The effect of oxygen vacancy of alkaline-earth metal Sr doped Sm2Zr2O7 catalysts in the oxidative coupling of methane

Article abstract of DOI:10.1016/j.cplett.2021.138562

A series of the Sr/Sm₂Zr₂O₇ catalysts with different Sr contents prepared by the coprecipitation method were used for OCM. The results revealed that the basicity and oxygen vacancies concentration on the surface of the Sm₂Zr₂O₇ catalyst were notably increased because of the addition of Sr. Activation of O₂ by the oxygen vacancies was responsible for the formation of C₂. Specifically, the 7.6% Sr/Sm₂Zr₂O₇ catalyst possessed 18.4% C₂ yield at 750 °C due to the presence of moderate basicity and the highest ratio of the oxygen species O₂^– to lattice oxygen O^2–. More importantly, the 7.6%Sr/Sm₂Zr₂O₇ catalyst possessed high stability.

Full text of DOI:10.1016/j.cplett.2021.138562

Chemical Physics Letters 771 (2021) 138562
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Research paper
The effect of oxygen vacancy of alkaline-earth metal Sr doped Sm2Zr2O7
catalysts in the oxidative coupling of methane
,
,
,
,c,*
Jie Haoa b, Fufeng Caia, Jiyang Wanga b, Yu Fua, Jun Zhanga *, Yuhan Suna
a CAS Key Laboratory of Low-Carbon Conversion Science and Engineering, Shanghai Advanced Research Institute, Chinese Academy of Science, Shanghai 201210, PR
China
b University of the Chinese Academy of Sciences, Beijing 100049, PR China
c ShanghaiTech University, Shanghai 201210, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
A series of the Sr/Sm2Zr2O7 catalysts with different Sr contents prepared by the coprecipitation method were
used for OCM. The results revealed that the basicity and oxygen vacancies concentration on the surface of the
Sm2Zr2O7 catalyst were notably increased because of the addition of Sr. Activation of O2 by the oxygen vacancies
was responsible for the formation of C2. Specifically, the 7.6% Sr/Sm2Zr2O7 catalyst possessed 18.4% C2 yield at
750 ◦C due to the presence of moderate basicity and the highest ratio of the oxygen species O–2 to lattice oxygen
O2–. More importantly, the 7.6%Sr/Sm2Zr2O7 catalyst possessed high stability.
Oxidative coupling of methane
Sr doped Sm2Zr2O7 catalysts
Oxygen vacancy
Oxide surface basicity
1. Introduction
temperature activity [13].
Mechanistic studies demonstrated that activating O2 into desirable
In the decade years, methane, as a main component of natural gas, is
expected to be converted into different high-value chemicals, such as
lower alcohols, olefins, and aromatics [1,2]. Among them, the oxidative
coupling of methane (OCM, see Eq. (1)) to C2 hydrocarbons, especially
ethylene, is attractive route to convert natural gas directly [3]. Since
Keller and Bhasin [4] published a paper on oxidative coupling of
methane to ethylene in 1982, many researchers have done a lot of
research on OCM [5].
species on the catalyst surface is a pivotal step and OCM initiated by two
possible active oxygen species were identified, namely: (I) molecular
species such as O–2 and O22– for lanthanide oxides [14-16], and (II)
dissociated oxygen species such as nucleophile ions O.- of the catalyst
lattice. Schwach and coworkers [17] showed that polycrystalline MgO
doped with Fe and Au can effectively activate O2 to peroxide (O22–
)
species, and thus enhanced the C2 formation. According to the literature
[18,19], for Ln2Zr2O7 compounds, the surface electrophilic O–2 species is
a critical factor influencing the OCM reaction performance. Lattice ox-
ygen anion on the perovskite oxides can be translated to the active sites
for the OCM reaction [20]. It can be seen that the catalyst with oxygen
vacancy can control the activation of oxygen and played an important
role in OCM reaction [21,22].
CH4 + O2→C2H6 + C2H4 + COx + H2O
(1)
It is well documented that the OCM reaction involves two typical
steps, one is the heterogeneous catalytic step involving the activation of
CH4 on the catalyst surface to generate methyl (CH3⋅) radicals [6], the
other is combining two CH3⋅ radicals to form C2H6 molecule and then
dehydrogenation to form C2H4 molecule [7,8]. The representative
catalyst is Li/MgO with [Li+ O-] active site, where [Li+ O-] centers are
formed to effectively generate CH3⋅ from CH4 [9], but evaporation of Li
from the bulk to the surface was observed for Li/MgO catalysts during
the OCM reaction [10,11]. Wang et al.[12] reported a TiO2-doped
Mn2O3-Na2WO4/SiO2 catalyst because the formation of MnTiO3 trig-
gered the low-temperature MnTiO3 ↔ Mn2O3 chemical cycle for O2
activation, thereby leading to a marked improvement of the low-
The A2B2O7 pyrochlore compounds generally own high thermal
stability, intrinsic 8a oxygen vacancies and certain surface alkalinity,
which is beneficial to OCM reaction [18,23]. The lattice defects change
the adsorption and desorption properties of oxygen, which favors to
increase the catalytic activity of pyrochlore catalysts for OCM [24,25]. If
alkali and alkaline earth metals (such as Li, Na, Sr) are introduced to
modify the catalyst, the increase of alkalinity will help to increase the
number of active sites and inhibit the deep oxidation of methane to
CO2[26,27]. According to the literature [28], Mg2+, Ca2+, and Sr2+ into
* Corresponding authors.
E-mail addresses: zhangj@sari.ac.cn (J. Zhang), sunyh@sari.ac.cn (Y. Sun).
https://doi.org/10.1016/j.cplett.2021.138562
Received 4 February 2021; Received in revised form 17 March 2021; Accepted 18 March 2021
Available online 22 March 2021
0009-2614/© 2021 Elsevier B.V. All rights reserved.

J. Hao et al.
Chemical Physics Letters 771 (2021) 138562
CeO2 as substitution ions on cerium created oxygen species on the oxide
surface, leading to an abundance of electrophilic oxygen species on the
catalyst surface. Song and co-workers [29] found that the addition of Sr
changed the ratio of (O–2+ O-)/O- and had a great influence on the per-
formance of the catalyst. Therefore, the methane conversion and C2
selectivity can be effectively improved.
(TEM) system that was equipped with a Brooke quanta X400 detector
operated at 300 keV.
Raman spectra of the catalysts were measured using excitation
wavelength of 514 nm in Renishaw in Via instrument. The measured
Raman shift range was from 50 to 1600 cmꢀ 1
.
Thermogravimetric (TG) experiments were performed under 30 mL/
min Air (20% O2 + 80% N2) with a heating rate of 5 ◦C/min on a Netzsch
STA 449C thermoanalyzer.
In this study, we synthesized a series of Sr/Sm2Zr2O7 catalysts with
different Sr contents for OCM. The catalysts were characterized by using
XRD, STEM, Raman, TPD, EPR and so on. It was found that the basicity
and oxygen vacancy of the catalyst were changed in the case of Sr. The
synergistic effect of the basicity and oxygen vacancy played an impor-
tant role in the Sr/Sm2Zr2O7 catalyst. Among the catalysts prepared, the
7.6%Sr/Sm2Zr2O7 catalyst exhibited the best catalytic activity for OCM.
2.3. Reaction performance
The OCM reaction was performed at atmospheric pressure in a quartz
tube reactor with an inner diameter of 6 mm. In a typical experiment,
200 mg catalyst and the same weight of quartz sand was mixed into the
reaction tube, and the reaction temperature was controlled by inserting
the K-type thermocouple into the catalyst bed. The flow rates of CH4, O2
reactants and N2 balance gas were controlled by three mass flow con-
trollers and CH4: O2: N2 volume ratio was 4:1:5. The total rate of the flow
is 60 mL minꢀ 1, respectively with GHSV 18000 mL hꢀ 1 gꢀ 1. The reac-
tivity test generally started at 600 ◦C with a gap of 50 ◦C to 800 ◦C. In
order to obtain stable kinetic data, all measurements were carried out at
a certain temperature for 1 h. The mixed gas produced at the outlet of
the reactor was cooled completely through the condenser to capture
unreacted CH4 and O2. After drying, Inficon 3000 micro gas chro-
matograph was used to analyze H2, CO2, CO, C2H4 and C2H6 on-line
automatically. In OCM reaction, methane conversion (XCH4), C2 selec-
tivity (SC2), COX selectivity (SCOX) and C2 yield (YC2) were calculated as
follows:
2. Experimental
2.1. Catalyst preparation
Sm2Zr2O7 pyrochlore was synthesized by coprecipitation method
using Sm (NO3)3⋅6H2O (99.9%) and Zr (NO3)4⋅5H2O (99.9%) as pre-
cursors. In a typical experiment, 15 mmol Sm (NO3)3⋅6H2O and 15 mmol
Zr (NO3)4⋅5H2O were dissolved in 200 mL ddH2O to form stable solu-
tions. Then, the NH3⋅H2O solution (22–25 wt%) was slowly dripped in
until the pH reached about 10. After that, the precipitate was filtered
and thoroughly cleaned with ddH2O. The resulting precipitates were
dried overnight at 120 ◦C and then calcined in air at 800 ◦C for 4 h to
obtain the Sm2Zr2O7 catalyst.
The Sr/Sm2Zr2O7 catalysts were prepared by using the incipient
wetness method. Firstly, deionized water was added slowly to 1 g
Sm2Zr2O7 sample, then, white powder was continuously stirred during
the dropping process to make the sample and water mixed evenly. The
saturated water absorption of the sample was measured to be m g H2O/g
sample. Then, a certain amount of Sr (NO3)2 (99.9%) was weighed and
dissolved in m g deionized water, and slowly dropped into 1 g Sm2Zr2O7
sample. After the powder sample was mixed with Sr (NO3)2 solution, the
Sm2Zr2O7 samples were dried in vacuum oven at 60 ◦C for 12 h, then
calcined at 800 ◦C for 4 h in an air atmosphere to obtain the x% Sr/
Sm2Zr2O7 catalysts.
(CH4)in ꢀ (CH4)out
XCH4(%) =
× 100
(2)
(CH4)in
2(C2H4 + C2H6)
SC2(%) =
× 100
(3)
2(C2H4 + C2H6) + CO + CO2
COX
SCOX(%) =
× 100
(4)
(5)
2(C2H4 + C2H6) + CO + CO2
YC2(%) = XCH4 × SC2 × 100
2.2. Catalyst characterization
3. Results and discussion
The crystalline structure of the catalyst was analyzed by Rigaku Ul-
tima IV X-ray powder diffractometer, which was operated at 40 kV and
30 mA with a Cu target and K a irradiation. The scans were recorded in
the 2θ range from 10◦ to 90◦ with a step of 2◦/min.
3.1. Textural properties
As shown in Fig. 1a, the Sm2Zr2O7 catalyst showed the diffraction
peaks at 2θ = 29.4◦, 34.1◦, 49.0◦, and 58.3◦, corresponding to the fea-
tures of the (111), (200), (220), and (311) planes of Sm2Zr2O7 (JCPDS
78–1291) respectively. However, no impurity peaks corresponding to
single Sm2O3 or ZrO2 oxide were detected. These results suggested that
pure pyrochlore phase was successfully prepared in this study.
In the case of Sr/Sm2Zr2O7 catalysts, the diffraction peaks at 2θ =
30.5◦, 43.7◦ and 54.3◦ were attributed to the presence of SrZrO3 phase.
As displayed in Fig. 1b, the diffraction peak intensity of SrZrO3 phase for
Sr/Sm2Zr2O7 catalysts was gradually enhanced with the increase in Sr
content, but that of Sm2Zr2O7 phase was significantly decreased. Spe-
cifically, the diffraction peak intensity of Sm2Zr2O7 phase for 7.6%Sr/
Sm2Zr2O7 catalyst was the same as that of and SrZrO3 phase. However,
the diffraction peak intensity of SrZrO3 phase for the Sr/Sm2Zr2O7
catalyst was obviously stronger than that of Sm2Zr2O7 phase when the Sr
content was increased to 15.4%. Therefore, the diffraction peak in-
tensity of SrZrO3 phase was increased with the increase of Sr content,
while that of Sm2Zr2O7 phase was decreased, which may be due to the
substitution of Sr for Sm.
N2 adsorption–desorption analyses were conducted at 77 K with
ASAP 2420 physisorption analyser. And then the surface area was
calculated by Brunauer-Emmett-Teller (BET) method.
The CO2-TPD experiment was carried out on the Auto ChemII2920
chemisorption instrument. Typically, 50 mg sample was placed in a U-
tube. Next, the sample was heated to 200 ◦C and then cooled to 50 ◦
C
and kept there for 1 h in flowing CO2. After that, the sample was purged
by a flowing He for 30 min. Subsequently, the CO2 temperature pro-
grammed desorption was started from 50 ◦C to 800 ◦C in flowing He to
get the CO2 desorption amount.
The surface elements and oxygen species of the samples were
analyzed by Thermo Fisher k-alpha X-ray photoelectron spectrometer
with A1 KGC excitation source and accelerating voltage of 50 eV. The
binding energy of each species was corrected by 284.6 eV of C 1 s.
EPR technique was used to characterize the oxygen vacancy and the
oxygen species with electron paramagnetic activity. The sample was
pretreated with 100 torr oxygen at 450 ◦C for 30 min, then cooled to
room temperature and evacuated at the same temperature at 77 K in
liquid N2, and then analyzed by EPR.
To further investigate the structural information, Raman spectra of
the Sr/Sm2Zr2O7 catalysts were recorded at 50–1600 cmꢀ 1 in Fig. S1.
For Sm2Zr2O7, the F2g modes at 395, 520, 601 cmꢀ 1 were most likely due
The HAADF-STEM images and elemental phase mapping were ob-
tained by using a Hitachi s-4800 transmission electron microscopy
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J. Hao et al.
Chemical Physics Letters 771 (2021) 138562
Fig. 1. X-ray diffractograms of (a) fresh Sm2Zr2O7 and Sr/Sm2Zr2O7 catalysts, and (b) fresh Sm2Zr2O7 and Sr/Sm2Zr2O7 catalysts from 27.5◦ to 32◦.
to the mixed Zr-O and Sm-O bond stretching with O-Zr-O bending vi-
brations [30]. Based on group theory, an ordered pyrochlore typically
had six active Raman modes distributed as A1g + Eg + 4F2g [31,32]. Only
the F2g peak was still detected with the disappearance of other bands,
testifying that the Sm2Zr2O7 pyrochlore phase was still present but
become less ordered [33]. With Sr doping in Sm2Zr2O7, the bands at 304
cmꢀ 1 were most likely due to Sr-O bond vibration. This was consistent
with the results of XRD.
associated with desorption of CO2 at the temperatures 100 ◦C,
300–600 ◦C and >600 ◦C, respectively. These curves in Fig. 3a provided
information on the strength of basic sites on the catalysts. For the
Sm2Zr2O7 catalyst, the desorption peak of CO2 was at 500–600 ◦C.
However, incorporation of Sr significantly altered the CO2 desorption
behavior compared to Sm2Zr2O7. The low temperature CO2 desorption
was not increased, and CO2 mainly desorbed between 300 ◦C and
800 ◦C. The 7.6%Sr/Sm2Zr2O7 based materials showed a broad
desorption peak around 400 ◦C, and the intermediate basic sites of the
catalyst surface reached the maximum. When the Sr content exceeded
7.6 wt%, the desorption peak of CO2 increased significantly at
600–800 ◦C. Whereas the strong alkaline sites were not conducive to the
OCM reaction, which was easily to capture CO2 and form stable car-
bonate [35].The quantity of CO2 evolved, as well as the temperatures of
desorption indicated that doping the Sm2Zr2O7 with Sr greatly affected
the strength and the amounts of basic sites.
As illustrated in Fig. 2, the distribution of Sm, Zr or O elements in the
Sm2Zr2O7 samples was very homogeneous, which proved the formation
of homogeneous and chemically stable Sm2Zr2O7 pyrochlore oxide.
Similarly, the Sr was uniformly distributed on the surface of 7.6%Sr/
Sm2Zr2O7catalyst. Besides, according to the N2-BET analysis results in
Table S1, compared with the fresh catalysts, all the spent catalysts had
only insignificant surface area drop, indicating that these Sr/Sm2Zr2O7
catalysts were physically stable, which can stand the high temperature
impact during the OCM reaction process.
3.3. EPR study
3.2. CO2-TPD measurement
As displayed in Fig. 3b, the oxygen vacancies of Sm2Zr2O7 and Sr/
Sm2Zr2O7 were characterized by EPR. Firstly, the samples were pre-
treated with 100 Torr oxygen at 450 ◦C for 30 min, then cooled to room
temperature and evacuated at the same temperature, finally analyzed by
As shown in Fig. 3a, the temperature-programmed desorption (TPD)
of CO2 was performed. Based on the literature [34], there were three
types of basic sites: the weak, medium and strong basic sites, which were
Fig. 2. HAADF STEM mapping images of Sm2Zr2O7 and 7.6%Sr/Sm2Zr2O7.
3

J. Hao et al.
Chemical Physics Letters 771 (2021) 138562
Fig. 3. (a) CO2-TPD profiles of pure Sm2Zr2O7 and Sr/Sm2Zr2O7 catalysts; (b) EPR spectra of the Sm2Zr2O7 and Sr/Sm2Zr2O7 catalysts.
EPR. It can be observed from Fig. 3b that the Sm2Zr2O7 catalysts showed
two evident peaks at g = 1.997, 2.009. With increasing the loading
content of Sr, the EPR signals were gradually strengthened, which
indicated the concentration of oxygen vacancies was gradually
increased. The results showed that the key step to prepare Sr/Sm2Zr2O7
with the more concentration of oxygen vacancies was to change the
amount of Sr (NO3)2 in the synthesis process. It was obvious that the
peak intensity of g value for the catalysts followed the sequence of 7.6%
Sr/Sm2Zr2O7 > 12.1%Sr/Sm2Zr2O7 > 15.4%Sr/Sm2Zr2O7 > 3.8%Sr/
Sm2Zr2O7 > Sm2Zr2O7, which was consistent with the performance of
the catalyst.
oxygen species CO23–, and lattice oxygen species O2–. According to the
literature, the researchers [36,37] suggested that the electron deficient
oxygen species O–2 were responsible for the C2 selectivity of OCM,
whereas the lattice oxygen species O2– promoted the complete oxidation
of the reaction. Therefore, the ratio of electron deficient oxygen species
to lattice oxygen (O–2/O2–) can reflect the catalytic performance of OCM
catalyst. The ratio of the peak intensities of the electron deficient species
to lattice oxygen, O–2 /O2– can be calculated using the 80% Gaussian-
20% Lorentzian peak shape (Fig. 4). In Table 1, as can be seen, the ratio
of O–2/O2– on the surface of Sm2Zr2O7 catalyst was 0.08. With the in-
crease of Sr content in Sr/Sm2Zr2O7 catalysts, the ratio of O–2/O2–
gradually increased. However, the ratio of O–2/O2– decreased gradually
when the content of Sr was increased from 7.6% to 15.4%. As can be
seen from Fig. 4f, it should be noted that the 7.6%Sr/Sm2Zr2O7 catalyst
had the highest O–2/O2– ratio and possessed the highest C2 yield at
750 ◦C.
3.4. XPS analysis of catalyst
As displayed in Fig. 4, the surface oxygen species on the catalyst were
investigated by XPS analyses. The O 1s spectra of Sm2Zr2O7 catalyst was
fitted with three peaks, which were superoxide species O–2, carbonate
Fig. 4. XPS O 1s spectra of the Sm2Zr2O7 and Sr/Sm2Zr2O7 catalysts: (a) Sm2Zr2O7, (b) 3.8%Sr/Sm2Zr2O7, (c) 7.6%Sr/Sm2Zr2O7, (d) 12.1%Sr/Sm2Zr2O7, (e) 15.4%
Sr/Sm2Zr2O7, (f) C2 yield at 750 ◦C vis O2–/O2– ratios.
4

J. Hao et al.
Chemical Physics Letters 771 (2021) 138562
in the oxygen uptake [41]. The role of oxygen vacancy was to activate
oxygen into ideal species, so as to promote methane activation to methyl
radical and subsequent oxidative dehydrogenation of ethane [13].
Therefore, the increase of oxygen vacancy content was helpful for the
better OCM catalytic performance of the catalyst. Besides, according to
XRD results, with the increase of Sr content, SrZrO3 perovskite phase
was formed. According to the OCM reaction performance of SrZrO3 in
Fig. S3, SrZrO3 had little contribution to the reaction.
Table 1
Curve-Fitting results from XPS Data.
Catalyst
O 1s binding energies (eV)
1) O2–
O–2/O2–
2) CO23–
3) O–2
Sm2Zr2O7
528.5
530.3
533.09
532.58
532.16
533.16
532.78
0.08
0.16
0.54
0.31
0.30
3.8%Sr/Sm2Zr2O7
7.6%Sr/Sm2Zr2O7
12.1%Sr/Sm2Zr2O7
15.4%Sr/Sm2Zr2O7
528.51
528.43
528.32
528.38
530.45
530.32
530.62
530.64
Among the Sr/Sm2Zr2O7 catalysts tested, the 7.6%Sr/Sm2Zr2O7
catalyst possessed the highest C2 yield 18.4% at 750 ◦C of reaction
temperature. The reason was that the 7.6%Sr/Sm2Zr2O7 catalyst
possessed the highest intrinsic oxygen vacancies concentration and the
ratio of O–2/O2– which were favorable for activation of CH4 and pro-
duction of C2. According to the TGA results in Fig. S4, no obvious weight
loss was detected in the spent 7.6%Sr/Sm2Zr2O7 catalyst, suggesting no
carbon formed. Therefore, it can be understood that the 7.6%Sr/
Sm2Zr2O7 catalyst displayed best OCM performance.
3.5. Catalytic activity test
As shown in Table 2, the catalytic activity of Sr/Sm2Zr2O7 catalysts
for OCM performance was tested at 750 ◦C of reaction temperature. It
was found that the addition of Sr into Sm2Zr2O7 catalyst significantly
increased the methane conversion, indicating the presence of promoting
effect in Sr-modified Sm2Zr2O7 catalysts [29]. Among the catalysts
tested, the 7.6%Sr/Sm2Zr2O7 exhibited the highest methane conversion
of 39.1%. However, further increases in Sr loading notably decreased
the methane conversion. Regarding to the product selectivity, the
Sm2Zr2O7 catalyst showed 21.7% C2 selectivity at 750 ◦C. When the Sr
species was added to the Sm2Zr2O7 catalyst, the C2 selectivity was
remarkable enhanced. For example, in the case of 3.8%Sr/Sm2Zr2O7
catalyst, the C2 selectivity was increased to 36.3%. But it should be
pointed out that the Sr/Sm2Zr2O7 catalysts with high contents of Sr
displayed similar C2 selectivity, stabilizing at about 47%.
Fig. 5 displayed the catalytic performance for OCM on Sr/Sm2Zr2O7
catalysts with a WHSV of 18000 mL hꢀ 1 gꢀ 1. By increasing the tem-
perature from 600 ◦C to 800 ◦C, CH4 conversion, C2 product selectivity,
and yield of Sr/Sm2Zr2O7 catalysts gradually improved. Besides, with
the increase of temperature, the selectivity of CO and CO2 decreased
gradually. The rise of temperature inhibited the deep oxidation of
methane to CO and CO2, which contributed to the formation of methyl
radical and C2 production. In all cases, XCH4, SC2 and YC2 showed a
progressive increase with increasing reaction temperature up to 750 ◦C.
The growth rate of the reaction selectivity and yield to C2 decreased
between 750 and 800 ◦C, most likely because of kinetic limitations. In a
conventional packed bed continuous feed reactor, the upper limit of
OCM yield was determined by the enthalpy of hydrogen adsorption of
CH4 and the enthalpy of oxygen dissociation [42].
It can be observed from Table 2 that the C2 yield of Sm2Zr2O7 at
750 ◦C was only 3.6%. The reason was that the deep oxidation of
methane produced a lot of CO and CO2, which deteriorated the C2
selectivity. It had been proposed in the literature [35] that the formation
of CO2 reaction product took place on acid type as well as on amphoteric
sites showing low affinity for CO2. According to the results of CO2-TPD
in Fig. 3a, the amount of CO2 desorption increased with the addition of
Sr, indicating that there were more basic sites on the surface of the Sr/
Sm2Zr2O7 catalysts than Sm2Zr2O7 catalyst. When the Sr species was
introduced into Sm2Zr2O7 catalyst, the selectivity of CO and CO2
decreased, thus the yield of C2 gradually increased. In addition, it was
pointed out that the surface alkalinity was closely related to the chem-
isorption of oxygen anions such as O–2, O22–, O- and surface defects
[35,38,39]. The results of XPS O1s curve fitting confirmed that the
addition of Sr increased the ratio of O–2/O2– on the surface of Sr/
Sm2Zr2O7 catalyst.
3.6. Stability test
As displayed in Fig. 6, the Sm2Zr2O7 and 7.6%Sr/Sm2Zr2O7 catalysts
was selected to study the stability. In the case of Sm2Zr2O7 catalyst, the
methane conversion and C2 selectivity was remarkably decreased after
76 h time on stream. Conversely, the 7.6%Sr/Sm2Zr2O7 catalyst had a
relatively stable catalytic performance for OCM. After 100 h time on
stream, the methane conversion and C2 selectivity were still higher than
35% and 43% respectively. This indicated that the 7.6%Sr/Sm2Zr2O7
catalyst possessed good stability, which may be due to the lattice
distortion caused by the increase of oxygen vacancy concentration, thus
improving the temperature stability of the sample.
It was generally believed that the higher the oxygen vacancy con-
centration on the surface of the catalyst, the more favorable it was for
OCM to generate C2 hydrocarbons. To encourage the formation of ox-
ygen vacancies, the inclusion of a dopant can be used to introduce lattice
defects, the nature of which varied with dopant and its concentration
[40]. According to the results of EPR, the addition of Sr into Sm2Zr2O7
catalyst increased oxygen vacancies concentration. The ability of ma-
terials to uptake oxygen was governed by the concentration of defect
sites/vacancies in which an increase in vacancies resulted in an increase
4. Conclusion
In summary, the Sr-doped Sm2Zr2O7 catalysts provided an enhanced
activity and selectivity in combination with good stability for the OCM
reaction. Incorporation of Sr2+ ions into Sm2Zr2O7 as substitution ions
on Sm created more oxygen species on the oxide surface, leading to an
abundance of electrophilic oxygen species on the catalyst surface. OCM
reaction performance of Sr/Sm2Zr2O7 with different Sr contents was
controlled by the relative amount of electrophilic oxygen species to
lattice oxygen on the surface of the catalyst. Among them, the 7.6%Sr/
Sm2Zr2O7 catalyst with moderate basicity and the highest oxygen va-
cancies concentration had the best catalytic performance for OCM with
39.1% methane conversion and 47.0% C2 selectivity at 750 ◦C. Through
the stability test at 750 ℃ for 100 h, the 7.6%Sr/Sm2Zr2O7 catalyst
showed good stability. It was apparent that alkali-doped pyrochlore
catalysts exhibited a wide range of catalytic activities upon optimiza-
tion. This, therefore, represented a major aspect that was worthy of
further detailed study. It was anticipated that further investigation of the
solid state and surface chemistry of such doped pyrochlore compounds,
together with relationship between active oxygen species and C2 yield,
should provide the basis for the identification of improved activity
Table 2
The performance of OCM reaction over Sm2Zr2O7 and Sr/Sm2Zr2O7 at 750 ◦C
(Reaction conditions: 200 mg catalysts, CH4: O2: N2 = 4:1:5 flow rate, WHSV =
ꢀ 1
18 000 mL hꢀ 1
Catalyst
g
).
cat
XCH4
(%)
SCOx
(%)
S2 (%)
C2H4/
C2H6
C2 yield
(%)
Sm2Zr2O7
16.7
27.4
39.1
32.9
78.3
63.7
53.0
52.0
21.7
36.3
47.0
48.0
0.3
0.7
1.1
0.9
3.6
3.8%Sr/Sm2Zr2O7
7.6%Sr/Sm2Zr2O7
12.1%Sr/
9.9
18.4
15.8
Sm2Zr2O7
15.4%Sr/
25.3
53.4
46.6
0.7
11.8
Sm2Zr2O7
5

J. Hao et al.
Chemical Physics Letters 771 (2021) 138562
Fig. 5. OCM reaction performance over the Sm2Zr2O7 and Sr/Sm2Zr2O7 catalysts: (a) CH4 conversion, (b) C2 selectivity, (c) CO + CO2 selectivity, and (d) C2 yield.
ꢀ 1
Reaction conditions: 200 mg catalysts under 60 mL minꢀ 1 CH4: O2: N2 = 4:1:5 flow rate, WHSV = 18 000 mL hꢀ 1
g
.
cat
Fig. 6. Stability test for C2 yield, selectivity and CH4 conversion (Reaction conditions: 200 mg catalysts under T = 750 ◦C, CH4: O2: N2 = 4:1:5, WHSV = 18 000 mL
hꢀ 1
g
).
ꢀ 1
cat
catalysts containing an enhanced concentration of active surface sites.
Declaration of Competing Interest
CRediT authorship contribution statement
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Jie Hao: Investigation, Writing - original draft. Fufeng Cai: Data
curation, Writing - review & editing. Jiyang Wang: Software. Yu Fu:
Writing - review & editing. Jun Zhang: Conceptualization, Writing -
review & editing. Yuhan Sun: Conceptualization, Supervision.
6

J. Hao et al.
Chemical Physics Letters 771 (2021) 138562
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Acknowledgements
This work was financially supported by the National Key R & D
Program of China (2016YFA0202802), the National Natural Science
Foundation of China (21908236 and 21805302) and Shanghai Func-
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