Functional Conjugated Polymers for CO2 Reduction Using Visible Light

Article abstract of DOI:10.1002/chem.201804496

The reduction of CO₂ with visible light is a highly sustainable method for producing valuable chemicals. The function-led design of organic conjugated semiconductors with more chemical variety than that of inorganic semiconductors has emerged a

Full text of DOI:10.1002/chem.201804496

A Journal of
Accepted Article
Title: Functional Conjugated Polymers for CO2 Reduction Using Visible
Light
Authors: Can Yang, Wei Huang, Lucas Caire da Silva, Kai A. I. Zhang,
and Xinchen Wang
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Functional Conjugated Polymers for CO2 Reduction Using Visible
Light
Can Yang,[a] Wei Huang, [b] Lucas Caire da Silva,[b] Kai A. I. Zhang, *[b] and Xinchen Wang*[a]
Abstract: The reduction of CO2 with visible light is a highly
sustainable method for producing valuable chemicals. The function-
led design of organic conjugated semiconductors with more
chemical variety than that of inorganic semiconductors has emerged
as a method for achieving carbon photofixation chemistry. Here, we
report the molecular engineering of triazine-based conjugated
microporous polymers to capture, activate and reduce CO2 to CO
with visible light. The optical band gap of the CMPs is engineered by
varying the organic electron-withdrawing (benzothiadiazole) and
electron-donating units (thiophene) on the skeleton of the triazine
rings while creating organic Donor-Acceptor junctions to promote the
charge separation. This engineering also provides control of the
texture, surface functionality and redox potentials of CMPs for
achieving the light-induced conversion of CO2 to CO ambient
conditions.
because of its basic sites, tunable structure and N-rich surface
functional groups. For example, Wang et al. reported that g-CN
copolymerized from urea and organic monomers was used to
promote the photochemical conversion of CO2. [25] Very recently,
Maeda et al. designed a novel catalytic system for CO2 reduction
with a turnover number of 2000 and a selectivity of 98% based
on Ag-modified g-CN with Rull binuclear complexes. [26]
Conjugated polymers (CPs), another emerging class of
heterogeneous and metal-free photocatalysts, are of particular
interest due to their highly tunable electronic and optical
properties, morphology and porosity on account of the large
synthetic variety in their structure formation. [27,28] In particular,
the semiconductor redox potentials of CPs can be precisely
modified via electron donor or electron acceptor combinations,
which in principle also create organic dyadic structure to
facilitate charge separation. Recent research has demonstrated
the use of CMPs in a variety of organic photoredox reactions. [29-
The conversion of CO2 by light into valuable C1/C2 chemicals
32]
Cooper et al. reported the use of CMPs with an optical band
provides a sustainable solution to addressing environmental and
gap ranging from 2.95 eV to 1.94 eV for photocatalytic hydrogen
[1-5]
energy issues.
However, the activation of the stable carbon-
[33]
production via water splitting.
We recently reported a series
oxygen bond in a CO2 molecule to achieve the subsequent
reduction has proven highly challenging. Enormous research
of benzothiadiazole-containing CMPs as heterogeneous
[34]
photocatalysts for hydrogen production.
Other conjugated
efforts have been devoted to the development of catalytic
systems containing benzene and spirobifluorene or triazine-
[6-9]
systems for direct CO2 conversion.
Inspired by photosystem
[35-37]
based polymer networks
have been developed for
Ⅰ (PS I) in natural photosynthesis, which directly converts CO2
into hydrocarbons by using sunlight at room temperature and
ambient pressure, two different reduction pathways, i.e.,
electroreduction or photoreduction, have been developed to
activate the carbon-oxygen bond of CO2. For example, atomic
utilization in photocatalytic hydrogen production. Owing to their
[38-41]
strong CO2-philicability,
the incorporation of triazine
functional groups in polymeric networks should facilitate CO2
photoreduction process. What’s more, photocatalytic CO2
reduction using CPs-based semiconductors has barely been
reported. It is therefore time to examine the feasibility of applying
CMPs in the direct conversion of CO2 into valuable chemicals.
[10]
[11]
[12,13]
metal,
metal oxides,
metal chalcogenides,
metal-
[14]
organic complexes
and heteroatom-doped carbon materials
[15,16] have been used to catalytically induce the electroreduction
of CO2. To achieve the direct conversion of CO2 by
[17]
photoreduction,
semiconductors
transition
metal
complexes
or
have
[18,19]
[20,21]
and metal-decorated zeolites
been investigated; however, conjugated organic semiconductors
have been rarely studied during the last 50 years.
Recently, conjugated polymer systems have gained
considerable attention as visible-light-active and metal-free
photocatalysts due to their easy ability to influence various
[22,23]
properties by varying their semiconductor characteristics.
One of the most promising examples is binary graphitic carbon
nitride (g-CN), [24] which has been utilised in CO2 photoreduction
Scheme 1. Illustrated design and synthesis pathway of three triazine-based
polymers with different molecular structures for photocatalytic CO2 reduction.
[a]
[b]
Dr. C. Yang, Prof. X. Wang
State Key Laboratory of Photocatalysis on Energy and Environment,
College of Chemistry
Fuzhou University, Fuzhou 350116, P. R. China
E-mail: xcwang@fzu.edu.cn
Here, we have studied the utilization of CPs for the
reduction of CO2 and investigated the impact of the structure on
their catalytic efficiency. In this study, three CPs were designed
and obtained via coupling 2,4,6-tris(4-bromophenyl)-1,3,5-
triazine (TBPT-Br) with 1,4-phenylenediboronic acid (B), 2,5-
thiophenediboronic acid (Th) and 2,1,3-benzothiadiazole-,4,7-bis
(boronic acid pinacol ester) (BT) as electron donor and acceptor
building blocks. This coupling was done using palladium-
Dr. W. Huang, Dr. L. C. Silva, Dr. K. A. I. Zhang,
Max Planck Institute for Polymer Research
Ackermannweg 10, 55128 Mainz, Germany
E-mail: kai.zhang@mpip-mainz.mpg.de
Supporting information for this article is given via a link at the end of
the document.
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catalysed Suzuki cross-coupling polycondensation reaction to
form the corresponding conjugated polymer networks CPs-B,
CPs-Th and CPs-BT, respectively. The structures of the three
CPs are illustrated in Figure 1. The detailed synthetic
procedures and characterization data are described in the
Supporting Information (SI). By the incorporation of different
electron donor or acceptor units, the efficiency of photocatalytic
CO2 reduction into CO could be enhanced, giving CO formation
rates of up to 18.2 μmol h-1. A selectivity of 81.6% was achieved
with a maximal apparent quantum yield (AQY) of 1.75% at 405
nm in a water/acetonitrile/triethanolamine reaction medium. As a
side product, the formation of H2 gas was also identified with a
hydrogen evolution rate (HER) of 4.1 μmol h-1.
The triazine-based CPs were obtained as powders, which
were insoluble in all common organic and aqueous solvents
tested. Interestingly, scanning electron microscopy (SEM) and
transmission electron microscopy (TEM) both revealed a hollow
nanotubular morphology. The CPs-BT hollow nanotubes had an
average length of approximately 25 µm with an inner diameter of
approximately 135 nm and a wall thickness of approximately 60
nm (Fig. 1a, b).The elemental mapping images (Fig. 1c) and
EDX (Fig. S1) of CPs-BT clearly illustrated that it contained C, N
and S. The morphology of polymer CPs-B and CPs-Th was also
nanotubes but distinguished from CPs-BT in the size (Fig. S2).
This observation of the nanotubular morphology of the CPs was
similar to that of previous reports on CPs containing triazine
units. [42] The exact formation mechanism is unclear. However,
we believe that it might be caused by the low solubility of the
triazine monomer (TBPT-Br) and the assembly effect, which
resulted in a dimethylformamide (DMF)/water mixture during the
polymerization process.
as seen in the similar signals at 170 ppm for the sp2 carbons of
three polymers (Fig. 2b).30, 34 The chemical shifts at 127 ppm
and 134 ppm could be attributed to the phenyl carbons. The
specific peak for CPs-BT at 152.7 ppm can be assigned to the
carbon adjacent to the nitrogen atom in the BT units. The
chemical structures of another two CPs were also demonstrated
by NMR spectrum (Fig. S4). Thermogravimetric analysis (TGA)
revealed high thermal stabilities for the three polymers up to
approximately 400 °C (Fig. S5). The powder X-ray diffraction
(PXRD) pattern shown in Fig. S6 revealed the amorphous
character of the three CPs. In addition, the porosity of the CPs
nanotubes was measured by N2 gas sorption at 77 K, and the
Brunauer-Emmett-Teller (BET) surface areas were 409, 52 and
37 m2 g-1 for CPs-B, CPs-Th and CPs-BT, respectively (shown in
Fig. S7). In addition to the micropores, pronounced pore size
distributions ranging between 2.3 and 5 nm were observed for
CPs-Th and CPs-BT. This result could possibly be caused by
the low cross-linking degree of both CPs, which also explains
their low BET surface areas.
Figure 2. Characteristics of Chemical Structure and Optical Properties. (a) FT-
IR spectra of three CP materials; (b) Solid state 13C CP/MAS spectra of three
CPs; (c) UV/Vis Diffuse Reflectance Spectra of CMPs and their corresponding
photograph.
The UV/vis diffuse reflectance spectra (DRS) of the polymers
were displayed in Fig. 2c. The polymers exhibited different
optical properties. CPs-B showed an absorption range of up to
423 nm with an optical band gap of 2.93 eV. CPs-Th, which
contained thiophene as electron donor units, showed an
absorption range of up to 510 nm and an optical band gap of
2.43 eV. With the incorporation of electron-withdrawing units, i.e.,
benzothiadiazole, into the CPs backbone, an absorption range
of up to 554 nm was observed, resulting in an optical band gap
of 2.24 eV. Cyclic voltammetry (CV) measurements were
conducted (Figure S6) to enable precise study of the highest
occupied molecular orbital (HOMO) and the lowest unoccupied
molecular orbital (LUMO) of the polymers. The HOMO levels
were observed at 1.81, 1.42 and 1.31 V, and the LUMO levels
were observed at -1.12, -1.01 and -0.93 V for CPs-B, CPs-BT
and CPs-Th, respectively (Figs. 3d and S8), which is enough for
the reduction of CO2. Furthermore, we examined the (photo)-
Figure 1. Characteristics of CPs-BT Nanotube Morphology. (a) The SEM and
(b) TEM images and (c) Element Mapping of C , N and S.
The Fourier transform infrared (FT-IR) spectra of the three
CPs contained two characteristic and intense peaks at 1505 and
1360 cm-1 in Fig. 2a, which could be ascribed to the aromatic C-
N stretching and breathing modes of the triazine unit. The
signals at 1572 and 1603 cm-1 could be assigned to the skeletal
vibration of the typical aromatic rings in the polymers. Solid-state
13C cross-polarization magic-angle-spinning (CP-MAS) NMR
spectroscopy unambiguously confirmed the presence of triazine,
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electrochemical properties of the polymers by electrochemical
impedance spectroscopy and photocurrent measurements (see
Fig. S9). The minimum Nyquist radius and an enhanced
photocurrent generated on CPs-BT suggested an improved
charge separation and transfer in photocatalysis.
1,4-dioxane (DOA), tetrahydrofuran (THF), dichloromethane
(DCM) and trifluorotoluene (TFT), were first tested without using
water as a co-solvent. A clear tendency was observed showing
that strong, polar and nitrogen- or oxygen-containing solvents
could enhance the reduction of CO2 to CO. Among the solvents,
the highest CO formation rate (8.7 μmol h-1) was observed in
DMF. This indicates a possible enhanced solubilizing effect of
CO2 resulting from the Lewis acid-base interactions in those
solvents (Fig. S11a). Significantly, by adding water into the
solvents, a general increase in the CO formation rate was
achieved, with a 1:4 ratio of MeCN/H2O being the best solvent
combination (Fig. S11b and Table S1). The addition of water as
a co-solvent ought to have a combined solubilizing and further
reducing effect for CO2. This effect was achieved through the
formation of reactive protons [H] via water splitting by the
polymer photocatalyst, as demonstrated before. [23]
The activities of the CPs in CO2 reduction was investigated in
a water/acetonitrile/TEOA reaction medium. TEOA acted as an
extra sacrificial electron donor. CoCl2 and dipyridyl were added
into the catalytic system as an electron-mediating co-catalyst to
enhance charge separation and promote the transfer kinetics
and interfacial interactions. The results were listed in Table 1.
Gas chromatography mass spectrometry analysis (GC-MS) with
isotope labelling in an extra control experiment under 12CO2 and
13CO2 atmospheres was conducted to confirm that the CO
originated from CO2 reduction (Fig. S10). Among the three CPs,
CPs-BT appeared to be the best photocatalyst, with a CO
formation rate of 18.2 μmol h-1 (In Table 1, Entries 1-3) and a
high selectivity of 81.6%. Interestingly, the formation of H2 gas
with an evolution rate of 4.1 μmol h-1 was also observed. The
highest photocatalytic efficiency of CPs-BT could result from the
incorporation of electron-withdrawing BT units into the triazine
poly-network. This in turn could enhance charge separation and
further improve light-induced electron transfer reactions.
Table 1. Photocatalytic CO2 reduction into CO by the CMPs.
Entry
1
Cat.
CO/μmol•h-1
4
H2/μmol•h-1 Sel.CO [b] /%
CPs-B
CPs-Th
CPs-BT
CPs-BT
/
2
66.7
2
10
3
76.9
3
18.2
n. d.
n. d.
n. d.
0.35
n. d.
4.1
81.6
4[c]
n. d.
n. d.
0.18
2.51
1.04
/
5
/
6[d]
7[e]
8[f]
CPs-BT
CPs-BT
CPs-BT
/
12.2
/
[a] Standard reaction conditions: CMP (15 mg), CoCl2 (1 μmol), dipyridyl (5
μmol), solvent (5 mL, acetonitrile/water = 4:1), TEOA (1 mL), CO2 (1 atm.),
white light (λ ≥ 420nm), 30 ℃ 1h. [b] Sel.CO%= n (CO)/n(CO+H2) ×100%. [c]
in dark. [d] Without TEOA. [e] Without CoCl2/dipyridyl. [f] Replace CO2 with Ar
gas. n. d. = not detectable.
To explain the mechanics of light-induced CO2 reduction, we
conducted a series of control experiments using CPs-BT as a
photocatalyst. There was no detectable formation of CO and H2
when the experiments were performed in the absence of the
CPs or light (Entries 4-5). Without using TEOA, no CO was
determined however, a small amount of H2 (Entry 6) was
detected. In the absence of Co2+, decreases in the CO and H2
formation rates were determined (Entry 7). This demonstrates
the positive role of Co2+ as an electron mediator, which can
enhance the catalytic efficiency of CO2 photoreduction. When
the reaction mixture was exposed to an Ar atmosphere instead
of CO2, only H2 formation was observed, demonstrating the
activity of the polymer photocatalyst for H2 evolution (Entry 8).
We then turned our attention to studies of the solvent effect
to reveal the reaction mechanism (Figure S9). Different solvents,
such as acetonitrile (MeCN), N,N-dimethylformamide (DMF),
Figure 3. Conversion of CO2 to CO by photocatalyst CPs-BT.(a) Time
dependence of CO and H2 generation rate using CPs-BT (15 mg) as a
photocatalyst under visible light irradiation (λ>420 nm) for 4 h. (b) Stability and
Reusability tests for CO2 reduction. (c) Wavelength dependence of AQY for
the H2 and CO production at 405, 420, 470, 490 and 546 nm, respectively.
The time dependence of the CO2 reduction rate using CPs-
BT as a photocatalyst was shown in Fig. 3a. After a reaction
period of 4 h, 60 μmol of CO gas was obtained. Repeated
experiments showed that similar amounts of CO were produced
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after six additional cycles, demonstrating the high stability and
reusability of CPs-BT as a photocatalyst (Fig. 3b). During the
recyclability experiments, more than 400 μmol of CO gas was
generated, corresponding to a catalytic turnover number of ~20
for CPs-BT and ~400 for the cobalt co-catalyst. No apparent
change was observed in both FT-IR (Fig. S12) and DRS (Fig.
S13) spectrum of CPs-BT to demonstrate the excellently stable
chemical structure in conjugated networks. The wavelength
dependence of the apparent quantum yield of CO and H2
production was illustrated for CPs-BT in Figure 3c, with the
highest AQY of ca. 1.75% at 405 nm. In addition, some reported
modified polymeric photocatalysts with similar chemical
structures (such as carbon nitride and BCN systems) for CO2
reduction were listed in Table S2, the triazine-based CPs
polymer especially CPs-BT possessed a superior photocatalytic
activity with high selectivity.
In summary, we designed triazine-based CPs to be used as
emerging photocatalysts for visible-light-promoted reduction of
CO2 to CO. By varying the organic electron-withdrawing and
electron-donating units in the triazine-based polymer backbone,
the charge separation kinetics and catalytic reaction kinetics can
be promoted to enable the stable photoconversion of CO2 to CO
under visible light irradiation, giving an AQY of 1.75% at 405 nm.
By studying the reaction mechanism, we were able to observe
the enhancing effect of the strong polar and nitrogen- or oxygen-
containing solvents on the efficiency. This work demonstrates a
correlation between the structure of conjugated polymer
photocatalysts to their photocatalytic performance in CO2
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This work was financially supported by the National Key
Technologies R & D Program of China (2018YFA0209301), the
National Natural Science Foundation of China (21802022,
21425309, 21761132002, and 21861130353) and the 111
Project (D16008). C. Y. thanks the support of Guest PhD
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Keywords: organic semiconductor • conjugated polymers•
trizaine-based polymers• CO2 reduction
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The optical band gap of the
triazine-based conjugated
polymers were engineered
by varying the organic
C. Yang, W. Huang, L. C.Silva, K. A. I. Zhang,*
and X. Wang*
Page No. – Page No.
electron-withdrawing and
electron-donating units on
the skeleton of the triazine
rings while controlling of the
texture, surface functionality
and redox potentials for
achieving the conversion of
CO2 to CO.
Functional Conjugated Polymers for CO2
Reduction Using Visible Light
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