Enhanced Ethylene Formation from Carbon Dioxide Reduction through Sequential Catalysis on Au Decorated Cubic Cu2O Electrocatalyst

Article abstract of DOI:10.1002/ejic.202100229

Electrochemical reduction of carbon dioxide to fuels has been recognized as a perspective way to address the environmental and energy issues. Herein, the novel Au decorated Cu₂O electrocatalysts were synthesized via the galvanic replacement reaction and the successive pre-reduction by linear voltammetry (LSV) method. In contrast to Cu₂O, CO formation became dominant over HCOOH and H₂ on Au_xCu₂O, and a remarkably enhanced selectivity of C₂H₄ was obtained beyond ?1.1 V vs. RHE. Among them, Au_0.02Cu₂O exhibited the highest Faraday efficiency (FE) of C₂H₄ of 24.4 % at ?1.3 V vs. RHE, which was as high as 2～2.5 and 5 times of those on other two Au_xCu₂O and bare Cu₂O, respectively. Meanwhile, it is demonstrated that the optimal ratio of Au/Cu was essential for effective sequential catalysis between Au and Cu₂O to enhance C?C coupling. Furthermore, the effect of copper states resulting from different pre-reduction methods on the selectivity of C₂H₄ was explored. A half decrease of C₂H₄ FE was observed on HPR-Au_0.02Cu₂O (high potential reduced) relative to the LSV reduced Au_0.02Cu₂O, which was ascribed to the different content of Cu⁰ and residual Cu⁺ in two catalysts. Our results demonstrate an effective approach to construct Cu₂O based bimetallic catalysts for ethylene formation from CO₂.

Full text of DOI:10.1002/ejic.202100229

Full Papers
doi.org/10.1002/ejic.202100229
Enhanced Ethylene Formation from Carbon Dioxide
Reduction through Sequential Catalysis on Au Decorated
Cubic Cu O Electrocatalyst
2
[a]
[a]
[a]
[a]
[a]
[b]
Xuerui Cao, Guangwei Cao, Mei Li, Xinli Zhu, Jinyu Han, Qingfeng Ge, and
[a]
Hua Wang*
Electrochemical reduction of carbon dioxide to fuels has been
recognized as a perspective way to address the environmental
bare Cu O, respectively. Meanwhile, it is demonstrated that the
optimal ratio of Au/Cu was essential for effective sequential
2
and energy issues. Herein, the novel Au decorated Cu O
electrocatalysts were synthesized via the galvanic replacement
reaction and the successive pre-reduction by linear voltamme-
catalysis between Au and Cu O to enhance CÀ C coupling.
2
2
Furthermore, the effect of copper states resulting from different
pre-reduction methods on the selectivity of C H was explored.
2
4
try (LSV) method. In contrast to Cu O, CO formation became
A half decrease of C H FE was observed on HPR-Au Cu O
2 4 0.02 2
2
dominant over HCOOH and H on Au Cu O, and a remarkably
(high potential reduced) relative to the LSV reduced Au Cu O,
0.02 2
2
x
2
0
enhanced selectivity of C H was obtained beyond À 1.1 V vs.
which was ascribed to the different content of Cu and residual
2
4
+
RHE. Among them, Au Cu O exhibited the highest Faraday
Cu in two catalysts. Our results demonstrate an effective
0
.02
2
efficiency (FE) of C H of 24.4% at À 1.3 V vs. RHE, which was as
approach to construct Cu O based bimetallic catalysts for
ethylene formation from CO2.
2
4
2
high as 2~2.5 and 5 times of those on other two Au Cu O and
x
2
[
11,20–22]
Introduction
controlling the copper states,
introducing a second
[23–24]
metal,
forming grain boundaries, low coordination
and regulating the exposed facet.
[25–27]
[28–30]
The electrochemical CO reduction reaction (CO RR) presents an
sites,
The former
2
2
attractive approach for the sustainable production of chemicals
and fuels, and holds great potential for the effectively
two have recently gained much interests. Regarding to the role
of copper states, several groups have proved that the preserve
of Cu O is essential to achieve high C selectivity in the oxide
[1–6]
regulation of carbon loop.
Great progress has been achieved
2
2
[20,22,31–32]
for a variety of effective electrocatalysts, which can reduce CO2
derived Cu system,
and copper oxidation states make
to C products (such as CO, HCOOH) with high FEs at high
contribution to stabilize the C intermediate to increase C H
1 2 4
1
[7–8]
[33–37]
current density.
proton-electron transfer steps and the difficulty in CÀ C
coupling, there is still a great challenge to selectively
However, due to the involvement of multiple
production.
Meanwhile, recent reports demonstrated that
+
0
the synergistic effect between Cu and Cu accelerated CO2
activation and *CO dimerization, thereby the thermodynamics
and kinetics of C2 production were promoted.
[9–10]
[22,38–40]
generate high energy-dense C products, such as ethylene
The
2
+
[11]
[12]
[13]
(
C H ),
ethanol (C H OH),
acetic acid (CH COOH),
n-
improvement had been reported on an oxygen-plasma-treated
Cu electrode achieving one of the highest ethylene Faradaic
2
4
2
5
3
[9]
[14]
propanol (C H OH) or 1-butanol (C H OH).
3
7
4
9
[41]
Copper (Cu) has been proved to be the only active metal for
electro-reduction of CO to C products owing to its optimized
efficiency (~60%). In addition, an anodized copper (ANÀ Cu)
catalyst exhibited ~40% Faradaic efficiency for ethylene and
high stability over 40 h, which were ascribed to the mixed
2
2+
À
binding energy with *CO, as the key intermediate to form >2e
[
15–17]
[11]
reduction products.
Nevertheless, the active sites of bulk
oxidized copper species along with high oxygen content.
Cu catalysts showed high selectivity to H and HCOOH, resulting
Noteworthy, one usual method to fabricate oxygen-Cu combi-
nation catalysts is to prepare a Cu-oxide precursor firstly, and
2
[
18–19]
in an inferior FEs of C2+ products.
Up to now, various
[42]
strategies have been developed to improve the selectivity of
multiple carbon products on Cu-based catalysts, including
then the subsequent in situ or ex situ redox cycles. Using the
resulted catalysts, the activity and product selectivity were
indeed improved, especially for the ratio of C H and CH4,
2
4
although the catalytic mechanism has yet to be fully
understood.
[
a] X. Cao, G. Cao, M. Li, Prof. X. Zhu, Prof. J. Han, Prof. H. Wang
Key Laboratory for Green Chemical Technology of Ministry of Education
Collaborative Innovation Center of Chemical Science and Engineering
School of Chemical Engineering and Technology, Tianjin University
Tianjin 300072, China
[11,18,20,43]
Furthermore, combining Cu with a second metal (M) to
form alloys or separated CuÀ M composites is also an efficient
E-mail: tjuwanghua@tju.edu.cn
approach to enhance the activity and selectivity for CO RR. In
2
[b] Prof. Q. Ge
particularly, introducing the metal (Au, Zn, Ag, etc.) with high
selectivity for CO but weak binding ability for CO has been
applied to improving the C2+ product selectivity on Cu-based
Department of Chemistry and Biochemistry, Southern llinois University
Carbondale, IL 62901, United States
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/ejic.202100229
catalysts. As is reported, the dimerization of *CO or other C to
1
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form CÀ C bond is a pivotal step in the generation of C
2
+
[15,24]
products.
zation was easier to occur in the high coverage of *CO.
Moreover, Huang confirmed that C H would be generated only
Theoretical studies have revealed that CO dimeri-
[44–45]
2
4
when there was sufficient *CO coverage on the Cu surface
[46]
through experiments. When the second metal (Au, Zn, Ag) is
introduced as an active promoter that forms more CO and
could help generate a high flux of CO, so that neighboring Cu
may have more opportunity to participate in CÀ C coupling.
Therefore, a sequential-sites mechanism was adopted to
illustrate the increased FEs of C products on CuAu, CuAg,
2
[47–49]
Cu Zn, Ag/Cu O catalysts.
Beyond that, the stability of Cu O
2
x
2
could be improved by introducing metal nanoparticles on Cu O
2
surface, such as our previously reported Cu OÀ Au catalyst and
2
[12,23]
nanoporous CuÀ Ag alloy catalyst.
2 x 2
Scheme 1. Schematic illustration of the synthesis of Cu O and Au Cu O
catalysts.
In present work, we prepared the novel Au Cu O (x
x
2
represents the molar mass of the introduction of Au) catalysts
by the galvanic replacement reaction between cubic Cu O and
2
+
0
HAuCl , and then the co-existence active sites of Cu and Cu
planes (Figure 1c). For Au0.02Cu O sample (Figure 1d), the
4
2
on Au Cu O catalysts were generated by performing a repeated
original cubes were well maintained but with a slightly larger
size of 170 ~200 nm and the rougher surface. As shown in
Figure1f, the presence of Cu O and Au of Au Cu O were
x
2
linear voltammetry (LSV) reduction before CO RR. For Au Cu O
2
x
2
catalysts, Au nanoparticles were used as the active CO
formation sites, and then CO molecules spillover from Au to
adjacent Cu sites, where CÀ C coupling reaction was accelerated
to form C H . The correlations between the FEs of C H and the
2
0.02
2
confirmed by the crystal plane spacing distances of 0.246 nm,
0.213 nm and 0.234 nm corresponding to Cu O (111), Au (200)
2
and Au (111), respectively. In addition, it was indicated that the
average size of Au nanoparticles was 11�2 nm, which exactly
accounted for the increase of cube size after loading Au
nanoparticles. Besides, from the elemental mapping images in
Figure 1g-i, it was found that the density of Au elements
seemed higher at the tips and edges than that on the facets,
which might be ascribed to the higher surface energy at the
tips and edges of cubes, facilitating the nucleation and growth
2
4
2
4
Au/Cu atomic ratio were investigated to illustrate the impor-
tance of the right balance between Au atoms (forming CO) and
Cu atoms (adsorbing CO and further coupling) for effective
sequential catalysis. Furthermore, we also discussed the effect
of pre-reduction on the selectivity of C H by comparing the
2
4
CO RR performance of Au Cu O samples undergoing different
2
0.02
2
reduction methods, namely repeated LSV cycles and high
potential reduced by chronoamperometry (À 1.5 V). Our results
revealed the importance of the enhanced CO generation at Au
sites for further hydrogenation of the CÀ C products on Cu sites,
and the balance of two sequential sites as well as the content
[51–52]
of Au nanoparticles.
Moreover, the distribution of Au
nanoparticles and composition of Au0.01Cu O and Au Cu O
2
0.04
2
samples were also characterized. The images in Figure S1-S2
demonstrated that the average size of Au nanoparticles
gradually increased from Au0.01Cu O to Au Cu O samples.
+
0
of Cu and Cu in improving C selectivity in CO RR.
2
2
2
0.04
2
Meantime, the compositions determined by SEM-EDS and ICP-
OES (Figure S3-S5 and Table S1-S2), indicated that both the
amount of Au and the average atomic ratio of Au/Cu exhibited
a proportional increase with the increasing addition of HAuCl4
solution. It was also suggested that the Cu atoms exposed on
the surface might decrease due to the covering of the
increasing Au nanoparticles, which would lead to their different
Results and Discussion
Characterization of Cu O and Au Cu O composites
2
x
2
As shown in Scheme 1, CuSO , SA and SDS were respectively
4
used as the metal precursor, the reducing agent and the
catalytic performance in CO reduction.
2
[50]
surfactant to synthesize Cu O cubes. The chemical reactions
The crystal phase and composition of the as-prepared Cu O,
2
2
2
4
À
were as following: (1) Cu(OH) and Cu(OH) formed during the
Au Cu O samples and the corresponding LSV reduced ones
2
x
2
reaction of CuSO with NaOH; (2) SA reacted with Cu(OH) or Cu
were analyzed by XRD. As shown in the Figure S6, the as-
4
2
2
À
(
OH)4 to form Cu O cubes in the presence of SDS. Then the
prepared Cu O exhibited the diffraction peaks at 36.42°, 42.30°,
2
2
Au Cu O composites were fabricated through a rapid galvanic
61.34° and 73.53° assigned to the planes of (111), (200), (220)
x
2
replacement reaction between the Cu O cubes and the different
and (311) of Cu O (PDF#5-0667). For the as-prepared Au Cu O,
2
2
x
2
amount of HAuCl4.
new diffraction peaks appeared at 38.18°, 44.39°, 64.57° and
77.5° assigned to the (111), (200), (220) and (311) crystal planes
of face-center cubic (FCC) Au crystals (PDF#4-0784). Besides, the
diffraction peak intensities of Au gradually increased with the
increasing content of Au, indicating that Au nanoparticles were
The as-prepared Cu O and Au Cu O samples were charac-
2
x
2
terized by SEM and TEM. As shown in Figure 1a-b, the sample
displayed a well-defined cubic structure with a width of 150~
1
80 nm. High-resolution TEM demonstrated the high crystal-
linity of the as-prepare Cu O with resolved lattice fringe of (111)
successfully loaded on Cu O cubes. By contrast, as shown in
2
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Figure 1. (a) SEM, (b) TEM, and (c) HRTEM images of as-prepare Cu
Elemental mapping of Cu, Au and O of Au0.02Cu O catalyst.
2 2
O cubes, (d) SEM, (e) TEM, and (f) HRTEM images of as-prepare Au0.02Cu O catalyst, (g–i)
2
Figure 2b, the LSV reduced Cu O and Au Cu O samples
displayed the existence of Cu by the typical diffraction peaks at
replacement reaction. By contrast, for Au0.02Cu O sample after
2
x
2
2
0
LSV reduction (Red line in Figure 2c), the satellite peaks of Cu
(II) nearly disappeared, and the other two peaks distinctively
shifted to the low binding energies of 932.6 eV and 952.6 eV,
4
3.34° and 50.43° indexed to the (111) and (200) planes of
metallic Cu (PDF#4-0836), which indicated the occurrence of
reduction reaction from Cu to Cu during the repeated LSV
+
0
0
which could be assigned to the mixed species of Cu /Cu(I)
cycles in the potential range of À 0.1~À 1.2 V. Interestingly, it
according to the XRD result in Figure 2a–b. Therefore, it was
0
was also found that the intensity of the diffraction peak at
revealed that Cu O was partially reduced to the metallic Cu
2
0
4
3.34° of Cu decreased with the increasing content of Au,
during the LSV reduction process, again approving the co-
0
relative to the intensity of Cu O peak at 42.30°, indicating that
existence of Cu and Cu(I) in Au Cu O. The detailed analysis of
2
0.02
2
0
the presence of Au nanoparticles prevented the Cu O from
Cu /Cu(I) species by Cu LMM Auger spectra was presented and
discussed in latter section. In addition, Figure 2d displayed the
2
completely reduced, thus resulting in a surface with a mixed
copper states of Cu and Cu ions. As reported in previous
0
+
Au 4f spectra of Au0.02Cu O before and after LSV reduction.
2
+
0
studies, the co-existence of Cu and Cu was beneficial to CÀ C
coupling reactions through electrostatic interaction between
two adjacent CO*, leading to the enhancement of C
Both exhibited two peaks at 84.0 eV and 87.7 eV corresponding
to Au 4f7/2 and Au 4f5/2 of pure metallic Au, indicating the
[54]
chemical state of Au maintained the same.
2
+
[
22,38–39]
products.
Furthermore, the surface composition and chemical states
of Au0.02Cu O (before and after LSV reduction) were character-
Surface Cu chemical states examined by CV method
2
ized by XPS. The XPS survey spectrum in Figure S7 confirmed
the presence of the elements Cu, Au and O, indicating that Au
The chemical states of Cu were further probed through the
surface Cu redox property of the samples before (Black lines)
and after LSV reduction (Red lines) using CV method. The
obtained CV curves at the potential range from À 0.75 to 1.25 V
were shown in Figure S8. The samples all exhibited two anodic
peaks at 0.46 V and 0.72 V corresponding to the oxidation
reactions of Cu to Cu O (A1) and Cu O to CuO (A2), and three
nanoparticles were successfully loaded on the surface of Cu O.
2
The high-resolution spectra of the Cu 2p in the as-prepared
Au0.02Cu O (Black line in Figure 2c) displayed two peaks at
2
9
33.2 eV and 953.5 eV, which could be ascribed to Cu (I) species
based on the XRD result in Figure S6. In addition, two satellite
peaks at the 942.9 eV and 962.1 eV were obviously observed,
2
2
[53]
implying the existence of Cu (II) species, which might result
cathodic peaks attributed to the reduction reactions of CuO to
2
+
from the residual Cu ions on the surface during the galvanic
Cu O (C1, 0.39 V), further reduction to Cu (C2, 0.07 V) and
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Figure 2. (a) XRD patterns of different samples after LSV reduction. (The presence of C different peak is due to carbon paper as the electrode substrate), (b)
the corresponding locally magnified XRD patterns, (c) Cu 2p XPS spectra, (d) Au 4f of Au0.02Cu O before (black) and after (red) LSV reduction.
2
2
À
[12,55–56]
HCuO (C3, À 0.33 V).
To distinguish the reduction level
Electrocatalytic CO RR performance
2
of different samples due to LSV reduction, the two oxidation
curves of each sample were compared as displayed in Figure 3.
In contrast with the as-prepared state (Black lines), the LSV
reduced samples all presented the larger A1 peak area (Red
lines), indicating the increased content of Cu due to the
reduction of Cu O. In particular, A1 peak of Cu O increased
The electrochemical activity of CO RR of the Cu O, Au Cu O
2
2
x
2
were evaluated through LSV method between À 0.1 V and
À 1
À 1.2 V at a rate of 50 mVs in N - and CO -saturated 0.1 M
2
2
0
KHCO solutions in sequence. Under the condition of CO
3
2
À 2
saturation, the required for the current density of 1 mAcm
2
2
[60]
significantly after LSV pre-reduction, revealing the worse
was defined as the CO RR onset potential.
As shown in
2
stability of Cu O in absence of Au nanoparticles, in agreement
Figure 4, the CO RR onset potential of Cu O was À 0.62 V, which
2
2
2
with the above XRD results. Moreover, based on the less
changes of A1 peak of Au Cu O samples with the increasing Au
was more negative than those of the Au Cu O, Au Cu O and
0.01 2 0.02 2
Au0.04Cu O (À 0.56 V, À 0.48 V and À 0.45 V), indicating that
x
2
2
loading (Figure 3b-d), it was demonstrated that the present of
introducing Au nanoparticles improved the activity toward
Au improved the resistance to reduction of Cu O to a different
CO RR. In addition, the current discrepancy between two curves
2
2
extent. Importantly, it can be found that the area of A2 peak
was larger than those of A1 peak for both Cu O and Au Cu O
obtained under N2 and CO2 was used to represent partial
current densities of CO RR. Compared to Cu O, the Au Cu O
2
x
2
2
2
x
2
electrodes after LSV reduction, indicating the dominant compo-
sition of Cu(I) in the mixed Cu states of these LSV reduced
catalysts all exhibited the enhanced partial current densities.
Among these, Au0.02Cu O exhibited the best activity toward
2
samples. The above results demonstrated that Cu O and
CO RR. Moreover, the results of in Figure S9–S10 indicated that
2
2
Au Cu O samples possessed the different surface copper states
Au0.02Cu O had the largest electrochemical active surface area,
which contributed to its better performance.
x
0
2
2
[61]
Cu /Cu(I) and the different amount of accessible copper atoms
due to the presence of Au nanoparticles, which would lead to
the different performance regarding the absorption of inter-
Furthermore, the product selectivity of Au Cu O were
x
2
examined between À 0.9 V and À 1.3 V in CO -saturated 0.1 M
2
mediates and the coupling of C intermediates, and then the
KHCO and compare with the Cu O cubes and spherical Au
1
3
2
[
22,33,48,57–59]
various product distribution.
nanoparticles (an average size of 9 nm as shown in TEM images
in Figure S11). All tests were performed for 4 h chronoamper-
ometry at each potential. As shown in Figure S12–S13, the gas
phase products detected were CO, C H , CH and H through
2
4
4
2
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2 2 2 2 2
Figure 3. The oxidation peaks of CV curves of (a) Cu O, (b) Au0.01Cu O, (c) Au0.02Cu O and (d) Au0.04Cu O before (black) and after (red) LSV reduction in N -
À 1
3
saturated 0.1 M KHCO solution, scan rate=10 mVs .
1
GC, while only HCOOH in electrolyte was detected by H NMR.
CO was adsorbed by its oxygen atom or its carbon atom. CO
2
could dominantly produce if the carbon was bound.
[
64–65]
The potential-dependent FEs of the obtained products on Cu O,
2
Au Cu O catalysts and control sample Au nanoparticles were
Therefore, it was supposed that the addition of Au atoms on
Cu O surface made CO molecules adopt *CO2 adsorption
x
2
summarized in Figure 5. For Cu O and Au catalysts, the products
2
2
2
distributions consistent with the reported results were obtained
structure, instead of COO*, leading to favorable *COOH
formation and then CO formation. As a result of the enhanced
CO production on Au Cu O catalysts, the maximum FEs of C H
4
as shown in Figure 5a–b. In other words, Cu O electrode
2
exhibited the high selectivity of HCOOH, moderate CO
formation, small quantify of C H and minus CH . Whereas Au
x
2
2
increased from 5.9% on Cu O to 24.4% on Au Cu O, 12.7%
2
4
4
2
0.02
2
electrode displayed an exclusive product CO from CO reduc-
tion and a gradually increasing trend of FE CO with the
on Au0.04Cu O at À 1.3 V. These results demonstrated that
2
2
increased concentration of *CO intermediate effectively
boosted the formation of C H . By contrast, the formation of
potentials from ~55% to ~85%. Besides, HER on Cu O was
2
2
4
more competitive than that on Au, approving Au had
another hydrocarbon product CH always kept very low level
4
significantly weak absorption ability for H O comparing with
either on Cu O or on Au Cu O catalysts, which was in agree-
2
2
x
2
Cu O. Moreover, considering that CO was the key intermediate
ment with the result of previously reported oxygen-Cu combi-
2
[11,41]
to form C H , it could be expected to elevate C H amount by
nation catalysts.
Their results indicated the selectivity of
2
4
2
4
combing the advantages of Au for selective CO formation and
C H to CH in OÀ Cu combination catalysts was influenced by
2
4
4
Cu O for CÀ C coupling, which was referred to the two-sites
the electrochemical reduction environment related to the
mixed valences. Based on the above characterization results of
surface Cu states in our work, it could be explained by high CO*
coverage and the preference of CÀ C coupling over further
2
[48,62–63]
sequential catalysis mechanism.
As shown in Figure 5c–e,
in contrast with Cu O, Au Cu O catalysts exhibited the inhibited
2
x
2
generation of H in varying degrees. Importantly, CO formation
2
[66]
became prevailed over HCOOH on Au Cu O catalysts. Especially
hydrogenation on the mixed Cu sites. In consequence, the
highest FE of C H reached 24.4% on Au Cu O catalyst at a
x
2
for Au0.02Cu O and Au Cu O, the maximum CO FEs increased
2
0.04
2
2
4
0.02
2
À 2
to over 50% whereas HCOOH FEs decreased to ~15%
throughout the test potentials. As it was reported, the key to
total current density of ~5.7 mA·cm at À 1.3 V (Table S3), 4
times that of single Cu O, while CH generation was remarkably
2
4
whether CO reduced to HCOOH or CO depended on whether
suppressed. This result means C H production accounted for
2 4
2
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À 1
Figure 4. LSV curves of (a) Cu
2
O, (b) Au0.01Cu
2
O, (c) Au0.02Cu
2
O, (d) Au0.04Cu
2
2
O in N /CO
2
-saturated 0.1 M KHCO
3
solution, scan rate=50 mVs
.
Figure 5. The faradaic efficiencies of H
2
, CO, HCOOH and C
2
H
4
on (a) Cu
2
O, (b) Au, (c) Au0.01Cu
2
O, (d) Au0.02Cu
2
2
O, (e) Au0.04Cu O at different potentials ranging
from À 0.9 V to À 1.3 V in CO -saturated 0.1 M KHCO
2
3
solution.
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~
43% of the total carbonaceous gas products. Meanwhile,
ences were found between the two LSV curves, indicating that
Au Cu O electrocatalysts maintained their activity well. Mean-
compared with recent reports listed in Table S4, both the total
x
2
current density and the FE of C H4 in our work were at a
while, slighter changes of the current density between the two
LSV curves of Au Cu O catalysts were observed with the
increasing Au loading. It is consistent with the results of surface
Cu property by CV method shown in Figure 3, which demon-
2
comparable level using H-type electrolytic cell. In addition, from
the perspective of kinetics, more negative potentials were
supposed to favor CÀ C coupling, and then the higher FE of
x
2
[11,17,25,28,62,67]
C H .
However, due to HER prevailing, higher
strated that the presence of Au nanoparticles on Cu O is helpful
2
4
2
cathodic potential was not definitely conducive to the accel-
eration of C H . As shown in the Figure S14, Au Cu O
to keep the better stability of the electrocatalysts than
[12,23]
Cu O.
In addition, the total current densities and the FEs of
2
4
0.02
2
2
presented the declined FE of C H to 19.4% when further
the products were stable for each Au Cu O catalyst under
2
4
x
2
increasing the potential to À 1.4 V.
different applied potentials, which also confirmed that the
Au Cu O possessed good stability (Figure S17–S19). Moreover,
Furthermore, the morphology and composition of
x
2
Au0.02Cu O catalyst after 4 h electrolysis were characterized by
the current density and the FE of each product on Au0.02Cu O to
2
2
SEM and XRD (Figure S15) to evaluate stability. It was indicated
that the used Au0.02Cu O maintained the cubic structure of Cu O
and the similar size and distribution of Au nanoparticles.
remained stable when the electroreduction reaction time was
prolonged to 5 h at À 1.3 V (Figure S18e). All the above results
proved that the Au Cu O catalysts had desirable stability.
2
2
x
2
Meanwhile, the co-existence of Cu O and metallic Cu was also
Based on a mechanism of the sequential catalysis model to
2
preserved in the Au0.02Cu O after the electrolytic process, which
produce C H from CO2 on bimetallic AuÀ Cu based electro-
2
2
4
was similar to the XRD pattern of Au0.02Cu O before electrolysis.
catalysts, the first step is the formation of CO intermediate on
Au. Therefore, the FEs of CO for Au Cu O electrocatalysts were
2
In order to further evaluate the stability of Au Cu O electro-
x
2
x
2
catalysts, we compared the discrepancy between the two LSV
curves before (red) and after (blue) 4 h electrolysis of each
Au Cu O catalyst. As shown in the Figure S16, negligible differ-
compared with the respective bare Cu O and Au nanoparticles
at different potentials as displayed in Figure 6a. Obviously, Au
nanoparticles presented high selectivity to CO, since the
2
x
2
Figure 6. The faraday efficiencies of (a) CO on Cu
À 0.9 V to À 1.3 V in CO -saturated 0.1 M KHCO solution. (c) The enhancement factor for C
enhancement factors equal to 1 are shown for the Cu
Cu of Au Cu O catalysts.
2
O, Au and Au
x
Cu
2
O catalysts, (b) C
2
H
4
on Cu
2
O and Au
x
Cu
2
O catalysts at different potentials ranging from
Cu
2
3
2
H
4
of Cu O and Au
2
x
2
O catalysts at À 1.2 V and À 1.3 V, the
2
O catalyst at each potential. (d) The relationship between the FE of C
2
H
4
and surface atomic ratio of Au/
x
2
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unfavorable thermodynamics for CO adsorption on Au sites
Since the above results clearly approved the importance of
introduced Au atoms in increasing CO formation, it was
[12,24,68]
caused the easy release of CO as the gas product.
In
contrast, bare Cu O gave the least CO FEs, which could be
necessary to discuss the effect of copper states on C H4
2
2
attributed to that the competitive C pathway to HCOOH was
formation. The above XRD and XPS characterizations have
1
+
0
dominant, and that a small amount of formed CO could bind
stronger with copper surface. As expected, by introducing Au to
Cu O, Au Cu O electrocatalysts produced the intermediate
confirmed the co-existence of Cu and Cu states in the
Au Cu O catalyst after LSV reduction. In the previous studies, it
x
2
has been proposed that copper oxidation states could contrib-
ute to keep the intermediate stable and increase C H
2
x
2
amount of CO, which were remarkably higher than those on
2
4
[
10–11,29,58]
bare Cu O. This demonstrated that Au nanoparticles could
formation.
There were other studies reporting the
synergism between Cu and Cu in facilitating CO adsorption
Notably, it was presented that the initial
reduction conditions applied on the oxide Cu catalysts were
critical to achieve selective and stable C H production from
2
+
0
provide enough in-situ CO sources to adsorb on Cu, probably
leading to an increase of *CO coverage on copper surface,
which was crucial to promote the CÀ C coupling reaction rate,
and then the enhanced formation of C H . Herein, it can be
[22,38]
and CÀ C coupling.
2
4
2
4
observed that the FEs of CO on Au Cu O exhibited a volcanic
CO RR, because the initial reduction condition affected chemical
2
x
2
[11,69]
trend with potentials changing more negative and peaked at
À 1.1 V, in contrast with the continuously boosted selectivity of
CO on Au nanoparticles when increasing the cathodic poten-
tials. Corresponding to the decreased CO FEs, the FEs of C H
states of Cu.
Herein, we explored the effect of copper states
resulting from different pre-reduction conditions on the
selectivity of C H . Except the above testing Au Cu O (here
2
4
0.02
2
referred to as LSV-Au0.02Cu O for clear denotation), the other
2
4
2
on Au Cu O increased sharply beyond the potential of À 1.1 V,
two electrodes were selected to perform control experiments.
x
2
as shown in Figure 6b. Notably, the occurrence of an inflection
potential of À 1.1 V might be rationalized by that the CO-
binding strength on copper is affected by the applied potential.
At the potentials less than À 1.1 V, no significant improvement
in selectivity towards C H , since large number of CO molecules
One is the as-prepared Au0.02Cu O catalyst without LSV reduc-
2
tion. The other one denoted as HPR-Au0.02Cu O was prepared
2
under the high potential reduction (HPR) under À 1.5 V for 2 h
in N -saturated 0.1 M KHCO . Their initial copper states were
2
3
compared through XRD patterns and Cu LMM Auger spectra as
shown in Figure 7a–b. It can be seen that both LSV-Au0 Cu O
2
4
formed on Au desorb from the catalysts surface as a gas
product evidenced by the higher CO FEs. Beyond À 1.1 V, more
negative potentials facilitate the adsorption of CO and then the
enough high *CO coverages on copper, which results in the
improved kinetics of CÀ C coupling, thereby the remarkable
.02
2
+
0
and HPR-Au0.02Cu O displayed the co-existence of Cu and Cu
2
but with different composition. Namely, HPR-Au Cu O pos-
0
.02
2
0
sessed much more content of Cu than LSV-Au Cu O. It should
0
.02
0
2
+
be note that the co-existence of Cu and Cu would occur
during CO RR, although the as-prepared Au Cu O only
[11,23–24,67]
increase of C H .
2
4
2
0.02
2
+
Moreover, the change of C H selectivity on Au Cu O
exhibited the presence of Cu before electrolysis. Their catalytic
2
4
x
2
catalysts was further investigated through the enhancement
performances toward CO RR were investigated under the same
2
factors of C H at À 1.2 V and À 1.3 V, where sequential catalysis
condition. As shown in Figure S18e and Figure S20, the three
2
4
was the most effective. As shown in the Figure 6c, the
enhancement factors of C H showed a volcanic variation with
catalysts had similar total current densities in CO RR process.
2
However, as shown in Figure 7c, the FEs of C H followed the
2
4
2
4
increasing Au loading, which demonstrated that there is an
optimal amount of the Au loading. As thus the relationship
between the selectivity of C H and the surface atomic ratio of
decreasing order from LSV-Au0.02Cu O, as- prepared Au Cu O
2 0.02 2
to HPR-Au0.02Cu O. Especially, a half decrease of C H FE and an
2
2
4
increase of 25% of H2 were observed on HPR-Au0.02Cu O in
2
4
2
Au/Cu was analyzed, as shown in the Figure 6d. It can be seen
that Au Cu O catalysts all exhibited the increasing FEs of C H
4
under more negative potentials. Whereas, a different trend with
the increasing surface atomic ratio of Au/Cu was observed with
comparison to LSV-Au0.02Cu O. Whereas the FEs of CO followed
2
the reversed order. Since they have the same Au loading,
supposed that they had the equal ability to form CO, the
differences in C H formation among them might be attributed
x
2
2
2
4
a peak value on Au0.02Cu O under the testing potentials. It is
to the role of copper states. Initially, three catalysts represented
2
+ +
revealed that an optimal Au/Cu ratio was a crucial for effective
three typical copper states as Cu , dominant Cu and much
[23]
0
sequential catalysis to elevate the selectivity of C H . Namely,
Cu estimated by the peak areas. After CO RR, as shown in
2
4
2
on the one side, insufficient density of Au atoms could not
provide enough CO source for Cu atoms, and then the not
enough coverage of *CO intermediates to enhance CÀ C
coupling reaction. On the other side, since Au is not active for
further CO reduction, excessive Au atoms might block copper
active sites. These were evidenced by the less improved
formation of CO, C H on Au Cu O, and significantly higher FE
Figure 8, the Cu O patterns nearly disappeared for HPR-
2
Au0.02Cu O, while both as-prepared Au0.02Cu O and LSV-
2
2
Au0.02Cu O maintained the distinct characteristic peak of Cu O
2
2
+
0
at 36.42°, preserving the co-existence of Cu and Cu . Our
+
results revealed that the role of residual Cu on the product
distributions, particularly on C H selectivity, which might be
explained by the synergy between Cu and Cu .
approved that the C atom of CO@Cu was positively charged
whereas the C atom of CO@Cu was negatively charged, thus
2
4
+
0 [22,39]
They
2
4
0.01
2
+
of CO but lower FE of C H on Au0.04Cu O. As a result, the
2
4
2
0
highest C H production was achieved on Au Cu O. This was
2
4
0.02
2
consistent with the selectivity of C products (C H or C H OH)
the two carbon atoms of *CO intermediates facilitated the *CO
dimerization through the electrostatic interaction. Therefore, it
was suggested that the mixed stated of Cu and Cu in
2
2
4
2
5
on the NGQ/Cu-NR, Cu Zn and Au/Cu catalysts in previous
x
[12,24,48,59,63]
+
0
reports.
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Figure 7. (a) the XRD patterns (the presence of C different peak is due to carbon paper as the electrode substrate) and (b) Cu LMM Auger spectra of as-
prepared Au0.02Cu
CO -saturated 0.1 M KHCO .
2
2
O, LSV- Au0.02Cu
2
O and HPR-Au0.02Cu
2
O, (c) the product distributions of Au0.02Cu
2
O catalysts with different reduction degrees at À 1.3 V in
3
Au Cu O composites synergistically played the role in elevating
x
2
the selectivity of C H .
with the pristine Cu O. Namely, Au Cu O catalysts had signifi-
2 x 2
2
4
To sum up, the enhanced generation of C H on Au Cu O
cantly increased the FEs of CO and C H , while highly inhibited
2 4
2
4
x
2
was ascribed to a combined effect of the optimal availability of
the generation of H and HCOOH. What’s more, the selectivity
2
CO due to Au atoms and the promoted CÀ C coupling due to
of C H on Au Cu O catalyst reached 24.4% with the total
2
4
0.02
2
+
0
À 2
the synergy of Cu and Cu . As illustrated in the Figure 9, Au
provides enough in-situ CO sources and more negative
potentials facilitate the adsorption of CO and then the enough
high *CO coverages on Cu sites, so that the adjacent Cu had
more chance to participate in CÀ C coupling. Further dimeriza-
tion of two *CO molecules to form C H was favored in the
current density ~5.7 mAcm at À 1.3 V. It is worth noting that
the Au Cu O catalysts exhibited different C H selectivity with
x
2
2
4
the Au loading, suggesting an optimal of Au was required for
the effective sequential catalysis to enhance the formation of
C H . In addition, we examined the effect of mixed copper
2
4
states on the selectivity of C H by comparing the CO RR
2
+
4
2
4
2
0
presence of mixed copper states of Cu and Cu . It is approved
that the concept of sequential catalysis via bifunctional inter-
face is an attractive strategy to design the efficient electro-
catalysis for CO reduction to C products.
performance of Au0.02Cu O with different pre-reduction con-
2
ditions. A half decrease of C H FE and an increase of 25% of H
2
2
4
were observed on HPR-Au0.02Cu O in comparison to LSV-
2
Au0.02Cu O. This might be related to their different content of
2
2+
2
0
residual Cu O species and the generated Cu as measured by
XPS and XRD, since the synergism between Cu and Cu had
been considered a crucial parameter affecting C H selectivity.
2
+
0
Conclusion
2
4
Overall, the increased FE of C H was achieved on Au Cu O
2
4
x
2
In summary, we successfully prepared Au Cu O catalysts using
catalysts. This remarkable enhancement could be attributed to
the interplay of different factors affecting C H selectivity,
including the presence of Au providing enough in-site source of
x
2
the galvanic replacement reaction and followed by the
subsequent repeated LSV reduction, which performed im-
proved activity of CO RR and selectivity of C H in comparison
2
4
CO, the optimal balance between Au and Cu sites and the
2
2
4
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Experimental Section
Materials and methods
KHCO3 (99.5%) and CuSO ·5H O were purchased from J&K
4
2
Scientific LTD (China). sodium sulfate (SDS, AR) was purchased from
Kemiou Chemical Reagent (China). L-Sodium ascorbate (SA, 98%),
HAuCl4 (99%) and Sodium citrate (99%) were purchased from
HEOWNS (China). Sodium hydroxide (99%) were purchased from
Aladdin (China). Sodium borohydride (98%) were purchased from
Innochem (China). Deionized water was used in all experiments. All
chemicals were used as received without further purification.
Preparation of Cu O cubes
2
Cu O cubes were prepared by the liquid-phase reduction method
2
[50]
using SDS as the surfactant. In a typical procedure, 2.88 g SDS
was dissolved in 400 mL deionized water under stirring for 10 min.
Then 5 mL of 0.1 M CuSO4 aqueous solution was added and
continued stirring for 5 min. When 8.5 mL of 1 M NaOH solution
was injected into the mixed solution, the color immediately turned
to blue. Next, 85 mL of 0.2 M SA aqueous solution was added and
stirred for 5 min. After, aging for 10 min, the yellow precipitate of
Cu O was separated through centrifugation, and then washed three
2
times with deionized water and ethanol in sequence. Finally, the
sample of Cu O cubes was obtained after drying in the vacuum
2
oven at 60°C for 12 h.
Preparation of Au Cu O
x
2
Au Cu O samples were prepared through the galvanic replacement
x
2
reaction. In a typical synthesis, 20 mg as-prepared Cu O cubes were
2
dispersed into 15 mL deionized water under sonic oscillation for
1
5 min. Then 4 mL of 5 mM HAuCl aqueous solution were added
4
dropwise. The Au0.02Cu O was collected by centrifugation after
2
Figure 8. (a) XRD patterns (the presence of C different peak is due to carbon
reaction for 10 min followed by washing with ethanol and water for
paper as the electrode substrate), (b) the corresponding locally magnified
several times. Finally, the sample Au0.02Cu O was obtained after
2
XRD patterns of as-prepared Au0.02Cu
after CO -saturated 0.1 M KHCO
RR process at À 1.3 V in CO
2
O, LSV-Au0.02Cu
2
O and HPR-Au0.02Cu
2
O
drying in the vacuum oven at 60°C for 12 h. The samples denoted
2
2
3
.
as Au Cu O and Au Cu O were obtained by changing the
0
.01
2
0.04
2
amount of HAuCl aqueous solution (2 mL and 8 mL), respectively.
4
Characterization
Multiple analytical techniques were used to characterize different
materials. Scanning electron microscopy (SEM, Japan, Hitachi
Limited, S 4800) and transmission electron microscopy (TEM, Japan,
JEOLJ, EM-2100 F) were used to characterize the morphology,
structure and particle size. The element composition and distribu-
tion were characterized by energy dispersive X-ray spectroscopy
(
EDS) on the Hitachi S 4800. The element composition was also
characterized by inductively coupled plasma optical emission
spectrometer (ICP-OES, America, Varian, VISTA-MPX). The metal (Au,
Cu) concentration in standard solution and the sample solution
were 100 ppm and 50–100 ppm, respectively. The X-ray diffraction
(
XRD, Japan, Rigaku Corporation, D/ Max-2500) was used to detect
the composition and crystal structures of the samples. The test
À 1
range was 25°~85° at scanning speed of 6 °·min . The surface
Figure 9. Schematic illustration of proposed sequential catalysis mechanism
of C
2
H
4
formation from CO
2
on Au
x
Cu
2
O catalysts.
chemical valence of the samples was detected by X-ray photo-
electron spectroscopy (XPS, Britain, Thermo Fisher Scientific, K-
+
Alpha ), and the binding energy was calibrated with carbon C1s
(284.8 eV as the correction value).
+
0
mixed copper states Cu /Cu as well. This will provide a
strategy to develop the efficient Cu oxide based bimetallic
catalysts for selective C formation during CO RR.
2
2
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Electrochemical Measurements
day efficiency of each product is calculated as the follow
equations (2–4):
All electrochemical tests were performed in a H type cell and 0.1 M
KHCO3 solution was used as electrolyte. The cathode chamber
Gas phase products:
(
90 mL of 0.1 M KHCO solution, ~30 mL of headspace) and the
3
anode chamber (45 mL of with 0.1 M KHCO solution, ~15 mL of
3
FP0
RT
headspace) were separated by a proton exchange Nafion mem-
brane. The electrochemical measurements were controlled with an
AutoLab 302 N electrochemical workstation (Switzerland, Metrohm).
The Pt sheet (1.0 cm×2.0 cm) and saturated calomel electrode (SCE,
saturated KCl) served as the counter electrode and the reference
electrode, respectively. Unless stated otherwise, all the potentials in
this work were converted into the reversible hydrogen electrode
potential (RHE) by the follow formula:
i
x
¼ V
x
ꢀ q ꢀ 2
(2)
(3)
ix
FE ¼
ꢀ 100%
x
itotal
Liquid phase products:
ZnF
Q
FEHCOOH
¼
ꢀ 100%
(4)
EðRHEÞ ¼ EðSCEÞ þ 0:0591 ꢀ PH þ 0:24
(1)
Where V represents the gas phase products (CO, H , CH , C H ), i is
x
2
4
2
4
x
the partial reduction current of different gas phase products, itotal
Preparation of the working electrodes
represents the total reduction current of the reaction, q represents
À 1
the flow rate of CO in the reduction process (20 mLmin ), F is the
2
Firstly, the catalyst ink solution was prepared by adding 5 mg
catalyst and 50 μL Nafion solution (5 wt%) into 300 μL ethanol/
water mixture (volume ratio 1:1). Then the carbon paper (1.0 cm
À 1
Faraday constant (96485.3 Cmol ), P is atmospheric pressure, R
stands for ideal gas constant (8.314 Jmol K ), T stands for room
0
À 1 À 1
temperature (297.15 k), Z is the molar mass of the product, n
represents the number of electrons transferred during the reaction,
Q is the total transfer charges of the reaction.
×
1.0 cm) was used as the substrate and coated with 100 μL catalyst
ink solution. Finally, it was mildly reduced through the repeated
LSV reduction process (À 0.1~À 1.2 V) for 1 h at the scan rate of
À 1
5
0 mVs in N -saturated 0.1 M KHCO solution. The resulting
2
3
electrode was used as the working electrode. In addition, the
control experiments were conducted to investigate the effects of
the reduction condition. The electrode without the above repeated
LSV reduction process was marked with word “as-prepared”. And
the electrode undergoing the severe reduction at À 1.5 V for 2 h
was marked with word “HPR” meaning high potential reduction.
Acknowledgements
This research was funded by National Natural Science Foundation
of China (Grant No.21873067 and 21206117). We are grateful to
the analysis and test center of Tianjin University for providing
XRD, SEM, XPS characterizations.
Surface redox property of Cu atoms through Cyclic Voltage
(CV) method
The surface redox property of Cu atoms for each working electrode
À 1
Conflict of Interest
was studied by CVs (10 mVs ) in N -saturated 0.1 M KHCO
2
3
solution at the potential range of À 0.75~1.25 V. The Cu redox
properties were compared for each working electrode before and
after LSV treatment. The differences of redox peak intensities were
The authors declare no conflict of interest.
used to evaluate the different reduction degree of Cu O after
stabilization. In addition, it was used as an indicator of the exposed
Cu atoms on the surface involved in the electrochemical reaction.
2
Keywords: Electrochemical CO2 reduction
electrocatalysts · Sequential catalysis · Ethylene · Au decorated
·
Cu based
Cu O
2
Electrochemical reduction of CO (CO RR)
2
2
To compare CO RR activity of the catalysts, LSV tests between
2
À 1
À 0.1 V and À 1.2 V at scanning tares of 50 mVs were performed
on each as-prepared catalyst in N and CO saturated 0.1 M KHCO
3
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These are not the final page numbers!

FULL PAPERS
X. Cao, G. Cao, M. Li, Prof. X. Zhu,
Prof. J. Han, Prof. Q. Ge, Prof. H.
Wang*
1
– 13
Enhanced Ethylene Formation
from Carbon Dioxide Reduction
through Sequential Catalysis on
Au Decorated Cubic Cu O Electro-
2
catalyst
Novel Au Cu O electrocatalysts were
lectivity in CO electrocatalytic
x
2
2
synthesized via the galvanic replace-
reduction, which were 5 and 2 times
ment reaction between Cu O
higher than those of bare Cu O and
2
2
nanocubes and HAuCl solution, and
Au0.02Cu O pre-reduced by chronoam-
4
2
the pre-reduction by linear voltamme-
perometry at high potential, respec-
tively.
try (LSV). The Au0.02Cu O pre-reduced
2
by LSV significantly boosted C H se-
2
4