Highly Efficient CO2 Electroreduction to Methanol through Atomically Dispersed Sn Coupled with Defective CuO Catalysts

Article abstract of DOI:10.1002/anie.202108635

Using renewable electricity to drive CO₂ electroreduction is an attractive way to achieve carbon-neutral energy cycle and produce value-added chemicals and fuels. As an important platform molecule and clean fuel, methanol requires 6-electron transfer in the process of CO₂ reduction. Currently, CO₂ electroreduction to methanol suffers from poor efficiency and low selectivity. Herein, we report the first work to design atomically dispersed Sn site anchored on defective CuO catalysts for CO₂ electroreduction to methanol. It exhibits high methanol Faradaic efficiency (FE) of 88.6 % with a current density of 67.0 mA cm^?2 and remarkable stability in a H-cell, which is the highest FE(methanol) with such high current density compared with the results reported to date. The atomic Sn site, adjacent oxygen vacancy and CuO support cooperate very well, leading to higher double-layer capacitance, larger CO₂ adsorption capacity and lower interfacial charge transfer resistance. Operando experiments and density functional theory calculations demonstrate that the catalyst is beneficial for CO₂ activation via decreasing the energy barrier of *COOH dissociation to form *CO. The obtained key intermediate *CO is then bound to the Cu species for further reduction, leading to high selectivity toward methanol.

Full text of DOI:10.1002/anie.202108635

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
International Edition
www.angewandte.org
Accepted Article
Title: Highly efficient CO2 electroreduction to methanol via atomically
dispersed Sn coupled with defective CuO catalysts
Authors: Buxing Han, Weiwei Guo, Shoujie Liu, Xingxing Tan, Ruizhi
Wu, Xupeng Yan, Chunjun Chen, Qinggong Zhu, Lirong
Zheng, Jingyuan Ma, Jing Zhang, Yuying Huang, and Xiaofu
Sun
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202108635
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10.1002/anie.202108635
Angewandte Chemie International Edition
RESEARCH ARTICLE
Highly efficient CO2 electroreduction to methanol via atomically
dispersed Sn coupled with defective CuO catalysts
Weiwei Guo[1,2], Shoujie Liu[3], Xingxing Tan[1,2], Ruizhi Wu[1,2], Xupeng Yan[1,2], Chunjun Chen[1],
Qinggong Zhu[1], Lirong Zheng[6], Jingyuan Ma[7], Jing Zhang[6], Yuying Huang[7], Xiaofu Sun*[1,2], and
Buxing Han* [1,2,4,5]
[1]
Dr. W. Guo, X. Tan, R. Wu, X. Yan, Dr. C. Chen, Dr. Q. Zhu, Prof. Dr. X. Sun, Prof. Dr. B. Han
Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Colloid and Interface and Thermodynamics, CAS Research/Education Center
for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences Zhongguancun North First Street 2, Beijing, 100190, China
E-mail: sunxiaofu@iccas.ac.cn; hanbx@iccas.ac.cn
[2]
[3]
Dr. W. Guo, X. Tan, R. Wu, X. Yan, Prof. Dr. X. Sun, Prof. Dr. B. Han
School of Chemistry and Chemical Engineering University of Chinese Academy of Sciences Yuquan Road, ShijingshanDistrict, Beijing, 100049, China
Dr. S. Liu
Chemistry and Chemical Engineering of Guangdong Laboratory, Shantou 515063, China.
[4,5] Prof. Dr. B. Han
Physical Science Laboratory, Huairou National Comprehensive Science Center, Beijing 101400, China and Shanghai Key Laboratory of Green Chemistry
and Chemical Processes, School of Chemistry and Molecular Engineering, East China Normal University, Shanghai 200062, China.
[6]
[7]
Dr. L. Zheng, Prof. Dr. J. Zhang
Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China.
Dr. J. Ma, Prof. Dr. Y. Huang
Shanghai Synchrotron Radiation Facility, Zhangjiang Laboratory (SSRF, ZJLab), Shanghai Advanced Research Institute, Chinese Academy of Sciences,
Shanghai 201204, China
Abstract: Using renewable electricity to drive CO2 electroreduction
methanol, are highly desirable among the formed products.
Methanol is an important platform molecule in chemical industry,
and it can be also used as clean fuel directly. CO2
electroreduction to methanol requires complicated 6-electron
transfer process and sluggish kinetics. Some catalysts, such as
RuO2/TiO2,7 Pd/SnO2,8 Mo-Bi sulphides,9 Pd-Cu aerogels10 and
Cu2-xSey11 have been designed to achieve considerable
progress for methanol production from CO2 electroreduction.12,
is an attractive way to achieve carbon-neutral energy cycle and
produce value-added chemicals and fuels. As an important platform
molecule and clean fuel, methanol requires 6-electron transfer in
the process of CO2 reduction. Currently, CO2 electroreduction to
methanol suffers from poor efficiency and low selectivity. Herein,
we report the first work to design atomically dispersed Sn site
anchored on defective CuO catalysts for CO2 electroreduction to
methanol. It exhibits high methanol Faradaic efficiency (FE) of
88.6% with a current density of 67.0 mA cm-2 and remarkable
stability in a H-cell, which is the highest FE(methanol) with such
high current density compared with the results reported to date. The
atomic Sn site, adjacent oxygen vacancy and CuO support
cooperate very well, leading to higher double-layer capacitance,
larger CO2 adsorption capacity and lower interfacial charge transfer
resistance. Operando experiments and density functional theory
calculations demonstrate that the catalyst is beneficial for CO2
activation via decreasing the energy barrier of *COOH dissociation
to form *CO. The obtained key intermediate *CO is then bound to
the Cu species for further reduction, leading to high selectivity
toward methanol.
13
However, the low efficiency, high overpotential, and poor
stability of catalysts hinder its practical use to some degree.7, 14,
15 Therefore, there is a strong need for developing highly active
and robust electrocatalysts that can generate methanol with
high selectivity and high current density.
In the past decades, the various nanocatalysts with well-
defined compositions, sizes, morphologies and structures have
been developed to improve activity, enhance selectivity, and
clarify the structure-activity relationships for a wide range of
reactions.5, 16-19 It is known that the size of catalysts is a key
parameter governing the catalytic activity and selectivity, due to
the change of their local geometries and electronic structures.20
With the decrease of catalyst size, the coordination
environment of the surface atoms changes dramatically,
particularly the single-atom-site catalysts (SASCs).21 SASCs
are defined as the catalysts in which the assembly consist of
metal active site and its coordinating atoms (such as N, O, S)
provided by the support.22 Their atomically distributed active
sites can present excellent selectivity and maximum atom
efficiency. To date, many efforts have been devoted to
developing SASCs for CO2 electroreduction.23-28 Nevertheless,
so far majority of SASCs applied in CO2 electroreduction just
produce CO through 2-electron transfer. Multi-electron
reduction to generate high value-added products has been
rarely reported.
Introduction
Converting CO2 into value-added chemicals and fuels is
one of the most attractive approaches to mitigate our pressing
energy and environmental threats.1 With the aid of renewable
electricity, CO2 electroreduction reaction with water is
a
potential strategy for CO2 transformation under mild conditions.2,
3
In recent years, many studies have been carried out for CO2
electroreduction to different products by the development of
various electrocatalysts, electrolyte, and electrochemical cells.3-
6
Hydrocarbons and alcohols with high energy density, such as
1
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RESEARCH ARTICLE
10.1002/anie.202108635
Angewandte Chemie International Edition
The main group metals, such as In, Sn, Pb and Bi, have
been considered as efficient catalysts for CO2 electroreduction,
due to the high overpotential for H2 evolution reaction (HER).8,
different concentration of oxygen vacancies. We named them
as Sn1/Vo-CuO-x, where x is the H-plasma treatment time in
seconds. As measured by the inductively coupled plasma
optical emission spectroscopy (ICP-OES), the actual Sn
loading on Sn1/Vo-CuO-x was 0.9-1.2 wt% (Table S2). The
scanning electron microscopy (SEM) images in Fig. 1B and 1C
and transmission electron microscopy (TEM) image in Fig. 1D
show that the packed Sn1/Vo-CuO-90 were formed uniformly
and had an irregular sheet-like morphology. Meanwhile, it can
be seen that there are some visible pits on the nanosheet,
which suggests that the formation of nanoscale defects
resulting from a collection of many atomic defects due to H-
plasma-treatment. It was further proved by high-resolution TEM
(HRTEM) in Fig. 1E. The spacing of the lattice fringe is 0.27 nm,
which can be assigned to the (110) plane of CuO.39 The
powder X-ray diffraction patterns (XRD) of Sn1/Vo-CuO-x are
shown in Fig. S1. All the diffraction peaks can fit well with CuO,
which are indexed with a pure monoclinic symmetry with a
space group C2/c.39 It was also proven by the typical selected-
area electron diffraction (SAED)40 in Fig. 1F. The dispersion of
atomic Sn was then identified by using aberration-corrected
high-angle annular dark-field scanning transmission electron
microscopy (HAADF-STEM). The isolated bright dots can be
confirmed as the Sn species (highlighted by brown circles),
which were atomically dispersed on CuO support (Fig. 1G). No
nanoparticles or clusters were observed. Element distribution
mappings and element distribution diagram in Fig. 1H revealed
a uniform distribution of Cu, O and Sn, indicating that Sn atoms
are homogeneous over the entire architectures. The Sn content
determined by TEM energy dispersive X-ray spectroscopy
(TEM-EDX) was 1.3 wt%, which is close to that obtained by
ICP-OES (1.19 wt%). We also synthesized a series of defective
CuO supported atomic Sn catalysts using the same method
except the treatment time of H-plasma. The SEM and TEM
images of Sn1/CuO and other Sn1/Vo-CuO-x are shown in Fig.
S2, and their morphologies were essentially the same.
Moreover, EDX analysis (Fig. S3) indicated that the surface
oxygen content decreases with the increasing treatment time of
H-plasma. However, long exposure of the samples to plasma
destroyed the morphology, which became mixed and disorder.
Besides, the Brunauer-Emmett-Teller (BET) surface areas of
the as-synthesized catalysts were determined by the N2
adsorption/desorption method (Fig. S4). Sn1/Vo-CuO-90 had
the largest specific surface area, leading to more space for
mass diffusion.
29-31
Among them, the low cost and low toxicity Sn-based
catalysts have been widely studied, and they can convert CO2
into formate or CO (Table S1). The Sn electronic and
coordination environment can be modified once dispersed into
the atomic morphology, which could modulate the catalytic
behaviors dramatically.32 It is important to note that both
oxygen species in catalysts and active site-support interactions
play critical roles in the adsorption of intermediates during CO2
electroreduction.33-34 Metal oxides, especially CuO, may be
used as the appropriate support. The surface defects, such as
oxygen vacancies (Vo), can be viewed as the anchoring sites of
SASCs. Moreover, Cu species has moderate adsorption and
activation ability of CO intermediate, which is usually involved
in multi-electron transfer during CO2 electroreduction.17,35,36
Diverse strategies can be also easily applied to improve the
catalytic activity, including controlling oxidation state,
constructing surface/interface defects, doping, alloying and
decoration.17,37,38 Thus, choosing CuO as the support of Sn
SASCs may realize the production of value-added chemicals
beyond 2-electron transfer process in CO2 electroreduction.
Herein, we report the first work to construct atomically
dispersed Sn catalysts on defective CuO for CO2
electroreduction to methanol. It was discovered that the
Faradaic efficiency (FE) of methanol could reach 88.6% with a
current density of 67.0 mA cm-2. This is the highest
FE(methanol) with such high current density compared with
those of state-of-the-art electrocatalysts. The superior
performance of the electrocatalysts can be attributed to the
formation of local Lewis acid-base between atomic Sn and its
adjacent
oxygen
vacancy,
which
exhibited
higher
electrochemical activity and electronic conductivity, resulting in
faster charge transfer in CO2 reduction process. Importantly, in
situ X-ray absorption (XAS) spectra, in situ Raman spectra and
density-functional theory (DFT) studies indicated that the
synergistic effect of atomic Sn site and defective CuO support
could promote CO2 activation and further reduction via
decreasing the energy barrier of *COOH dissociation to form
*CO. The resulting *CO intermediate was then bound to the Cu
species, which facilitates the production of methanol during
CO2 reduction.
The quasi-operando X-ray photoelectron spectroscopy
(XPS) revealed the composition and valence state of the
catalysts. The binding energies of 495.4 and 486.9 eV for Sn
3d3/2 and Sn 3d5/2 indicated the oxidation state of Sn41 (Fig. S5-
6). Fig. S7 showed that the Cu 2p XPS spectra of Sn1/Vo-CuO-
x exhibited significantly distinct Cu features compared with that
of Sn1/CuO. It can be seen that the Cu2+ peaks disappeared
gradually as the increasing H-plasma treatment time. Cu+ or
Cu0 species exists in Sn1/Vo-CuO-x, but it is hard to distinguish
from XPS spectra due to the similarity in the binding energies.
Therefore, the Cu Auger L3M45M45 transition was also
performed to provide more insight into the electronic structures
and surface state (Fig. S8),14 indicating the coexistence of Cu+
and Cu0 species on Sn1/Vo-CuO-x.
Results and Discussion
The fabrication procedures of the electrocatalysts are
illustrated in Fig. 1A, and the experimental details can be found
in the Methods section. Briefly, tin (IV) chloride pentahydrate
(SnCl4·5H2O) and copper
(II) sulfate pentahydrate
(CuSO4·5H2O) were dissolved in deionized water with a 1:100
mass ratio and placed in an ice water bath with vigorous stirring
to form a homogeneous blue solution. NaOH aqueous solution
(1.2 M) was then slowly injected into the mixture. After being
refrigerated (~3 oC) for 24 h, the mixture was transferred into a
Teflon-lined autoclave and heated at 130 oC for 18 h. The
o
obtained powder was subsequently calcined at 400 C for 3 h,
as denoted by Sn1/CuO. Finally, Sn1/CuO was treated using H-
plasma for different times to synthesize the catalysts with
2
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RESEARCH ARTICLE
10.1002/anie.202108635
Angewandte Chemie International Edition
Figure 1. Morphology characterization of Sn1/Vo-CuO-90. A) Illustration of the procedures to prepare Sn1/Vo-CuO-90. B) and C) SEM images. D) HR-TEM image.
E) HAADF-STEM image. F) SAED pattern (scale bar:10 1nm-1). G) HAADF-STEM images with the atomic dispersion of Sn highlighted in circles. H) EDS images
and distribution diagram of the elements.
To further get the detailed structural information of Sn1/Vo-
CuO-x, X-ray absorption near-edge structure (XANES) and
sample after H-plasma treatment. The pre-edge features
became broader, and the absorption edge shifted to lower
energy.34 On the other hand, the intensity of Cu-O coordination
(1.5 Å) decreased gradually. These indicated the existence of
Cu species with low oxidation state, which is in line with XPS
results.
extended
X-ray
absorption
fine structure (EXAFS)
characterizations were then performed. As presented in Fig. 2A,
the Sn K-edge XANES spectra showed that the near-edge
absorption energy of Sn1/Vo-CuO-90 was between that of Sn
foil and SnO2, suggesting a lower positive charge state of Sn in
the sample. The Fourier transform (FT) of the EXAFS spectra
for Sn1/Vo-CuO-90 in R space exhibited the dominant Sn-O
coordination with a peak at 1.4 Å (Fig. 2B). No obvious signal
can be assigned to Sn-Sn coordination in comparison with Sn
foil and SnO2 reference samples, which indicated the formation
of atomically dispersed Sn stabilized by surrounding O sites on
support.32 Moreover, the coordination configurations and local
structural parameters of Sn1/Vo-CuO-90 were quantitatively
analyzed by the least squares EXAFS fittings (Fig. S9 and
Afterwards, we conducted a series of characterizations to
determine the concentration of oxygen vacancies. It can be
seen that Sn1/Vo-CuO-x had a similar electron paramagnetic
resonance (EPR) signal (g = 2.001), suggesting the electrons
captured by oxygen vacancies (Fig. S11).42 The O 1s XPS
spectra in Fig. 2F showed two peaks at 532.0 and 529.7 eV,
corresponding to the O in the vicinity of oxygen vacancies and
O bond of Cu-O-Cu,33 respectively. We can find that the
integral-area ratios of the two peaks increased with the
increasing H-plasma treatment time. The photoluminescence
(PL) spectra were also used to verify the relative concentration
of oxygen vacancies (Fig. 2G).33 A peak was observed at
approximately 608 nm, which can be ascribed to the
recombination of two-electron-trapped oxygen vacancies with
photo-generated holes. Sn1/Vo-CuO-90 showed the highest
intensity, leading to the highest concentration of oxygen
vacancies.
2
Table S3). The obtained amplitude reduction factor (S0 ) was
0.80. The fitting results indicated that the Sn atoms coordinated
by three O atoms on CuO support. We also carried out wavelet
transform (WT)-EXAFS (with high resolution in both k and R
space) (Fig. 2C). A strong WT signal focused at 4.8 Å-1 for
Sn1/Vo-CuO-90, which was derived from Sn-O contribution. No
signal derived from Sn-Sn contribution can be seen, further
suggesting the atomically dispersed Sn on CuO. Additionally,
XANES and EXAFS spectra for Cu K-edge were also recorded
(Fig. 2D-2E and Fig. S10). Obvious changes took place in the
3
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10.1002/anie.202108635
Angewandte Chemie International Edition
RESEARCH ARTICLE
Figure 2. Structure characterization of Sn1/Vo-CuO-x. A) Sn K-edge XANES spectra. B) Sn K-edge FT of the EXAFS spectra. C) WT-EXAFS plots of Sn1/Vo-CuO-
90, Sn foil and SnO2. D) Cu K-edge XANES spectra. E) Cu K-edge FT of the EXAFS spectra. F) O 1s XPS spectra. G) PL spectra. H) Electrostatic potential
figures of Sn1/Vo-CuO and Vo-CuO.
The electrostatic potentials were then calculated to
investigate the surface electron density of Sn1/Vo-CuO and Vo-
CuO (Fig. 2H). The presence of Sn atom caused the
reconfiguration of electron density, leading to create a local
alkaline environment. Meanwhile, the oxygen vacancy remained
positively charged. Therefore, a local Lewis acid-base pair was
formed between the Sn and its adjacent oxygen vacancy, which
can play a critical role in CO2 reduction.43
To characterize the electrocatalytic performance for CO2
reduction, Sn1/Vo-CuO-x was dispersed in acetone to form
homogeneous ink and then spread onto carbon paper to be
used as the working electrode. It was first evaluated in a CO2-
saturated [Bmim]BF4/H2O electrolyte with a 1:3 molar ratio by
linear sweep voltammetry (LSV). Sn1/Vo-CuO-90 exhibited a
more positive onset potential and higher current density than
those of the others (Fig. S12), indicating the highest activity
among the as-synthesized catalysts. A cathodic peak appears at
approximately -2.0 V (vs. Ag/Ag+) with a current density of 60.2
mA cm-2, which was roughly 1.2, 1.5 and 2.6 times higher than
that of Sn1/Vo-CuO-60, Sn1/Vo-CuO-30 and Sn1/CuO,
respectively. Moreover, the much lower current density and the
absence of cathodic peak in N2-saturated electrolyte also
indicated the reduction of CO2 on the cathode.
spectroscopy and gas chromatography (GC) with a total FE of
around 100% (Fig. 3A and Fig. S13). No other reduction product
was detected. We recorded the current density for methanol
production, and the equilibrium potential for CO2-to-methanol
was -1.56 V vs. Ag/Ag+ via the extrapolation method calculation
(Fig. S14). It can be seen that all the catalysts exhibited a
volcano-shaped dependence of FE(methanol) at the applied
potentials. Significantly, Sn1/Vo-CuO-90 showed the best
performance with the highest selectivity for methanol production.
The maximum FE(methanol) could reach up to 88.6% with a
current density of 67.0 mA cm-2 at -2.0 V vs. Ag/Ag+ (Fig. 3A-3B
and Fig. S15). To the best of our knowledge, this is the highest
FE(methanol) with very high partial current density (current
density times FE) compared with other reported catalysts in the
literature (Fig. 3C and Table S4). The N2 blank experiment was
carried out during the electrolysis to confirm the carbon source
of the products, and no methanol can be detected. 13CO2 isotope
labeling experiments was then conducted by using the same
set-up over Sn1/Vo-CuO-90 catalyst. From 1H NMR spectra of
the electrolyte after reaction (Fig. S16), we can further confirm
that the carbon source of methanol was from CO2. The long-
term stability of Sn1/Vo-CuO-90 was evaluated by continuous
CO2 reduction at -2.0 V vs Ag/Ag+ for 36 h. There was no
obvious decay in both current density and FE(methanol) in Fig.
3D. We also characterized Sn1/Vo-CuO-90 after electrolysis by
To further analyze the reduction products, controlled
potential electrolysis of CO2 was performed in the same
electrolyte for 2 h at various potentials using a typical H-type cell. TEM, HAADF-STEM, EDX, XPS (Figs. S17-19). The
Methanol, formate, H2 and
a
trace amount of CO were
composition and structure of the catalyst did not change
obviously after the reaction, indicating the excellent stability of
determined by 1H nuclear magnetic resonance (NMR)
4
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10.1002/anie.202108635
Angewandte Chemie International Edition
RESEARCH ARTICLE
Sn1/Vo-CuO-90. Furthermore, we also calculated the cell voltage
and energy efficiency (EE) for methanol production using the
reported method.11 The cell voltage was the combination of the
half reactions potentials for water oxidation and CO2 reduction.
The values were iR corrected and the ohmic drop (EiR) was from
electrolyte resistance (Rs), which was determined by
electrochemical impedance spectroscopy (EIS) at frequencies
ranging from 10−2 to 105 Hz. Fig. S20 shows that the cell voltage
for Sn1/Vo-CuO-90 in this work was 3.28 V with the highest EE
of 64.5%, which is comparable to many reported systems (Table
S5).
The electrochemically active surface areas (ECSAs) of
these catalysts were determined by measuring double layer
capacitance (Cdl).44 From the results in Fig. 3E, Sn1/Vo-CuO-90
had a larger ECSA, which was estimated to be roughly 1.7, 2.5
and 4.1 times higher than that of Sn1/Vo-CuO-60, Sn1/Vo-CuO-
30 and Sn1/Vo-CuO, respectively. Combined with the CO2
adsorption isotherms (Fig. S21) and the CO2 temperature-
programmed desorption (CO2-TPD, Fig. S22), we can conclude
that Sn1/Vo-CuO-90 had higher CO2 adsorption capability, and
its higher CO2 reduction activity was partially attributed to the
larger number of active sites. Moreover, Sn1/Vo-CuO-90 showed
the lowest interfacial charge transfer resistance (Rct), which was
confirmed by the Nyquist plots in Fig. 3F and the electrical
equivalent circuit used for simulating the experimental
impedance data was shown in Fig. S23, hence owning high
overall electronic conductivity and ensuring fast electron transfer
to CO2 for forming the intermediates and products.The excellent
catalytic CO2 reduction performance of Sn1/Vo-CuO-90 was
closely related to its unique structure. When Vo-CuO-90 (without
Sn) was used as the catalysts, the maximum FE(methanol) was
only 46.8%. We also synthesized some Sn/Vo-CuO-90 catalysts
with different amounts of Sn via similar methods, and measured
the CO2 activity for comparison. It was found that both
FE(methanol) and current density increased with the increasing
Sn content until it was 1.19 wt% (Fig. S24). When the Sn
content increased further, the FE(methanol) and current density
became lower than that of Sn1/Vo-CuO-90 (Fig. S25). Therefore,
the atomically dispersed Sn can be regarded as the key active
site for CO2 reduction. On the other hand, high concentration of
oxygen vacancies cooperated very well with atomic Sn sites in
catalyzing CO2 electroreduction to methanol. All the H-plasma
treatment catalysts showed higher selectivity to methanol under
the same applied potential (Fig. 3A). The maximum
FE(methanol) for Sn1/Vo-CuO-60, Sn1/Vo-CuO-30, and Sn1/Vo-
CuO were 75.0%, 58.9% and 40.7%, respectively, suggesting
that high concentration of oxygen vacancies could
Figure 3. CO2 electroreduction in [Bmim]BF4/H2O electrolyte with a 1:3 molar ratio. A) The FE(methanol) and B) Total current density (j) at different applied
potentials with 2 h electrolysis. C) The FE(methanol) and partial current density of Sn1/Vo-CuO-90 compared with other CO2-to-methanol catalysts. D) Long-term
stability of Sn1/Vo-CuO-90 at -2.0 V vs Ag/Ag+. E) Charging current density differences plotted against the square root of scan rates. F) Nyquist plots for Sn1/Vo-
CuO-x.
5
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10.1002/anie.202108635
Angewandte Chemie International Edition
RESEARCH ARTICLE
favor multi-electron transfer during CO2 reduction. Interestingly,
the maximum FE(methanol) can only reach 65.1% and 36.9%
over Sn1/Vo-CuO-120 and Sn1/Vo-CuO-180, which indicated that
the ordered morphology and moderate oxygen content in Cu
species also played important roles in CO2 reduction to
methanol45. Therefore, we can conclude that the cooperative
effect of atomic Sn, oxygen vacancy and CuO support could
promote multi-electron transfer during CO2 reduction.
Sn1/Vo-CuO-90 at an applied potential of -2.0 V vs. Ag/Ag+ (Fig.
S28), we can find that there is no change in the spectra,
indicating that [Bmim]BF4 can keep stable during the electrolysis.
To gain insight into the mechanism for CO2 reduction to
methanol, a series of further experiments were carried out.
Operando X-ray adsorption spectroscopy (XAS) at the Sn K-
edge and Cu K-edge was first performed to monitor the
electronic structure change of Sn1/Vo-CuO-90 during CO2
reduction. Fig. 4A shows that the chemical state of Sn had no
obvious changes in the reaction process, suggesting high
stability of the atomic Sn sites on support. The Sn K-edge
The electrolysis of CO2 was also performed in 0.5 M KHCO3
aqueous solution over Sn1/Vo-CuO-90. Fig. S26 shows that FE
and current density of all products at different applied potentials
with 2 h electrolysis. Unfortunately, no methanol can be detected. EXAFS spectra of the catalysts after reaction are shown in Fig.
The maximum CO2 reduction occurred at -0.6 V vs. RHE, where
formate (15.4%) and very little of CO (< 0.5%) were the products
with a current density of 16.4 mA cm-2. These values were much
lower than those in [Bmim]BF4/H2O electrolyte with a 1:3 molar
ratio, indicating the significant role of IL during the electrolysis.
The presence of [Bmim]BF4 enhances the CO2 solubility, which
alleviates mass transport limitation in the reaction.46 On the
other hand, the imidazolium cation can be electrostatically
arranged on the cathode to promote CO2 adsorption via [Bmim-
CO2]+, which could decrease the activation energy for CO2
reduction.47,48 We also studied the catalytic performance using
[Bmim]BF4/H2O electrolyte with different ratios (Fig. S27). It can
be seen that the activity and selectivity for methanol production
were the highest when adding moderate amount of water into IL.
This may result mainly from it can depress the HER and boost
the CO2 reduction efficiency.48,49 Based on the 1H NMR
spectrum of [Bmim]BF4/H2O catholyte after 36 h electrolysis over
S29. It can be seen that no obvious signal can be assigned to
Sn-Sn coordination in comparison with Sn foil and SnO2
reference samples in Fig. 2B, indicating that the Sn species can
keep the atomic morphology after CO2 reduction. However, it is
clear that significant changes took place on CuO support once a
negative potential had been applied (Fig. 4B). The oxidation
state of Cu became lower as the increasing potential from -1.6 V
to -2.0 V vs. Ag/Ag+ throughout CO2 reduction. A linear
combination fitting analysis of the operando XANES data were
shown in Fig. 4C and Fig. S30 using Cu foil and CuO as the
reference spectra. The results indicated that the fraction of Cu0
species was 15%, 28% and 35% at -1.6 V, -1.8 V and -2.0 V,
respectively (inset of Fig. 4D). In the corresponding FT-EXAFS
spectra (Fig. 4D), the Cu local environment exhibited a rapid
evolution of Cu-Cu coordination peak at 2.2 Å and a decay of
Cu-O coordination peak at 1.5 Å, which were in agreement with
the XANES data.34,50,51
Figure 4. Operando characterization of Sn1/Vo-CuO-90 during CO2 electroreduction. A) XANES spectra at the Sn K-edge under different potentials. B) XANES
spectra at the Cu K-edge under different potentials. C) Cu K-edge XANES data fitting with a linear combination of reference spectra (at an applied potential of -2.0
V vs. Ag/Ag+). D) FT EXAFS spectra at the Cu K-edge under different potentials. E) In situ Raman spectra under different potentials.
6
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We used the possible intermediates as the reactants to
further study the reaction pathway of CO2-to-methanol. The
control experimental results in Table S6 showed that CO and
formaldehyde promoted the formation of methanol, while no
methanol can be detected when formate was used as the
reactant. It demonstrated that CO and formaldehyde were the
possible intermediates for CO2 reduction to methanol. We then
acquired Raman spectra in situ to reveal the interactions
between Cu species and the reaction intermediates. Two
regions in Raman spectra are associated with the presence of
adsorbed *CO (Fig. 4E). The appearance of peaks located at
about 298 and 2083 cm-1 can be assigned to the frustrated
rotation of adsorbed *CO on Cu and C≡O stretching,
respectively. The broad peak around 608 cm-1 corresponded to
CuOx species,52-54 which are further showed that the low
oxidation state of Cu existed during CO2 reduction. It is believed
that CO is critical for the formation of methanol. Thus, the
electrochemical CO stripping voltammetry tests were performed
to probe the CO desorption ability on Sn1/Vo-CuO-x. As shown
in Fig. S31, a sharp CO stripping profile with a dominant peak
appeared around 0.76 V vs. RHE for Sn1/Vo-CuO-90, whereas it
was around 0.71 V, 0.68 V and 0.61 V vs. RHE for Sn1/Vo-CuO-
60, Sn1/Vo-CuO-30 and Sn1/Vo-CuO, respectively. The positive
shift indicated that Sn1/Vo-CuO-90 had stronger CO binding
ability, leading to higher activity for further reduction to methanol.
DFT calculations were conducted to study the intrinsic
property of the remarkable electrocatalytic activity of Sn1/Vo-
CuO-x for CO2 reduction to methanol. All the computational
structure models and the detailed data were shown in the
supporting information (Fig. S32 and Table S7). The density of
states (DOS) of Sn, Cu and O atoms in the models were first
analyzed and depicted in Fig. 5A, we can find that a new defect
level appeared around the valence band maximum in the
presence of oxygen vacancies. From the partial charge density
in Fig. 5B, the electrons are inclined to accumulate around the
valence band maximum of Sn1/Vo-CuO, but not in Sn1/CuO. The
abundant localized electrons around the valence band maximum
can be transferred into the antibonding orbitals of CO2 molecules,
which is beneficial to promote CO2 activation and further
reduction.33
Figure 5. DFT calculations for CO2 reduction to methanol. A) The DOS of Sn, Cu and O atoms for Sn1/Vo-CuO and Sn1/CuO models. B) The partial charge
density around the valence band maximum for Sn1/Vo-CuO and Sn1/CuO models. C) Gibbs free-energy diagrams for CO2-to-methanol over different simulated
Sn1/Vo-CuO model. D) The catalytic pathway on Sn1/Vo-CuO based on the optimized structures of adsorbed intermediates. Grey, blue, red, brown and white
spheres in the insets represent Sn, Cu, O, C and H atoms, respectively.
Furthermore, the catalytic pathway over Sn1/Vo-CuO was
proposed based on DFT calculations and control experiments.
As depicted in Figs. 5C-5D and Figs. S33-S34, we find that CO2
was more favorably adsorbed onto Sn site in the first activation
step. Compared with other two models (Sn1/CuO and Vo-CuO),
the formation of *COOH on Sn1/Vo-CuO can reach a stable
configuration with lower free energy (0.08 eV), where one of the
O atoms was directly bound to the adjacent oxygen vacancy of
the Sn atom. This indicated the synergistic effect of Sn1-Ov
Lewis acid-base pair for CO2 activation. It also enabled a
decrease in the energy barrier for the dissociation of *COOH to
form *CO. The resulting *CO intermediate as thought to be
bound to the Cu species, which facilitates the production of
value-added products beyond 2-electron transfer in CO2
reduction process. Following *CO formation, *CHO formation
occurred, which was exergonic due to its strong binding onto the
Cu surface. The free energy of *CHO over Sn1/Vo-CuO was
more negative than that over Sn1/CuO and Vo-CuO, indicating
that the moderately strongest binding energy among the three
models. We can find that the formation of *OCH2 from *CHO
was highly endothermic and likely acted as the rate-determining
step (RDS) since the highest energy potential (0.35 eV) was
needed in this step. However, it is quite different from Sn1/CuO
and Vo-CuO. According to the simulation, the Gibbs free energy
for the RDS over Sn1/Vo-CuO was 0.31 eV and 0.11 eV lower
than that over Sn1/CuO and Vo-CuO (Fig. S33, S35),
respectively. It demonstrated better performance for CO2
reduction to methanol with lower onset potential over Sn1/Vo-
CuO. After that, *OCH2 was adsorbed to further accept electrons
and protons to form *OCH3, and then was reduced to methanol.
Additionally, all of the steps became downhill in energy when a
potential of 0.35 V is applied. These results indicated that the
design of atomically dispersed Sn on the defective CuO favored
CO2 electroreduction to methanol.
7
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RESEARCH ARTICLE
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Entry for the Table of Contents
We have prepared atomically dispersed Sn catalysts on defective CuO for CO2 electroreduction to methanol. The Faradaic efficiency
of methanol could reach 88.6% with a current density of 67.0 mA cm-2. The catalyst was beneficial for CO2 activation via decreasing
the energy barrier of *COOH dissociation to form *CO. The *CO was then bound to the Cu species for further reduction, leading to
high selectivity toward methanol.
10
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