DSpace at VNU: 3DOM-LaSrCoFeO6-delta as a highly active catalyst for the thermal and photothermal reduction of CO2 with (HO)-O-2 to CH4

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3DOM-LaSrCoFeO6-δ as a highly active catalyst for thermal and
photothermal reduction of CO2 with H2O to CH4
Published on 21 July 2016. Downloaded by LA TROBE UNIVERSITY on 29/07/2016 02:21:59.

Minh Ngoc Haa,b,c, Guanzhong Lu*a,b, Zhifu Liub, Lichao Wangb and Zhe Zhao*b

Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/

The double perovskite LaSrCoFeO6-δ (LSCF) and LaSrCoFeO6-δ with three-dimensionally ordered macroporous structure
(3DOM-LSCF) were successfully synthesized by a facile combustion process. The crystal structure, morphology, BET surface
area, band gap and catalytic properties were characterized in details. Phase pure of the double perovskite LSCF and 3DOMLSCF can be obtained by calcination at 550-950 oC for 4 h. The ordered and interconnected pore structure generated by

PMMA template can be remained successfully in the 3DOM-LSCF catalyst. Both catalysts had good catalytic performance in
either CH4 selectivity and total yield. Production of CH4 from CO2 and H2O can reach 351.32 µmol g-1 for LSCF and 557.88
µmol g-1 for 3DOM-LSCF under photothermal (350 oC + Vis-light) in 8 h. The high solar-to-methane (STM) energy conversion
efficiency was 1.217% of LSCF and 1.933% of 3DOM-LSCF under photothermal mode. The results also show that the yield of
CH4 in photothermal mode is 5 times of that in thermal reduction. The double perovskite LSCF and 3DOM-LSCF are promising
photothermal catalytic materials for CO2 reduction to hydrocarbon fuels.

Introduction
The rapid development of the industry has been accompanied
by increasing concentrations of atmospheric pollutants. Global
warming caused by emissions of greenhouse gases such as
carbon dioxide (CO2), chlorofluorocarbons (CFCs), and nitrous
oxide (N2O) to the atmosphere, is widely regarded as one of the
most severe environmental issues of recent years. The
atmospheric concentration of CO2 has gradually increased
mainly owing to human activities.1 Beside, thermal pollution is
also the most current pollution and it is a result of large-scale
industrialization. The extremely large amounts of these waste
heat will be useful if they can be harvested and used for
sustainable energy generation. In addition, as we know solar
energy can be used not only for thermal power generation, but
also for chemical manufacture.2 The discovery of new costeffective and highly active catalysts for directly energy
conversion using solar energy, transforming CO2 and heat
emission into hydrocarbon fuels and storage is of prime
importance to address climate change challenges and develop
storage options for renewable energies.

a. Key

Laboratory for Advanced Materials, Research Institute of Industrial Catalysis,

East China University of Science and Technology, Shanghai 200237, China. Email:

b. School of Materials Science and Engineering, Shanghai Institute of Technology,
Shanghai 201418, China. E-mail:
c. Faculty of Chemistry, Hanoi University of Science, Vietnam National University,
Hanoi 10000, Vietnam
Electronic Supplementary Information (ESI) available: [Experimental details and
catalytic measurements]. See DOI: 10.1039/x0xx00000x

Photocatalysis, as an efficient, green, and promising solution
to the current energy crisis and environmental deterioration,
has attracted considerable interest. In general, the
photocatalytic reduction of CO2 is a possible avenue to convert
CO2 into hydrocarbon fuels, because reducing the amount of
CO2 will not only meet the purpose of environmental protection
but also provide raw materials for chemical industry. Since
Halmann discovered the photoelectrochemical reduction of CO2
into organic compounds in 19783 and Hiroshi and co-worker
reported that the photocatalytic reduction of CO 2 into organic
compounds over suspending semiconductor particles in water, 4
a growing interest in the development of semiconductor
photocatalyst has evolved. The present invention combines
photo, thermal, electric and chemical processes to develop a
new method, maximizing the efficiency and the conversion rate
of thermal radiation to chemical potential, in the form of CO2
reduction to CO, C and O2 and H2O reduction to H2 and O2 in the
same system. The dissociation of CO2 and H2O may occur in the
same system simultaneously or either one of them can be
performed alone. Photothermal combines photo and thermal
reaction conditions in one way to reduction of CO2 with H2O

vapor to CH4 had more attention.2,20 In our previous study have
shown that the photothermal process has improved catalytic
performance better than reduction of CO2 with H2O vapor to
CH4 under thermal only.20 The photothermal process was good
to be combined advantages of photochemical and
thermochemical catalytic, while it promoted and supported
together in the one reaction system to provide high efficiency
and reaction rate.
To date, many kinds of photocatalyst have been investigated
to catalyze the CO2 reduction.3,5-9 For heterogeneous
photocatalyst, many efforts still focus on TiO 2-based6,8,10,11

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materials while other catalysts such as SrTiO 3,12 Zn2GeO4,13
ZnGa2O4,14 CaFe2O4,15 ALa4Ti4O15 (A = Ca, Sr, and Ba),16
NiO/InTaO4,17 and BiVO4,18 ZnO@Cu-Zn-Al,19 WO3,20 NaNbO321
and so forth have also been reported. Among them, perovskite
oxides with general formula ABO3 possess unique properties,
such as metal-insulator transition, spin blockade, colossal
magnetoresistance, ferroelectricity, and superconductivity,
which make them attractive in technological applications such
as electrocatalysis, catalysis, sensor devices, magnetoresistance
devices, and spintronics.22-24 Perovskite-type La1-xSrxCo1-yFeyO3δ oxides with mixed electronic and ionic conductivities are
known mainly as good candidates for cathode materials used in
solid oxide fuel cells25 and for membrane materials with high
oxygen permeability as well as phase/chemical stability. 26
Excellent catalytic properties of La1-xSrxCo1-yFeyO3-δ, as powders
intended for membrane reactors, were found for partial
oxidation of natural gas.25 It is also highly efficient catalyst
towards methane and propane combustion, 27 toluene
combustion28 and methanol decomposition to CO and H 2,29 VOC
combustion,30 catalysts in automobile exhaust systems, and as
gas sensors.31 La1-xSrxCo1-yFeyO3-δ perovskite possess oxygen
vacancies,32-35 which may act as Lewis acid sites necessary for
the reaction of phenol catalytic alkylation.36 In addition, double
perovskite oxides with a general formula AA’BB’O6 or A2BB’O6
(where A and A’ are alkaline-earth and/or rare-earth metals and
B and B’ are transition metals) have been widely investigated
for their catalytic, magnetic, dielectric properties and colossal
magnetoresistance (CMR).37,38 After the discovery of room
temperature CMR and tunnelling magnetoresistance (TMR) in

the double perovskite Sr2FeMoO6 and Sr2FeReO6,
respectively,39,40 there have been growing interests worldwide
in researching for effective methods to make double perovskite
materials.41-43 Unfortunately, the traditional methods involve in
high-temperature solid-state reactions, leading to the
destruction of pore structures and hence to low surface areas,
unfavorable for enhancement in the catalytic performance of
the obtained perovskite materials. Therefore, it is highly
desirable to develop an effective strategy for the controlled
preparation of porous perovskite materials that are high in
surface area. Recently, this problem has been solved using the
colloidal crystal templating method, by which one can create a
three-dimensionally ordered macroporous (3DOM) structure.
Perovskite-type oxides with 3DOM structure possess relatively
large surface areas, high thermal stability, and good catalytic
performance.44,45 The unique ordered macroporous structure
can provide easy mass transfer to the reactant molecules, facile
accessibility to the active sites, and convenient loading of active
components.46 Therefore, 3DOM-structured ABO3 is considered
to be one of the most promising catalytic materials. 47-49
Therefore, we report the preparation, characterization, and
comparing the catalytic properties of the double perovskite
LSCF and 3DOM-LSCF for thermal and photothermal reduction
of CO2 with H2O vapor to CH4. The aim of this work was to
investigate the effect of temperature on morphology, crystal
structure, band gap, catalytic performance and the thermal,
photothermal reaction mechanism of the double perovskite
LSCF and 3DOM-LSCF for the CO2 reduction.

In a typical experiment, the double perovskite

and
View LSCF
Article Online
DOI: 10.1039/C6TA05402A
3DOM-LSCF were prepared by a convenient
and efficient
modified combustion process. The samples were calcined in air
for 4 h at different temperatures between 550 and 950 oC. The
3DOM-LSCF catalyst with well-defined 3DOM structure could be
prepared using the PMMA template. The catalytic experiments
were carried out in a gas-closed circulation system. The volume
of the reaction system was about 150 mL. The evaluation of
catalytic activity was performed at 150, 250, 350 oC without
light (thermal) and 350 oC with visible light (photothermal), the
light source was used a 300 W Xe lamp with a UV-light filter
(λ>420 nm). Taking samples per hour and quantitative analysis
was performed on a GS-Tek (Echromtek A90) equipment with a
capillary column. The quantification of CH4 yield product was
based on the external standard and the use of calibration curve
(ESI S1).

Results and discussion
The prepared double perovskite LSCF and 3DOM-LSCF powders
were calcined in air for 4 h at different temperatures between
550 and 950 °C to investigate the evolution of crystalline
phases. X-ray diffraction patterns (XRD) for the heat-treated
double perovskite LSCF and 3DOM-LSCF powders are shown in
Fig. 1a and Fig. 1b, respectively. The diffraction peaks of two
samples are in good agreement with the standard file, which
corresponds to pure perovskite phase with a cubic system

(space group Pm-3m, Ref. Code 01-089-5720). All the
characteristic diffraction peaks, which belong to the double
perovskite LSCF and 3DOM-LSCF are observed in the patterns of
all the samples indicating that the obtained catalysts possessed
AA’BB’O6 double perovskite-type structure with disordered
cubic structure. Fig. 1a shows the XRD pattern of LSCF calcined
in air at 750 oC for 4 h with diffraction peaks at 2θ = 22.97°,
32.77°, 40.41°, 47.04°, 52.96°, 58.52°, 68.73°, 73.57°, and
78.25°, which could be perfectly indexed to the (1 0 0), (1 1 0),
(1 1 1), (2 0 0), (2 1 0), (2 1 1), (2 2 0), (3 0 0), and (3 1 0) crystal
faces of double perovskite, respectively. Fig. 1b shows the XRD
pattern of 3DOM-LSCF calcined at the same condition with
diffraction peaks at 2θ = 23.10°, 32.81°, 40.47°, 47.10°, 53.10°,
58.59°, 68.84°, 73.60°, and 78.38°, which could be perfectly
indexed to the (1 0 0), (1 1 0), (1 1 1), (2 0 0), (2 1 0), (2 1 1), (2
2 0), (3 0 0), and (3 1 0) crystal faces of double perovskite,
respectively. The XRD pattern of 3DOM-LSCF with main peak
indexed to the (1 1 0) crystal face shifted to higher angle than
peak of LSCF, it means lattice parameter of 3DOM-LSCF
decrease and diffraction peaks move to the high angle side.
Furthermore, the diffraction peaks shifted to higher angle,
higher intensity and sharper when increasing temperature,
indicating that perovskite crystal structure affected by
temperature. The crystallite size of the double perovskite LSCF
and 3DOM-LSCF also increase when increasing temperature. It
was affected the surface electronic structure, electrical
transport properties of the catalysts. The results of Rietveld
structure refinement for the double perovskite LSCF and 3DOMLSCF are summarized in Table 1. The stability of complex
perovskite structures can be well explained with the use of

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tolerance factors (t). For the materials studied here, the
tolerance factors can be determined by equation (eqn) (1):

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t

rLa  rSr
 rO
2
r r
2( Co Fe  rO )
2

(1)

where rLa, rSr, rCo, rFe and rO are the ionic radii of La, Sr, Co, Fe
and O ions, respectively.51,52 Shannon’s ionic radii52 are
frequently employed to determine the tolerance factors. Hines
et al. suggested (solely by analysis of the tolerance factor) that
the perovskite will be cubic if 0.9 < t < 1.0, and orthorhombic if
0.75 < t <0.9.53 For the double perovskite LaSrCoFeO6-δ, the
tolerance factor is 0.9785, which is at the cubic structure.
According to the XRD patterns, the double perovskite
LaSrCoFeO6-δ crystal structure is cubic and it was modelled as
Fig. 1c (super cell) and Fig.1d for one cell, with a lattice constant
of 3.86 Å. Moreover, earlier investigation of synthesis of LSCF
by solid-state reaction method indicated that the perovskite
phase was formed after calcination at 1200 °C for 6 h.50 The
products calcined at this temperature will have low porosity and
non-ideal microstructure. In this study, the double perovskite
LSCF and 3DOM-LSCF were obtained pure phase at lower
temperature and short time. Therefore, in this method
prepared perovskite using PMMA template had good phase and

high porosity promised for great catalytic performing.
The specific surface area, pore structures, and size
distributions of the double perovskite LSCF and 3DOM-LSCF are
characterized by nitrogen adsorption-desorption isotherms at
77 K on a Micrometrics ASAP 2020 HD88 system (Fig. 2). It is
seen that the two samples shown a mesoporous structure. The
nitrogen adsorption-desorption isotherms can be classified as a
type IV isotherm, typical of mesoporous materials. According to
IUPAC classification, the hysteresis loop is type H3.54 This type
of hysteresis is usually found on solids consisting of aggregates
or agglomerates of particles forming slit shaped pores, with a
non-uniform size and/or shape. The BET specific surface of the
3DOM-LSCF is 21.68 m2 g−1 higher of 2.6 times than LSCF sample
of 8.46 m2 g−1 and characteristic of mesoporous double
perovskite 3DOM-LSCF with an adsorption average BarretlJoyner-Halenda (BJH) pore width 17.50 nm and a total pore
volume of 0.095 cm3 g−1. The result also shows that the BET
surface area and the total pore volume of 3DOM-LSCF are much
higher than LSCF. The high BET surface area and large total pore
volume strongly support the fact that the 3DOM-LSCF has a
mesoporous structure. The enlarged specific surface area would
create more reaction sites to facilitate the access of reactants.
It is reasonable to believe the 3DOM-LSCF would be favorable
for the improvement of photothermal reduction of CO2 with
H2O vapor to CH4 reaction activity more than double perovskite
LSCF synthesized without PMMA template.
The morphologies of the PMMA template, the prepared
double perovskite LSCF at 750 oC and 3DOM-LSCF at different
temperatures for 4 h in air were observed by SEM. Fig. 3 (a, b)
shows the SEM images of the colloidal crystal template
assembled by PMMA microspheres. It can be seen that the

template is uniform and orderly with particles size 720 nm. In
preparing process of PMMA template, Bragg diffraction will be

occurred by orderly array in the visible wavelength
range.
A
View Article
Online
DOI: obtained
10.1039/C6TA05402A
clear color change can be observed for the
colloidal
crystal template while changing the viewing angle, which is the
typical diffraction behaviour of an orderly array. The
appearance of Bragg diffraction phenomenon also
demonstrates the good order and uniform assembly of the
template.

Fig. 1 XRD patterns of double perovskite (a) LSCF, (b) 3DOMLSCF prepared at different temperatures for 4 h in air, (c) double
perovskite LaSrCoFeO6-δ crystal structure and (d) one cell
structure.
Table 1 The results of Rietveld structure refinement for the
double perovskite LSCF and 3DOM-LSCF at different
temperatures.
Catalyst
LSCF-550
LSCF-650
LSCF-750
LSCF-850
LSCF-950

3DOM-LSCF-550
3DOM-LSCF-650
3DOM-LSCF-750
3DOM-LSCF-850
3DOM-LSCF-950

2-Theta
(degree)

d110
(nm)

Unit cell*
(a, b, c, nm)

Crystallite
size (nm)

32.751
32.959
32.773
32.84
32.912
32.697
32.773
32.807
32.761
32.971

0.27327

0.27285
0.27304
0.27252
0.27192
0.27366
0.27304
0.27277
0.27314
0.27145

0.38646
0.38588
0.38602
0.38524
0.38470
0.38672
0.38589
0.38555
0.38614
0.38479

17.298
18.386
20.063
43.297
54.539
15.866
20.107
25.792
31.321

41.775

* Rietveld structure refinement for LaSrCoFeO6-δ (JCPDF 01-089-5720): Cubic, Pm-3m
The synthesis route of double perovskite 3DOM-LSCF is
shown schematically in Scheme 1 and details can be found in ESI
S1. SEM images of the double perovskite LSCF and 3DOM-LSCF
are shown in Fig. 3. As shown in Fig. 3, all samples had high
porous, agglomerated structures with an estimated particle size
between 50 and 120 nm, the small particle sizes might lead to
the higher catalytic activity for CO2 conversion. From the SEM
images of Fig. 3 (e-m), it could be seen that all the 3DOM-LSCF
catalysts obtained by colloidal crystal template method have
the 3DOM structure. All the PMMA colloidal templates were

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completely removed after calcination at 550 °C and this
temperature was not affected the formation of the 3DOM
structure. The macropores structure of 3DOM-LSCF are almost
ordered hemispherical shape and connected with each other
through the small windows. Their pore sizes estimated from the
SEM image are about 450-550 nm, which corresponds to
shrinkage of 23.6-37.5% compared with the initial sizes of
PMMA microspheres about 720 nm. This shrinkage is caused by
melting of the microspheres and sintering of the produced
perovskite-type compound. Nevertheless, the long-range
orderly and uniform pore structure of the 3DOM-LSCF catalyst
is not destroyed by this large shrinkage. The wall thickness of
macroporous double perovskite 3DOM-LSCF catalyst estimated
from the SEM images are about 100-150 nm. The wall seems to
be composed of linearly fused grains of the produced
perovskite-type compound. Three small windows in the
macropores formed as a result of the contact between the
PMMA microspheres template removed after calcination could
be seen. These inner connected macropores are favorable for
internal part of 3DOM materials to exchange substance outside.
Through SEM analysis results, it could be seen that the 3DOM
structure of 3DOM-LSCF deformed and broken when the
temperature is higher than 850 °C. Thus, combined with the
XRD analysis results, the double perovskite LSCF and 3DOMLSCF prepared at 750 °C with good morphology and fine
crystalline phase were selected for thermal and photothermal
catalytic performance.

good agreement with SEM observations. The wallView
thickness
of
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DOI: 10.1039/C6TA05402A
the 3DOM-LSCF is in range of 100-200 nm.
Besides, nanovoids
with diameter of 20-30 nm, which are randomly distributed on
the wall of macropores. Moreover, the Fig. 4 (c, f) TEM images
also shown clear lattice spacing (d values) of the double
perovskite LSCF and 3DOM-LSCF. The interplanar spacing of (1
1 0) of the double perovskite LSCF and 3DOM-LSCF were 0.273
nm and 0.272 nm, respectively, corresponding to the (1 1 0)
lattice spacing of the cubic phase of perovskite crystal structure
and it is the same with XRD Rietveld structure refinement for
double perovskite LaSrCoFeO6-δ results. While, with 3DOMLSCF, the interplanar spacing of (1 1 0) planes become smaller
than LSCF sample.

Scheme 1. Schematic illustration of the 3DOM-LSCF catalyst

Fig. 2 N2 adsorption/desorption isotherm curves of the double
perovskite LSCF and 3DOM-LSCF prepared at 750 °C for 4 h in
air.
To get more detail information on the double perovskite
LSCF and 3DOM-LSCF, TEM images are presented in Fig. 4. From
the TEM images of Fig. 4 (a, b, d, e) it could be seen that the
small nanoparticles with an average size of 50-120 nm were
aligned together. Fig. 4 (d, e) is obvious that 3DOM-LSCF
possessed a high-quality 3DOM structures which was composed

of interconnected macropores with nanocrystal skeletons, in

Fig. 3 SEM images of (a, b) PMMA, (c, d) LSCF-750, (e, f) 3DOMLSCF-550, (g, h) 3DOM-LSCF-650, (i, k) 3DOM-LSCF-750, (l, m)
3DOM-LSCF-850.
A Bruker-AXS 133 eV XFlash 4010 Detector attached to the
SEM is used to measure the element composition and
distribution of the double perovskite LSCF and 3DOM-LSCF.
From the EDS spectrum and the elements mapping images in

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Fig. 4 g, h, they are clear to see that these elements La, Sr, Fe,
Co and O dominates the composition of the double perovskite
LaSrCoFeO6-δ. Those mapping images are solid proofs that these
La, Sr, Fe, Co and O are uniformly distributed in the double
perovskite LSCF and 3DOM-LSCF.

calculated to be +2.43 V and +1.97 V for the double
Viewperovskite
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DOI:5d).
10.1039/C6TA05402A
LSCF and 3DOM-LSCF, respectively (Fig.
These results
clearly confirm that the ECB and EVB of the double perovskite
LSCF and 3DOM-LSCF suitable for CO2 photoreduction. The edge
of the VB of the double perovskite LSCF and 3DOM-LSCF were
more positive than Eo(H2O/H+) (H2O → 1/2O2 + 2H+ + 2e–, Eoox =
0.82 V vs NHE) (Fig. 5d). The edge of the CB was thus estimated
to be −0.42 V, which is more negative than Eo(CO2/CH4) (CO2 +
8e– + 8H+ → CH4 + 2H2O, Eored = −0.24 V vs NHE). This indicates
that the photogenerated electrons and holes in the irradiated
double perovskite LSCF and 3DOM-LSCF can react with
adsorbed CO2 and H2O to produce CH4, as described in the

following equation: CO2 + H2O → CH4 + O2.58 Moreover, the
color of the double perovskite LSCF and 3DOM-LSCF are black,
which indicates that these catalysts could absorb more visible
light and would exhibit higher photocatalytic efficiency.

Fig. 4 TEM images and EDS images of (a, b, c, and g) LSCF, (d, e,
f, and h) 3DOM-LSCF prepared at 750 °C for 4 h
An optical property is one of the most important properties
of any material for evaluation of its photocatalytic activity. Fig.
6a shows the UV-vis diffuse reflectance spectra (DRS) of the
double perovskite LSCF and 3DOM-LSCF. The double perovskite
LSCF and 3DOM-LSCF had good light absorption properties in
both ultraviolet and visible light region. Hence, the
photocatalytic activity of the double perovskite LSCF and
3DOM-LSCF can be performed under UV light and visible light.
The lower cut off wavelength of the double perovskite LSCF and
3DOM-LSCF were observed at 436 nm and 439 nm, respectively.
No other peak related with impurities and structural defects
were observed in the spectra which confirms that the
synthesized crystals have good crystallinity. Further band gap
energy was calculated on the basis of the maximum absorption
band of the crystal and found to be 2.84 eV and 2.83 eV for the
double perovskite LSCF and 3DOM-LSCF, respectively (Fig. 5 a).
As shown in Fig. 5 (b, c), the Mott-Schottky measurements were
performed to determine the relative positions of the CB and VB
edges. The positive slope of the plot revealed typical n-type
characteristics of semiconductors.55 As for n-type
semiconductors, the flat-band potentials (Efb) can be used to
approximately estimate the CB potentials (ECB).56,57 The ECB of
the double perovskite LSCF and 3DOM-LSCF were about -0.41

and -0.86 V, respectively. Based on the above results and the
band gap energy obtained by DRS, the VB potentials (E VB) were

Fig. 5 a) UV–vis spectra and band gap of the double perovskite
LSCF and 3DOM-LSCF, b) Mott-Schottky plots of the LSCF
catalyst, and c) Mott-Schottky plots of the 3DOM-LSCF catalyst,
d) A schematic illustration of the band structures of the double
perovskite LSCF and 3DOM-LSCF.
The XPS was performed in order to study the chemical state
and surface composition of the double perovskite LSCF and
3DOM-LSCF (Fig. 6 a, b, c, d). Table 2 reports the binding
energies of elements constituting the double perovskite LSCF
and 3DOM-LSCF. As shown in Fig. 6 (a, b) the double perovskite
LSCF and 3DOM-LSCF with both Fe and Co regions shown the
evidence of the coexistence of at least two oxidation states and
had the same results.59-61 In the double perovskite LSCF and
3DOM-LSCF, the B position is occupied by iron or cobalt, in the
two oxidation states 2+ and 3+ whose relative amount could be
estimated only for the ion present in higher concentration. The
Fe 2p peak fitting of the double perovskite LSCF and 3DOM-LSCF
were performed according to the constraints for the Fe 2+ and
Fe3+ components and the respective shake up satellites
indicated by Liu.62 Both catalysts shown the two-oxidation
states characterized by the components at 709.99, 713.61 eV of
Fe2+ and Fe3+ respectively for LSCF and 709.90, 713.49 eV of Fe2+
and Fe3+ respectively for 3DOM-LSCF. The high binding energy
of the second component of iron in figure attributed to Fe 3+,

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could account also for the presence of small amount of Fe 4+.71
Analogously, the Co 2p region was fitted with the Co 2+, Co3+ and
the respective shake up satellites.59,60 In the case of cobalt, the
Co2+ component is located at higher binding energy than the
Co3+. As shown in Fig. 6 the binding energy values for Co 2p
(Co2+) are obtained at 781.38 eV and 779.77 eV for LCSF and
783.08 eV and 779.90 eV for 3DOM-LCSF catalyst.59 In Figure 6c
are shown the experimental and fitted O 1s photoelectron
spectra for the double perovskite LSCF and 3DOM-LSCF. The O
1s peaks typical of all catalysts consists of three components at
about 528.37, 530.65 and 532.19 eV attributed to lattice,
surface and adsorbed oxygen, respectively.61 The thermal and

photothermal catalytic activity for CO2 conversion with H2O
vapor to CH4 over double perovskite LSCF and 3DOM-LSCF
catalysts has been compared and shown that the catalytic
activity toward this reaction depends on Co3+/Co2+, Fe3+/Fe2+
ratio, Oads/Olattice and exposure of lattice planes of the
catalysts.61,63
Table 2 XPS binding energies relative to the double perovskite
LSCF and 3DOM-LSCF.
Catalyst

Co 2p (eV)
Co 2p3/2
Co 2p1/2

Fe 2p (eV)
Fe 2p3/2
Fe 2p1/2

528.37
(64.45
%)

779.77
(Co 3+)
(76.13
%)

795.05
(Co 3+)
(91.20

%)

709.99
(Fe 2+)
(68.96
%)

722.92
(Fe 2+)
(70.01
%)

530.47
(35.55
%)

781.38
(Co 2+)
(23.87
%)

797.36
(Co 2+)
(8.80 %)

713.61
(Fe 3+)
(31.04
%)

726.07
(Fe 3+)
(29.99
%)

528.44
(64.11
%)

779.90
(Co 3+)
(78.42
%)

794.91
(Co 3+)
(68.61
%)

709.90
(Fe 2+)
(69.61
%)

722.77
(Fe 2+)
(73.54
%)

530.65

(35.89
%)

783.08
(Co 2+)
(21.58
%)

796.68
(Co 2+)
(31.78
%)

713.49
(Fe 3+)
(30.39
%)

725.64
(Fe3+)
(26.46
%)

O 1s
(eV)

LSCF

3DOMLSCF

Fig. 6 The XPS spectra of a) Co 2p, b) Fe 2p c) O 1s and d) XPS
survey spectra of the double perovskite LSCF and 3DOM-LSCF.

At the same time, BE values of La 3d5/2 and SrView
3dArticle
the
5/2 ofOnline
DOI:6d)
10.1039/C6TA05402A
double perovskite LSCF and 3DOM-LSCF (Fig
no more differ.
Considering basic character of perovskite surface one should
take into account that in ABO3 structure, lattice oxygen anions
have only two coordinations with small and strongly polarizing
B cations and they are only weakly polarized by big A cations.
When chemisorbed oxygen species (oxygen ions) lie on the
perovskite surface they are coordinatively unsaturated and
their coordination with B is lowered to one.36 So, one could
expect that if relative concentration of La and Sr in A position
and Co and Fe in B position are different, the formal charge on
the oxygen species chemisorbed on this surface will also be
different. Moreover, XPS analysis results confirmed the double
perovskite LaSrCoFeO6-δ with mixed ions and mixed valence
state of A, A’-site (La3+/Sr2+) and B, B’-site (Co2+/Co3+, Fe2+/Fe3+)
possess self-formed oxygen vacancies. The oxygen vacancies in
the double perovskite LSCF and 3DOM-LSCF could be controlled
by change crystal structure, A, A’, B, B’ site, mole ratios of
elements and 3DOM structure of the double perovskite LSCF
and 3DOM-LSCF.
The thermal and photothermal catalytic activity for

reduction of CO2 with H2O vapor to CH4 over double perovskite
LSCF and 3DOM-LSCF catalysts were evaluated under thermal
only at 150, 250, and 350 °C and photothermal (350 °C + Vislight) (Fig.7 and Table 3). Fig. 7 (a, b) shows the compared
catalytic activity of thermal catalytic at different temperatures
150, 250, 350 °C and photothermal reduction of CO2 with H2O
vapor to CH4 over double perovskite LSCF and 3DOM-LSCF
catalysts. The yield, turn over number (TON) and solar-tomethane (STM) energy conversion efficiency of thermal and
photothermal reduction of CO2 with H2O vapor to CH4 over
double perovskite LSCF and 3DOM-LSCF catalysts summarized
in Table 4, 5 (ESI S3). Fig. 7 (a, b) shows that the enhancing
temperature greatly increased the thermal catalytic activity. As
shown in Fig. 7 (a, b) after 8 h, the methane production yields
of the double perovskite LSCF and 3DOM-LSCF are in the order
of 350 °C + Vis-light > 350 °C > 250 °C > 150 °C. The detail values
of catalytic performance under photothermal after 8 h was
arranged by 557.88, 120.86, 39.02, and 2.81 µmol g-1 for 3DOMLSCF and 351.32, 65.88, 24.94 and 1.89 µmol g-1 for LSCF. Figure
7b shown the yield of methane over 3DOM-LSCF catalyst under
photothermal after 8h (557.88 µmol g-1) is about 1.6 times of
LSCF catalyst (351.32 µmol g-1). The results also shown that the
best catalytic performance is under photothermal and it is
higher 5 times than catalytic performance under thermal only.
The results may consider on the comparative surface area, pore
volume, and crystallite size.63-65 The band gap energy is also
correlated to the photocatalytic activity. The double perovskite
LSCF and 3DOM-LSCF have similar band gap but the 3DOM-LSCF
catalyst has a positions of the CB and VB more suitable for CO2
photoreduction than LSCF catalyst. It has a more negative CB
and less positive VB than LSCF catalyst. In addition, the BET
specific surface area of 3DOM-LSCF catalyst is 21.86 m2 g-1,
which is larger than LSCF catalyst with BET specific surface area

of 8.46 m2 g-1. In addition, the high catalytic performance of the
double perovskite LSCF and 3DOM-LSCF may consider on the
photo-thermal coupling effect, self-formed oxygen vacancies,

6 | J. Name., 2012, 00, 1-3

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Table 3 Thermal and photothermal catalytic activity and
physical properties of the double perovskite LSCF and 3DOMLSCF.
BET

µmol g-1 h-1
Samples
150
o

C

LSCF
3DOM-

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LSCF

250
o

C

350
o

C

Band

350 oC

gap

+ Vis-

(eV)

Crystallite
size (nm)

light

Total

surface

pore

area

volume

(m2 g-

(cm3 g-

1

)

1

)

1.35

18.09

53.35

300.69

2.84

20.06

8.46

0.048

2.11

28.03

93.91

467.63

2.83

25.79

21.68

0.095

small crystallite size and high porous material. Furthermore, the
3DOM-LSCF may process self-formed heterostructures with
3DOM structure all play positive role in the separation process
of photogenerated electrons and holes. In such a way, the
presence of heterostructures interface the recombination of
photogenerated electrons and holes were suppressed
effectively, and the photocatalytic activity is greatly enhanced.
The reusability of the catalyst is important for its practical
application. In order to evaluate the activity stability of the
catalyst, the reuse experiment was carried out. From Fig. 7c, it
can be seen that the catalytic activity of the double perovskite
LSCF and 3DOM-LSCF remain high catalytic activity after reuse
of 5 times and the catalytic activity of 3DOM-LSCF catalyst is
better than LSCF catalyst. The double perovskite 3DOM-LSCF
shown considerable stability in the catalytic process.
Addition, it is difficult to directly compare the methane
production rate of the double perovskite LSCF and 3DOM-LSCF
with rates reported for other photocatalysts because of the
variance in experimental conditions (such as light intensity,
illumination area, and photocatalyst dosage), morphological
features, surface areas, and co-catalysts. However, the catalytic
performance of the double perovskite LSCF and 3DOM-LSCF are
comparable to, and perhaps better than other reported

photocatalysts that convert CO2 into methane using solar
irradiation and without using noble metal co-catalysts, including
in Table 6 (ESI S3).
The reaction mechanism for the thermal and photothermal
reduction of CO2 with H2O vapor to CH4 over double perovskite
LSCF and 3DOM-LSCF catalysts were proposed base on last
study20,66,67 and illustrated in Fig. 9. The rate of photocatalytic
reaction can be controlled by several steps: photoexcitation of
the double perovskite LaSrCoFeO6-δ surface, creating electronhole pairs, followed by their transfer to CO 2 and H2O. The
surface defects and hydration are often considered to be
important for heterogeneous catalysis as well, since these
particular factors also play important roles in the reactantsurface binding and the formation of bonds between the
surface atoms and H2O, CO2 molecules. To correlate surface
structures with photocatalytic activity, interaction between H 2O,
CO2 molecules and the surface of the photocatalyst was
examined.68-73 An understanding of the interaction between
catalyst surface and the CO2 and H2O molecules is vital for
developing its role in the photocatalytic reduction of CO 2. The
CO2 and H2O molecules could be adsorbed on the double
perovskite LaSrCoFeO6-δ surface. A variety of possible binding

Fig. 7 Thermal and photothermal catalytic activity of a) LSCF, b)
3DOM-LSCF and d) Reuse of the catalyst.
configurations of H2O and CO2 on the perfect and defective
catalysts surfaces in terms of geometries, energies, and net
charges were explored. Five models were constructed to
determine the adsorption energy of the system (Fig. 8). Li Liu
and co-workers70,71 shown that the adsorbed CO2 molecules are
partially negatively charged, indicating that CO2 accepted
electrons from the surface and formed a partially and negatively

charged CO2δ- species. This negatively charged CO2δintermediate has also been described in experimental 74,75 and
theoretical work.76,77 For defective surfaces, surface oxygen

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Journal of Materials Chemistry A Accepted Manuscript

Rate of CH4 evolution

View Article Online

DOI: 10.1039/C6TA05402A


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defects were found to play an important role and can
significantly influence the interaction of CO 2 with the surface:
the oxygen vacancies are the active sites on the defective

surfaces; the nearby oxygen vacancies can significantly enhance
the adsorption energy of CO2 molecule compared to the perfect
surfaces; CO2 can not only be activated but can also be further
dissociated into CO and O on the surface oxygen defect site and
the oxygen vacancy defect can be healed by the oxygen atom
released during the dissociation process. Through analysis of
the dissociative adsorption mechanism of CO 2 on defective
surfaces, the results shown that the dissociative adsorption of
CO2 favours the stepwise dissociation mechanism and the
dissociation process can be described in eqn (2):
CO2 + Vo  CO2δ- /Vo  COadsorbed + Osurface
(2)
Furthermore, H2O adsorbed on perfect surfaces could
spontaneously dissociate into an H atom and an OH group. The
presence of oxygen defects was found to strongly promote H2O
dissociation on the (0 1 0) surface. The results revealed that the
interaction of CO2 and H2O with catalyst surfaces was
dependent on the structure, crystal plane and active site on
surface.70,71

Fig. 8 Possible configurations of adsorbed CO2 (a, b, c, d, e) and
H2O (f, g, h, i, k) molecule on the double perovskite LaSrCoFeO 6δ surface.
In order to understand the reaction process, a possible
catalytic mechanism of the double perovskite LSCF and 3DOMLSCF for the reduction of CO2 with H2O vapor to CH4 is shown in
Fig. 9 and equations. Photocatalytic reduction of CO2 with H2O
vapor on semiconductor oxide catalyst surfaces using solar
energy to yield fuels/chemicals (CH4, CH3OH, etc.) involves two
major steps, splitting of H2O to yield H2, which in turn helps in
the reduction of CO2 to different hydrocarbon products in the
second step. The complex sequence of process steps that follow,

involving two, four, six or eight electrons for reduction, lead to
the formation of formic acid/CO, formaldehyde, methanol and

methane respectively7 depending on the type ofView
catalyst
and
Article Online
DOI: 10.1039/C6TA05402A
reaction conditions employed.
The first step involving photocatalytic splitting of water
follows the well-accepted elementary steps as shown in eqn (3)(8):
LaSrCoFeO6-δ + hυ  e− + h+
(3)
H2Oads + h+ OH− + H+
(4)
H+ + e−  •H
(5)
OH− + h+  •OH
(6)
2•OH  H2O2 + h+  O2− + 2H+
(7)
O2− + h+  O2
(8)
The second step for activation and reduction of CO2 to CH4
could then follow20,67,78,79 it shows in eqn (9)-(14):
CO2ads + e−  •CO2−
(9)
•CO2− + •H  CO +OH−
(10)
CO + e−  •CO−

(11)
•CO− + •H  •C +OH−
(12)
•C +H+ +e−  •CH  •CH2  •CH3
(13)
•CH3 + H+ + e−  CH4
(14)
Possible thermocatalysis mechanism activation and reduction
of CO2 to CH4 shows in eqn (15)-(19) and the total reaction under
photothermal coupling effected shows in eqn (20)
LaSrCoFeO6-δ + H2O  LaSrCoFeO6 + H2
(15)
LaSrCoFeO6-δ + δ/2CO2  LaSrCoFeO6 + δ/2C(s) (high Vo) (16)
LaSrCoFeO6-δ + CO2  LaSrCoFeO6 + CO (low Vo)
(17)
Vo (oxygen vacancies)
CO + H2  C + H2O
(18)
C + 2H2  CH4
(19)
The total reaction under photo-thermal coupling
CO2 + 2H2O  CH4 + 2O2
(20)
Tabata and co-worker88 reported that CO2 could be
decomposed completely to carbon with oxygen-deficient
ferrites, Zn(II), Mn(II) and Ni(II) bearing ferrites 81-84 at low
temperature near 300 °C. In this study, water used as a
hydrogen source, under optimized reaction conditions the
double perovskite LaSrCoFeO6-δ with self-formed oxygen
vacancy could split H2O into element H under 350 °C. The

combination of the two splitting reactions improved conversion
of CO2 to CH4 of high selectivity and high yield. High selectivity
was due to the splitting of CO2 more tend to form C (eqn (16))
as intermediate product of CH4 under low temperature (<500 °C)
circumstances, and the intermediate product is single.
Moreover, when temperature increases, the electrical
conductivity increases85,86, it means the yield of CH4 is affected
by the electrical conductivity of catalysts. Furthermore, the high
temperature improved mass transfer and reaction kinetics.
Usually conducting a reaction at a higher temperature delivers
more energy into the system and increases the reaction rate by
causing more collisions between particles. However, the main
reason that temperature increases the rate of reaction is that
more of the colliding particles will have the necessary activation
energy resulting in more successful collisions. The simultaneous
thermochemical reaction of CO2 and H2O with the oxygen
deficient in double perovskite LaSrCoFeO6-δ at a relative low
temperature can convert CO2 into CH4 with high efficient as well
as solving the problem of catalytic carbon deposition that
catalyst surface might be covered by a carbon layer in the

8 | J. Name., 2012, 00, 1-3

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catalytic reaction process. The generated carbon from the
splitting of CO2 will react with the element H to convert into CH4.
(eqn (19)). Thus, CO2 can be converted to CH4 through the two
reaction steps by oxygen deficient. In summary, through
demonstrate the simultaneous thermochemical reaction of CO2
and H2O with oxygen deficient double perovskite LaSrCoFeO6-δ
catalyst at a relative low temperature to achieve a high efficient
of CO2 converting into CH4. The exploration of these catalysts
with oxygen vacancies confirmed that transformation of CO2 to
CH4 was achieved by active oxygen vacancies. The repeatability
of the catalyst decreased because of the decrease of the oxygen

vacancies concentration.

performance of 3DOM-LSCF might be associated with
its 3DOM
View Article
Online
DOI:
10.1039/C6TA05402A
structure with high porous structure, high
surface
areas, small
crystallite size, mixed-ionic, mixed valence, self-formed
heterostructures, self-formed oxygen vacancies can improve
CO2 and H2O absorption and reaction on the catalyst surface.
This material are considered as promising catalytic materials for
photothermal conversion of CO 2 to hydrocarbon fuels,
environmental cleaning, energy storage, catalysis and cathode
materials for solid oxide fuel cells.

Acknowledgments
We gratefully acknowledge the financial support by the
program for young scientists (YangFan Program, 14YF1410800)
at Science and Technology Commission of Shanghai
Municipality, young teachers training scheme of Shanghai
Municipal Education Commission (ZZyy15085, ZZyy15086), the
program of introducing talents of Shanghai Institute of
Technology (YJ2014-42) and the special fund to support the
development of local colleges of Ministry of Finance of China.

References

1
2
Fig. 9 Schematic diagram of combined photo- and thermalcatalytic reduction of CO2 with H2O vapor to CH4 in one system
over double perovskite LSCF and 3DOM-LSCF catalysts.

Conclusions
The double perovskite LSCF and 3DOM-LSCF were successfully
synthesized by a convenient and efficient modified combustion
process. The 3DOM-LSCF catalyst with 3DOM structure could be
prepared using the PMMA template. The crystal structure,
morphology, BET surface area, band gap and catalytic
properties were characterized in detail. The double perovskite
LSCF and 3DOM-LSCF produced good phase after calcined at
550-950 °C for 4 h. The slow light effect of the 3DOM structure
can enhance absorption efficiency of solar irradiation.
Moreover, the 3DOM-LSCF may self-doped and self-formed
heterostructures can effectively extend the spectral response
from UV to visible region owing to surface plasmon resonance
and it is favorable for enhancing the separation of
photoinduced electron-hole pairs. The results shown both LSCF
and 3DOM-LSCF catalysts had good catalytic performance, high
selectivity for reduction of CO2 with H2O vapor to CH4 under
thermal and photothermal condition reaction. The catalytic
activity for reduction of CO2 with H2O vapor to CH4 under
photothermal condition is better of 5 times than thermal only.
Under the same reaction condition, the double perovskite
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mechanism of photothermal reduction of CO2 with H2O to CH4
over double perovskite LSCF and 3DOM-LSCF was proposed in
more detail. From results, we believe that the excellent catalytic

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