Morphology Effects of Nanoscale Er2O3 and Sr-Er2O3 Catalysts for Oxidative Coupling of Methane

Article abstract of DOI:10.1007/s10562-020-03503-6

Abstract: Er₂O₃ nanorods were prepared by a hydrothermal method, and Sr-modified Er₂O₃ nanorods (Sr-Er₂O₃) were synthesized using an impregnation method. Their catalytic performance for oxidative coupling of methane was investigated. The catalysts were characterized by several techniques such as XRD, N₂ adsorption, TEM, XPS, O₂-TPD and CO₂-TPD. Compared with Er₂O₃ and Sr-Er₂O₃ nanoparticles, Er₂O₃ and Sr-Er₂O₃ nanorods exhibit higher CH₄ conversion and C₂–C₃ selectivity. This is caused by higher (O^? + O₂^?)/O^2? ratio, a higher number of chemisorbed oxygen species and moderate basic sites achieved on the nanorods catalysts. The Sr-Er₂O₃ nanorods afford a 23.2% conversion of CH₄ with 50.3% selectivity to C₂–C₃ at 650?°C. Graphic Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-020-03503-6

Catalysis Letters
https://doi.org/10.1007/s10562-020-03503-6
Morphology Efects of Nanoscale Er2O3 and Sr‑Er2O3 Catalysts
for Oxidative Coupling of Methane
Yuqiao Fan1 · Mingxing Sun2 · Changxi Miao3 · Yinghong Yue1 · Weiming Hua1 · Zi Gao1
Received: 16 October 2020 / Accepted: 16 December 2020
© The Author(s), under exclusive licence to Springer Science+Business Media, LLC part of Springer Nature 2021
Abstract
Er2O3 nanorods were prepared by a hydrothermal method, and Sr-modifed Er2O3 nanorods (Sr-Er2O3) were synthesized
using an impregnation method. Their catalytic performance for oxidative coupling of methane was investigated. The catalysts
were characterized by several techniques such as XRD, N2 adsorption, TEM, XPS, O2-TPD and CO2-TPD. Compared with
Er2O3 and Sr-Er2O3 nanoparticles, Er2O3 and Sr-Er2O3 nanorods exhibit higher CH4 conversion and C2–C3 selectivity. This is
−
caused by higher (O− +O2 )/O2− ratio, a higher number of chemisorbed oxygen species and moderate basic sites achieved on
the nanorods catalysts. The Sr-Er2O3 nanorods aford a 23.2% conversion of CH4 with 50.3% selectivity to C2–C3 at 650 °C.
Graphic Abstract
Keywords Er2O3-based catalysts · Oxidative coupling of methane · Morphology efect · Crystal plane
1 Introduction
Methane is the main component of natural gas, coal-bed
Supplementary Information The online version contains
gas and shale gas, which can be converted into high value-
supplementary material available at https://doi.org/10.1007/s1056
added products through non-direct conversion and direct
conversion. On the one hand, methane can be frst converted
into syngas, and further transformed into important basic
chemical materials and liquid fuels by Fischer–Tropsch
reaction [1, 2]. On the other hand, in the direct conversion
technology of methane, most of the research is focused on
2-020-03503-6.
* Changxi Miao
miaocx.sshy@sinopec.com
* Weiming Hua
wmhua@fudan.edu.cn
Extended author information available on the last page of the article
Vol.:(0123456789)
1 3

Y. Fan et al.
the direct partial oxidation of methane to methanol [3] or
formaldehyde [4], oxidative coupling of methane (OCM)
reaction to C2 hydrocarbons (ethylene and ethane) [5–10],
and methane anaerobic aromatization [11, 12]. Owing to the
fast consumption of petroleum, the OCM reaction is of great
signifcance and attractive.
nanoparticles (labelled as Er2O3-NP) were synthesized by a
conventional precipitation method. 3.0 ml of ammonia water
was added dropwise to 100 ml of 0.1 M Er(III) nitrate solu-
tion under stirring. After stirring for additional 0.5 h, the
resulting precipitate was washed with deionized water, and
dried at 110 °C for 12 h. Finally, the dried Er(OH)3 nano-
particles were calcined in air at 750 °C for 4 h to obtain the
Er2O3 nanoparticles.
OCM was frst developed in 1982 by Keller and Bhasin
[5]. Since then, hundreds of catalysts have been attempted
for OCM. Among them, Li/MgO and Mn-Na2WO4/SiO2 are
reported to be the most promising catalysts, and have been
extensively studied over the past 30 years [13–28]. However,
both types of catalysts generally require high temperature
(above 800 °C) to efectively catalyze the OCM reaction. In
addition, Li/MgO catalysts still sufer from severe deactiva-
tion, due to the vaporization of Li+ ions at high temperature
[19]. Researchers have made an efort to develop the low-
temperature OCM catalysts. Wang et al. reported that the Ti-
doped Mn-Na2WO4/SiO2 catalyst by using Ti-MWW zeolite
as Ti dopant enabled OCM with 26% methane conversion
and 76% C2–C3 selectivity at 720 °C [29]. Rare earth oxides,
such as La2O3, Sm2O3 and their promoted forms with special
morphology, can efectively catalyze the OCM reaction at
relatively low temperatures (500–650 °C), although methane
conversion and C2 selectivity and yield are not so satisfac-
tory [30–34].
The Sr-modified Er2O3 nanorods and nanoparticles
(denoted as Sr-Er2O3-NR and Sr-Er2O3-NP, respectively)
were prepared by impregnating an aqueous solution of
Sr(NO3)2 on the dried Er(OH)3 nanorods or nanoparticles
via an incipient wetness method. The impregnated samples
were dried at 110 °C for 12 h and calcined at 750 °C in air
for 4 h to obtain the Sr-Er2O3-NR and Sr-Er2O3-NP catalysts.
The molar ratio of Sr to Er in both catalysts was 0.02.
2.2 Catalyst Characterization
X-ray difraction (XRD) patterns were recorded on a Bruker
D2 PHASER X-ray difractometer using nickel-fltered Cu
Kα radiation at 40 kV and 30 mA. The BET surface areas
of the samples were analyzed by N2 adsorption at−196 °C
using a Micromeritics Tristar 3000 instrument. Transmis-
sion electron microscopy (TEM) data were obtained on an
FEI Tecnai G2 F20 S-TWIN electron microscope equipped
with an EDX instrument. X-ray photoelectron spectroscopy
(XPS) measurements were carried out on a Perkin–Elmer
PHI 5000C spectrometer with Mg Kα radiation as the excita-
tion source. The O 1 s XPS spectra were ftted using the soft-
ware XPSPEAK 41, assumed at 20% Lorentzian and 80%
Gaussian peak shape. All binding energy values were refer-
enced to the C1s peak located at 284.6 eV. Temperature-pro-
grammed desorption of CO2 (CO2-TPD) and temperature-
programmed desorption of O2 (O2-TPD) were performed
on a Micromeritics AutoChem II analyzer. 0.2 g of sample
(40–60 mesh) was pretreated at 750 °C for 1 h in a He fow.
For the CO2-TPD experiment, the temperature was cooled
to 80 °C. Then the fow was switched to 5 vol.% CO2/He
(30 ml/min) and kept at this temperature for 1.5 h, followed
by purging with He (30 ml/min) for 2 h. Finally, the tem-
perature was increased from 80 to 900 °C at a heating rate of
10 °C/min. For the O2-TPD experiment, the temperature was
cooled to 50 °C. Then the fow was switched to O2 (30 ml/
min) and kept at this temperature for 1 h, followed by purg-
ing with He (30 ml/min) for 2 h. Finally, the temperature
was raised from 50 to 700 °C at a ramping rate of 10 °C/min.
Er2O3 is one kind of rare earth oxide, and there is no
report dealing with the use of Er2O3 related catalysts for
the OCM reaction so far. In this work, Er2O3 nanorods,
nanoparticles and their respective Sr-modifed counterparts
were synthesized to catalyze the OCM reaction. The main
purpose of this work is to study the morphology efects of
Er2O3 related catalysts for the OCM reaction. The enhanced
reaction performance of Er2O3 and Sr-Er2O3 nanorods than
their nanoparticles counterparts were elucidated based on
the physicochemical properties.
2 Experimental
2.1 Catalyst Preparation
The Er2O3 nanorods (designated as Er2O3-NR) were pre-
pared by a modifed hydrothermal method according to the
procedures described by Yoon et al. [35]. In a brief, 4.43 g
of Er(NO3)3•5H2O was dissolved in 100 ml of deionized
water, and 5.0 ml of ammonia water was added dropwise
to the above 0.1 M Er(III) nitrate solution under stirring.
After stirring for additional 0.5 h, the suspension was trans-
ferred into a Tefon-lined stainless steel autoclave, sealed
and placed in an oven of 200 °C for 12 h. The resulting solid
was washed with deionized water, and dried at 110 °C for
12 h. Finally, the dried Er(OH)3 nanorods were calcined in
air at 750 °C for 4 h to obtain the Er2O3 nanorods. The Er2O3
2.3 Catalytic Testing
The OCM reaction was carried out in a fxed-bed quartz tube
reactor with an internal diameter of 6 mm under ambient
pressure. 0.2 g of catalyst (40–60 mesh) was loaded in the
1 3

Morphology Efects of Nanoscale Er2O3 and Sr‑Er2O3 Catalysts for Oxidative Coupling…
quartz tube, and the downstream of the catalyst was fxed
with quartz wool. Prior to the reaction, the catalyst was pre-
treated at 750 °C for 1 h in a Ar fow. A gas mixture of CH4
and O2 (4:1 molar ratio) fowed through the catalyst at a
fow rate of 60 ml/min, corresponding to a gas hourly space
velocity (GHSV) of 18,000 ml/(g•h). The reaction tempera-
ture (that is, catalyst bed temperature) was monitored by
a thermocouple placed in the middle of the catalyst bed.
The hydrocarbon products were analyzed on-line with a gas
chromatograph (GC) equipped with a FID and a 50-m Pora-
PLOT Q capillary column (for the separation of CH4, C2H4,
C2H6, C3H6 and C3H8), and N2 was used as the carrier gas.
CH4, H2, O2, CO and CO2 were analyzed on-line by another
GC equipped with a TCD and a 2-m Shincarbon ST packed
column, and Ar was used as the carrier gas. Before analyz-
ing by TCD, the products were passed through a cold trap
at−3 °C to remove most of water produced in the reaction.
The CH4 conversion and C2–C3 selectivity were calculated
using the standard normalization method based on carbon
atom balance.
3 Results and Discussion
3.1 Catalyst Characterization
Figure 1 shows the XRD patterns of Er2O3 nanorods, nano-
particles and their respective Sr-modified counterparts.
These samples display the characteristic difractions that
match the cubic crystal structure of Er2O3 (PDF #43–1007).
The difraction peaks at 2θ = 20.6°, 29.3°, 34.0°, 36.1°,
40.1°, 43.7°, 47.2°, 48.8°, 53.5°, 56.5°, 57.9°, 59.4° and
60.8° can be assigned to the (211), (222), (400), (411),
(332), (134), (521), (440), (611), (145), (622), (136) and
(444) planes of cubic phase Er2O3, respectively. The domi-
nant peak located at 29.3° corresponds to the (222) plane of
the crystal phase. It is worth noting that there are no other
crystal phases such as SrO and SrCO3 (produced by the com-
bination of SrO with CO2 in air) which can be detected in all
catalysts. One reason is due to the lower content of Sr in the
catalysts. Another reason could be that Sr is highly dispersed
in the catalysts. The cubic lattice parameters of Er2O3-NP
and Er2O3-NR are 1.0538 nm and 1.0534 nm, respectively,
whereas those of Sr-Er2O3-NP and Sr-Er2O3-NR are some-
what increased to 1.0546 nm and 1.0541 nm, respectively
(Table 1). This result is indicative of doping of Sr2+ ions into
the crystal lattice of cubic Er2O3, taking into account the
larger ionic radius of Sr2+ (0.118 nm) than Er3+ (0.089 nm).
Similar phenomena were reported for the incorporation of
Sr to La2O3 and Sm2O3 [32, 34, 36].
The TEM images of Er2O3-NR, Sr-Er2O3-NR, Er2O3-NP
and Sr-Er2O3-NP are shown in Fig. 2. Both Er2O3-NR and
Sr-Er2O3-NR display the nanorod morphology, whereas the
morphology of nanoparticles with irregular shape can be
observed for both Er2O3-NP and Sr-Er2O3-NP. The aver-
age length and width for Er2O3-NR nanorods are 3.34 μm
Fig. 1 XRD patterns of (a) Er2O3-NP, (b) Er2O3-NR, (c) Sr-Er2O3-
NP, and (d) Sr-Er2O3-NR
Table 1 Textural properties and XPS data of the Er2O3-based catalysts
−
Catalyst
SBET
Average
Lattice
O 1 s BEc, FWHMd (eV)
(O− +O2−)/O2
2−
−
(m2/g)
size (μm)
parameter
(nm)
O2−
O−
CO3
O2
Er2O3-NP
Er2O3-NR
5.6
8.2
0.022 0.003
1.0538
1.0534
529.2/1.8
529.4/1.6
530.6/1.6
530.5/1.7
531.7/1.4
532.0/1.5
532.7/1.4
533.0/1.4
0.9
1.9
3.34 0.75a
0.346 0.065b
Sr-Er2O3-NP
Sr-Er2O3-NR
4.5
8.0
0.022 0.004
1.0546
1.0541
529.3/1.6
529.3/1.5
530.4/1.7
530.3/1.9
531.7/1.4
531.7/1.5
532.7/1.5
532.7/1.5
2.0
2.5
3.29 0.77a
0.380 0.063b
aAverage length of nanorods. bAverage width of nanorods
cBinding energy. dfull width at half maximum
1 3

Y. Fan et al.
Fig. 2 TEM graphs of a
Er2O3-NP, b Sr-Er2O3-NP, c
Er2O3-NR, and d Sr-Er2O3-NR
Fig. 3 TEM (top) and HR-TEM (bottom) graphs of Er2O3-NR. Insets are the fast Fourier transfer (FFT) patterns of the HR-TEM images
1 3

Morphology Efects of Nanoscale Er2O3 and Sr‑Er2O3 Catalysts for Oxidative Coupling…
Fig. 4 TEM (top) and HR-TEM (bottom) graphs of Sr-Er2O3-NR. Insets are the fast Fourier transfer (FFT) patterns of the HR-TEM images
in Figs. 3 and 4, the HRTEM images combined with a fast
Fourier transform (FFT) analysis reveal that Er2O3-NR and
Sr-Er2O3-NR nanorods predominantly expose (440) and
(222) planes. A homogeneous distribution of the Sr element
in the Sr-modifed Sr-Er2O3-NR and Sr-Er2O3-NP samples is
demonstrated by HAADF STEM mapping (Fig. S1).
The BET specifc surface areas of the catalysts are given
in Table 1. All catalysts possess a low surface area between
4.5 and 8.2 m2/g, which is typical for the majority of the
OCM catalysts. Er2O3-NR has a higher surface area than
Er2O3-NP (8.2 vs 5.6 m2/g). Both Er2O3-NR and Sr-Er2O3-
NR have equivalent surface areas. Modifcation of Er2O3-NP
with Sr leads to a slight decrease in surface area.
XPS was employed to elucidate the surface oxygen spe-
cies on the catalysts. Fig. S2 depicts the XPS spectra of O
1 s on Er2O3-NR, Sr-Er2O3-NR, Er2O3-NP and Sr-Er2O3-NP.
The XPS spectra were deconvoluted into four peaks at ca.
529.3, 530.5, 531.7 and 532.7 eV which can be assigned to
four diferent oxygen species, i.e. lattice oxygen O2−, per-
Fig. 5 O2-TPD profles of (a) Er2O3-NP, (b) Er2O3-NR, (c) Sr-Er2O3-
NP, and (d) Sr-Er2O3-NR
−
oxide ions O−, carbonate CO32− and superoxide ions O2 ,
respectively [31, 37, 38]. It is generally considered that the
surface electrophilic oxygen species O− and O2− are benef-
cial to the formation of C2 hydrocarbons of OCM, whereas
the lattice oxygen species O2− is responsible for the com-
plete oxidation of CH4 to CO and CO2 [31, 33, 39, 40].
and 346 nm, respectively. Er2O3-NP nanoparticles have a
mean size of 22 nm. It is clear that modifcation of Er2O3
nanorods and nanoparticles with a small amount of Sr has a
little impact on their sizes (Fig. 2 and Table 1). As illustrated
1 3

Y. Fan et al.
Table 2 O2-TPD and CO2-
TPD data of the Er2O3-based
catalysts
Catalyst
Peak temperature
(oC)
Amount of desorbed
Amount of basic sites (μmol/g)
O2 (μmol/g)
I
II
I
II
Weak
Moderate
Strong
Er2O3-NP
Er2O3-NR
Sr-Er2O3-NP
Sr-Er2O3-NR
98
98
102
109
540
425
397
402
7.5
11.8
32.1
64.1
80.7
3.5
9.4
3.8
8.9
4.2
8.2
4.2
–
–
11.7
12.0
27.2
11.2
32.4
11.4
counterparts which plays a key role in OCM reaction, the
O2-TPD measurements were performed. As illustrated in
Fig. 5, there are two desorption peaks on the TPD profles.
The low temperature peaks at about 100 °C correspond to
the desorption of molecular oxygen, whereas the high tem-
perature ones at 400–540 °C are assigned to the desorption
−
of chemisorbed oxygen species (such as O− and O2 [34,
42]) which are helpful for the activation of CH4 and C2
selectivity in OCM reaction [31, 38, 42, 43]. Thus, higher
CH4 conversion of OCM would be expected over the cata-
lysts possessing more sites for chemisorbed oxygen adsorp-
tion. The O2-TPD data (Table 2) shows that the amount of
chemisorbed oxygen species is higher over Er2O3-NR than
Er2O3-NP (32.1 vs 11.8 μmol/g), and over Sr-Er2O3-NR than
Sr-Er2O3-NP (80.7 vs 64.1 μmol/g). It can be also seen that
the amount of chemisorbed oxygen species over Er2O3-NR
and Er2O3-NP is enhanced upon Sr modifcation. The similar
phenomena were found in the case of incorporation of Sr to
La2O3 [32, 34].
Fig. 6 CO2-TPD profles of (a) Er2O3-NP, (b) Er2O3-NR, (c) Sr-
Er2O3-NP, and (d) Sr-Er2O3-NR
The TPD of CO2 was conducted to investigate the sur-
face basic sites of Er2O3 nanorods, nanoparticles and their
respective Sr-doped counterparts which are important for an
efective OCM catalyst [36, 44]. These basic sites originate
−
Hence, an increase in the ratio of (O− +O2 )/O2− over the
catalysts would favor the C2 selectivity of OCM [32, 34, 40].
The XPS data presented in Table 1 indicate that Er2O3-NR
−
from surface O2−, O2 and O− [14, 36, 45]. As presented
−
has a higher ratio of (O− + O2 )/O2− than Er2O3-NP (1.9
in Fig. 6, there are three peaks of CO2 desorption at about
140 °C, 370 °C and 760 °C for Sr-Er2O3-NR and Sr-Er2O3-
NP which correspond to the weak, moderate and strong
basic sites present on the catalysts, respectively. Er2O3-NR
and Er2O3-NP possess only weak and medium basic sites.
It is believed that the surface basic sites with moderate and
strong strength are benefcial to both CH4 activation and
C2 product formation in OCM reaction [31, 34, 36, 38, 43,
44, 46, 47]. The CO2-TPD data (Table 2) shows that the
number of moderate basic sites is greater on Er2O3-NR than
Er2O3-NP (8.2 vs 4.2 μmol/g), and on Sr-Er2O3-NR than
Sr-Er2O3-NP (11.4 vs 4.2 μmol/g). Both Sr-Er2O3-NR than
Sr-Er2O3-NP have equivalent strong basic sites. The addi-
tion of Sr to Er2O3-NR and Er2O3-NP results in an enhanced
number of strong or moderate basic sites.
vs 0.9). Furthermore, Sr-Er2O3-NR has also a higher
−
(O− + O2 )/O2− ratio than Sr-Er2O3-NP (2.5 vs 2.0). This
could be related to the fact that the predominantly exposed
planes over Er2O3-NR and Sr-Er2O3-NR nanorods are (440)
and (222) (Figs. 3, 4). Computer simulations have revealed
that the energy required to generate oxygen vacancies over
CeO2 is lower on the plane of (110) than (111) [41]. That
is to say, oxygen vacancies are more easily to create on the
former plane. The interaction of O2 with oxygen vacancies
generates surface oxygen species such as O− and O2− which
play a signifcant role in OCM reaction. Wan and co-authors
reported that the exposed (110), (1 2 0), and (2 1 0) facets
on the La2O2CO3 catalysts were benefcial to the formation
of the chemisorbed oxygen species [38]. On the other hand,
modifcation of Er2O3-NR and Er2O3-NP with Sr leads to an
−
improvement in the (O− +O2 )/O2− ratio. Reportedly, dop-
ing Sr into La2O3 brought about the similar result [32, 34].
In order to discern the oxygen activation over Er2O3
nanorods, nanoparticles and their respective Sr-doped
1 3

Morphology Efects of Nanoscale Er2O3 and Sr‑Er2O3 Catalysts for Oxidative Coupling…
Fig. 7 CH4 conversion, C2-C3 selectivity and C2-C3 yield as a function of reaction temperature for the Er2O3-based catalysts. (■) Er2O3-NP, (▲)
Er2O3-NR, (●) Sr-Er2O3-NP, (▼) Sr-Er2O3-NR
Table 3 Reaction data of the
Catalyst
Conversion Selectivity (%)
Selectivity
Yield
Er2O3-based catalysts at 650 °C
of CH4 (%) C2H4 C2H6 C3H6 C3H8 CO2 CO
of C2-C3 (%) of C2-C3 (%)
Er2O3-NP
Er2O3-NR
Sr-Er2O3-NP 16.7
Sr-Er2O3-NR 23.2
15.3
19.0
4.9
14.5
7.0
10.4
16.6
13.8
21.9
0.6
1.0
0.7
2.2
0.4
0.5
0.5
0.8
47.8 35.9 16.3
43.9 23.5 32.6
53.5 24.5 22.0
2.5
6.2
3.7
25.4
39.9
9.8 50.3
11.7
As the reaction temperature is increased from 600 °C to
750 °C, the CH4 conversion improves gradually. In con-
trast, the selectivity of C2–C3 (C2H4, C2H6, C3H6 and C3H8)
increases more obviously. Correspondingly, the C2–C3 yield
also rises with the reaction temperature. Regardless of CH4
conversion, C2–C3 selectivity or yield, Er2O3-NR and Sr-
Er2O3-NR nanorods perform better than Er2O3-NP and
Sr-Er2O3-NP nanoparticles, respectively, which is indica-
tive of shape efects of Er2O3 and Sr-Er2O3 nanocatalysts
3.2 Catalytic Performance
Figure 7 shows the results of OCM reaction over Er2O3
nanorods, nanoparticles and their respective Sr-doped
counterparts at diferent temperatures. The main hydro-
carbon products are C2H4 and C2H6, and minor amounts
of C3H6 and C3H8 were also detected. The by-products are
CO and CO2. The typical product distribution over the cata-
lysts at a reaction temperature of 650 °C is given in Table 3.
1 3

Y. Fan et al.
performance of Er2O3-NR and Er2O3-NP catalysts upon Sr
modifcation, as can be seen from Fig. 7.
The stability of the best catalyst Sr-Er2O3-NR was evalu-
ated at 650 °C, and the results are shown in Fig. 8. As can
be seen, the Sr-Er2O3-NR catalyst displays good stability in
OCM reaction for 60 h of time on stream, afording ca. 23%
conversion of CH4 with around 50% selectivity to C2–C3.
As revealed in Figs. S3 and S4a, almost no changes can be
observed in TEM images and XRD patterns of the fresh and
spent Sr-Er2O3-NR catalysts. After the stability test, the sur-
face area of Sr-Er2O3-NR is 7.8 m2/g which is equivalent to
that of the fresh catalyst (8.0 m2/g). These facts demonstrate
the maintenance of the catalyst structure after the stability
test. Moreover, there are almost no diferences in the O 1 s
XPS spectra of fresh and spent Sr-Er2O3-NR catalysts (Fig.
S4b), indicating that the surface species did not alter during
the OCM reaction.
Fig. 8 CH4 conversion (■) and C2-C3 selectivity (●) with time on
stream over Sr-Er2O3-NR at 650 °C
A reference catalyst (Sr-La2O3 nanofbers) was prepared
according to the literature [32]. The Sr content was selected
to be 8.6 wt.%, because Sr-La2O3 nanofbers with this con-
tent of Sr exhibited the highest methane conversion and
C2 selectivity for OCM reaction [32]. We tested the per-
formance of Sr-La2O3 nanofbers under our reaction condi-
tions, and compared with the Sr-Er2O3-NR catalyst in this
work. As illustrated in Fig. 9, Sr-Er2O3-NR exhibits higher
C2–C3 selectivity and a bit greater methane conversion than
Sr-La2O3 at 600 and 650 °C. However, the former catalyst
displays lower C2-C3 selectivity and a bit lower methane
conversion than Sr-La2O3 at 700 and 750 °C.
for OCM reaction. For example, Er2O3-NR nanorods give a
20.2% conversion of CH4 with 39.9% selectivity and 8.1%
yield to C2–C3 at 700 °C, while Er2O3-NP nanoparticles
aford a 17.0% conversion of CH4 with 25.5% selectiv-
ity and 4.3% yield to C2–C3. The CH4 conversion, C2–C3
selectivity and yield are 24.0%, 52.6% and 12.6%, respec-
tively, for Sr-Er2O3-NR nanorods at 700 °C, whereas they
are 19.4%, 38.6% and 7.5%, respectively, for Sr-Er2O3-NP
nanoparticles. Reportedly, La2O3 catalysts also displayed
morphology efects for OCM reaction [31, 33]. Taking into
account the above XPS, O2-TPD and CO2-TPD results, the
better reaction performance over Er2O3-NR and Sr-Er2O3-
−
NR nanorods is caused by their higher (O− +O2 )/O2− ratio,
more chemisorbed oxygen species and moderate basic sites.
These reasons are also responsible for the enhanced reaction
Fig. 9 Comparison of Sr-Er2O3-NR (■) and Sr-La2O3 (●) catalysts for CH4 conversion and C2-C3 selectivity as a function of reaction tempera-
ture
1 3

Morphology Efects of Nanoscale Er2O3 and Sr‑Er2O3 Catalysts for Oxidative Coupling…
17. Amin NAS, Pheng SE (2006) Chem Eng J 116:187
4 Conclusions
18. Tang L, Yamaguchi D, Wong L, Burke N, Chiang K (2011)
Catal Today 178:172
In this work, we have demonstrated that Er2O3 and Sr-Er2O3
nanorods outperform Er2O3 and Sr-Er2O3 nanoparticles,
respectively, for oxidative coupling of methane. The XRD
and HAADF STEM mapping results reveal the homogene-
ous distribution of Sr element in Sr-Er2O3 nanorods and
nanoparticles. The HRTEM images reveal that Er2O3 and
Sr-Er2O3 nanorods predominantly expose (440) and (222)
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−
planes. The (O− +O2 )/O2− ratio, amount of chemisorbed
oxygen species and moderate basic sites are greater on
Er2O3 nanorods than Er2O3 nanoparticles, and on Sr-Er2O3
nanorods than Sr-Er2O3 nanoparticles, as revealed by XPS,
O2-TPD and CO2-TPD, respectively. These reasons are
responsible for the superior reaction performance of Er2O3
and Sr-Er2O3 nanorods to their nanoparticles counterparts.
A 23.2% conversion of CH4 with 50.3% selectivity to C2–C3
can be achieved over Sr-Er2O3 nanorods at 650 °C.
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Acknowledgements This work was supported financially by the
National Key R&D Program of China (2017YFB0602200), the
National Natural Science Foundation of China (91645201), the
Science and Technology Commission of Shanghai Municipality
(19DZ2270100), the Shanghai Research Institute of Petrochemi-
cal Technology SINOPEC (19ZC06070005), and the Science and
Technology Project of General Administration of Customs of China
(2019HK059).
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Y. Fan et al.
Afliations
Yuqiao Fan1 · Mingxing Sun2 · Changxi Miao3 · Yinghong Yue1 · Weiming Hua1 · Zi Gao1
1
Yuqiao Fan
Shanghai Key Laboratory of Molecular Catalysis
18110220090@fudan.edu.cn
and Innovative Materials, Department of Chemistry, Fudan
University, Shanghai 200438, PR China
Mingxing Sun
2
3
sunmxing@vip.163.com
Shanghai Customs Technology Center of Inspection and Test,
Shanghai 200120, PR China
Yinghong Yue
yhyue@fudan.edu.cn
Shanghai Research Institute of Petrochemical Technology
SINOPEC, Shanghai 201208, PR China
Zi Gao
zigao@fudan.edu.cn
1 3