Structural effect of one-dimensional samarium oxide catalysts on oxidative coupling of methane

Article abstract of DOI:10.1166/jnn.2018.14647

We report an interesting structural effect of one-dimensional Sm₂O₃ catalysts such as nanorods, nanobelts and nanotubes synthesized by a simple solvothermal method on oxidative coupling of methane. The Sm₂O₃ nanobelts showed the 28% CH4 conversion and 42% C₂ selectivity at 500 °C. The different spatial structures and surface structures of these Sm₂O₃ catalysts indeed brought about the distinct exposed facets, surface active oxygen species and surface active sites, which could account for their diverse activity and products selectivity in OCM reaction. Otherwise, the Sm₂O₃ nanobelts doped with Sr increased the C₂ selectivity to 48% at 500 °C, which enhanced the C₂ yield sharply.

Full text of DOI:10.1166/jnn.2018.14647

Article
Journal of
Nanoscience and Nanotechnology
Vol. 18, 3398–3404, 2018
Copyright © 2018 American Scientific Publishers
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Printed in the United States of America
Structural Effect of One-Dimensional Samarium Oxide
Catalysts on Oxidative Coupling of Methane
Bo Fu1ꢀ2, Tao Jiang1ꢀ2, and Yan Zhu1ꢀ∗
1CAS Key Laboratory of Low-Carbon Conversion Science and Engineering, Shanghai Advanced Research Institute,
Chinese Academy of Sciences, 201210, China
2University of Chinese Academy of Sciences, Beijing 100049, China
We report an interesting structural effect of one-dimensional Sm2O3 catalysts such as nanorods,
nanobelts and nanotubes synthesized by a simple solvothermal method on oxidative coupling of
methane. The Sm2O3 nanobelts showed the 28% CH4 conversion and 42% C2 selectivity at 500 ꢀC.
The different spatial structures and surface structures of these Sm2O3 catalysts indeed brought
about the distinct exposed facets, surface active oxygen species and surface active sites, which
could account for their diverse activity and products selectivity in OCM reaction. Otherwise, the
Sm2O3 nanobelts doped with Sr increased the C2 selectivity to 48% at 500 ꢀC, which enhanced the
C2 yield sharply.
Keywords: Samarium Oxide, Oxidative Coupling of Methane, One-Dimensional Structure.
IP: 85.70.228.150 On: Tue, 19 Jun 2018 10:33:02
Copyright: American Scientific Publishers
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1. INTRODUCTION
performed excellently.13ꢁ14 Our previous studies15 also
investigated that La2O3 with one-dimensional nanorods
had higher catalytic activity and C2 selectivity at low
temperatures compared to conversional spherical parti-
cles. No further significant advance in the correlate other
one-dimensional structures like belt, tube, and wire with
catalysis in OCM reaction has no reported in the related
literature. Therefore, it is worthwhile exploring the struc-
ture effect of one-dimensional morphologies of rare earth
oxides on the catalytic activity and C2 selectivity. Sm2O3
with well-defined one-dimensional structure has rarely
been considered in this catalytic reaction, and their catal-
ysis for OCM is not unheard. Based on these questions,
herein we elaborately controlled the synthetic conditions of
Sm2O3 catalysts to obtain different one-dimensional struc-
tures, and study the effect of the different one-dimensional
structure of the catalysts on the methane conversion and C2
selectivity. Our studies indicate that belt-like Sm2O3 cata-
lyst achieved higher conversion of methane and selectivity
of C2, when compared with rod-like and tube-like ones,
which is attributed to the more surface activity specites
and sites of the former induced by the structural factor.
In this work, one-dimensional Sm2O3 nanocatalysts
including belt, rod and tube were synthesized to cat-
alyze the oxidative coupling of methane. Methane activity
Over the past few decades, researchers have concentrated
on the direct conversion of methane,1 whose conversion to
more useful carbon chemicals is highly desired. Oxidative
coupling of methane (OCM) is a considerably attentive
reaction to produce high valuable hydrocarbons such as
ethylene and ethane (C2ꢀ from methane directly.2ꢁ3 How-
ever, methane is a symmetrical molecule and a very stable
hydrocarbon and most of methane conversions are ther-
modynamically unfavorable at 298 K. Until now the yield
toward ethylene cannot meet the industrial requirement
(C2 yield is about 25%) in a real world, due to the undesir-
able surface reaction and gas-phase combustion reaction to
improve the deeper oxidation of methane and C2 products,
although researchers endeavored to enhance the selectiv-
ity of C2 hydrocarbons. In particular, the direct conver-
sion of methane to form ethylene and ethane is not easy
at mild temperatures, and the activation of CH4 and cou-
pling of methyl rꢀadicals to produce C2 are usually pro-
ceeding at ∼800 C.4–6 Among all kinds of catalysts for
OCM, the rare earth oxides7–11 have exhibited excellent
properties. The OCM reaction has been found to be struc-
ture sensitive,12 whereas one-dimensional structure catalyst
∗Author to whom correspondence should be addressed.
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1533-4880/2018/18/3398/007
doi:10.1166/jnn.2018.14647

Fu et al.
Structural Effect of One-Dimensional Samarium Oxide Catalysts on Oxidative Coupling of Methane
and C2 hydrocarbons selectivity descend as the following
sequence: Sm2O3 nanobelts > Sm2O3 nanorods > Sm2O3
nanotubes. The enhanced catalysis of Sm2O3 nanobelts
should be attributed to the more open exposed face, more
relative amount of active oxygen species and stronger
surface basic sites, compared to Sm2O3 nanorods and
nanotubes.
The catalyst power (170 mg) was heatedꢀ a He flow from
ꢀ
60 to 800 C with a heating rate of 10 C/min and then
keep for 60 min at 700 ꢀC, and cooled down to 60 ꢀC. The
ꢀ
CO2 was injected at 60 C for 60 min. after that He was
injected and flow for 60 min, finalꢀly the temperature was
ꢀ
raised to 800 C with a rate of 10 C/min.
2.3. Catalytic Activity Measurement
The catalytic activity test of the oxidative coupling of
methane (OCM) was evaluated in fixed bed quartz tubu-
lar reactor under atmospheric pressure. In a typical test,
0.2 g fresh Sm2O3 catalyst (40–60 mesh) and 0.8 g silica
sand were mixed together and placed in quartz. The cat-
alyst was treꢀated in N2 atmosphere with 30 mL/min flow
rate at 800 C for 2 h prior to the reaction. When the
feed temperature was cooled to room temperature, a flow
(240 mL/min) of mixed reactant gas, consisting of methane
and oxygen (CH4/O2 = 3) passed the catalyst bed. In order
to separate the condensed water vapor produced during the
reaction, a cold trap was needed at the outlet of the quartz
tube. The outlet products were detected by a micro gas
chromatogragh (3000 Micro GC; Inficon) with two ther-
mal conductivity detectors (TCD), one Molecular sieve 5A
and one Plot U columns.
2. EXPERIMENTAL DETAILS
2.1. Catalyst Preparation
Synthesis of Sm2O3 nanobelts: The 4.44 g Samarium
nitrate (Sm(NO)3 ·6H2O) was dissolved into 75 mL deion-
izer water with stirring, the precursor was obtained after
the 5 mL ammonia (28%∼30%) being added slowly and
stirring for 1 h. The precursor solution was transferred
ꢀ
into a 100 mL Teflon-lined autoclave and kept 110 C for
12 h in an oven. When the solution was cooled to room
temperature, the precipitate was centrifuged and washed
by ethanol and ultra-pure water several times before being
ꢀ
dried in a vacuum drying oven at 60 C. Finally the pow-
der samples were calcined in air at 800 ꢀC for 1 h to obtain
the product.
Synthesis of Sm2O3 nanotubes: The 0.5460 g CTAB was
put into 30 mL deionized water, and then the 0.7372 g
samarium nitrate was added into the solution under stirring
at room temperature and stirred for about 1 h. The 0.2 mL
ammonia (25%∼28%) was added dropwise into the above
3. RESULTS AND DISCUSSION
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To gain insight into the influence of the precise structure
solution and stirred for 2 h to get the precursor. Then we
Copyright: American Scientific Publishers
of Sm O3 on the nature of catalytic active sites, the cata-
lysts were evaluated for the oxidative coupling of methane.
As presented in Figure 1(a), the activities of Sm2O3 cata-
lysts for OCM process descend as the following sequence:
nanobelts > nanorods > nanotubes. Although the initial
ignition temperature of methane over the three catalysts
was 500 ꢀC, Sm2O3 nanobelts could give rise to 28% con-
version of methane and 42% selectivity toward C2 hydro-
carbons at 500 ꢀC. At that condition, Sm2O3 nanorods
gave 38% C2 selectivity and tube-like catalyst exhibited
only 34% C2 selectivity in Figure 1(b). From Figures 1(c)
and (d), the selectivity for CO and CO2 over Sm2O3 nano-
tubes is the highest among the three catalysts, revealing
that the deep oxidation process is easy to be carried out
on the tube-like catalyst. The possible reason might be
ascribed to the presence of the surface active species on
the tubes to favor the complete oxidation of methane or C2
products, which will be confirmed by the following stud-
ies. If the feed temperature is high, C2 hydrocarbons are
put at a serious disadvantage, since methane is more ther-
modynamic stable than C2 products. This phenomena has
been proved by this result that with the increase of feed
temperature, the C2 selectivity decreased while CO2 selec-
tivity went up for the three catalysts. Therefore, it is worth
noting that valuable products such as ethylene and ethane
can be improved by keeping the reaction temperature at a
favorable range.
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put the precursor into a 50 mL Teflon-lined autoclave and
kept it at 120 ꢀC for 24 h in an oven. The nanotubes were
ꢀ
centrifuged and drieꢀd at 60 C, then the final nanotubes
was calcined at 800 C for 1 h to obtain.
Synthesis of Sm2O3 nanorods: After we obtained the
precursor of nanotubes, 1.8 mL ammonia (25%∼28%) was
put into it and stirred for 10 min, and then we got the pre-
cursor of nanorods. Next, the nanorods precursor was put
ꢀ
into a 50 mL Teflon-lined autoclave and heated at 120 C
for 24 h. Nanorods were also gotten by centrifuging and
ꢀ
calcining at 800 C for 1 h in the same way.
2.2. Catalyst Characterization
The crystalline microstructure of the as-prepared cata-
lyst was characterized by X-ray powder diffraction (XRD)
with a RigakuD/Max-RB X-ray diffractometer with Cu
Kꢂ radiation. SEM characterization was detected by
ZEISS Supra 55. TEM characterization was observed
using JEOL JEM-2100 Electron Microscope (JEOL). The
Brunauer-Emmett-Teller (BET) was recorded by nitro-
gen adsorption–desorption isotherm measurements at 77 K
(ASAP 2010). X-ray photoelectron spectra (XPS) were
performed by a Thermo Fisher Scientific K-Alpha X-ray
photoelectron spectrometer. The binding energy values
were corrected using the C1s peak at 284.80 eV. CO2
temperature-programmed desorption measurements were
measured on Micromeritics AutoChem II 2920 instrument.
J. Nanosci. Nanotechnol. 18, 3398–3404, 2018
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Structural Effect of One-Dimensional Samarium Oxide Catalysts on Oxidative Coupling of Methane
Fu et al.
Figure 1. Catalytic performance of Sm2O3 catalysts with different one-dimensional catalysts for oxidative coupling of methane (space velocity:
72000 mL·g−1 ·h−1, CH4/O2 = 3:1, 1 atm).
We now turn to find the reasons why Sm2O3 nanobelts
that their crystal facets are indeed different. Figure 2(a)
shows that the average width of Sm2O3 nanobelts is about
200 nm and the length is up to 1–2 ꢃm with thick-
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showed better catalytic performance for OCM reaction in
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comparison of nanorods and nanotubes. Above all, the
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focus on the surface structures of the three catalysts shows
ness of 30 nm. For the Sm2O3 nanorods, the diameter is
Figure 2. SEM images (a, b, c), TEM images (d, e, f) and HRTEM images (g, h, i) of Sm2O3 catalysts.
J. Nanosci. Nanotechnol. 18, 3398–3404, 2018
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Fu et al.
Structural Effect of One-Dimensional Samarium Oxide Catalysts on Oxidative Coupling of Methane
Figure 3. XRD patterns of Sm2O3 catalysts.
approximate 20 nm and not longer than 400 nm shown in
Figure 2(b). Especially, the nanotube is constructed with
many tunnels rather than one tunnel. Each tube is up to
several micro-scale meters and the diameter of a tunnel is
in the range of 50–100 nm. XRD patterns of these cat-
alysts shown in Figure 3 display dominating diffraction
peaks at the 2ꢄ of 28.1, 32.6, 47.0 and 55.6ꢀ, which are
typically indexed as the (222), (400), (440), and (622)
diffractions well matching with Sm2O3 with face centered
cubic phase (JCPDS, PDF No. 43-1029). Further TEM
Figure 5. O1s XPS spectra of Sm2O3 catalysts.
look like multi-faced pillars with inner tunnels. The lattice
fringes of nanotubes are 0.32 nm and 0.26 nm apart, which
agree with the d values of (111) and (200). It is obvious
that the side enclosed facets are paralleled to the lattice
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fringes of (111), in that (111) facets are mainly exposed on
Copyright: American Scientific Publishers
nanotubes. For nanorods, it’s clearly that there is more than
and HRTEM images of these catalysts were analyzed to
make clear the distinct surface faces. According to the
crystal lattice fringes and the measure angles between the
faces, shown in Figure 2(g), the belt was detected along
the [111] direction, which means that (111) crystal planes
are mainly exposed in Sm2O3 thick-nanobelts. Here we
need to take the vertical faces into account, consisting of
the side and terminal ends of the nanobelts. Based on the
formula: r∗1 · r2∗ = r1∗ · r2∗ · cosꢂ (ꢂ: angle between crystal
facets, M∗: reciprocal vector, M = r1, r2).
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one kind of diffraction lattice inset the Figure 2(h), reveal-
ing that the nanorods were formed with multi crystals. As
a result, nanobelts enclosed by (110) facets are more effec-
tive for methane activation than nanorods and nanotubes,
which is well agreement with the results reported.16–18
Further SEM also offered insight into the structural
information of spent Sm2O3 catalysts after catalytic tests.
Three structures of Sm2O3 samples appeared to have no
significant aggregation after catalytic reaction (Fig. 4).
The electronic properties of the three catalysts were also
recorded via X-ray photoelectron spectroscopy (XPS). In
the O1s profiles in Figure 5, four components, i.e., super-
oxide ions O−2 , peroxide ions O−, lattice oxygen O2− and
carbonate CO23−, can be decomposed from the two peaks
observed for the three catalysts. It is well known that
surface-absorbed oxygen species (O− and O2−ꢀ benefit to
Not only the angle between the vertical side face and
(110) is 30ꢀ, but also the angle between the vertical side
face and (011) planes is 30ꢀ, which implies the presence
of (121) facets in vertical sides. In addition, the terminal
ends are enclosed by (110) facets since they are paral-
lel to the (110) facets combining the TEM and HRTEM
from Figures 2(d) and (g). In terms of the nanotubes, they
Figure 4. SEM images of Sm2O3 nanobelts (a), nanorods (b), and nanotubes (c) after the OCM reaction.
J. Nanosci. Nanotechnol. 18, 3398–3404, 2018
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Structural Effect of One-Dimensional Samarium Oxide Catalysts on Oxidative Coupling of Methane
Fu et al.
Table I. Surface area, porous textures, surface oxygen species, and
exposed facets of Sm2O3 samples.
obviously that the specific surface area is not consistent
with the catalytic performance trends, which suggests that
catalytic activities are not mainly influenced by the spe-
cific area on OCM reaction but by the active sites of
the catalysts, which is in keeping with the speculation in
our previous work.22 In this work, it is estimated that the
poor catalytic performance for OCM might be attributed
to a higher specific surface space exceeding certain opti-
mal condition, which means a higher residence time of
the intermediate species at the surface of the catalyst and
destroy of surface, leading to the complete oxidation of
methane to yield relative high amount of CO2 and CO. In
view of above analysis, next we transfer our point to the
surface basic sites for powerful explanation of the distinct
properties of the three catalysts.
Pore
size
Pore
volume
SBET
(m2 ·g−1
(O−+O−2 )
Catalyst
)
(nm) (cm3 ·g−1
)
/O2−
Exposed facets
Nanobelt
17ꢅ2
14.5
0.06
1.4
(121), (101)
and (111)
–
Nanorod
Nanotube
27ꢅ6
8ꢅ2
38.5
11.1
0.27
0.02
1.1
0.9
(111)
enhance the C2 selectivity, whereas lattice oxygen might
result in the formation of CO and CO2 through complete
oxidation process. The relative amount ratio of the elec-
trophilic oxygen species (O− and O2−ꢀ to lattice oxygen
(O2−ꢀ, which is pointed to be relative effectivity on C2
selectivity,19–21 is calculated as 0.90 for nanotubes, 1.11
for nanorods and 1.38 for nanobelts, respectively, listed in
Table I. As a comparison of the above ratio values, it can
be concluded that nanobelts should show higher selectivity
toward ethane and ethylene, which is in accordance with
the catalytic results.
The roles played by surface basic sites, which made
such a significant effect on OCM reaction, are clarified by
CO2-TPD analysis. The description by CO2-TPD told us
the nature surface basic sites on the surface of catalysts.
Generally speaking, the weak, medium and strong basic
sites can be found by the CO2 detected at lowꢀtemper-
ꢀ
ature (<200 C), middle temperature (300–600 C), and
ꢀ
high temperature (>600 C), respectively. From Figure 7,
The surface area and pore feature of catalysts could
play a significant role on the catalytic activities. Thus, the
information of specific surface area, pore size and pore
volume of catalysts were obtained from the N2 absorption–
desorption characterization. See from Figure 6 and Table I,
the specific surface area of nanobelts, nanorods and nano-
tubes are 17.2, 27.6 and 8.2 m2/g, respectively. It is
medium strong basic sites are existed on the surface of the
three Sm2O3 catalysts, obtained from two peaksꢀaround
ꢀ
250–600 C, in which one isꢀ located at 250–300 C, and
the other is located at 500 C. It should point out that
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the value of peak area represents the quantity of surface
basic sites, following as the order: nanobelts > nanorods >
nanotubes. The result has expressed a fact that Sm2O3
nanobelts have moderately strong surface basic sites which
are crucial for OCM.23–26
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In addition, the above result also gave us a clue to
improve C2 selectivity of OCM reaction to add the basic
metal as a promoter. The three Sm2O3 catalysts doped with
Sr indeed gave the enhanced methane conversion and C2
hydrocarbons selectivity, which has been approved by our
experiments (Fig. 8). It’s clearly that the conversions of
methane of Sm2O3 nanorods and nanotubes go up to 28%,
nanobelts lift the conversion of methane to 29% after dop-
ꢀ
ing with Sr at 500 C. Moreover, the C2 selectivity of
Sm2O3 nanobelts doped with Sr have been boosted from
ꢀ
42% to 48% at 500 C, in the meantime, both nanorods
Figure 6. N2 absorption–desorption isotherm of the three Sm2O3 cata-
lysts, and insets are the corresponding pore size distribution.
Figure 7. CO2-TPD profiles of the three Sm2O3 catalysts.
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Fu et al.
Structural Effect of One-Dimensional Samarium Oxide Catalysts on Oxidative Coupling of Methane
Figure 8. Catalytic performance of Sm2O3 catalysts doped Sr with different one-dimensional catalysts for oxidative coupling of methane (space
velocity: 72000 mL·g−1 ·h−1, CH4/O2 = 3:1, 1 atm).
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and nanotubes doped with Sr have strengthen the C2 selec-
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Copyright: American Scientific Publishers
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Received: 5 February 2017. Accepted: 31 March 2017.
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J. Nanosci. Nanotechnol. 18, 3398–3404, 2018