Metallic Cobalt–Carbon Composite as Recyclable and Robust Magnetic Photocatalyst for Efficient CO2 Reduction

Article abstract of DOI:10.1002/smll.201800762

CO₂ conversion into value-added chemical fuels driven by solar energy is an intriguing approach to address the current and future demand of energy supply. Currently, most reported surface-sensitized heterogeneous photocatalysts present poor activity and selectivity under visible light irradiation. Here, photosensitized porous metallic and magnetic 1200 Co?C composites (PMMCoCC-1200) are coupled with a [Ru(bpy)₃]Cl₂ photosensitizer to efficiently reduce CO₂ under visible-light irradiation in a selective and sustainable way. As a result, the CO production reaches a high yield of 1258.30 μL with selectivity of 64.21% in 6 h, superior to most reported heterogeneous photocatalysts. Systematic investigation demonstrates that the central metal cobalt is the active site for activating the adsorbed CO₂ molecules and the surficial graphite carbon coating on cobalt metal is crucial for transferring the electrons from the triplet metal-to-ligand charge transfer of the photosensitizer Ru(bpy)₃²⁺, which gives rise to significant enhancement for CO₂ reduction efficiency. The fast electron injection from the excited Ru(bpy)₃²⁺ to PMMCoCC-1200 and the slow backward charge recombination result in a long-lived, charge-separated state for CO₂ reduction. More impressively, the long-time stability and easy magnetic recycling ability of this metallic photocatalyst offer more benefits to the photocatalytic field.

Full text of DOI:10.1002/smll.201800762

COMMUNICATION
Photocatalysis
www.small-journal.com
Metallic Cobalt–Carbon Composite as Recyclable and
Robust Magnetic Photocatalyst for Efficient CO Reduction
2
Kun Zhao, Shenlong Zhao, Chao Gao, Jian Qi, Huajie Yin, Ding Wei,
Megasia Feyissa Mideksa, Xiaoli Wang, Yan Gao, Zhiyong Tang,* and Ranbo Yu*
Thanks to the depletion of fossil fuels
CO conversion into value-added chemical fuels driven by solar energy is
2
and the increased daily requirements for
energy fuels, large amounts of anthro-
pogenic CO2 emissions associated with
industrial production and daily life have
led to global warming and environmental
an intriguing approach to address the current and future demand of energy
supply. Currently, most reported surface-sensitized heterogeneous photo-
catalysts present poor activity and selectivity under visible light irradiation.
Here, photosensitized porous metallic and magnetic 1200 CoC composites
[
1–3]
crisis.
Therefore, CO conversion into
2
(
PMMCoCC-1200) are coupled with a [Ru(bpy) ]Cl photosensitizer to effi-
3 2
value-added chemical fuels is an intriguing
approach to address the current and
future demand of energy supply. The solar
light-driven photocatalytic CO2 reduction
into chemical fuels is believed to be one
of the best methods to address energy
and environment problems. Although
a number of solar-active catalysts have
been reported for CO2 reduction, most
of them suffer from low energy conver-
sion efficiency, uncontrollable selectivity,
ciently reduce CO under visible-light irradiation in a selective and sustain-
2
able way. As a result, the CO production reaches a high yield of 1258.30 µL
with selectivity of 64.21% in 6 h, superior to most reported heterogeneous
photocatalysts. Systematic investigation demonstrates that the central metal
cobalt is the active site for activating the adsorbed CO molecules and the
2
surficial graphite carbon coating on cobalt metal is crucial for transferring
the electrons from the triplet metal-to-ligand charge transfer of the photo-
sensitizer Ru(bpy) 2 , which gives rise to significant enhancement for CO
+
3
2
+
reduction efficiency. The fast electron injection from the excited Ru(bpy)32
to PMMCoCC-1200 and the slow backward charge recombination result in a
[
4–6]
and instability.
Therefore, the design
and fabrication of highly active and inno-
vative photocatalytic systems with high
conversion efficiency and selectivity of
long-lived, charge-separated state for CO reduction. More impressively, the
2
long-time stability and easy magnetic recycling ability of this metallic photo-
catalyst offer more benefits to the photocatalytic field.
CO reduction remain a great challenge.
2
Heterogeneous semiconductor cata-
lysts reported to exhibit unsatisfactory
activity and selectivity in photocatalytic CO reduction, due to
2
Dr. K. Zhao, Prof. R. Yu
their wide energy band gap nature only allowing for absorp-
tion of a small fraction of solar spectrum and kinetically more
Department of Physical Chemistry
School of Metallurgical and Ecological Engineering
University of Science and Technology Beijing
Beijing 100083, P. R. China
+
favorable photocatalytic reduction of H . Simultaneously,
homogeneous molecular catalysts also offered an alternative
E-mail: ranboyu@ustb.edu.cn
strategy for photocatalytic CO reduction owing to their better
2
Dr. K. Zhao, Dr. S. Zhao, Dr. C. Gao, Dr. D. Wei, Dr. M. F. Mideksa,
Dr. X. Wang, Prof. Y. Gao, Prof. Z. Tang
CAS Key Laboratory of Nanosystem and Hierarchical Fabrication
National Center for Nanoscience and Technology
Beijing 100190, P. R. China
light utilization efficiency and low activation requirement for
[
6]
CO2 reduction. However, homogeneous catalysis systems
have own shortcomings such as involvement of high cost, time-
consuming synthesis procedure, instability, and nonreusable,
which severely impede their practical applications in photo-
E-mail: zytang@nanoctr.cn
Dr. K. Zhao, Dr. H. Yin
Centre for Clean Environment and Energy
Gold Coast Campus
Griffith University
QLD 4222, Australia
[7]
catalysis. Indeed, some new systems are being developed to
construct molecular combined with semiconductor, to improve
the performance of CO converting into chemical fuels. Among
2
different candidates, the cobalt composites have been dem-
[
8–10]
Dr. J. Qi
onstrated as one of the most promising catalysts.
Cobalt-
State Key Laboratory of Biochemical Engineering
Institute of Process Engineering
Chinese Academy of Sciences
Beijing 100190, P. R. China
containing zeolitic imidazolate framework or Co O metal
3
4
oxides have been recently reported as novel photocatalytic sys-
tems with the assistance of a ruthenium-based photosensitizer
[
11–14]
for CO reduction.
It is surprising to recognize that heter-
2
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/smll.201800762.
ogeneous photocatalytic photosensitized mechanisms to access
excited-stated magnetic recyclable and metallic photocatalysts
[
14,15]
DOI: 10.1002/smll.201800762
have never been paid more attentions.
Small 2018, 1800762
1800762 (1 of 8)
© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

www.advancedsciencenews.com
www.small-journal.com
A detailed description of the sample PMMCoCC-1200 is
shown in Figure 2. The specific high-resolution TEM (HRTEM)
image (Figure 2a) reveals that the dominant exposed surfaces
of PMMCoCC-1200 are (111) facets of cobalt metal, which
is corresponding to a lattice space of 0.208 nm. Meanwhile, it is
found that the specific structural feature of PMMCoCC-1200
is further presented by selected area fast Fourier transform
(
SAFFT) image (the inset of Figure 2a) to evidence the metal
cobalt existence. Furthermore, the Raman spectrum (Figure 2b)
of graphite carbon shows a sharp G2g peak, declaring its better
2
crystallinity with a long-range conjugation structure of sp
Scheme 1. Schematic illustration of the synthesis procedure for
PMMCoCC-1200. a) Synthesis of Co74s with CoCl2 and 2, 5-dihydroxy-
terephthalic acid in a mixture solution. b) Morphology of Co74s and
the corresponding crystalline structure. c) Post–treatment of Co74s to
carbon, rather than that obtained from the modified Hum-
[
17–19]
mer’s method.
small D peak near 1325 cm contributed to defect carbon and
In addition, PMMCoCC-1200 also shows a
−
1
−
1
[17a]
produce PMMCoCC for photocatalytic CO reduction by following.
sharp G peak (1590 cm ) contributed to graphite carbon.
More importantly, the ratio of I /I is 0.474, which indicates
2
D
G
Herein, an artificial photosystem based on photosensitized
that PMMCoCC-1200 owns the relatively better crystallinity of
graphite. The last point should be figured out that the Raman
spectrum (Figure 2b) showed no peak at the 400–600 cm−
to give the evidence that PMMCoCC-1200 owns the metallic
property instead of cobalt oxide.
porous metallic and magnetic CoC composite is proposed
1
to perform the CO reduction. Interestingly and impressively
2
enough, porous metallic magnetic CoC composites derived
from metal organic frameworks (MOFs) coupled with ruthe-
nium photosensitizer is very rare to apply in photocatalytic
CO2 reduction. We surmise that a breakthrough could be
achieved by replacing homogeneous cobalt complexes with
surficial photosensitized porous metallic and magnetic 1200
CoC composite (PMMCoCC-1200), which was used to
realize the surface excited-stated electrons superfast transpor-
tation, resulting in the CO2 reduced into CO. It is the first
time to propose the recycling metallic and magnetic compos-
ites mixed with organic molecule complex to fulfill the high
After that, the content of PMMCoCC-1200 is obtained from
the TG measurement (Figure 2c) including Co metal 90.5 and
C 9.5 wt% (graphite carbon, 7.5 wt%; defect carbon, 2.0 wt%).
Moreover, the series of post-treatment samples under different
calcination temperatures (Figure S3, Supporting Information)
show a general regular that the cobalt metal content increases
slightly and the carbon content decreases slightly with the
temperature increasing. The mainly reason is that the carbon
is easy to be removed with the temperature increasing, but at
the same time part of defect carbon will transfer into graphite
carbon, which is formed due to the metal cobalt catalytic effect,
efficiency of CO reduction.
2
PMMCoCC-1200 was synthesized by calcination of
Co-MOF74 sheets (Co74s) precursors which are obtained by
[
17b]
to improve the electron conductivity.
mixture of CoCl solution with 2, 5-dihydroxyterephthalic acid
The typical X-ray photoelectron spectroscopy (XPS) analysis
reveals that Co2p peak is corresponding to cobalt metal peak
derived from carbonized Co74s, which is consistent with
2
in a mixture uniform solution. Schematic illustration of the syn-
thesis procedure for PMMCoCC-1200 is shown by Scheme 1.
The scanning electron microscope (SEM) image (Figure 1a)
of Co74s shows the sheet-like morphology and typical powder
X-ray diffraction (XRD) patterns (Figure 1b) of Co74s is shown
by the characteristic diffraction peaks (100) and (300) which
are assigned to Co74s. And also the pore size (1.30 nm) dis-
[
18]
the reported literatures.
And also the peak of absorbed
oxygen on the surface of PMMCoCC-1200 is present for XPS
analysis of O 2p (Figure 2d), which also give the evidence for
PMMCoCC-1200 without the surficial oxidation. In addition,
the band gap calculation of PMMCoCC-1200 is confirmed
with diffuse reflection spectrum (DRS) and K–M transforma-
tion spectrum (Figure 2e). Intriguingly, it is clear that there is
tribution and Brunauer–Emmett–Teller (BET) surface area
2
(
237.30 m g) image in the Figure S1a,b in the Supporting
[16]
Information is the similar with the reported Co74s. Mean-
while, the thermogravimetric (TG) analysis spectrum of Co74s
is shown in Figure S1c in the Supporting Information to claim
largely absorption ranged from 400 to 2000 nm, and the E is
g
obtained (≈0.5 eV) via the K–M transformation spectroscopy.
Besides, the hysteresis loop pattern demonstrates that this type
catalyst is magnetic recycling for reusability and more con-
venient for collection (Figure 2f). Finally, it is given that BET
surface areas and pore sizes analysis are shown in Table S1 in
the Supporting Information. With the calcination temperature
increasing, the pore size becomes large and the BET surface
areas show a downtrend. It is worth noting that the samples
after HCl aching have the larger BET surface areas and more
typical diversity of pore sizes (microporous and nanoporous
structure).
the H O removing and carbonized process. In addition, the
2
images of SEM and transmission electron microscope (TEM)
for the PMMCoCC-1200 derived from Co74s (Figure 1c,d) show
well-dispersed sizes distribution of 100–200 nm (Figure S2,
Supporting Information). It is noted that there are two types
selected area electron diffraction (SAED) patterns of (111) peak
from metallic cobalt and (002) peak from graphene (Figure 1d,
inset), respectively. Subsequently, the image Figure 1e perfectly
matches the face-centered cubic phase of cobalt metal crystal-
line model pattern (Figure 1e, inset image). Finally, the element
mapping images (Figure 1f) show that the cobalt metal and
graphite carbon are component of the PMMCoCC-1200, and
also the graphite carbon is surrounding outside of cobalt metal.
In order to illuminate the relationship between mate-
rial structure and photocatalytic CO2 reductive property,
Figure 3a shows the high yield of CO product (1258.30 µL)
after 6 h long-time reaction with a high selectivity of 64.21%
Small 2018, 1800762
1800762 (2 of 8)
© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

www.advancedsciencenews.com
www.small-journal.com
irradiation of λ = 450 nm. Very impressively,
the external quantum efficiency is estimated
to be 0.049%, significantly outperforming
the reported high performance photocata-
lysts for CO reduction (Table S2, Supporting
2
Information). Simultaneously, in order to
evidence that the photocatalytic CO2 reduc-
tion is based on the whole system, when any
controlled experimental condition is absent,
the reaction cannot be approached (Table S3,
Supporting Information).
Besides the activity, the recycle usability
is another key performance parameter
for the photocatalytic system. To evaluate
the photocatalytic reusable capacity of
PMMCoCC-1200, the used catalysts were
recovered and redispersed in a fresh reac-
tion solution with photosensitizer for recy-
cling tests (Figure 3b). It should be noted
that the catalysts can be separated by magnet
conveniently, due to the magnetic property
(
Figure S5, Supporting Information), which
is evidenced by the Figure 3b inset image. As
demonstrated in Figure 3b, even 3 months
later, the reused catalysts for the third run
still retain ≈90% of their original activity, sig-
nifying an excellent stability and reusability
of PMMCoCC-1200.
Summarized to further explore the key
factors that determine the performance of
PMMCoCC-1200, according to Figure 3c,
a series of control experiments are given to
declare the relationship between Raman
spectra (Figure S6, Supporting Information)
characterization of carbon (I /I ) and CO
D
G
2
photocatalytic reduction activity of carbon-
ized samples under different calcination
temperature (Figure 3c, Supporting Informa-
tion). 50 mg CoC composites photocatalytic
materials obtained from different calcination
temperatures (Figures S7 and S8, Supporting
Information) were added into the photocata-
lytic reaction system, all of samples obtained
from 600 to 1600 °C respectively, showed
the high performance in photocatalytic CO2
reduction. The optimal top point is clearly
shown that PMMCoCC-1200 reaches at the
best performance of CO yield. In order to
explain why PMMCoCC-1200 with general
Figure 1. a) TEM image of Co74s. b) XRD pattern of Co74s. (The inset is the Co74s of crystalline
structure.) c) Large areas of SEM image of PMMCoCC-1200 (The inset is the enlarge image of
PMMCoCC-1200). d) TEM image and corresponding inserted SAED image of PMMCoCC-1200.
e) XRD pattern of PMMCoCC-1200. (Inset is the crystalline structure of metallic cobalt.)
f) Element mapping images of PMMCoCC-1200.
over competing H2 generation by using surface sensitized
PMMCoCC-1200. Impressively, with respect to the yield
of CO (20 µL only sustaining 0.5 h) for the homogeneous
absorption ability of CO (Figure S9, Supporting Information)
2
owns the best photocatalytic performance of CO reduction? As
2
it is shown in Raman spectra, the degree of graphitization of
PMMCoCC-1200 has been enhanced tremendously to leading
the photoexcited electrons transferring more efficiently. So, it
is reasonable to believe that PMMCoCC-1200 has the best gra-
phitized degree (I /I , 0.474) with the comparison of other
[
Ru(bpy) ]Cl photocatalyst which is limited by the stability of
3 2
the chromophore in aqueous solution (Figure S4, Supporting
Information),[
7,8]
PMMCoCC-1200 can continuously gen-
erate CO and increase with three orders of magnitude when
the reaction time arrives at 6 h. In addition, we further deter-
mined the external quantum efficiency (QE) of photocatalytic
CO2 reduction upon PMMCoCC-1200 at a monochromatic
D
G
CoC composites (Figure 3c), as evidenced by the electrochem-
ical impedance spectroscopy (Figure S10 and Table S4, Sup-
porting Information), the higher electrochemical impedance of
Small 2018, 1800762
1800762 (3 of 8)
© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

www.advancedsciencenews.com
www.small-journal.com
Figure 2. The basic characteristics of PMMCoCC-1200. a) HRTEM image (inset is the corresponding typical SAFFT pattern). b) Raman spectroscopy
excited wavelength of 514 nm). c) TG analysis spectra. d) XPS spectra for O 1s and Co 2p. e) Absorption spectrum from DRS (inset image is band
(
gap calculation = 0.5 eV). f) Hysteresis loop pattern.
PMMCoCC-1200 is benefit for electron transportation. In addi-
tion, based on the analysis of Raman spectra, it is concluded
that the detailed information of different samples I /I ratio
is shown in Table S5 in the Supporting Information. Another
important contrast is shown in Figure S11 in the Supporting
Information to evident when the Co74s is calcined under the
of commercial cobalt metal without the coated layered gra-
phene carbon, hindering the absorption of Ru complexes and
the electrons transferring. Meanwhile, compared to the iron
powder, a plausible discovery can be concluded that this type
system metal composited with graphene carbon is benefit
D
G
for photocatalytic CO reduction, when the Ru complexes are
2
Ar-H mixture gas and finally obtain the cobalt metal particle
added in photocatalytic system.
2
with less carbon coated, and the performance is worse than that
of PMMCoCC-1200. An obvious conclusion can be given that
when the conductivity and crystalline graphite carbon compos-
ited with metal cobalt was likely to realize the efficient photo-
For the innovative photocatalytic system, it is crucial to
develop the general materials for that whether this type
carbonized MOF derived can be used efficiently for phtocata-
lytic CO reduction. According to the Figure 3f, because this
2
catalytic CO reduction.
type structure reflects that the electrons being in direct contact
with the metal and graphene carbon support possess active
photocatalytic properties, these systems are generally carbon-
ized to enhance the activity toward kinetic favored photocata-
2
The photocatalytic active site is the key point for cata-
lytic reaction, so exploration of the active site becomes more
and more important. So here, the etching experiments were
designed to corrode the metal cobalt under the different
condition (Figure 3d). With the increase of the degree for cobalt
metal removing, it is found that the photocatalytic performance
becomes worse, and even there is no CO product when the
cobalt metal nanoparticles cores almost disappear after etching
for 5 h (Figure S12, Supporting Information). This phenom-
enon gives the strong evidence that the cobalt metal is the
lytic CO reduction. It has inspired the use of metal organic
2
frameworks (Fe-, Ni-, Mn-, or Co-Fe-MOF74) to take post-
treatment for obtaining the metal nanoparticles embedded
into the graphene carbon within several layers or inside their
channels, to influence the activity of the entire nanoparticle
surface while maintaining efficient reactant and product trans-
port owing to the porous nature of the material. Under the
same condition, we try to test other kinds of carbonized MOF
materials for photocatalytic CO2 reduction. Although other
type metal–carbon composites showed low activity for CO2
reduction, FeC composites owned good performance for
excellent photocatalytic CO2 reduction property. In sharp
contrast to the reported homogeneous organometallic cata-
lysts that are thermodynamically unstable and subjected to
active site for the photocatalytic CO reduction.
In order to explain the excellent CO reduction property only
2
2
obtained from CoC composites combined with Ru complex
photosensitizer, a series of contrast experiments were car-
ried out in following test for commercial cobalt metal nano-
particles, Co74s, commercial iron metal nanoparticles, and
graphene carbon(Figure 3e). It is expected when commercial
cobalt metal powder instead of PMMCoCC-1200 is adopted
as catalyst under the same conditions; a relative low CO yield
of 405.59 µL is obtained (the first column in Figure 3e). Such
low catalytic activity should be ascribed to the structural nature
[
7]
oxidative degradation, PMMCoCC-1200 system has advan-
tages of low-cost, ease of synthesis and readily recoverable for
reuse. In summarized Table S6 in the Supporting Information,
PMMCoCC-1200 system also owns the relatively good
Small 2018, 1800762
1800762 (4 of 8)
© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

www.advancedsciencenews.com
www.small-journal.com
Figure 3. a) CO (red square dots) and H (black spherical dots) evolution with reaction prolonging upon irradiation of visible light using PMMCoCC-1200.
2
b) Yields of CO and H in 6 h with PMMCoCC-1200 in recycling tests. (Inset image shows recycling capacity in magnetic absorption method.) c) The
2
relationship between Raman spectrum characterization of carbon (I /I ) and CO photocatalytic reduction activity of carbonized samples under
D
g
2
−
1
different calcination temperature. d) PMMCoCC-1200 after etching in 1 mol L HCl for yields of CO and H . e) Average yields of CO and H in first
2
2
6
h for control experiments under the same conditions using commercial cobalt powder, Co74s, iron powder, graphene, respectively. f) Carbonized
products derived from Fe-MOF74, Ni-MOF74, Mn-MOF74, and Co-Fe-MOF74 under 1200 °C in Ar atmosphere for photocatalysis. (Test condition:
3
00 W Xe lamp, λ > 420 nm; catalyst: 50 mg; photosensitizer: [Ru(bpy) ]Cl , 10 mg; reaction solution: water: CH CN: TEOA = 1:3:1, 30 mL solvent
3
2
3
under mild reaction conditions (15 °C and 1 atm CO2).
selectivity. More importantly, it simultaneously possesses supe-
rior activity, selectivity, and highly promising for large-scale
HCOOH, were detected (Figure S13a, Supporting Informa-
tion). Second the isotopic 13CO2 was used as the reactant
in the photocatalytic reaction and the obtained products
were identified by gas chromatography-mass spectrometry
(Figure S13b, Supporting Information). The peak appearing
at 2.4 min with m/z = 29 can be assigned to the gener-
photocatalytic CO reduction system.
2
After quantitatively evaluating the catalytic performance,
two additional experiments were carried out to ascertain the
photocatalytic products CO from CO . First, the solvent after
2
photocatalytic reaction was analyzed by gas chromatograph
ated 13CO, while the peak with m/z = 45 is ascribed to the
1
13
and H-NMR spectroscopy. No measurable hydrocarbon
original CO . This confirms that the produced CO is origi-
2
products derived from CO , such as CH OH, HCOH, and
nated from CO2.
2
3
Small 2018, 1800762
1800762 (5 of 8)
© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

www.advancedsciencenews.com
www.small-journal.com
Figure 4. a) Proposed mechanism for the photosensitized catalytic reduction of CO to CO. b) Schematic representation of energy level diagram
2
2
+
showing electron transfer from Ru(bpy)3 to PMMCoCC-1200. E : Fermi level; LUMO: lowest unoccupied molecular orbital; HOMO: highest occupied
F
molecular orbital. (See Supporting Information for more details for calculation of E , HOMO and LUMO).
F
On the basis of previous works in homogeneous systems[1]
and the above results, we propose the following mecha-
nism (Figure 4a). Upon light excitation, the photosensitizer
Ru(bpy)3 is promoted to the excited state. This excited state
is then reductively quenched by a sacrificial electron donor
owns the ultraquick transferring function for electrons conduct
and gives the CO perfectly reactive condition, leading to the
2
efficient photocatalytic CO reduction.
2
2
+
In summary, tunable structure and contents of PMMCoCC
composites are derived from Co74s fabricated by ultrasoni-
cation methods. PMMCoCC-1200 with more uniform mor-
phology, high degree of graphitization, and more convenient
magnetic recyclability is demonstrated as excellent heteroge-
[
triethanolamine (TEOA)] to form a reduced photosensi-
tizer. The reduced photosensitizer transfers an electron to
PMMCoCC-1200, where CO2 molecule is absorbed and acti-
vated. During above reduction process, it has been evidenced
that PMMCoCC-1200 owns a lot of pores on the surface, this
porous structure is benefit for small molecule going through,
and also the excited photoelectrons can easily transfer between
graphite carbon and cobalt metal. Finally, CO2 is reduced to
CO and subsequently released from PMMCoCC-1200 surface.
neous photocatalyst for CO reduction; for instance, highly effi-
2
cient activity with orders of magnitudes higher than single Ru
complexes, outstanding stability with negligible activity decay
after three repetitive reactions. Due to the existence of metal
cobalt active site and the graphene carbon with superelectronic
conductivity, the synergetic effect from metal cobalt and gra-
As the catalytic reduction of CO by PMMCoCC-1200 is initi-
phene carbon promotes the photocatalytic CO reduction into
2
2
ated by the photosensitizer, the electrons transferring from
photosensitizer to PMMCoCC-1200 is a prerequisite, which
is feasible as demonstrated in the corresponding energy level
CO more effectively. It is worth noting that it is the first time to
propose porous metallic and magnetic material combined with
Ru(bpy)32 , which is used to obtain an optimized photocata-
lytic CO2 reduction. Our understanding for PMMCoCC-1200
will not only provide a guide for the development of efficient
and durable magnetic photocatalysts for solar energy conver-
sion, but also offer important insight into the exact identity of
active sites in such noble-metal-free composites for photocata-
+
diagram (Figure 4b). The fermi level (E ) of PMMCoCC-1200
F
relative to the vacuum level is measured to be 5.04 eV, respec-
tively (Figure S14, Supporting Information). Also, the highest
occupied molecular orbital (HOMO) and lowest unoccupied
molecular orbital (LUMO) energy levels of Ru(bpy)32 with
respect to the vacuum level are determined as 3.19 and 5.68 eV,
+
lytic CO reduction. This study also gives a new angle through
2
respectively (Figure S15, Supporting Information). Since the E
a novel strategy for developing high performance of the surface
photosensitized heterogeneous photocatalysts. More impres-
sively, it paves the way to deep insight into understanding the
key factors for improving the photocatalytic performance for
F
of PMMCoCC-1200 is lower than the LUMO of Ru(bpy)32 , the
+
electrons in the LUMO of photosensitizers can be preferentially
transferred to the E of PMMCoCC-1200, enabling the subse-
F
quent reduction of CO . The employed PMMCoCC-1200 here
CO reduction.
2
2
play two important roles: 1) hosting the metal cobalt active sites
for CO reduction; 2) accepting electrons from the LUMO of
2
3
[
Ru(bpy) ]Cl and transferring the received electrons to reacti-
2
Experimental Section
[
20,21]
vated CO for reduction at the metal cobalt active sites.
It is supposed that CO2 molecular and Ru complexes
absorbed on the surface of PMMCoCC-1200 and reactivated,
2
Chemicals: All chemicals used were of analytical grade and were used
as received without any further purification. MnCl ·4H O, NiCl ·6H O,
2
2
2
2
FeCl ·6H O, and CoCl ·6H O was purchased from Sinopharm Chemical
3
2
2
2
then the visible light excited Ru complex due to the effect of
Reagent. 2, 5-dihydroxyterephthalic acid and [Ru(bpy) ]Cl ·6H O
3
2
2
metal–ligand–charge–transferring,[
22–27]
the separated electrons
(
bpy = 2, 2-bipyridine) were obtained from Sigma-Aldrich. Ethanol,
were transferred to PMMCoCC-1200 and the holes take part in
the TEOA oxidation. It can be imagined that PMMCoCC-1200
triethylamine (TEA), N, N-dimethylformamide (DMF), TEOA, and
acetonitrile (MeCN) were purchased from Alfa Aesar. Carbon dioxide
Small 2018, 1800762
1800762 (6 of 8)
© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

www.advancedsciencenews.com
www.small-journal.com
gas, supplied by Beijing Beiwen Gases Company, was of super grade
purity (99.999%). The 13CO2 was 99% enriched providing by Shanghai
Wusheng Bio. Company. Ultrapure water (Millipore Milli-Q grade) with a
resistivity of 18.2 MΩ·cm was used in all the experiments.
cyclic voltammogram were recorded using a CHI 660C computer-
controlled potentiostat (ChenHua Instruments Co., Shanghai, China)
in a standard three-electrode system. The measured BET surface
area was obtained by ASAP2020 (M+C). Magnetic hysteresis loop
1
The Synthesis of Porous Metallic and Magnetic 1200 CoC Composites
patterns were measured by PPMS-9 H-NMR spectra were obtained
(
PMMCoCC-1200): First, DMF (30 mL), ethanol (2.5 mL), and water
by AVANCE III HD 400. Fourier transform-infrared spectroscopy was
performed using a PerkinElmer Spectrum One spectrometer. Ultraviolet
photoelectron spectroscopy (UPS) and XPS were carried out using a
UPS/XPS system (Thermo-Fisher, ESCLAB 250, USA). UV–vis diffuse
reflection absorption spectra of the samples were recorded by an UV–
vis spectrometer (Lambda 950, PE, USA) equipped with an integrating
sphere accessory in the diffuse reflectance mode (R) and BaSO4 as
reference material. UV–vis absorption spectra were obtained by a
Hitachi U-3010 Spectrophotometer.
(
2.5 mL) were mixed in a 100 mL polytetrafluoroethylene (PE) tube.
Next, 0.0775 g 2, 5-dihydroxyterephthalic acid was dissolved into
the mixed solution under ultrasonication. Subsequently, 0.375
CoCl ·6H O was added and dissolved, then, 0.8 mL TEA was quickly
g
2
2
injected into the solution. And then, the solution was stirred for 5 min
to obtain a uniform colloidal suspension. Afterward, the colloidal
solution was continuously ultrasonicated for 8 h (40 kHz) under airtight
condition. Finally, the obtained clay bank precipitates Co-MOF74
sheets (Co74s) were centrifuged and washed with deionized water and
DMF for three times, and finally dried by a vacuum oven under 80 °C
for 24 h.
Photocatalytic Test: The photocatalytic experiments were carried
out in a 300 mL pyrex reactor equipped with a Quartz lid under the
photoirradiation condition coupled with the gas chromatograph at
15 °C as controlled by cooling water (Figure S16). The reaction system
containing [Ru(bpy) ]Cl ·6H O (10 mg), PMM(Co, Fe, Mn, Ni, and
The product of CoC composites were respectively prepared by
calcination of the Co74s under 400, 600, 800, 1000, 1200, 1400, and
3
2
2
1
600 °C in an Al O tube furnace for 3 h with the speed of raising
Co-Fe)CC-1200 materials (50 mg), solvent [30 mL, MeCN/TEOA/H O =
2
3
2
−
1
temperature 2 °C min . The final product CoC composites were
collected. Especially, well-dispersed PMCoCC-1200 was obtained through
centrifuging the intermediate mixture ethanol solution with 3000 rmp for
3:1:1 (Vol. ratio)], by stirring with a magnetic stirrer and irradiated
under a 300 W Xenon lamp (Beijing Perfect Light Company) with a
420 nm cutoff filter. The whole reaction setup was vacuum degassed
and then the high-purity CO2 gas was filled into the reaction setup
to reach a pressure of 1 bar. Measured by an optical power meter
(PM100D, Thorlabs), the optical density of the light source with a
3
min and finally dried by a vacuum oven under 80 °C for 24 h.
The PMCoCC-1200 with Posttreatment for Etching: In
a
typical
procedure, the obtained sample CoC composites under different
calcination temperatures were prepared with 5.0 g, respectively.
Subsequently, the former related samples were used HCl solution (the
concentration of 1 mol L ) to etching them under different condition
with keeping 1, 3, and 5 h, respectively. Then, the obtained products
were washed with water and ethanol several times with centrifugating
at 14 ░0 00 rpm. Finally, all the etching samples dried in the vacuum
oven with 80 °C for 24 h, cooled to room temperature in the vacuum
environment.
2
420 nm cutoff filter was 293.61 mW·cm with an illuminated area of
18.08 cm2 and a distance of 11 cm between the liquid level and light
source.
−
1
Quantum Efficiency Values Measurement: External QE values were
calculated using the following equation:
(The number of CO molecules)×2
(The number of incident photons)
QE =
×100%
(1)
The Porous Metallic 1200 M-C (M = Fe, Ni, Mn, and Co (50 wt%)-Fe)
Composites (PMMCC-1200): First, DMF (30 mL), ethanol (2.5 mL),
and water (2.5 mL) were mixed in a 100 mL PE tube. Next, 0.0775 g
The external quantum efficiency values for CO generation were
determined at 450 nm using a single band pass filter by using the same
photochemical experimental setup. The flux of incident photons from
the power was measured by a power meter (PM100D, Thorlabs).
2
,5-dihydroxyterephthalic acid was dissolved into the mixed solution
under ultrasonication. Subsequently, 0.375 g M [M = Fe, Ni, Mn, and
Co (50 wt.%)-Fe] chloride was added and dissolved, then, 0.8 mL
TEA was quickly injected into the solution. And then, the solution
was stirred for 5 min to obtain an uniform colloidal suspension.
Afterward, the colloidal solution was continuously ultrasonicated for
2
The lamp intensity at 450 nm was measured to be 37.3 mW·cm , the
2
illuminated area was 18.08 cm , and the incident photon flux was
−
1
calculated to be 5.49 × 1021 h . The molecules of CO generated were
.66 µmol after 3 h photoirradiation.
6
8
h (40 kHz) under airtight condition. Finally, the obtained clay bank
precipitates were centrifuged and washed with deionized water and
DMF for three times, and finally dried by a vacuum oven under 80 °C,
for 24 h. The product of MC composites was prepared by calcining
the corresponding precursors under 1200 °C in an Al O tube furnace
for 3 h with the speed of raising temperature 2 °C min . The final
PMMCC-1200 was collected.
Supporting Information
2
3
Supporting Information is available from the Wiley Online Library or
from the author.
−
1
Characterization: Transmission electron microscope (TEM),
HRTEM and SAED were performed on a FEI Tecnai G2 F20 electron
microscope operated at 200 kV, and the morphology was observed Acknowledgements
under a Hitachi S4800 scanning electron microscope (SEM). Powder
K.Z. and S.Z. contributed equally to this work. The authors acknowledge
financial support from National Key Basic Research Program of China
2014CB931801 and 2016YFA0200700, Z.Y.T.), National Natural Science
Foundation of China (21475029 and 91427302, Z.Y.T.), Frontier Science
Key Project of the Chinese Academy of Sciences (QYZDJ-SSW-SLH038,
Z.Y.T.), Instrument Developing Project of the Chinese Academy of
Sciences (YZ201311, Z.Y.T.), CAS-CSIRO Cooperative Research Program
XRD patterns were recorded on a Rigaku D/MAX-TTRIII (CBO) X-ray
power diffract meter by using Cu Ka radiation (λ = 1.5418 Å). Gas
chromatographic analysis was conducted using a gas chromatograph
(
(
Tianmei GC-7900) equipped with a TDX-01 packed column, a thermal
conductivity detector and a flame ionization detector (FID). The FID
detector was fitted with a methanizer to detect CO. The product of
1
3
the CO2 isotopic experiment was analyzed by a Trace DSQ GC-MS
Thermo Fisher Scientific, Inc.). The liquid phase of the reaction system
(GJHZ1503, Z.Y.T.), and “Strategic Priority Research Program” of
(
Chinese Academy of Sciences (XDA09040100, Z.Y.T.).
was analyzed by a gas chromatograph (Shimadzu GC-2010 Plus) with
1
an Agilent DB-Wax column and H-NMR spectra (Bruker Advance
4
00 spectrometer, 400 MHz), respectively. CO2 sorption study was
performed in a Micromeritics TriStar II 3020 adsorption apparatus at
88 K up to 1 bar. Prior to sorption measurements, all samples were
degassed at 120 °C for 6 h. Electrochemical impedance spectra and
Conflict of Interest
2
The authors declare no conflict of interest.
Small 2018, 1800762
1800762 (7 of 8)
© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

www.advancedsciencenews.com
www.small-journal.com
Y. Weng, X. Wang, X. Fu, J. Long, Angew. Chem., Int. Ed. 2016, 55, 1;
d) S. Oh, J. R. Gallagher, J. T. Miller, Y. Surendranath, Green Chem.
2015, 17, 1605.
Keywords
magnetic recyclable materials, metallic composites, photocatalytic CO2
reduction
[
15] a) H. Zhang, J. Wei, J. Dong, G. Liu, L. Shi, P. An, G. Zhao, J. Kong,
X. Wang, X. Meng, J. Zhang, J. Ye, Angew. Chem., Int. Ed. 2016, 55,
Received: February 24, 2018
Revised: June 4, 2018
Published online:
1
; b) O. Anjaneyulu, K. Takeda, S. Ishii, S. Ueda, T. Nagao, X. Peng,
T. Fujita, M. Miyauchi, H. Abe, Mater. Chem. Frontiers 2018, 2, 580;
c) J. Zhang, G. Xiao, F. Xiao, B. Liu, Mater. Chem. Frontiers 2017,
1
2
, 231; d) Y. Xu, W. Wen, M. Tang, J. Wu, Mater. Chem. Frontiers
017, 1, 1453; e) Z. Shen, X. Zhang, F. Giordano, Y. Hu, J. Hua,
S. M. Zakeeruddin, H. Tiana, M. Gratzel, Mater. Chem. Frontiers
2017, 1, 181.
[16] S. Zhao, Y. Wang, J. Dong, C. He, H. Yin, P. An, K. Zhao,
X. Zhang, C. Gao, L. Zhang, J. Lv, J. Wang, J. Zhang, A. M. Khattak,
N. A. Khan, Z. Wei, J. Zhang, S. Liu, H. Zhao, Z. Tang, Nat. Energy
2016, 1, 16184.
[17] a) L. G. Cancado, A. Jorio, E. H. Martins Ferreira, F. Stavale,
C. A. Achete, R. B. Capaz, M. V. O. Moutinho, A. Lombardo,
T. S. Kulmala, A. C. Ferrari, Nano Lett. 2011, 11, 3190;
b) M. Z. Rahman, J. Zhang, Y. Tang, K. Daveya, S. Qiao,
Mater. Chem. Frontiers 2017, 1, 562.
[18] a) P. Pachfule, D. Shinde, M. Majumder, Q. Xu, Nat. Chem. 2016, 8,
718; b) X. Yan, Y. Liu, J. Lan, Y. Yu, J. Murowchick, X. Yang, Z. Peng,
Mater. Chem. Frontiers 2018, 2, 96.
[19] a) Y. Wang, Y. Nie, W. Ding, S. G. Chen, K. Xiong, X. Q. Qi, Y. Zhang,
J. Wang, Z. D. Wei, Chem. Commun. 2015, 51, 8942; b) W. Jiang,
L. Gu, L. Li, Y. Zhang, X. Zhang, L. J. Zhang, J. Q. Wang, J. S. Hu,
Z. Wei, L. Wan, J. Am. Chem. Soc. 2016, 138, 3570.
[20] S. A. Didas, M. A. Sakwa-Novak, G. S. Foo, C. Sievers, C. W. Jones,
J. Phys. Chem. Lett. 2014, 5, 4194.
[
1] J. Schneider, H. Jia, J. T. Muckermana, E. Fujita, Chem. Soc. Rev.
012, 41, 2036.
2
[
[
[
2] D. M. Schultz, T. P. Yoon, Science 2014, 343, 1239176.
3] W. Wang, S. Wang, X. Ma, J. Gong, Chem. Soc. Rev. 2011, 40, 3703.
4] Y. Zhao, B. Zhao, J. Liu, G. Chen, R. Gao, S. Yao, M. Li,
Q. Zhang, L. Gu, J. Xie, X. Wen, L. Wu, C. Tung, D. Ma, T. Zhang,
Angew. Chem., Int. Ed. 2016, 55, 4215.
[
[
[
5] a) P. McMorn, G. J. Hutchings, Chem. Soc. Rev. 2004, 33, 108;
b) Q. Yin, J. M. Tan, C. Besson, Y. V. Geletii, D. G. Musaev,
A. E. Kuznetsov, Z. Luo, K. I. Hardcastle, C. L. Hill, Science 2010,
3
28, 342.
6] a) T. Morimoto, T. Nakajima, S. Sawa, R. Nakanishi, D. Imori,
O. Ishitani, J. Am. Chem. Soc. 2013, 135, 16825; b) K. Sekizawa,
K. Maeda, K. Domen, K. Koike, O. Ishitani, J. Am. Chem. Soc. 2013,
135, 4596.
7] a) U. S. Akhtar, E. L. Tae, Y. S. Chun, I. C. Hwang, K. B. Yoon,
ACS Catal. 2016, 6, 8361; b) Y. Hou, X. Chen, S. Yang, Y. Zhong,
C. Lia, H. Zhao, H. Yang, Nano Energy 2017, 36, 102; c) S. Martha,
S. Mansingh, K. M. Parida, A. Thirumurugan, Mater. Chem. Frontiers
2
017, 1, 1641.
[21] X. Chang, T. Wang, J. Gong, Energy Environ. Sci. 2016, 9, 2177.
[22] a) N. Kornienkoa, K. K. Sakimotoa, D. M. Herlihya, S. C. Nguyena,
A. P. Alivisatosa, C. B. Harrisa, A. Schwartzbergd, P. D. Yang,
Proc. Natl. Acad. Sci. USA 2016, 113, 11750; b) D. L. Ashford,
M. K. Gish, A. K. Vannucci, M. K. Brennaman, J. L. Templeton,
J. M. Papanikolas, T. J. Meyer, Chem. Rev. 2015, 115, 13006.
[23] a) Y. Kou, Y. Nabetani, D. Masui, T. Shimada, S. Takagi,
H. Tachibana, H. Inoue, J. Am. Chem. Soc. 2014, 136, 6021;
b) J. Bonin, M. Robert, M. Routier, J. Am. Chem. Soc. 2014,
136, 16768; c) J. Schneider, H. Jia, J. T. Muckerman, E. Fujita,
Chem. Soc. Rev. 2012, 41, 2036; d) A. J. Morris, G. J. Meyer, E. Fujita,
Acc. Chem. Res. 2009, 42, 1983.
[
8] a) T. Ogata, S. Yanagida, B. S. Brunschwig, E. Fujita,
J. Am. Chem. Soc. 1995, 117, 6708.
9] J. Hawecker, J. M. Lehn, R. Ziessel, J. Chem. Soc., Chem. Commun.
[
1
983, 55, 536.
[
[
10] J. M. Lehn, R. Ziessel, Proc. Natl. Acad. Sci. USA 1982, 79, 701.
11] a) C. Gao, Q. Meng, K. Zhao, H. Yin, D. Wang, J. Guo, S. Zhao,
L. Chang, M. He, Q. Li, H. Zhao, X. Huang, Y. Gao, Z. Tang,
Adv. Mater. 2016, 28, 6485; b) C. Gao, S. Chen, Y. Wang, J. Wang,
X. Zheng, J. Zhu, L. Song, W. Zhang, Y. Xiong, Adv. Mater. 2018, 30,
1704624.
[
12] a) S. Wang, W. Yao, J. Lin, Z. Ding, X. Wang, Angew. Chem. 2014,
1
2
3
26, 1052; b) S. Wang, X. Wang, Angew. Chem., Int. Ed. 2016, 55,
308; c) S. Wang, B. Y. Guan, X. Lou, Energy Environ. Sci. 2018, 11,
06.
[24] K. Li, B. Peng, T. Peng, ACS Catal. 2016, 6, 7485.
[25] Z. Lin, Z. Zhang, Y. Chen, W. Lin, Angew. Chem., Int. Ed. 2016, 55,
13739.
[
13] D. J. Boston, C. Xu, D. W. Armstrong, F. M. MacDonnell,
J. Am. Chem. Soc. 2013, 135, 16252.
14] a) F. Lakadamyaliy, E. Reisner, Chem. Commun. 2011, 47, 1695;
b) P. Kumar, A. Bansiwal, N. Labhsetwar, S. L. Jain,
J. Am. Chem. Soc. 2016, 138, 1820; c) H. Huang, J. Lin, G. Zhu,
[26] a) M. T. Greiner, M. G. Helander, W. Tang, Z. Wang, J. Qiu, Z. Lu,
Nat. Mater. 2012, 11, 76; b) A. Listorti, J. Durrant, J. Barber,
Nat. Mater. 2009, 8, 929.
[27] S. Wallin, J. Davidsson, J. Modin, L. Hammarstro, J. Phys. Chem. A
2005, 109, 4697.
[
Small 2018, 1800762
1800762 (8 of 8)
© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim