SnO2 Based Catalysts with Low-Temperature Performance for Oxidative Coupling of Methane: Insight into the Promotional Effects of Alkali-Metal Oxides

Article abstract of DOI:10.1002/ejic.201701440

To develop reactive catalysts for oxidative coupling of methane at low temperatures, different alkali-metal oxides are adopted in this study to modify the surface of SnO₂. In comparison with the unmodified SnO₂, the reaction performance of all of the modified catalysts can be significantly improved, among which lithium oxide shows the best promotional effects, and the optimal catalyst is achieved with a Sn/Li molar ratio of 5:5. With this catalyst, the highest C₂ product yield of 16 % is achieved at 750 °C. The XPS and CO₂-TPD results reveal that for those catalysts with evident enhanced OCM reaction performance, the coexistence of a suitable amount of surface alkaline and electrophilic oxygen sites is indispensable. Furthermore, with the catalysts modified by different amounts of lithium oxide, the C₂ yield at different temperatures is nearly proportional to the amounts of both surface intermediate alkaline sites and electrophilic oxygen species. Therefore, it is concluded that the abundance and the concerted interaction of these two types of surface active sites are the major factors determining the reaction performance of the SnO₂ based catalysts. Last, but not least, the optimized catalyst, which has a Sn/Li molar ratio of 5:5 and also contains equal amounts of SnO₂ and Li₂SnO₃ crystalline phases, exhibits much better reaction performance than Mn/Na₂WO₄/SiO₂, the most promising catalyst at present, in the low-temperature region (below 750 °C). After all, this may give people some new insight into developing new types of OCM catalysts that can operate at low temperatures and thereby facilitate the industrialization process of this important chemical engineering reaction.

Full text of DOI:10.1002/ejic.201701440

A Journal of
Accepted Article
Title: SnO2-based catalysts with low temperature performance for
oxidative coupling of methane: insight into the promotional effects
of alkali metal oxides
Authors: Liang Peng, Junwei Xu, Xiuzhong Fang, Wenming Liu,
Xianglan Xu, Liang Liu, Zhongchen Li, 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.201701440
Link to VoR: http://dx.doi.org/10.1002/ejic.201701440

European Journal of Inorganic Chemistry
10.1002/ejic.201701440
FULL PAPER
SnO -based catalysts with superior low temperature performance for
2
oxidative coupling of methane: insight into the promotional effects of
alkali metal oxides
[a]
[a ]
[a]
Junwei Xu,
[a]
[a]
[a]
[a]
Liang Liu,
[a]
[b]
Liang Peng,
Xiuzhong Fang,
Wenming Liu,
Xianglan Xu,
Zhongchen Li, Renyang Zheng,
Xiang Wang*
Abstract: To develop reactive catalysts for oxidative coupling of
developing catalysts to directly convert methane into
hydrocarbons with higher value is desirable and have been paid
much attention. Catalytic oxidative coupling of methane to higher
hydrocarbons is regarded as an effective direct way to utilize
methane, which has aroused great interest around the world
over recent ten years.
methane at low temperature, different alkali metal oxides have been
adopted to modify the surface of SnO
2
in this study. In comparison
with the un-modified SnO , the reaction performance of all the
2
modified catalyst can be significantly improved, among which lithium
oxide shows the best promotional effects, and the optimal catalyst
was achieved with a Sn/Li molar ratio of 5/5. Over this catalyst, the
Since Keller and Bhasin first reported their work on OCM
[
6-8, 10-15]
highest C
2
product yield of 16 % has been achieved at 750 ℃. XPS
in early 1980s,
a lot of work on screening active and
and CO -TPD results have revealed that for those catalysts with
2
selective catalysts have been performed. The investigated
catalysts can be divided roughly into two groups, the non-redox
and the redox type. As a typical example for the non-redox type
catalysts, Li/MgO catalysts have been reported to be effective
evident enhanced OCM reaction performance, the co-existence of
suitable amount of surface alkaline and electrophilic oxygen sites is
indispensable. Furthermore, over the catalysts modified by different
[
16-18]
amount of lithium oxide, the C
2
yield at different temperatures is
for the reaction,
oxygen species are regarded to be the active sites.
However, Li/MgO catalysts still suffer from deactivation due to
on which the surface alkaline sites and
[3, 19-22]
nearly proportional to the amounts of both surface intermediate
alkaline sites and electrophilic oxygen species. Therefore, it is
concluded that the abundance and the concerted interaction of these
two types of surface active sites are the major factors determining
+
[23]
the vaporization of Li cations.
redox type catalysts, Mn-Na WO
tremendously studied over past 30 years.
oxides can store and release oxygen cyclically between multiple
As a typical example for the
2
4
/SiO catalysts have been
2
[
24-30]
the reaction performance of the SnO
the least, the optimized catalyst, which has a Sn/Li molar ratio of 5/5
and also contains equal amounts of SnO and Li SnO crystalline
phases, exhibits much better reaction performance than
Mn/Na WO /SiO , the most promising catalyst at present, at low
2
-based catalysts. Last but not
Reducible metal
[
19]
2−
2
2
3
valence states.
proposed to be the active sites for the OCM .
both groups of catalysts, surface electrophilic oxygen species
The surface lattice oxygen (O ) has been
[
20]
Whereas，for
2
4
2
−
−
2−
temperature region (below 750 ℃). After all, this may give people
some new insight into developing novel type of OCM catalysts that
can be operated at low temperature and faciliate the industiliaztion
process of this important chemical enginering reaction.
(O , O
2 2
and O ) are believed to be crucial for the reaction and
could decide the selective formation of the coupling products.
Up to date, Mn-Na
reported to be the most promising catalysts for OCM reaction.
Unfortunately, over both of the two catalysts, the one way
2
product yield is still below 30%, which is regarded as the
[
3,
3
1-34]
2 4 2
WO /SiO and Li/MgO have been
[
17,
2
7, 35-36]
C
lowest yield requirement for the industrialization of this
Introduction
[20, 26]
reaction.
In addition, on both types of catalysts, it generally
requres high temperature above 750 ℃ to proceed the reaction
effectively, though some interesting work has been reported
Due to the fast consumption of crude oil, much attention has
been transferred to the utilization of abundant natural gas, which
recently to decrease the reaction temperature over Mn-
[
1-2]
usually consists of 83 % ~ 99 % methane.
Over the past
[28, 36]
Na
2
WO
4
/SiO
2
catalyst evidently.
Therefore, there is still
decades, both the direct and indirect ways to convert methane
into value-added chemical products have been intensively
strong incentive to find more reactive and selective catalysts to
overcome these disadvantages.
It is usually accepted that for a good OCM catalyst,
suitable alkaline sites, selective oxygen sites and high thermal
[
3]
investigated. The typical indirect ways include methane steam
reforming, dry reforming and partial oxidation, during which,
methane is first converted into syngas, and then synthesized
[19]
stability are required.
atom from a CH
oxygen site is considered as the first and rate-determining
The abstraction of the first hydrogen
[
3]
into higher value hydrocarbons by Fischer-Tropsch reaction.
The typical direct ways include non-oxidative methane
4
molecule on an active oxygen site or a basic
[
4-5]
conversion to aromatics,
and oxidative coupling of methane
[19, 37-38]
step.
gas phase to couple with each other to form C
surface to be oxidized further into CO and CO. As a secondary
product, C is formed by C dehydrogenation on the surface
Then the formed methyl radicals either diffuse to the
[
6-10]
(
OCM) to C hydrocarbons.
2
For indirect conversion of
2
6
H , or stay on the
methane, multiple steps are required, which obviously makes
the technological process more complicated. Therefore,
2
2
H
4
2 6
H
of the catalysts. The hydrogen abstraction process produces
hydroxyl groups, which react with each other to form water and
generate oxygen vacancies at the same time. The surface
[
[
a]
b]
*
L Peng, X Fang, J Xu, W Liu, X Xu, Z Fu, H Peng, X Wang.
Institute of Applied Chemistry, College of Chemistry,
Nanchang University, Nanchang, 330031, P. R. China
R. Zheng.
Research Institute of Processing(RIIP), SINOPEC, 18
Xueyuan Road Haidian district, Beijing 100083,China
Corresponding author, E-mail: xwang23@ncu.edu.cn(X.
Wang).
2
oxygen vacancies will then be restored by the gas phase O .
Tin dioxide is an n-type semiconductor, which contains
abundant metastable surface deficient oxygen species and its
[
39-42]
lattice oxygen is also reducible and reactive.
In addition, its
o
[43]
melting point is as high as 1630 C, testifying that it has both
good chemical and thermal stability. Tin dioxide has been
1
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European Journal of Inorganic Chemistry
10.1002/ejic.201701440
FULL PAPER
[
44-48]
20
reported previously to be very good oxidation catalysts,
(A)
SnO
2
especially after its lattice matrix is doped with secondary metal
cations to form solid solution structures, as found by this
Sn9Li1
Sn9Na1
Sn9K1
1
5
[
49-50]
Sn9Cs1
group.
Nevertheless, SnO
2
related catalysts have been
is
Based on the above
mentioned former studies, it is postulated that by using alkali
metal oxides to tune the surface of SnO , catalytic materials with
rarely used for the oxidative coupling of methane, and SnO
mainly adopted as a catalyst additive.
2
[
3, 21]
10
5
2
both abundant alkaline sites and redox sites might be achieved,
which could match well the active site requirements for an
effective OCM catalysts.
Therefore, to develop more effcient catalysts that can be
operated at lower temperature and to investigate the influence of
0
650
675
700
725
750
Temperature (℃)
alkali metal oxides on the reaction performance of SnO
work, different alkali metal oxides have been adopted to modify
SnO to prepare catalysts for OCM reaction. It has been
revealed that in comparison with the un-modified SnO , all the
modified catalysts demonstrate significantly improved
performance for the target reaction due to the concerted
interaction between the alkaline sites and the electron deficient
oxygen species. Lithium oxide displays the best promotion
effects among all the alkali metal oxides, on which the highest
2
, in this
100
80


2
2
60
40
20
0
SnO
2
Sn9Li1
Sn9Na1
Sn9K1
2
C product yield of 16 % can be obtained by adjusting an
appropriate Sn/Li molar ratio. It is particularly mentioned here
that the optimal catalysts also exhibits much better reaction
Sn9Cs1
650
675
700
725
750
Temperature (℃)
2 4 2
performance than Mn-Na WO /SiO , the most promising catalyst
at present, at low temperature region between 650-700 ℃ ,
which could give people some new insights on how to develop
catalysts that can be operated at low temperatures. With
different characterization methods, the relationship between the
reaction performance and the structure of the catalysts has been
elucidated.
20
SnO2
(C)
Sn9Li1
Sn9Na1
Sn9K1
1
5
0
5
0
Sn9Cs1
1
Results and Discussion
Reaction performance of the catalysts modified by different
alkali metal oxides
650
675
700
725
750
Temperature (℃)
The reaction performance of the catalysts modified by different
alkali metal oxides has been evaluated for oxidative coupling of
methane, with the results demonstrated in Figure. 1. Compared
Figure 1. Reaction performance of the catalysts modified by different alkali
metal oxides. (A) CH
4
conversion, (B) C
2
product selectivity, (C) C
2
p
-1ro
－
du
1
ct
,
yield. Reaction conditions: cat 0.2g, CH
50% Nitrogen balanced.
4
/O =4:1, GHSV=18000mLh g
2
with the un-modified SnO
2
support, the addition of alkali metal
conversion, C product
oxides obviously increases the CH
4
2
selectivity and yield over all the modified catalysts. With the
increasing of the reaction temperature from 650 to 750 ℃, the
2
XRD and N -BET measurements of the catalysts modified by
CH
4
conversion, C
2
product selectivity and yield improve slightly,
-based
different alkali metal oxides
testifying that the reaction performance of these SnO
catalysts is not sensitive to the reaction temperatures. It is
mentioned here that though Sn9Na1, the catalyst modified by
2
The phase composition of the catalysts was analyzed by XRD
techniques, with the patterns displayed in Figure. 2. For all the
modified catalysts, similar to the un-modified SnO
2
support,
4 2
sodium oxide, has the highest initial CH conversion, C product
tetragonal rutile SnO is the only crystalline phase detected. No
2
selectivity and yield among all the catalysts, they decay
gradually with the increasing of the temperature, indicating its
surface active sites are not as stable as those on other catalysts.
Comparing the stable reaction performance above 700 ℃, the
any diffraction feature related to alkali metal oxides can be
observed. Taking into account of the fact that all the catalysts
were prepared with the traditional impregnation method, it is
believed that after calcination the formed alkali metal oxides are
dispersed finely on the surface of the SnO
escaping the detection by XRD.
CH
sequence of Sn9Li1 > Sn9Cs1 > Sn9K1 > Sn9Na1 > SnO
testifies strongly that all the alkali metal oxides have evident
promotion effects on SnO for oxidative coupling of methane but
4
conversion, C
2
product selectivity and yield follow the
2
support, thus
2
. This
[
51]
2
with different extents, among which lithium oxide displays the
best enhancement effect.
2
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European Journal of Inorganic Chemistry
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FULL PAPER
4
+

and Figure. S2(B) proves that Sn is fully oxidized as Sn , in


-SnO
2
agreement with the XRD results.
(A)
Sn9Cs1
Sn9K1
(a) SnO2
(b) Sn9Li1
2
-
2-
O
O
2-
O
2
-
O
22-
CO32-
OH
5
35
534
533
532
531
530
529
529
529
528
528
528
527
527
527
535
534
533
532
531
530
529
528
527
Binding Energy (eV)
Binding Energy (eV)
Sn9Na1
Sn9Li1
(
c) Sn9Na1
2-
(d) Sn9K1
2-
O
O
2
-
O
2
2-
O
2
2
-
2-
CO
3
CO
3
SnO
2
535
534
533
532
531
530
535
534
533
532
531
530
529
528
527
Binding Energy (eV)
Binding Energy (eV)
(
e) Sn9Cs1
2-
O
20
40
60
80


2
-
O
2
2
-
CO
3
5
35
534
533
532
531
530
Binding Energy (eV)
Figure 2. XRD patterns of the catalysts modified by different alkali metal
oxides.
15
6
7
7
50℃
00℃
50℃
2-
2- 2-
/(O +O ) ratio
(B)
O
2
2
22% 35% 36% 25%
31%
The texture properties of the catalysts were analysed by
means of N
2
adsorption-desorption technique, with the results
10
3
5%
3
6%
31%
displayed in Table 1. As shown in Figure. S1, all the catalysts
exhibit typical type V isotherms with a capillary condensation
step, proving the presence of mesoporous structure. The
2
5%
5
0
hysteresis loop in the relative pressure (P/P ) range of 0.6-1.0
belongs to H3-type, which is typical for the presence of inter-
particle meso-pores, in line with the mono-modal pore
distribution profiles in Figure. S1 (B). As indicated in Table 1,
after the addition of alkali metal oxides, the pore volumes of the
catalysts decrease more or less in comparison with the
2
2%
0
SnO
Sn9Li1
Sn9Na1
Sn9K1
Sn9Cs1
2
2
individual SnO support, implying the blockage of some initial
Figure 3. XPS analysis of the surface oxygen properties for the catalysts
modified by different alkali metal oxides
pores. While the average pore diameters of the two catalysts
modified by lithium and sodium oxides have little change, the
other two samples modified by potassium and cesium oxides
have some evident decrease. Moreover, compared with pure
To understand the surface oxygen properties of the
catalysts, the O 1s spectra of the catalysts have been analyzed
in detail, with the quantified results listed in Table 2. As shown in
Figure. 3(A), all the catalysts displays bi-modal O 1s peaks,
indicating the presence of surface oxygen species with different
chemical environments. According to the former publications,
the major O 1s peak at approximately 530.0 eV can be assigned
2
SnO support, the surface areas of all the modified catalysts is
also nearly intact. All these experimental facts testify that the
addition of different alkali metal oxides with a 9/1 molar ratio has
little alteration on the texture properties of the catalysts, and
which should not be the major reason influencing the reaction
performance of the catalysts.
2
-
to the surface lattice oxygen species (O ); and the shoulder
Table 1. Physical-chemical properties of the catalysts modified by different
peak above 532.0 eV is believed to be composed of the loosely
alkali metal oxides
[52-53]
bonded surface oxygen species.
For pure SnO
2
, surface
2
3
[
54-55]
Sample
SnO
S
BET (m /g)
Pore volume (cm /g) Pore diameter (nm)
hydroxyl groups are generally present.
In contrast, for the
samples modified by alkali metal oxides, the formation of
carbonate is observed, as proved by Figure. S2(C). Based on
these findings, the O 1s peaks of the catalysts have been
deconvoluted and fitted, with the detailed results summarized in
Table 2. It is revealed that an electron deficient oxygen species,
2
12
13
0.73
0.72
10.0
9.9
Sn9Li1
Sn9Na1
Sn9K1
12
11
9
0.50
0.60
0.61
10.3
9.0
2
-
O2 , is detected on the surface of all the catalysts. Former
reports have testified that several types of surface electrophilic
Sn9Cs1
8.8
−
−
2−
oxygen species (O , O
formation of the coupling products.
subtracting the contribution from hydroxyl group on pure SnO
and the contribution from carbonate on other modified samples,
2 2
and O ) are critical for the selective
[
31-32, 34, 37]
Therefore, by
2
XPS study on the catalysts modified by different alkali metal
oxides
2
-
2-
2-
the O
2 2
/(O +O ) molar ratios of the catalysts have been
quantified in Table 2. Apparently, after modification by alkali
XPS was used to investigate the surface properties of the
catalysts modified by different alkali metal oxides, with the
spectra shown in Figure. 3 and S2. Figure. S2(A) testifies that
all the alkali metals are fully oxidized with a valence state of +1;
2-
2-
2-
metal oxides,
2 2
O /(O +O ) molar ratio of each sample
increases in comparison with that of the individual SnO
2
,
indicating the formation of more mobile and selective oxygen
species.
3
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European Journal of Inorganic Chemistry
10.1002/ejic.201701440
FULL PAPER
.
15
6
50℃
700℃
50℃
Intermediate basic sites amount (a.u)
14 12 10
(B)
Table 2. Physical-chemical properties of the cat alysts modified by
6
9
7
different alkali metal oxides
O1s Binding Energy (eV)
2-
2-
2-
1
0
5
0
Catalysts
2 2
O /(O +O )
2-
2-
2-
14
CO
3
O
2
O
1
2
1
0
5
(
32.3
OH-)
SnO
2
531.4
530.9
22%
9
Sn9Li1
Sn9Na1
Sn9K1
532.0
532.0
532.2
531.9
531.2
530.7
530.6
530.6
530.6
530.1
530.0
529.6
35%
36%
25%
31%
6
SnO
Sn9Li1
Sn9Na1
Sn9K1
Sn9Cs1
Sn9Cs1
2
2
Figure 4. CO -TPD profiles of the catalysts modified by different alkali metal
To discern the relationship between the OCM reaction
oxides.
2
−
performance and the surface O
2
species, the C
2
2-
product yield
2
-
2-
of each catalyst is plotted versus its O
2
/(O
2
+O ) molar ratio.
As displayed in Figure. 3 (B), for those catalysts with better C
2
2
On the profile of individual SnO , a broadened peak center
at 400 ℃ is observed, indicating the presence of a small amount
2
-
yield, the surface O
2
species is generally more abundant. The
un-modified SnO
2
possesses the smallest amount of surface
of alkaline sites. In contrast, with the addition of different alkaline
species, thus displaying the lowest C
2
yield. On the contrary, metal oxides, the profiles of the modified catalysts become much
2
-
O
2
Sn9Li1, the sample modified by lithium oxide, possesses the
more complicated, testifying the formation of a larger amount of
alkaline sites with different strength. For clear comparison, the
2
-
largest amount of surface O
highest C yield. However, it is noticed that the C
proportional to the relative surface O
2
species, hence displaying the
yield is not
amount, indicating other
alkaline sites are divided into two groups according to the CO
desorption temperatures. In detail, it is defined that the CO
2
2
2
2
-
2
2
factors might also influence the reaction performance of the
catalysts. Most probably, the surface basicity induced by the
addition of different alkali metal oxides could be another reason
affecting the OCM performance of the catalysts.
desorption peak below 230 ℃ is correspondent to weak alkaline
sites and the peak above 230 ℃ to alkaline sites with
intermediate strength (intermediate alkaline sites). The amount
of weak basic sites is observed to increase in the order of SnO
2
<
Sn9Li1< Sn9Na1< Sn9Cs1 < Sn9K1; and the amount of
CO
2
-TPD analysis of the catalysts modified by different
intermediate alkaline sites increases in the order of SnO
Sn9Cs1 < Sn9K1< Sn9Na1< Sn9Li1.
2
<
alkali metal oxides
As demonstrated previously, for an effective OCM catalyst, the
presence of surface alkaline sites is necessary, which can
Table 3. Quantified CO
2
-TPD results of the catalysts modified by different alkali
[
19]
activate the potent methane molecules. Many early literatures metal oxides.
have reported the correlations between the number of basic
[
19, 56]
Catalysts
Weak basic
sites
Intermediate
basic sites
Total basic sites
amount (a.u.)
sites and the C
2
yield in the OCM reaction.
While all the
alkaline sites are beneficial for the activation of methane
molecules, the basic sites with intermediate strength are
amount (a.u.)
amount (a.u.)
[3, 19]
favorable for the selective formation of C
Therefore, CO -TPD technique is adopted to probe the alteration
of the surface alkaline sites of SnO support after the
2
coupling products.
SnO2
0
5
6
6
2
Sn9Li1
Sn9Na1
Sn9K1
Sn9Cs1
14
12
9
19
22
29
27
2
modification by different alkali metal oxides, with the profiles
shown in Figure. 4.
10
20
85
10
(A)
17
403
91
Sn9Cs1
K1Sn9
3
50
440
Although all the alkaline sites with different strength might
contribute to the OCM reaction performance, the sites with
74
3
30
5
10
intermediate strength are found relating to the selective
350
Sn9Na1
[3, 19, 31, 57]
formation of the coupling products.
Therefore, the C
2
9
1
Sn9Li1
yield of each catalyst is plotted versus the amount of the
intermediate alkaline sites in Figure. 4 (B). It is evident that a
catalyst with better C2 yield generally has a larger amount of
intermediate alkaline sites, indicating the intermediate alkaline
sites are critical for the formation of the coupling products.
Whereas, although Sn9K1 possesses slightly larger amount of
intermediate alkaline sites than Sn9Na1, it displays still evidently
4
00
SnO
2
1
00
200
300
Temperature (℃)
400
500
4
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European Journal of Inorganic Chemistry
10.1002/ejic.201701440
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1
00
80
60
40
2
lower C yield than Sn9Na1. The major reason is due to the
(B)
presence of a larger amount of electrophilic oxygen species on
the surface of Sn9Na1 (Table 2). According to these
experimental facts, it is reasonable to conclude that both of the
abundance of electrophilic oxygen species and intermediate
alkaline sites are important for the OCM reaction. Therefore, the
concerted interaction between these two types of surface active
2
sites could determine the reaction performance of the SnO -
SnO
2
based catalysts in this study. Due to the optimal concerted
interaction of the two types of surface sites, Sn9Li1 displays the
best reaction performance among all the catalysts modified by
2
0
0
Sn9Li1
Sn7Li3
Sn5Li5
Sn3Li7
650
675
700
725
750
different alkali metal oxides, over which the highest C
yield around 10 % can be obtained.
2
product
Temperature (℃)
25
(C)
SnO
The influence of lithium oxide amount on the OCM reaction
performance of the SnO -based catalysts
With the purpose to develop catalysts having better reaction
performance, SnO -based catalysts modified with different
amounts of lithium oxide have been prepared and used for OCM
reaction. As displayed in Figure. 5(A), pure SnO itself has very
low activity for the reaction, as evidenced by the low methane
conversion in the whole tested temperature region. The addition
of lithium oxide improves the activity of the resulted catalysts
obviously. Moreover, with the increase of the lithium oxide
amount from Sn/Li molar ratio of 9/1 to 5/5, the methane
conversion on the catalysts keeps on increasing. The highest
activity can be obtained on Sn5Li5 catalyst. Further increasing
the lithium oxide amount to Sn/Li molar ratio of 3/7 degrades the
activity abruptly, implying the change of the active site property
2
Sn9Li1
Sn7Li3
Sn5Li5
Sn3Li7
20
2
1
5
0
5
0
2
1
2
6
50
675
700
725
750
Temperature (℃)
1
00
CH Conversion
4
(D)
C
Selectivity
C Yield
2
2
80
60
40
20
0
2
drastically. Interestingly, the C product selectivity of all the
catalysts with different amount of lithium oxide is similar, which
is around 60-70 % in the whole temperature region, and much
higher than that on the pure SnO
2
support, as shown in Figure.
5(B). As a result, Figure. 5 (C) exhibits that all the modified
2 2
catalysts have higher C yield than the pure SnO catalyst. On
Sn5Li5, the modified catalyst with a Sn/Li molar ratio of 5/5, the
highest C yield around 15.3 % can be achieved at 750 ℃. For
2
0
20
40
Time
60
80
100
more detailed information, the product distribution collected at
different temperatures on Sn5Li5, the best catalyst in this study,
( )
h
is shown in Table S1. C
products. Moreover, the selectivity of C
than that of C . It is noted here that the similar product
distribution is observed also on other catalysts in this study.
2
H
4
, C
2
H
6
and CO
2
are the major
Figure 5. Reaction performance of the catalysts modified by different amount
of lithium oxide. (A) CH conversion, (B) C product selectivity, (C) C product
yield, (D) Stability test at 750℃ over Sn5Li5 catalyst. Reaction conditions: cat
2
H
6
is generally higher
4
2
2
2
H
4
-1 -1
4 2
0.2g, CH /O =4:1, GHSV=18000mLh g , 50% Nitrogen balanced.
For a catalyst, the stability is an important issue to estimate
25
20
15
10
5
0
(
A)
its application potential. Therefore, Sn5Li5 was subjected to a
100 h stability test at 750 ℃. As demonstrated in Figure. 5 (D),
the methane conversion, C2 selectivity and yield are constant
during the testing without any decrease, indicating this catalyst
is stable for the high temperature OCM reaction.
SnO
2
Sn9Li1
Sn7Li3
Sn5Li5
Sn3Li7
2 4 2
At present, Mn/Na WO /SiO is still the most promising
OCM catalyst for industrial application, on which the highest one
way yield close to 30 % has been reported by some researchers
[
58-59]
above 800℃.
Whereas, this catalyst has very low activity
below 700 ℃. To save the cost, it is always desirable to find
catalysts that can be operated at lower temperatures. As
compared in Figure. S3, Sn5Li5, the best catalyst in this study,
650
675
700
725
750
Temperature (℃)
displays
Mn/Na WO
much
better
reaction
performance
than
2
4
/SiO
2
catalyst under the same condition at low
5
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European Journal of Inorganic Chemistry
10.1002/ejic.201701440
FULL PAPER
temperature region. At 700 ℃, the C
2
yield over Sn5Li5 is
of excess amount of Li
of the catalyst.
2
O is actually harmful for the performance
1
2.2 % and that over Mn/Na WO /SiO is only 3.0 %. It is
2 4 2
summarized here that with the addition of appropriate amount of
lithium oxide, SnO -based catalysts with not only significantly
2
Table 4. Physical-chemical properties of the catalysts promoted by different
improved reaction performance, but also very high stability can
be achieved, which may give people some new thoughts on
developing OCM catalysts operated at lower temperature region.
amount of lithium oxide..
Sample
S
BET
2
Pore
Pore
The theoretical
crystalline phase
composition
The actual
crystalline
phase
(
m /g) volume diameter
3
(
cm /g)
(nm)
2
XRD and N -BET analysis of the catalysts modified by
different amount of lithium oxide
a
composition
SnO
2
12
13
0.73
0.72
10.0
9.9
SnO
2
SnO
2
To understand the reaction performance of the catalysts
modified by different amount of lithium oxide, they were Sn9Li1
8.5 SnO +0.5 Li SnO
2
2
3
SnO2
subjected to XRD analysis, with the patterns exhibited in Figure.
Sn7Li3
8
5
2
0.14
0.11
0.02
35.1
34.7
14.0
5
.5SnO
2
+1.5Li
+2.5Li
2
SnO
3
SnO
2
+Li
2
SnO
SnO
3
3
6
. While Sn9Li1 shows still only the diffraction feature of
tetragonal rutile SnO phase, starting from Sn7Li3, a new
Li SnO phase appears, whose amount increases with the
addition amount of lithium oxide, as evidenced by the improved
peak intensity of Li SnO . Sn7Li3 and Sn5Li5 consist of both
tetragonal rutile SnO and Li SnO phases. In contrast, with the
further increasing of lithium oxide amount, Li SnO is the only
Sn5Li5
Sn3Li7
2.5SnO
2
2
SnO
3
SnO
2
+Li
2
2
2
3Li SnO
3
+0.5Li
2
O
2 3
Li SnO
2
3
a. identified by XRD.
2
3
2
2
3
The texture property of the catalysts modified by different
amount of lithium oxide was also analyzed with N adsorption-
desorption technique. As displayed in Figure. S4 and Table 4,
while Sn9Li1 still shows similar sorption isotherm, pore
distribution profile, pore volume, pore diameter and specific
2
3
2
detected phase for Sn3Li7 catalyst. This testifies that during the
high temperature calcination process, solid phase reaction
between lithium and tin oxides takes place according to the
following equation:
2
surface area to the un-modified SnO , those of Sn7Li3, Sn5Li5
and Sn3Li7 are completely different, testifying further the change
of the bulk phase composition of these samples containing
larger amounts of lithium oxide. With the formation of more
Li
2
O + SnO
2
= Li
2
SnO
3
(1)
Li
disappear abruptly starting from Sn7Li3 catalyst. Sn3Li7, the
catalyst without any SnO and with excess Li O possesses extra
low surface area and pore volume, testifying it is nearly non-
porous. Indeed, the N adsorption-desorption results are well
consistent with the XRD results, testifying the change of the bulk
phase compositions of the catalysts with different amount of
lithium oxide.
2 3 2
SnO in the catalysts, the initial interparticle pores of SnO

Li SnO
2
3


SnO
2
2
2


Sn3Li7
Sn5Li5
2



Sn7Li3
Sn9Li1
Raman characterization of the catalysts modified by
different amount of lithium oxide
SnO
2
Raman technique was used to further confirm the phase
composition of the catalysts modified by different amount of
lithium oxide, with the profiles exhibited in Fig. 7. For
20
40
60
80


comparison, the spectroscopy of the individual SnO
2
was also
collected, which shows three typical Raman peaks at 480, 635
Figure 6. XRD patterns of the catalysts modified by different amount of lithium
oxide.
-1
and 775 cm , corresponding to the E
g
, A1g and B2g vibration
While Sn9Li1 displays the same profile
, starting from Sn7Li3, the typical Raman peaks
[
60-64]
modes in sequence.
to pure SnO
2 3
belonging to Li SnO are observed at 588, 370 and 320 cm ,
2
-1 [65]
Based on the stoichiometry expressed by this equation and
supposing the reaction is very complete, the phase composition
of the catalysts modified with different amount of lithium oxide is
estimated in Table 4. Sn9Li1, Sn7Li3 and Sn5Li5 should be
whose intensities increase with the increasing amount of lithium
oxide. Same to the XRD phase analysis results, in Sn7Li3 and
Sn5Li5 catalysts, both of SnO
with varied ratios. However, in Sn3Li7, Li
2
and Li
2
SnO
3
phases co-exist but
SnO is the only
composed of both SnO
SnO /Li SnO molar ratio of 8.5/0.5, 5.5/1.5 and 2.5/2.5,
respectively. However, Sn3Li7 should be composed of both
Li SnO and Li O phases with a Li SnO /Li O molar ratio of
.0/0.5. The excess Li O is believed to be finely dispersed on
the surface of Li SnO
2
and Li
2
SnO
3
phases, with a theoretical
2
3
[
66]
detected phase. As lithium oxide is Raman silent, the excess
lithium oxide on this catalyst is not detectable. In brief, the
Raman results are in good agreement with the XRD results,
testifying the addition of different amount of lithium oxide
induced the phase composition change of the catalysts, which
2
2
3
2
3
2
2
3
2
3
2
[
51]
2
3
, thus escaping the detection by XRD.
Apparently, for the catalysts with both SnO
the reaction performance is much better, and the best catalyst in
fact consists of equal amount of the two phases. The existence
2
and Li
2
SnO
3
phases, could subsequently influence the reaction performance of the
catalysts.
6
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European Journal of Inorganic Chemistry
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FULL PAPER
It is noted here that some of the modified catalysts show a
minor reduction peak below 350 ℃, as indicated by the enlarged
profiles in Figure. 8. It was reported previously that as an n-type
B
A
2g
1g
Li SnO
2
E
g
3
480
7
75
635
588
semiconductor, SnO
2
possesses a certain amount of deficient
but which can be depleted mostly if
[
44, 49]
Li SnO
oxygen species,
calcined above 300 ℃ .
amounts of lithium oxide, this deficient oxygen species is
obviously stabilized at higher temperature. Therefore, on the H
2
3
[
69]
3
70 320
With the addition of appropriate
Sn3Li7
Sn5Li5
2
-
TPR profiles of Sn9Li1, Sn7Li3 and Sn5Li5, a small reduction
peak corresponding to this active oxygen species can be
observed. Sn5Li5, the best catalyst in this study, possesses the
largest amount of this active oxygen species, as identified from
the enlarged profiles. The abundance of this active surface
deficient oxygen species could indeed be an important factor
Sn7Li3
Sn9Li1
SnO
2
900
800
700
600
500
400
300
200
-
1
Raman shift (cm
)
[37, 70]
accounting for its outstanding OCM reaction performance.
Table 5. Quantified H
2
-TPR results of the catalysts modified by different
Figure 7. The Raman spectra of the catalysts modified by different amount of
lithium oxide.
amount of lithium oxide.
Catalysts
H
2
uptake amount
H
2
uptake
O/Sn atomic
ratio
above 350℃
amount above
-
1
H
2
-TPR study on the catalysts modified by different amount
of lithium oxide
-TPR experiments were performed to investigate the redox
(mmol g cat
)
350℃
-
1
(
mmol g SnO2)
SnO
2
13.22
13.22
1.99
1.92
H
2
Sn9Li1
Sn7Li3
Sn5Li5
Sn3Li7
12.41
12.76
12.56
11.16
0
property of the catalysts modified by different amount of lithium
oxide, with the profiles shown in Fig. 8 and the quantified results
11.11
1.89
1.68
0
in Table 5. Pure SnO
be assigned to the reduction of SnO
2
has a reduction peak at 653 ℃, which can
10.04
[49, 67]
2
to metallic Sn.
The
quantified O/Sn atomic ratio around 2 also testifies this. With the
addition of lithium oxide, this major reduction peak shifts to lower
temperature gradually. However, the profile of Sn3Li7 shows no
any reduction peak. As identified by XRD and Raman results
0
above, this catalyst consists of Li
Apparently, after the formation of Li
cations become non-reducible. Therefore, for Sn9Li1, Sn7Li3
and Sn5Li5, the observed major peak should be contributed by
2
SnO
SnO
3
and Li
compound, the Sn
2
O phases.
4
+
XPS study on the catalysts modified by different amount of
lithium oxide
2
3
XPS was used again to measure the surface property of the
catalysts modified by different amount of lithium oxide, with the
results displayed in Figure 9 and Table 6. Figure S5 (A) exhibits
the Sn 3d spectra of the catalysts. With the increasing of the
added lithium oxide amount, the binding energies shift gradually
to lower region, testifying the increase of electron density of the
Sn cations. However, the binding energies are still typical for
the reduction of the SnO
understanding, the H -TPR results have been quantified and
listed in Table 5. Compared with pure SnO , the H consumption
amount based on each gram catalyst, based on each gram
SnO and the O/Sn atomic ratio decrease gradually with the
increase of lithium oxide amount. Moreover, the H consumption
amount Sn3Li7, the catalyst consisting of Li SnO and Li
phases, becomes zero, testifying further the Sn cations in the
2
phase in the bulk. To gain deeper
2
2
2
2
2
O
4+
2
3
2
Sn , testifying that Sn is fully oxidized in the catalysts.
4
+
[
68]
lattice of Li
2
SnO
3
compound are non-reducible.
(A)
(
a) SnO
(b) Sn9Li1
2-
2
2-
O
O
6
46
Sn3Li7
2-
O22-
O2
-
CO32-
OH
Sn3Li7
579
2
60
535
534
533
532
531
530
529
528
527
535
534
533
532
531
530
529
O
528
527
Sn5Li5
Sn7Li3
6
44
Binding Energy (eV)
Binding Energy (eV)
Sn5Li5
Sn7Li3
260
(
c) Sn7Li3
(d) Sn5Li5
2-
2
-
O
2-
2
50
CO3
6
30
250
2-
2
-
O2
O2
2
-
CO3
533
Sn9Li1
2
25
Sn9Li1
225
6
53
535
534
532
531
530
529
528
527
535
534
533
532
531
530
529
528
527
SnO
2
Binding Energy (eV)
Binding Energy (eV)
SnO
2
(
e) Sn3Li7
2
-
2
-
O
CO3
100
200
300
400
500
600
700
800 150 200 250 300 350
2
-
Temperature (℃)
O
2
Temperature (℃)
535
534
533
532
531
530
529
528
527
Figure 8. H
2
-TPR profiles of the catalysts modified by different amount of
Binding Energy (eV)
lithium oxide.
7
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European Journal of Inorganic Chemistry
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FULL PAPER
2
1
1
0
5
0
5
0
larger amount of surface electrophilic oxygen species, thus
having better reaction performance. Sn3Li7 sample, as a
catalyst contains Li SnO phase but lacks SnO phase, its
2 3 2
amount of surface electrophilic oxygen species is less than other
modified catalysts, thus having lower OCM reaction performance.
Therefore, the presence of surface electrophilic oxygen species
is important, and could be a crucial factor to decide the OCM
6
7
7
50℃
00℃
50℃
2-
2- 2-
O
/(O +O ) ratio
2
(B)
2
22% 35% 39% 41%
30%
4
1%
3
9%
3
5%
3
0%
2
reaction performance on the SnO -based catalysts in this study.
CO
amount of lithium oxide
As discussed above, the presence of surface alkaline sites are
2
-TPD analysis of the catalysts modified by different
2
2%
SnO
Sn9Li1
Sn7Li3
Sn5Li5
Sn3Li7
2
[19, 33]
important for an effective OCM catalyst.
influence of surface basic sites by the addition of different
amount of lithium oxide has been probed by means of CO -TPD
method. As shown in Figure. 10 (A), both of SnO and Sn9Li1
consist predominantly of tetragonal SnO phase, thus they
possesses very small amount of surface alkaline sites. In
comparison with pure SnO , a new weak CO desorption peak at
90 ℃ is observed on the profile of Sn9Li1 catalyst. Whereas,
Therefore, the
Figure 9. XPS analysis of the surface oxygen property for the catalysts
modified by different amount of lithium oxide.
2
2
2
For the catalysts modified by different amount of lithium
oxide, a doublet O 1s peak is still observed, as shown in Figure
(A). Besides the major O 1s peak belonging to lattice O at ~
30 eV, a shoulder peak assigned to loosely bond oxygen
species is observed, whose area increases evidently with the
increasing of lithium oxide amount. Therefore, the O1s peak of
each sample is deconvoluted and quantified in Table 6. It is
particularly noted that Figure S5 (B) proves that carbonate is
observed for all the modified catalysts, especially those
containing larger amount of lithium oxide. Therefore, when
2
2
~
2
-
9
5
further increasing the amount of lithium oxide obviously induces
the formation of more complicated surface alkaline sites with
also much larger amount. Sn7Li3 and Sn5Li5, the two catalysts
composed of both large quantities of SnO
display similar CO desorption profiles, but the latter possesses
evidently more abundant surface alkaline sites with intermediate
strength. Sn3Li7, the catalyst containing Li SnO and Li
phases, owns even more abundant and stronger alkaline sites,
2 2 3
and Li SnO phases,
2
2
3
2
O
2
-
2-
2-
quantifying the
2 2
O /(O +O ) ratio of each catalyst, the
2
as evidenced by the CO desorption peak at 528 ℃
contribution from the carbonate is subtracted. As listed in Table
6
, the addition of lithium oxide improved this ratio apparently,
.
2
-
5
28
testifying the formation of more abundant electron deficient O
2
(A)
1
57
species, which is found to be important for the selective
formation of the coupling products.
4
95
5
50
8
5
1
67
Sn3Li7
4
87
Table 6. Quantified H
2
-TPR results of the catalysts modified by different
8
7
amount of lithium oxide.
167
583
Sn5Li5
2-
2-
2-
87
91
Catalysts
O 1s Binding Energy (eV)
O
2
/(O
2
+O )
459
5
57
2-
3
2-
2
2-
CO
O
O
Sn7Li3
Sn9Li1
3
50
SnO
2
532.3
532.0
531.8
531.6
531.7
531.4
531.2
530.6
530.7
530.8
530.9
530.6
529.9
529.9
529.7
22%
35%
39%
41%
30%
4
00
SnO
2
Sn9Li1
Sn7Li3
Sn5Li5
Sn3Li7
100
200
300
400
500
600
700
Temperature (℃)
20
6
7
7
50℃
00℃
50℃
Intermediate basic sites amount (a.u)
(B)
6
14
16
33
25
3
3
1
5
0
5
0
1
6
To understand the relationship between the reaction
1
2
−
14
performance and the surface O
each catalyst is plotted versus its O
Apparently, with the increasing of the lithium oxide from Sn9Li1
2 2
amount, the C product yield of
2
-
2-
2-
25
2
2
/(O +O ) molar ratio.
2
-
2-
2-
2 2
to Sn5Li5, the surface O /(O +O ) molar ratio improves.
6
Sn5Li5 has the highest ratio, testifying it possesses the most
abundant surface electrophilic oxygen species, which explains
its best performance among all the catalysts. The un-modified
SnO
Sn9Li1
Sn7Li3
Sn5Li5
Sn3Li7
2
2
SnO possesses the least amount of surface electrophilic
oxygen species, thus having the lowest OCM performance
among all the catalysts. It is also noticed that if a catalyst
Figure 10. XPS analysis of the surface oxygen property for the catalysts
modified by different amount of lithium oxide.
2 2 3
contains both of SnO and Li SnO phases, it usually has a
8
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European Journal of Inorganic Chemistry
10.1002/ejic.201701440
FULL PAPER
For a more accurate comparison, the alkaline sites are
rutile SnO
2
is the predominant crystalline phase for all the
divided into three groups according to the CO
temperatures and quantified in Table 7. In detail, it is defined
that the CO desorption peak below 250 ℃ is correspondent to
2
desorption
catalysts, and the alkali metal oxides disperse highly on the
SnO surface. As a consequence, the amounts of surface
alkaline sites and electrophilic oxygen species have been
evidently improved, which are believed to be the major factors
influencing the OCM performance of the catalysts.
(2) By tuning the addition amount of lithium oxide, the best
reaction performance can be obtained over a catalyst with a
2
Sn/Li molar ratio of 5/5, on which 15.3 % C yield has been
2
2
weak alkaline sites, the peak between 250-550 ℃ to alkaline
sites with intermediate strength (intermediate alkaline sites) and
[
3, 33]
the peak above 550 ℃ to strong alkaline sites.
While all the
alkaline sites are favorable for the activation of methane
molecules, the intermediate alkaline sites are important for the
[3, 31-33]
selective formation of coupling products.
Therefore, the C
2
achieved at 750 ℃. XRD and Raman results have disclosed that
yield on the catalysts is plotted against the amount of their
intermediate alkaline sites in Figure. 10(B). It is apparent that the
for those catalysts with evident improved OCM reaction
performance, the co-existence of both SnO and Li SnO phases
2 2 3
C
2
product yield increases with the amount of intermediate
is necessary and important. The best catalyst consists of equal
amounts of these two phases.
(3) Based on the results achieved over the catalysts
modified by different amount of lithium oxide, it has been
alkaline sites, testifying that the abundance of surface
intermediate alkaline sites is another factor determining the
OCM reaction performance of the SnO
study.
2
-based catalysts in this
discovered that the C
2
yield at different temperatures is
proportional to the amount of both surface intermediate alkaline
sites and electrophilic oxygen species. Therefore, it is concluded
that the abundance and the concerted interaction of these two
types of surface active sites are the major factors determining
2
Table 7. Quantified CO -TPD results of the Catalysts modified by different
amount of lithium oxide.
Catalysts
Weak basic sites
amount (a.u.)
Intermediate basic
sites
Total basic
sites
the OCM reaction performance of the SnO
this study.
2
-based catalysts in
amount (a.u.)
amount (a.u.)
(
4) Last but not the least, the optimized catalyst, Sn5Li5,
which has a Sn/Li molar raio of 5/5 and consists equal amount of
SnO and Li SnO phases, exhibits much better reaction
performance than Mn/Na WO /SiO , the most promising OCM
SnO
2
0
6
6
19
Sn9Li1
Sn7Li3
Sn5Li5
Sn3Li7
5
14
16
33
25
2
2
3
2
4
2
61
67
17
92
catalyst at present, at low temperature region (below 750 ℃),
and its reaction performance is very stable. Therefore, we trust
information provided here may give people some new
enlightenment on how to develop novel OCM catalysts that can
be operated at low temperature and faciliate the industiliaztion
process of this important chemical enginering reaction.
100
85
In summary, it has been disclosed in this study that for
2
-based catalysts modified by different alkaline metal oxides,
SnO
the surface electrophilic oxygen sites and intermediate alkaline
sites are the two major factors influencing the OCM reaction
performance. The concerted interaction of the two types of
surface sites determines the reaction performance of a catalyst.
As an example, Sn5Li5 possesses the most abundant two types
of surface sites, thus displaying the best reaction performance
among all the catalysts.
Experimental Section
Catalyst preparation
The SnO2 supports were prepared by Precipitation method. In
detail, 0.05 mol SnCl
distilled deionized (DDI) water to form stable solution. Then NH
25%-28 wt. %) aqueous solution was added dropwise into it until the pH
4 2
·5H O (A.R.) was dissolved into a certain amount of
3
.H O
2
(
reached 7. Afterwards the precipitate was vacuum filtered and washed
thoroughly with DDI water until Cl free, with a total dissolved solid (TDS)
less than 10 ppm. The resultant precipitate was dried at 120 ℃ for 12 h，
and then calcined at 800 ℃ for 4 h in air atmosphere and ground into
Conclusions
With the objective to develop reactive catalysts for
oxidative coupling of methane at low temperature, different alkali
2
powder, which is named SnO .
metal oxides have been adopted to modify the surface of SnO
2
The supported alkali metal oxide catalysts were prepared by a
traditional impregnation method with aqueous nitrate solution as their
with a traditional impregnation method. Since lithium oxide
displays very positive modification effects, the influence of its
addition amount has also been investigated and tuned.
Furthermore, all the catalysts have been characterized with
different techniques to study its surface and bulk property
change, and correlated with the OCM reaction performance.
precursor. With Li-modified catalyst as an example, LiNO
3
(A.R.) was first
-
dissolved into DDI water to prepare a stable aqueous solution (0.5 mol.L
1
). Afterwards, a certain amount of SnO
2
support prepared in the last step
was added into this LiNO solution. The mixture was then dried at 120 ℃
3
-1
for 12 h and calcined again at 800 ℃ with a heating rate of 2 ℃.min in
air atmosphere for 4 hours to get the final catalyst, which was named
Sn9Li1 according to the Sn/Li molar ratio. For the preparation of the
catalysts modified with other alkali metal oxides having the same molar
(
1) With the addition of various alkali metal oxides, the
methane conversion, C product selectivity and yield can be
significantly improved over all the modified catalysts in
comparison with the un-modified SnO . Among all the alkali
2
ratios, the same procedure was used except that NaNO
3 3
(A.R.), KNO
(
A.R.) and CsNO (A.R.) solutions were used as the precursors. The
3
2
catalysts are named Sn9Li1 ， Sn9Na1, Sn9K and Sn9Cs1. For the
preparation of the catalysts modified with different amount of lithium
metal oxides, lithium oxide shows the most positive promotion
effects. XRD results have demonstrated that that tetragonal
9
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European Journal of Inorganic Chemistry
10.1002/ejic.201701440
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oxide, the same procedure was used and the catalysts are named
Sn7Li3, Sn5Li5 and Sn3Li7 according to the Sn/Li molar ratios.
The methane conversion (XCH4), C
this study are calculated with the following equations:
2 2
selectivity (SC2) and C yield (YC2) in
ꢀ
ꢄꢅꢀꢄꢆꢅꢇ(ꢈꢆꢉꢂꢅꢈꢆꢉꢊ)ꢅꢋ(ꢈꢌꢉꢊꢅꢈꢌꢉꢍ)
Catalyst characterization
ꢃ
ꢁꢂ
ꢐꢑꢒꢒꢓ
(2)
(3)
(4)
ꢀ
(ꢈꢉꢂ)ꢎꢏ
ꢇ(ꢈꢆꢉꢂꢅꢈꢆꢉꢊ)
The bulk phase composition of the fresh catalysts were examined
by using a Bruker AXS D8Focus diffractometer (XRD) that was operated
at 40 kV and 30 mA with a Cu target and Kα-ray irradiation. The Scans
ꢔꢈꢆ ꢃꢀ
ꢄꢅꢀꢄꢆꢅꢇ(ꢈꢆꢉꢂꢅꢈꢆꢉꢊ)ꢅꢋ(ꢈꢌꢉꢊꢅꢈꢌꢉꢍ)
ꢐꢑꢒꢒꢓ
ꢕ
ꢃ
ꢖꢔꢈꢆ ꢐꢑꢒꢒꢓ
ꢈꢆ
ꢈꢉꢂ
o
-1
were collected over a 2θ range from 10° to 90° with step of 2 min .
Nitrogen adsorption-desorption experiments of the all samples
were performed at 77 K on ASAP2020 instrument. The specific surface
areas of the catalysts were calculated using the Brunauer-Emmett-Teller
Acknowledgements
This work is supported by the Natural Science Foundation of
China (21567016, 21566022, 21666020), the Natural Science
0
(BET) method in the relative pressure (P/P ) range of 0.05-0.25. The
pore size distribution of samples was calculated with the Barrett-Joyner-
Halenda (BJH) method and the average pore sizes of the samples were
obtained from the peak positions of the distribution curves.
Foundation
of
Jiangxi
Province
(20171BAB213013,
2
0151BBE50006)， the Education Department of Jiangxi
2
Hydrogen temperature programmed reduction (H -TPR)
experiments were carried out on a FINESORB 3010C system equipped
with a thermal conductivity detector (TCD). 10 mg catalyst was loaded
and pretreated in a high purity N stream at 120 ℃ for 0.5 h to remove
2
any surface impurities. After that the hydrogen temperature-programmed
reduction was started from room temperature to 800 °C at a rate of
Province (GJJ150016, GJJ150085, KJLD14005), and the
Graduate Student Creativity Funding of Jiangxi Province
(
YC2016-S004) which are greatly acknowledged by the authors..
1
2 2
0 °C/min in a 30 mL/min 10% H /Ar gas mixture flow. The H
Keywords: oxidative coupling of methane • alkali metal oxides •
consumption was quantified by using CuO (99.99%) as the calibration
standard sample.
SnO
2
catalysts • electrophilic oxygen sites • intermediate alkaline
sites
2 2
CO temperature programmed desorption (CO -TPD) was carried
out with a micromeritics Auto Chem 2920 apparatus. Typically, 50 mg
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the sample was heated to 400 C and kept for 30 min to remove any
o
possible impurities. Afterwards, the sample was cooled down to 50
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European Journal of Inorganic Chemistry
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European Journal of Inorganic Chemistry
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Lithium oxide boosts: Due
Liang Peng,[a ] Junwei
Xu,[a] Xiuzhong Fang,[a]
Wenming Liu,[a] Xianglan
Xu,[a] Liang Liu,[a]
Zhongchen Li,[a] Honggen
Peng,[a] Renyang Zheng,
[b] Xiang Wang*[a]
to the balance between SnO
2
and Li SnO phases, both the
2
3
amounts of surface active
oxygen and alkaline sites
have been increased, thus
improving significantly the
low temperature activity and
C
2
product yield for methane
Page No. – Page No.
oxidative coupling.
2
SnO -based catalysts
with superior low
temperature performance
for oxidative coupling of
methane: insight into the
promotional effects of
alkali metal oxides
1
2
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