Highly Selective Production of Ethylene by the Electroreduction of Carbon Monoxide

Article abstract of DOI:10.1002/anie.201910662

Conversion of carbon monoxide to high value-added ethylene with high selectivity by traditional syngas conversion process is challenging because of the limitation of Anderson-Schulz–Flory distribution. Herein we report a direct electrocatalytic process for highly selective ethylene production from CO reduction with water over Cu catalysts at room temperature and ambient pressure. An unprecedented 52.7 % Faradaic efficiency of ethylene formation is achieved through optimization of cathode structure to facilitate CO diffusion at the surface of the electrode and Cu catalysts to enhance the C?C bond coupling. The highly selective ethylene production is almost without other carbon-based byproducts (e.g. C₁–C₄ hydrocarbons and CO₂) and avoids the drawbacks of the traditional Fischer–Tropsch process that always delivers undesired products. This study provides a new and promising strategy for highly selective production of ethylene from the abundant industrial CO.

Full text of DOI:10.1002/anie.201910662

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Accepted Article
Title: Highly Selective Production of Ethylene by Electroreduction of
Carbon Monoxide
Authors: Ruixue Chen, Hai-Yan Su, Deyu Liu, Rui Huang, Xianguang
Meng, Xiaoju Cui, Zhong-Qun Tian, Dong H. Zhang, and
Dehui Deng
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201910662
Angew. Chem. 10.1002/ange.201910662
Link to VoR: http://dx.doi.org/10.1002/anie.201910662
http://dx.doi.org/10.1002/ange.201910662

10.1002/anie.201910662
Angewandte Chemie International Edition
Highly Selective Production of Ethylene by Electroreduction of Carbon
Monoxide
Ruixue Chen+1,2, Hai-Yan Su+3, Deyu Liu1, Rui Huang2, Xianguang Meng2, Xiaoju
Cui1,2, Zhong-Qun Tian1, Dong H. Zhang3 and Dehui Deng1,2*
1Collaborative Innovation Center of Chemistry for Energy Materials, College of
Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China.
2State Key Laboratory of Catalysis, Collaborative Innovation Center of Chemistry for
Energy Materials, Dalian Institute of Chemical Physics, Chinese Academy of Sciences,
Dalian 116023, China.
3State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical
Physics, Chinese Academy of Sciences, Dalian 116023, China.
+These authors contributed equally.
*Email: dhdeng@dicp.ac.cn
Abstract: Conversion of carbon monoxide to high value-added ethylene with high
selectivity by traditional syngas conversion process is always challenging because of
the limitation of Anderson-Schulz-Flory distribution. Herein we report a direct
electrocatalytic process for highly selective ethylene production from CO reduction
with water over the Cu catalysts at room temperature and ambient pressure. An
unprecedented 52.7% Faradaic efficiency of ethylene formation from CO
electroreduction has been achieved through rational optimization of cathode structure
to facilitate CO diffusion at the surface of electrode and Cu catalysts to enhance the C-
C bond coupling. Moreover, the highly selective ethylene production almost without
other carbon-based byproducts such as C1-C4 hydrocarbons and CO2 refrains from the
drawback of traditional Fischer-Tropsch process that always delivers massive undesired
products. This study provides a new and promising strategy for highly selective
production of ethylene from the abundant industrial CO.
Ethylene (C2H4) is an important building block for chemical industry and usually
produced by steam cracking of naphtha feedstocks at 800-900 oC worldwide. However,
considering the utilization of the nonpetroleum carbon resources, converting the
1
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10.1002/anie.201910662
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abundant industrial carbon resource, such as CO from syngas (CO + H2) to C2H4
through energy-efficient strategy is regard as one of the key process. Over the years,
much attention has been attracted in C2-C4 olefins production by Fischer-Tropsch
synthesis (FTS) under high temperatures (200-450 oC) and high pressures (5-50 bar).[1]
Apart from these harsh reaction conditions and consuming hydrogen resources, the
products from FTS process are often limited by the Anderson-Schulz-Flory (ASF)
distribution, which leads to 30% selectivity of C2 hydrocarbons (ethylene and ethane)
at most.[2] The resultant gas mixture containing C1-C4 hydrocarbons requires further
separation to get high-purity ethylene.[2b, 3] Besides, there is still as high as 30-50% CO2
selectivity accompanying with the products in FTS to olefins,[2b, 3a-c] which causes
undesired carbon emission and loss. Therefore, the development of highly selective and
energy-saving ethylene production route without additional separation and waste of
carbon resources is of great technological, economical and environmental interest.
In contrast to the electrocatalytic systems, cracking of naphtha and FTS to olefins
both suffer from high energy consumption and complex processes of product separation.
Electrocatalytic systems with the spatially separated oxidation and reduction processes
allow for a different reaction equilibrium and product selectivity via designing special
electrodes in electrolytes at mild conditions. Taking the advantage of the rational
designed anode, recently we have succeeded in high-purity hydrogen production via
electrochemical water-gas shift reaction at room temperature.[4] This inspires us to
adopt electrocatalytic strategy for another reaction, i.e. electrocatalytic CO reduction to
ethylene (CORTE) process. Electrocatalytic CO reduction has recently emerged as a
promising approach to attain multi-carbon alcohols[5] with high energy conversion
efficiency. However, the direct electrocatalytic CORTE process had rarely been
reported due to the low Faradaic efficiency (FE) and selectivity of ethylene.
Herein we report a highly selective and efficient production of ethylene process
from electrocatalytic CO reduction with water at room temperature and ambient
pressure. Through rational optimization of polytetrafluoroethylene (PTFE) content to
increase CO concentration at the surface of electrode and Cu particle catalysts to
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10.1002/anie.201910662
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enhance the C-C bond coupling, this electrocatalytic CORTE process can achieve an
unprecedentedly high ethylene FE of 52.7%. Moreover, the high selectivity of ethylene
production without any CO2 emission breaks through the 30% selectivity limitation
from CO to C2 hydrocarbons in traditional FTS process, which always delivers
uncontrollable mixture of C1-C4 hydrocarbons and massive CO2.
Figure 1. Electrode structure and the related CORTE performance. a) Faradaic
efficiency and geometric current density for CO electroreduction over Cu-Ps loaded on
four types of carbon papers tested in 1 M KOH at 25 oC. Bottom pictures show contact
angles of water droplets on the corresponding surface of carbon papers. b) SEM images
of Cu-Ps/CP-MPL electrode. c) Schematic illustration for CORTE process on Cu
catalysts with the assistant of the hydrophobic micro-porous layer to improve the CO
diffusion.
The low solubility of CO in electrolyte leads to the low local concentration of CO
on the surface of catalysts, which therefore limits the efficiency of the electrocatalytic
CORTE process. It is believed that increasing the hydrophobicity of the electrode
surface can reduce the affinity of water to the electrode and promote the diffusion of
CO to the water-electrode interface. Besides, the carbon papers themselves are inert in
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CO electroreduction reaction (Figure S1). Hence, carbon papers with different
hydrophobicity (CP-5%, CP-13%, CP-25% and CP-MPL) were selected as the supports
of Cu particles (Cu-Ps) to optimize the CO diffusion on the surface of Cu-Ps. The
amounts of PTFE of CP-5%, CP-13% and CP-25% are 5 wt%,13 wt% and 25 wt%,
respectively. CP-MPL is composed of hydrophobic carbon fiber and 25 % PTFE
treatment micro-porous layer (MPL) while other three types of carbon paper are without
MPL.
The electrocatalytic CORTE process is greatly affected by the hydrophobicity and
porous structure of catalyst supports. As shown in Figure 1a and Figure S2, with the
hydrophobicity increasing from CP-5% to CP-MPL reflected by their contact angle, the
corresponding FE of C2H4 will increase from 2.46% to 52.7%. It suggests that the high
hydrophobicity of electrode is one of the key factors to enhance CO to C2H4. Moreover,
the geometric current density of C2H4 (jCꢀHꢁ) over Cu-Ps/CP-MPL is 2-fold as that of
Cu-Ps/CP-25% when the amounts of PTFE in both samples is 25 wt%. It indicates that
micro-porous layer (MPL) structure is another key factor to enhance the reaction rate
of CORTE. Accordingly, scanning electron microscopy (SEM) images (Figure 1b)
show Cu-Ps/CP-MPL electrode has a special granular micro-porous structure (around
100 nm). This micro-porous structure of Cu-Ps/CP-MPL can efficiently increase the
CO diffusion at the interface of gas/liquid/solid as depicted in Figure 1c which is similar
with the report where CO2 electroreduction is enhanced by using carbon paper with
MPL.[6] Compared with CP-MPL, carbon paper without MPL (Figure S3) does not
favor the construction of three-phase interface. Besides, the C2H4 production
performance of Cu-Ps on carbon papers with higher PTFE content was also tested
(Figure S4). However, the activity slightly decreases with PTFE increasing to 35 wt%.
It is probably caused by a reduction in the conductivity of electrode at higher PTFE
content, which is in agreement with the conductivity test reported by Kenis et al.[6b]
Thus, optimizing the hydrophobicity and micro-porous layer structure of carbon paper
provides an efficient strategy to construct hydrophobic microenvironment in
gas/liquid/solid interfaces, which enhances the diffusion of CO molecules and the
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corresponding electrocatalytic CORTE process.
a
80
70
60
50
40
30
20
10
0
Cu-Ps
CuO-syn
CuO
C2H4
AcO-
n-PrOH
EtOH
-0.4 -0.5 -0.6 -0.7 -0.8 -0.9 -0.4 -0.5 -0.6 -0.7 -0.8 -0.9 -0.4 -0.5 -0.6 -0.7 -0.8 -0.9
Potentianl (V versus RHE)
b
c
18
Cu(111)
Cu-Ps
CuO-syn
CuO
Cu-Ps
16
14
12
10
8
Cu(200)
Cu(220)
CuO-syn
CuO
CuO/CP-MPL-after
6
4
CuO-syn/CP-MPL-after
CP-MPL
2
0
-0.4 -0.5 -0.6 -0.7 -0.8 -0.9
Potential (V versus RHE)
30
40
50
60
70
80
2Theta (degree)
Figure 2. Copper oxidation-state effect on CORTE performance. a) Faradaic
efficiencies of products over different Cu-based catalysts at applied potentials versus
o
the reversible hydrogen electrode (RHE) in 1 M KOH at 25 C. b) XRD patterns of
freshly prepared catalysts and that after 2 hours CO electroreduction process at -0.7 V
o
versus RHE (conditions: 1 M KOH, 25 C). c) Geometric current densities for C2H4
production over different copper-based catalysts.
Since initial oxidation state of copper-based catalysts plays key role in CO/CO2
electroreduction,[7] in particular for yielding multi-carbon oxygenates, it is unclear
whether the oxidation state of initial catalysts benefits CORTE process. Then three
typical copper-based samples were selected as catalysts to examine the initial oxidation-
state effect of copper on the CORTE reactions, including Cu-Ps catalyst, copper oxide
(CuO-syn) synthesized by an oxidation procedure of Cu-Ps, and commercial copper
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oxide (CuO). As shown in Figure 2b, the samples of both CuO-syn and CuO share
similar X-ray diffraction (XRD) patterns containing copper oxide phase, whereas the
freshly prepared Cu-Ps has a face-centered cubic copper phase. During the CO
electroreduction process, there are hydrogen and five carbon-based products detected
on the cathode including ethylene, ethanol (EtOH), acetate (AcO-), n-propanol (n-PrOH)
and trace amount of methane (Figure S5 and Table S1).
Comparisons in Figure 2a clearly show that the Cu-Ps delivers the highest C2H4
formation FE and total CO electroreduction FE among all three samples.
Correspondingly, the geometric current density of C2H4 production over Cu-Ps is larger
than that over CuO-syn (except at -0.9 V) and commercial CuO (Figure 2c). In addition,
geometric current density (Figure 2c) and the intrinsic activity normalized by
electrochemical surface area (ECSA, see Figure S6 for more details) of three catalysts
are similar at -0.4 V and -0.5 V. However, the C2H4 FE of CuO-syn are much like that
of Cu-Ps at lower potentials (< -0.6 V). It implies that the electrocatalytic CORTE
performance is recovered to some extent when CuO-syn is mostly in situ reduced into
metallic copper at lower potentials as evidenced by XRD in Figure 2b. This suggests
that the oxidized copper could make against the CORTE reaction and the metallic
copper should be the real active species for CORTE process. The optimal FE of C2H4
and total FE of CO electroreduction over Cu-Ps can reach 52.7 % and 72.5% at -0.7 V
in 1 M KOH, respectively. Ethylene is the only hydrocarbon product from CO
electroreduction at wide potentials. Besides, a small amount of liquid products
including EtOH, AcO-, and n-PrOH are concomitantly generated, which are widely
considered as value-added products and will not influence the separation of ethylene
gas in practice. To the best of our knowledge, the FE of C2H4 is unprecedented in
comparison to all previous reports of CO electroreduction listed in Table S2, which
could benefit from both the excellent catalytic performance of Cu-Ps and the
hydrophobic micro-porous structure of the electrode.
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0.1M KOH
80
1M KOH
3M KOH
70
60
50
40
30
20
10
0
-0.8
-0.7
-0.7
C2H4
30
25
20
15
10
5
AcO-
n-PrOH
EtOH
0
-0.5 -0.6 -0.7 -0.8 -0.9 -1.0
-0.4 -0.5
-0.6 -0.7 -0.8 -0.9
-0.6 -0.7 -0.8 -0.9
-0.4 -0.5
Potential (V versus RHE)
Figure 3. OH- concentration effect on the performance of CORTE. Faradaic efficiencies
(top bars) and geometric current densities (bottom bars) over Cu-Ps for CO
electroreduction to C2H4, EtOH, AcO- and n-PrOH at applied potentials versus RHE.
o
Reaction temperature: 25 C. KOH concentration: 0.1, 1, 3 M. The potential value
highlighted in red on the bottom of each panel corresponding to the optimal value of
each condition, respectively.
Apart from the effect of electrode structure and initial oxidation-state of copper-
based catalysts, we find that the OH- concentration also has an important influence on
the performance of electrocatalytic CORTE over Cu-Ps. As shown in Figure 3, the
highest geometric current density of CORTE increases from 7.2 mA cm-2 to 22.4 mA
cm-2 with increasing the concentration of KOH electrolyte from 0.1 M to 3 M, and the
total geometric current density correspondingly has a two-fold increase at all potentials.
One possible reason of reaction rate enhancement is that increasing OH- concentration
can accelerate the ion migration in aqueous electrolyte. The increasing FE of CO
electroreduction with the increasing concentration of OH- is possibly caused by
inhibiting the competitive hydrogen evolution in higher pH (Figure S7), leading to more
reaction sites for CO reduction and a higher FE of C-C coupling reaction. The
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10.1002/anie.201910662
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selectivity of C2H4 reaches the highest value in 1 M KOH at -0.7 V while the high the
concentration of OH- in favor of higher selectivity of AcO- at all tested potentials.
Besides, the selectivity based on CO conversion to ethylene (Figure S8) is around 70%
(in 1 M KOH), breaking through the 30% selectivity limitation from CO to C2
hydrocarbons in traditional FTS process. Through optimizing the OH- concentration
together with the catalyst and the electrode structure, we can achieve a C2H4 FE of up
to 52.7% with a geometric current density of 14.9 mA cm-2 in 1 M KOH at -0.7 V.
Besides, the Cu-Ps also has a good stability within 24 hours during CORTE process
(Figure S9).
For better understanding the nature of CO electroreduction reaction, the in-situ X-
ray absorption spectroscopy (XAS) experiment and density functional theory (DFT)
simulations were carried out. As shown in Figure 4a-b, the X-ray absorption near-edge
structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra both
prove that the catalyst keeps the metallic copper during the CO electroreduction
reaction. Note that the peak of Cu-Cu bond of Cu-Ps becomes higher after applying
negative potentials comparing with the initial catalyst (Figure 4b), which may be caused
by the reduction of the surface oxide layer on the Cu particles during the reaction. It
further demonstrates that the metallic Cu should be the active phase for the CORTE
process, which provides the basis for constructing the DFT model. Then we investigated
the reaction mechanism of electrochemical reduction of CO on three Cu surfaces (111),
(100) and (110) by using DFT calculations.
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10.1002/anie.201910662
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Figure 4. Insights into the reaction mechanism towards CO electroreduction on the Cu
surfaces. a, b) The in-situ normalized (a) Cu K-edge XANES spectra and (b) Cu K-
edge k2-weighted EXAFS spectra of Cu-Ps for no voltage and at -0.7 V in CO or Ar
compared with the reference Cu foil and Cu2O. c) Schematic representations of CO
electroreduction reaction pathways. d) Free energy diagram for CO electroreduction on
Cu(100) at U = 0 V, and at the potentials that all reaction steps are exothermic except
for the first step. e) Comparison of adsorption energy of key intermediates that affect
activity (upper panel) and free energies of competitive pathways for C2H4 and EtOH
formation (lower panel) on Cu(100), Cu(111) and Cu(110).
The key intermediates and favorable reaction pathways in the present work are
shown in Figure 4c (see Figures S10-S11 for more intermediates and pathways
investigation). Figure 4d and Figures S12-S13 show the free energy diagrams on
Cu(100), Cu(111) and Cu(110). It can be seen that formation of the C2O2* is the most
endergonic electrochemical step and determines the overpotentials of the cathode
reaction, which follows the order 0.89 V on Cu(111) > 0.70 V on Cu(110) > 0.21 V on
Cu(100). As shown in Figure 4e, the high activity of Cu(100) may originate from its
stronger binding with C2O2*relative to 2CO* (by 0.92 eV). Since Cu(111) and Cu(110)
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10.1002/anie.201910662
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are unfavorable to yield C2O2* intermediate, we only investigate the C2O2* coupling
with CO* for the formation of n-PrOH (Figure 4c and Figures S11f and S11g) on
Cu(100). The step is endothermic by 0.57 eV, indicating that the C3 product is not
preferred to be formed on the Cu single crystalline surfaces.
The proton-electron transfer of CHCOH* splits the pathway to C2H4 from the
pathway to EtOH. The competitive CCH* and CHCHOH* formation largely
determines the selectivity of C2H4 and EtOH on the three surfaces (Figure 4d and
Figures S12-S13). As seen from Figure 4e, CCH* formation is thermodynamically
more favorable than CHCHOH* formation by 0.32 eV on Cu(100), which therefore
results in the higher selectivity towards C2H4 than EtOH. On Cu(111), the two
competitive steps have equivalent ΔG, suggesting similar selectivity towards C2H4 and
CH3CH2OH. However, CHCHOH* instead of CCH* is preferred to be formed (by 0.17
eV) on Cu(110), which indicates a higher selectivity towards EtOH. The structure
dependent selectivity may be due to the distinct water arrangement on the Cu surfaces.[8]
Cu(110) prefers the five-membered ring of water molecules, which is more sparse than
the six-membered ring arrangement on Cu(100) and Cu(111), thereby stabilizing the
adsorption of large CHCHOH* intermediates by 0.67 and 0.35 eV compared to other
two Cu surfaces.
In addition, we also investigated the CH3COOH selectivity, which is few
addressed in the literatures. Following previous work,[9] we studied the pathway via
CH2COHO* and (HO)(CH2)OC* (see Figure S14 for more details) intermediates.
However, no stable structures are identified on Cu(100) in water environment. Based
on the consideration of both activity and selectivity as shown above, Cu(100) is the
most preferred surface for C2H4 formation. This agrees with the results of online
electrochemical mass spectroscopy experiments reported previously where Cu single
crystals were used as the model catalysts to explore the active sites.[10] Thus, it could
be speculated that exposed {100} planes in Cu-Ps dominate the performance of CORTE
process.
In summary, we report a direct and efficient electrocatalytic process by using the
10
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Cu particle catalysts to enhance the CO conversion to ethylene at room temperature and
atmospheric pressure. Through optimizing the structure of gas diffusion electrode, the
intrinsic activity of Cu-based catalyst and the OH- concentration, we achieve an
ultrahigh CORTE performance with the C2H4 Faradaic efficiency of up to 52.7%.
Besides, theoretical calculations suggest that Cu(100) is the most preferred plane for
C2H4 formation via considering all possible products. This electrocatalytic CORTE
process provides a highly selective, energy-efficient and eco-friendly way to convert
the abundantly industrial CO to C2H4 in comparison with the Fischer-Tropsch synthesis
process.
Acknowledgements
We gratefully acknowledge the financial support from the Ministry of Science and
Technology of China (No. 2016YFA0204100, 2017YFA0204800 and
2016YFA0200200), the National Natural Science Foundation of China (No. 21890753,
21988101 and 21872136), the Key Research Program of Frontier Sciences of the
Chinese Academy of Sciences (No. QYZDB-SSW-JSC020), and the DNL Cooperation
Fund, CAS (No. DNL180201). We thank the staff at the BL14W1 beamline of the
Shanghai Synchrotron Radiation Facilities for assistance with the EXAFS and XANES
measurements. We thank Pengyang Zhang for the assistance in contact angle tests.
Conflict interest
The authors declare no conflict interest.
Keyword: CO reduction; Ethylene production; Cu catalyst; Electrocatalysis;
Heterogeneous catalysis
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10.1002/anie.201910662
Angewandte Chemie International Edition
Table of Contents
This work reports a direct electrocatalytic process for highly selective ethylene
production from CO reduction with water over Cu catalysts at room temperature and
ambient pressure. An unprecedented 52.7% Faradaic efficiency of ethylene formation
from CO electroreduction has been achieved through rational optimization of cathode
structure to facilitate CO diffusion and Cu catalysts to enhance the C-C bond coupling.
14
This article is protected by copyright. All rights reserved.