Photoreduction of carbon dioxide of atmospheric concentration to methane with water over CoAl-layered double hydroxide nanosheets

Article abstract of DOI:10.1039/c8ta01309h

The photoreduction of CO₂ to carbon energy sources over catalysts is of great interest and significance. However, though many studies have been conducted at high CO₂ concentrations, research on CO₂ photoreduction at atmosp

Full text of DOI:10.1039/c8ta01309h

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
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Photoreduction of carbon dioxide of atmospheric concentration
with water to methane over CoAl-Layered Double Hydroxide
nanosheets†
Received 00th January 20xx,
Accepted 00th January 20xx
Kefu Wang,ab Ling Zhang,a Yang Su,ab Dengkui Shao,ab Shuwen Zeng,ab Wenzhong Wang*a
DOI: 10.1039/x0xx00000x
Photoreduction of CO2 to carbon energy resources over catalysts is of great interest and significance. However, many works
are conducted under high CO2 concentration, research on CO2 photoreduction under atmospheric concentration remains
rare. Here, CoAl-Layered Double Hydroxide (CoAl-LDH) nanosheets have been synthesized as a platform to explore low-
concentration CO2 photoreduction. The CoAl-LDH nanosheets exhibit efficient photocatalytic activity for CO2
photoreduction of atmospheric concentration (400 ppm) to CH4 under simulated solar light and proceed without
deactivation after 55 hours. A CH4 production rate of 4.3 μmol·g-1·h-1 is achieved without the assistant of any sacrificial
agent or noble metal. CO2 adsorption experiments show that CoAl-LDH nanosheets exhibit an adsorption capacity of 2.95
cm3·g-1, which is about two times than that of P25 with a comparative specific surface area. XPS analysis indicates that CH4
generation relates closely to the divalent cobalt and the CH4 yield decreases sharply when the divalent cobalt is oxidized,
which could likely be ascribed to the divalent cobalt for dissociating water to release H for hydrogenation of the
intermediates through further experimental analysis. This work emphasizes the importance of alkaline OH group for the
efficient adsorption of low-concentration CO2 and reveals the unique property of the divalent cobalt for CO2 photoreduction.
www.rsc.org/
15
while most of these research is implemented under CO2
1. Introduction
atmosphere with high concentration. Converting CO2 of
atmospheric concentration into carbon energy sources is rare
The atmospheric CO2 concentration has been monotonically
growing with the increasing consumption of fossil fuels,
bringing about adverse environmental issue, and the overusing
of fossil fuels could lead to greater energy shortage ulteriorly.1
Artificial photosynthesis, which utilizes semiconductor
photocatalysts to convert CO2 and water into substances of
energy sources (such as CO, CH3OH, CH4), is possibly one of the
most sustainable and ideal solutions to simultaneously address
the problems of the global warming and energy crisis.2
However, CO2 is pretty stable with a high dissociation energy of
~750 kJ mol-1 for C=O bond under normal conditions.3 In
addition, the photoreduction of CO2 is a multi-electron process
and usually involves the activation of water, which has not been
commendably resolved at the moment. Searching for an
efficient photocatalyst with multi-function seems to be
necessary to meet the challenge. Up till now, various
photocatalysts for efficient CO2 reduction have been reported,4-
and deserves to be explored in view of the low concentration of
CO2 in the atmosphere. Besides, the primary products of CO2
photoreduction are CO or HCOOH, and a high selectivity for CH4
product remains difficult, possibly hindered by the further
conversion of intermediates. Improving the CO2 adsorption and
promoting hydrogenation process of the intermediates seem to
be the key to solve the problem.
In consideration of the Lewis acidity of CO2, alkaline
catalysts will benefit the adsorption and activation of CO2.
Surface modification of TiO2 with NaOH has been reported to
greatly promote the photoreduction of CO2 to CH4, which is
considered to derive from efficient chemisorption and
activation by forming carbonate species.4 Moreover, artificial
photosynthesis contains the processes of CO2 reduction and
H2O oxidation simultaneously, demanding the photocatalysts
dual functions. Recently, layered materials have attracted much
attention.16 Layered materials usually possess high specific
surface area, meaning affluent active sites for catalytic reaction.
Among the layered materials, layered double hydroxides (LDHs)
are common but important photocatalysts, with their host
structure based on brucite-like layers with edge-sharing MO6
octahedra. Edge-sharing MO6 octahedra units result in the
formation of 2D sheets, in which the metal cations are six-fold
coordinated by OH groups.17 It is proposed that the alkaline OH
groups of layered double hydroxides are in favor of the
adsorption of CO2 and may also help to activate CO2 molecules.
Besides, LDHs have been reported to be efficient photocatalysts
a.
State Key Laboratory of High Performance Ceramics and Super fine
Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences,
1295 Dingxi Road, Shanghai 200050, P. R. China.
b.University of Chinese Academy of Sciences, Beijing 100049, P.R. China.
*E-mail: wzwang@mail.sic.ac.cn
† Electronic Supplementary Information (ESI) available: EDS and EDS mapping, UV-
visible absorption spectrum, Blank experiments, CO and H2 evolution of CoAl-LDH
nanosheets. CH4 evolution of CoAl-LDH and P25. Summary of the activities of some
widely used semiconductor catalysts. XRD patterns of CoAl-n (n=0, 0.25, 0.5, 2).
CO2 adsorption capacities of CoAl-n (n=0, 2) and some other kinds of LDH. XRD
patterns, UV-visible absorption spectra, FT-IR spectra and CH4 evolution of some
other LDHs. See DOI: 10.1039/x0xx00000x
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for water oxidation.18-21 It is expected that LDHs might possess LABRAM HR, with a 532 nm excitation laser line. Brunauer-
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the dual properties of CO2 activation and H2O oxidation. Hence, Emmett-Teller (BET) specific surface areDaOaI:n1d0.1B0a39rr/Cet8tT-AJ0o1y3n0e9rH-
LDHs could be desired catalysts for CO2 photoreduction.
Halenda (BJH) pore size distribution were measured by a surface
In this work, CoAl-LDH nanosheets were synthesized. They area and pore size analyzer (V-sorb 2008 P, Gold APP).
were found to be active and stable in the photoreduction of CO2
to CH4 without obvious deactivation even after 55 hours. The 2.3 Photocatalytic reactions
CH4 production rate of 4.3 μmol·g-1·h-1 was achieved under
atmospheric CO2 concentration (about 400 ppm) without the
The experiments of gas-phase CO2 photoreduction with H2O on
CoAl-LDH nanosheets were carried out in a closed pyrex cell
assistant of any sacrificial agent or noble metal. The good
(600 mL) with a quartz window. First, CoAl-LDH powder (30 mg)
performance could likely be attributed to the surface alkaline
was sprinkled on bottom of the pyrex cell. Water (0.5 mL) was
OH groups for efficient adsorption of low-concentration CO2
then sprayed around the catalyst. Prior to the irradiation of a Xe
and the divalent cobalt for the unique and effective electronic
lamp (500 W), the cell was purged with dry nitrogen, followed
structure configuration, which could be demonstrated that the
by injection of CO2 (0.25 mL, about 400 ppm). The gaseous
catalytic efficiency decreased sharply when the divalent cobalt
products (CO, CH4) were analyzed by a gas chromatograph (GC,
was oxidized. Further experimental study indicated that the
Tianmei 7890) equipped with a flame ionization detector (FID).
divalent cobalt oxidized might lose the ability to dissociate
The H2 product was analyzed by a GC (Tianmei 7890II) equipped
water and no hydrogen was released for the hydrogenation
with a thermal conductivity detector (TCD).
process of the intermediate products.
2.4 Electrochemical measurements
The electrochemical analysis was performed on a CHI 660D
2. Experimental
electrochemical workstation (Shanghai Chenhua, China) using a
2.1 Catalyst preparation
standard three-electrode quartz cell with a working electrode,
All reagents were of analytical purity and received from
Shanghai Chemical Company without further purification. CoAl-
LDH nanosheets were synthesized in a simple co-precipitation
method. Typically, cobalt nitrate hexahydrate (0.582 g, 2 mmol)
and aluminum nitrate nonahydrate (0.375 g, 1 mmol) were
dissolved in de-ionized water (20 mL) (noted as solution A).
Stirring the solution for 15 min. Sodium hydroxide (0.24 g, 6
mmol) and anhydrous sodium carbonate (0.053 g, 0.5 mmol)
were dissolved in deionized water (20 mL) with stirring (noted
as solution B). Then solution A was added dropwise into solution
B and the suspension was transferred to a Teflon-lined
a platinum slice as the counter electrode, and a standard
saturated calomel electrode (SCE) in saturated KCl as the
reference electrode. To make a working electrode, catalyst
powders were deposited on a fluorine-doped tin oxide (FTO)
substrate by coating. Briefly, 10 mg of catalyst was suspended
in 1 mL ethanol solution and the mixtures were ultrasonically
scattered for 15 min. Then, 100 μL of above slurry was coated
on the FTO glass. After natural evaporation of ethanol, the
catalyst coated on FTO substrate was used as the working
electrode. The area (S) for the coating layer is about 0.8 cm2,
while the thickness (L) is about 0.02 cm. The electrical resistivity
(ρ) is calculated by ρ = R*S / L, where R could be measured by
the electrochemical impedance spectroscopy (EIS). During the
measurements, the electrolyte of 0.1 M Na2SO4 solution (pH =
7) was utilized.
o
autoclave (50 mL), sealed and heated at 110 C for 9 h. The
products were filtered and washed thoroughly with de-ionized
water and dried in vacuum oven at 60 oC.
2.2 Materials characterization
2.5 CO2 adsorption measurements
The crystallinity of the as-prepared catalyst samples was
characterized by powder X-ray diffraction (XRD) with a Rigaku
D/MAX 2250 V diffractometer using monochromatized Cu Kα (λ
= 0.15418 nm) radiation while the voltage and electric current
were held at 40 kV and 100 mA. The morphologies and
microstructures characterizations were performed on the
transmission electron microscope (TEM, JEOL JEM-2100F,
accelerating voltage 200 kV). UV−Vis diffuse reflectance spectra
(DRS) of the samples were obtained on an UV−Vis
spectrophotometer (Hitachi U-3010). X-ray photoelectron
spectroscopy (XPS) was performed on an ESCALAB 250, Thermo
Scientific Ltd. with a 320 μm diameter spot of monochromated
aluminum Kα X-rays at 1486.6 eV under ultrahigh-vacuum
conditions. FTIR spectra presented in this work were recorded
on a Bruker Tensor 27 spectrometer. In situ infrared spectra
presented in this work were recorded on a NICOLET iS10
apparatus with a cavity (about 20 cm3) for adjusting
atmosphere. The Raman spectra were recorded on a Horiba
In order to accurately characterize the CO2 adsorption capacity
of catalysts under the experimental environment, CO2
adsorption measurements were conducted in a closed pyrex
cell (600 mL). Typically, the cell was purged with dry nitrogen
for half an hour, followed by injection of CO2 (0.25 mL), then the
CO2 concentration was detected (noted as N0) by gas
chromatograph to obtain the ratio K between the amount of
CO2 and the CO2 concentration (The detected CO2
concentration by gas chromatograph keeps a linear relationship
with the amount of injected CO2). Control experiments were
conducted when catalysts (30 mg) were sprinkled on bottom of
the pyrex cell, followed by injection of CO2 (0.25 mL), then the
CO2 concentration was detected (noted as N1). The CO2
adsorption capacity could be obtained by the formula A = (N0 –
N1) * K. The experiments were repeated for three times to
ensure the veracity.
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groups of the brucite-like sheets, analogous toVtiehweArtiinclferOanrleinde
spectra. The sharp band at 1056 cm is assigned to in-plane OH
bending vibrations, with a second band at a slightly lower
Raman shift of 1044 cm-1 assigned to ν1 mode of the
carbonate.25 With regard to the low frequency region lower
than 1000 cm-1, the bands at 545 cm-1 and 508 cm-1 stems from
Al-OH vibrations, while the band centered at 153 cm-1 could be
due to the framework vibrations.27
The specific surface area and porosity property of the CoAl-
LDH nanosheets were studied by nitrogen Brunauer-Emmett-
Teller (Figure 2D), which shows a type IV isotherm with H3
hysteresis loop, indicating the presence of mesopores. The
sample illustrates a wide pore-size distribution in the range of
2-200 nm and a high surface area of 37.7 m2·g-1. The high
specific surface area of CoAl-LDH nanosheets is expected to be
in favor of CO2 uptake and to provide more active sites, so as to
improve the photocatalytic activity.
3. Results and discussion
3.1 Structure and morphology
-1
DOI: 10.1039/C8TA01309H
CoAl-LDH nanosheets have been prepared by a simple co-
precipitation method. TEM images (Figure 1A-1C) show that the
CoAl-LDHs possess a hexagonal sheet-like morphology with a
lateral size of about 80 nm and a thickness of about 18 nm. From
the SAED pattern in Figure 1D, hexagonally arranged spots are
observed and the corresponding spots could be allotted to (012)
and (110) planes of CoAl-LDH. The EDS profile of CoAl-LDH
nanosheets (Figure S1) indicates the presence of Co, Al, O
elements, and the elemental mapping exhibits a relatively
uniform distribution of these constituent elements.
Powder XRD data (Figure 2A) shows the product to be a
typical rhombohedral structure with the lattice parameters of
a=b=0.306 nm and c=2.263 nm (a=b=2d110, c=3d003).22 The Bragg
reflections in the XRD could be indexed as (003), (006), (012),
(110), (113) and (116), respectively, which are consistent with
those of well-known LDH materials.23 To analysis the surface
and interior structure of CoAl-LDH nanosheets, FT-IR spectra
and Raman spectra were exploited. As shown in Figure 2B, the
broad strong absorption band around 3430 cm-1 is attributed to
the stretching vibrations of surface hydroxyl groups and
interlayer water molecules, indicating that hydrogen bonds
with a wide range of strength existed, while the weak band
centered at 1628 cm-1 is due to the bending mode of water.24
The absorption band at 1358 cm-1, 1038 cm-1, 808 cm-1 could be
assigned to ν3 mode (asymmetric stretching), ν1 mode
(symmetric stretching), ν2 mode (out-of-plane deformation) of
the carbonate, respectively.25,26 Although the ν1 mode is IR-
inactive in the free carbonate, it becomes activated owing to
the lowing of symmetry of carbonate anion in the interlayer.26
The band remained in the low-frequency region could be
interpreted as the lattice vibration modes, such as 617 cm-1, 547
cm-1, 427 cm-1 corresponding to the Co-OH stretching, O-Co-O
stretching, Al-OH stretching, respectively.24,25 As for the Raman
spectra (Figure 2C), the broad band around 3480 cm-1 is
attributed to the H-banded stretching vibrations of hydroxyl
The UV-visible absorption spectrum of CoAl-LDH nanosheets
is displayed in Figure S2. The absorption peak around 500 nm
corresponds to the 4T1g(F)
octahedrally coordinated by weak-field ligands, while the peak
around 600 nm could be due to the 3A2g(F) 3T1g(F) transition

4T1g(P) transition of Co2+

arising from spin-orbit coupling.28 The UV absorption around
230 nm may stem from ligand to metal charge transfer within
the CoAl-LDH layer. The associated Tauc plot (Figure 3A) reveals
that the optical band gap for CoAl-LDH nanosheets is 2.1 eV.15,21
To determine the positions of band edge related to redox
potentials of CO2 photoreduction, electrochemical experiment
for Mott-Schottky plot was performed to measure the flat-band
potential (Efb) of CoAl-LDHs. As shown in Figure 3B, the negative
slope of the plot indicates that CoAl-LDH is a characteristic p-
type semiconductor. The Efb of CoAl-LDH measured from the
intercept of the axis with potential value is 0.66 V vs NHE (0.42
V vs SCE) at pH 7. Generally, the position of Efb is considered to
be about 0.3 V above the top of the valence band (Evb) for the
p-type semiconductor whose electrical resistivity is in the range
of 103 - 104 Ω·cm, while the electrical resistivity of CoAl-LDH is
Figure 2. (A) XRD pattern, (B) FT-IR spectrum, (C) Raman spectrum, (D) N2 adsorption–
desorption isotherm and pore size distribution of CoAl-LDH nanosheets.
Figure 1. (A-C) TEM images and (D) SAED pattern of CoAl-LDH nanosheets.
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DOI: 10.1039/C8TA01309H
Figure 5. CO2 adsorption measurements of CoAl-LDHs and P25 for three times.
Figure 3. (A) Tauc plot of CoAl-LDH nanosheets. (B) Mott-Schottky plot of CoAl-LDH
nanosheets. (C) Electrochemical impedance spectroscopy of CoAl-LDH nanosheets. (D)
Band structure of CoAl-LDH nanosheets.
Table 1. CO2 adsorption capacities of CoAl-LDHs and P25 for three times.
calculated to be about 9*103 Ω·cm (Figure 3C, ρ = R*S / L, R ≈
230 Ω, S = 0.8 cm2, L = 0.02 cm).29,30 Therefore, the Evb of CoAl-
LDH is supposed to be 0.96 V vs NHE, and the corresponding
conduction band (Ecb) is then calculated to be about -1.14 eV vs
NHE according to the band gap (2.1 eV). As can be seen in Figure
3D, the positions of the conduction band edge and the valence
band edge sit astride the redox potentials of CO2 reduction and
water oxidation, indicating the feasibility of CO2 photoreduction
for CoAl-LDH nanosheets.
Sample
First
test
Second
test
Third
test
Average
(cm3 g-1)
Standard
deviation
(cm3 g-1)
0.0129
CoAl-
LDHs
P25
2.97
1.53
2.94
1.57
2.95
1.55
2.95
1.55
0.0163
4A. Meanwhile, a small amount of CO and H2 were also detected
(Figure S5). Interestingly, the yield of CO and H2 became
balanced after several hours of reaction and did not increase
anymore, which might indicate that it reached an equilibrium
between the production rate and consumption rate. The CH4
yield gradually increased with the reaction time and reached an
overall value of 238 μmol·g-1 in 55 hours (corresponding to
approximately 4.3 μmol·g-1·h-1), which was efficient for a pure
semiconductor without any modifications, and the average CH4
formation rate of CoAl-LDH nanosheets (4.2 μmol·g-1·h-1) was 13
times higher than that of P25 (0.3 μmol·g-1·h-1) for five hours’
reaction (Figure S6, supporting information). The CH4 selectivity
3.2 Photocatalytic activity
To exclude the possible influence of environmental
contaminations on photocatalysts and the reaction equipment,
photocatalysts in closed pyrex cell were first illuminated for
three hours without injecting water and CO2 to affirm no CO and
CH4 product was released before measuring photocatalytic
activity. Blank experiment was conducted with CoAl-LDH
nanosheets in the absence of CO2 under photoillumination.
Neither CO nor CH4 was detected, confirming CO2 was indeed
the carbon source (Figure S3, S4). Another control experiment
was done without either illumination or CoAl-LDH nanosheets,
no products were detected, indicating that both the light
irradiation and CoAl-LDH nanosheets were necessary for CO2
photoreduction. The predominant reaction product was CH4,
whose yield as a function of irradiation time is plotted in Figure
for CO2 conversion reached 92% (selectivity = NCH4 / N(CH4+CO)
100%) in 55 hours, while the CH4 selectivity for whole reduction
products was calculated to be 77% (selectivity = NCH4
*
/
N(CH4+CO+H2) * 100%). Table S1 (supporting information)
summarizes the activities of some widely used semiconductor
catalysts. It can be seen that CoAl-LDH nanosheets are of high
efficiency for the photoreduction of CO2 to CH4. More
importantly, the CO2 photoreduction activity did not show any
significant loss even after 55 hours, indicating the good
photostability of CoAl-LDH nanosheets. Besides, CoAl-LDH
nanosheets were conducted for five cycles of repetition test
(Figure 4B), the CH4 generation rate remained almost
unchanged, which confirms the stability further.
3.3 Mechanism and discussion
So the question is why the CoAl-LDH nanosheets exhibited high
activity for photoreduction of CO2 to CH4 under low-
concentration CO2 atmosphere. The origins are considered to
Figure 4. (A) Time course of CH4 evolution for CoAl-LDH nanosheets in the
photocatalytic conversion of CO2 with water. (B) Cycling curves of photocatalytic
production of CH4 on CoAl-LDH nanosheets.
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Figure 7. (A) Time course of CH4 evolution for CoAl-n (n=0, 0.25, 0.5, 2). (B) DRS spectra
of CoAl-n (n=0, 0.25, 0.5, 2). (C) XPS spectra of the Co 2p region for CoAl-n (n=0, 0.25,
0.5, 2). (D) The magnified XPS spectra of Co 2p1/2 region for CoAl-n (n=0, 0.25, 0.5, 2).
adsorption further. First, the spectrum for CoAl-LDHs (noted as
CoAl-LDH-initial) was recorded when the apparatus was purged
with dry argon. Dry CO2 was then introduced for 10 min (20
mL/min) and the spectrum was recorded (noted as CoAl-LDH-
CO2 (10 min)). Argon was subsequently used to expel the CO2
for 1 min, 10 min (20 mL/min, noted as CoAl-LDH-Ar (1 min) and
CoAl-LDH-Ar (10 min), respectively). As shown in Figure 6A,
several absorption bands at 2230-2380 cm-1 appeared after the
introduction of CO2, which are typical of the ν3 anti-symmetric
stretching modes of CO2 gas, while the new-emerging bands at
682 cm-1, 669 cm-1 and 649 cm-1 could be attributed to the
splitting of the v2 bending mode of adsorbed CO2.31,32 After the
expelling of CO2 by argon, it was observed that the absorption
bands at 2230-2380 cm-1 became weakened and vanished at
last, indicating that CO2 gas was nearly expelled. However, we
could find that the bands around 669 cm-1 remained and had
not been blown away, revealing a good adsorption of CO2. With
regard to the spectra for P25 (Figure 6B), similar absorption
bands were observed after the introduction of CO2.
Nevertheless, the bands decreased rapidly and vanished at last
after expelling by argon. Therefore, it is proposed that the
efficient adsorption of CO2 for CoAl-LDHs could contribute to
CO2 photoreduction.
Figure 6. (A) In situ FT-IR spectra for CO2 adsorption on CoAl-LDHs. (B) In situ FT-IR
spectra for CO2 adsorption on P25.
derive from the surface alkaline OH groups for efficient
adsorption of low-concentration CO2 and the divalent cobalt for
dissociating water to release hydrogen for the hydrogenation
process of the intermediates, which will be discussed in detail
in this section.
3.31. Efficient adsorption of CO2
To accurately present the CO2 adsorbing capacity of the
catalysts in the experimental environment, CO2 adsorption
measurements were conducted in the closed pyrex cell used for
photocatalytic reaction. As shown in Figure 5, CoAl-LDHs
exhibited an admirable adsorption capacity of 2.95 cm3·g-1,
which was about two times than that of P25 (1.55 cm3·g-1) with
a comparative specific surface area (50 m2·g-1), indicating the
efficient adsorption of low-concentration CO2 for CoAl-LDHs.
The experiments were repeated for three times to ensure the
veracity. Table 1 shows that the measured values kept almost
the same, and the standard deviation of CO2 adsorption
capacity was much smaller than the CO2 adsorption capacity,
manifesting the reliability of the experimental method. In
addition, some other LDHs were conducted on CO2 adsorption
experiments as well, and all of them exhibited good adsorption
of CO2 molecules (Figure S7), probably due to the surface OH
groups.
3.32. Divalent cobalt for the unique and effective property
Further experiments have been conducted to demonstrate the
unique effect of the divalent cobalt in CoAl-LDHs for CO2
photoreduction to CH4. The divalent cobalt was oxidized by
o
heating the CoAl-LDHs in oxygen atmosphere at 85 C for 0 h,
0.25 h, 0.5 h, 2 h (donated as CoAl-0, CoAl-0.25, CoAl-0.5, CoAl-
2), respectively. It is expected that cobalt (II) would be oxidized
to cobalt (III) and a series of CoAl-LDHs with different cobalt (II)
concentrations were prepared.33 As shown in Figure 7A, the
catalytic activity for CO2 photoreduction to CH4 decreased
sharply with the oxidation time of CoAl-LDHs increasing, while
there was no significant difference between CoAl-n (n=0, 0.25,
0.5, 2) samples in the X-ray diffraction (Figure S8), indicating
In situ FT-IR spectra were implemented to investigate CO2
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that the phase change of CoAl-LDHs had not occurred during the emission for CoAl-0 gradually reached a platform with time,
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DOI: 10.1039/C8TA01309H
oxidation for different time, which could also be confirmed by which has been exhibited in Figure S5. In comparison with CoAl-
the UV-visible diffuse reflectance spectra. The DRS spectra of 0, the CO emission for CoAl-2 increased steadily with reaction
CoAl-n (n=0, 0.25, 0.5, 2) did not show much difference (Figure time. It is proposed that the decrease for CH4 product could
7B). XPS analysis was then carried out to obtain the surface likely be in some connection with the sustaining CO emission,
information of the samples. As shown in Figure 7C, the two which was considered to be ascribed to the relative decrease of
strong peaks in the Co region around 797.3 eV and 781.4 eV consumption rate compared with the production rate.
could be assigned to Co 2p1/2 and Co 2p3/2, while the other two Meanwhile, the H2 emission for CoAl-n (n=0, 2) displayed
peaks around 803.7 eV and 787.5 eV could be assigned to the distinct difference from each other (Figure 6B). The CoAl-0
satellite peaks of Co 2p1/2 and Co 2p3/2, respectively.34 exhibited relatively stable H2 yield for the initial several hours,
Generally, the peaks at 798 eV and 797 eV in Co 2p1/2 band were while little H2 emission was detected using CoAl-2 as the
severally indicative of cobalt (II) and cobalt (III).35-38 On account photocatalyst. As is known, H in the CH4 originates from the H
of the oxidation of divalent cobalt for different times, it was in H2O for CO2 photoreduction with water. Hence, the activation
found that the peak position of Co 2p1/2 had moved to lower of water to release H is of great significance for CH4 formation.
binding energy with the prolongation of oxidation time (Figure H2 emission was experimentally observed for CoAl-0 catalyst,
7D), indicating that the oxidation degree of divalent cobalt indicating that CoAl-0 must have the ability to dissociate water
improved from CoAl-0 to CoAl-2 (CoAl-n, n=0, 0.25, 0.5, 2). to release H, while CoAl-2 is likely to have negligible activity to
Figure 7C also shows that the ratio between cobalt (II) and dissociate water to release H for the further reaction with
cobalt (III) decreased when extending the oxidation time. All carbon intermediates like CO, which might account for the
these above make it clear that CoAl-n (n=0, 0.25, 0.5, 2) just decrease of CO consumption rate and the sustaining increase of
distinguished from each other with the oxidation degree of CO yield. It should be noted that the divalent cobalt is
divalent cobalt on the surface, but with bulk phase to be the frequently-used element employed in water splitting by virtue
same. However, CoAl-n (n=0, 0.25, 0.5, 2) differed greatly on the of the unique and effective electronic structure configuration.40-
activity of CO2 photoreduction, and it does not take much effort 42 Therefore, it is proposed that the poor CH4 yield of CoAl-2
to find that the catalytic activity of CO2 photoreduction to CH4 could likely stem from the weak ability to dissociate water to
decreased sharply with the oxidation degree of divalent cobalt release H when the divalent cobalt was oxidized. In addition, it
increasing, which strongly confirms that the divalent Cobalt has been demonstrated that Co (II) could convert into Co (I)
plays a key role in CH4 generation.
under photo-excitation, and then the photoexcited electrons
CO2 adsorption experiments showed that the CO2 adsorbing could be transferred to CO2 from Co (I).43 All above indicate the
capacity of CoAl-n (n=0, 2) remained almost unchanged (Figure unique and effective property of the divalent cobalt for CO2
S9), declaring that the decrease of catalytic activity had no photoreduction.
relation with the CO2 adsorption. To gain deeper insights into
In addition to CoAl-LDHs, we further explored the CO2
the reason for the poor photocatalytic activity of CoAl-LDHs photoreduction activity of CoM-LDHs (M=Cr, Fe) and NAl-LDHs
once the divalent cobalt was oxidized, some traces were seized (N=Zn, Ni) synthesized by the same method as CoAl-LDHs. XRD
through other products (such as CO, H2) for CO2 patterns, UV-visible absorption spectra and FT-IR spectra of
photoreduction. CO or HCOOH is generally considered to be the these LDHs were displayed in Figure S10, Figure S11, and Figure
primary intermediate for CO2 photoreduction to CH4.39 In this S12. The CH4 formation rates for CoFe-LDHs and CoCr-LDHs
reaction system of CoAl-LDHs, CO was expected to be the were 2.2 μmol·g-1·h-1 and 3.3 μmol·g-1·h-1, respectively (Figure
primary intermediate in view of that abundant CO was S13). CoM-LDHs (M=Cr, Fe) exhibited efficient activity for CO2
generated and could be further consumed as the reactant. If a photoreduction to CH4 while there was little CH4 yield for NAl-
large amount of HCOOH was produced as the primary LDHs (N=Zn, Ni), possibly indicating the universality of the
intermediate, CoAl-LDH catalyst would be gradually corroded divalent cobalt for CO2 photoreduction.
with reaction time. However, CoAl-LDHs were of good stability
in photocatalytic process.
The mechanism of CO2 photoreduction is complex, and CH4
generation is considered to be a multi-step process. Generally,
CoAl-0 and CoAl-2 were taken as samples to test the yields
of CO and H2 with the reaction time. As shown in Figure 8A, CO
Figure 8. (A) Time course of CO evolution for CoAl-n (n=0, 2). (B) Time course of
H2 evolution for CoAl-n (n=0, 2).
Scheme 1. CO2 photoreduction on CoAl-LDH nanosheets with water.
6 | J. Name., 2012, 00, 1-3
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Journal of Materials Chemistry A
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Journal Name
ARTICLE
10 R. Kuriki, M. Yamamoto, K. Higuchi, Y. Yamamoto, M.
Akatsuka, D. Lu, S. Yagi, T. Yoshida, O.DIsOhIi:t1a0n.1i0a3Vn9ied/wCK8AT.rtAiMc0le1aO3e0ndl9inaHe,
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the first step to activate CO2 over catalysts is of great
significance for the subsequent reactions, since efficient binding
could greatly reduce the high potential barrier and accelerate
the dissociation of CO2. CO2 photoreduction over CoAl-LDHs is
suggested to begin with the chemisorption between CO2 and
the OH groups. The linear CO2 molecules become bent and
activated after binding with surface OH Lewis bases, forming
carbonate-like species. The carbonate-like species could then
possibly be converted into CO accompanied by the break of C=O
under illumination. Meanwhile, water is dissociated to proton
and hydroxyl by the divalent cobalt. The adsorbed CO could
likely be further converted into CH4 via the proton-assisted
multi-electron transfer mechanism while some H2 was
generated by direct combination of two protons and two
electrons.
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4. Conclusions
In summary, CoAl-LDH nanosheets were synthesized by a facile
co-precipitation method and showed a hexagonal sheet-like
morphology with a lateral size of about 80 nm and a thickness
of about 18 nm. The CoAl-LDH nanosheets exhibited efficient
catalytic activity and good stability for low-concentration CO2
photoreduction to CH4 with water under simulated solar light.
The origins are considered to derive from the surface alkaline
OH groups for efficient adsorption of low-concentration CO2
and the divalent cobalt for dissociating water to release
hydrogen for the hydrogenation process of the intermediate
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Graphical abstract
High CH4 selectivity for CO2 photoreduction of atmospheric concentration on CoAl-LDHs
1