Eosin Y-Functionalized Conjugated Organic Polymers for Visible-Light-Driven CO2 Reduction with H2O to CO with High Efficiency

Article abstract of DOI:10.1002/anie.201812790

Visible-light-driven photoreduction of CO₂ to energy-rich chemicals in the presence of H₂O without any sacrifice reagent is of significance, but challenging. Herein, Eosin Y-functionalized porous polymers (PEosinY-N, N=1–3), with high surface areas up to 610 m² g^?1, are reported. They exhibit high activity for the photocatalytic reduction of CO₂ to CO in the presence of gaseous H₂O, without any photosensitizer or sacrifice reagent, and under visible-light irradiation. Especially, PEosinY-1 derived from coupling of Eosin Y with 1,4-diethynylbenzene shows the best performance for the CO₂ photoreduction, affording CO as the sole carbonaceous product with a production rate of 33 μmol g^?1 h^?1 and a selectivity of 92 %. This work provides new insight for designing and fabricating photocatalytically active polymers with high efficiency for solar-energy conversion.

Full text of DOI:10.1002/anie.201812790

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: EosinY-Functionalized Conjugated Organic Polymers for Visible-
Light Driven CO₂ Reduction with H₂O to CO with High Efficiency
Authors: Zhimin Liu, Xiaoxiao Yu, Zhenzhen Yang, Bing Qiu, Shien
Guo, Peng Yang, Bo Yu, Hongye Zhang, Yanfei Zhao,
Xinzheng Yang, and Buxing Han
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201812790
Angew. Chem. 10.1002/ange.201812790
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http://dx.doi.org/10.1002/ange.201812790

10.1002/anie.201812790
Angewandte Chemie International Edition
COMMUNICATION
EosinY-Functionalized Conjugated Organic Polymers for Visible-
Light Driven CO2 Reduction with H2O to CO with High Efficiency
Xiaoxiao Yu,[a, b],+ Zhenzhen Yang,[a],+ Bing Qiu,[a, b] Shien Guo,[a, b] Peng Yang,[a, b] Bo Yu,[a] Hongye
Zhang,[a] Yanfei Zhao,[a] Xinzheng Yang,[a, b],* Buxing Han,[a, b] and Zhimin Liu[a, b],
*
Abstract: Visible-light driven photoreduction of CO2 to energy-
rich chemicals in the presence of H2O without any sacrifice
reagent is of significance, but challenging. Herein, we report
EosinY-functionalized porous polymers (PEosinY-N, N=1~3)
with high surface areas up to 610 m2 g-1, which exhibit high
activity for photocatalytic reduction of CO2 to CO in the presence
of gaseous H2O without any photosensitizer or sacrifice reagent
under visible light irradiation. Especially, PEosinY-1 derived
from coupling of EosinY with 1,4-diethynylbenzene shows the
best performance for the CO2 photoreduction, affording CO as
the sole carbonaceous product with a production rate of 33 μmol
g-1 h-1 and a selectivity of 92%. It is indicated that the EosinY
sites and conjugated structure of PEosinY-N could realize the
light absorption and efficient separation of photo-generated
electron/hole, as well as the adsorption and activation of CO2
and H2O. This work provides new insight for designing and
fabricating photocatalytically active polymers with high efficiency
for solar-energy conversion.
by varying the organic precursors and synthetic methods.[6] For
example, COPs with CO2-philic groups have been reported for
efficient capture and further conversion of CO2.[7] Due to their
tunable optical and electronic properties, 1,3-diyne-linked COP
nanosheets and diacetylene-functionalized ketoenamine COPs
have been applied in photocatalytic water splitting, showing
higher or comparable activities compared to the most of
8]
inorganic photocataysts.[6b,
Nonetheless, COPs have been
rarely reported for photocatalytic reduction of CO2.
The visible-light driven CO2 photoreduction into energy-rich
chemicals, such as CO, HCOOH, CH3OH and CH4, has
attracted tremendous attention, considering its double benefits
for utilization of CO2 as a renewable and green C1 building block
and conversion of solar energy.[1] In this research area, various
photocatalysts such as transition-metal complexes,[2] metal
oxides/sulfides[3] and metal-organic frameworks[4] have been
developed for the photocatalytic reduction of CO2. However, in
most cases sacrifice reagents rather than H2O are required to
realize the photoreduction of CO2. Inspired by the natural
photosynthesis that realizes the transformation of CO2 and H2O
simultaneously under ambient conditions, the photoreduction of
CO2 combined with H2O oxidation has been paid much attention,
however, still remained a significant challenge due to the lack of
efficient photocatalysts.[5]
Figure 1. Synthetic procedure for PEosinY-N (N=1~3).
Herein, we present a kind of EosinY-functionalized COPs
(PEosinY-N, N=1~3) prepared via direct Sonogashira-Hagihara
cross-coupling of Eosin Y with aromatic alkynes (Figure 1, A1,
A2 and A3). The resultant materials possessed porous
structures with Brunauer–Emmett–Teller (BET) surface areas up
to 610 m2g-1, and could adsorb and activate CO2 and H2O
simultaneously, confirmed by in-situ diffuse reflectance infrared
Fourier transform spectroscopy (DRIFTS) analysis and density
functional theory (DFT) calculations. Moreover, PEosinY-N
could absorb visible light with suitable band structures
(Eg=2.42~2.53 eV), which makes them be capable of catalysing
the photoreduction of CO2 with gaseous H2O without using
sacrifice reagent under visible light irradiation. Especially,
PEosinY-1 showed the best performance, affording CO as the
sole carbonaceous product with a production rate of 33 μmol g-1
h-1 and a selectivity of 92%. It was indicated that the EosinY
sites and conjugated structure of PEosinY-N were responsible
for the light absorption as well as efficient electron/hole
separation.
In recent years, conjugated organic polymers (COPs) have
emerged as a kind of materials with promising application
potentials because they can be designed with specific properties
[a]
X. Yu, Dr. Z. Yang, B. Qiu, S. Guo, P. Yang, Dr. B. Yu, Dr. H. Zhang,
Dr. Y. Zhao, Prof. Dr. X. Yang, Prof. Dr. B. Han and Prof. Dr. Z. Liu
Beijing National Laboratory for Molecular Sciences, Key Laboratory of
Colloid, Interface and Thermodynamics, Institute of Chemistry, Chinese
Academy of Sciences, Beijing 100190, China
E-mail: xyang@iccas.ac.cn
E-mail: liuzm@iccas.ac.cn
As shown in Figure 1, PEosinY-N were dark red powders,
which were insoluble in common solvents (e.g. H2O, ethanol,
CH2Cl2 and tetrahydrofuran). Their formation was verified by
Fourier transform infrared spectroscopy (FTIR), cross-
polarization magic-angle spinning (CP/MAS) 13C NMR and X-ray
photoelectron spectroscopy (XPS) (Figure S1, S2 and S3, SI).
[b]
[+]
X. Yu, B. Qiu, S. Guo, P. Yang, Prof. Dr. X. Yang, Prof. Dr. B. Han and
Prof. Dr. Z. Liu
University of Chinese Academy of Sciences, Beijing 100049, China.
These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of the
document.
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10.1002/anie.201812790
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COMMUNICATION
FTIR analysis showed that PEosinY-1 had the least unreacted
C≡CH (3296 cm-1) groups, suggesting that it had the highest
polymerization degree and the best conjugated structure, while
PEosinY-3 was contrary, probably due to the difference in
chemical structures of their monomers. The powder X-ray
diffraction analysis indicated that PEosinY-N was amorphous
(Figure S4, SI), which was composed of tiny particles with
irregular shapes and sizes confirmed by field-emission scanning
electron microscopy (SEM) and transmission electron
microscopy (TEM) observation (Figure S5, SI). The
thermogravimetric analysis indicated that PEosinY-N was stable
in air up to 300 oC (Figure S6, SI).
The N2 sorption analysis indicated that each PEosinY-N
displayed type I sorption isotherm (Figure S7A, SI), indicating
the presence of micropores in the polymeric matrices, as shown
by the curves of the pore size distribution (Figure S7B, SI).
Based on the sorption isotherms, the BET surface areas of
PEosinY-1, PEosinY-2 and PEosinY-3 were estimated to be
445, 131, 610 m2g-1, respectively (Figure S8, SI). The
differences in their BET surface areas resulted from the
chemical structure and activity of their monomers. The CO2
adsorption capacity of PEosinY-N was measured at 273 K and
pressure range of 0.002~1 bar (Figure S9, SI). It was
demonstrated that their CO2 adsorption capacities were related
to their BET surface areas and reached 39, 24 and 62 mg g-1 at
1.0 bar for PEosinY-1, PEosinY-2 and PEosinY-3, respectively.
The porous structures and high surface areas of these polymers
may be favorable to the capture and activation of CO2 or H2O
molecules at the active sites of the polymers. [9]
appropriate for photocatalytic reduction of CO2 to CO and
oxidation of H2O to O2.
The charge-transfer ability of PEosinY-N was examined by
electrochemical impedance spectroscopy (EIS), photo-current
spectra, and time-resolved fluorescent decay spectroscopy
(TRFDS). The EIS spectrum of each PEosinY-N (Figure S14
and Table S1, SI) showed that the radius of semicircular Nyquist
plot obtained under visible light irradiation was much smaller
than that obtained in dark, elucidating that the visible light
illumination on PEosinY-N induced the generation of electron-
hole pairs that separated rapidly, and PEosinY-1 had the best
charge transfer efficiency. As shown in Figure 2C, each
PEosinY-N exhibited fast, uniform and repeatable photo-current
responses upon cycling the light on and off, and PEosinY-1
displayed the highest photocurrent of 0.401 μA/cm-2. The
TRFDS spectra were collected to illustrate the interfacial
transition of the photogenerated charge carriers. The average
photoluminescence lifetime was 254 ps for PEosinY-1, which
was the longest (Figure 2D), indicating that PEosinY-1 could
offer more opportunities for free charges to participate in the
surface photoreaction, thereby enhancing its photocatalytic
activity.
The optical and electronic properties of PEosinY-N were
examined to evaluate their potential application in photocatalytic
CO2 reduction and H2O oxidation. EosinY as one of the building
monomer of the polymers showed absorption in the range of
220~640 nm (Figure 2A), while PEosinY-N displayed obviously
red-shifted absorption edges with enhanced intensity over a
wide range of 250~750 nm, possibly ascribed to the formation of
conjugated structures in the polymers. In comparason with metal
A1
A2
τa
τ1
τ2
(%)
(%)
(ps)
(ps)
(ps)
PEosin-Y-1 87 49.0 414 51.0 254
PEosin-Y-2 81 52.4 373 47.6 220
PEosin-Y-3 79 52.3 302 47.7 185
[11]
oxide semiconductors [10], g-C3N4 and some organic polymer
photocatalysts[6a] with the light absorption edges up to 380, 400
and 450 nm, respectively, PEosinY-N had stronger capability to
absorb visible light. The optical bandgaps of PEosinY-N were
estimated from Tauc plots, which were 2.45, 2.53 and 2.42 eV
for PEosinY-1, PEosinY-2 and PEosinY-3, respectively (Figure
S10, SI). The bandgap of PEosinY-3 was the smallest, while
DFT calculations indicated that PEosinY-3 should have the
largest bandgap (Figure S11 and S12, SI). This discrepancy
may result from the low polymerization degree of PEosinY-3,
and DFT result could not give the actual information. Mott-
Schottky plots analysis was carried out to estimate the relative
conduction band (CB) positions of PEosinY-N, and the positive
slopes of the plots for PEosinY-N (Figure S13) demonstrated
their typical characters of n-type semiconductors.[12] The valence
band (VB) positions of PEosinY-N were obtained by combining
the acquired bandgaps from Taus plots with the relative CB
positions, and their relative electronic band structures are shown
in Figure 2B. Obviously, the CB and VB energies of each
PEosinY-N straddle the potentials of CO2 reduction to CO (-0.69
eV vs. Ag/AgCl, pH=6.8) and H2O oxidation to O2 (0.64 eV vs.
Ag/AgCl, pH=6.8). This suggests that PEosinY-N is likely to be
Figure 2. (A) UV–Vis light absorption spectra of EosinY and PEosinY-N
(N=1~3). (B) CB and VB positions of PEosinY-N. (C) Transient photocurrent
responses of PEosinY-N (λ>420). (D) TRFDS spectra of PEosinY-N, which
were recorded at 640 nm under excitation of 405 nm. (E) Average production
rates of CO from CO2 photoreduction over PEosinY-N (λ>420) for 50h. (F)
DRIFT spectra of PEosinY-1 for CO2 and H2O adsorption and their
photoreaction under visible light irradiation. The bands with apparent changes
are marked with full and imaginary lines, in which the full lines refer to the
decreased or the gradually disappeared bands, and the imaginary lines denote
the new appeared or increased bands.
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10.1002/anie.201812790
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COMMUNICATION
With good CO2 uptake, visible-light absorption, and high
separation and transfer efficiency of photoinduced charge
carries, PEosinY-N were tested for the photoreduction of CO2
with gaseous H2O under visible light (>420 nm) irradiation at
ambient conditions. It was indicated that each PEosinY-N was
effective for the photoreduction of CO2 with H2O, producing CO
as the sole carbonaceous product together with H2 (Figure 2E).
As expected, PEosinY-1 showed the best performance,
affording CO with a production rate of 33 μmol g-1 h-1 and
selectivity of 92%, even superior to some metal
oxides/sulfides[13] or MOF[14] photocatalysts in the presence of
sacrifice reagents. For comparison, EosinY was tried to catalyze
the CO2 photoreduction under the otherwise identical conditions,
and only H2 was obtained with a production rate of 5 μmol g-1 h-1
without any carbonaceous product detectable. The coupling of
EosinY with aryl alkynes made the resultant polymer be capable
of catalyzing CO2 photoreduction, which suggestes that the
combination of the EosinY sites with CC and the conjugated
structure in the polymers jointly contribute to the photocatalytic
activity of the polymers. The cyclic runs of CO2 photoreduction
over PEosinY-1 indicated that the CO productivity and
selectivity slightly decreased to 25 μmol g-1 h-1 and 73.5%,
respectively, after 5 runs (Figure S15, SI). However, comparing
the TEM, SEM images and FTIR, UV–Vis light absorption
spectra of PEosinY-1 before and after the cycle (Figure S16, SI),
no obvious changes were observed in the morphology, and the
FTIR, UV–Vis absorption spectra changes little.
decrease in the catalytic efficiency of PEosinY-1 as it was
reused .
In-situ electron paramagnetic resonance (EPR) analysis was
carried out to detect the spin reactive species dissolved in water
or adsorbed on PEosinY-1 using 5,5-dimethyl-1-pyrroline N-
oxide (DMPO) as a free radical trap. As shown in Figure S20, no
obvious signals of radicals were observed in the case of
performing the reaction in dark, while characteristic signals of
•OH with relative intensities of 1:2:2:1 were clearly observed
under light illumination. Radical signals with six characteristic
peaks of equal intensities were also detected, which might be
attributed to the [PEosinY]• radicals originated from the EosinY
active sites in PEosinY-1.[17] These results indicate that visible
light irradiation resulted in the formation of reactive •OH and
[PEosinY]• radicals. Moreover, H2O2 was also detected by
FeCl2/KSCN and UV–vis spectrophotometry (Figure S21, SI).
Considering that the VB position of PEosinY-1 is higher than the
redox potential of H2O oxidation to H2O2, it is thermodynamically
unfavorable for PEosinY-1 to catalyzing H2O oxidation to H2O2.
We performed cyclic voltammetry (CV) detection on PEosinY-1
in 0.2 M Na2SO4 aqueous solutions saturated with O2 or CO2,
respectively. An O2 reduction peak was observed at -0.52 V in
the collected CV curve under CO2 atmosphere though it was
very weak compared to that observed in the case of O2
atmosphere (Figure S22, SI). After potentiostatic reduction at -
0.52 V for 1 h, H2O2 was detected in the electrolyte. Therefore,
we speculated that the formation of H2O2 in the CO2
photoreduction may undergo the following processes. Under the
visible light irradiation, H2O was oxidized to O2, which was
further reduced to H2O2 on the surface of the catalyst.[18] This
could also explain why no O2 was detected in the CO2
photoredution process over PEosinY-N.
To gain insight into the CO2 reduction with H2O over PEosinY-
1, control experiments were performed under Ar atmosphere or
in dark, and no carbonaceous species were detected in these
two cases. In the isotope-labeling experiment using 13CO2
instead of CO2, 13CO was obtained confirmed by the GC-MS
spectrum with a peak at m/z =29 (Figure S17, SI) and GC
spectrum (Figure S18, SI). These results indicate that PEosinY-
N catalysed the photoreduction of CO2 to CO under visible-light
irradiation.
To understand the mechanism of photogenerated electron
transport during the CO2 photoreduction, DFT calculations were
performed taking M-1 (as shown in Figure S23A and S23B, SI),
the typical structure unit of PEosinY-1, as a module compound.
The highest occupied molecular orbital (HOMO) (Figure S23C,
SI) and the lowest unoccupied molecular orbital (LUMO) (Figure
S23D, SI) were used to elucidate the transfer of photoexcited
charge carrier in M-1. It was indicated that the HOMO of M-1
mainly consisted of 2pz orbitals of O and C (center 27, 62, 63)
as shown in Table S2, which could serve as the light-absorption
sites and the electron donors. The LUMO of M-1 mainly
composed of the 2pz orbitals of C (center 1, 5, 15) as shown in
Table S3, suggesting that the photogenerated electrons could
transfer to or through these atoms to realize the photoreduction
of CO2. Hence, it can be deduced that during the CO2
photoreduction process over PEosinY-N, EosinY building block
and CC within the polymer backbone serve as light-absorption
sites for generation of electrons and holes, as well as for the
subsequent electron transfer.
The interaction sites for CO2 and H2O on M-1 were explored
as well. The coordination of -OH and –COO– of M-1 with CO2
were taken into consideration (Figures 3A and 3B). After
optimization, the binding energies of –OH, and –COO- with CO2
were calculated to be -3.99 and –4.50 Kcal/mol, respectively,
suggesting that both –OH and –COO– could adsorb CO2, and –
COO– had better capability. Likewise, –OH and –COO- could
also adsorb H2O, with binding energies of -9.22, and -5.99
To further reveal the reaction mechanism, in-situ DRIFTS of
PEosinY-1 under experimental conditions were performed, and
the DRIFTS spectra are shown in Figure 2F. The DRIFTS
spectrum of PEosinY-1 under N2 atmosphere (Figure S19, SI)
was taken as the background, and the other DRIFTS spectra
were collected after CO2 and gaseous H2O was injected. After
adsorption for 15 min, the DRIFTS spectrum was collected, in
which the bands at 3729-3597 and 1643 cm-1 were assigned to
the isolated or physically adsorbed H2O on the surface of
PEosinY-1, while the wide and strong bands at 3427 and 3237
cm-1 were ascribed to the hydrogen bonding of the surface OH
on PEosinY-1 with H2O. The bands at 1789, 1763 cm-1 were the
characteristic signals of C=O on PEosinY-1 interacting with CO2.
These results indicate that the EosinY-functionalized polymers
can capture CO2 and H2O by the C=O and –OH groups, which is
beneficial to the activation of CO2 and H2O. After visible light
irradiation, in the DRIFT spectra of PEosinY-1 collected at
different time, new bands appeared at 1606, 1405 and 1294 cm-
-
1, ascribing to CO2 , which indicates that the CO2-• might be the
intermediate in the process of CO2 photoreduction.[15] Notably, a
new band appeared at 1719cm-1, which may be attributed to the
bending vibration of C=O derived from CO.[16] The intensity of
this band increased with time, suggesting the accumulation of
CO on the surface of PEosinY-1. The strong adsorption of CO
on the surface of PEosinY-1 might be resposible for the slight
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10.1002/anie.201812790
Angewandte Chemie International Edition
COMMUNICATION
Kcal/mol, respectively, and –OH showed the better ability to
interact with H2O (Figures 3C and 3D). The adsorbed CO2 and
H2O may be activated by the interaction sites to some extent.
Hence, it can be deduced that CO2 is mainly adsorbed and
activated at –COO– sites, getting electrons from LUMO of
PEosinY-N and further being reduced to CO, while H2O can be
mainly adsorbed and activated at –OH sites, getting holes from
HOMO and further being oxidized to oxygen.
We thank the National Natural Science Foundation of China (Nos.
21673256, 21533011) and the Chinese Academy of Sciences
(QYZDY-SSW-SLH013-2).
Keywords: porous organic polymer • photocatalysis • CO2 reduction
• H2O oxidation • CO
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Figure 3. The binding distance and binding energy between –OH or –COO- in
M-1 and CO2 (A) (B) and H2O (C) (D).
Based on the above experimental and calculation results, the
mechanism for photocatalytic CO2 reduction over PEosinY-N
was proposed as shown in Scheme S1. Upon visible light
irradiation, the photo-generated electrons (e-) migrate from VB to
CB of the catalyst, leaving holes (h+) at VB. The absorbed H2O
on the catalyst surface captures h+, generating H+ and O2.
Subsequently, O2 captures e- and H+, forming •OH, which further
converts to H2O2. The adsorbed CO2 accepts electron to form
-•
CO2 intermediate, which further combines with H+ to form
[COOH]ads. After accepting another pair of H+/e-, [COOH]ads
undergoes dehydration, generating adsorbed CO, which then
disengages from the catalyst surface into free CO.
In summary, the EosinY-functionalized porous polymers
with high surface areas were designed, which achieved the
photocatalytic reduction of CO2 with gaseous H2O under visible
light irradiation, affording CO as the sole carbonaceous product
with a production rate up to 33 μmol g-1 h-1 and a selectivity of
92%. DFT calculations indicate that the EosinY sites could
adsorb and activate the CO2 and H2O molecules, and the
EosinY sites and CC bonds together with the conjugated
structure of PEosinY-N could realize light absorption and
efficient separation of electron/hole, as well as the adsorption
and activation of CO2 and H2O. This work provides new insight
for designing and fabricating photocatalytically active polymers
with high efficiency for solar-energy conversion.
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Acknowledgements
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10.1002/anie.201812790
Angewandte Chemie International Edition
COMMUNICATION
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COMMUNICATION
EosinY-functionalized organic polymers
Xiaoxiao Yu,[a, b],+ Zhenzhen Yang,[a],+ Bing
Qiu,[a, b] Shien Guo,[a, b] Peng Yang[a, b] Bo
Yu,[a] Hongye Zhang,[a] Yanfei Zhao,[a]
Xinzheng Yang,[a, b],* Buxing Han,[a, b] and
exhibit high catalytic efficiency for CO2
photoreduction in the presence of H2O
under visible light irradiation, affording
CO as the sole carbonaceous product
with a rate of 33 μmol g-1h-1 and a
selectivity of 92%.
CO2
e- H+
CO, H2O
H2O2
e-
Zhimin Liu[a, b],
*
O2, H+
h+
H2O
Page No. – Page No.
*No metal *No sacrifice reagent *No photosensitizer
EosinY-Functionalized Conjugated
Organic Polymers for Visible-Light
Driven CO2 Reduction with H2O to CO
with High Efficiency
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