Metal-organic framework derived copper catalysts for CO2to ethylene conversion

Article abstract of DOI:10.1039/d0ta02395g

The electrochemical reduction of CO₂to ethylene provides a carbon-neutral avenue for the conversion of CO₂to value-added fuels and feedstocks, so contributing to the storage of intermittent renewable electricity. The exploration of efficient electrocatalysts with high ethylene selectivity and productivity is highly desirable but remains challenging. Here, we present a Cu-based catalyst derived from a metal-organic framework (Cu-MOF) which shows enhanced performance due to its porous morphology, complex oxidation states and strong lattice strain. X-ray diffraction, X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy are utilized to track the evolution of the crystal structure and oxidation states during the reaction, and the results reveal that Cu²⁺ions are rapidly reduced to Cu⁺and then slowly to Cu⁰, resulting in a Cu?Cu_xO core?shell structure. The tensile strain caused by the distorted grain is beneficial for the activation of CO₂. Cu⁺/Cu⁰interfaces formed through stabilized Cu⁺facilitate *CO-CO dimerization, promoting conversion to C₂₊products and suppressing conversion to C₁products. The optimized catalyst exhibits a 51% Faraday efficiency (FE) for ethylene and a 70% FE for C₂₊products, with 20 h operational stability in an H-cell configuration, and a partial ethylene current density of 150 mA cm^?2in a flow-cell configuration.

Full text of DOI:10.1039/d0ta02395g

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Materials for energy and sustainability
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N. Wang, J. Han, C. C. Gao, M. Han, G. Shen, Y. Liu, A. Seifitokaldani, X. Sun and H. Liang, J. Mater. Chem.
A, 2020, DOI: 10.1039/D0TA02395G.
Volume
Number 12
March 2018
Pages 4883-5230
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ISSN 2050-7488
COMMUNICATION
Zhenhai Wen et al.
An electrochemically neutralized energy-assisted low-cost
acid-alkaline electrolyzer for energy-saving electrolysis
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Page 1 of 7
Jo Pu l rena as le o df oM na ot et rai ad lj su Cs th me ma ri gs ti nr ys A
View Article Online
DOI: 10.1039/D0TA02395G
Communication
Metal Organic Framework Derived Copper Catalysts
for CO to Ethylene Conversion
2
a,b
c
d
a
a
Kaili Yao, Yujian Xia, Jun Li, Ning Wang, Jingrui Han, Congcong
a
a
e
a
f
Gao, Mei Han, Guoqiang Shen, Yongchang Liu, Ali Seifitokaldani,
c
a,b,
Received 00th February 20xx,
Accepted 00th February 20xx
Xuhui Sun, and Hongyan Liang *
Electrochemical reduction of CO
2
to ethylene provides a carbon-neutral avenue to convert
DOI: 10.1039/x0xx00000x
CO to value-added fuels and feedstocks powered using the intermittent renewable
2
www.rsc.org/
electricity. The exploration of efficient electrocatalysts with high ethylene selectivity and
productivity are highly desirable but remain challenging. Here, we present a Cu-based
catalyst derived from metal-organic framework (Cu-MOF) with enhanced performance in
terms of their porous morphology, complex oxidation state and strong lattice strain. X-ray
diffraction, X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy are
utilized to track the evolution of crystal structure and oxidation state during reaction, the
2
+
+
0
results reveal that the Cu were rapidly reduced to Cu then slowly to Cu , resulting in
Cu@Cu O core@shell structure. The tensile strain caused by distorted grain can benefit the
activation of CO
dimerization, resulting in promoted C2+ products conversion and suppressed C
conversion. The optimized catalyst exhibits a 51% Faraday efficiency (FE) for ethylene and
0% FE for C2+ products, with 20 h operation stability in H-cell configuration and a partial
x
+
0
+
2
. Cu /Cu interfaces formed through stabilized Cu facilitate *CO-CO
products
1
7
-2
ethylene current density of 150 mA cm in flow-cell configuration.
the commercial demands. The exploration on improving the
selectivity and productivity of ethylene is crucial for the CO RR
technologies’ marketization.
1
. Introduction
2
Electrochemical CO
attractive paradigm to convert CO
contributed to storing the intermittent renewable electricity and
closing the carbon cycle. Compared with C
such as ethylene (C
2
reduction reaction (CO
2
RR) provides an
to valuable fuels and feedstocks,
CO
2
RR to ethylene is 12-electron reduced process, and only
2
5
copper-based catalysts are reported to show high selectivity. The
surface of copper plays a crucial role in determining the reaction
pathway, strategies such as enlarging surface area, promoting
oxidation states, exposing high-index facets, introducing strain
effects,
1
-3
1
products, C
2
products
6
-8
2 4
H ), offer higher energy density and market
9
10
4
value. Despite recent breakthroughs in catalysts and reaction
configuration design, the energy efficiency of CO RR cannot fulfill
2
1
1, 12
and engineering surface reconstruction13-15 have been
explored to improve the product selectivity and lower the reaction
energy barrier. However, the synthetic control and characterization
of surfaces are challenging.
a. School of Materials Science and Engineering, Tianjin University, Tianjin 300350, P. R.
China
b. Key Laboratory of Efficient Utilization of Low and Medium Grade Energy, Ministry of
Copper oxidation states on surface has shown effects on activity
16
and selectivity for CO RR to C H
2 2 4
conversion. During the CO
2
RR
process, the copper oxide can be reduced at negative potentials,
resulting in distorted grain, roughened surfaces and complex
Education, Tianjin University, Tianjin 300350, P. R. China
c.
Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for
oxidation states, which are proposed to favor the CO
2
RR. Sargent et
Carbon-Based Materials and Devices, Soochow University, Suzhou, Jiangsu, 215123,
P.R. China
d. Institute of Chemical Sciences and Engineering, École polytechnique fédérale de
+
al. reported that residual Cu species can stabilize the OCCOH*
intermediate, then promote the ethylene conversion while
17
suppress the methane production. Currently, copper oxide
Lausanne, Lausanne 1015, Switzerland
catalysts have an average Faradaic efficiency (FE) for ethylene range
e.
School of Chemical Engineering and Technology, Tianjin University, Tianjin 300350,
9, 17-21
from 30% to 40%.
However, due to the complex changes of
P. R. China
crystal structure, morphology and chemical state on the surface of
f.
Department of Chemical Engineering, McGill University, Montreal, H3A 0C5, Canada
copper oxide, the mechanism of their enhancement in CO
under debate.
2
RR is still
†
Electronic Supplementary Information (ESI) available: Experimental section and
supplementary Fig.s. See DOI:
Surface area of catalyst also has a significant effect on enhancing
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DOI: 10.1039/D0TA02395G
CO
2
RR. Copper catalysts have been fabricated in different shapes to
enlarge their active surface area. Compared with traditional copper
film, sputtered copper particles, or nanoparticles,
2
1
22
23-25
metal-
organic framework (MOF) has a large surface area with abundance
of catalytic sites due to their porous structure, which can provide
2
6-28
stronger adsorption with reaction intermediates.
Moreover,
benefiting from the chemical tunability and the porosity/surface
topography controllability, the formation of undercoordinated Cu
2
sites in MOF can be promoted to enhance CO RR efficiency, leading
to a 45% FEC2H4,which is the record for MOF-derived Cu catalysts
2
9
based on our knowledge.
Inspired by the merits of copper oxide and MOF structure in
CO RR, here, we synthesized a Cu-based MOF (HKUST-1) derived
catalysts for CO to C conversion. The calcination was utilized to
2
2
2 4
H
simultaneous modify the morphology and oxidization state of
catalysts. The pristine catalyst has an octahedral MOF structure, in
which copper dimers are separated by adjacent benzene
tricarboxylate. After calcination at high temperature, the
dissociation of organic ligand happens and the copper oxide phase
occurs. When the calcination temperature was 265 ° C, all MOF
converted to CuO and Cu
grains with high tensile strain, which might promote the activation
of CO molecules. X-ray diffraction (XRD), X-ray photoelectron
2
O, the mixed phase caused distorted
2
spectroscopy (XPS), and X-ray absorption spectroscopy (XAS) were
used to track the evolution of catalysts during reaction. The results
2
+
+
show that under a negative applied bias, the Cu is reduced to Cu
+
Fig. 1 a) Schematic illustration of synthesis process (red oxygen atom refers to CuO,
rapidly, then the Cu is reduced to metallic Cu slowly, leading to a
2
pink oxygen atom refers to Cu O). Structural investigations of H-265. b) XRD patterns,
c) XPS of Cu 2p, and d) XPS of Cu LMM. e) SEM image, f) TEM bright field image and g-
h) TEM EDS images of a single particle. Nanoparticles size analysis. i) TEM and j) HRTEM.
Atomic-resolution HAADF-STEM images of the H-265: k) without and l) with lattice
strain.
Cu@Cu
x
O core@shell structure. After 10 h operation, trace amount
+
+
of Cu species remains. The stabilization of Cu benefits the *CO
coupling at Cu /Cu interface to promote the ethylene conversion
and suppress the methane production. As a result, the MOF-derived
0
+
between H-230 and H-0 in XRD because of the retained organic
bonding of HKUST-1. However, the morphology of H-230 was
minimally changed and the appearance of porous surface revealed
partial degradation of carboxylic moieties. When the temperature
increased to 265°C, all HKUST-1 features in XRD disappeared and a
Cu@Cu
1.58 V versus reversible hydrogen electrode (vs. RHE), with a
stability of 20 h in an H-cell configuration. To accelerate the reaction,
a flow-cell configuration was performed and a C partial current
density of 150 mA cm can be achieved with a FEC2H4 of 49.8%.
x 2 4
O exhibited 51% FE for C H and 70% FE for C2+ products at
-
2 4
H
-2
2
mixture of CuO and Cu O phase presented, indicating that MOF
2
3,
phase was completely converted to copper oxide phase (Fig. 1b).
2
. Results and discussion
32
While, the octahedral outline of H-265 remained consistent with
H-230, excepting enlarged pore size (Fig. 1e). Further to increase
the calcination temperature to 500°C, only CuO features can be
observed in XRD, and the sharp peaks demonstrate high crystallinity.
SEM image shows the octahedral template was destroyed to
irregular aggregated particles and the porosity was lost. Noted that,
residual carbon was observed in the calcinated sample (Fig. S5-6†),
Precursor HKUST-1 was used for preparing catalysts as described
in Fig. 1a. In HKUST-1, Cu cations are stabilized by adjacent oxygen
atoms in carboxylate groups (Fig. S1†). The calcination process
was used to remove the organic ligands, as described by
thermogravimetric analysis (Fig. S2†). XRD patterns at different
temperature showed that when the calcination temperature was
3
0
2
7, 28
which agreed with other reports.
2
65ºC or above 265ºC, HKUST-1 was completely converted to
The crystal structure, oxidation state, and atomic-resolution
images of H-265 were further characterized in detail. XRD patterns
inorganic copper oxide (Fig. S3†). Therefore, 230, 265, and 500ºC
was chosen as calcination temperature for pristine HKUST-1 (H-0),
and the obtained catalysts were denoted as H-230, H-265 and H-
in Fig. 1b clearly reveal the mix CuO and Cu
broadening peaks and the 0.1-0.2º negative shifts compared to H-
00 demonstrate the diminished crystallinity with smaller grain size.
2
O phase. The
5
00, respectively.
5
To determine the crystal structure and morphology of catalysts,
+
2+
XPS results in Fig. 1c-d confirms that Cu and Cu coexist in H-265,
which agrees with the XRD results. Element mapping images
demonstrate that the Cu and O elements distribute homogeneously
throughout the whole octahedra (Fig. 1f-h). To further understand
the microstructure of H-265, transmission electron microscope
XRD and scanning electron microscopy (SEM) measurements were
performed on four samples as shown in Fig. 1 and supplementary
Fig. S3-4†. XRD patterns of H-0 show representative peaks
corresponding to HKUST-1. The shape of H-0 is octahedral crystals
of ∼1 μm in size with nonporous flat surface, which is typical MOF
morphology. After calcinated at 230°C, the color of H-0 changed
from light blue to dark green, but there were no distinct differences
3
1
(TEM) was performed. High-resolution TEM (HRTEM) images reveal
the porous octahedra is constituted of tightly packed ~10 nm
nanoparticles (Fig. 1i-j). Atomic-resolution HAADF-STEM images
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DOI: 10.1039/D0TA02395G
after 4 h operation and the FEC2H4 dropped from 40% to 30% after
10 h with an increased H production (Fig. S13b†). H-0 had poorest
2
stability with a steady low FEC2H4 and showed a markedly increased
of FEH2 to 43% after 5 h due to its bad water stability (Fig. S13c†).
show that some nanoparticle has perfect lattice arrangement, while
others have bending lattices due to the distorted grain, which can
lead to tensile strain and abundance of uncoordinated Cu sites (Fig.
operation (Fig. S13a†). H-500 exhibited a significant FEC2H4 decline
1
1
1
k-l). The lattice tensile strain also can be confirmed by the
3
3
negative peak shifts and broadening peaks of XRD patterns, and
the origin of this inhomogeneous lattice bending might be from the
These results suggest that mixed Cu
cally stable than pristine MOF or pure CuO.
2
O/CuO are more electrochemi-
3
4
2
lattice mismatch between CuO and Cu O.
In summary, when the calcinated temperature was 230ºC, the
octahedral mainframe and partial organic bonding remained. When
the calcinated temperature was 265 ℃, HKUST-1 was completely
2
converted to inorganic Cu O/CuO phase with poor crystallinity and
bending lattices. After calcinated at 500°C, the HKUST-1 octahedra
was destroyed into irregular aggregated CuO particles with highly
crystallinity.
To evaluate the electrocatalytic activity of four samples, CO
2
RR
was performed in CO saturated 0.1 M KHCO in a standard three-
2
3
electrode H-cell setup, with graphite rod as counter electrode and
Ag/AgCl as reference electrode. The applied potential was in the
range from -0.78 to -1.78 V vs. RHE. The gas chromatography (GC)
was used to determine the component of gas products and the
calibration were shown in Fig. S7†. NMR analysis of electrolyte after
reaction was used to analyze the liquid products. All samples
produced mostly gas products such as H
small amounts of liquid products such as formic acid, acetate,
ethanol and propanol. For the target product C , H-265 shows a
maximum FEC2H4 of 51 ± 3% at a potential of -1.58 V vs. RHE (Fig.
a), which is comparable to current record (Table S1†). Interestingly,
the FECH4 was suppressed to 1%, resulting in a C :CH ratio of 60,
which is two order of magnitude higher than that of the pristine H-
(0.38) as shown in Table S2†. The catalytic performance of other
2 4 2 4
, CO, CH , and C H , and
2 4
H
2
H
2 4
4
Fig. 2 Electrochemical CO
potentials. b) accumulated bar graph for quantification of products at -1.58 V vs. RHE
compared with other catalysts of our experiment, Cu foil and Cu O. c) FEs at various
. d) FEs for products in a flow-cell
2
RR performances of H-265. a) FEs for gas products at virous
0
samples was shown in Fig. S8† and supplementary Table S2†. These
results suggest that thermal treatment increased the selectivity of
CO RR towards C to C , and C :C ratio reach maximum when the
2 2 1 2 1
2
2 1 2 3
potentials for total products classified by H , C , C +C
configuration, catalytic runs were performed with constant current, the pink dots show
the partial current density of ethylene. (e) stability of ethylene FE on H-0, H-265, and
H-500 samples.
catalysts calcinated at 265°C, and then declined with further
increasing treated temperature. When compared with Cu foil and
To investigate the high catalytic activity of H-265 is whether from
their intrinsic nature or just owing to the enhanced electrochemical
surface area (ECSA), we conducted electrochemical double-layer
capacitance measurement of these samples (Fig. S14† and Table
S5†). It was found that H-265 did not show highest ECSA as
suggested by the double-layer capacitance, excluding the influence
of their ECSA.
Cu
2
O samples (Fig. S9-10†), MOF-derived catalysts H-265 showed
higher selectivity towards C2+ product and suppressed the C
1
conversion, resulting in a 70% total FE toward C2+ products (Fig. 2b-c
and Table S3†). More interesting, a n-propanol FE of 3.1% based on
our NMR analysis (Fig. S11†) was achieved of H-265, while no n-
propanol product could be detected for H-0, H-230, Cu-foil and
The excellent CO
assess the alteration of chemical state of copper before and after
CO RR process with XRD, XPS and XAS studied. As shown in Fig. 3a,
XRD patterns show that initial H-265 had a prominent CuO phase
mixed with Cu O phase. After 1 h CO RR operation, no discernible
CuO peaks remained while the enhanced Cu O peaks and features
of metallic Cu peak appeared, suggesting all CuO was reduced to
Cu O and a small part of Cu. When the reaction continued, Cu
was gradually reduced to Cu, but the XRD features of Cu O still can
2
RR performance of H-265 prompted us to
Cu
2
O.
In traditional H-cell configuration, the low solubility of CO
2
and
2
the carbonate electrolyte limit the mass transfer and the current
density is low. In order to increase the productivity, the flow-cell
2
2
configuration with a gas diffusion electrode (GDE) (favors the CO
transfer) and alkaline electrolyte (favors C2+ selectivity) was utilized
Fig. S12†). In this system, the CO and electrolyte contact on the
2
2
(
2
2
2
O
surface of catalysts, and feed the reaction at the gas-solid-liquid
three-phase interface continuously. As shown in Fig. 2d, the
2
be observed after 10 h. The similar trend was reproduced by using
a flow-cell configuration (Fig. S15†). These results suggested that
production of CH
4 2
and H was effectively suppressed. The current
-2
density was 320 mA cm , and C
2 4
H partial current density was 150
+
the Cu can be stabilized under an applied potential of -1.58 V vs.
-2
mA cm with a FEC2H4 of 49.8% (Table S4†).
RHE and a high current density for several hours, which might
To evaluate the electrochemical stability of the catalysts, the
durability during the long-term operation were investigated at
constant potential. As shown in Fig. 2e, the results suggest that H-
1
7, 35
2 4
facilitate the selectivity towards C H .
XPS spectra were measured to further distinguish the copper’
states on the surface of H-265. Cu 2p spectrum clearly shows that
2
65 was most stable, with negligible decrease of FEC2H4 after 20 h
2
+
Cu with a binding energy at 933.8 eV is the dominated state of the
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DOI: 10.1039/D0TA02395G
2 3
Fig. 3 Characterization of H-265 before and after 1 and 10 h of CO RR. a) XRD analysis. b) XPS Cu 2p spectra. c) Auger spectra of Cu LMM. d) Ex-situ Cu L -edge soft X-ray absorption
spectroscopy (sXAS). e) Cu k-edge XANES and EXAFS spectra, and f) The Fourier transform magnitudes in R space.
9
+
0
of high intensity of Cu2+ signal in XAS after CO
In addition to the surface sensitive Cu L -edge TEY XAS spectrum,
we also collected Cu K-edge X-ray absorption near edge structure
(XANES) and extended X-ray absorption fine structure (EXAFS)
initial H-265, which is reduced to Cu or Cu state during the CO
2
RR
2
RR operation.
process (suggested by the negative shift of binding energy) (Fig. 3b).
A small content Cu residue after 1 and 10 hours of operation were
observed, which might originate from the surface oxidation in air.
Because the difference of binding energies between Cu and Cu is
only 0.1 eV in the Cu 2p peak, it is difficult to distinguish them.
Therefore, auger electron spectroscopy (AES) of Cu LMM signal was
conducted, which can provide higher spectrum resolution, allowing
for acquisition of greater contrast with oxidation state change. It
clearly shows after 1 h and 10 h CO
pristine sample transferred into Cu located at 570.0 eV (Fig. 3c).
To provide a more qualitative investigation of oxidation state
2 3
evolution during CO RR, Cu L -edge XAS were collected at various
reaction time. The transitional metal L-edge (i.e. 2p→3d transition)
3
2
+
+
0
2
spectra which have deeper probing depth. Cu foil, CuO and Cu O
standards were taken as reference. Cu K-edge EXAFS oscillations in
3
k space (k -weighting) image (Fig. S16†) show clear oscillations up
to k=11, which suggests high data quality. As shown in XANES (Fig.
3e), before reaction, the white line of H-265 at 8979(OR) eV is
consistent with CuO, while after 1 h CO RR operation, it is similar to
2
2
+
2
+
RR, Cu located at 568.5 eV in
36
that of the Cu foil, indicating the dominant metallic copper phase in
the core of the nanoparticles. Linear combination fitting were
conducted as shown in Fig. S17† and Table S6†. The results
+
suggested the presence of Cu after reduction reaction. In addition,
is highly sensitive to 3d electronic states which is suitable for the
the Fourier transform magnitudes in R space were used to track the
associated Cu bonding environment. As shown in Fig. 3f, the Cu-O
binding was dominant in pristine H-265, and Cu-Cu binding was
3
7
electronic structure study here. Total electronic yield (TEY) signal
3
8
were collected with an average probing depth of around 10 nm.
As shown in Fig. 3d, the Cu L -edge XAS spectrum of CuO standard
exhibited a single peak at 931 eV, both peak position and line shape
3
2
dominant after CO RR operation, suggesting the transformation
between CuO to Cu. First shell extended edge fitting results suggest
consistent transformation in radical distance (Fig. S18† and Table
S7†). Combined with XRD, XPS and sXAS measurements, we can get
3
9
3
match well with previously reported results. The Cu L -edge XAS
spectrum of H-265 before reaction has same line shape with that of
CuO standard but 0.2 eV energy shift, clearly indicating +2 oxidation
state. After applying a constant potential of -1.58 V vs. RHE for 5
min, another higher energy peak at 933.6 eV began to emerge,
2
+
a conclusion that the initial Cu in pristine H-265 was reduced to
+
0
Cu and Cu during CO
2
RR process, accompanying with a surface
+
reconstruction resulting in a Cu@Cu
x
O core@shell structure. Cu
+
2+
which is consistent with Cu , suggesting the transition from Cu to
was stabilized for over 10 h operation.
+
39
Cu occurs quickly. Remarkably, we found that the peak intensity
of Cu increased gradually and then remained stable, indicating Cu
may be stabilized under an negative potential for over 10 h during
In order to explore the reasons for the existence of oxidation
valence state, we monitored the chemical state change of oxygen
during the reaction process. XPS O 1s peak of pristine H-265 shows
feature of the lattic oxygen species (Olatt) corresponding to CuO
+
+
9
, 35
CO
2
RR process.
Noted that, the re-oxidation in air cannot be
totally exclude in our experiments, which might be the origination
2 2
phase, which changed into Cu O phase after CO RR
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+
Fig. 4 Proposed reaction pathways starting from *CO on Cu surface in the presence of Cu . Green, blue, and yellow routes are competitive reactions
reaction (Fig. S19†).9, 40 Besides, the atomic concentration ratio of
O to Cu (O/Cu) of pristine H-265 was 1.19 and then decreased to
C
H
OH generation (green routes).48 In our experiments, the
products were dominated C and small amounts of C OH, n-
OH, while no C and CH OH was detected, which match the
products from route 3. Moreover, our Cu@Cu O catalyst has
3
7
2
H
4
2 5
H
0
.46 after 1 h reaction (Table S8†). For the control sample H-500,
C
H
3 7
2
H
6
3
the O 1s peak only showed an adsorbed state after 1 h reaction, and
the O/Cu ratio was 0.07, almost 1 order of magnitude lower than
that of H-265 (Table S8†). Both XRD and XPS of H-500 showed that
x
+
0
abundance of Cu /Cu interface, which have been proved to assist
*CO-CO dimerization due to the opposite electrostatic attraction of
charged carbon atoms (+0.11 on Cu and -0.31 on Cu ). Based on
the final products and catalysts’ properties, we propose our
reaction pathway follows route 3, in which *CO-CO coupling
promotes the C2+ conversion while impedes C
selectivity towards C may be caused by the suitable formation
energy barrier, compared with other product such as C
n-C H OH.
3 7
2
+
0
+
0 49
Cu was completely reduced to Cu after 1 h reaction (Fig. S20-21†).
These results demonstrated that the stabilized oxide state on H-265
surface benefits from the oxygen-rich environment, which maybe
contributed by the small size nanoparticles and their compact
arrangement, thus facilitating rapid oxygen migration along the
dense boundaries, then retard the reduction of copper (Fig. S22-
1
production. The high
2 4
H
2
H
5
OH and
4
1
45
2
4†).
To provide mechanistic insights into the high C
H-265, we firstly used Tafel analysis to explore whether the rate-
2 4
H selectivity of
3
. Conclusions
Here, we synthesized MOF derived copper catalyst via calcination
determining step (RDS) is the initial one-electron reduction CO
2
to
-
CO
2
· or a chemical reaction occurring after the one-electron
in air. XRD, XPS and XAS analysis indicated that the catalyst
underwent oxidation state transition and surface reconstruction
during the CO
distorted grains lead to lattice strain, which might lower CO
activation barrier. The Cu /Cu interface facilitates *CO-CO
dimerization, which promotes the C2+ products conversion and
suppresses the C
towards C . The Cu@Cu
FEC2+ with an operation stability of 20 hours in H-Cell. Moreover, a
partial ethylene current density of 150 mA cm in a flow-cell
4
2
activation of an inert CO
Tafel slope of H-265 sample was 119.5 mV dec , which was close to
2
molecule. According to Fig. S25†, the
-1
2 x
RR process and formed a Cu@Cu O structure. The
-1
1
18 mV dec , implying that the RDS is activation of CO
a lower electron transfer resistance during CO RR process as
confirmed by the electrochemical impedance spectroscopy (EIS)
2
, which has
2
2
+
0
4
3
measurements (Fig. S26†). Besides, tensile strain on the surface
of catalyst has been reported to shift up the d-band center of
1
products conversion, leading to a high selectivity
O catalyst reached 51% FEC2H4 and 70%
H
2 4
x
4
4
surface atoms, thus strengthening the adsorption of CO
Therefore, tensile strain caused by the bending lattice on H-265 is
supposed to lower the CO activation energy barrier (Fig. 4).
For the second RDS of C formation, there are three proposed
reaction pathways based on previous mechanistic studies as
2
.
-2
2
configuration with a 49.8% FEC2H4 was achieved. This study will
provide new strategies to enhance the selectivity and durability of
MOF-derived catalysts for electrochemical CO
2 4
H
2
conversion to value-
4
5, 46
described in Fig. 4.
dimerize to form C
CH to form C , or through a Fischer-Tropsch like CO insertion
pathway to form C
COH (or *CO-CHO) coupling can lead to C
competed with the CH or CH OH formation (yellow routes), and no
liquid C2+ products can be achieved. In route 3, the dimerization of
CO-CO will be further protonated to facilitate C , C OH, or n-
In route 1, *CH
2
intermediate can directly
added chemicals.
2 4
H , or via the parallel pathway to bond with
*
3
H
2 6
Conflict of interest
The authors declare no conflict of interest.
-
47
2
H
4
or CH
3
COO (blue routes). In route 2, *CO-
formation, which is
2 4
H
4
3
Acknowledgements
This work was supported by the National Natural Science
*
H
2 4
H
2 5
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Journal of Materials Chemistry A
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Communication
View Article Online
DOI: 10.1039/D0TA02395G
Hofmann, J. Bisquert, S. Gimenez and W. A. Smith, ACS Appl. Mater.
Foundation of China (NSFC No. 51771132) and the Recruitment
Program of Global Youth Experts of China; The author thank Taiwan
Light Source (TLS) beamline 17C1, 20A1 and Taiwan Photon Source
22 K. Liu, M. Ma, L. Wu, M. Valenti, D. Cardenas-Morcoso, J. P.
Interfaces, 2019, 11, 16546-16555.
(
TPS) beamline 44A1 of NSRRC for providing synchrotron
23 H. Jung, S. Y. Lee, C. W. Lee, M. K. Cho, D. H. Won, C. Kim, H.-
S. Oh, B. K. Min and Y. J. Hwang, J. Am. Chem. Soc., 2019, 141, 4624-
beamtimes.
4
2
633.
4
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