Ambient-temperature oxidative coupling of methane in an electric field by a cerium phosphate nanorod catalyst

Article abstract of DOI:10.1039/c9cc00174c

CePO₄ nanorods with uniform surface Ce sites could work as a durable catalyst and showed the highest C₂ yield of 18% in an electric field without the need for external heating, which was comparable to that reported for high-performance catalysts at high temperature (>900 K).

Full text of DOI:10.1039/c9cc00174c

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DOI: 10.1039/C9CC00174C.
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Ambient‐temperature Oxidative Coupling of Methane
in an Electric Field by a Cerium Phosphate Nanorod Catalyst
Ayaka Sato,a Shuhei Ogo,*ab Keigo Kamata,*bc Yuna Takeno,a Tomohiro Yabe,a
Received 00th January 20xx,
Accepted 00th January 20xx
Tomokazu Yamamoto,d Syo Matsumura,d Michikazu Hara,c Yasushi Sekinea
DOI: 10.1039/x0xx00000x
www.rsc.org/
A CePO4 nanorod with the uniform surface Ce sites could work as a proposed the formation of active oxygen species involving the
durable catalyst and showed the highest C2 yield of 18% in the redox reaction of a Ce cation.18 However, these systems (e.g.,
electric field without the need for external heating, which was Ce2(WO4)3/CeO2) have several drawbacks; (i) the need for a
comparable to those for the reported high performance catalysts at CeO2 support leads to difficult structure control of surface
high temperature (>900 K).
active Ce sites, (ii) and low surface areas (~4 m2/g), and (iii)
which results in insufficient C2 yield (~6%). Herein, we focus on
the OCM activity of a monoclinic CePO4 nanorod catalyst with
structurally controlled uniform active sites.19 The present
system has significant advantages of high C2 yields (18%) and
durability in sharp contrast to other cerium‐based catalysts. This
study provides the first example of the catalytic application of
CePO4 to OCM at low‐temperature in the electric field (Scheme
1).
Methane, a principle component of natural gas, has attracted
much attention because it is abundant and distributed world‐
wide.1 However, methane is mostly combusted for heating and
little are used for hydrogen production by steam reforming;1, 2
thus, methane is an underutilized resource to produce chemical
products. Since oxidative coupling of methane (OCM) is a useful
reaction to directly convert methane to high value‐added C2
hydrocarbons,1−6 various non‐catalytic and catalytic processes
have been developed for OCM reactions from the 1980s.2−10
However, most systems typically require high reaction
temperatures over 900 K for OCM reaction,3−6 leading to low
selectivity to C2 hydrocarbons due to an overoxidation of the
more reactive C2 hydrocarbon products than methane.3−6
Despite extensive studies on the developments of highly active
and selective OCM catalysts over 40 years, the C2 hydrocarbon
yields have been limited so far to less than 25%, the majority
was below 20%, in the high temperature range (900–1200 K).4−6,
10−14 Therefore, the development of novel catalytic systems to
efficiently promote the selective OCM reaction even in low
temperature region remains a challenging subject of research.
We have very recently reported that the application of
electric field to Ce‐W‐O catalyst systems enables the selective
OCM reaction even at low temperatures such as 423 K 15−17 and
Scheme 1. Low‐temperature OCM by a monoclinic CePO4 nanorod catalyst
combined with an electric field.
Monoclinic CePO4 synthesized using
a hydrothermal
method could act as an efficient acid‐base bifunctional solid
catalyst for chemoselective acetalization.19 Although the IR
measurements for a sample with adsorbed probe molecules
(pyridine, chloroform, acetone, and methanol) indicated the
presence of uniform Lewis acid and weak base sites, the
detailed surface structure of CePO4 was still unclear.
Aberration‐corrected high‐angle annular dark‐field scanning
transmission electron microscopy (HAADF‐STEM) image and
a. Department of Applied Chemistry, Waseda University, 3‐4‐1 Okubo, Shinjuku,
Tokyo, 169‐8555 Japan. E‐mail: ogo@aoni.waseda.jp (S. Ogo)
b. PRESTO, Japan Science and Technology Agency (JST), 4‐1‐8 Honcho, Kawaguchi,
Saitama, 332‐0012 Japan.
c. Laboratory for Materials and Structures, Institute of Innovative Research, Tokyo
Institute of Technology, Nagatsuta‐cho 4259, Midori‐ku, Yokohama, 226‐8503
Japan. E‐mail: kamata.k.ac@m.titech.ac.jp (K. Kamata)
d. Department of Applied Quantum Physics and Nuclear Engineering, Kyushu
University, Motooka 744, Nishi‐ku, Fukuoka 819‐0395, Japan.
† Electronic Supplementary Informaꢀon (ESI) available: Experimental details, Tables
S1‐S2 and Figure S1‐S8. See DOI: 10.1039/x0xx00000x
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fast Fourier transform (FFT) pattern are shown in Figures 1(a) activity without the electric field even at high temperature of
and S1. The HAADF‐STEM results proved that the dominant 1073 K. Therefore, the effect of Joule heat by the imposed
facets exposed on CePO4 nanorods are (110) and (100), electric field was not important in this low‐temperature
especially (110) is more dominant (see the details in the Figure catalytic system, and the CePO4 catalyst can activate methane
S1). The amount of surface Ce cations at (110) and (100) facets only when the electric field is applied.
are estimated to be 1.6 and 2.2 nm–2, respectively (Figures 1(b)
20
and 1(c)), in good agreement with the experimental one (1.6
nm–2) calculated from the BET surface area of CePO4 (37 m2 g–1)
and the surface Ce cations with Lewis acid sites measured using
pyridine‐IR (96 µmol g–1)19 (see the details in the Electronic
15
CePO
Supplementary Information). These results support the
presence of uniform surface Ce species on CePO4 nanorods.
4
Ce (WO )
4 3
2
Ce (MoO )
10
5
2
4
3
CeVO
4
CeO
2
0
0
10 20 30 40 50 60 70
Time on stream / min
Figure 2. Time course of the C2 hydrocarbon yield for OCM over various cerium‐
based catalysts at ambient temperature in the electric field: CH4:O2:Ar = 15:12:33,
total flow rate 60 SCCM; catalyst, 100 mg; current, 7.0 mA; furnace temperature,
without external heating (299 K).
Figure 1. (a) The TEM image with FFT pattern and (b) the (110) surface and (c) the
(100) surface structure of monoclinic CePO4 catalyst.
To investigate the reaction mechanism, periodic operation
tests over the CePO4 catalyst with the electric field were
conducted; First, the catalyst was pretreated with O2 in the
electric field, and then the methane was introduced in the
electric field (see the details in the Electronic Supplementary
Information). Figure S4 shows time course of the formation rate
of C2 hydrocarbons during methane supply in the periodic
operation test with the electric field. The C2 hydrocarbons
formation gradually decreased and almost completed within 5
min. After the catalyst was regenerated by the O2 treatment in
the electric field, the C2 formation rate was almost the same as
that of the first cycle. In addition, there was no significant
difference in the catalytic activity and C2 selectivity (>75%) for
the periodic operation tests (5 cycles) with the electric field as
shown in Figure 3. In contrast, for the periodic operation test
without the electric field, C2 selectivity (<20%) and the CH4
conversion (<1%) were very low even at high temperature of
1073 K as presented in Figure S5. These results indicated that
the active oxygen species was formed on the catalyst surface by
the reaction with O2 in the electric field and such species were
consumed by the reaction with methane in the electric field.20
The amounts of active oxygen species for productions of C2
and COx were determined from the integrated product yields in
the periodic operation tests during methane supply after O2
pre‐treatment in the electric field (Figures 4 and S6). The
consumed active oxygen species for C2 production was
estimated to be 93 µmol‐O2 g‐cat−1 during methane supply, and
the value was almost equal to the number of surface Ce site on
the CePO4 catalyst (96 µmol g‐cat−1) as mentioned above.19 The
Raman measurements were conducted for the recovered CePO4
Then, activity tests for OCM in the electric field (7 mA) over
various cerium‐based catalysts including CePO4 were conducted
without external heating. Figure 2 shows the time course of the
C2 hydrocarbon yield for OCM over various Ce‐based catalysts
(CePO4, Ce2(WO4)3, Ce2(MoO4)3, CeVO4, and CeO2) and the
details are shown in Table S1. XRD patterns and BET surface
area of the catalysts are shown in Figure S2. The full details
about the characterization of CePO4 including IR spectroscopy
for a sample with adsorbed probe molecules, SEM image and
XPS (Ce species in CePO4 was Ce3+) have been already reported
in the previous paper.19 Among the catalysts tested, CePO4
catalyst showed the highest C2 hydrocarbon yield (about 18%)
in the electric field without external heating. The OCM activity
of CePO4 catalyst at low temperature (542 K) with the electric
field is comparable to that of Mn‐Na2WO4/support system
catalysts at 1073 K 11 which is one of the highest performance
catalysts. Although the catalytic activity of CePO4 did not change
for 1 h, the C2 hydrocarbon yields rapidly decreased in the case
of Ce2(WO4)3 and Ce2(MoO4)3. In addition, CeVO4, and CeO2
were almost ineffective; thus, the CePO4 catalyst exhibited high
and stable OCM activity in the electric field. There was no
significant difference in the XRD patterns between the fresh and
spent CePO4 (Figure S3), which indicates the high durability of
monoclinic CePO4 during the reaction even in the electric field.
As shown in Table S2, the catalyst‐bed temperature increased
to 542 K in the electric field (7 mA) because of Joule heat and
reaction heat. However, the CePO4 catalyst showed low OCM
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catalysts after both O2 treatment and reaction with CH4 in the
In conclusion, CePO4 catalyst with the uniform surface Ce
electric field to evaluate active oxygen species on the surface Ce species showed the high activity and stability for low‐
sites of CePO4.21 As shown in Figure S7, the Raman band at 538 temperature oxidative coupling of methane (C2 yield = 18%) in
cm−1 appeared after the O2 treatment in the electric field while the electric field without external heating. In the electric field,
it completely disappeared after the reaction with CH4 in the formation and regeneration of the surface active oxygen
electric field. It has been reported that the Ce–O bands of active species over the surface Ce sites of CePO4 proceeded easily
oxygen species on CeO2 are observed in the range of 340–540 even at low‐temperature, and thereby OCM proceeds
cm–1.22−24 Therefore, the 538 cm−1‐band would be related to effectively through consumption and reproduction of the active
active oxygen species on CePO4. The potentials of Ce–O species oxygen species. The present study provide design for high
for the activation of small hydrocarbons including CH4 have performance catalysts: The Ce‐based catalyst with high specific
been proposed;25−27 thus, such species suitable for C2 surface area and uniform active sites will show a high OCM
production is possibly formed on the CePO4 catalyst through the activity. The combination of uniform active sites and electric
activation of O2 molecule on the surface Ce sites assisted by the field can be an effective approach to avoid undesirable
electric field, which results in OCM reaction even at low reactions thanks to the lowering reaction temperature and will
temperature.
open the door for highly selective catalytic oxidation reaction
under mild conditions.
This study was supported by JST PRESTO (Grant No.:
JPMJPR16S4, JPMJPR15S3).
100
8
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6
5
4
3
2
1
0
80
60
40
20
0
Conflicts of interest
There are no conflicts to declare.
Notes and references
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