Constructing La2B2O7 (B = Ti, Zr, Ce) Compounds with Three Typical Crystalline Phases for the Oxidative Coupling of Methane: The Effect of Phase Structures, Superoxide Anions, and Alkalinity on the Reactivity

Article abstract of DOI:10.1021/acscatal.9b00022

To probe the phase structure-reactivity relationship of A₂B₂O₇ catalysts for the oxidative coupling of methane (OCM), three model La₂B₂O₇ compounds with Ti⁴⁺, Zr⁴⁺, or Ce⁴⁺ at the B-site have been purposely designed. By decreasing the r_A/r_B ratios in the order of La₂Ti₂O₇ > La₂Zr₂O₇ > La₂Ce₂O₇, typical monoclinic layered perovskite, cubic ordered pyrochlore, and disordered defective cubic fluorite phase are formed, respectively. The reaction performance of the catalysts based on CH₄ conversion and C₂ product yield follow the order of La₂Ce₂O₇ > La₂Zr₂O₇ > La₂Ti₂O₇. It has been discovered that superoxide O₂^- is the active oxygen species detected on all the catalysts and is responsible for the OCM reaction, whose amount follows also the sequence of La₂Ce₂O₇ > La₂Zr₂O₇ > La₂Ti₂O₇. Moreover, the surface alkalinity related to the superoxide anions observes the same order. This testifies that the amount of surface superoxide O₂^- determines the OCM reaction performance over the La₂B₂O₇ compounds. On the basis of the characterization results, the formation of active O₂^- species could follow two pathways. For La₂Zr₂O₇ and La₂Ce₂O₇ possessing intrinsic 8a oxygen vacancies, O₂^- anions are formed by activating the oxygen species entering into the vacancies in the bulk and then migrating to the catalyst surface. For La₂Ti₂O₇ possessing no oxygen vacancies, they are formed directly by transforming the O₂ molecules adsorbed on its surface. Usually, the former pathway generates more abundant O₂^- species than the latter one. La₂Ce₂O₇ displays not only promising reaction performance in the lowerature region, but also potent sulfur and lead poisoning resistance, thus having the potential for application after further optimization.

Full text of DOI:10.1021/acscatal.9b00022

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Article
Constructing La2B2O7 (B = Ti, Zr, Ce) Compounds with Three Typical
Crystalline Phases for Oxidative Coupling of Methane: the Effect of
Phase Structures, Super Oxide Anions and Alkalinity on the Reactivity
Junwei Xu, Yan Zhang, Xianglan Xu, Xiuzhong Fang, Rong Xi, Yameng Liu, Renyang Zheng, and Xiang Wang
ACS Catal., Just Accepted Manuscript • Publication Date (Web): 26 Mar 2019
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Constructing La B O (B = Ti, Zr, Ce) Compounds
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with Three Typical Crystalline Phases for Oxidative
Coupling of Methane: the Effect of Phase Structures,
Super Oxide Anions and Alkalinity on the Reactivity
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Junwei Xu , Yan Zhang , Xianglan Xu , Xiuzhong Fang , Rong Xi , Yameng Liu ,
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Renyang Zheng , Xiang Wang *
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Key Laboratory of Jiangxi Province for Environment and Energy Catalysis, College of
Chemistry, Nanchang University, Nanchang, 330031, PR China
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Research Institute of Petroleum Processing (RIPP), SINOPEC,18 Xueyuan Road,
Haidian district, Beijing 100083, China
 Corresponding author. E-mail: xwang23@ncu.edu.cn (X. Wang)
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Abstract: To probe the phase structure-reactivity relationship of A B O catalysts for oxidative
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coupling of methane (OCM), three model La B O compounds with Ti , Zr or Ce B site have
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La Ce O , typical monoclinic layered perovskite, cubic ordered pyrochlore and disordered
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defective cubic fluorite phase are formed, respectively. The reaction performance of the catalysts
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La Ti O . It has been discovered that superoxide O is the active oxygen species detected on all
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the catalysts and responsible for the OCM reaction, whose amount follows also the sequence of
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anions observes the same order. This testifies that the amount of surface superoxide O determines
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results, the formation of active O species could follow two pathways. For La Zr O and
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La Ce O possessing intrinsic 8a oxygen vacancies, O anions are formed by activating
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the oxygen species entering into the vacancies in the bulk and then migrating to the
catalyst surface. For La Ti O possessing no oxygen vacancies, they are formed directly
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by transforming the O molecules adsorbed on its surface. Usually, the former pathway
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generates more abundant O species than the latter one. La Ce O displays not only
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promising reaction performance at low temperature region, but also potent sulfur and lead
poisoning resistance, thus having the potential for application after further optimization.
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Keywords: Oxidative coupling of methane, surface O anions, La B O compounds,
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different crystalline phases, structure-reactivity relationship.
1. Introduction
Oxidative coupling of methane into ethylene is an attractive route to convert natural gas
directly into value-added chemical products.1 Over past decades, people have
investigated many catalysts to achieve sufficient CH conversion and C selectivity
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simultaneously to industrialize this important resource utilization reaction. Among these
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catalysts, Li/MgO, which was first reported by Ito et al., is a representative one with
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promising performance, but it deactivates quickly due to the fast Li cation vaporization at
elevated temperatures.4, 5 Rare earth oxide-based catalysts also show good activity for
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OCM reaction, due to the existence of reactive oxygen sites generated by surface oxygen
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vacancies, but they have low C product yield because of low selectivity. At present, it
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is commonly agreed that Mn/NaWO /SiO is the most applicable catalyst, over which the
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In addition, the nature of the catalytic active sites is not yet
understood clearly due to its complex compositions. Summarizing the results over all the
studied OCM catalysts up to now, the one-way C yield is still below 30%, the minimum
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still strong incentive to understand deeper the active sites requirements and seek
catalysts with higher performance for this reaction, especially those can function at a
relatively lower temperature region.
It is commonly accepted that two typical steps are involved in an OCM reaction, which
include a heterogeneous step to activate CH molecules into gas phase CH · radicals on
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step to combine two CH · radical into an C H molecule, which is then dehydrogenated
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into an C H molecule.
In spite of some still existing controversies on the active sites
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for catalytic OCM, most of the researchers agreed that the surface active oxygen sites is
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of great importance for methane conversion and C product formation.14-16Activation of
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O molecules to generate desirable surface oxygen species plays a key role to govern
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the formation of CH · radicals and the subsequent oxidative dehydrogenation of C H .
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It was reported previously that surface electrophilic oxygen anions, e.g., O , O and O
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are very important oxygen species contributing to C product selectivity 14-16. In contrast,
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the surface O lattice oxygen usually results in deep oxidation of the products.
However, the mechanisms and pathways for the generation of surface active electrophilic
oxygen species involving the cycle between gas phase O and surface oxygen vacancies
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have not been understood very clearly.
A B O compounds possess some unique properties, such as superior thermal stability,
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good oxygen mobility and oxygen ion conductivity etc. Therefore, they have been
investigated as functional materials for various applications such as magnetic, electrical
and optical materials.17-21 The investigation on their catalytic properties are much less
documented. Our former studies have demonstrated that A B O compounds with
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different A or/and B site compositions displayed very good performance for several high
temperature reactions, including methane reforming and OCM reaction.22-24 It has been
substantiated that an effective OCM catalyst usually possesses surface electrophilic
oxygen sites and also has certain surface alkalinity.15, 16, 25, 26 A typical A B O compound
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containing trivalent rare earth A site can provide surface alkalinity, which matches mostly
the requirement of a good OCM catalyst, together with its mobile oxygen species.
Therefore, our former work has testified that a La Ce O catalyst displays very good
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performance for OCM reaction, especially at low temperature region. The study on using
A B O compounds for OCM can be traced back to 1980s but with very limited
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publications.27-31 Although people have correlated their catalytic performance to the
substitution of A sites, the change of B-O bond strength and the deficiency in B sites, the
OCM reaction performance and phase structure relationship has not yet been
investigated in detail and elucidated disntinctly.28, 30, 31
It has been proved that the phase structure of A B O oxides can be altered by adjusting
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the r /r radius ratio. In detail, if the ratio is over 1.78, monoclinic layered perovskite
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crystalline phase (space group P2 ) will be formed; If the ratio is between 1.46 and 1.78,
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a stably ordered face-centered cubic pyrochlore phase (space group Fd3m) will be
formed;17, 22, 32 and if the ratio is below 1.46, a disordered cubic defective-fluorite phase
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space group Fm3m) will be formed.17, 22, 32 In the lattice matrix of the latter two phases,
intrinsic 8a oxygen vacancies are present, which can enhance the oxygen migration in
the structure evidently and is believed to be beneficial to the OCM reaction.25, 26
Aiming to understand the above mentioned fundamental issues and seek for improved
catalysts more applicable in industry, we have purposely designed and prepared three
model La B O catalysts in this work with three typical crystalline phases by changing the
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B site cations (Ti , Zr and Ce ) but fixing the La cation A site. It has been proved that
La B O catalysts possessing a typical monoclinic layered perovskite (La Ti O ), cubic
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pyrochlore (La Zr O ) or disordered defective cubic fluorite (La Ce O ) phase has been
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successfully synthesized. The OCM reaction performance over the samples follows the
sequence of La Ce O > La Zr O > La Ti O . By using various characterization methods,
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the effects of phase structure change on the surface property and reactivity have been
discussed and correlated.
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. Experimental
2.1 Catalyst synthesis
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A series of La B O catalysts with varied B site cations (Ti , Zr and Ce ) were
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synthesized by a citric acid sol-gel method, using La(NO ) .6H O (99.99%), Ti(OC H )
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(99.9%), Zr(NO ) .5H O (99.99%), Ce(NO ) .6H O (99.99%) and citric acid (99.8%) as
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the precursors. While La(NO ) .6H O, Zr(NO ) .5H O and Ce(NO ) .6H O were
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dissolved directly into a certain amount of DDI water directly to get stable solutions,
Ti(OC H ) was dissolved first in an appropriate quantity of nitric acid aqueous solution to
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prevent the hydroxylation of Ti , which can generate Ti(OH) precipitation. Afterwards,
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the stoichiometric molar ratio of B site salts: La(NO ) .6H O: citric acid = 1:1:1.2 were
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mixed into uniform aqueous solutions and then stirred completely till clear solutions were
formed. NH .H O solution (22 ~ 25 wt%) was then dripped slowly to adjust their pH around
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until viscous gels were formed. The gels were aged for 12 h at 130 C, followed by
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final La B O (B= Ti, Zr and Ce) catalysts. The catalysts are named La Ce O , La Zr O
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and La Ti O according to the element compositions.
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2.2. Reaction performance evaluation
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OCM reaction was used to test the reactivity of the three model La B O (Ti , Zr and
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Ce ) catalysts. The reaction was carried out in a fixed-bed straight quartz tube reactor
having a length of 300 mm and an internal diameter of 10 mm at 1 atm. Since OCM is a
strong exothermic reaction, to avoid any hot-spot effects, typically, 200 mg catalyst was mixed
with the same weight of quartz sand before loading into the reactor, and the reaction
temperature was controlled by inserting the thermal couple into the reactor, with the head
touching the catalyst bed. The flow rates of the CH , O reactants and N balance gas
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were controlled separately by three mass flow controllers to get a mixed flow with a CH4
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O : N volume ratio of 4:1:5. The total rate of the flow is 60 mL min , resulting in a
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WHSV of 18000 mL h g . The reaction generally starts from 550 C with a 50 C gap
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until 800 C for the reactivity tests. To get steady state kinetic data, all measurements
were taken after 1 hour’s stabilizing at a certain temperature. The OCM products were
monitored by two online GC, with a GC9310 equipped with a TDX-01 column and a TCD
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to analyze CO, CO and CH ; and a GC9310II equipped with a Propak Q column and a
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The methane conversion (XCH4), C selectivity (S ) and C yield (Y ) were quantified
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by using the standard normalization method on the basis of carbon balance, which are
defined in the following equations:
[
2 ∗
(
C H + C H6) + 3 ∗
(
C H + C H8) + CO + CO2
]
2
4
2
3
6
3
XCH4
=
× 100%
(1)
(
CH4
)
in
2 ∗ C2H4 + C2H6
( )
Sc =
× 100%
(2)
2
( ) ( )
∗ C2H4 + C2H6 + 3 ∗ C3H6 + C3H8 + CO + CO2
2
YC2 = XCH4 × S × 100%
(3)
C2
2.3. Catalyst characterization
The bulk and surface properties of the catalysts were explored by different means to
discern the effects induced by the replacement of the B site cations. The texture
properties of the catalysts were measured with N sorption method at 77K on an
2
ASAP2020 instrument. The phase compositions of the catalysts were analyzed by XRD
on a Bruker AXS D8 Focus X-ray diffractometer system with Cu Kα radiation (40kV and
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0mA). Raman spectra were performed on a Renishaw in Via instrument with a 532 nm
semiconductor laser as excitation. The HAADF-STEM images and elemental phase
mapping were obtained by using a Tecnai F30 transmission electron microscope
1
1
1
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2
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8
9
0
equipped with an Oxford EDX detector operated at 300 keV. H -TPR was performed on
2
a FINESORB 3010C instrument equipped with a thermal conductivity detector (TCD). In
o
each experiment, 50 mg sample was calcined at 400 C in a high purity air flow for 30 min
to remove any possible impurities, and then cooled down to room temperature in an ultra-
high purity Ar flow and kept for 30 minutes. Afterwards, it was heated from room
o
o
temperature to 800 C with a rate of 10 C/min in a 10% H /Ar gas mixture flow. O -TPD
2
2
and CO -TPD were measured on a micromeritics Auto Chem 2920 apparatus equipped
2
o
with a TCD. In each experiment, 50 mg sample was calcined at 400 C in a He flow for
o
30 min to remove any possible impurities, and then cooled down to 50 C. At the same
temperature, it was exposed to an O or CO flow for 1h to saturate the surface completely.
2
2
After purged by a He flow to remove any physically absorbed O or CO for 30 min, it was
2
2
o
o
heated from room temperature to 800 C with a rate of 10 C/min in an ultra-high purity
He flow. EPR technique was performed on a JEOL FA-200 EPR Spectrometer. For the
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experiment, a sample was pretreated with 100 Torr oxygen at 450 °C for 30 min and then
cooled down to room temperature, which was followed by evacuation before collecting
0
1
2
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8
9
0
1
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8
9
0
EPR spectra at 77K in liquid N . In situ DRIFT experiments were conducted on a Bruker
2
Vertex equipped with an MCT detector and KBr windows. XPS spectra were performed
with a PerkinElmer PHI1600 system using a single MgKα X-ray source operated at 300
W and 15 kV voltage.The equipment models, operation conditions, experimental
procedures and parameters are described in detail in the accompanied supporting
information file. Depending on necessity, some of the sample treatment details are also
included in the related results and discussion section.
3. Results and discussion
3
.1La B O phase structure analysed by N sorption and XRD
2
2
7
2
N sorption has been emplyed to identify the texture properties of the La B O catalysts.
2
2
2
7
Figure 1 displays that both La Ce O and La Zr O possess a Type II isotherm and an
2
2
7
2
2
7
H3 hysteresis loop, which indicates that these two samples have similar texture structure.
However, La Ti O exhibits a Type IV isotherm and with a hysteresis loop type between
2
2
7
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H1 and H2, testifying that it has different texture property from the other two samples. The
presence of hysteresis loops indicates the mesoporous structure of the samples, which
is also confirmed by the pore size distribution profile. As listed in Table 1, all the samples
have slightly different average pore volumes and pore sizes. In contrast, the difference in
specific surface area is more distinct, among which La Ce O possesses the largest and
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
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2
2
2
3
3
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3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
2
7
La Zr O possesses the smallest surface area. This implies that the changing of B site
2
2
7
cations has some impact on the texture peroperties of the prepared catalysts. Compared
with the fresh catalysts, Figure S1 displays that the N sorption isotherms and pore size
2
distribution profiles have no essential change for all the used catalysts after high
temperature OCM reaction. However, the surface areas have some decrease, the
average pore volumes have nearly no change and the average pore sizes have some
increase for all the spent samples. This implies that all the samples have relatively stable
structure and good thermal resistance due to the formation of A B O compounds with a
2
2
7
fixed formula and stable crystalline phases.
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2
2
2
2
2
2
2
2
2
3
3
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3
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3
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2
1
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1
1
00
80
60
40
20
00
0.008
(b)
La Ti O
2 2 7
(
a)
0
.007
.006
La Zr O
7
2
2
0
La Ce O
7
2
2
La Ce O
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
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9
0
1
2
3
4
5
6
7
8
9
0
2
2
7
0.005
0.004
0.003
0.002
0.001
0.000
8
0 La Zr O
2 2 7
60
40
20 La Ti O
7
2
2
-0.001
0
0.0
0.2
0.4
0.6
0.8
1.0
0
20
40
60
80
100
Relative pressure(P/P0)
Pore size(nm)
Figure 1 N sorption measurement of the fresh catalysts (a) isotherms, (b) pore size
2
distribution profiles.
Table 1 Physical chemical properties of the fresh and spent La B O catalysts
2
2
7
Catalysts
BET surface area
Pore volume
Pore size
(nm)
2
-1
3
-1
(m g )
(cm g )
fresh
0.09
fresh
16.3
9.7
spent
10.6
7.4
spent
0.08
0.06
0.13
fresh
15.2
spent
17.6
22.5
26.1
La Ti O
2
2
7
La Zr O
0.06
16.9
2
2
7
La Ce O
22.5
16.8
0.11
16.4
2
2
7
The phase compositions of the La B O catalysts were identified by XRD. As presented
2
2
7
in Figure 2, all the fresh samples display intensive diffraction peaks, indicating they are
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well crystallized. After careful analysis, it is concluded that no diffraction features relating
to the individual metal oxides, such as La O , TiO , ZrO and CeO , are detected,
2
3
2
2
2
1
1
1
1
1
1
1
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0
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9
0
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9
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9
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8
9
0
o
testifying that complete solid reaction occurred during the calcination process at 800 C.
As depicted in Figure S2(a), XRD patterns of the used catalysts display no evident change
in comparison with that of the corresponding fresh samples, indicating these La B O
2
2
7
compounds are very stable for the high temperature OCM reaction.
As discussed in the introduction section, the bulk phase structures of A B O
2
2
7
32
compounds are governed by the r /r radius ratios. For all the La B O synthesized here,
A
B
2
2
7
the radius of the 8-fold coordinated A site La3+ is 1.16 Å and the radii of the 6-fold
4+
4+
4+
coordinated B site Ti , Zr and Ce are 0.61, 0.72 and 0.87 Å, respectively. Hence, the
r /r ratios of La Ti O , La Zr O and La Ce O are 1.90, 1.61 and 1.33. According to the
A
B
2
2
7
2
2
7
2
2
2
7
3
phase formation rule discussed above, monoclinic layered perovskite, cubic pyrochlore
and disordered defective cubic fluorite phase will be theoretically generated for La Ti O ,
2
2
7
La Zr O and La Ce O in sequence. Indeed, the diffraction features of the samples
2
2
7
2
2
7
matches well with the standard JCPDS cards of monoclinic layered perovskite La Ti O
2
2
7
(JCPDS #28-0517), cubic pyrochlore (JCPDS #71-2363) and disordered defective cubic
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fluorite (JCPDS #04-12-6396), proving that the desired model catalysts with typical phase
compositions have been successfully synthesized. XPS results in Figure S3 show that La
in all the samples is in trivalent state, Ti, Zr and Ce in the related samples are
predominantly in tetravalent state, which testifies indirectly the formation of La B O
0
1
2
3
4
5
6
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9
0
1
2
3
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9
0
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compounds possessing varied crystalline phases for all the catalysts.
2
2
1
1
500
000
500
000
3000
(
a) La Ti O
2
2
7
(b) La Zr O
2 2 7
2500
2000
1
500
000
500
0
1
5
00
0
JCPDS #28-0517
70 80
JCPDS #71-2363
10
20
30
40
2
50
60
10 20
30
40
2  ()
50
60
70
80
 ()
2500
2000
1500
1000
(
c) La Ce O
2 2 7
500
0
JCPDS #04-12-6396
20 30
1
0
40
2
50
60
70
80
 ()
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Figure 2 XRD patterns of the freshly calcined La B O catalysts. (a) La Ti O , (b)
2
2
7
2
2
7
La Zr O , (c) La Ce O .
2
2
7
2
2
7
1
1
1
1
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1
1
1
1
2
2
2
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2
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2
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9
0
To further confirm the phase structure, XRD Rietveld refinement method is used to
identify accurately the lattice parameters of the samples in Table 2, with the refinement
patterns depicted in Figure S4. As demonstrated by the three same side lengths of 5.573
o
Å and intersection angles of 90 , La Ce O has obviously a cubic unit cell similar to cubic
2
2
7
fluorite CeO2,33-35 which matches its disordered defective cubic fluorite phase composition.
In contrast, La Zr O has doubled side lengths of 10.746 Å but has still three intersection
2
2
7
angles of 90 , which matches the typical characteristics of a cubic pyrocholre phase.17
o
However, La Ti O possesses three different side lengths and a special β intersection
2
2
7
o
angle of 98.6 , which are the typical lattice parameters for a monoclinic layered perovskite
36
unit cell. In summary, the XRD Rietveld refinement results are in good accordance to
the phase analysis results, indicating that the phase structure of La B O compounds can
2
2
7
be strongly influenced by changing the metal cations at B site.
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1
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0
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Table 2 Lattice proprieties of the fresh La B O catalysts obtained by XRD Rietveld
2
2
7
refinement.
Lattice parameter
Catalyst
s
r /r
Crystallit Crystallite
A
B
a (Å)
b (Å)
c (Å)
α (o)
β (o)
γ (o)
e sizes a
ratio
phase
(nm)
Monoclini
c layered
perovskite
Cubic
La Ti O
1.90
12.971
5.524
7.787
90.0
98.6
90.0
17.4
2
2
7
La Zr O
2
2
1
.61
.33
10.746 10.746 10.746
90.0
90.0
90.0
90.0
90.0
90.0
20.2
16.1
pyrochlor
7
e
Disordered
defective
cubic
La Ce
2
2
1
5.573
5.573
5.573
O7
fluorite
a Calculated by Scherrer equation from the XRD patterns
3
.2 Structure identification of the catalysts by Raman and STEM Mapping
To discern the structure change of the La B O compounds by B site replacement, the
2
2
7
catalysts have been further studied by Raman, with the spectra depicted in Figure 3(a).
Previous studies have indicated that Raman spectroscopy is very effective to provide
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explicit information to distinguish A B O compounds with a monoclinic layered perovskite,
2
2
7
cubic pyrocholre and disordered defective cubic fluorite phase.37
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
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4
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5
5
5
5
5
6
0
1
2
3
4
5
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9
0
1
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1
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9
0
1
2
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0
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9
0
As formerly reported, La Ti O shows theoretically 37 Raman bands due to the phonon
2
2
7
36
modes of multiple La-O and Ti-O vibrations. The peaks at 342, 371, 405, 430, 448, 520,
5
41, 559, 609, 793 cm-1 correspond to the typical monoclinic layered perovskite
38
-1
crystalline structure. The bands at 154, 244 and 342 cm are most likely due to La-O
bond vibration. The stretching of octahedral O-Ti-O bonds can be found at 445 (E mode),
g
-1
-1
610 and 793 cm . The peaks at 130 (B mode) and 820 (B mode) cm are similar to
1g
2g
the Ti-O bond stretching vibration for the six-fold coordinated Ti in rutile TiO2.39 The
detection of these fine Raman peaks has confirmed the formation of monoclinic layered
perovskite phase in La Ti O catalyst.
2
2
7
Based on group theory, an ordered pyrochlore has typically 6 active Raman modes
40, 41
-1
distributed as A + E + 4F .
The most intensive peak at 300 cm can be ascribed
1g
g
2g
-1
to the vibrations of B-O bond in E mode. The peak at 500 cm can be assigned to the
g
A mode relating to the O-B-O bending vibrations in octahedral BO unit. The four F
1g
6
2g
-1
modes at 395, 520, 601 cm are most likely due to the mixed B-O and A-O bond
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stretching with O-B-O bending vibrations.22 The evident observation of all the active
Raman modes for La Zr O strongly testifies the existence of a well ordered pyrochlore
2
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phase in its bulk structure.
-1
For La Ce O catalyst, the Raman band at 453 cm is assigned to the F mode of cubic
2
2
7
2g
-1
fluorite structure, and the band at 576 cm is ascribed to the surface oxygen vacancies,
which is generated by the random distribution of La3+ and Ce4+ cations in the lattice
structure.35, 36 The weak band at 256 cm could be assigned to the disordering of oxygen
ions in the sublattices.35
-1
In summary, the Raman results support the XRD analysis, proving the successful
preparation of the desired model catalysts with the three typical crystalline phases for
A B O compounds by varying the B site cations. It is mentioned here that Figure S2(b)
2
2
7
proves that the Raman spectra of the spent catalysts have no any evident difference from
that of the corresponding fresh samples, confirming that these La B O catalysts are very
2
2
7
stable with potent heat resistance during the OCM reaction process operating at high
temperatures.
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As discussed above, surface electrophilic oxygen species could be in different forms
-
and are indispensible for OCM reaction. For instance, O species is observed on the
1
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surface of Li/MgO and Na/CaO catalysts;42 O superoxide species is detected on the
-
2
43, 44
and O22- peroxide species is observed on
surface of Y O -CaO and LaOF catalysts;
2
3
45-47
the surface of some lanthanide oxide-based catalysts such as La O and Sm O .
All
2
3
2
3
these surface oxygen species in different forms are proposed to be important active and
selective sites for OCM reaction.
100000
2000
(b)
1075
(a)
Oxygen vacancy
F2g
576
453
8
6
4
0000
0000
0000
1
500
La Ce O
7
2
2
Eg
1186
1
078
1360
1320
302
1000
A1g F2g
520 500
396
La Ce O
2 2 7
F2g
F2g
596
La Zr O
7
2
2
1295
1170
La Zr O
7
2
2
1065
20000
B2g
B1g
130
5
00
793
244
154
1116
820
A1g 559 540
1184
E
405371 342
1290
g
1310
520
610
445430
La Ti O
7
2
2
0
La Ti O
7
2
2
0
1000 900 800
700 600
Raman shift (cm )
500 400
300 200
100
1400 1350 1300 1250 1200 1150 1100 1050 1000
-1
-1
Raman shift (cm )
Figure 3 (a) Raman spectra of the freshly calcined La B O catalysts, (b) Magnified
2
2
7
-1
Raman spectra from 1400 to 1000 cm .
To identify the surface active oxygen species clearly, the Raman spectra of the three
o
catalysts are characterized. In detail, the catalysts were first heated to 450 C and kept
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for 30 min to remove any possible impurities in a 30 ml min He flow, which were then
-1
exposed to a 30 ml min 5% O /Ar gas mixture flow for 30 min to saturate the surface
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completely at the same temperature. Afterwards, the samples were cooled to the room
-1
temperature in a He flow and analyzed by Raman. The shift region of 1400-1000 cm are
particularly enlarged and displayed in Figure 3(b). Several peaks attributed to the surface
adsorbed dioxygen species can be detected for all the samples. For La Ti O , the signals
2
2
7
-1
-
at 1065, 1116 and 1184 cm can be attributed to O species, and the signals at 1290
2
and 1310 cm could be attributed to the surface CO species.48, 49 For La Zr O , the
-1
2-
3
2
2
7
-1
-
signals at 1078 and 1170 cm can be ascribed to O species, and the signals at 1295
2
cm could be assigned to the surface CO species.48, 49 For La Ce O , the signals at
-1
2-
3
2
2
7
-1
-
1
075 and 1186 cm can be attributed to O species, and the signals at 1360 and 1320
2
cm could be assigned to the surface CO species.48, 49 Therefore, on all the three
-1
2-
3
-
catalysts, surface superoxide O anions have been detcted.
2
It was formerly reported that O22- peroxide species showed Raman bands at ~750 cm-
1,
~800 cm and ~900 cm .
-1
-1 48, 49
Apparently, no any Raman band matchs O22- species
is observed for all the three catalysts. On the basis of Raman results, it is proposed that
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-
superoxide O is the only type of electrophillic active oxygen sites present on the surfaces
2
of all the three catalysts, which plays the crucial role for the OCM reaction.
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2 2 7
(b) La Zr O
(a) La
2
Ti
2
O
7
La
La
5
0 nm
50 nm
Zr
Ti
O
O
(c) La
2
Ce
2
O
7
La
200 nm
Ce
O
Figure 4 HAADF STEM mapping images of (a) La Ti O , (b) La Zr O and (c) La Ce O .
2
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7
2
2
7
2
2
7
To identify the element distribution in the three catalysts with varied B sites, HAADF-
STEM mapping images have been obtained and displayed in Figure 4. Obviously, the
images have demonstrated that Ti, Zr or Ce element in the corresponding samples
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distributes very homogeneously with La and O elements, testifying strongly the formation
of uniform and chemically stable A B O composite oxides in the bulk of all the samples.
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The particle morphology and size distribution of the catalysts were investigated by TEM
technique. As shown in Figure S5, the mean particle sizes of the catalysts are 24.7, 32.8
and 21.2 nm for La Ti O , La Zr O and La Ce O , respectively. In comparison with the
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crystallite sizes of the samples measured by XRD in Table 2, the average particle sizes
are a little larger, indicating that slight aggregation occurred for all the samples.
3.3Reaction performance tests of the catalysts
Figure 5 displays the reaction performance on the catalysts. By increasing the
o
temperature from 550 to 800 C, CH conversion, C product selectivity and yield follows
4
2
the order of La Ce O > La Zr O > La Ti O . Apparently, La Ce O possesses the best
2
2
7
2
2
7
2
2
7
2
2
7
OCM reaction performance in all the samples, over which 17.2% C yield can even be
2
o
gotten at 800 C. In contrast, La Ti O exhibits the lowest performance, on which only 6.0%
2
2
7
C yield is achieved at the same temperature.
2
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To better understand the reaction behaviors, the C H selectivity over all the catalysts
2
4
is compared particularly in Figure 5(d). Interestingly, the selectivity also follows the order
of La Ce O > La Zr O > >La Ti O at different temperatures, substantiating that the
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surface active site composition of La Ce O and La Zr O is much more favorable than
2
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2
7
that of La Ti O for the direct production of valuable ethylene, which comes from the
2
2
7
11, 25
dehydrogenation of the initially formed C H .
2
6
In order to estimate the contribution of the B site metal oxides to the OCM reaction, the
reactivity of the individual TiO , ZrO and CeO , which were also prepared by the same
2
2
2
o
sol-gel method and calcined at 800 C, have been tested. As depicted in Figure S6, XRD
and Raman results have proven that pure phase rutile TiO , baddeleyite ZrO and cubic
2
2
fluorite CeO have been successfully achieved, but with very low surface area (Table S1)
2
due to high temperature calcination. Figure S7 demonstrates that all the pure oxides
display extremely low CH conversion, C product selectivity and yield. This strongly
4
2
testifies that without forming La B O compounds, all the individual B site metal oxides
2
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7
4+
4+
4+
possess low reaction performance for OCM. In other words, only if Ti , Zr and Ce
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cations combine with La to form La B O compounds, active OCM catalysts can then
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La Zr O
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La Ce O
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La Ti O
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La Zr O
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La Ti O
7
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2
5
50
600
650
700
750
800
550
600
650
700
750
800
(d)
o
o
Temperature ( C)
Temperature ( C)
30
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10
0
50
40
La Ti O
7
(c)
2
2
La Zr O
7
2
2
La Ce O
7
2
2
30
20
10
0
La Ce O
7
2
2
La Zr O
7
2
2
La Ti O
7
2
2
5
50
600
650
700
750
800
550
600
650
700
750
800
o
o
Temperature ( C)
Temperature ( C)
Figure 5 The OCM reaction performance over the La B O catalysts. (a) CH
2
2
7
4
conversion, (b) C selectivity, (c) C yield, (d) C H selectivity. Reaction conditions:
2
2
2
4
-1
-1
0
.200g catalysts under 60 mL min CH : O : N =4:1:5 flow rate, WHSV=18000 mL h
4
2
2
-
1
gcat .
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Sulfur and lead compounds are often found at low percentage in natural gas, which
are well known poisoning components for many catalysts. Since La Ce O catalyst shows
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o
good OCM performance at low temperature region (below 750 C) and has certain
application potential, its resistence to these poisoning agents has been tested. Figure
6
(a) displays the sulfur tolerance test of La Ce O . In detail, the catalyst was kept first at
2
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7
o
7
50 C for 5 hours in the typical oxidative coupling feed adopted in this study, which
exhibited stable CH conversion, C product selectivity and yield. Afterwards, 100 ppm
4
2
SO was introduced into the reaction flow, which decreases the reaction performance
2
slightly. In addition, during the continued ~35 hours‘ test, no futher performance dropping
was observed. After removing SO from the feed, the CH conversion, C product
2
4
2
selectivity and yield can be restored completely. This indicates that SO molecules
2
adsorbed reversibly on La Ce O surface and occupied only a small amount of active
2
2
7
sites for the reaction, which can be desorbed easily when SO is removed from the
2
reaction feed, and poses no any permanent damage to the catalyst. Generally, individual
La and Ce oxides can react with sulfur oxides at high temperatures to form sulfates or
sulfites.50, 51 It is apparent that after forming La Ce O compound, the La and Ce cations
2
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7
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have been protected by the potent and stable La B O lattice, thus La Ce O showing
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potent sulfur resistance during high temperature OCM reaction.
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Figure 6(b) shows the lead poisoning test of the catalyst. Prior to the experiment, a
certain amount of La Ce O was mixed with Pb(NO ) acqueous solution, followed by
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3 2
o
o
drying at 120 C and calcining at 800 C to get the final sample polluted by ~1% PbO. By
o
increasing the reaction temperature from 550 to 750 C, it is observed the CH conversion,
4
C product selectivity and yield have only negligible decrease (dashed lines),
2
demonstrating that this catalysts possesses also potent resistance to lead deactivation.
1
00
100
80
60
40
20
0
CH conversion
(a)
CH conversion
(b)
4
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C selectivity
C selectivity
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0
0
C yield
C yield
2
2
SO on
SO off
2
2
6
40
2
0
0
0
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55
550
600
650
700
750
o
Time on stream (h)
Temperature ( C)
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Figure 6 Poisoning tests of La Ce O catalyst. (a) Sulfur poisoning test at 750 C with
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1
00 ppm SO , (b) Lead poisoning test. Dashed line: catalyst polluted by 1% PbO; Solid
2
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line: fresh catalyst.
3.4 H -TPR study on the redox property
2
H -TPR was adopted to probe the redox behaviours of the freshly calcined La B O
2
2
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7
samples. Figure 7 displays that the three La B O catalysts show similar reduction
2
2
7
behaviours but also have some differences. La Ti O displays one broadened reduction
2
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o
o
peaks at ~ 342 C and La Zr O exhibits one reduction peak at ~ 345 C. La Ce O shows
2
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o
two wide reduction peaks at ~ 346 and ~ 538 C.
Theoretically, if all the lattice oxygen of an A B O compound can be reduced
2
2
7
completely, the O/(A+B) molar ratio is 1.75. However, the quantified O/(La+B) ratios of all
the samples in Table 3 are below 0.1, much less than 1.75, substantiating that only a
small quantity of active surface oxygen can be reduced for the catalyst. Our former study
has shown that the originally reducible Sn-O bonds in pure SnO become almost non-
2
o
24
reducible below 800 C when La Sn O pyrochlore compound is formed. The reason
2
2
7
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