Optimizing the Reaction Performance of La2Ce2O7-Based Catalysts for Oxidative Coupling of Methane (OCM) at Lower Temperature by Lattice Doping with Ca Cations

Article abstract of DOI:10.1002/ejic.201801250

To optimize the reaction performance of La₂Ce₂O₇ for OCM, catalysts with varied La/Ce molar ratios and doped by Ca additive have been synthesized and characterized by different techniques. It is discovered that a La₂Ce₂O₇ compound having a disordered cubic defective fluorite phase is predominantly formed in all the samples. Varying the La/Ce ratio influences the fine crystalline structure of the catalysts. As a result, the abundance of the surface facile oxygen and alkaline sites increase by increasing the La/Ce ratio. Moreover, the Ca additive exists on the surface of Ca_0.5La₂Ce₂O₇ and Ca_0.5La_1.5Ce₂O₇ as dispersed CaCO₃, which covers the active sites and is harmful to the OCM reaction. However, for La₂Ce_1.5Ca_0.5O₇, the major part of the Ca additive has entered into the lattice of La₂Ce₂O₇ phase as Ca²⁺ cations, thus creating more oxygen vacancies and improving the surface alkalinity of the catalyst, which is favorable to the OCM reaction. Both the active surface oxygen and alkaline sites are vital for OCM, and their synergism determines the reactivity. By increasing La/Ce ratio and doping Ca²⁺ into La₂Ce₂O₇ lattice, the quantities of both kinds of active sites can be significantly improved, thus obtaining catalysts with much improved reaction performance. La₂Ce_1.5Ca_0.5O₇ owns the largest amount of both types of active sites, hence depicting the best reaction performance, over which the highest C₂ yield of 22.5 % can be achieved even at 750 °C.

Full text of DOI:10.1002/ejic.201801250

A Journal of
Accepted Article
Title: Optimizing the Reaction Performance of La2Ce2O7-based
Catalysts for Oxidative Coupling of Methane (OCM) at Lower
Temperature by Lattice Doping with Ca Cations
Authors: Junwei Xu, Yan Zhang, Yameng Liu, Xiuzhong Fang,
Xianglan Xu, Wenming Liu, Renyang Zheng, and Xiang Wang
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201801250
Link to VoR: http://dx.doi.org/10.1002/ejic.201801250

10.1002/ejic.201801250
European Journal of Inorganic Chemistry
FULL PAPER
Optimizing the Reaction Performance of La2Ce2O7-based
Catalysts for Oxidative Coupling of Methane (OCM) at Lower
Temperature by Lattice Doping with Ca Cations
Junwei Xu,[a] Yan Zhang,[a] Yameng Liu,[a] Xiuzhong Fang,[a] Xianglan Xu,[a] Wenming Liu,[a] Renyang
Zheng,[b] Xiang Wang*[a]
Abstract: To optimize the reaction performance of La2Ce2O7 for
OCM, catalysts with varied La/Ce molar ratios and doped by Ca
additive have been synthesized and characterized by different
techniques. It is discovered that a La2Ce2O7 compound having a
disordered cubic defective fluorite phase is predominantly formed in
all the samples. Varying the La/Ce ratio influences the fine
crystalline structure of the catalysts. As a result, the abundance of
the surface facile oxygen and alkaline sites increase by increasing
the La/Ce ratio. Moreover, the Ca additive exists on the surface of
Ca0.5La2Ce2O7 and Ca0.5La1.5Ce2O7 as dispersed CaCO3, which
covers the active sites and is harmful to the OCM reaction. However,
for La2Ce1.5Ca0.5O7, the major part of the Ca additive has entered
into the lattice of La2Ce2O7 phase as Ca2+ cations, thus creating
more oxygen vacancies and improving the surface alkalinity of the
catalyst, which is favorable to the OCM reaction. Both the active
surface oxygen and alkaline sites are vital for OCM, and their
synergism determines the reactivity. By increasing La/Ce ratio and
doping Ca2+ into La2Ce2O7 lattice, the quantities of both kinds of
active sites can be significantly improved, thus obtaining catalysts
with much improved reaction performance. La2Ce1.5Ca0.5O7 owns the
largest amount of both types of active sites, hence depicting the best
reaction performance, over which the highest C2 yield of 22.5% can
be achieved even at 750 oC.
still remains a big challenge for both fundamental understanding
and industrialization of this important process though thirty years
have passed. It is commonly accepted that, to solve this problem,
the development of more effective catalysts to get a one-way C2
yield not below 30% is still the crucial issue.
So far, Mn/Na2WO4/SiO2 and Li/MgO catalysts have been
extensively studied and the reaction mechanisms have been
researched. Although Li/MgO exhibits good initial catalytic
performance, its stability is suffered from the vaporization of Li+
cations, the structure modifier, at elevated temperatures.[2-4]
Mn/Na2WO4/SiO2 and its promoted forms have both good
catalytic performance and long-term stability.[5-14] However, the
property of the active sites in this complicated catalyst has not
yet been understood very clearly. Additionally, on this catalyst,
o
OCM reaction can only take place efficiently above 800 C. On
the basis of former studies, it is generally recognized that OCM
reaction includes typically two steps, that is, the generation of
CH3· radicals on active surface oxygen and alkaline sites, which
is followed by the coupling of the CH3· radicals in gas phase to
form C2H4.[14-16] Therefore, both active surface electrophilic
oxygen and alkaline sites are indispensible for the reaction.[17, 18]
Currently, Mn/Na2WO4/SiO2 is regarded as the catalyst with the
highest potential for industrial application. However, its
industrialization is still hampered by the low C2 yield below the
30% minimum requirement.[17, 18] Therefore, there is still a strong
driving force to search better catalysts that can be operated at
relatively lower temperature with enough C2 yield to industrialize
this important resource reaction.
Introduction
Methane is a widely utilized and cheap natural resource for the
manufacturing of platform chemicals, and also an energy source.
Exploring potential technologies for the value-added
transformation of methane is always an attractive topic. Ethylene
A2B2O7 composite oxides usually possess high thermal
stability, intrinsic structure oxygen vacancies and certain surface
alkalinity, which meets most of the active site requirements for
an applicable OCM catalyst.[19] Therefore, A2B2O7 compounds
with different chemical compositions have been preliminary
studied by several researchers at early 1990s, which shows
certain reactivity and selectivity to this reaction.[19-24] However,
people still lack systematic understanding on their structure and
reaction relationship. Our former studies on Ln2Ce2O7 catalytic
materials with varied A sites have testified that a La2Ce2O7
catalyst possessing a disordered defective cubic fluorite phase
exhibits good performance for OCM reaction because of the
optimal mutual interaction between the surface electrophilic
oxygen and alkaline sites, which relates intimately to its highly
disordered crystalline structure and abundant 8a inherent
oxygen vacancies.[19] Compared with the state-of-art
Mn/Na2WO4/SiO2, La2Ce2O7 displays much better reaction
performance at lower temperature, though its highest C2 product
yield is still lower. For instance, even at 650 oC, 15.3% C2
product yield can be gotten over this catalyst, but the reaction is
just initiated over Mn/Na2WO4/SiO2 with a yield below 1%.[19]
Therefore, with the purpose to obtained improved catalysts, in
this study, we have intentionally prepared La2Ce2O7-based
is
a vital intermediate chemical in industry for higher
hydrocarbons manufacturing. The production and consumption
volumes of ethylene are an important criterion to measure the
development level of a country’s chemical industry. Over past
decades, OCM reaction has drawn a lot of attention because it is
a simple route that can transform methane directly into ethylene.
However, since the pioneering work of Keller et al in 1982,[1] it
[a]
J. W. Xu, Y. Zhang, Y. M. Liu, Dr. X. Z. Fang, Dr. X. L. Xu, Prof. Dr.
W. M. Liu, Prof. Dr. X. Wang
Key Laboratory of Jiangxi Province for Environment and Energy
Catalysis
College of Chemistry，Nanchang University
Nanchang, Jiangxi 330031 (P.R. China)
E-mail: xwang23@ncu.edu.cn
http://chem.ncu.edu.cn/index.php?c=channel&molds=szdw&id=39
Dr. R. Y. Zheng,
[b]
Research Institute of Petroleum Processing (RIPP)
SINOPEC
18 Xueyuan Road, Haidian District, Beijing 100083, China
Supporting information for this article is given via a link at the end of
the document.
1
This article is protected by copyright. All rights reserved.

10.1002/ejic.201801250
European Journal of Inorganic Chemistry
FULL PAPER
catalysts with varied La/Ce molar ratios to adjust the quantities
of surface oxygen species and alkaline sites, which believed to
be critical for the reaction performance. In addition, Ca2+, which
has a close radius to both La3+ and Ce4+ at the same
coordination numbers (Table S1), has been adopted to dope the
crystalline lattice of La2Ce2O7 to improve the oxygen mobility
and alkalinity of the prepared catalysts. It has been disclosed
that for a La2Ce1.5O7 sample with a La/Ce molar ratio below the
stoichiometry, the reaction performance can be significantly
enhanced in comparison with the stoichiometric La2Ce2O7.
Furthermore, by doping an appropriate amount of Ca2+ into the
lattice matrix of the La2Ce2O7 phase, the performance of the
achieved La2Ce1.5Ca0.5O7 catalyst can be further improved, over
which the highest C2 yield around 23% has been achieved at
750 oC. By using various techniques, the bulk and surface
structures of the samples have been explored, which has been
correlated with their OCM reaction performance.
Table 1. ICP-AES elemental analysis and N2-BET specific surface area
results for the catalysts.
Catalysts
La/Ce
Molar
ratio
Ca/La
Molar
ratio
Ca/Ce
Molar
ratio
Specific surface
areas
Fresh
29
Spent
28
La2Ce2O7
2.10/2.00
2.06/2.00
1.52/2.00
1.54/2.00
2.04/1.50
2.01/1.50
-
-
Ca0.5La2Ce2O7
La1.5Ce2O7
0.48/2.00
0.50/2.00
16
15
-
-
18
18
Ca0.5La1.5Ce2O7
La2Ce1.5O7
0.52/1.50
-
0.54/2.00
-
13
12
17
16
La2Ce1.5Ca0.5O7
0.49/2.00
0.49/2.00
13
12
.
Results and Discussion
As listed in Table 2, in comparison with the stoichiometric
La2Ce2O7 sample, while the 2θ angles of the typical diffraction
peaks of La1.5Ce2O7 are evidently increased with reduced inter-
planar spacing distances (d), the 2θ of La2Ce1.5O7 are
decreased with increased d values, testifying that the change of
the La/Ce molar ratio affects the exquisite crystalline phase
structure of the samples. On the other hand, in comparison with
the un-modified La2Ce2O7 and La1.5Ce2O7, the addition of Ca2+
cations has no detectable influence on the 2θ angles and d
values of the corresponding Ca0.5La2Ce2O7 and Ca0.5La1.5Ce2O7
catalysts. Whereas, the addition of Ca2+ cations into La2Ce1.5O7
shifts obviously the 2θ of La2Ce1.5Ca0.5O7 to lower angles with
slightly increased d values. These results imply that the added
calcium species could be distributed in the samples with
different forms.
N2-BET and ICP-AES analysis
The element compositions have been verified by ICP-AES
analysis for the final catalysts, with the results summarized in
Table 1. Compared with the initial formulations used for sample
preparation, it is clear that during the synthesis process, there is
no loss of the metal elements.
The surface areas of the fresh and used samples have been
analyzed by N2-BET. The original La2Ce2O7 sample with a
stoichiometric La/Ce atomic ratio of 2/2 possesses the highest
surface area among all the catalysts. By deviating the
stoichiometric ratio, the surface areas of La1.5Ce2O7 and
La2Ce1.5O7 decrease evidently. With the addition of Ca2+ cations,
the surface areas of all the modified samples decrease further in
comparison with that of the counterpart un-modified samples.
Interestingly, compared with the fresh samples, all the spent
catalysts have only insignificant surface area drop, indicating
that these La2Ce2O7-based catalysts are physically stable, which
can stand the high temperature impact during the OCM reaction
process. In fact, the tolerance of high temperatures is an
important parameter to decide the application potential of an
OCM catalyst. Therefore, from the point of view of physical
stability, it is believed that these La2Ce2O7-based catalysts are
appropriate for the reaction.
The phase compositions and fine structures of the
catalysts identified by XRD Rietveld refinement method
XRD has been employed first to identify the phase compositions
of the fresh and spent catalysts. As depicted in Figure 1(a), all
the fresh catalysts, with or without Ca2+ cation doping, exhibit
the diffraction feature of a disordered defective cubic fluorite
La2Ce2O7 phase having no any impurity peaks from calcium
related compounds. This indicates that all the catalysts consist
predominantly of La2Ce2O7 phase, and the calcium related
species either has dispersed finely onto the catalyst surfaces as
oxide or carbonate, or have entered into the lattice matrix of
La2Ce2O7 as Ca2+ cations to substitute part of the La3+ and Ce4+
cations, hence avoiding the detection by XRD. Notably, the
distribution of calcium species will be discussed in more detail in
the following section with additional experimental evidences.
Compared with the fresh catalysts, Figure.1(b) demonstrates
that no detectable change is observed, indicating that these
catalysts are very stable for the high temperature OCM process
111
(a)
220
311
222
200
La2Ce1.5Ca0.5O7
331
420
400
La2Ce1.5O7
Ca0.5La1.5Ce2O7
La1.5Ce2O7
Ca0.5La2Ce2O7
La2Ce2O7
10
20
30
40
50
2 (°)
60
70
80
2
This article is protected by copyright. All rights reserved.

10.1002/ejic.201801250
European Journal of Inorganic Chemistry
FULL PAPER
111
(b)
331
Table 3. XRD Rietveld refinement results.
220
200
311
222
La2Ce1.5Ca0.5O7
La2Ce1.5O7
420
400
Samples
Lattice parameter
a=b=c(Å)
5.573
5.571
5.546
5.544
5.586
5.597
5.411
α=β=γ(°)
Ca0.5La1.5Ce2O7
La1.5Ce2O7
La2Ce2O7
90
90
90
90
90
90
90
Ca0.5La2Ce2O7
La1.5Ce2O7
Ca0.5La1.5Ce2O7
La2Ce1.5O7
La2Ce1.5Ca0.5O7
CeO2
Ca0.5La2Ce2O7
La2Ce2O7
10
20
30
40
50
60
70
80
2 (°)
Figure 1. XRD patterns of the catalysts. (a) freshly calcined, (b) spent.
Table 2. Crystalline phase properties of the fresh catalysts measured By
XRD.
As shown in Table 3, the side length of the catalysts follows
the sequence of La2Ce1.5Ca0.5O7> La2Ce1.5O7 > La2Ce2O7
≈
Samples
(111)
(220)
2θ(°)
(311)
2θ(°)
Ca0.5La2Ce2O7 > La1.5Ce2O7 ≈ Ca0.5La1.5Ce2O7. It is well known
that doping metal cations with a smaller radius into the
crystalline lattice of a host oxide usually leads to its lattice
contraction with decreased side lengths. On the contrary, the
doping of metal cations with a larger radius will increase the side
lengths, both accompanying the formation of oxygen vacancies
because of electrostatic repulsion.[28] To help the discussion of
the structure change of the La2Ce2O7-based catalysts, the radii
of the related cations with different CN are listed in Table S1.
As formerly reported, in some A2B2O7 compounds, excess A
or B site cations can substitute some portion of B or A site
cations in the structure. The expansion or shrinking of the lattice
parameters will usually take place, which depends on the
substitution amount.[23] For La1.5Ce2O7, the excess Ce4+ cations
with a smaller radius could substitute some portion of La3+
cations in its La2Ce2O7 phase lattice, hence resulting in cell
shrinking and smaller side lengths compared to pure La2Ce2O7.
On the contrary, for La2Ce1.5O7, the excess La3+ cations with a
larger radius could substitute some portion of Ce4+ cations in its
La2Ce2O7 phase lattice, thus resulting in cell expansion and
larger side lengths compared to pure La2Ce2O7.
As to calcium doping effects, it is apparent that both
Ca0.5La2Ce2O7 and Ca0.5La1.5Ce2O7 have the same side lengths
to their counterpart un-modified catalysts, indicating that the
Ca2+ cations could disperse finely on the catalyst surface as
calcium oxide or calcium carbonate, instead of entering into the
lattice matrix of the formed La2Ce2O7 phase. Whereas,
La2Ce1.5Ca0.5O7 has obviously larger side lengths than the un-
modified La2Ce1.5O7, proving that a portion of Ca2+ cations with a
larger radius have been incorporated into the lattice of the
La2Ce2O7 phase in this sample. Indeed, the good fitness
between Ca2+ and La3+/Ce4+ in Table S1 supports this
assumption.
2θ(°)
d(Å)
d(Å)
d(Å)
La2Ce2O7
27.741
27.739
27.980
27.982
27.682
27.639
28.663
3.213
3.211
3.186
3.185
3.220
3.225
3.112
46.035
46.032
46.342
46.340
45.981
45.901
47.591
1.970
1.968
1.958
1.958
1.972
1.975
1.909
54.568
54.566
54.941
54.939
54.461
54.422
56.433
1.680
1.678
1.670
1.672
1.683
1.684
1.629
Ca0.5La2Ce2O7
La1.5Ce2O7
Ca0.5La1.5Ce2O7
La2Ce1.5O7
La2Ce1.5Ca0.5O7
CeO2
To accurately discern the fine structure transformation of the
samples with varied La/Ce ratios and modified by calcium
additive, XRD Rietveld refinement method has been adopted to
evaluate the lattice parameters of the samples (Table 3 and
Figure S1). For comparison purpose, the 2θ and d values of
pure CeO2 possessing an ordered cubic fluorite phase structure
are also listed here. Based on crystallography study, the
disordered defective cubic fluorite phase structure of La2Ce2O7
is similar to the ordered cubic fluorite phase structure of CeO2,
but they have also some difference from each other, as
evidenced by the different 2θ angles, d values and unit cell side
lengths between them in Table 2 and 3. For example, compared
with the ordered cubic fluorite phase, the disordered defective
cubic fluorite phase typically possesses an 8a intrinsic oxygen
vacancy in a unit cell.[19, 25, 26] As a consequence, the average
oxygen coordination number (CN) of the total A and B metal
cations in a disordered defective cubic fluorite phase is 7, which
is less than CN=8 in a perfect cubic fluorite phase.[27] In addition,
the A and B cations are distributed randomly in the sublattices,
and the inherent oxygen vacancies are highly disordered, thus
resulting in evident improvement of lattice oxygen mobility, and
In summary, all the studied samples are composed majorly of
La2Ce2O7 phase with a disordered defective cubic fluorite
structure in the bulk. By increasing or decreasing the La/Ce
ratios, the fine crystalline structure can be altered due to the
substitution of the A site La3+ or B site Ce4+ cations by the
excess Ce4+ or La3+ cations, respectively. Moreover, while the
addition of Ca2+ cations has no impact on the exquisite
eventually generating abundant facile surface oxygen sites.[19, 25,
26]
3
This article is protected by copyright. All rights reserved.

10.1002/ejic.201801250
European Journal of Inorganic Chemistry
FULL PAPER
crystalline structures of La2Ce2O7 and La1.5Ce2O7 samples, it
has evident influence on the structure of La2Ce1.5O7. This proves
that in the former two samples, calcium oxide or carbonate might
be formed and dispersed on the catalyst surface, but in
La2Ce1.5O7, Ca2+ cations could have been doped into the lattice
matrix of its La2Ce2O7 phase.
HAADF-STEM Mapping of the Modified Catalysts
To further identify the distribution of the calcium species in the
modified samples, Ca0.5La0.5Ce2O7, Ca0.5La1.5Ce2O7 and
La2Ce1.5Ca0.5O7 have been analyzed by HAADF-STEM mapping.
The images of Ca0.5La2Ce2O7 in Figure 2(a) and the images of
Ca0.5La1.5Ce2O7 in Figure 2(b) demonstrate the uniform
distribution of La, Ce and O elements in the scanned zones.
However, the distribution of Ca in these two samples is not
homogeneous with other elements especially for Ca0.5La1.5Ce2O7,
as evidenced by some observed brighter areas, indicating the
accumulation of calcium species on the surfaces. This is in good
accordance to the XRD results, substantiating that calcium oxide
or carbonate could have been formed and dispersed uniformly
on the catalyst surfaces.
Figure 2. HAADF-STEM mapping images of (a) Ca0.5La2Ce2O7, (b)
Ca0.5La1.5Ce2O7 and (c) La2Ce1.5Ca0.5O7.
Structure analysis with Raman spectroscopy
In contrast, as depicted in Figure.2(c), for La2Ce1.5Ca0.5O7, Ca
distributes very uniformly with all the other elements, which
testifies strongly that the Ca2+ cations in this sample have been
incorporated into the crystalline lattice. In conclusion, the
mapping results are well consistent with the XRD measurements,
further confirming the distribution of calcium species is different
in the catalysts with varied La/Ce molar ratios.
The surface structures of the fresh and spent catalysts have also
been investigated by Raman spectroscopy. Compared with the
spectra of the fresh catalysts in Figure 3, no any evident change
can be observed for those of the used catalysts in Figure S2,
proving again that these catalysts are very stable during the high
temperature OCM process.
As displayed in Figure 3(a), two characteristic peaks at ~445
and ~575 cm-1 are observed for all the fresh samples. It was
reported previously that a well crystallized CeO2 with an ideal
cubic fluorite structure exhibits only one Raman active F2g mode
at ~460 cm-1, corresponding to the Ce-O bond vibration.
However, for poorly crystallized CeO2 possessing a lot of
surface and bulk defects, another peak at ~600 cm-1
corresponding to surface oxygen vacancies can also be
observed.[29, 30] In addition, the doping of La3+ into CeO2 lattice
can shift the F2g peak to a lower wavenumber.[31, 32] Therefore,
the Raman peak of the La2Ce2O7-based samples at ~445 cm-1
is assigned to the F2g mode of the defective cubic fluorite phase,
and the band at ~575 cm-1 is assigned to the surface oxygen
vacancies originated from the incorporation of La3+ into CeO2
lattice. In detail, to maintain the overall charge, oxygen
vacancies are formed, which can dissociatively adsorb O2
molecules and generate surface mobile oxygen species.[31, 32]
To confirm this, the Raman spectra in the region of 950-1200
cm-1are enlarged for the samples and shown in Figure 3(b). A
band located at 1075 cm-1 is detected for all the catalysts, which
-
can be attributed to the electrophilic O2 species, and indeed
verifies the formation of facile oxygen sites over the surface of
all the samples.[33]
Notably, for the three modified catalysts, a sharp peak at
1088 cm-1 can be obviously observed, which is attributed to the
A1 mode of aragonite CaCO3.[34,35] This proves the existence of
dispersed CaCO3 on the catalyst surface, especially for
Ca0.5La2Ce2O7 and Ca0.5La1.5Ce2O7, which is in line with the
XRD and STEM-mapping analysis. It is particularly explained
here that for La2Ce1.5Ca0.5O7, though most of the calcium
species has been doped into the lattice matrix, a small amount
of CaCO3 could still be un-avoidably present on its surface. In
comparison with Ca0.5La2Ce2O7 and Ca0.5La1.5Ce2O7, its surface
CaCO3 amount is much less, as testified by the much weaker
4
This article is protected by copyright. All rights reserved.

10.1002/ejic.201801250
European Journal of Inorganic Chemistry
FULL PAPER
aragonite CaCO3 peak.
surface oxygen vacancies improves. Second, both La2Ce2O7
and La1.5Ce2O7 have similar A575/A445 ratios to their
corresponding samples modified by calcium, further testifying
that Ca is present as CaCO3 on their surface instead of doping
into their crystalline lattice. Therefore, the concentration of the
surface oxygen vacancies between the modified and un-
modified catalysts remains nearly unchanged within the
experimental error. However, La2Ce1.5Ca0.5O7 exhibits evidently
higher A575/A445 ratio than that of the un-modified La1.5Ce2O7,
confirming the doping of Ca2+cations into the lattice matrix of this
sample and resulting in improved amount of surface oxygen
vacancies. In brief, the varying of the La/Ce molar ratio and the
addition of calcium affects the surface defects and oxygen
vacancy amount of the prepared catalysts significantly, which
might eventually impact the OCM performance over the
catalysts.
50000
(a)
Oxygen vacancy
F2g
45000
40000
35000
30000
25000
20000
15000
10000
5000
La2Ce1.5Ca0.5O7
La2Ce1.5O7
Ca0.5La1.5Ce2O7
La1.5Ce2O7
Ca0.5La2Ce2O7
La2Ce2O7
0
1400
1200
1000
800
600
400
200 100
1500
Raman shift (cm-1
)
5500
5000
4500
4000
3500
3000
2500
2000
1500
1000
500
2-
-
CO3
O2
Catalytic performance tests
(b)
La2Ce1.5Ca0.5O7
La2Ce1.5O7
The OCM reaction performance has thus been evaluated for the
catalysts (Figure 4). As to the un-modified catalysts, both
La1.5Ce2O7 and La2Ce1.5O7 display higher CH4 conversion, C2
selectivity and C2 yield than the stoichiometric La2Ce2O7 below
650 oC. Above this threshold temperature, the reaction
Ca0.5La1.5Ce2O7
La1.5Ce2O7
performance follows the order of La2Ce1.5O7 > La2Ce2O7
>
La1.5Ce2O7, proving that the increase of La/Ce molar ratio
improves the reaction performance of the catalysts. Furthermore,
with the addition of calcium, the reaction performance of
La2Ce2O7 and La1.5Ce2O7 is significantly dropped, which could
be attributed to the covering of the active sites by the dispersed
CaCO3 on the catalyst surface. On the contrary, the addition of
calcium improves the reaction performance of La2Ce1.5O7
apparently, especially above 650 oC. Over the promoted
La2Ce1.5Ca0.5O7, the highest C2 yield of 22.5% can be gotten
even at 750 oC. As discussed above, for this catalyst, the
Ca2+cations have been majorly incorporated into the lattice
matrix of the La2Ce2O7 phase to generate more bulk and surface
defects, which improves the surface active oxygen abundance
significantly, an important active species for OCM.
Ca0.5La2Ce2O7
0
-500 La2Ce2O7
-1000
1200
1150
1100
Raman shift (cm-1
1050
1000
950
)
Figure 3. Raman spectra of the catalysts. (a) Complete spectra from 100 to
1500 cm-1, (b) Magnified spectra from 950 to 1200cm-1
Table 4. Quantified Raman results for oxygen vacancies.
Samples
Quantified Raman results (A575/A445)
The reaction stability of La2Ce1.5Ca0.5O7, the best catalyst in
this study, has thus been performed to estimate its application
potential. Figure 4(d) proves that during 100 h test at 750 oC, the
CH4 conversion, C2 selectivity and C2 yield remained constant,
implying that it possesses very stable reaction performance.
Fresh
0.74
0.71
0.64
0.61
2.09
2.50
Spent
La2Ce2O7
0.71
0.65
0.59
0.54
2.01
2.42
Ca0.5La2Ce2O7
La1.5Ce2O7
(a)
30
Ca0.5La1.5Ce2O7
La2Ce1.5O7
La2Ce1.5Ca0.5O7
20
La2Ce2O7
Ca0.5La2Ce2O7
La1.5Ce2O7
Ca0.5La1.5Ce2O7
La2Ce1.5O7
For pure CeO2, the integrated area ratio between the ~450
and ~600 cm-1 peak has formerly been used to estimate the
quantity of surface oxygen vacancies.[29] Therefore, we have
quantified the integrated area ratios (A575/A445) of the fresh and
spent samples in Table 4. The A575/A445 ratios of the catalysts
follow the order of La2Ce1.5Ca0.5O7 > La2Ce1.5O7 > Ca0.5La2Ce2O7
≈ La2Ce2O7 > Ca0.5La1.5Ce2O7 ≈ La1.5Ce2O7. First, with the
increasing of La/Ce molar ratio, the A575/A445 ratios of the un-
modified catalysts increase, proving the concentration of the
La2Ce1.5Ca0.5O7
10
550
600
650
700
750
Temperature (oC)
5
This article is protected by copyright. All rights reserved.

10.1002/ejic.201801250
European Journal of Inorganic Chemistry
FULL PAPER
100
Therefore, this seems to imply that the enrichment of La on the
catalyst surface is related to the generation of oxygen vacancies.
Interestingly, while the Ca/La molar ratios of Ca0.5La2Ce2O7
and Ca0.5La1.5Ce2O7 is higher than the original ratio in the bulk,
that of La2Ce1.5Ca0.5O7 is less than its original bulk ratio. This
confirms the Raman results that in the former two catalysts, the
Ca additive is present as dispersed CaCO3 on the surface. In
contrast, Ca2+ cations have been doped into the crystalline lattice
of La2Ce1.5Ca0.5O7 instead of staying on the surface, as have
been testified by the STEM Mapping results.
(b)
La2Ce2O7
Ca0.5La2Ce2O7
La1.5Ce2O7
80
Ca0.5La1.5Ce2O7
La2Ce1.5O7
La2Ce1.5Ca0.5O7
60
40
20
0
550
600
650
700
750
Table 5. Quantified surface element compositions for the catalysts.
Temperature (oC)
Catalysts
La/Ce Molar
ratio
Ca/La Molar
ratio
Ca/Ce Molar
ratio
30
20
10
La2Ce2O7
(c)
Ca0.5La2Ce2O7
La1.5Ce2O7
La2Ce2O7
3.04/2.00
3.08/2.00
2.64/2.00
2.74/2.00
3.41/1.50
3.72/1.50
-
-
Ca0.5La1.5Ce2O7
La2Ce1.5O7
Ca0.5La2Ce2O7
La1.5Ce2O7
0.72/2.00
0.55/2.00
La2Ce1.5Ca0.5O7
-
-
Ca0.5La1.5Ce2O7
La2Ce1.5O7
0.67/1.50
-
0.61/2.00
-
La2Ce1.5Ca0.5O7
0.32/2.00
0.60/1.50
0
550
600
650
700
750
Temperature (oC)
100
The Ce 3d spectra of the catalysts are specially depicted and
Methane conversion
C2selectivity
(d)
analyzed in Figure 5. By fixing the FWHM of a single Ce peak in
36]
the range of 2.9-3.3 eV,[30,
each spectroscopy can be
C yield
80
60
40
20
0
2
deconvoluted into 10 groups related to various surface Ce
species. Typically, five 3d5/2 peaks attributed to v0 (881.7eV), v
(882.9eV), v′ (885.7eV), v″ (889.0eV) and v‴ (897.6eV), and five
3d3/2 peaks assigned to u0 (899.1eV), u (901.2eV), u′ (903.0eV),
u″ (907.7eV), u‴ (916.8eV) are clearly observed for the samples.
[30]
According to former reports, the formation of oxygen
vacancies is closely related to the oxidation state change of Ce
cations in Ce-related oxide catalysts, which will influence the
quantities of vacancy defects or partially reduced oxygen
species.[30, 36] In other words, the quantity of oxygen vacancies
on CeO2 surface is intimately related to the surface Ce3+
concentration.
0
20
40
60
80
100
Time on stream(h)
Figure 4. OCM reaction performance over the catalysts. (a) CH4 conversion,
(b) C2 selectivity, (c) C2 yield, (d) 100 h durability test of La2Ce1.5Ca0.5O7 at 750
oC.
Therefore, the surface concentration of Ce3+ on the catalysts
has been quantified by using the following equations:
Ce3+
Ce3+ + Ce4+
Ce3+ % =
× 100%
XPS and EPR analysis of the catalysts
(
)
The surface characteristics of the samples have been studied by
XPS. Table 5 lists the quantified surface element compositions.
The surface La/Ce ratios of all the catalysts are more than their
stoichiometric ratio in the bulk, which suggests the migration of
La species onto the surface. As shown in Figure S3, the spectra
of all the catalysts contain two C1s peaks, that is, a 284.8 eV
peak for the carbon standard and a 289.2 eV peak for carbon
from carbonate,[19] testifying that La could be present on the
surface as La2(CO3)3. For the three Ca doped catalysts, the
surface carbonate is also contributed partly by CaCO3. The
La/Ce ratio of all the catalysts follows the sequence of
La2Ce1.5Ca0.5O7 > La2Ce1.5O7 > Ca0.5La0.5Ce0.5O7 ≈ La2Ce2O7 >
Ca0.5La1.5Ce2O7 ≈ La1.5Ce2O7, which is well consistent with the
concentration of oxygen vacancies characterized by Raman.
Where Ce3+= v0 + v′ + u0 + u′ and Ce4+= v + v″ + v‴ + u + u″ + u‴,
which represent the corresponding sums of the integrated peak
areas of Ce3+ and Ce4+ XPS peaks.[30] As displayed in Table 6,
the surface Ce3+ concentrations of all the catalysts follow the
order of La2Ce1.5Ca0.5O7
> La2Ce1.5O7 > Ca0.5La2Ce2O7 ≈
La2Ce2O7 > La1.5Ce2O7 ≈ Ca0.5La1.5Ce2O7, in good accordance to
the concentration of surface oxygen vacancies measured by
Raman. For clarification, the A575/A445 Raman peak ratios used
to characterize the surface oxygen vacancies on the catalysts is
plotted versus the Ce3+ concentration in Figure 6(a). It is
apparent that the A575/A445 ratio increases with the improvement
of the surface Ce3+concentration. Furthermore, the C2 yield
obtained over the catalysts at 750 °C is plotted against the
6
This article is protected by copyright. All rights reserved.

10.1002/ejic.201801250
European Journal of Inorganic Chemistry
FULL PAPER
surface Ce3+ concentration in Figure 6(b), which also increases
with the increasing of the surface Ce3+ concentration. This
strongly testifies that the formation of surface oxygen vacancies
and active electrophilic oxygen sites on the catalyst surfaces are
intimately related to the surface Ce3+concentration, which can
enhance the OCM reaction performance of the catalysts
evidently.
64000
62000
60000
58000
56000
54000
52000
50000
48000
46000
44000
(e) La2Ce1.5O7
u'''
u
0
u
v'''
u''
u'
v
v'
v''
v
0
190000
(a) La2Ce2O7
180000
u'''
u
170000
160000
150000
140000
130000
120000
v'''
u'
920
910
900
890
880
u''
Binding Energy (eV)
v'
v
0
160000
155000
150000
145000
140000
135000
130000
125000
120000
115000
110000
105000
u
v''
0
(f) La2Ce1.5Ca0.5O7
v
u'''
u
v'''
u''
v
0
u'
920
910
900
890
880
v''
v'
u
Binding Energy (eV)
0
180000
170000
160000
150000
140000
130000
120000
110000
v
(b) Ca0.5 La2Ce2O7
u'''
920
910
900
890
880
v'''
Binding Energy (eV)
u'
u''
u
v'
v
v
u
v''
0
0
Figure 5. Analysis of Ce3d spectra for the sampless. (a) La2Ce2O7, (b)
Ca0.5La2Ce2O7, (c) La1.5Ce2O7, (d) Ca0.5La1.5Ce2O7, (e) La1.5Ce2O7, (f)
La2Ce1.5Ca0.5O7.
3
920
910
900
890
880
(a)
La Ce O
7
2
2
Ca La Ce O
7
2.50
0.5
2
2
Binding Energy (eV)
La Ce O
7
1.5
2
56000
54000
52000
50000
48000
46000
44000
42000
40000
38000
36000
34000
32000
Ca La Ce O
0.5
1.5
2
7
2.09
La Ce
O
7
2
1.5
2
1
(c) La1.5Ce2O7
La Ce Ca
O
2
1.5
0.5
7
u'''
u
v'''
u''
u'
0.74
u
0
0.71
0.64
0.61
v''
v
v'
v
0
0
24.6 25.8
28.7 30.0
32.6
38.8
[Ce(III)%]
30
920
910
900
890
880
La Ce O
7
(b)
2
2
Binding Energy (eV)
Ca La Ce O
7
0.5
2
2
La Ce O
7
1.5
2
Ca La Ce O
0.5
1.5
2
7
160000
150000
140000
130000
120000
110000
100000
90000
La Ce
O
7
(d) Ca0.5La1.5Ce2O7
2
1.5
20
10
0
La Ce Ca
O
7
u'''
2
1.5
0.5
u
v'''
u''
u'
v
v''
0
v'
u
0
v
38.8
24.6 25.8
28.7
30.0
32.6
[ Ce(III)% ]
920
910
900
890
880
Figure 6. (a) A575/A445 versus the concentration of Ce3+, (b) C2 yield at 750 °C
versus the concentration of Ce3+
Binding Energy (eV)
.
7
This article is protected by copyright. All rights reserved.

10.1002/ejic.201801250
European Journal of Inorganic Chemistry
FULL PAPER
90000
80000
70000
60000
50000
40000
30000
20000
It has been commonly accepted that surface electrophilic
(a)
La2Ce2O7
-
oxygen species such as O2 , O22-, O- generated by adsorbing
O2-
gas phase O2 molecules at the oxygen vacancies on a catalyst
surface is beneficial to methane activation and C2 product
selectivity during OCM reaction.[17-19] On the contrary, surface
lattice oxygen (O2-) usually contributes to hydrocarbon deep
oxidation into COx.[17-19]
2-
O2
As described above, Raman results have testified the
2-
CO3
-
presence of electrophilic O2 in all the samples but with varied
-
O2
quantities. To confirm this, EPR technique has been used to
measure the un-doped La2Ce2O7 and the doped La2Ce1.5Ca0.5O7
possessing the best catalytic performance, which are two typical
catalysts in this study. It is noted that EPR is an amenable
536
534
532
530
528
O2-
526
Binding Energy (eV)
-
method to detect species having unpaired electrons, such as O2 ,
80000
70000
60000
50000
40000
30000
20000
O- species and electrons trapped by oxygen vacancies, hence
having been extensively used.[37-40] Figure 7 shows the EPR
spectra of both catalysts, on which signals at g=1.995, 2.002,
2.046 can be evidently observed. The g values can be
2-
Ca0.5La2Ce2O7
(b)
O2
-
compared favorably with O2 signals reported by Lunsford et al at
gxx=1.994, gyy=2.006, gzz=2.040 on La2O3[38]; and Wang et al at
gxx=2.0001, gyy=2.0045, gzz=2.0685 on SrF2/La2O3.[37] Therefore,
it is concluded that the signals observed on both La2Ce2O7 and
La2Ce1.5Ca0.5O7 in this study can be ascribed to surface
2-
CO3
-
O2
-
electrophilic O2 species.[37] In addition, all the signals of
La2Ce1.5Ca0.5O7 are slightly stronger than those of La2Ce2O7.
536
534
532
530
528
526
Binding Energy (eV)
1000
La2Ce2O7
24000
22000
20000
18000
16000
14000
12000
10000
8000
gyy=2.002
800
600
La2Ce1.5Ca0.5O7
La1.5Ce2O7
(c)
O2-
400
2-
200
O2
gzz=2.046
0
-200
-400
-600
-800
-1000
2-
CO3
-
O2
gxx=1.995
6000
310
315
320
325
330
335
340
536
534
532
530
528
526
Magnetic field (mT)
Binding Energy (eV)
Figure 7. EPR spectra of La2Ce2O7 and La2Ce1.5Ca0.5O7 catalysts.
80000
70000
60000
50000
40000
30000
20000
(d)
Ca0.5La1.5Ce2O7
O2-
The O1s spectra of the samples are deconvoluted and
analyzed in detail by assuming that a single oxygen peak has a
FWHM of 1.8-2.0 eV and with 80% Gaussian plus 20%
Lorentzian peak shape.[17-19,41] Considering the results in Figure
S3, the contribution from carbonate has been excluded in the
deconvolution and quantification. Four peaks corresponding to
2-
O2
2-
CO3
-
O2 (533.0-533.1eV), CO32- (531.9-532.2eV), O22-(530.9-531.2eV),
-
O2
O2-(528.6-529.2eV) can be fitted and obtained, as shown in
-
Figure 8. (O2 +O22-)/O2-, the ratio between the surface
536
534
532
530
528
526
electrophilic oxygen and lattice oxygen amount are compared in
Binding Energy (eV)
-
Table 6 for all the samples. Interestingly, the (O2 +O22-)/O2- ratios
follow the sequence of La2Ce1.5Ca0.5O7
> La2Ce1.5O7 >
Ca0.5La2Ce2O7 ≈ La2Ce2O7 > Ca0.5La1.5Ce2O7 ≈ La1.5Ce2O7. On
the base of this, it is concluded that catalysts with different
concentration of oxygen vacancies are generated by varying the
La/Ce ratio and the addition of Ca, which can subsequently
impactthe OCM performance of the catalysts.
8
This article is protected by copyright. All rights reserved.

10.1002/ejic.201801250
European Journal of Inorganic Chemistry
FULL PAPER
30000
that an improved catalyst can be achieved by increasing the
La/Ce molar ratio and doping Ca2+ cations into La2Ce2O7 lattice
matrix to create more abundant surface Ce3+ cations and
surface oxygen vacancies.
(e)
La2Ce1.5O7
25000
2-
O2
O2-
20000
15000
10000
5000
30
2-
La Ce O
7
CO3
2
2
Ca La Ce O
7
0.5
2
2
La Ce O
7
1.5
2
-
O2
Ca La Ce O
0.5
1.5
2
7
7
20
10
0
La Ce
O
7
2
1.5
La Ce Ca
O
2
1.5
0.5
536
534
532
530
528
526
Binding Energy (eV)
100000
90000
80000
70000
60000
50000
40000
30000
20000
La2Ce1.5Ca0.5O7
(f)
2-
O2
O2-
0.77
0.81
1.56
1.95
1.16 1.18
(O22-+O2 )/O2-
2-
-
CO3
2
2
Figure 9. C2 Yield at 750 °C against (O2 −+O2 )/O − molar ratio.
−
-
O2
536
534
532
530
528
526
Binding Energy (eV)
Figure 8. Analysis of
O
1s peaks for the catalysts. (a) La2Ce2O7, (b)
Ca0.5La2Ce2O7, (c) La1.5Ce2O7, (d) Ca0.5La1.5Ce2O7, (e) La1.5Ce2O7, (f)
La2Ce1.5Ca0.5O7.
To gain deeper understanding on the relationship between the
reaction performance and surface active oxygen abundance, the
−
C2 yield at 750 °C against surface (O22−+O2 )/O2− ratio is plotted
in Figure 9 for the catalysts. It is evident that a catalyst owning a
larger quantity of active electrophilic oxygen sites displays better
OCM performance, and vice versa. Therefore, it is concluded
Table 6. Curve-fitting and quantification of O1s spectra for the catalysts.
-
Catalysts
O1s B.E (eV)/ FWHM (eV) / relative amount (%)
(O2 +O22-)/O2-
[Ce(III)%]
-
2-
2-
O2
CO3
O2
O2-
La2Ce2O7
533.0/2.0/6.2
533.0/1.8/4.9
533.0/2.0/7.8
533.0/2.0/5.9
532.9/2.0/4.7
532.9/2.0/4.7
532.0/1.8/11.2
532.1/2.1/20.2
531.9/1.7/10.3
532.3/1.9/15.3
532.0/2.0/25.9
532.0/2.0/25.9
531.0/1.7/42.0
531.1/2.0/38.0
531.2/1.7/32.3
531.2/1.6/45.7
531.2/1.6/44.3
531.2/1.6/44.3
528.7/2.0/40.6
528.7/1.8/36.9
528.6/2.0/49.6
529.0/1.7/33.1
529.2/1.9/25.1
529.2/1.9/25.1
1.18
1.16
0.81
0.77
1.56
1.95
30.0
28.7
25.8
24.6
32.6
38.8
Ca0.5La2Ce2O7
La1.5Ce2O7
Ca0.5La1.5Ce2O7
La2Ce1.5O7
La2Ce1.5Ca0.5O7
9
This article is protected by copyright. All rights reserved.

10.1002/ejic.201801250
European Journal of Inorganic Chemistry
FULL PAPER
Decomposition of the carbonates analyzed by TGA
sites, respectively. According to the TGA results, the peaks
above 500 oC for Ca0.5La2Ce2O7, Ca0.5La1.5Ce2O7 and
La2Ce1.5Ca0.5O7 can be ascribed to the decomposition of CaCO3
on the catalyst surface. By excluding the contribution from
CaCO3, the alkaline site amount is compared in Table 7. The
total quantity of the alkaline sites obeys the sequence of
XPS results indicate the presence of La2(CO3)3 and/or CaCO3 on
the surface of the catalysts, which is un-detectable by XRD due
to their high dispersion. To further clarify this issue, the catalysts
have been analyzed by TGA (Figure 10). Thermal
decomposition process of La2(CO3)3 can be divided into two
stages. The first stage can be assigned to the decomposition of
La2(CO3)3 into La2O2CO3, which occurs in the range of 366 -
566 °C . The second stage can be assigned to the decomposition
of La2O2CO3 into La2O3, which occurs in the range of 566 -
1000 °C .[42] For all the catalysts without Ca additive, no any
weight loss stage can be observed in the whole tested
temperature region, indicating that the amount of La2(CO3)3 is
too small to give any detectable signal. For all the catalysts
modified by Ca, a small weight loss stage at 550 - 650 °C is
observed, which can thus be assigned to the decomposition of
CaCO3 in the catalysts. As formerly reported, the decomposition
temperature of CaCO3 is above 750 oC.[43] Due to the highly
dispersion of CaCO3 as amorphous phase on the surface of the
catalysts, its decomposition temperature is reduced because of
lowered thermal stability. In detail, a 3.1%, 2.3% or 1.7% weight
loss is detected in the TGA curves of Ca0.5La2Ce2O7,
Ca0.5La1.5Ce2O7 and La2Ce1.5Ca0.5O7, respectively. The much
smaller weight loss of La2Ce1.5Ca0.5O7 has substantiated again
the incorporation of a major portion of Ca2+ cations into its lattice
matrix, in good accordance to the XRD and STEM-Mapping
analysis.
La2Ce1.5Ca0.5O7 > La2Ce1.5O7 > La2Ce2O7 > Ca0.5La2Ce2O7
>
La1.5Ce2O7> Ca0.5La1.5Ce2O7. In contrast, the quantity of the
intermediate alkaline sites, which are crucial for the generation
of C2 products, follows the similar order of La2Ce1.5Ca0.5O7
>
La2Ce1.5O7 La2Ce2O7 Ca0.5La2Ce2O7 La1.5Ce2O7>
>
>
>
Ca0.5La1.5Ce2O7. For comparison, the C2 product yield achieved
at 750 °C is plotted against the quantity of intermediate surface
alkaline sites in Figure 11(b) for the catalysts. It is disclosed that
the C2 yield rises apparently with the quantity of the intermediate
basic sites. Therefore, it is rational to propose that the
intermediate alkalinity of a catalyst is another crucial element to
control its OCM performance.
3.51
(a)
3.50
3.49
3.48
3.47
3.46
3.45
552
729
198
277
85
641
170
La2Ce1.5Ca0.5O7
La2Ce1.5O7
289
450
420
84
760
169
136
85
85
622
Ca0.5La1.5Ce2O7
La1.5Ce2O7
100
173
172
83
82
431
303
Ca0.5La2Ce2O7
La2Ce2O7
440
2.3%
100
200
300
400
500
600
700
800
900
3.1%
95
90
85
1.7%
Temperature (oC)
30
20
10
0
La2Ce2O7
(b)
La2Ce2O7
Ca0.5La2Ce2O7
La1.5Ce2O7
Ca0.5La2Ce2O7
La1.5Ce2O7
Ca0.5La1.5Ce2O7
La2Ce1.5O7
Ca0.5La1.5Ce2O7
La2Ce1.5O7
La2Ce1.5Ca0.5O7
La2Ce1.5Ca0.5O7
0
100
200
300
400
500
600
700
800
Temperature (oC)
Figure 10. TGA profiles of the samples.
Probing the surface alkaline sites with CO2-TPD
36
46
14
18 20 22
Intermediate basic sites amount (a.u.)
Quite some previous studies have proved the close relations
between the surface alkaline sites and C2 yield for OCM.
Therefore, CO2-TPD experiments have been carried out to
characterize the surface alkalinity of the catalysts (Figure 11a). It
is reported that not only the overall alkalinity, but also the
number of surface alkaline sites can affect significantly the C2
product selectivity and yield.[19, 41, 44, 45] While all the alkaline sites
can play the role to activate CH4 molecules, the intermediate
strength alkaline sites are especially important to improve the
coupling product selectivity.[19, 41, 44, 45]
Figure 11. (a) CO2-TPD curves of the catalysts, (b) C2 yield at 750 °C against
the amount of intermediate alkaline sites.
Based on the CO2 desorption temperatures, the CO2
desorption peaks are divided into three groups. In detail, the
o
o
peaks below 250 C, in the region of 250 ~ 500 C and above
o
500 C are assigned to weak, intermediate and strong alkaline
10
This article is protected by copyright. All rights reserved.

10.1002/ejic.201801250
European Journal of Inorganic Chemistry
FULL PAPER
(99.8%) and citric acid (99.8%) as the precursors. La(NO3)3.6H2O,
Table 7. The quantification of the CO2-TPD results.
Ce(NO3)3.6H2O and Ca(NO3)2.4H2O were dissolved into
a desired
amount of deionized water according to the calculated ratio for the
preparation of a catalyst. The molar ratio of metal nitrates/citric acid =
1/1.2 was mingled into homogeneous aqueous solution and stirred
completely until clear solution was finally formed. The excess water was
volatilized in a 80 oC water bath by continuous stirring until viscous gel
was formed, which was kept at 130 oC for 12 h, and then calcined at 800
oC for 4 h in air atmosphere to get rid of the citric acid and obtained the
final catalysts. The catalysts are named according to the La, Ce and Ca
molar ratios.
Catalysts
CO2-TPD results
Weak
alkaline
site
amount
(a.u.)
Intermediate
alkaline
site
Strong
alkaline
site
Total
alkaline
site
amount
(a.u.)
amount
(a.u.)
amount
(a.u.)
La2Ce2O7
27
26
25
23
30
24
22
20
18
14
36
46
4
-
53
46
Catalyst characterization and reaction performance evaluation
Ca0.5La2Ce2O7
La1.5Ce2O7
4
47
OCM reaction has been used to evaluate the performance of the
catalysts. The bulk and surface properties of the catalysts has also been
probed by different means. The equipment models, operation conditions,
experimental procedures and parameters are listed in detail in the
supporting information.
Ca0.5La1.5Ce2O7
La2Ce1.5O7
-
37
12
30
78
La2Ce1.5Ca0.5O7
100
Acknowledgements
The authors deeply acknowledge the financial support from the
National Natural Science Foundation of China (21567016,
21566022, 21666020), the Natural Science Foundation of
Conclusions
Jiangxi
Province
(20181ACB20005,
20181BCD40004,
In this study, with the target to optimize the catalytic
performance of La2Ce2O7 for OCM, a series of catalysts with
varied La/Ce molar ratios and doped by calcium additive have
been designed and fabricated with a sol-gel method and
characterized by different means:
20171BAB213013) and the Education Department of Jiangxi
Province (GJJ150016, GJJ150085, KJLD14005).
Keywords: Ca2+ doped La2Ce2O7 catalysts • electron-deficient
compounds • nonstoichiometric compounds • OCM at low
temperature • solid-state structures
(1) XRD has demonstrated that
a La2Ce2O7 compound
possessing a disordered cubic defective fluorite structure
has been formed as the predominant phase in the bulk of
all the catalysts. The surface oxygen vacancy concentration,
facile oxygen abundance and alkalinity increase with the
increasing of La/Ce molar ratio, as testified by Raman, EPR,
XPS and CO2-TPD results.
[1]
[2]
G.E. Keller, M.M. Bhasin, J. Catal. 1982, 73, 9-19.
S. Arndt, U. Simon, K. Kiefer, T. Otremba, K. Siemensmeyer, M.
Wollgarten, A. Berthold, F. Schmidt, O. Gorke, R. Schomacker, K.P.
Dinse, Chemcatchem 2017, 9, 3583-3596.
[3]
U. Simon, S.A. Villaseca, H.H. Shang, S.V. Levchenko, S. Arndt, J.D.
Epping, O. Gorke, M. Scheffler, R. Schomacker, J. van Tol, A.
Ozarowski, K.P. Dinse, Chemcatchem 2017, 9, 3597-3610.
L. Luo, Y. Jin, H. Pan, X. Zheng, L. Wu, R. You, W. Huang, J. Catal.
2017, 346, 57-61.
(2) For La2Ce2O7 and La1.5Ce2O7 catalysts, the calcium additive
is present on the surface as finely dispersed CaCO3, which
covers the surface active sites and is harmful to the OCM
reaction performance. However, for La2Ce1.5O7, the major
portion of the Ca additive has entered into the lattice matrix
of its La2Ce2O7 phase as Ca2+ cations, thus creating more
bulk and surface defects and improving both the quantities
of surface active oxygen and alkaline sites of the modified
catalyst, which is favorable to the reaction.
(3) It is concluded that both the active surface electrophilic
oxygen and intermediate alkaline sites are vital for OCM
reaction, and their synergistic interaction determines the
reactivity. La2Ce1.5Ca0.5O7 possesses the largest amount of
both types of active sites, hence showing the best
performance for OCM reaction, over which the highest C2
yield of 22.5% can be achieved even at 750 oC.
[4]
[5]
[6]
[7]
W. Jeon, J.Y. Lee, M. Lee, J.-W. Choi, J.-M. Ha, D.J. Suh, I.W. Kim,
Appl. Catal., A 2013, 464, 68-77.
H. Wang, R. Schmack, B. Paul, M. Albrecht, S. Sokolov, S. Rümmler,
E.V. Kondratenko, R. Kraehnert, Appl. Catal., A 2017, 537, 33-39.
M.G. Colmenares, U. Simon, M. Yildiz, S. Arndt, R. Schomaecker, A.
Thomas, F. Rosowski, A. Gurlo, O. Goerke, Catal. Commun. 2016, 85,
75-78.
[8]
[9]
G.D. Nipan, Inorg. Mater. 2015, 51, 389-395.
N. Hiyoshi, K. Sato, Fuel Process. Technol. 2016, 151, 148-154.
[10] S. Ji, J. Catal. 2003, 220, 47-56.
[11] L. Chou, Y. Cai, B. Zhang, J. Niu, S. Ji, S. Li, Appl. Catal., A 2003, 238,
185-191.
[12] J. Wu, H. Zhang, S. Qin, C. Hu, Appl. Catal., A 2007, 323, 126-134.
[13] S. Gu, H.-S. Oh, J.-W. Choi, D.J. Suh, J. Jae, J. Choi, J.-M. Ha, Appl.
Catal., A 2018, 562, 114-119.
[14] P. Wang, G. Zhao, Y. Wang, Y. Lu, Sci Adv. 2017, 3, e1603180.
[15] C. Karakaya, H.Y. Zhu, B. Zohour, S. Senkan, R.J. Kee, Chemcatchem
2017, 9, 4538-4551.
Experimental Section
[16] D. Noon, B. Zohour, S. Senkan, J. Nat. Gas Sci. Eng. 2014, 18, 406-
411.
Catalyst synthesis
[17] V.J. Ferreira, P. Tavares, J.L. Figueiredo, J.L. Faria, Ind. Eng. Chem.
Res. 2012, 51, 10535-10541.
La2Ce2O7-based catalysts with varied La/Ce molar ratios and modified by
calcium additive were synthesized with
a sol-gel method, using
La(NO3)3.6H2O (99.99%), Ce(NO3)3.6H2O (99.99%), Ca(NO3)2.4H2O
11
This article is protected by copyright. All rights reserved.

10.1002/ejic.201801250
European Journal of Inorganic Chemistry
FULL PAPER
[18] P. Huang, Y. Zhao, J. Zhang, Y. Zhu, Y. Sun, Nanoscale 2013, 5,
10844-10848.
[19] J. Xu, L. Peng, X. Fang, Z. Fu, W. Liu, X. Xu, H. Peng, R. Zheng, X.
Wang, Appl. Catal., A 2018, 552, 117-128.
[20] A.T. Ashcroft, A.K. Cheetham, M.L.H. Green, C.P. Grey, P.D.F. Vernon,
J. Chem. Soc., Chem. Commun. 1989, 1667-1669.
[21] A.C. Roger, C. Petit, L. Hilaire, J.L. Rehspringer, A. Kiennemann, Catal.
Today 1994, 21, 341-347.
[22] C. Petit, J.L. Rehspringer, A. Kaddouri, S. Libs, P. Poix, A. Kiennemann,
Catal. Today 1992, 13, 409-416.
[23] C.A. Mims, A.J. Jacobson, R.B. Hall, J.T. Lewandowski, J. Catal. 1995,
153, 197-207.
[24] A.C. Roger, C. Petit, A. Kiennemann, J. Catal. 1997, 167, 447-459.
[25] X.H. Zhang, X.Z. Fang, X.H. Feng, X. Li, W.M. Liu, X.L. Xu, N. Zhang,
Z.X. Gao, X. Wang, W.F. Zhou, Catal. Sci. Technol. 2017, 7, 2729-
2743.
[26] M. Lang, F. Zhang, J. Zhang, J. Wang, J. Lian, W.J. Weber, B.
Schuster, C. Trautmann, R. Neumann, R.C. Ewing, Nucl. Instrum.
Methods Phys. Res., Sect. B 2010, 268, 2951-2959.
[27] J. Shamblin, C.L. Tracy, R.I. Palomares, E.C. O'Quinn, R.C. Ewing, J.
Neuefeind, M. Feygenson, J. Behrens, C. Trautmann, M. Lang, Acta
Mater. 2018, 144, 60-67.
[28] D. Filkova, D. Wolf, G. Gayko, M. Baerns, L. Petrov, Appl. Catal., A
1997, 159, 33-44.
[29] W. Liu, W.Z. Wang, K. Tang, J.X. Guo, Y.Q. Ren, S.P. Wang, L.J. Feng,
Y.Z. Yang, Catal. Sci. Technol. 2016, 6, 2427-2434.
[30] Z. Hu, X. Liu, D. Meng, Y. Guo, Y. Guo, G. Lu, ACS Catalysis 2016, 6,
2265-2279.
[31] F.X. Zhang, C.L. Tracy, M. Lang, R.C. Ewing, J. Alloys Compd. 2016,
674, 168-173.
[32] K. Singh, R. Kumar, A. Chowdhury, Ultrason. Sonochem. 2017, 36,
182-190.
[33] H. Wan, Z. Chao, W. Weng, X. Zhou, J. Cai, K. Tsai, Catal. Today 1996,
30, 67-76.
[34] S. Martinez-Ramirez, S. Sanchez-Cortes, J.V. Garcia-Ramos, C.
Domingo, C. Fortes, M.T. Blanco-Varela, Cem. Concr. Res. 2003, 33,
2063-2068.
[35] L.J. Simpson, Electrochim. Acta 1998, 43, 2543-2547.
[36] V.J. Ferreira, P. Tavares, J.L. Figueiredo, J.L. Faria, Catal. Commun.
2013, 42, 50-53.
[37] J.X. Wang, J.H. Lunsford, J. Phys. Chem. 1986, 90, 3890-3891.
[38] L. Wang, X. Yi, W. Weng, C. Zhang, X. Xu, H. Wan, Catal. Lett. 2007,
118, 238-243.
[39] M. Anpo, M. Che, B. Fubini, E. Garrone, E. Giamello, M.C. Paganini, Top.
Catal. 1999, 8, 189-198.
[40] P. Schwach, M. Eichelbaum, R. Schlögl, T. Risse, K.P. Dinse, J. Phys.
Chem. C 2016, 120, 3781-3790.
[41] L. Peng, J. Xu, X. Fang, W. Liu, X. Xu, L. Liu, Z. Li, H. Peng, R. Zheng, X.
Wang, Eur. J. Inorg. Chem. 2018, 17, 1787-1799.
[42] X.-h. Zhang, C. He, L. Wang, J. Liu, M. Deng, Q. Feng, T Nonferr Metal
Soc 2014, 24, 3378-3385.
[43] S.-W. Lee, Y.-J. Kim, Y.-H. Lee, H. Guim, S.M. Han, Mater Design
2016, 112, 367-373.
[44] X. Yide, Y. Lin, G. Xiexian, Appl. Catal., A 1997, 164, 47-57.
[45] Z. Wang, G. Zou, X. Luo, H. Liu, R. Gao, L. Chou, X. Wang, J. Nat. Gas
Chem. 2012, 21, 49-55
12
This article is protected by copyright. All rights reserved.

10.1002/ejic.201801250
European Journal of Inorganic Chemistry
FULL PAPER
Table of Contents
FULL PAPER
Lattice Ca2+ Boosts: Ca2+ cations
doped into the lattice matrix of
La2Ce2O7 phase improve both the
quantities of surface active oxygen
and alkaline sites of the promoted
catalyst, hence boosting its OCM
reaction performance remarkably.
Junwei Xu,[a] Yan Zhang,[a] Yameng
Liu,[a] Xiuzhong Fang,[a] Xianglan
Xu,[a] Wenming Liu,[a] Renyang
Zheng,[b] Xiang Wang*[a]
Page No. – Page No.
Optimizing the Reaction
Performance of La2Ce2O7-based
Catalysts for Oxidative Coupling
of Methane (OCM) at Lower
Temperature by Lattice Doping
with Ca Cations
Keywords: Ca2+ doped La2Ce2O7 catalysts • electron-deficient compounds •
nonstoichiometric compounds• OCM at low temperature • solid-state structures
13
This article is protected by copyright. All rights reserved.