Electrochemical Reduction of Carbon Dioxide to Methanol on Hierarchical Pd/SnO2 Nanosheets with Abundant Pd–O–Sn Interfaces

Article abstract of DOI:10.1002/anie.201804142

Electrochemical conversion of CO₂ into fuels using electricity generated from renewable sources helps to create an artificial carbon cycle. However, the low efficiency and poor stability hinder the practical use of most conventional electrocatalysts. In this work, a 2D hierarchical Pd/SnO₂ structure, ultrathin Pd nanosheets partially capped by SnO₂ nanoparticles, is designed to enable multi-electron transfer for selective electroreduction of CO₂ into CH₃OH. Such a structure design not only enhances the adsorption of CO₂ on SnO₂, but also weakens the binding strength of CO on Pd due to the as-built Pd–O–Sn interfaces, which is demonstrated to be critical to improve the electrocatalytic selectivity and stability of Pd catalysts. This work provides a new strategy to improve electrochemical performance of metal-based catalysts by creating metal oxide interfaces for selective electroreduction of CO₂.

Full text of DOI:10.1002/anie.201804142

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Accepted Article
Title: Electrochemical Reduction of CO2 to CH3OH on hierarchical Pd/
SnO2 nanosheets with abundant Pd-O-Sn interfaces
Authors: Wuyong Zhang, Qing Qin, Lei Dai, Ruixuan Qin, Xiaojing
Zhao, Xumao Chen, Daohui Ou, Tracy T Chuong, Binghui
Wu, and Nanfeng Zheng
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201804142
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Angewandte Chemie International Edition
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COMMUNICATION
Electrochemical Reduction of CO
2
to CH OH on hierarchical
3
Pd/SnO nanosheets with abundant Pd-O-Sn interfaces
2
Wuyong Zhang, Qing Qin, Lei Dai, Ruixuan Qin, Xiaojing Zhao, Xumao Chen, Daohui Ou, Jie
Chen,Tracy T Chuong, Binghui Wu* and Nanfeng Zheng*
Abstract: Electrochemical conversion of CO
2
into fuels using
Previous studies have demonstrated that CO intermediate is
usually involved in multi-electron transfer during CO
RR.[1a]
electricity generated from renewable sources helps to create an
artificial carbon cycle. However, the low efficiency and poor stability
hinder the practical use of most conventional electrocatalysts. In this
2
Considering that Pd has high chemical affinity to CO and is thus
easily poisoned by it,[ the manipulation of both adsorption and
activation of CO is crucial in the design of new Pd-based
nanocatalysts to realize the production of value-added chemicals
8]
work, a 2D hierarchical Pd/SnO
partially capped by SnO nanoparticles, is designed to enable multi-
electron transfer for selective electroreduction of CO into CH OH.
Such a structure design not only enhances the adsorption of CO on
SnO , but also weakens the binding strength of CO on Pd due to the
2
structure, ultrathin Pd nanosheets
2
-
2
3
beyond 2e process. We now report in this work a two-dimensional
2
2
(2D) hierarchical Pd/SnO structure to meet such a demand on
2
balancing CO adsorption and activation. 2D ultrathin Pd
nanosheets (Pd NSs) with thickness of several atomic layers and
thus large surface area[9] were chosen to provide abundant active
sites for electrocatalysis.[ The surface of Pd NSs were modified
as-built Pd-O-Sn interfaces, which is demonstrated to be critical to
improve the electrocatalytic selectivity and stability of Pd catalysts.
This work provides a new strategy to improve electrochemical
performance of metal-based catalysts by creating metal-oxide
10]
by depositing fine SnO
catalyst,[11] to form the hierarchical structure.[12] The partial
capping of Pd NSs with SnO helped to enhance the adsorption
of CO but weaken the CO binding on Pd. The as-built Pd-O-Sn
interfaces was demonstrated to play a key role in electro-
reduction of CO to CH OH via a multi-electron transfer process.
2 2
NPs, another widely-studied CO RR
2
interfaces for selective electroreduction of CO .
2
2
2
Transferring carbon dioxide (CO ) to useful carbon-based fuels
through electrochemistry using renewable electricity is an
attractive way to alleviate the environment and energy crisis.[1]
Over the past few decades, numerous catalysts have been
2
3
developed and applied for electrochemical CO
2
reduction reaction
(
CO
2
RR). Early work mostly focused on bulk metal catalysts,[2]
which usually suffered from poor activity with low current density,
poor selectivity with multiple products, and depressed stability
during electrocatalysis. To tackle these problems, a series of
nanostructured catalysts including metal, alloy, metal-oxides and
metal chalcogenides have been developed to replace traditional
bulk metal catalysts.[3] Among these nanocatalysts, palladium-
2
based nanomaterials have shown great potential in CO RR due
to their high mass activity.[4] When pure Pd nanocatalysts are
used, the selectivity of products are often regulated by controlling
the size of Pd nanoparticles (NPs) or the reduction potentials.[5]
As for alloyed-Pd catalysts, the proper ratio of second metal to Pd
is critical to obtain a high-selectivity product.[6] Nevertheless, so
far majority of Pd-based nanomaterials just produce CO or
-
-
2
HCOO through 2e transfer in CO RR. Multi-electron reduction to
obtain value-added products such as methanol has been rarely
reported.[
7]
[*]
W. Zhang, Q. Qin, Dr. L. Dai, R. Qin, Dr. X. Zhao, X. Chen, Dr. J.
Chen, Prof. N. Zheng
State Key Laboratory for Physical Chemistry of Solid Surfaces,
Collaborative Innovation Center of Chemistry for Energy Materials,
National & Local Joint Engineering Research Center of Preparation
Technology of Nanomaterials, College of Chemistry and Chemical
Engineering, Xiamen University, Xiamen 361005 (China)
E-mail: nfzheng@xmu.edu.cn
D. Ou, Dr. T. T Chuong, Prof. B. Wu
Pen-Tung Sah Institute of Micro-Nano Science and Technology,
Xiamen University, Xiamen 361005 (China)
E-mail: binghuiwu@xmu.edu.cn
Figure 1. Structure characterization of Pd and hierarchical Pd/SnO
Illustration for the synthesis of Pd/SnO NSs. (b) TEM image of Pd NS seeds.
c) TEM image, (d) HAADF-STEM and EDX mapping, and (e) HRTEM images
2
NSs. (a)
2
(
Supporting information for this article is given via a link at the end of
the document.
2
of Pd/SnO NSs (Pd:Sn=1:1). Insets of b and e are corresponding HRTEM
images showing interplanar lattice fringe spacing.
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Angewandte Chemie International Edition
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COMMUNICATION
Partial capping of Pd NSs with SnO
2
NPs (denoted as
test in 0.1 M NaHCO
Pd/SnO NSs is much larger than those of Pd NSs and SnO
(Figure 2c), clearly confirming the close correlation between
active sites and strong interaction of Pd and SnO . Moreover, as
indicated in the Nyquist plots (Figure S10), CO molecules have
a smaller interfacial charge-transfer resistance on Pd/SnO NSs.
for
3
solution saturated with CO
2
. ECSA of
Pd/SnO
2
NSs) was carried out using the precedure illustrated in
2
2
NPs
Figure 1a (details in Supporting Information). Pd NS seeds were
first dispersed in ethanol and water by adequate sonication in a
2
round-bottom flask. SnCl
mixture, followed by dropwise addition of NH
The black product was obtained after refluxing at 120 oC. The
microstructure of as-prepared Pd/SnO NSs was studied by
2
·2H
2
O was then added into the above
2
3
2
·H O under stirring.
2
As a result, electrons can transfer easily to adsorbed CO
2
forming the intermediate.[ In order to confirm the necessity of
3a]
2
transmission electron microscopy (TEM). As shown in Figure 1b-
c, while surface roughness was observed, the structure and
dimension of the Pd NSs remained unchanged after the
strong interaction between Pd and SnO
2
, we mixed SnO
RR performance of the mixture.
OH upon the electrolysis was
2
NPs
with Pd NSs physically to test CO
No liquid product of HCOO or CH
2
-
3
deposition of SnO
showed that the product was uniform and free of unattached SnO
NPs (Figure S1). The attachment of these Pd/SnO NSs on
carbon nanotubes allowed the direct measurement of their overall
thickness by TEM which is 4-6 nm, consistent with atomic force
microscopy results (Figure S2). As verified by high-angle annular
dark field-scanning transmission electron microscopy (HAADF-
STEM) image and energy-dispersive X-ray (EDX) mapping
2
NPs. Low magnification TEM images also
obtained (Figure S11). Besides activity, durability is another
important measure of an excellent catalyst. Pd/SnO NSs did not
2
2
2
show obvious attenuation on the current densities in
chronoamperometry test (Figure 2d), and held the high FE of 54%
over 24 h to produce CH
(Figure S12). Moreover, the Pd/SnO
their original morphology and crystalline structure after the 24-h
electrolysis (Figure S13). In addition, Pd/SnO NSs could transfer
CO to methanol with similar performance in different
electrolytes (Figure S14). All these results suggested that the
modification of Pd with SnO greatly enhanced the catalysis of Pd
in CO RR.
3
OH from the time-tracking 1H NMR
catalyst also maintained
2
2
(
Figure 1d and S3), SnO
It should be noted that the surface of Pd NSs capped by SnO
still partially exposed. An obvious CO stripping peak was
observed on Pd/SnO NSs although the intensity was lower than
that of uncapped Pd NSs (Fig S4). The EDX analysis revealed
that the Pd:Sn atomic ratio of Pd/SnO NSs is 0.52:0.48 (Figure
2
NPs are well-dispersed on each Pd NS.
2
a
2
is
2
2
2
2
S5), which was further confirmed by the inductively coupled
plasma mass spectroscopy (ICP-MS) result (0.54:0.46). The X-
ray diffraction (XRD) pattern of Pd/SnO
indexed to fcc Pd (JCPDS#46-1043) and rutile SnO
445). The size of SnO NPs analyzed by XRD is about 3-5 nm,
consistent with the high-resolution TEM analysis (Figure 1e). As
shown in Figure 1e, the single-crystalline nature of SnO NPs with
interplanar lattice fringe spacing of 0.33 nm, corresponding to the
d-spacing of (110) plane of rutile SnO , was clearly observed.
In view of the unique microstructure of Pd/SnO NSs, we
conducted CO RR to investigate whether the as-fabricated Pd-
SnO interfaces would influence the catalysis of Pd. The Pd/SnO
2
NSs (Figure S6) is clearly
2
(JCPDS#41-
1
2
2
2
2
2
2
2
NSs were loaded onto carbon paper to serve as a working
electrode. Linear sweep voltammetry (LSV) test was performed in
a H-cell with CO
Poor electrochemical activity was found on both isolated Pd NSs
and SnO NPs (Figure S7). By contrast, Pd/SnO NSs showed a
remarkable enhancement of CO reduction. As illustrated in
Figure 2a, the Pd/SnO NSs displayed a current density of -1.45
mA cm-2 at -0.24 V versus the reversible hydrogen electrode
RHE), significantly higher than those of Pd NSs and SnO NPs.
reduction product, gas chromatography and
2 3
-saturated 0.1 M NaHCO solution (pH=6.8).
2
2
2
2
(
2
Figure 2. Electrochemical studies on Pd/SnO
with Pd NSs and SnO NPs. (a) LSV curves in 0.1 M NaHCO
Faradaic efficiencies for CH OH at various applied potentials. (c) Charging
current density differences plotted against scan rates. (d) Chronoamperometry
results of different catalysts.
2
NSs (Pd:Sn=1:1) in comparison
To identify the CO
2
2
3
solution. (b)
1
H NMR spectroscopy (Figure S8) were performed to quantify the
gaseous and liquid products, respectively. To our surprise,
3
different from the situations on single Pd NSs and SnO
multi-electron transfer in CO RR took place on Pd/SnO NSs to
produce CH OH as a major product, along with hydrogen and
formate (Figure S9). As shown in Figure 2b, a maximum faradaic
efficiency (FE) of 54.8±2% for CH OH was achieved at -0.24 V
versus RHE. Compared to other reported catalysts (Table S1),
Pd/SnO NSs showed a decent current density with an ideal FE
and low overpotential. To verify that the modification of SnO on
Pd created new active sites for CO RR, the electrochemical active
surface area (ECSA) was measured via double-layer capacitance
2
NPs,
2
2
3
The combination of Pd and SnO
effect for the formation of CH OH, suggesting that the built-up Pd-
SnO interface should modulate the CO and CO adsorption
capacities of the catalyst. First of all, we found that the adsorption
capacity of CO on the Pd/SnO NSs was significantly improved
2
created an unexpected
3
3
2
2
2
2
2
2
as compared to that of Pd NSs. As clearly revealed by the in situ
diffuse reflectance infrared Fourier transform (DRIFT)
2
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Angewandte Chemie International Edition
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COMMUNICATION
spectroscopy data (Figure 3a and Figure S15), Pd/SnO
2
NSs
As revealed by extend X-ray absorption fine structure (EXAFS)
spectra (Figure 4a), there is one smaller peak of Pd/SnO NSs at
2
displayed a much stronger absorption at 2,350 cm-1 that those on
SnO
desorption) measurements (Figure S16) also confirmed the
excellent capacity of CO adsorption on Pd/SnO NSs, which is
beneficial to CO RR. Secondly, Pd atoms on Pd/SnO NSs are
partially exposed. As clearly shown in the CO stripping (Figure
b), SnO NPs are unable to bind CO, but the Pd/SnO NSs
inherit the CO binding ability of Pd NSs. Since CO has been well-
documented as an important intermediate in CO electroreduction
to CH OH, such a CO-binding capability of Pd/SnO NSs made it
is possible to further reduce CO during CO
RR.[1a] More
importantly, the modification of SnO helped to weaken CO
poisoning on Pd. Compared to that on Pd NSs, the CO stripping
potential on Pd/SnO NSs is slightly shifted towards a lower
potential (Figure S17a). To further evaluated the effect of SnO
the coverage of SnO on Pd/SnO NSs was tuned by different
Pd:Sn ratio in synthesis (Figure S18). Figure 3c shows the
potential-dependent FEs of Pd/SnO NSs with different Pd/Sn
ratios. While the Pd/SnO NSs with low ratio of SnO (Pd:Sn=3:1)
content on Pd
2
NPs and Pd NSs. The CO
2
-TPD (temperature programmed
~1.5 Å from the Pd-O contribution, and a peak at ~2.5 Å from the
Pd-Pd contribution referring to the standard sample (Figure S19).
Further EXAFS fitting analysis (Table S2) revealed that the
2
2
2
2
coordination number (C.N.) of Pd-Pd in Pd/SnO
lower than that of 9 in Pd NSs, indicating the partial oxidation of
Pd in Pd/SnO NSs. The oxidation state of Pd in Pd/SnO NSs
2
NSs is 4.5, much
3
2
2
2
2
was also confirmed by the white-line intensity in the X-ray near-
edge structure (XANES) (Figure 4b), X-ray photoelectron
spectroscopy (XPS) and Raman spectra. As shown in Figure S20,
XPS peaks at 336.8 eV (Pd 3d5/2) and 342.4 eV (Pd 3d3/2) are
assigned to Pd(II).[14] In the Raman spectra (Figure S21), the
hump peak near 628 cm-1 happens to be the overlap of the A1g
2
3
2
2
2
-
1
2
vibration mode of SnO
at 636 cm .
2
at 635 cm and B1g vibration mode of PdO
As for Sn in Pd/SnO NSs, the electronic structure
is close to that of pure SnO NPs from the EXAFS and XANES
spectra (Figure S22 and S23), but has a lower coordination
number than SnO NPs (Table S3). In view of this, we speculated
the formation process of Pd/SnO NSs as that Sn(II) went through
-1 [15]
2
,
2
2
2
2
2
2
2
2
2
had H
2
as the major product, the increase of SnO
2
the hydrolysis and oxidation to Sn(IV) under the alkaline
condition,[16] and Pd NSs acted as a support.[17] With the
-
NSs increased the FEs of HCOO and CH
3
OH. No matter what
OH reached a
-
potential applied, the total FE of HCOO and CH
3
deposition of SnO
2
, some surface Pd atoms on Pd NSs were
maximum value when Pd:Sn ratio is 1:1. As confirmed by the CO
stripping data (Figure S17), the increased content of SnO
reduced Pd exposure. It is thus reasonable to tune the Pd/Sn ratio
in Pd/SnO NSs for maximizing the FE of CH OH.
inevitably oxidized to create Pd-O-Sn interfaces.
2
2
3
Figure 4. Evaluation of Pd-O interfaces in Pd/SnO
edge (a) FT-EXAFS and (b) XANES spectra. (c) XRD patterns and (d) HNMR
2
NSs (Pd:Sn=1:1). Pd K-
1
spectra of liquid products before and after electrolysis of Pd/PdO catalyst.
Figure 3. Adsorption of CO
2
/CO and effects of Pd/Sn ratio on FEs. (a) In situ
DRIFTS of CO saturated adsorption. (b) CO stripping voltammetry. (c) Faradaic
2
efficiencies for methanol, formate, and hydrogen at different voltages.
After proving the presence of Pd-O-Sn interfaces in the
Pd/SnO
interfaces on CO
catalyst to simulate Pd/SnO
Multi-potential step electrolysis was conducted on the catalyst in
.1 M NaHCO solution under CO atmosphere (Figure S24).
Interestingly, there was a current fluctuation in the initial 2-hour
2
NSs, we further explored the effect of the built the
RR. For comparison, we synthesized a Pd-PdO
NSs by heating Pd(OAc) in air.
Considering that many metal-oxide interfaces have been
reported to play important roles in electrocatalysis,[13] we
2
2
2
proposed that the Pd-SnO
promotional effect in this work. We thus conducted X-ray
absorption studies to evaluate the interface structure of Pd/SnO
2
interface is crucial for the observed
0
3
2
2
.
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Angewandte Chemie International Edition
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COMMUNICATION
electrolysis (Figure S24a), which was probably due to the
reduction of PdO to Pd. Indeed, the XRD analysis (Figure 4c)
revealed that the initial Pd-PdO was completely reduced and
transferred to metallic Pd after the electrolysis. Meanwhile, both
Keywords: CO reduction reaction (CO RR) • palladium • tin
2
2
oxide • methanol • interface
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electrolyte. We thus speculate the Pd-oxide interface plays a
catalytic role on CO electroreduction to CH OH. As proposed in
Figure S25, the electroreduction of CO on Pd/SnO NSs takes
first adsorbs on the surface of SnO and reduces
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3
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2
3
2
2
place with CO
2
2
•- [3a]
•-
to CO
2
.
Then CO
2
COads.[3b] While competing with Pd(II) for electrons, the CO
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intermediate has priority to be further reduced to CH OH. The
process is consistent with the mechanism for CO
2
1a, 1b]
electroreduction to CH
3
OH as reported.[
Thinking about the
-
[1a]
possibility of HCOO as an intermediate to get methanol, CO
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proving that the CO pathway was fit for Pd/SnO
the general importance of Pd-O-Sn interfaces on CO
synthesized the Pd Black/SnO via the similar SnCl
Figure S27). The catalyst was found to be highly effective for the
electroreduction CO to CH OH (Figure S28) with a FE of 34%.
The stability of Pd-O-Sn interfaces in Pd/SnO NSs was also
2
NSs. To prove
RR, we also
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1
2
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interaction between Pd and SnO
interfaces gave rise to the electrocatalytic selectivity and stability
for the formation of CH OH.
In conclusion, we have built a hierarchical 2D material by
partially capping Pd NSs with SnO NPs for the electrochemical
reduction of CO . Compared to single Pd NSs and SnO NPs, the
hierarchical Pd/SnO NSs displayed a much better performance
in the multi-electron transfer reduction of CO into CH OH. The
selectivity and long-term stability were readily optimized by tuning
the coverage of SnO on Pd NSs. The high selectivity toward
CH OH are attributed to three factors: (1) Pd/SnO NSs possess
excellent adsorption capacity of CO ; (2) partially exposed Pd on
Pd/SnO NSs adsorbs CO* intermediate for further
electroreduction; (3) the existence of Pd-O-Sn interfaces assists
further reduction of CO* intermediate to CH OH. Given current
2
and as the built-up Pd-O-Sn
3
2
2
2
2
2
3
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Acknowledgements
1
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The authors acknowledge support from the Ministry of Science
and Technology of China (2017YFA0207302, 2015CB932300),
the National Natural Science Foundation of China (21731005,
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research funds for central universities (20720160080). We also
acknowledge support from beamline BL14W1 (Shanghai
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Angewandte Chemie International Edition
10.1002/anie.201804142
COMMUNICATION
COMMUNICATION
A 2D hierarchical Pd/SnO
based on ultrathin Pd nanosheets
partially capped by SnO
nanoparticles, was designed to enable
multi-electron transfer for
2
structure,
Wuyong Zhang, Qing Qin, , Lei Dai,
Ruixuan Qin, Xiaojing Zhao, Xumao
Chen, Daohui Ou, Jie Chen, Tracy T
Chuong, Jie Chen, Binghui Wu* and
Nanfeng Zheng*
2
electroreduction of CO
The upgraded adsorption of CO
the SnO surface, proper binding of
CO to Pd, and unique Pd-O-Sn
interfaces were critical factors for the
electrocatalytic selectivity and stability.
2
to CH
3
OH.
on
Page No. – Page No.
2
2
Electrochemical reduction of CO
2
to
CH OH on hierarchical Pd/SnO
3
2
nanosheets with Abundant Pd-O-Sn
interfaces
This article is protected by copyright. All rights reserved.