni on the CeO2 support,
which enhances their catalytic activity and resistance to carbon
coking in the partial oxidation of methane [19].
transferred to the RF thermal plasma using induction from an
RF power supply (Trumpf, IG30/3000, Ditzingen, Germany),
which has a maximum power level of 60 kVA. A water‐cooled
quartz tube with an inner diameter of 50 mm was used to con‐
fine the generated RF thermal plasma inside an RF plasma
torch, which was mounted on a synthesis reactor. The synthesis
reactor contains a cylindrical guide with an inner diameter of
300 mm and a length of 500 mm to expand the confined RF
plasma flame as presented in Fig. 1. By expanding the plasma
flame, the nucleation and condensation of the evaporated spe‐
cies can be enhanced as discussed in the simulation by Mendo‐
za‐Gonzalez et al. [20]. In this work, nanosized Ni‐CeO2 cata‐
lysts were synthesized by injecting a solid precursor containing
Ni and CeO2 powders into the RF thermal plasma. For this pur‐
pose, the feeding of the solid precursor was along the center‐
line of the RF plasma torch using a water‐cooled injector, which
is connected to the powder feeder as shown in Fig. 1. For the
effective heating of the mixed solid precursor, Ni (99.9%;
YeeYoung Cerachem, Seoul, Korea) and CeO2 (99.9%; YeeYoung
Cerachem) powders with diameters of 5 µm and 200 nm, re‐
spectively, were weighed at a molar ratio of 1:1, and mixed for
48 h using a ball mill. Fig. 2 presents the field emission scan‐
ning electron microscopy (FE‐SEM) image of the mixed solid
precursor, where the CeO2 powders with diameters of 100–200
nm were distributed uniformly on the surface of microsized Ni
metal. After RF thermal plasma treatment of the precursor, the
nanocomposite powders were retrieved by filtration (Fig. 1).
The operating conditions for this process are listed in Table 1.
2. Experimental
2.1. Catalyst preparation using RF thermal plasma
2.2. Catalyst characterization
The RF thermal plasma system used for the preparation of
Ni‐CeO2 catalysts is illustrated in Fig. 1. In this system, electro‐
magnetic energy with oscillation frequencies of 2–4 MHz is
The morphologies and particle sizes of the powders were
analyzed using FE‐SEM (S4300, Hitachi, Tokyo, Japan) and
transmission electron microscopy (TEM; JEM‐4010, Jeol, Tokyo,
Fig. 1. A schematic diagram of an RF thermal plasma system used for preparation of Ni‐CeO2 catalysts.

M. Y. Lee et al. / Chinese Journal of Catalysis 37 (2016) 743–749
745
Table 2
Experimental conditions for catalytic performance of as‐prepared
composite.
Test material
Ni‐CeO2 100 mg
Gas components
CH4:O2:Ar = 4:2:94,
Total flow rate of 100 cm3/min
Gas chromatography
(Yong Lin, YL6100)
Detector: TCD, FID
Column: Carboxen‐1000 packing column
Carrier gas: N2
Furnace condition
Reactor
Temperatures: room temperature to 800 °C
Holding time at measured temperature: 30
min
Quartz Tube with inner diameter of 6 mm
Fig. 2. FE‐SEM images of the solid precursor consisting of Ni (~5 m)
and CeO2 (100–200 nm) powders.
(2) CO2 conversion rate (%) = (moles of CO2 consumed/
moles of CO2 introduced) × 100
(3) CO selectivity = moles of CO produced/
(moles of CH4 consumed + moles of CO2 consumed)
(4) H2 selectivity = moles of H2 produced/
Table 1
Operating parameter values for synthesis of Ni‐CeO2 catalysts by RF
thermal plasma.
(2 × moles of CH4 consumed)
Operating parameter
Plasma forming gas (Ar)
Central gas
Sheath gas
Quenching gas
Operating pressure (torr)
Plate power (kVA)
Feeding rate of precursors (g/h)
Value
Flow rate (L/min)
From the H2‐TPR data and the partial oxidation of methane,
the catalytic performance of the Ni‐CeO2 nanocomposite was
estimated. The degree of carbon deposition was examined from
the variation in catalytic activities during 24‐h tests for the
partial oxidation of methane at 550 and 750 °C. Moreover,
FE‐SEM, TEM and thermogravimetric analysis, (SDT Q600, TA
Instruments, New Castle, DE, USA) after the 24‐h test were
performed to show the resistance to carbon deposition of the
catalysts and the amount of carbon deposition.
20
90
120
~400
~52
~120
Japan). The crystalline structures of the powders were investi‐
gated by X‐ray diffraction (XRD; X’pert PRO MPD, PANalytical
B.V., Almelo, The Netherlands) with Cu Kα radiation source (λ =
0.15406 nm). In addition, the composition changes were evalu‐
ated by comparing the inductively coupled plasma‐optical
emission spectrometry (Optima 5300DV, PerkinElmer, Wal‐
tham, MA, USA) results between the precursors and powders.
The local compositions of the particles were also examined
using energy dispersive X‐ray spectroscopy, (EDS; Link ISIS
6498, Oxford Instruments, Abingdon, UK) from the TEM imag‐
es. The morphological properties of the powders, such as sur‐
face area, pore volume and pore size were calculated from N2
adsorption‐desorption isotherms using the Brunauer‐Emmett‐
Teller method (ASAP 2020, Micromeritics, Norcross, GA, USA).
3. Results and discussion
Fig. 3 and Fig. 4 show the FE‐SEM and TEM images of the
composites, respectively. The inset in Fig. 4 presents the EDS
data corresponding to the numbered areas in the image. From
the comparison between FE‐SEM images in Fig. 2 and Fig. 3,
microsized solid precursors are formed into small particles
with sizes under 100 nm. In addition, the TEM image and the
EDS data in Fig. 4 show that the particles mainly consist of CeO2
particles with sizes of several tens of nm, which are covered
with very small particles of high Ni content. The solid precursor
was Ni powder of almost 5‐µm diameter covered with CeO2
2.3. Catalytic performance test
For the Ni‐CeO2 nanocomposite, temperature‐programmed
reduction (H2‐TPR; AutoChem II 2920, Micromeritics) data
were obtained to investigate the reducibility of Ni‐based mate‐
rials. The partial oxidation of methane was conducted over the
Ni‐CeO2 nanocomposite at atmospheric pressure and a reaction
mixture of molar ratio CH4:O2 = 2:1 with Ar diluent. The details
of the test conditions are presented in Table 2. The methane
conversion rate was calculated with the selectivity to H2 and CO
for the temperatures from 400 to 800 °C using to the following
definitions:
(1) CH4 conversion rate (%) = (moles of CH4 consumed/
moles of CH4 introduced) × 100
Fig. 3. FE‐SEM image of the as‐prepared Ni‐CeO2 composite.

746
M. Y. Lee et al. / Chinese Journal of Catalysis 37 (2016) 743–749
composites. From the comparison between these two XRD
graphs, the crystalline peaks for CeO2 were enhanced in the
composites. Because the partially evaporated CeO2 particles
condense into very small particles during the quenching pro‐
cess, their contribution to the crystalline peak enhancement is
negligible. Therefore, these enhanced peaks originated from
CeO2 particles with reduced sizes, which crystallized during the
RF thermal plasma heat treatment. However, Ni and NiO crys‐
talline peaks were decreased and increased, respectively, in the
composites, compared with those in the precursor. Because the
inert gas of Ar is used in this work as listed in Table 1, these
changes in Ni and NiO crystalline peaks indicate that reactions
may occur between Ni metal and the lattice oxygen of CeO2. For
example, NiO particles can form from Ni vapors and free oxy‐
gen escaping from the lattices of CeO2 during the RF thermal
plasma heating process. The NiO can condense on the surfaces
of CeO2 or into free NiO particles. With this in‐flight formation
of NiO particles, most Ni vapors can be directly nucleated into
Ni metal particles on the surface of CeO2. In addition, we expect
that NiO layers between the Ni metal and CeO2 supports can
form by incorporating Ni ions into the lattice of CeO2 because
the heated CeO2 surface is hot enough to cause a high temper‐
ature reaction between the nucleated Ni metal and CeO2 lattic‐
es. Thus, highly dispersed Ni/NiO particles on nanosized CeO2
supports were synthesized by the RF thermal plasma treatment
of Ni metal and CeO2 powders with sizes of almost 5 μm and
200 nm, respectively, as shown in Fig. 3 and Fig. 4.
To investigate the reduction properties of these highly dis‐
persed Ni/NiO particles on CeO2 supports, a H2‐TPR test was
conducted, and the results are shown in Fig. 6. For comparison,
Fig. 6 also presents the results of the H2‐TPR test for a commer‐
cial NiO powder (637130; Sigma‐Aldrich, St Louis, MO, USA)
with diameters of ~50 nm and the CeO2 powder used as a sup‐
port material in this work. For the nanocomposite shown in Fig.
6, high reduction peaks were observed at the temperatures of
225, 350 and 750 °C. Among these, the reduction peak at 350 °C
is extremely intense. Compared with the test results of the
commercial NiO and CeO2 powders, the consumption of H2 at
225 and 750 °C represents the reduction of free NiO and CeO2,
respectively. As reported in previous studies [14,21,22], the
Fig. 4. TEM image of the as‐prepared Ni‐CeO2 composite. The inset
shows the EDS analysis data corresponding to the numbered area in the
TEM image.
particles (~200‐nm diameter) as shown in Fig. 2, the electron
microscope observations of the composites indicate that the
microsized Ni powders in the solid precursors instantaneously
evaporated during their passage in the RF thermal plasma
flame. The produced vapors condensed into very small parti‐
cles on the surfaces of the ceria. However, the ceria particles in
the solid precursors reduced in size by RF thermal plasma at‐
trition and did not fully evaporate because of their high boiling
point and poor thermal conductivity. Therefore, ceria particles
with reduced sizes became the seeds for the nucleation of Ni
vapor during the quenching process at the central guide cylin‐
der shown in Fig. 1, which creates nanosized CeO2 supports
(< 100‐nm diameter) with highly dispersed Ni‐rich particles on
their surfaces as shown in Fig. 4.
Table 3 shows the inductively coupled plasma‐optical emis‐
sion spectrometry results of the composite. The Ni content was
reduced to 4.2 at.% compared with the original value. In our
experiment, the particles were retrieved at the filtration part
shown in Fig. 1, and the composition changes in Table 3 may
occur because of the complicated interactions between the
trajectories of the in‐flight treatment precursors during their
travel from the RF torch to filtration. If the loss of Ni results
from this transportation process, the composition change will
occur systematically in our equipment, and the difference can
be reduced by adjusting the starting Ni concentration to con‐
sider the loss of Ni in the system. In addition, the morphological
properties of the catalyst were measured by the Brunau‐
er‐Emmett‐Teller method. The surface area, pore volume and
pore size are 56.64 m2/g, 0.29 cm3/g and 21.04 nm, respec‐
tively. The as‐prepared Ni‐CeO2 composite has a high surface
area, indicating that the microsized precursor was successfully
formed into a nanosized composite by RF thermal plasma.
Fig. 5 presents the XRD data of the solid precursors and the
CeO2
NiO
Ni
Table 3
Atomic ratio of Ni to Ce for the as‐prepared Ni‐CeO2 composite obtained
from ICP‐OES analysis result.
(2)
(1)
20
*
Constituent (at.%)
Item
10
30
40
50
60
70
80
Ni
Ce
2 /(o )
Weighed (1)
As‐prepared (2)
Difference between (1) and (2)
50.0
45.8
–4.2
50.0
54.2
+4.2
Fig. 5. Comparison of XRD graphs between mixture precursor (1) and
the as‐prepared Ni‐CeO2 composite (2).

M. Y. Lee et al. / Chinese Journal of Catalysis 37 (2016) 743–749
747
1.0
100
NiO
0.8
80
Ni-CeO2
0.6
60
H2
CO
CeO2
0.4
40
0.2
20
0.0
0
0
200
400
Temperature (oC)
600
800
100 200 300 400 500 600 700 800 900
Temperature (oC)
Fig. 6. Comparison of H2‐TPR graphs between commercial NiO powders
(~50 nm), CeO2 powders (~200 nm) and the as‐prepared Ni‐CeO2
composite.
Fig. 8. Selectivity for H2 and CO at the temperatures from 400 to 800 °C.
The test conditions are listed in Table 2.
The figure shows that the conversion rates were maintained
with slight variation during the test despite the high Ni content
of 50 mol%. Because the degradation of catalytic performance
is accompanied by carbon deposition on the active metal sites,
the catalyst was analyzed by TEM after the 24‐h tests at 550
and 750 °C. Fig. 10 shows images for the test temperatures of
550 and 750 °C, respectively. These figures show a low amount
of carbon deposition on the catalyst used in the 24‐h test at 550
°C despite the high Ni content of 50 mol%. However, solid car‐
bon was coated on the Ni metal for the catalyst after the 24‐h
test at 750 °C.
The results of carbon deposition depending on temperature
are supported by the thermogravimetric analysis of the cata‐
lysts after 24‐h tests at 550 and 750 °C as shown in Fig. 11. In
this figure, the decrease of mass in the range of 200–300 °C was
caused by the hydrogenation reaction between water and
amorphous carbon. The burning of graphitic carbon causes a
mass decrease in the temperature range of 470–650 °C
[17,19,23]. The amounts of carbon deposition were estimated
as 0.58 and 2.39 wt.% for the catalysts tested at 550 and 750
°C, respectively. Therefore, these catalysts show good re‐
reduction peaks in the temperature range of 300–400 °C are
the reduction of NiO solid solution particles on a CeO2 surface,
and have a significant effect on catalytic performance at low
temperatures.
To examine the catalytic performance, we partially oxidized
methane using the nanocomposite under the test conditions
listed in Table 2. Fig. 7 shows the methane conversion rate
from 400 to 800 °C. The composite shows a high methane con‐
version rate of ~76% at a low temperature of 550 °C. For the
temperatures higher than 550 °C, the conversion rates were
elevated to the values higher than 90%. In addition, the H2 and
CO selectivities were ~90% at 550 °C as observed in Fig. 8,
which shows the selectivities to H2 and CO for the temperatures
between 400 and 800 °C. As expected from the H2‐TPR results
shown in Fig. 6, the high catalytic activities at a low tempera‐
ture (~550 °C) are because of the numerous active metal sites,
which are highly dispersed NiO solid solution. The high Ni con‐
tent of 50 mol% in the mixture precursor provides an abun‐
dance of active metal sites.
Fig. 9 presents the methane conversion rates for 24‐h test at
550 and 750 °C, showing the catalytic stability of the catalyst.
100
100
80
60
40
20
0
(2)
80
(1)
60
40
20
0
0
100 200 300 400 500 600 700 800
Temperature (oC)
0
2
4
6
8
10 12 14 16 18 20 22 24
Time (h)
Fig. 7. Methane conversion rate of the as‐prepared Ni‐CeO2 composite
for the temperature ranges from 400 to 800 °C.
Fig. 9. Methane conversion rate of the as‐prepared Ni‐CeO2 composite
for 24‐h test of CH4 partial oxidation at 550 (1) and 750 °C (2).

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M. Y. Lee et al. / Chinese Journal of Catalysis 37 (2016) 743–749
CeO2 support, which formed highly dispersed active metal sites
(a)
(c)
on the CeO2 surface despite the high Ni content of 50 mol%.
H2‐TPR and XRD results of the catalysts reveal that many active
metal sites are NiO solid solution on the CeO2 surface, and this
layer improves the resistance of the catalyst to carbon deposi‐
tion. The results indicate high catalytic activities at the temper‐
atures of ≥550 °C during the partial oxidation of methane. No
carbon deposition is occurred for the catalyst used in the 24‐h
test at 550 °C despite the high Ni content of 50 mol%. From
these advantages of the catalyst, the in‐flight treatment of Ni
metal and CeO2 support by the RF thermal plasma can be used
for the synthesis of Ni‐CeO2 catalysts with highly dispersed
active metal sites at a high Ni content of 50 mol%.
(b)
(d)
Acknowledgments
Fig. 10. FE‐SEM (a, c) and TEM (b, d) images for the as‐prepared
Ni‐CeO2 after 24‐h test of CH4 partial oxidation at 550 (a, b) and 750 °C
(c, d).
This research was supported by Renewable Energy Tech‐
nologies Development Program (No. 2008NFC02J0200002009)
and Technology Innovation Program (No. 10048910) funded
by the Ministry of Trade, Industry and Energy (MI, Korea).
100.0
-0.23%
-1.29%
(1)
(2)
99.5
99.0
98.5
98.0
97.5
97.0
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4. Conclusions
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M. Y. Lee et al. / Chinese Journal of Catalysis 37 (2016) 743–749
749
Graphical Abstract
Chin. J. Catal., 2016, 37: 743–749 doi: 10.1016/S1872‐2067(15)61071‐3
Synthesis of Ni‐CeO2 catalyst for the partial oxidation of methane using RF thermal plasma
M. Y. Lee, J. S. Nam, J. H. Seo*
Chonbuk National University, Korea
Epoxides were Ni‐CeO2 nano catalyst was prepared by in‐flight treatment of Ni and CeO2 powders in RF thermal plasma. The as‐prepared
catalyst showed high catalytic activities during the partial oxidation of methane.
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