Among them, perovskite oxides with general formula ABO3 possess unique properties, such as metal-insulator transition, spin blockade, colossal magnetoresistance, ferroelectricity, and su
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Liu, L wang and Z Zhao, J Mater Chem A, 2016, DOI: 10.1039/C6TA05402A.
Trang 2Journal Name
ARTICLE
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Minh Ngoc Haa,b,c, Guanzhong Lu*a,b, Zhifu Liub, Lichao Wangb and Zhe Zhao*b
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 3DOM-LSCF 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
cost-effective 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
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 CO2 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 TiO2-based6,8,10,11
a Key Laboratory for Advanced Materials, Research Institute of Industrial Catalysis,
East China University of Science and Technology, Shanghai 200237, China Email:
gzhlu@ecust.edu.cn
b School of Materials Science and Engineering, Shanghai Institute of Technology,
Shanghai 201418, China E-mail: zhezhao@kth.se
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
View Article Online DOI: 10.1039/C6TA05402A
Trang 3materials while other catalysts such as SrTiO3,12 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-yFeyO
3-δ 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 H2,29 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 LSCF and 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 3DOM-LSCF are summarized in Table 1 The stability of complex perovskite structures can be well explained with the use of
View Article Online DOI: 10.1039/C6TA05402A
Trang 4tolerance factors (t) For the materials studied here, the
tolerance factors can be determined by equation (eqn) (1):
whererLa, 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
Barretl-Joyner-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 clear color change can be observed for the obtained 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)
3DOM-LSCF 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 2-Theta
(degree)
d110
(nm)
Unit cell*
(a, b, c, nm)
Crystallite size (nm) LSCF-550 32.751 0.27327 0.38646 17.298 LSCF-650 32.959 0.27285 0.38588 18.386 LSCF-750 32.773 0.27304 0.38602 20.063 LSCF-850 32.84 0.27252 0.38524 43.297 LSCF-950 32.912 0.27192 0.38470 54.539 3DOM-LSCF-550 32.697 0.27366 0.38672 15.866 3DOM-LSCF-650 32.773 0.27304 0.38589 20.107 3DOM-LSCF-750 32.807 0.27277 0.38555 25.792 3DOM-LSCF-850 32.761 0.27314 0.38614 31.321 3DOM-LSCF-950 32.971 0.27145 0.38479 41.775
* Rietveld structure refinement for LaSrCoFeO 6-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
2
2
La Sr
O
O
r r
r t
r r
r
View Article Online DOI: 10.1039/C6TA05402A
Trang 5completely 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
3DOM-LSCF prepared at 750 °C with good morphology and fine
crystalline phase were selected for thermal and photothermal
catalytic performance
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
good agreement with SEM observations The wall thickness of 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 3DOM-LSCF, the interplanar spacing of (1 1 0) planes become smaller than LSCF sample
Scheme 1 Schematic illustration of the 3DOM-LSCF catalyst
Fig 3 SEM images of (a, b) PMMA, (c, d) LSCF-750, (e, f)
3DOM-LSCF-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
View Article Online DOI: 10.1039/C6TA05402A
Trang 6Fig 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
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 (EVB) were
calculated to be +2.43 V and +1.97 V for the double perovskite LSCF and 3DOM-LSCF, respectively (Fig 5d) 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 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 Fe2+ 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 Fe3+,
View Article Online DOI: 10.1039/C6TA05402A
Trang 7could account also for the presence of small amount of Fe4+.71
Analogously, the Co 2p region was fitted with the Co2+, 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 O 1s
(eV)
Co 2p (eV) Fe 2p (eV)
Co 2p3/2 Co 2p1/2 Fe 2p3/2 Fe 2p1/2
LSCF
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
%)
3DOM-LSCF
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
%)
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 Sr 3d5/2 of the double perovskite LSCF and 3DOM-LSCF (Fig 6d) 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 + Vis-light) (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-to-methane (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-1for 3DOM-LSCF and 351.32, 65.88, 24.94 and 1.89 µmol g-1for 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,
View Article Online DOI: 10.1039/C6TA05402A
Trang 8Table 3 Thermal and photothermal catalytic activity and
physical properties of the double perovskite LSCF and
3DOM-LSCF
Samples
Rate of CH 4 evolution
gap (eV)
Crystallite size (nm)
BET surface area (m 2 g
-1 )
Total pore volume (cm 3 g
-1 )
150
o C
250
o C
350
o C
350 o C + Vis-light
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
electron-hole pairs, followed by their transfer to CO2 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
reactant-surface binding and the formation of bonds between the
surface atoms and H2O, CO2 molecules To correlate surface
structures with photocatalytic activity, interaction between H2O,
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 CO2 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 CO2
δ-intermediate has also been described in experimental74,75 and theoretical work.76,77 For defective surfaces, surface oxygen
View Article Online DOI: 10.1039/C6TA05402A
Trang 9defects were found to play an important role and can
significantly influence the interaction of CO2 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 CO2 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
6-δ surface
In order to understand the reaction process, a possible
catalytic mechanism of the double perovskite LSCF and
3DOM-LSCF 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 of catalyst and 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 V o) (16) LaSrCoFeO6-δ + CO2 LaSrCoFeO6 + CO (low V o) (17)
V o (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 ferrites81-84 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
View Article Online DOI: 10.1039/C6TA05402A
Trang 10catalytic 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
Fig 9 Schematic diagram of combined photo- and thermal-
catalytic 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
3DOM-LSCF catalyst exhibits higher catalytic activity than LSCF
catalyst The double perovskite LSCF and 3DOM-LSCF remain
high catalytic activity after reuse of five times, it shown
considerable stability in the catalytic process The reaction
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
performance of 3DOM-LSCF might be associated with its 3DOM 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 CO2 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
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