Tricycloquinazoline-Based 2D Conductive Metal–Organic Frameworks as Promising Electrocatalysts for CO2 Reduction

Article abstract of DOI:10.1002/anie.202103398

2D conductive metal–organic frameworks (2D c-MOFs) are promising candidates for efficient electrocatalysts for the CO₂ reduction reaction (CO₂RR). A nitrogen-rich tricycloquinazoline (TQ) based multitopic catechol ligand was used to

Full text of DOI:10.1002/anie.202103398

Angewandte
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How to cite:
International Edition: doi.org/10.1002/anie.202103398
German Edition: doi.org/10.1002/ange.202103398
Electrocatalysis Very Important Paper
Tricycloquinazoline-Based 2D Conductive Metal–Organic Frameworks
as Promising Electrocatalysts for CO Reduction
2
+
+
+
Jingjuan Liu , Dan Yang , Yi Zhou , Guang Zhang, Guolong Xing, Yunpeng Liu, Yanhang Ma,*
Osamu Terasaki, Shubin Yang,* and Long Chen*
Abstract: 2D conductive metal–organic frameworks (2D c-
many research works have been documented focusing on the
MOFs) are promising candidates for efficient electrocatalysts
electrochemical reduction of CO into CO as the product
2
for the CO2 reduction reaction (CO RR). A nitrogen-rich
because of the efficient and simple reaction pathway. Never-
2
tricycloquinazoline (TQ) based multitopic catechol ligand was
used to coordinate with transition-metal ions (Cu and Ni ),
theless, transforming CO into other value-added chemicals
2
2
+
2+
or fuels, such as methane, ethane, methanol, and ethanol, is
thermodynamically sluggish due to the chemical passivation
of CO2 with a low standard molar Gibbs free energy of
which formed 2D graphene-like porous sheets: M (HHTQ)2
3
(
M = Cu, Ni; HHTQ = 2,3,7,8,12,13-Hexahydroxytricycloqui-
ꢀ
1
[4]
nazoline). M (HHTQ)2 can be regarded as a single-atom
formation (ꢀ394.4 kJmol ). Therefore, how to effectively
3
catalyst where Cu or Ni centers are uniformly distributed in the
hexagonal lattices. Cu (HHTQ) exhibited superior catalytic
activate inert CO molecule is a critical scientific issue. In
2
addition, compared with the two-electron involved process
for producing CO, converting CO2 into other C1 or C2
3
2
activity towards CO RR in which CH OH is the sole product.
2
3
[
5]
The Faradic efficiency of CH OH reached up to 53.6% at
products usually involves multiple electron transfer. For
example, electrochemically reducing CO into CH OH re-
3
a small over-potential of ꢀ0.4 V. Cu (HHTQ) exhibited larger
3
2
2
3
CO adsorption energies and higher activities over the
quires six-electron and six-proton transfer process: CO +
2
2
ꢀ
+
ꢀ
isostructural Ni (HHTQ) and the reported archetypical Cu -
6H + 6e !CH OH + H O
or
CO + 5H O + 6e !
3
2
3
3
ꢀ
2
2
2
(
HHTP) . There is a strong dependence of both metal centers
CH OH + 6OH , which bears an endothermic process, and
2
3
and the N-rich ligands on the electrocatalytic performance.
consequently features high overpotential, poor selectivity,
and low efficiency. In this context, developing efficient
[
4]
Introduction
electrocatalysts capable of converting CO into other valuable
2
liquid chemicals with high selectivity remains challenging.
Thus far, various heterogeneous catalysts, including
Overwhelmingly combusting fossil fuels has triggered
horrible environmental problems, for example, global warm-
ing which is severely impairing the ecosystem, mainly due to
[6]
[7]
[8]
metals, metal oxides, and graphene-based materials,
have been applied for CO RR to obtain value-added
2
[
1]
excessive CO emission. As such, great efforts have been
chemicals with high electrocatalytic activity. For large-scale
and practical applications, cost-effective transition metal
based electrocatalysts have been extensively investigated for
2
[2]
devoted to dispose and utilize CO2 in recent years. In
particular, converting CO into valuable chemicals driven by
2
electricity represents a viable strategy to accelerate carbon
recycling and alleviate the environmental problems. To date,
CO RR, among which Cu-based ones show promising
catalytic performance. However, the large over-potentials
2
[
3]
[9]
significantly hinder their widespread applications. Further-
more, preventing metal–cluster aggregation and accurately
controlling metal-particle sizes are challenging and are
[
+]
[
*] J. Liu, Dr. G. Zhang, Dr. G. Xing, Prof. L. Chen
Department of Chemistry, Tianjin Key Laboratory of Molecular
Optoelectronic Science, Tianjin University
Tianjin 300072 (China)
prerequisite to achieve highly efficient CO RR. A range of
2
strategies have been developed to boost the catalytic effi-
ciency by immobilizing metal atoms on the surface, pores, or
skeleton of porous materials, for example, MOFs and
covalent organic frameworks (COFs) to mediate single-atom
E-mail: long.chen@tju.edu.cn
[
+]
D. Yang, Prof. S. Yang
School of Materials Science and Engineering, Beihang University
Beijing 100191 (China)
[
10]
catalysts.
In particular, dispersing active metal ions uni-
E-mail: yangshubin@buaa.edu.cn
formly on the skeletons of two-dimensional (2D) MOFs
[
+]
Y. Zhou, Prof. Y. Ma, Prof. O. Terasaki
School of Physical Science and Technology, ShanghaiTech University
Shanghai 201210 (China)
represents a promising strategy to render superior CO RR
2
performance.
As a burgeoning subclass of 2D materials, 2D c-MOFs are
constructed by self-assembling transition metallic nodes with
conjugated organic ligands containing multitopic ortho-sub-
E-mail: mayh2@shanghaitech.edu.cn
Dr. Y. Liu
Institute of High Energy Physics, Chinese Academy of Sciences
Beijing 100049 (China)
[11]
stituted functional groups (-NH , -OH, -SH, or -SeH).
2
+
[
Thanks to the strong in-plane d-p conjugations and compact
out-of-plane p-p interactions, the 2D lattices are orderly
arranged to form layer-stacked periodic frameworks with
regular open channels, inherent porosities, high charge-
] These authors contributed equally to this work.
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202103398.
&
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carrier mobilities, exciting electrical conductivities, and tune-
insights into designing novel 2D c-MOFs as promising
[12]
able catalytic/redox active sites. These merits make 2D c-
electrocatalysts for CO RR through modulating the interplay
2
[13]
MOFs advantageous for electrocatalytic reduction of CO2.
Although several reports have successfully reduced CO into
of both the ligands and metal centers properly.
2
[
14]
CO using conductive MOFs as electrocatalysts, 2D c-MOF-
mediated CO -to-methanol transformation remains challeng- Results and Discussion
2
ing due to the complex electron-transfer pathways and high
energy barriers. To develop high-performance 2D c-MOF-
based electrocatalysts, both ligand engineering and judicious
selection of metals should be considered.
A solvothermal reaction between HHTQ and hydrated
Cu(NO3)2 or Ni(OAc)2 in a mixture of water and DMF
afforded crystalline M (HHTQ) (Figure 1a). Scanning elec-
3
2
Herein, the nitrogen-rich and electron-deficient tricyclo-
quinazoline (TQ) moiety was selected to integrate into the 2D
tron microscopy (SEM) reveals Cu (HHTQ) consists of rod-
3 2
like microcrystals (length: 1–10 mm), while Ni (HHTQ)
3
2
[15]
MOF skeleton for facilitating CO capture and catalysis.
appears as polycrystalline without a well-defined morphology
(Figure S1). The crystallographic information of M (HHTQ)
2
Accordingly, a C -symmetric 2,3,7,8,12,13-hexahydroxytricy-
3
3
2
[
16]
clo-quinazoline (HHTQ)
was synthesized to coordinate
was resolved through Pawley refinement of powder X-ray
diffraction (PXRD), three-dimensional electron diffraction
tomography (3D-EDT), high-resolution transmission elec-
tron microscopy (HRTEM), and selected-area electron
diffraction (SAED) methods. It can be inferred that both
M (HHTQ) are isostructural with little differences in unit
2
+
2+
with Ni and Cu respectively in square-planar geometry,
resulting in nicely honeycomb-like networks with uniform
hexagonal pores (Figure 1a). The resulted 2D MOFs, that is,
M (HHTQ) (M = Cu or Ni) feature high metal contents of
3
2
nearly 20% (wt) with metal ions regularly anchored and
distributed in the lattices, which suggests their potential as
single-atom catalysts. To our delight, Cu (HHTQ) exhibits
3
2
cell parameters, as revealed by the similarity of their PXRD
2
+
2+
patterns and similar ionic size between Ni and Cu . The
reconstructed 3D reciprocal lattice by 3D-EDT indicates the
unit cell parameters of Cu (HHTQ) are a = b = 26.10 ꢀ, c =
3
2
prominent activity toward electrochemical reduction of CO2
into methanol with high selectivity (up to 53.6%), decent
efficiency and good durability. In sharp contrast, the isostruc-
tural Ni (HHTQ) and the well-known archetypical Cu3-
3
2
3.34 ꢀ, and a = b = 908, g = 1208 (Figure 1h and Figure S2),
which are further refined to be a = b = 25.30 ꢀ, c = 3.23 ꢀ,
and a = b = 908, g = 1208 (Figure 1b) through Pawley refine-
ment against experimental PXRD pattern. The coordination
3
2
[17]
(
HHTP)2
showcased much worse CO RR performance.
2
The reason for the big performance variation and underlined
mechanism were further unraveled by systematic measure-
ments and computational studies. This work provides new
2
+
2+
between Ni /Cu and the triangular HHTQ ligands may
form honeycomb-like 2D nets with pore size of around 2.3 nm
Figure 1. a) Synthesis of M (HHTQ) (M=Cu, Ni). b) Experimental and Pawley refined PXRD patterns of Cu (HHTQ) . c) A structural model of
3
2
3
2
Cu (HHTQ) . C gray, N blue, O red, Cu purple, H white spheres. d) HRTEM image of Cu (HHTQ) taken along the c axis. e) Zoom-in view of
3
2
3
2
HRTEM image for Cu (HHTQ) taken along the c axis that shows a hexagonal pore and ligand termination, overlaid with a structure model.
3
2
ꢀ
f) Simulated TEM image along [001] direction. g) HRTEM image along ½120ꢁ direction. h) Projection view of 3D EDT data of Cu (HHTQ) along
3
2
[001] direction.
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ꢀ
using different space groups (P6/m or P62m), where the main
state as that of NiO (+ 2). In addition, Fourier transform (FT)
difference is the alignment of N atoms in two linkers that
connected to the same metal (Figure 1c; Supporting Infor-
mation, Figure S3). However, these two possibilities are hard
to be distinguished, so the one with higher symmetry P62m is
adopted for illustration and calculation in this work.
Moreover, HRTEM images and SAED patterns along
spectra of extended X-ray absorption fine structure (EXAFS)
3
oscillation k c(k) for M (HHTQ) and their corresponding
3
2
fitting parameters are also given in the Supporting Informa-
tion, Figures S10, S11 and Tables S1, S2. The first coordina-
tion peak of Cu (HHTQ) corresponds to the CuꢀO bond
ꢀ
3
2
with a distance of 1.93 ꢀ (2.05 ꢀ for NiꢀO) and the second
coordination peak is attributed to Cu···C interaction with
a distance of 2.67 ꢀ (2.69 ꢀ for Ni···O). Inspiringly, metallic
Cu or Ni are absent in these complexes. Therefore, the XAFS
spectra provide strong evidence on the formation of MO4
complexes in these M (HHTQ) .
[
001] direction show a honeycomb-like pore arrangement
throughout the whole nanocrystal with d100 = d010 = 21.90 ꢀ,
which is consistent with the crystal structure and simulation
(
Figure 1d–f; Supporting Information, Figure S4). It is noted
that the crystal morphology also suggests the possibility for
orthorhombic symmetry with the same structure. Although
the EM observation of MOF materials remains challenging
3
2
The chemical compositions of M (HHTQ) were surveyed
3
2
by Fourier-transform infrared spectroscopy (FTIR; Support-
ing Information, Figure S12). The significant attenuation of
[
18]
due to the severe beam-sensitivity, the good crystallinity
and conductivity enable the clear observation of crystal edge
with ligands as termination (Figure 1e; Supporting Informa-
tion, Figure S5). SAED patterns and HRTEM images of
Cu (HHTQ) nanocrystals perpendicular to c axis reveal an
ꢀ
1
O-H stretching vibration above 3000 cm suggests successful
coordination between metal ions and catechol ligands. Be-
sides, the characteristic peaks of TQ moiety locate at around
ꢀ1
1641, 1594, 1316, and 1262 cm . XPS spectra of Cu (HHTQ)
2
3
2
3
AA packing with an interlayer spacing of 3.3 ꢀ (Figure 1h;
Supporting Information, Figures S6, S7). Especially, the
and Ni (HHTQ) (Supporting Information, Figures S13, S14)
3 2
disclose the signals of C 1s, N 1s, O 1s, and M (Cu and Ni) 2p.
Inspecting high-resolution regions of M (2p) and O (1s)
indicates the formation of square-planar metal bis(dihydroxy)
complexes. The peak of Cu 2p 2/3 at 934.5 eV reveals the + 2
state of Cu. The bands of O 1s for Cu (HHTQ) at 531.13 and
distance between adjacent layers corresponding to d001
=
3
1
.3 ꢀ can be observed in the HRTEM image along the
20 zone axis (Figure 1g). Considering all of these, Cu3-
Â
Ã
ꢀ
(
HHTQ) are resolved to be honeycomb-like 2D networks
2
3
2
which are further stacked along the c axis by p-p interactions
and thereby affording regular and open 1D hexagonal
channels along the [001] direction. To further unravel the
valency states of coordinated metal ions and their atomic
neighboring structure, the X-ray absorption fine-structure
532.76 eV are assignable to oxygens from ligand and CuO4
segments, respectively (530.52 and 532.04 eV for Ni3-
(HHTQ) ). The high-resolution N 1s and C 1s spectra
2
manifest the existence of TQ core. Electron paramagnetic
resonance (EPR) spectrum of Cu (HHTQ) shows a strong
3
2
(
XAFS) spectra were collected at the Cu/Ni K edges (Figure 2
signal at g = 2.07 which corresponds to the characteristic
a,b; Supporting Information, Figure S9). Cu (HHTQ) dis-
metal-centered radicals, while no radical is detected in
3
2
plays almost the same white-line peak as that of CuO, which
Ni (HHTQ)2 (Supporting Information, Figure S15). The
3
suggests Cu ions exhibit + 2 oxidation state in Cu (HHTQ) .
porosities of M (HHTQ) were assessed by N adsorption/
3
2
3
2
2
Similarly, Ni ions in Ni (HHTQ) feature the same oxidation
desorption isotherms at 77 K, which reveal the Brunauer–
Emmett–Teller (BET) surface areas of Cu (HHTQ) and
3
2
3
2
2
ꢀ1
Ni (HHTQ) are 104 and 311 m g , respectively (Supporting
3
2
Information, Figure S16). Thermogravimetric analysis (TGA)
suggests both Cu (HHTQ) and Ni (HHTQ) display pro-
3
2
3
2
nounced weight losses above 2008C, probably due to the
structure decomposition (Supporting Information, Fig-
ure S17).
Interestingly, solid-state diffuse reflectance UV-vis-NIR
spectra of Cu (HHTQ)2 and Ni (HHTQ)2 powders show
3
3
broad absorption extending to the NIR region, corresponding
to small band gaps of around 1.33 and 1.15 eV respectively
owing to the strong p–d (in-plane) and p–p (out-of-plane)
conjugations for both MOFs (Supporting Information, Fig-
ure S18). We further evaluated electrical conductivities of
both MOFs in bulk samples with two-probe method. The
conductivities of Cu (HHTQ) and Ni (HHTQ) are about
3
2
3
2
ꢀ5
ꢀ1
ꢀ4
ꢀ1
2
.74 ꢂ 0.15 ꢁ 10 Scm and 2.27 ꢂ 0.19 ꢁ 10 Scm , respec-
tively under ambient conditions, which show negligible
change regardless of the pellet thickness (Supporting Infor-
mation, Figure S19). Noteworthily, a very interesting aniso-
tropic transport behavior was just recently demonstrated by
Dinc a˘ and co-workers based on the single-crystal conductivity
measurements via four-probe method showing much higher
Figure 2. a) XANES spectra of Cu K-edge for the standard Cu foil,
Cu O, CuO, and Cu (HHTQ) sample. b) Wavelet transforms of Cu -
2
3
2
3
(
HHTQ) and the standard Cu foil and CuO samples. c) Temperature-
2
ꢀ
1 [19]
dependent conductance of Cu (HHTQ) and Ni (HHTQ) pellets.
conductivity beyond about 10 Scm .
Furthermore, the
3
2
3
2
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conductivities exhibit nonlinear enhancement with increasing
only detected product throughout the entire range of
temperature, which implies a semiconducting nature (Fig-
ure 2c). The decent conductivities of both 2D c-MOFs
suggest their prospects in electrocatalysis.
potentials. In addition, the CO RR performance is strongly
2
dependent on the type of metal centers, organic linkers, and
the applied potentials. As shown in Figure 3c, Cu (HHTQ)
3
2
To investigate the performance of M (HHTQ)2 for
catalyst showcases the best CO RR performance among the
3
2
electrocatalytic CO RR, we performed the initial tests for
others and delivers the highest Faradic efficiency toward
2
Cu (HHTQ) , Ni (HHTQ) and Cu (HHTP)2 in a three-
CH OH (FE
CH3OH
about 100 times larger than those of Ni (HHTQ) and
3 2
) of 53.6% at ꢀ0.4 V vs. RHE, which is
3
2
3
2
3
3
electrode cell containing aqueous KHCO solution (0.1 M) as
3
electrolyte and 2D c-MOF/acetylene black (2:1, wt) compo-
Cu (HHTP)2 with only 0.54% and 0.15%, respectively.
3
site loaded on carbon fiber paper as the working electrode.
Besides, Ni (HHTQ)2 and Cu (HHTP)2 render the best
3
3
The linear sweep voltammetry (LSV) curve for Cu (HHTQ)2
FECH3OH of 5.4% and 1.9% respectively at ꢀ0.3 V vs. RHE
3
in CO -saturated electrolyte exhibits higher current densities
(Supporting Information, Figures S22, S23). Tafel slopes were
2
compared to that in Ar-saturated solution with a scan rate of
further used to evaluate the kinetics of CO RR catalyzed by
2
ꢀ
1
5
mVs (Figure 3a). Besides, in CO -saturated electrolyte,
these 2D c-MOFs. Cu (HHTQ) showcased the lowest Tafel
2
3
2
ꢀ
1
Cu (HHTQ) delivers the highest current densities compared
slope of 285.87 mVdec toward CH OH production com-
3
pared to that of Ni (HHTQ) (310.05 mVdec ) and Cu -
3 2 3
(HHTP)2 (305.7 mVdec ) (Supporting Information, Fig-
ure S24), further confirming its faster kinetics. Additionally,
the TON of Cu (HHTQ) , Ni (HHTQ) and Cu (HHTP) are
3
2
ꢀ
1
to Ni (HHTQ) and Cu (HHTP) , with a current density of
3
2
3
2
ꢀ2
ꢀ1
4
5 mAcm at ꢀ1.2 V (vs. RHE) (Figure 3b). These results
suggest the great potential of Cu (HHTQ) as promising
3
2
electrocatalyst for CO RR. Furthermore, the electrocatalytic
2
3
2
3
2
3
2
CO RR process using Cu (HHTQ) , Ni (HHTQ) and Cu -
calculated as 353, 30, and 38, respectively. Besides, Cu3-
2
3
2
3
2
3
(
HHTP) at different potentials (from ꢀ0.2 to ꢀ0.8 V vs.
(HHTQ) demonstrates the lowest overpotential of ꢀ0.4 V,
2
2
RHE) were investigated. The products were analyzed by gas
chromatography (GC) and nuclear magnetic resonance
which indicates its excellent energy efficiency for the electro-
chemical conversion of CO and is superior to most Cu- and
2
(
NMR) spectroscopy. Inspiringly, besides H , CH OH is the
MOFs-based electrocatalysts (see the Supporting Informa-
tion, Table S4 for a detailed comparison). The catalytic
2
3
durability of Cu (HHTQ)2 was evaluated by long-term
3
electrolysis process and the results indicated that a stable
CO RR performance of Cu (HHTQ) was maintained with-
2
3
2
out notable degeneration for at least ten hours at ꢀ0.4 V vs.
RHE (Figure 3e).
To understand the obvious difference on the catalytic
performance between Cu (HHTQ)2 and the isostructural
3
Ni (HHTQ) as well as the archetypical analogue of Cu -
3
2
3
[
17]
(
HHTP)2, the CO and proton adsorption capabilities of
2
MO sites in these three 2D c-MOFs were estimated through
4
density functional theory (DFT) calculations. As depicted in
Figure 4b, Cu (HHTQ) displayed the strongest CO adsorp-
3
2
2
Figure 3. CO RR performance of Cu (HHTQ) , Ni (HHTQ) , Cu -
2
3
2
3
2
3
(
HHTP) in 0.1 M KHCO . a) LSV curves for Cu (HHTQ) in Ar- (grey
2 3 3 2
line) or CO - (orange line) saturated 0.1 M KHCO electrolyte with
2
3
ꢀ1
a scan rate of 5 mVs . b) LSV curves of Cu (HHTQ) , Ni (HHTQ) ,
Figure 4. a) Atomistic structure of Cu (HHTQ) , Ni (HHTQ) , and
3 2 3 2
3
2
3
2
Cu (HHTP) in CO -saturated 0.1 M KHCO electrolyte with a scan rate
Cu (HHTP) . C gray, N blue, O red, H white, Cu orange, Ni lavender.
3 2
3
2
2
3
ꢀ1
of 5 mVs . c) CH OH FEs for Cu (HHTQ) , Ni (HHTQ) , Cu (HHTP)
2
The dark dashed circles indicate the catalytic active sites with MO4
units. The orange dashed circles indicate the nitrogen-rich organic
cores. b) Free-energy profiles of CO on MO units in the Cu (HHTQ) ,
3
3
2
3
2
3
at different potentials. d) The selectivity for each product (CH OH and
3
H ) for Cu (HHTQ) at different potentials. (e) Chronoamperometric
2
3
2
2
4
3
2
measurements and CH OH FEs for Cu (HHTQ) at ꢀ0.4 V vs. RHE
Ni (HHTQ) , Cu (HHTP) , respectively. c) Free-energy profiles of H on
3 2 3 2
3
3
2
with 10-hour tests.
MO units of Cu (HHTQ) , Ni (HHTQ) , Cu (HHTP) , respectively.
4
3
2
3
2
3
2
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tion ability with Gibbs free energy (DG) of ꢀ0.24 eV among
the three MOFs, followed the order of Ni (HHTQ) (DG:
second hydrogenation occurs at an oxygen atom to generate
*HCOOH with an energy release of 0.88 eV, rather than
3
2
ꢀ
0.19 eV) and lastly Cu (HHTP) (DG: ꢀ0.02 eV). More-
occurring at carbon atom to form *OCH O involving 1.32 eV
3
2
2
over, theoretical results suggest that it is not favorable to
adsorb proton for all three MOFs (Figure 4c). Thus, Cu3-
energy increase, which indicates converting *OCHO into
*HCOOH intermediate on Cu (HHTQ) is thermodynami-
3
2
(
HHTQ) represents the best CO RR electrocatalyst among
cally favorable. In the third step, hydrogenation occurs at an
2
2
the three MOFs, which exhibits lowest HER energy barrier
and fastest proton/electron transfer kinetics.
oxygen atom with eliminating a H O molecule and forming
2
a *CHO intermediate, which is accompanied by the free
energy uphill with the value of 0.67 eV. In this step, generating
*OCH moiety seems unfavorable due to its higher energy
DFT calculations were further performed to provide
insights into the mechanism of electrochemical production of
CH OH catalyzed by Cu (HHTQ) . A monolayer lattice of
+
ꢀ
injection than *CHO. Subsequently, the fourth H /e pair
3
3
2
Cu (HHTQ)2 was applied as the model to optimize the
reacts with *CHO to afford *CH O with a decreased energy
3
2
structures of the intermediates involved in the six-proton/six-
electron (6H /6e ) transfer process toward electrochemical
of 0.17 eV. The other possible product of this step is *CHOH,
which however, bears a relatively high energy injection of
+
ꢀ
+
ꢀ
reduction of CO2 into CH OH. The Gibbs free energies
1.09 eV. Afterwards, the reaction between the fifth H /e and
*CH O produces *CH OH with an energy reduction by
3
diagrams for optimized configurations of intermediates at
each step are summarized in Figure 5a, and the by-products
are also taken into account. The optimized structures at each
step with the lowest energy are shown in Figure 5b. Firstly,
two possible reaction pathways are proposed for the first
2
2
0.56 eV, in which the potential product of *CH O seems not
3
likely to dictate due to its obvious energy uphill of 0.96 eV.
Next, *CH OH is obtained with the reaction between the
3
ꢀ
+
sixth H /e pair and *CH OH with an energy release of
2
+
ꢀ
proton-electron (H /e ) pair transfer process. From an energy
0.33 eV. The final step is the desorption of CH OH from
3
+
ꢀ
point of view, a H /e attacking the carbon atom of CO to
Cu (HHTQ)2 with an energy increase by 0.21 eV. Conse-
2
3
form *OCHO intermediate renders a relatively lower energy
increase than that on oxygen atom to form *COOH (0.70 vs.
quently, the most favorable reacting pathway for Cu3-
(HHTQ) -electrocatalyzed CO RR is shown as follows: CO -
2
2
2
0
.72 eV). This *CO !*OCHO step is the potential determin-
(g)!*OCHO!*HCOOH!*CHO!*CH O!*CH OH!
2
2
2
ing step (PDS) during the CO RR process. Afterwards, the
*CH OH!* + CH OH (inset of Figure 5a). The correspond-
2
3
3
Figure 5. a) Free energy profiles for the CO RR on Cu (HHTQ) . The proposed catalytic mechanism for electrochemical reduction of CO to
2
3
2
2
CH OH by CuO sites of Cu (HHTQ) is shown at the bottom. b) Structures of the catalyst and key reaction intermediates involved in the
3
4
3
2
proposed reaction mechanism for the CO RR on Cu (HHTQ) . The bond lengths and distances are labeled in ꢀ. * represent to chemisorbed
2
3
2
species. c) Cu K-edge XANES spectra of Cu foil, Cu O, CuO, and Cu (HHTQ) samples before and after CO RR. d) Cu K-edge EXAFS oscillations
2
3
2
2
3
k c(k) for Cu foil, Cu O, CuO, and Cu (HHTQ) before and after CO RR. e) Cu K-edge FT spectra of Cu foil, Cu O, CuO, and Cu (HHTQ) before
2
3
2
2
2
3
2
and after CO RR.
2
&
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ing calculations for Ni (HHTQ) and Cu (HHTP) electro-
Keywords: CO RR · conductivity · electrocatalysis · metal–
3
2
3
2
2
catalysts are summarized as Section 15 in Supporting Infor-
mation.
organic frameworks · methanol
The structural stability of Cu (HHTQ) after long-term
3
2
testing was confirmed by the unchanged PXRD pattern, FT-
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3
504.
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2
[
12f]
(
Supporting Information, Figure S33).
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[
II
XAFS spectra disclose the oxidation state of Cu within
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3
2
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I
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4
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Conclusion
7
We successfully developed tricycloquinazoline based 2D
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2
2
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2
Cu (HHTQ)2 as electrocatalyst for CO RR shows high
3
2
selectivity toward CH OH with Faradic efficiency up to
3
5
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2 2 3
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roles in tuning the catalytic activity and selectivity towards
[
efficient electrochemical CO RR.
2
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[
Acknowledgements
This work was financially supported by the National Key
Research
and
Development
Program
of
China
(
2017YFA0207500), National Natural Science Foundation of
China (51973153, 92045303). This is also supported by the
center for high-resolution electron microscopy (C ꢀh EM),
ShanghaiTech University (EM02161943).
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3
Conflict of interest
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The authors declare no conflict of interest.
Angew. Chem. Int. Ed. 2021, 60, 2 – 9
ꢀ 2021 Wiley-VCH GmbH
www.angewandte.org
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Angewandte
Research Articles
Chemie
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Manuscript received: March 9, 2021
Version of record online: && &&
,
&&&&
&
&&&
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Angewandte
Research Articles
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Research Articles
Electrocatalysis
Tricycloquinazoline (TQ) based 2D con-
ductive MOFs were developed and
J. Liu, D. Yang, Y. Zhou, G. Zhang,
G. Xing, Y. Liu, Y. Ma,* O. Terasaki,
employed as electrocatalysts for convert-
ing CO into CH OH. Cu (HHTQ) with
2
3
3
2
S. Yang,* L. Chen*
&&&& — &&&&
electron-deficient but nitrogen-rich TQ
cores displays the best electrocatalytic
Tricycloquinazoline-Based 2D Conductive
Metal–Organic Frameworks as Promising
CO RR performance, showing high
2
selectivity toward CH OH with Faradic
3
Electrocatalysts for CO Reduction
efficiency up to 53.6% at a low over-
potential of ꢀ0.4 V and maintained good
durability.
2
Angew. Chem. Int. Ed. 2021, 60, 2 – 9
ꢀ 2021 Wiley-VCH GmbH
www.angewandte.org
&&&&
These are not the final page numbers!