Ln2Zr2O7 compounds (Ln = La, Pr, Sm, Y) with varied rare earth A sites for low temperature oxidative coupling of methane

Article abstract of DOI:10.1016/j.cclet.2019.03.031

With the objective to develop catalysts having application potential for oxidative coupling of methane (OCM) at relatively lower temperature. A series of Ln₂Zr₂O₇ compounds with varied rare earth A sites have been prepared by a co-precipitation method. XRD and Raman have proved that pure Ln₂Zr₂O₇ compounds have been successfully prepared for all the catalysts. By decreasing the r_A/r_B ratio, their crystalline structure transform from an ordered pyrochlore (La₂Zr₂O₇) to a less ordered pyrochlore (Pr₂Zr₂O₇ and Sm₂Zr₂O₇) and eventually to a defective cubic fluorite phase (Y₂Zr₂O₇). H₂-TPR, O₂-TPD and XPS have testified that the amount of surface active O₂ ^? species follows the order of La₂Zr₂O₇ > Pr₂Zr₂O₇ > Sm₂Zr₂O₇ > Y₂Zr₂O₇, which is well consistent with the reaction performance, indicating that the abundance of surface O₂ ^? sites is a critical factor influencing the reaction performance. CO₂-TPD has demonstrated that a better catalyst generally possesses a larger amount of surface moderate alkaline sites, which is another factor to affect the reaction performance. It is concluded that the concerted interaction between the two types of surface active sites controls the reaction performance of the Ln₂Zr₂O₇ catalysts. In comparison with the state-of-the art Mn/Na₂WO₄/SiO₂, La₂Zr₂O₇, the best catalyst, exhibits much improved reaction performance below 750 °C

Full text of DOI:10.1016/j.cclet.2019.03.031

Accepted Manuscript
Title: Ln2Zr2O7 compounds (Ln = La, Pr, Sm, Y) with varied
rare earth A sites for low temperature oxidative coupling of
methane
Authors: Xiuzhong Fang, Lianghui Xia, Liang Peng, Yuan
Luo, Junwei Xu, Luoji Xu, Xianglan Xu, Wenming Liu,
Renyang Zheng, Xiang Wang
PII:
DOI:
Reference:
S1001-8417(19)30126-3
https://doi.org/10.1016/j.cclet.2019.03.031
CCLET 4867
To appear in:
Chinese Chemical Letters
Received date:
Revised date:
Accepted date:
8 January 2019
16 February 2019
20 March 2019
Please cite this article as: Fang X, Xia L, Peng L, Luo Y, Xu J, Xu L, Xu X, Liu W,
Zheng R, Wang X, Ln2Zr2O7 compounds (Ln = La, Pr, Sm, Y) with varied rare earth
A sites for low temperature oxidative coupling of methane, Chinese Chemical Letters
(2019), https://doi.org/10.1016/j.cclet.2019.03.031
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Communication
Ln2Zr2O7 compounds (Ln = La, Pr, Sm, Y) with varied rare earth A
sites for low temperature oxidative coupling of methane
Xiuzhong Fang a, Lianghui Xia a, Liang Peng a, Yuan Luo a, Junwei Xu a, Luoji Xu a,
Xianglan Xu a, Wenming Liu a, Renyang Zheng b, Xiang Wang a,
*
a Key Laboratory of Jiangxi Province for Environment and Energy Catalysis, College of Chemistry, Nanchang University,
Nanchang 330031, China
b Research Institute of Processing (RIIP), SINOPEC, Beijing 100083, China
Corresponding author.
E-mail address: xwang23@ncu.edu.
Graphical abstract
The replacement of Ln site with different rare earth cations alters the crystalline phases of Ln2Zr2O7,
which in turn influences the surface active oxygen and alkaline sites significantly. The abundance and
the interaction of the two types of sites determine the OCM reaction performance.

ARTICLE INFO
Received 8 January 2019
Received in revised form 16 February 2019
Accepted 11 March 2019
Available online
ABSTRACT
With the objective to develop catalysts having application potential for oxidative coupling of
methane (OCM) at relatively lower temperature. A series of Ln2Zr2O7 compounds with varied rare
earth A sites have been prepared by a co-precipitation method. XRD and Raman have proved that
pure Ln2Zr2O7 compounds have been successfully prepared for all the catalysts. By decreasing the
rA/rB ratio, their crystalline structure transform from an ordered pyrochlore (La2Zr2O7) to a less
ordered pyrochlore (Pr2Zr2O7 and Sm2Zr2O7) and eventually to a defective cubic fluorite phase
-
(Y2Zr2O7). H2-TPR, O2-TPD and XPS have testified that the amount of surface active O2 species
follows the order of La2Zr2O7 > Pr2Zr2O7 > Sm2Zr2O7 > Y2Zr2O7, which is well consistent with the
-
reaction performance, indicating that the abundance of surface O2 sites is a critical factor
influencing the reaction performance. CO2-TPD has demonstrated that a better catalyst generally
possesses a larger amount of surface moderate alkaline sites, which is another factor to affect the
reaction performance. It is concluded that the concerted interaction between the two types of
surface active sites controls the reaction performance of the Ln2Zr2O7 catalysts. In comparison
with the state-of-the art Mn/Na2WO4/SiO2, La2Zr2O7, the best catalyst, exhibits much improved
reaction performance below 750 °C
Keywords:
Pyrochlore compounds
Low temperature OCM
A site substitution effects
Surface superoxide sites
Surface alkaline sites
Natural gas, which consists mainly of methane, is an important and cheap natural resource for the
production of platform chemicals and as an energy source. Since 2010, several countries have made
breakthroughs in the field of shale gas exploiting, thus a large amount of difficult-to-exploit methane can be
extracted [1, 2]. Therefore, the utilization of methane has attracted even more attentions and interests.
Oxidative coupling of methane (OCM) is regarded as an effective way to utilize methane with added-value,
since it can transform methane into C2 products, among which ethylene is a crucial platform organic
compound for chemical industry. Up to now, several kinds of catalysts have been tried for this reaction [3-6]. It
is commonly accepted that Mn/Na2WO4/SiO2 is still the most promising catalyst for industrial application, on
which 27% C2 product yield can be obtained at 850 oC [7, 8]. Whereas, the industrially implement of this vital
resource reaction is still limited by the one-way C2 yield below 30%, which is regarded as the minimum
requirement to industrialize this reaction economically and technically [9, 10]. Therefore, it is still of great
necessity to develop more efficient catalysts for this reaction.
A2B2O7 pyrochlore compounds generally own high thermal stability, intrinsic 8a oxygen vacancies and
certain surface alkalinity, which fit to the active site requirements for a good OCM catalyst. Therefore, since
late 1980s, some A2B2O7 pyrochlores have been tested for this reaction preliminarily, and found to display
certain reactivity [11-14]. But to date, only limited literatures are documented and people are still in short of
deep understanding on the structure and reactivity relationship for the application of pyrochlore
compounds for OCM reaction. Our former study has proved that La2Ce2O7 displays very good reaction
performance for OCM [15]. In particular, compared with the state-of-the-art Mn/Na2WO4/SiO2 catalyst, it
possesses much better low temperature reaction performance, though its highest C2 product yield is still lower
than that of Mn/Na2WO4/SiO2. For instance, even at 650 oC, 15.3% C2 yield can be achieved on La2Ce2O7, but
over Mn/Na2WO4/SiO2, the reaction is just started with a C2 yield below 1% [16, 17]. Furthermore, La2Ce2O7
demonstrates very potent resistance to sulfur and lead poisoning. It is revealed that the co-existence of both
active electrophilic oxygen and alkaline sites and the synergistic interaction between them are the major
reasons to control the reaction. With the target to develop better catalysts and gain deeper understanding on

structure and reactivity relationship, Ln2Zr2O7 pyrochlore compounds (Ln = La, Pr, Sm and Y) with a fixed
Zr4+ B site and varied rare earth A sites have been investigated for OCM reaction as a continued work. Indeed,
it is found that the reactivity of the catalysts changes with the change of the A site cation. La2Zr2O7 displays the
best reaction performance among all the catalysts, on which 15.1% C2 yield can be obtained even at 750 °C.
Moreover, at low temperature region between 650-700 °C, it also exhibits much better reaction performance
than Mn-Na2WO4/SiO2. These finding could give people some new insights on how to develop OCM catalysts
that can be operated at relatively lower temperature region. In addition, the bulk and surface structure changes
of Ln2Zr2O7 have been characterized by different means, and the relationship between the reactivity and
structure has been elucidated.
The Ln2Zr2O7 pyrochlore compounds have been prepared by a co-precipitation method. The detailed
activity evaluation and characterization experiment procedures of the catalysts are described in the
supporting information. For comparison purpose, a ZrO2 sample was also prepared by precipitation method,
and a Mn-Na2WO4/SiO2 catalyst was prepared with a traditional impregnation method according to the
references [18].
The reaction performance of the Ln2Zr2O7 catalysts has been tested and shown in Fig. 1. Pure ZrO2
displays very poor reaction performance for OCM, as evidenced by the very low methane conversion, C2
product selectivity and yield in the whole tested temperature region. In comparison, all the Ln2Zr2O7
catalysts exhibit significantly improved reaction performance, indicating the formation of pyrochlore
structure generates the active sites required for OCM reaction. With the increasing of the reaction
o
o
temperature from 650 C to 750 C, while the methane conversion improves, the C2 product selectivity is
nearly constant, proving that the C2 yield increase is contributed mainly by the methane conversion change.
In addition, the results imply that the C2 product selectivity on the catalysts is not sensitive to the reaction
temperatures. As shown in Fig. 1C, the C2 product yields achieved over the catalysts follow the sequence of
La2Zr2O7 > Pr2Zr2O7 > Sm2Zr2O7 ≈ Y2Zr2O7 > ZrO2. Among all the catalysts, La2Zr2O7 shows the best
o
reaction performance, on which the highest C2 yield around 15.1% can be achieved even at 750 C. For
o
clarification, the coupling product distribution on the catalysts collected at 750 C is presented in Table S1
(Supporting information). It is revealed that over all the catalysts, the percentage of C2H6 is higher than that
of C2H4. However, La2Zr2O7, the best catalyst in this study, owns the highest C2H4 selectivity in all the
catalysts, testifying that this catalyst also has more abundant active sites favorable for C2H6
dehydrogenation.
For an OCM catalyst, the stability is an important parameter to estimate its application potential, since this
reaction generally requires high temperature to proceed. Where upon, the long-term stability of La2Zr2O7, the
best catalyst, has been evaluated at 750 oC for 80 h. As demonstrated in Fig. 1D, the methane conversion, C2
selectivity and yield remains constant during the time span without any decrease, testifying this catalyst
possesses superior stability for the high temperature OCM process.
Currently, Mn-Na2WO4/SiO2 is still the most promising OCM catalyst from the point of view of industrial
o
application, on which the highest one way C2 product yield close to 27% can be obtained at 850 C [19].
o
However, this catalyst has very low activity below 700 C. To reduce production cost, it is always desirable
to find catalysts that can be operated at relatively lower temperatures region. As compared in Fig. S1
(Supporting information), La2Zr2O7, the best catalyst in this study, displays much better reaction
performance than Mn-Na2WO4/SiO2 under the same condition in the temperature region of 650~700 oC. For
o
instance, even at 650 C, 5.4% C2 yield has already been achieved over La2Zr2O7, but the reaction over
o
Mn-Na2WO4/SiO2 is just started with an extremely low C2 yield of 0.2%. At 700 C, the C2 yield over
La2Zr2O7 increases to 9.8%, which is still much higher than that over Mn-Na2WO4/SiO2. However, starting
from 750 oC, the C2 yield over La2Zr2O7 becomes about 1% lower than that over Mn-Na2WO4/SiO2. In brief,
Ln2Zr2O7 pyrochlores possess surface active sites that can match the requirements of OCM reaction. With
suitable element combination, La2Zr2O7 and Pr2Zr2O7 catalysts having outstanding low temperature reaction
performance and high stability has been attained. This implies that with further optimization, pyrochlore type
of OCM catalysts that can function at lower temperature could be eventually obtained.
The phase compositions of the catalysts have been analysed by XRD, with the patterns shown in Fig. 2.
Pure ZrO2 shows the typical diffraction feature of monoclinic phase structure, as also testified by the lattice
parameters in Table S2 (Supporting information). In contrast, for all the Ln2Zr2O7 catalysts, cubic pyrochlore
is the only detectable crystalline phase, no any impurity diffraction belonging to the individual oxides can be
observed, indicating that pure pyrochlore phase has been successfully synthesized. The intensive peaks
observed indicate that all the smaples are well crystallized. With the decreasing of rA/rB ratio from La2Zr2O7
to Y2Zr2O7, the peak position of the Ln2Zr2O7 pyrochlores shifts gradually to higher 2θ angles, as dispalyed

in the enlarged Fig. 2B, and the side length becomes smaller, indicating the presence of cell shrinking effect
caused by the decrease of the radii of the A site cations. It was reported formerly that with the decreasing of
the rA/rB ratio, the crystalline structure of the samples could transform from ordered pyrochlore (La2Zr2O7) to
less ordered pyrochlore (Pr2Zr2O7 and Sm2Zr2O7) and eventually to defective cubic fluorite phase (Y2Zr2O7)
[20]. As a result, the surface oxygen property related to the 8a intrinsic oxygen vacancies could be changed,
which subsequently influence the OCM reaction performance [14]. The XPS and O2-TPD results indeed
testify the change of the oxygen property, which will be discussed in more detail in the following sections.
The texture properties of the Ln2Zr2O7 catalysts have also been analysed by N2 adsorption-desorption,
with the isotherms and pore size distribution profiles displayed in Fig. S2 (Supporting information). Pure
ZrO2 depicts a typical type IV isotherm with an H1-type hysteresis loop, and its pore size distribution profile
shows a wide peak, which corresponds to an average pore size of 21 nm (Table S2), implying the presence of
inter-particle pores. In contrast, all the Ln2Zr2O7 catalysts possess also a typical type IV isotherm but with an
H2-type hysteresis loop, indicating that they have the same texture structure with each other but which is
different from that of the individual ZrO2. However, the quantification results in Table S2 manifest that both
Sm2Zr2O7 and Y2Zr2O7 have much higher pore volumes than that of La2Zr2O7 and Pr2Zr2O7. In addition, the
surface areas of the former two catalysts are higher than the latter two. Theoretically, the higher pore
volumes and surface areas are generally favourable for the reactants diffusion and contacting with the
surface active sites, which might promote the reaction performance of the catalysts. Whereas, as depicted in
Fig. 1, the reaction performance Sm2Zr2O7 and Y2Zr2O7 is much worse than La2Zr2O7 and Pr2Zr2O7, which
indicates that the texture structure change is not the crucial factor to affect the OCM reaction performance on
the catalysts.
Raman technique was adopted to investigate the structure change of Ln2Zr2O7 compounds, with the
profiles manifested in Fig. S3 (Supporting information). It is particularly noted here that the XRD patterns in
Fig. 2 have indicated that all the Ln2Zr2O7 samples are well crystallized due to high temperature calcination.
Therefore, it is rational to propose that the change of Raman spectra can be ascribed to the phase change
instead of crystalinity change of the samples. It is noticed that all the Ln2Zr2O7 catalysts show complete
different spectra from pure ZrO2, which confirms the structure difference observed by XRD. A well-ordered
cubic A2B2O7 pyrochlore generally has six typical Raman peaks at 299, 395, 500, 520, 601 and 750 cm-1 [21,
22]. The Raman peaks at 520 cm-1 is assigned to A1g mode related to the O-B-O banding vibrations. The
most intense Raman band at about 299 cm-1 is assigned to Eg mode. Moreover, the Raman band at around
395, 500, 601 and 750 cm-1 can be assigned to the F2g mode. It is well known that Raman spectroscopy
can provide explicit information to distinguish between a pyrochlore, biphasic mixture and a defect-fluorite
material [23]. The presence of all the six Raman bands in the profile of La2Zr2O7 testifies the formation of
ordered pyrochlore phase in its bulk. In contrast, on the profiles of Pr2Zr2O7 and Sm2Zr2O7, only the
strongest F2g peak is still detected with the disappearance of other bands, testifying that the pyrochlore phase
is still present but become less ordered [24]. On the profile of Y2Zr2O7, all the Raman bands assigning to
pyrochlore phase disappear completely, indicating the complete transformation of the crystalline phase. As
discussed above for the XRD results, it is believed that a defective cubic fluorite phase could have eventually
been formed in this sample.
H2-TPR was used to investigate the redox properties of the Ln2Zr2O7 catalysts, with the profiles shown in
Fig. S4A (Supporting information). For all of the Ln2Zr2O7 catalysts with different A site cations, a small
o
reduction peak is observed in the temperature region of 300-500 C, as better indicated by the enlarged
profiles in Fig. S4B (Supporting information). It is noted that pure ZrO2 shows also a minute reduction peak
o
at ~345 C. Theoretically, if all the lattice oxygen of a Ln2Zr2O7 compound can be reduced, the O/(Ln+Zr)
atomic ratio should be 1.75. However, the quantified results in Table S3 prove that on all the catalysts with
varied A sites, the ratios are much lower than 1.75, proving the peak can be assigned to the reduction of a
very small amount of surface deficient oxygen species. In addition, the oxygen amount follows the order of
La2Zr2O7 > Pr2Zr2O7 > Sm2Zr2O7 = Y2Zr2O7, which is nearly in accordance to the reaction performance of
the catalysts. Therefore, it is reasonable to believe that the amount of surface active oxygen sites could be
important for the reaction performance.
To further understand the oxygen property of the catalysts, O2-TPD experiments were also conducted for
all the Ln2Zr2O7 catalysts, with the profiles shown in Fig. S5 (Supporting information). Two groups of
desorption peaks are observed for all the Ln2Zr2O7 samples and the pure ZrO2. The low temperature peak at
~100 °C, which is labbled as  peak, is assigned to the desorption of loosely bonded surface oxygen species
[25]. The broad desorption peak above 200 °C, which is labelled as  peak, is obviously composed of two
overlapped peaks. Therfore, it was deconvoluetd into two peaks at ~300 and ~410 °C, respectively. The peak
at ~300 °C may correspond to the desorption of surface electrophilic oxygen species formed by the inducing

of the inherent 8a oxygen vacancies in the lattice [26], and the peak at ~410 °C could be assigned to the
desorption of surface mobile lattice oxygen [27]. Since the  group oxygen sites are close to the OCM
reaction temperature range, we hence believe that they should contribute mainly to the reaction. As
quantified in Table S4 (Supporting information), the amount of  group oxygen sites follows the sequence of
La2Zr2O7 > Pr2Zr2O7 > Sm2Zr2O7 > Y2Zr2O7, which is in line with the H2-TPR results. It is worth noting here
that La2Zr2O7 possesses the largest amount of surface active oxygen species, thus having the highest OCM
performance among all the catalysts. This testifies again that the amount of surface active oxygen species is
an important factor to decide the OCM reaction performance.
To further investigate the impact of A site change on the surface oxygen property of the Ln2Zr2O7
catalysts, XPS experiments were also performed, with the spectra shown in Fig. 3. For all the Ln2Zr2O7
catalysts, including the comparison pure ZrO2, two overlapped O 1s peaks are obviously observed. Fig. S6
(Supporting information) proves that carbonate is formed on the surface of all the catalysts, as indicated by
the XPS peak at ~288.9 eV. Therefore, part of the surface oxygen could be contributed by surface carbonates,
which is generally inert for any oxidation reaction [28]. Based on this, the O1s spectra of the samples are
deconvoluetd into three peaks according to the typical binding energies. The major peak at ~529.0 eV can be
assigned to surface lattice O2-, the peak at ~531.0 eV can be ascribed to oxygen species related to surface
-
carbonates, and the peak at ~532.8 eV can be attributed to surface superoxide O2 cations [28]. Former
-
studies have substantiated that superoxide O2 sites are important for the activation of methane molecules and
the selective formation of coupling products during OCM reaction [29, 30]. Therefore, the surface
-
superoxide O2 site percentages of the catalysts are quantified and listed in Table S5. Apparently, the
percentages of the Ln2Zr2O7 catalysts follow the sequence of La2Zr2O7 > Pr2Zr2O7 > Sm2Zr2O7 > Y2Zr2O7,
which is well consistent with the H2-TPR and O2-TPD results.
-
For better understanding, the surface superoxide O2 percentages of the catalysts and C2 product yields
achieved at 650, 700 and 750 °C are plotted in Fig. 3F. It is apparent that with the decreasing of the
-
superoxide O2 percentages in the order of La2Zr2O7 > Pr2Zr2O7 > Sm2Zr2O7 > Y2Zr2O7, the C2 product yields
degrade with the same order. This strongly demonstrates again that the amount of surface active electrophilic
oxygen species is a major factor to decide the OCM reaction performance of the La2Zr2O7 catalysts.
It is particularly noted here that although ZrO2 also possesses a small amount of surface active oxygen
species, our reaction results in Fig. 1 have testified that it displays very poor reaction performance for OCM,
with the formation of negligible amount of C2 product due to the short of surface alkaline sites. Many
previous studies have testified that both surface active electrophilic oxygen species and alkaline sites are
indispensible for OCM reaction. The surface alkaline sites are active for methane molecules adsorption and
activation, and the surface electrophilic oxygen sites are responsible for the formation of coupling products,
although sometimes they are also involved into methane activation [29]. Our former work has also testified
that the concerted interaction between surface alkaline sites and active oxygen species indeed controls the
OCM reaction performance of a catalyst [31].
According to this discussion, the amount of surface alkaline sites could correlate to the OCM reaction
performance of a catalyst intimately. Therefore, to probe the effect of the differed A sites on the surface
alkalinity of the Ln2Zr2O7 catalysts, they are subjected to CO2-TPD analysis. As manifested by Fig. 4, pure
ZrO2 displays no observable CO2 desorption in the tested temperature region, indicating the absence of
alkaline sites on its surface, which explains its very low OCM activity with the formation of negligible
amount of C2 products. In contrast, all Ln2Zr2O7 catalysts depict two CO2 desorption peaks. The first peak
emerges in the temperature region of 50-200 °C, which is assigned to weak surface alkaline sites. The second
peak emerges in the temperature range of 200-500 °C, which is assigned to alkaline sites with moderate
strength. It is worth noting here that the alkaline sites with moderate strength are particularly beneficial to
the selective formation of C2 coupling products [15, 31-33]. Therefore, the CO2 desorption amount is
quantified in Table S6 for all the catalysts, which indicates that the amount of the moderate alkaline sites
decreases in the order of La2Zr2O7 > Pr2Zr2O7 > Y2Zr2O7 > Sm2Zr2O7. In brief, the substitution of the A sites
of Ln2Zr2O7 compounds affects not only its surface active oxygen amount, but also its surface alkaline sites
quantity, which could eventually influencing the reaction performance of the catalysts.
For clarification, the amount of moderate surface alkaline sites is plotted against the C2 product yields
achieved at 650, 700 and 750 °C in Fig. 4B. It is typically observed that the C2 product yields on the
catalysts increase with the increasing of the moderate alkaline sites amount. Whereas, though Y2Zr2O7
possesses a larger amount of surface moderate alkaline sites than Sm2Zr2O7, it displays similar reaction
performance to the latter. This proves again that surface alkalinity is not the only factor deciding the reaction

performance. Taking into account of the surface oxygen property explored by H2-TPR, O2-TPD and XPS, it
is rational to propose that both the surface oxygen species and alkaline sites are critical for OCM reaction,
and their concerted interaction determines the reaction performance. By tuning the combination of A and B
sites of A2B2O7 compounds, catalysts with suitable amount of both active sites could be achieved, thus
developing catalysts having industrialization potential for OCM.
In conclusion, with the objective to develop catalysts having application potential for OCM, especially at
relatively lower temperature region. A series of Ln2Zr2O7 compounds with a fixed Zr4+ cation B sites but
with varied rare earth A sites (Ln = La, Pr, Sm and Y) have been prepared by a co-precipitation method in
this study. The surface and bulk properties of the catalysts have been characterized by different means, and
correlated with their reaction performance.
(1) XRD and Raman results have proved that pure Ln2Zr2O7 compounds have been successfully prepared
for all the catalysts. With the decreasing of the rA/rB ratio, the crystalline structure of the compounds
transform from an ordered pyrochlore (La2Zr2O7) to a less ordered pyrochlore (Pr2Zr2O7 and Sm2Zr2O7)
and eventually to a defective cubic fluorite phase (Y2Zr2O7).
-
(2) As testified by H2-TPR, O2-TPD and XPS, the amount of surface active superoxide O2 species follows
the order of La2Zr2O7 > Pr2Zr2O7 > Sm2Zr2O7 > Y2Zr2O7, which is well consistent with the reaction
performance of the catalysts, and indicates that the abundance of surface active oxygen sites is a critical
factor influencing the reaction performance. CO2-TPD results have demonstrated a better catalyst
generally possesses a larger amount of surface moderate alkaline sites, which is another factor to affect
the reaction performance.
(3) It is concluded that the concerted interaction between the two types of surface active sites controls the
reaction performance of the Ln2Zr2O7 catalysts. La2Zr2O7 possesses the largest amount of both kinds of
surface sites, hence displaying the optimal reaction performance among all the catalysts. In comparison
with the state-of-the art Mn/Na2WO4/SiO2, La2Zr2O7 exhibits much better reaction performance at a low
temperature region (＜750°C). Therefore, the information included in this study could give people some
new insights on how to develop improved OCM catalysts that can be operated at relatively lower
temperatures.
Acknowledgements
This work is supported by the Natural Science Foundation of China (Nos. 21567016, 21566022,
21666020), the Natural Science Foundation of Jiangxi Province (Nos. 20181ACB20005, 20171BAB213013
and 20181BAB203017), the Key Laboratory Foundation of Jiangxi Province for Environment and Energy
Catalysis (No. 20181BCD40004), the Education Department of Jiangxi Province (Nos. GJJ150016,
GJJ150085 and KJLD14005), the China Postdoctoral Science Foundation (No. 2018M631294) and the
Graduate Student Creativity Funding of Nanchang University (No. 201802062), which are greatly
acknowledged by the authors.
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30
25
20
15
10
5
100
80
60
40
20
0
(A)
La2Zr2O7
Pr2Zr2O7
Sm2Zr2O7
Y2Zr2O7
ZrO2
(B)
La2Zr2O7
Pr2Zr2O7
Sm2Zr2O7
Y2Zr2O7
ZrO2
0
650
675
700
725
750
650
675
700
725
750
Temperature (oC)
Temperature (oC)
30
25
20
15
10
5
100
CH4 conversion
C2 selectivity
C2 yield
(C)
(D)
La2Zr2O7
Pr2Zr2O7
Sm2Zr2O7
Y2Zr2O7
ZrO2
80
60
40
20
0
0
650
675
700
725
750
0
20
40
60
80
Temperature (oC)
Time (h)
Fig. 1. Reaction performance of the Ln2Zr2O7 catalysts. (A) CH4 conversion, (B) C2 product selectivity, (C) C2 product yield, (D)
Stability test at 750 oC over La2Zr2O7 catalyst.


(B)
(A)
- Ln2Zr2O
7



La2Zr2O7
La2Zr2O7
Pr2Zr2O7
Pr2Zr2O7
Sm2Zr2O7
Sm2Zr2O7
Y2Zr2O7
ZrO2
Y2Zr2O7
ZrO2
10 20 30 40 50 60 70 80 90 27 28 29 30


Fig. 2. XRD patterns of the Ln2Zr2O7 catalysts.

110000
100000
90000
80000
70000
60000
50000
40000
30000
20000
10000
90000
80000
70000
60000
50000
40000
30000
20000
10000
O2-
(C)
(B)
Pr2Zr2O7
O2-
70000
60000
50000
40000
30000
20000
10000
O2-
La2Zr2O7
(A)
Sm2Zr2O
7
CO32-
2-
2-
CO3
CO3
O2-
534
-
-
O2
O2
534
532
530
528
526
534
532
530
528
526
532
530
528
526
Binding energy (eV)
Binding energy (eV)
Binding energy (eV)
120000
108000
96000
84000
72000
60000
48000
36000
24000
12000
20
15
10
5
O2-
150000
135000
120000
105000
90000
75000
60000
45000
30000
15000
O2 /(O2 +CO32-+O2-
)
(F)
(D)
-
-
Y2Zr2O7
(E)
ZrO2
O2-
650 o
700 o
750 o
C
4.0%
C
C
3.6%
2-
CO3
3.0%
3.4%
2-
CO3
-
-
O2
O2
534
2.3%
Sm2Zr2O7 Y2Zr2O7 ZrO2
534
532
530
528
526
0
532
530
528
526
La2Zr2O7 Pr2Zr2O7
Binding energy (eV)
Binding energy (eV)
Fig. 3. Analyzing surface oxygen property of the Ln2Zr2O7 catalysts by XPS. (A) O 1s of La2Zr2O7, (B) O 1s of Pr2Zr2O7, (C) O
1s of Sm2Zr2O7, (D) O 1s of Y2Zr2O7, (E) O 1s of ZrO2, (F) the relationship between surface active oxygen amount and C2
product yields.
20
15
10
5
320
(A)
(B)
C
Moderate alkaline site amount
La2Zr2O7
95
105
650 o
73
700 oC
750 o
370
C
52
Pr2Zr2O7
315
100
58
35
370
Sm2Zr2O7
Y2Zr2O7
ZrO2
110
100
1
ZrO2
0
200
300
400
500
600
Y2Zr2O7
La2Zr2O7 Pr2Zr2O7
Sm2Zr2O7
Temperature (oC)
Fig. 4. CO2-TPD profiles of the Ln2Zr2O7 catalysts.