Freestanding Millimeter-Scale Porphyrin-Based Monoatomic Layers with 0.28 nm Thickness for CO2 Electrocatalysis

Article abstract of DOI:10.1002/anie.202006899

Developing two-dimensional (2D) and single atomic layered materials is a fascinating challenge. Here we successfully synthesize porphyrin-based monoatomic layer (PML), a freestanding 2D porphyrin-based material of monomer-unit thickness (2.8 ?). The solvothermal method provides a bottom-up approach for tailoring the monoatomic layer from the nanoscale to the milliscale. PMLs containing accurately tailorable M-N₄ units (M=Cu and Au) were synthesized, which present metal center-dependent performance for CO₂ electrocatalysis. PML with Cu-N₄ centers performs high faradaic efficiencies of HCOO^? and CH₄ (80.86 % and 11.51 % at ?0.7 V, respectively) while PML with Au-N₄ centers generates HCOO^? and CO as major products (40.90 % and 34.40 % at ?0.8 V, respectively). Irreversible restructuring behavior of Cu sites is also observed. Based on the graphene-like properties and metal center-selectivity relationships, we believe that PML will play a distinct role in various applications.

Full text of DOI:10.1002/anie.202006899

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Accepted Article
Title: Freestanding millimeter-scale porphyrin-based monoatomic
layers with 0.28 nm thickness for CO2 electrocatalysis
Authors: Xun Wang, Deren Yang, Shouwei Zuo, Haozhou Yang, and
Yue Zhou
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202006899
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Angewandte Chemie International Edition
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COMMUNICATION
Freestanding millimeter-scale porphyrin-based monoatomic
layers with 0.28 nm thickness for CO electrocatalysis
2
Deren Yang, Shouwei Zuo, Haozhou Yang, Yue Zhou, [a] and Xun Wang*[a]
[
a]
[b]
[a]
[
a]
D. Yang, H. Yang, Y. Zhou, Prof. X. Wang*
Key Lab of Organic Optoelectronics and Molecular Engineering, Department of Chemistry
Tsinghua University
Haidian District, Beijing 100084, P. R. China
E-mail: wangxun@mail.tsinghua.edu.cn
S. Zuo
[
b]
Beijing Synchrotron Radiation Facility
Institute of High Energy Physics, Chinese Academy of Sciences
19B Yuquan Road, Shijingshan District, Beijing 100049, P. R. China
S. Zuo
University of Chinese Academy of Sciences
19B Yuquan Road, Shijingshan District, Beijing 100049, P. R. China
Supporting information for this article is given via a link at the end of the document.
Abstract: Developing novel two-dimensional (2D) and single atomic
layered materials is a fascinating challenge. Here we successfully
synthesize porphyrin-based monoatomic layer (PML), a freestanding
particularly catalysis due to several advantages. First, the
atomically thin 2D nanostructure makes each metal center easily
accessible during catalysis. Second, the completely uniform and
periodic structure makes all of the catalytic sites are equally
accessible and equally active, which is similar as homogeneous
catalyst. Third, the desired product can be well controlled via
altering the type of metal centers. However, several challenges
such as instability, low yields, various thicknesses (usually about
2D porphyrin-based material of monomer-unit thickness (2.8 Å). The
solvothermal method provides a bottom-up approach for tailoring the
monoatomic layer from the nanoscale to the milliscale. PMLs
4
containing accurately tailorable M–N units (M=Cu and Au) were
1
nm) and a need for substrate persist. Therefore, a facile method
synthesized, which present metal center-dependent performance for
to synthesize large-area freestanding 2D PMLs is highly desired.
In this work, we firstly report on the scalable fabrication of PMLs
using a solvothermal method and study their metal center-
CO
2
electrocatalysis. PML with Cu-N
4
centers performs high faradaic
-
efficiencies of HCOO and CH
4
(80.9% and 11.5% at -0.7 V,
-
respectively) while PML with Au-N
4
centers generates HCOO and CO
selectivity relationship for CO
2
electroreduction. PML is a
as major products (40.9% and 34.4% at -0.8 V, respectively). The
irreversible restructuring behavior of Cu sites is also observed. Based
on the graphene-like properties and metal center-selectivity
relationships, we believe that PML will play a distinct role in various
applications.
freestanding 2D monoatomic layer of porphyrin units strictly
arranged into a square lattice. The Cu-O and M-N (such as Cu-
N and Au-N ) act as metal nodes within the paddlewheels and
4 4
coordination sites inside the porphyrin rings, respectively. All
evidences reveal that both its structure and property are
analogous to graphene. Its size can be well tailored from the
nanometer scale to the millimeter scale via the bottom-up growth
while the thickness maintains ~2 Å. Thus, the atomically thin 2D
structure will maximize atom utilization and molecular diffusion
during catalytic process. Different porphyrin units feature different
4
4
2D materials were widely presumed to be thermodynamically
unstable and discontinuous at a single atomic thicknesses until
[
1]
the graphene films were prepared by mechanical exfoliation.
2
4
types of M-N center as a catalytic site. As a result, PML will
Graphene, a 2D monolayer of sp -hybridized carbon atoms gives
rise to a series of unique properties,[2] and thereby exhibits great
display a drastic dependence of product selectivities on the metal
center. Meanwhile, the monoatomic thickness makes PML as an
active atomic plane for the formation of desired products. Here,
porphyrin-Cu and porphyrin-Au units are choosed as building
blocks for preparing PMLs, which are named as PML-Cu and
PML-Au, respectively.
potential in optoelectronic, energy storage, catalysis, sensors and
composites.[3] The success and promise of graphene have
motivated researchers to extend the graphene-like family, such
[
4]
[5]
[6]
[7]
as graphdiyne,
phosphorene,
2D transition metal carbides and nitrides
MXene),[ and even transition metal dichalcogenides (TMDs).
silicene,
stanene,
[
8]
germanene,
9]
PML is a freestanding 2D porphyrin-based material with
graphene-like structure and property (Figure S1). Figure 1a
shows the typical planar morphology at the nanoscale. It is
composed of a monoatomic layer of porphyrin rings, in which they
are coordinatively linked together via Cu-O bond between 4-
connecting porphyrin units and Cu2 links (Figures 1b, c). Owing
to the ultrathin character, it is difficult to directly observe its
existence during TEM observation but we can clearly identify its
edge from HRTEM image (Figure 1d). As expected, PML will
exhibit surface corrugations and crumples when the size is
increased to the milliscale (Figure 1e). SEM images at larger
scales also show the 2D nature, while only surface crumples can
be observed at smaller scales (Figure S2). EDS mapping and line-
scanning spectra show the homogeneous distribution of C, Cu
and N throughout the monoatomic layer (Figure 1f and Figure S3).
(
These 2D monoatomic layers composed of one or two other
elements also achieve tremendous progress over the past few
years. [
10,11]
Recognizing the uniqueness of the single atomic layer,
it is necessary to explore novel monolayer 2D materials, which
may further reveal unexpected properties and innovative
opportunities.
+
As previously reported, the monolayer of porphyrin molecules
can be obtained on metal substrates and silicon substrates while
[
12-
the application is still in its infancy due to the discrete structure.
1
4]
Recently, the porphyrin-based ultrathin nanosheets with a
thickness of ~1 nm have emerged as a novel 2D materials for
photocatalysis and electrical capacitors.[15-17] Accordingly,
porphyrin-based monoatomic layer (PML) with a theoretical
thickness of 2.7 Å also holds great promise in various applications,
1
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COMMUNICATION
Approximately, we can identify PML as a carbon monolayer
orderly and completely doped with O, Cu and N elements. The
average thickness was measured to be 0.28 nm by atomic force
microscopy (AFM), in good agreement with the theoretical
thickness of 2.7 Å for single porphyrin ring (Figures 1g,h). More
interestingly, PML exhibits a graphene-like X-ray diffraction (XRD)
curve with a slight negative shift of 3° (Figure 1i), which is
assigned to a higher interlayer spacing (d-spacing: d=λ/2sinθ).[18]
All these evidences demonstrate that PML possesses the
graphene-like structure and properties.
adsorption-desorption isotherm of PML exhibits a type IV curve.
The pore size distribution in Figure S7b indicates the
simultaneous existence of both micropores (1.0 nm) and
mesopores (2.1 nm). The specific surface area of PML is 1254
2
2
m /g, similar as the value of 1374 m /g for graphene
nanosheets.[ Raman scattering is strongly sensitive to the
electronic structure and proves to be a powerful tool to
characterize one-atom-thick materials. In the Raman spectrum,
the structure-sensitive bands at 1563, 1355, 1238, 1078, 1014,
20]
830 and 389 cm have been identified (Figure S8). The 1355 cm-
-1
1
band is approximately assigned to the breathing mode of the
pyrrole C-N bonds. Another two intense polarized bands of PML
c
-1
b
are observed at 1563 and 407 cm , whose frequency pattern
a
parallels that of the 1355 cm-1 band. Furthermore, the short Cu-O
-
bond is fully consistent with the frequency of the Cu-O (389 cm
1
[21]
-1
)
.
In addition, the bands at 1238, 1078 and 1014 cm
correspond to ν(C
m
-Ph), δ(C
β
-H) and δ(CPh-H) patterns of
porphyrin units, respectively.[
22]
5
00 nm
a
b
2.7 Å
PML-Au
N 1s
Cu2p3/2
d
e
f
C
N
PML-Cu
TCCP-Cu
Cu2p1/2
2
+
Strong Cu
satellite
2
+
Strong Cu
satellite
Cu
11 00 nn mm
500 nm
0.4
390
395
400
405
410 970
960
950
940
930
h0.3
i
g
Bind Energy (eV)
Bind Energy (eV)
PML
0.2
Graphene oxide
c
d
0
.28 nm
0.1
3
0
2.23
CuCl2
0
.0
.4
0.25 nm
0
200
400
nm
600
800
PML-Au
0
Cu foil
PML-Au
0
0
0
0
.3
.2
.1
.0
20
CuCl2
10
0
200
400
nm
600
800
20
40
60
80
1.56
Cu foil
2
.02
Figure 1. a) TEM image of PML. b) The schematic model. c) Top and side views
of the molecular structure. d) HRTEM image. e) STEM image. f) EDS mapping.
g) AFM image. h) Corresponding height profiles. i) XRD patterns of PML and
graphene oxide.
0
8
800
9000
9200
Energy (eV)
9400
9600
0
2
4
6
8
10
R ( Å )
e
f
8
6
4
2
0
PML-Au
Fitting
1
.56
PML was synthesized using a facile solvothermal method, in
which
a
homogeneous solution of porphyrin monomers
, PVP, formic acid and
(
porphyrin-Cu or porphyrin-Au), Cu(NO )
3 2
o
DMF was heated at 80 C. Up to 100 mg PML powder was easily
obtained for each batch when scaling up the reaction (Figure S4).
The monolayer structure, molecular structure and composition
are investigated by various characterization techniques. The FTIR
spectrum show two strong characteristic bands at 1676 and 1614
-1
cm that are associated with ν8a of the N-substituted pyridinium
) and ν
of the pyridyl units, respectively (Figure S5).[12]
combination of ν(C -N) and δ(CCN) of the porphyrin ring
0
1
2
R (Å)
3
4
(
a
1
4
A
β
Figure 2. a) N 1s XPS spectra of PML-Cu, PML-Au and TCPP-Cu. b) Cu 2p
XPS spectrum of PML-Cu. c) The normalized XANES spectra of PML-Au, CuCl2
and Cu foil. d) FT-EXAFS spectra at Cu K-edge. e) The FT-EXAFS r space
fitting curve of PML-Au. f) Schematic models of the structural unit of PML-Au,
Au (yellow), N (green), C (gray), Cu (pink), O (red) and H (white).
-1
contributes to an additional vibration at 1406 cm . The strong
vibrations appear at 1003, 841 and 771 (658) cm-1 correspond to
α m β p
the vibration of C -C , ν(C -N) and Π of the porphyrin skeleton,
respectively. All intensities of these vibrations are in the region of
a few milliabsorbance units, which substantially indicates a typical
monoatomic layer.[12] UV-vis diffuse reflectance spectrum was
also performed to understand the optical properties. The PML
shows a strong Soret absorption at λmax=421 nm and a weak α-
band at 540 nm (Figure S6), indicating that the π electrons in PML
are highly delocalized.[19] Using the Beer-Lambert law A=εlc,
where A, ε, l and c are the absorbance, the extinction coefficient,
the thickness of PML and the surface density of porphyrin units,
respectively, the estimated surface density within PML is 0.31
X-ray photoelectron spectroscopy (XPS) confirms that C, O and
N are the main elements (Figure S9). The C, O, N and Cu atom
percentages of PML-Cu are determined to be 76.38%, 19.50%,
7.24% and 4.11% respectively by XPS analysis, very close to the
theoretical value of the model structure (Table S1). As depicted in
Figures 2a, the N 1s spectra of PMLs and tetrakis (4-
carboxyphenyl)-porphyrin Cu complex (TCPP-Cu) show
a
singular peak at 397.3 eV, which is ascribed to Cu-N
[16]
coordination.
And Cu2p1/2 characteristic peaks correspond to
2
porphyrin ring per 100 Å , which is almost same as the theoretical
Cu (Figures 2b).[23] X-ray absorption fine structure (XAFS)
2+
value.[ Attributed to intermolecular π-electron interactions in
12]
spectroscopy of PML-Au was measured to confirm the linkage
structure (Figure 2c). The Cu K-edge in the X-ray absorption near
edge structure (XANES) curve is similar to CuCl , confirming the
2
conjugated monoatomic layers, PML exhibits a broad half-width
of 33 nm for the Soret band. As shown in Figure S7a, N
2
2
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COMMUNICATION
oxidation state of Cu links is around +2. Figure 2d shows Fourier
transform (FT) k3-weighted EXAFS spectra of PML-Au at Cu K-
edge. Compared with reference samples (Cu foil and CuCl ),
2
of PML-Au. Furthermore, PML-Cu displays a maximum FECH of
4
11.51% at a -0.7 V, whereas FECH4 on PML-Au slightly increases
from 1.02% at -0.9 V (Figure 4d). In contrast, FECO on PML-Au
displays a volcano trend with a maximum of 34.40% at -0.8 V
while FECO on PML-Cu remains below 7% at the same potential
PML-Au only shows a prominent peak of Cu-O at 1.56 Å,
indicating the distance of linkage bond is much lower than that of
Cu-Cu (2.23 Å) and Cu-Cl (2.02 Å). Figure 2f shows the fitting
curve of linkage structure, which is perfectly reproduced by the
experimental FT-EXAFS data. As shown in Table S2, the
coordination number (Cu-O) is 4.6, the bond length is 1.96 Å, and
Figure 4e). Overall, we can clearly observe a decrease in HCOO-
(
and CH
4
production and an increase in CO production when the
. The metal center and applied
Cu-N center is replaced by Au-N
4
4
[24-26]
potential synergistically determine the product distribution.
2
the disorder is 0.0058 Å . Based on the above results and
a
b
previous study, we propose the linkage model of porphyrin units
in the inset of Figure 2e. Top and bottom Cu ions are respectively
5
0
-10
PML-Cu
PML-Au
coordinated with four carboxylic groups to form a Cu
paddle-wheel structure as the monomer-monomer node.
2
(COO)
4
PML-Au
PML-Cu
-
5
a
b
-
5
0
-10
-15
-20
-1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0
5
10
15
20
c
E/V (vs.RHE)
d
Z'/ohm
1
5
0
5
0
-
8
0
0
0
PML-Cu
PML-Au
HCOO
PML-Cu
PML-Au
CH4
5
00 nm
2 μm
1
6
4
2
c
d
0
-
1.0
-0.9
-0.8
-0.7
-0.6
CO
-1.0
-0.9
E/V (vs.RHE)
-0.8
-0.7
-0.6
0
.6
.4
.2
.0
E/V (vs.RHE)
e
4
f
0
0
0
0
PML-Cu
PML-Au
2.5
2.0
-4
-5
-6
0
.1 mm
2 μm
0
2
4
6
8
10 12
Reaction time (h)
30
Figure 3. PML synthesized at a) 1 h, b) 2 h and c) 12 h. The inset shows the
simulated growth curve. d) In-situ TEM image of bottom-up growth of PML.
1
.5
.0
2
0
0
0
1
To study the growth mechanism of PML, we characterized its
morphology and size at different reaction times using TEM. As
shown in Figures 3a-c, PML collected at 1 h, 2 h and 12 h show
an average size of 245 nm, 4.38 μm and 0.63 mm, respectively.
The simulated growth curve (inset in Figure 3c) indicates the
bottom-up growth. According to in-situ TEM observation, the
porphyrin monomers gradually coordinate with Cu2+ ions present
in DMF, which then crosslink at the sharp edge of PML. Finally,
the long-time polymerization guarantees the successful assembly
of porphyrin monomers into millimeter-size monolayer structure.
The catalytic proformance on PML can be easily tuned when
the metal centers are accurately tailored. Millimeter-size PML-Cu
1
0.5
.0
-
7
PML-Cu
4 5
0
-
1.0
-0.9
-0.8
-0.7
-0.6
0
1
2
3
Time (h)
E/V (vs.RHE)
Figure 4. a) LSV curves of PML-Cu (blue line) and PML-Au (red line) scanned
-
at 5 mV/s in CO
2
-saturated 0.1 M KHCO
3
. b) EIS curves. c) FEHCOO , d) FECH
4
and e) FECO on PML-Cu (blue line) and PML-Au (red line). f) Long-term stability
-
of PML-Cu and HCOO concentration curve for CO
for 5 h.
2
electrocatalysis at -0.9 V
and PML-Au were studied as catalysts for CO
2
electroreduction in
The durability of PML-Cu catalyst was operated for 5 h at -0.9
V (Figure 4f). Intriguingly, the catalyst exhibits a stable current
density of -3.88 mA/cm2 during the first hour and then a
the CO - saturated 0.1 M KHCO electrolyte. Figure S10 displays
2
3
the chronoamperograms of two catalysts at different potentials.
LSV measurements reveal that PML-Au and PML-Cu can reach
the current densities of 18.9 and 18.1 mA/cm2 at -1.2 V,
respectively (Figure 4a). Apart from the similar current densities,
the overlapping EIS curves also evidence their approximate
2
successive increase of cathodic current density (-4.46 mA/cm at
5 h). The increase in current density suggests more and more
active sites become accessible to catalysis after the activation
-
process. The HCOO concerntation was recorded at 1 h intervals
-
values of resistance (Figure 4b). According to the original GC and
to measure the stability for producing HCOO .The red line
1
manifests a linear curve relationship between the HCOO-
2 4
H NMR traces in Figure S11, H , CO and a trace amount of CH
are detected over PML-Au at -1.0 V, and the only liquid product is
HCOO by H NMR. FEs on PML-Au and PML-Cu for each
product are listed in Table S3, which are calculated by the
concerntations and operation time, and proves that PML-Cu can
-
1
-
steadily convert CO
2
to HCOO .
There are two types of electrocatalytically active sites in PML-
Cu catalyst: (1) Cu-N site and (2) Cu-O site. Thus, it is necessary
to identify which one contributes to the catalysis. To gain insight
into the role of Cu-N site and Cu-O site, the model structures of
Cu porphyrin unit and Au porphyrin unit are illustrated in Figures
a, b. The observed trend clearly evidences that both Cu-N site
corresponding calibration curves (Figures S12-14). Faradic
4
4
-
efficiencies of HCOO , CO and CH
4
at each potential are
-
discussed below. As depicted in Figure 4c, FEHCOO on PML-Cu
attains a maximum of 80.86% at -0.7 V, 30.79% higher than that
4
4
5
4
3
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Angewandte Chemie International Edition
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COMMUNICATION
current densities of TCPP-Cu, PML-Cu and MOF-Cu are -6.43, -
2
8
.86 and -16.81 mA/cm , respectively (Figure 6b). As shown in
H C O N Cu Au
a
b
Figure S16, MOF-Cu catalyst exhibits a typical nanocube
structure with a size of ~500 nm. However, the bulk structure
suppresses the mass transfer and Cu utilization. Immobilizing
porphyrin Cu complex onto the highly conductive carbon black
can be an effective strategy to solve the aforementioned
problems.[
33-36]
Due to more easily accessible reaction sites,
TCPP-Cu exhibits a higher current density. As a bridge between
MOF material and molecular complex material, PMLs inherit the
merits of heterogeneous and homogeneous catalysts.[ The
monoatomic layered structure will minimize the mass transfer
resistance and maximum the catalytic site exposure, which
37]
FECH4=11.51%
FECO=34.40%
benefits for achieving
electrocatalysis.
a
highest current density in CO
2
-
-
FEHCOO =80.86%
FEHCOO =40.07%
c
d
a
b
-1.0 V
2+
Cu
0
Cu
-1.0 V
0
-0.6 V
-0.6 V
-
5
OCV
OCV
TCPP-Cu
Cu foil
-10
-15
-20
Self-assembly
Cu foil
8
MOF-Cu
TCPP-Cu
PML-Cu
9
000
9200
Energy (eV)
Figure 5. The catalytic pathway on the M-N
9400
9600
0
2
4
6
10
R ( Å )
-
1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0
PML-Cu
c
MOF-Cu
4
and Cu-O
4
sites on a) PML-Cu and
d
E/V (vs.RHE)
b) PML-Au. c) Cu K-edge XANES spectra and d) Fourier-transformed Cu K- 100
edge EXAFS spectra of PML-Cu before and after 0.5 h of electrocatalysis at -
-
TCPP-Cu
PML-Cu
MOF-Cu
TCPP-Cu
PML-Cu
MOF-Cu
HCOO
20
CH4
0.6 V and -1.0 V.
80
1
5
0
5
0
and Cu-O
individual products. The Au-N
with high selectivity, which in turn low the FEHCOO on Cu-O
Figure 5a). In contrast to the Au-N site, the Cu-N site can
generate CH with relatively low activity. Therefore, the FE profile 20
of PML-Cu is largely dominated by HCOO generated on the Cu-
sites (Figure 5b).
Wang et al. firstly offered insight into the true active site of
heterogeneous molecular complexes catalysts during CO
4
site play vital roles in the catalytic selectivity of
6
0
4
sites contribute to CO generation
1
-
4
sites 40
(
4
4
4
-
O
4
0
-1.0
-0.9
-0.8
E/V (vs.RHE)
-0.7
-0.6
-1.0
-0.9
-0.8
E/V (vs.RHE)
-0.7
-0.6
2
Figure 6. a) The schemical models of TCPP-Cu, PML-Cu and MOF-Cu. b) LSV
. c) FEHCOO and d) FECH on TCPP-Cu,
PML-Cu and MOF-Cu at the potentials of -0.6 V to -1.0 V.
electroreduction.[
27-30]
The copper(II) phthalocyanine displays a
-
curves in CO
2
-saturated 0.1 M KHCO
3
4
reversible restructuring behavior, thereby achieving a record high
FECH of 66% at −1.06 V.[ We also note that CH
28]
and a minute
were detectable for PMLs, which suggests that
4
4
-
amount of C
2
H
4
FE of HCOO and CH on three catalysts are studied at Figures
4
6c, d. MOF-Cu catalyst presents volcano trend in FEHCOO- with a
the single Cu sites may be transformed into metallic copper
clusters at high potentials (Table S3). The Cu K signals of PML-
Cu catalyst before and after 0.5 h of electrocatalysis were
analyzed by XAFS measurements. The characteristic Cu(II) peak
at 8998.0 eV dominates the normalized Cu K-edge XANES
spectra of PML-Cu at open circuit voltage (OCV), -0.6 and -1.0 V,
indicating that most of the Cu sites remain +2 (Figure 5c).
However, an apparent decrease in the peak intensity is still
observed after electrocatalysis. Compared to Cu foil with a Cu–
Cu peak at 2.26 Å, PML-Cu only exhibits a prominent Cu-O peak
at R =1.56 Å (Figure 5d). As the potential increases to -0.6 V, a
small Cu-Cu peak appears at 2.26 Å. Meanwhile, a pair of CV
waves is also observed at around –0.6 V, which may correspond
maximum of 62.25% at -0.7 V, whereas no CH peak is detected
at all potentials. In contrast, TCPP-Cu mainly generates CO at low
potentials while CH4 becomes the dominant product at high
4
potentials (Table S4). More detailed FEs on previously reported
single-Cu-site catalysts are compared in Table S5.[
23,24,26-28,38-43]
It
is interesting to discover that CO2 molecules can be deeply
reduced to CH4 on the Cu-N4 center of copper-porphyrin
molecular catalysts while HCOO is the major product over the
copper-porphyrin MOF catalysts. These observations suggest
that the catalytic nature of Cu-N centers on PML-Cu is closer to
-
4
that of molecular catalysts rather than MOF catalysts. Therefore,
we infer that the monoatomic layered structure may determine the
0
1+
to redox between Cu and Cu (Figure S15). The peak area ratios
selectivity of Cu-N center.
4
0
2+
of Cu /Cu are 0, 0.51 and 0.61 at 0 V, -0.6 V and -1.0 V,
respectively. Thus, it is reasonable to conclude that the gradual
In summary, freestanding porphyrin-based monoatomic layers
can be scalably synthesized using a facile solutionthermal
method. The porphyrin monomers periodically link each other in
a monolayer limit and finally assemble into a millimeter-size PML
via a bottom-up approach. The dependence of product selectivity
on metal center has been demonstrated. PML containing Cu-N4
centers exhibits a highest FEHCOO- (80.86%) and FECH4 (11.15%)
centers mainly generates HCOO-
and CO (40.9% and 34.4% at -0.8 V, respectively). EXAFS
measurements confirm the irreversible restructuring behavior of
formation of metallic Cu species is responsible for C
2 4
H
production.
TCPP-Cu and zirconium porphyrinic metal-organic framework
with Cu-N centers (MOF-Cu) were also synthesized for
4
comparison.[
31,32]
As shown in Figure 6a, the TCPP-Cu monomers
can assemble into PML-Cu via the bottom-up growth, and the
multi-layed stack and overlap of PML-Cu can also construct MOF-
Cu, of which porphyrin-Cu is the structural unit. At -1.1 V, the
at -0.7 V while PML with Au-N
4
4
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Angewandte Chemie International Edition
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COMMUNICATION
Cu sites. Combining the monoatomic layer with the tunable open
metal centers, we can expect that PML will reveal novel and
unexpected properties, which may offer a series of innovative
opportunities for the further.
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Keywords: CO conversion • electrocatalysis • porphyrin-based
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COMMUNICATION
Table of Contents
Freestanding porphyrin-based monoatomic layer can be easily synthesized from the nanometer scale to the millimeter scale via the
bottom-up growth. Both its structure and property are analogous to graphene. Interestingly, the monoatomic layers show remarkable
metal center dependence of the catalytic activity and selectivity for CO
2
electroreduction.
D. Yang, S. Zuo, H. Yang, Y. Zhou, X. Wang*
6
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