Isolated Square-Planar Copper Center in Boron Imidazolate Nanocages for Photocatalytic Reduction of CO2 to CO

Article abstract of DOI:10.1002/anie.201905869

Photocatalytic reduction of CO₂ to value-added fuel has been considered to be a promising strategy to reduce global warming and shortage of energy. Rational design and synthesis of catalysts to maximumly expose the active sites is the key to activate CO₂ molecules and determine the reaction selectivity. Herein, we synthesize a well-defined copper-based boron imidazolate cage (BIF-29) with six exposed mononuclear copper centers for the photocatalytic reduction of CO₂. Theoretical calculations show a single Cu site including weak coordinated water delivers a new state in the conduction band near the Fermi level and stabilizes the *COOH intermediate. Steady-state and time-resolved fluorescence spectra show these Cu sites promote the separation of electron–hole pairs and electron transfer. As a result, the cage achieves solar-driven reduction of CO₂ to CO with an evolution rate of 3334 μmol g^?1 h^?1 and a high selectivity of 82.6 %.

Full text of DOI:10.1002/anie.201905869

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Accepted Article
Title: Isolated Square Planar Copper in Boron Imidazolate Nanocages
for Photocatalytic Reduction of CO2 to CO
Authors: haixia Zhang, Qin-Long Hong, Jing Li, Fei wang, Xinsong
Huang, Shumei Chen, wenguang Tu, Dingshan Yu, Rong Xu,
Tianhua Zhou, and Jian Zhang
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201905869
Angew. Chem. 10.1002/ange.201905869
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10.1002/anie.201905869
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Isolated Square Planar Copper in Boron Imidazolate Nanocages
for Photocatalytic Reduction of CO2 to CO
Hai-Xia Zhang, † Qin-Long Hong, †Jing Li,† Fei Wang, Xinsong Huang, Shumei Chen, Wenguang Tu,
Dingshan Yu, Rong Xu, Tianhua Zhou* and Jian Zhang
Abstract: Photocatalytic reduction of CO2 to value-added fuel has
been considered to be a promising strategy to reduce global warming
and shortage of energy. Rational design and synthesis of catalysts to
maximumly expose the active sites is the key to activate CO2 molecule
and determine the reaction selectivity. Here, we synthesize a well-
defined copper-based boron imidazolate cage (BIF-29) with exposed
six mononuclear copper for photocatalytic reduction of CO2.
Theoretical calculations uncover single Cu site including weak
coordinated water delivers a new state on conduction band near the
Fermi level and stabilizes COOH intermediate. Steady-state and
time-resolved fluorescence spectra elucidate these Cu sites efficiently
promote the separation of electron-hole pairs and electron transfer.
As a result, the cage achieves solar-driven reduction of CO2 to CO
with an evolution rate of 3334 µmol g-1 h-1 and a high selectivity of
82.6%. Our work provides a new clue to design and synthesize cost-
effective catalysts for solar driven CO2 reduction.
developed but limited product selectivity due to the presence of
multiple neighboring sites for involving CO2 reduction. To
enhance product selectivity, a conventional approach is to tailor
the number of nearest neighboring accessible Cu atoms, such as
reducing the particle size[6] or hybridizing Cu with other
metals.[7]Besides these, introducing single active site in a catalyst
has been proven to be an effective strategy to study the reactive
pathway and enhance the activity for CO2 reduction reaction
(CO2RR).[8]In particular, the presence of unsaturated coordinated
single active sites or defect can modify the electron structure of
catalysts, which could stabilize the reaction intermediates and
thus depress the energy barrier of the photoreduction CO2.[9]
Recent studies demonstrate that this type of catalyst exhibits high
activity for reducing CO2 to CO.[10] However, such a catalyst is
generally obtained through pyrolysis of Ni, or Co-based metal
[10-11]
organic frameworks or coordination polymers.
By contrast,
few of Cu-based catalysts with single active site was developed
for the photocatalytic CO2 reduction. Therefore, constructing Cu-
based catalysts with isolated unsaturated coordinated active sites
is challenging, but it may provide new insights into the
development of high-performance and selective CO2RR catalysts.
With the development of human activities, the excessive emission
of carbon dioxide (CO2) results in an increasingly serious
environmental problem. To address this issue, one of the most
promising solutions is direct photochemical reduction of CO2 to
useful chemicals or fuels such as methane, methanol, carbon
monoxide, and formic acid.[1] Among these possible target
products, the two-electron reduction of CO2 to CO is a less
hindered process and plays an important role in chemical
industry.[1a, 2]To date, numerous work has been devoted to
developing selective catalysts for reduction of CO2 to CO.[3]
Precious metals have been identified as promising candidates for
CO2 reduction to CO but their practical applications are still limited.
Herein, we present a boron imidazole cage with six isolated
unsaturated coordinated Cu sites, [Cu6[BH(im)3]8](H2O)6-
·3(NO3)4H2O, im = imidazole, a M6L8 type cage), as a co-catalyst
for visible-light-driven photocatalytic reduction of CO2 to CO
(Figure 1). Compared to coordination saturated cage, it displays
higher efficiency and selectivity for CO2 to CO in CH3CN/H2O (v/v
= 4:1) solution using [Ru(bpy)3]Cl2 as the photosensitizer, and
triethanolamine (TEOA) as the sacrificial reagent. Experiments
and theoretical calculations indicate that these isolated
coordinative unsaturated Cu sites in cage not only improve
charge-separation efficiency of the whole composites, but also
stabilize COOH intermediate. The synergistic effect endows BIF-
29 with high efficiency and selectivity for the photocatalytic
reduction of CO2 to CO.
Copper as an earth-abundant metal is a critical metal in
photosynthesis[4] and Cu-based materials have been regarded as
promising catalysts for efficiently photo- or electro-catalyzing CO2
reduction.[5]To date, various Cu-based catalysts have been
[]
Dr. H.-X. Zhang, Q.-L. Hong, Dr. F. Wang, Prof. T. Zhou, Prof. J.
Zhang
The Cu-based cage (referred to as BIF-29, Figure S1) was
synthesized according to reported procedures.[12] Single-crystal
X-ray diffraction reveals a rhombic dodecahedral cage consisting
State Key Laboratory of Structural Chemistry
Fujian Institute of Research on the Structure of Matter, Chinese
Academy of Sciences, Fuzhou, Fujian 350002, P. R. China
E-mail: wangfei04@fjirsm.ac.cn; thzhou@fjirsm.ac.cn
Dr. J. Li, Prof. D. Yu
Key Laboratory for Polymeric Composite and Functional Materials of
Ministry of Education, Key Laboratory of High-Performance Polymer-
based Composites of Guangdong Province, School of Chemistry
Sun Yat-Sen University, Guangzhou, China
Dr. W. Tu, Prof. R. Xu
School of Chemical & Biomedical Engineering, Nanyang
Technological University 62 Nanyang Drive, Singapore.
Prof. S. Chen.
College of Chemistry, Fuzhou University, Fuzhou, Fujian 350108, P.
R. China
-
of six copper ions and eight BH(im)3 (Figure 1). Each Cu ion
shows a square planar configuration formed by four N atoms from
four BH(im)3- ligands. On the outside of the cage, each Cu center
is linked to a water by weak coordination bond (Cu-O: 2.538 Å) to
form unsaturated coordinated open metal site (Figure 1), which
provide an ideal configuration to adsorb and activate CO2.[13]
Powder X-ray diffraction patterns (PXRD) confirmed the phase
purity of BIF-29 (Figure S2a). X-ray photoelectron spectroscopy
(XPS) indicated co-existence of Cu2+ and Cu+ species in BIF-29
(Figure S3a and Table S1). To verify the chemical stability, we
investigate its acidic and alkaline resistance. Figure S2a indicates
the structure of BIF-29 remains unchanged after being soaking in
acidic (pH = 2, HNO3) and basic (pH =14, NaOH) media for 12 h
Supporting information for this article is given via a link at the end of
the document.
These authors contributed equally to this work.
[†]
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(a)
(b)
(d)
(c)
Figure 1. Schematic illustration of the synthesis of BIF-29 (Right) and the CO2
adsorption and activation over unsaturated coordinated Cu site (Left). Color
codes: Cu, cyan; B, yellow; O, red; C, gray; N, blue. H atoms are omitted for
clarity.
The UV-vis diffuse reflectance spectrum (DRS) of BIF-29
revealed a broad and strong absorption in the region of 450-800
nm (Figure S3b). According to the Tauc plot, the bandgap energy
(Eg) of 1.62 eV was obtained (Figure S2b). Mott-Schottky
measurements indicate that BIF-29 possesses the intrinsic
characteristics of n-type semiconductor and the flat-band
potential is -0.55 eV vs NHE (Figure S2c). For n-type
semiconductor, the bottom of the conduction band (CB) is
approximately close to the flat-band potential.[14] The CB of BIF-
29 (-0.55 eV vs. NHE) is negative than the reduction potential of
CO2 to CO (-0.51 eV vs. NHE) at pH = 7, implying that the photo-
excited electron can be transferred from BIF-29 to the adsorbed
Figure 2. (a) Photocatalytic reduction of CO2 to CO and H2 over BIF-29 (10 mg).
(b) GC-MS analysis of the photocatalytic products on BIF-29 using 13CO2 as
carbon source (m/z, mass/charge ratio). (c) The recyclability of the BIF-29 (30
mg) system after each ten hours of reaction. (d) Photocatalytic reduction of CO2
to CO over BIF-29 (10 mg) and BIF-33 (10 mg).
the reaction without BIF-29. This result further confirms the
photocatalytic CO2 reduction by BIF-29 itself. N2 instead of CO2
was introduced into the reaction system to further confirm the
origin of the produced CO. After 5 h of reaction, only a trace
amount (0.3 µmol) of CO was detected (Figure S9a). The 13CO2
isotopic experiment was also carried out under the identical
reaction condition and the generated 13CO was analyzed by mass
spectra. As shown in Figure 2b, the peak of m/z = 29 was
assigned to 13CO, confirming that the carbon source of CO
originates from CO2 rather than organic solvent or the possible
decomposition product from BIF-29 or photosensitizer.
CO2 on the active sites when using
a photosensitizer
[Ru(bpy)3]Cl2 with HOMO and LUMO of 1.22 eV and -1.27 eV with
respect to NHE, respectively (Figure S2d and Figure S4).
The photocatalytic activity of BIF-29 for CO2 reduction was
evaluated under the optimized reaction condition in CO2-
saturated CH3CN/H2O solution at pH 9.3 under visible light ( >
420 nm). As shown in Figure 2a and Figure S5, BIF-29 shows
photocatalytic activity for CO2 reduction. The amounts of CO and
H2 generated increase steadily with irradiation time at different
reaction rates, and then gradually reach the maximum at around
9 h. The average CO evolution rate reaches 3334 µmol g-1 h-1 in
the first 5 h of reaction with a high selectivity of 82.6% over
competing H2 generation, in which the H2 evolution rate is 700
µmol g-1 h-1. The highest quantum efficiency (QE) for CO
production was measured to be 0.078% at 440 nm (Figure S6).
To the best of our knowledge, the value of CO evolution rate
outperforms most photocatalytic systems for CO2 reduction
(Table S2). The products in the liquid phase was also detected by
1H nuclear magnetic resonance spectroscopy (1HNMR),
indicating no signals assigned to CH3OH, HCOH or CHOOH
(Figure S7-8). To confirm that CO is from photocatalytic CO2
reduction on BIF-29, a series of control experiments were carried
out under identical experimental conditions. The results indicate
that no product was detected in the absence of BIF-29,
photosensitizer, or TEOA in the solution (Figure S9a).
Furthermore, there was also no product detected in the dark,
suggesting the photocatalytic nature of the reduction reaction.
When BIF-29 was replaced by the mixture of Cu(NO3)2 and
KBH(im)3, only trace amounts of CO and H2 were detected. To
verify the effect of the released Cu2+ from BIF-29 on the
photocatalytic CO2 reduction, Cu(NO3)2 was adopted as the
catalyst (Figure S9a). No detectable products were observed in
To examine the photocatalytic stability of BIF-29, recycle
experiment was carried out and no noticeable change in the yield
rate of CO occurs during the three runs. As shown in Figure S9b
and Figure 3c-d, CO evolution increases almost lineally from the
initial irradiation, and the rate apparently slows down after 5 h due
to the photo-decomposition of the photosensitizer rather than the
inactivation of BIF-29.[15] Figure S9b indicates the CO production
can be fully recovered in the consecutive runs by adding a fresh
photosensitizer solution with subsequent purging of CO2,
suggesting the stability and reusability of BIF-29. Correspondingly,
PXRD, field emission-scanning electron microscope (FESEM),
and transmission electron microscope (TEM) also supported the
high durability of BIF-29 during photocatalysis (Figure S10 and
S11). The above results demonstrated that BIF-29 possesses
excellent stability for photocatalytic CO2 reduction to CO.
In order to understand the photoexcited electron transfer, the
photoluminescent (PL) quenching experiments were performed
on the system containing photosensitizer and BIF-29 or TEOA in
a degassed solution of CH3CN/H2O (v:v = 4:1). Figure 3a
indicates that the fluorescence intensity of [Ru(bpy)3]Cl2 gradually
decreases with the increase of the amount of catalyst BIF-29,
while the UV-vis spectrum of photosensitizer remains unchanged
by the addition of BIF-29, indicating the luminescence quenching
of [Ru(bpy)3]Cl2 is mainly attributed to the electron transfer from
the excited [Ru(bpy)3]Cl2 to BIF-29 (Figure S12a).[16] By contrast,
the PL of photosensitizer is not quenched by TEOA (Figure S12b).
Thus, the excited photosensitizer is quenched by BIF-29 via an
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oxidative quenching.[17] To further understand the dynamic charge
behavior, time-resolved PL decay spectroscopy was employed.
The emission of photosensitizer in aqueous solution follows one
exponential decay with a lifetime of 271 ns. An obvious decay of
the singlet excited state is observed when adding BIF-29 to the
solution (Figure S12c). This decay curve can be fitted with a
shorter lifetime of 252 ns. The shortened lifetime reveals the rapid
transfer of photogenerated electron from [Ru(bpy)3]Cl2 to BIF-29,
and thereby suppresses the recombination of photogenerated
electron and hole pair.[14, 18] These results indicate that BIF-29 can
effectively promote charge transfer and separation. To further
elucidate the charge transfer property, photoelectrochemical
measurements were performed on as-prepared samples in dark
and light at a bias of 0.7 V versus RHE. As shown in Figure S13,
the BIF-29/[Ru(bpy)3]Cl2 exhibits an increase in photocurrent
density by 2 times compared to the BIF-29, which could be
attribute to the efficient electron-hole pair separation.[19]
(a)
(b)
(d)
(c)
Figure 3. (a) Steady state fluorescence spectra of [Ru(bpy)3]Cl2 (66 µM, 10 mL)
upon the addition of BIF-29 (ex = 365 nm). (b) In situ IR spectra of N2-saturated
BIF-29, CO2-saturated BIF-29 and BIF-33 after 10 min or 60 min of the N2 purge.
(c) Calculated free energy of photocatalytic CO2 reduction. (d) Density of states
of BIF-29 and BIF-33.
The high performance of BIF-29 for photocatalytic CO2
reduction could be attributed to the presence of unsaturated
coordinated Cu sites of the catalyst. For comparison, BIF-33, a
3D structure based on the cross-linking of the Cu6B8 cage with
iodine-coordinated Cu sites, ([Cu6[BH(im)3]8]I48H2O), were
synthesized (detail characterization in Figure S14-18). As shown
in Figure 2d, the evolution rate of CO (3334 µmol g-1 h- for BIF-29
is 3 times of the value (1185.4 µmol g-1 h-1) for BIF-33, and the
selectivity of 82.6% for BIF-29 is also higher than that of 76% for
BIF-33. The high activity and selectivity of BIF-29 could derived
from the variation of valence state of the Cu sites. XPS spectra
indicate that both BIF-29 and BIF-33 possess similar ratio of
Cu2+/Cu+ on the surface (Figure S2, Figure S16 and Table S1).
Furthermore, the absorption edge of BIF-29 in the XANES curve
of Cu K-edge is close to that of the BIF-33, indicating the nearly
identical average oxidation state of Cu in both samples (Figure
S2d). Such noticeable differences in the photocatalytic
performance should be ascribed to the coordination geometry of
Cu site. As shown in Figure 1, for BIF-29 the external sites of Cu
ions are occupied by water molecules with a weak Cu-O bond,
while in BIF-33, I- ions bonded to Cu centers act as linkers to
bridge the adjacent cages (Figure S14). In BIF-29, the weak
coordinated water molecules can be readily removed to generate
unsaturated coordinated Cu sites.[20] Accordingly, CO2 could
favorably bind to the Cu center and be activated, thus benefiting
its conversion under visible-light irradiation.[13] To further reveal
the surface-active species, we carried out in-situ electron
paramagnetic resonance (EPR) experiments under visible light
irradiation. In general, the d-orbital of Cu2+ has nine electrons and
contribute to EPR signal, while the Cu+ species is EPR-silent.
Figure S19 indicated that the EPR signal for BIF-29 decrease in
density with irradiation time under nitrogen atmosphere. As a
comparison, the EPR signal of I-coordinated BIF-33 keep nearly
unchanged. The results indicate that the excited electrons from
photosensitizer can be injected to BIF-29 and trapped in the Cu
sites in BIF-29 to form Cu+ species.[20] Furthermore, when CO2 is
introduced, the EPR signal of BIF-29 exhibit slightly changed with
irradiation time, suggesting that the trapped electrons on Cu sites
can be further transferred to the adsorbed CO2 molecules on the
catalytic sites.[20-21] This observation implies that the unsaturated
coordinated Cu sites of BIF-29 could facilitate the photogenerated
charge transfer and suppress the charge recombination, in
agreement with the photo current and time-resolved FL. These
experiments further signify that the unsaturated coordinated Cu
sites in BIF-29 with the coexistence of Cu2+ and Cu+ are
responsible for the high activity and selectivity toward CO2
reduction. To verify this speculation, in situ Fourier transform
infrared (FTIR) experiments were performed on BIF-29 and BIF-
33. After pretreatment in CO2, both samples exhibit strong peaks
at about 2353 cm-1 which is assigned to the asymmetric stretching
of absorbed CO2 (Figure S20).[20, 22]Notably, BIF-29 presents a
new peak at about 1257 cm-1 which remain unchanged after more
than 60 min of N2 purge. By contrast, CO2-saturated BIF-33 lack
the corresponding peak in its in-situ IR spectra (Figure 3b). This
observations suggest that CO2 could be absorbed directly on the
unsaturated coordinated Cu sites in BIF-29.[20] In order to further
investigate the adsorption behavior of Cu sites, the CO2
temperature-programmed desorption (CO2-TPD) measurements
were performed. CO2-TPD is considered to be sensitive technique
to provide detailed information about the interaction between the
adsorbed gas molecules and the surface of catalysts.[23]As shown
in Figure S21, BIF-29 exhibits one strong desorption peak at 233
C whereas BIF-33 displays two weak desorption peaks at about
159 C and 226 C, respectively. This observation indicates BIF-
29 displays relatively stronger CO2 adsorption active sites
compared to BIF-33, although BIF-29 (Figure S2c) presents
smaller amount of adsorbed CO2 compared to BIF-33 (Figure
S15b), suggesting the catalysis mainly occur at the external
surface. Meanwhile, theoretical calculation about CO2 adsorption
energy demonstrated that BIF-29 exhibited more negative
adsorption energy of CO2 compared to BIF-33, suggesting much
stronger affinity of BIF-29 with unsaturated coordinated Cu sites
towards CO2 (Figure S22 and Table S3).[1e, 1f, 24]Moreover, DFT
calculations (Figure 3c) revealed that for BIF-29, the formation
energy of COOH is 1.26 eV for COOH intermediate adsorbed at
Cu site. In contrast, for I--coordinated BIF-33, the COOH
intermediate adsorb on Cu site with an extremely high formation
energy of 1.63 eV due to the saturated coordination mode of Cu
site. This result indicates that BIF-29 favors the *COOH
formation.[25] In addition, from the density of states of two models,
it is found that BIF-29 without I- delivers a new state on conduction
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reactivity in BIF-29 compared to the BIF-33. These observations
demonstrate that the unsaturated coordinated Cu sites in BIF-29
contribute to facilitating the adsorption of CO2 and promoting the
activity as well as selectivity towards CO2 reduction.
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In conclusion, a copper-based boron imidazolate cage with six
isolated copper atoms is found as an excellent visible-light
photocatalyst for CO2 reduction. Compared to Cu-based cage
with I- coordination, the cage with unsaturated coordinated Cu
sites displays higher activity for photocatalytic reduction of CO2 to
CO. A combination of DFT calculations, PL, in situ IR, EPR, and
CO2-TPD experiments confirms that the enhanced photocatalytic
activity is attributed to the presence of unsaturated coordinated
isolated Cu sites. These sites not only facilitate the transfer of the
excited electrons but also promote the CO2 adsorption and
activation. Our findings open up new avenues of cage-based
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This work is supported by NSFC (51772291, 21425102,
21773242 and 21603226), the Strategic Priority Research
Program of the Chinese Academy of Sciences (XDB20000000),
National Key Research and Development Program of China
(2018YFA0208600), the Youth Innovation Promotion of CAS
(2016276), and NSF of Fujian province (2016J05052). The
authors thank 1W1B beamline of BSRF,IHEP,CAS for the XAFS
experiments.
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Keywords: cage • carbon dioxide reduction • photocatalysis •
copper • carbon monoxide
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10.1002/anie.201905869
Angewandte Chemie International Edition
COMMUNICATION
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COMMUNICATION
A copper-based boron imidazolate
cage with unsaturated coordinated
single active sites was found as an
excellent co-catalyst for highly
efficient and selective solar-driven
CO2 reduction to CO.
H.-X. Zhang,† Q.-L. Hong,† J. Li, †F.
Wang, S. Huang, S. Chen, W. Tu,
D. Yu, R. Xu, T. Zhou* and J.
Zhang
Cu(II)
e-
CO
Page No. – Page No.
CO2
Title
TEOA TEOA• +
This article is protected by copyright. All rights reserved.