Nanoporous Gold Embedded ZIF Composite for Enhanced Electrochemical Nitrogen Fixation

Article abstract of DOI:10.1002/anie.201909770

The electrochemical nitrogen reduction reaction (NRR) offers an energy-saving and environmentally friendly approach to produce ammonia under ambient conditions. However, traditional catalysts have extremely poor NRR performances because of their low activity and the competitive hydrogen evolution reaction. The high catalytic activity of nanoporous gold (NPG) and the hydrophobicity and molecular concentrating effect of the zeolitic imidazolate framework-8 (ZIF-8) were incorporated in the NPG?ZIF-8 nanocomposite so that the ZIF-8 shell could weaken hydrogen evolution and retard reactant diffusion. A highest Faradaic efficiency of 44 % and an excellent rate of ammonia production of (28.7±0.9) μg h^?1 cm^?2 were achieved, which are superior to traditional gold nanoparticles and NPG. Moreover, the composite catalyst shows high electrochemical stability and selectivity (98 %). The superior NRR performance makes NPG?ZIF-8 one of the most promising water-based NRR electrocatalysts for ammonia production.

Full text of DOI:10.1002/anie.201909770

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Accepted Article
Title: Nanoporous Gold Embedded ZIF Composite for Enhanced
Electrochemical Nitrogen Fixation
Authors: Yijie Yang, Shu-Qi Wang, Haoming Wen, Tao Ye, Jing Chen,
Cheng-Peng Li, and Miao Du
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201909770
Angew. Chem. 10.1002/ange.201909770
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COMMUNICATION
Nanoporous Gold Embedded ZIF Composite for Enhanced Elec-
trochemical Nitrogen Fixation
Yijie Yang, Shu-Qi Wang, Haoming Wen, Tao Ye, Jing Chen, Cheng-Peng Li,* and Miao Du*
Abstract: Electrochemical nitrogen reduction reaction (NRR) offers
an energy-saving and environmentally-friendly approach to produce
ammonia under ambient condition. However, the traditional catalysts
have extremely poor NRR performances due to their low activity and
competitive hydrogen evolution reaction. In virtue of the high catalyt-
ic activity of nanoporous gold (NPG), as well as the hydrophobicity
and molecular concentrating effect of zeolitic imidazolate framework-
tive hydrogen evolution reaction (HER), resulting in unsatisfacto-
ry efficiency of NRR.[10] Thus, enormous efforts have been made
to suppress the occurrence of HER, including the utilization of
alloy nano-catalysts,[ constructing the hydrophobic surfaces,[12]
and incorporating Li+.[13] Nevertheless, it has become extremely
urgent to design the electrocatalytic systems that can simultane-
ously promote the N–H reaction for NRR and hinder the proton-
proton or proton-water interaction for HER.
11]
8
(ZIF-8), the NPG@ZIF-8 nanocomposite has been designed and
synthesized, in which the ZIF-8 shell can weaken hydrogen evolution
Metal–organic frameworks (MOFs) are a burgeoning type of
porous crystalline materials,[14] among which zeolitic imidazolate
frameworks (ZIFs)[15] are an extensive subclass with great appli-
cations in chemocatalysis of a variety of reactions.[16] However,
the electrocatalytic activity of ZIFs is generally poor due to fewer
active sites and lower electrical conductivity. Nevertheless, ZIFs
can serve as the matrix for immobilizing other active electrocata-
lysts.[17] First, their porosity facilitates the concentration of chem-
ical species near the active sites with less diffusion in electrolyte.
Second, the high chemical stability of ZIFs sustains the structur-
al integrity of catalysts. Moreover, most ZIFs show good hydro-
and retard the reactant diffusion. Significantly, the highest Faradaic
–
efficiency of 44% and excellent ammonia yield of (28.7 ± 0.9) ȝgxh
1
–2
xcm are achieved, which are obviously superior to traditional gold
nanoparticle and NPG. Moreover, the composite catalyst shows high
electrochemical stability and selectivity (98%). Notably, the superior
NRR performance places NPG@ZIF-8 in the most promising water-
based NRR electrocatalysts for ammonia production.
Ammonia is a key commodity chemical as the nitrogen source of
fertilizers and the carrier for fuels or clean energy, also providing
the feedstock to various chemical products.[1] Traditional Haber-
Bosch technique for ammonia synthesis involves the reaction of
high-purity nitrogen and hydrogen under harsh conditions,[2] thus
resulting in low efficiency, great energy consumption, and large
amount of greenhouse gases. Due to the chemical inertness of
nitrogen, nitrogen reduction reaction (NRR) at room temperature
and pressure is a great challenge.[3] Encouragingly, electrocata-
lytic NRR provides a sustainable method,[4] in which some metal
catalysts, such as Ru,[5] Pt,[6] and Au,[7] show better NRR activi-
ties than non-metal catalysts, owing to their stronger bonding to
chemicals and higher electrical conductivity. In this context, NRR
on Au is demonstrated to follow the associative mechanism, that
is, the breakage of NŁN bond and the hydrogenation of nitrogen
atom occur concurrently, thus realizing good NRR performance.
For example, Au tetrahexahedral particles and sub-nanoclusters
show nice NRR performance with Faradaic efficiency of 6% and
[18]
phobic properties, which thus are effective to suppress HER.
For example, ZIF-71 film has been introduced over Ag-Au plat-
[12]
form to reduce HER, achieving the Faradaic efficiency of 18%.
However, the catalytic activity is still to be improved for practical
applications and the use of tetrahydrofuran media in this case is
also not environmentally-friendly.
8%, due to more active sites on multifaceted surfaces and sub-
nano sites, respectively. However, the catalytic capacity of tradi-
tional Au has yet to be improved for large-scale ammonia syn-
thesis, mainly due to the following reasons. First, the low density
of active sites and serious aggregation render a poor electrocat-
alytic activity of Au particles.[8] In comparison, nanoporous gold
Scheme 1. Nitrogen reduction enhancement by NPG@ZIF-8 electrocatalyst.
(
NPG) shows superior electrocatalysis, owing to its high density
[
9]
of active sites distributed on the interconnected ligaments. On
the other hand, the reduction of nitrogen is inhibited by competi-
Herein, we report the design and preparation of a core-shell
nanostructured NPG@ZIF-8 composite (see Scheme 1) for high-
efficiency NRR electrocatalysis under ambient condition. Signifi-
cantly, NPG@ZIF-8 promotes the ammonia yield to (28.7 ± 0.9)
ȝgxh xcm under a potential of –0.8 V vs reversible hydrogen
electrode (RHE) and Faradaic efficiency to 44% under a poten-
tial of –0.6 V vs RHE. Comparing the performance of NPG@ZIF-
[
*]
Dr. Y. Yang, S.-Q. Wang, H. Wen, J. Chen, Dr. C.-P. Li, Prof. M. Du
College of Chemistry, Tianjin Key Laboratory of Structure and Per-
formance for Functional Molecules, MOE Key Laboratory of Inorgan-
ic-Organic Hybrid Functional Material Chemistry
Tianjin Normal University, Tianjin 300387, China
E-mail: hxxydm@mail.tjnu.edu.cn; hxxylcp@mail.tjnu.edu.cn
Dr. T. Ye
–
1
–2
8
with AuNPs@ZIF-8 (AuNPs: gold nanoparticles) indicates that
NPG exhibits the enhancement of catalytic activity via involving
the reaction at both outer and inner ligament surfaces bearing
numerous active sites. Moreover, comparison between NPG and
NPG@ZIF-8 catalysts reveals that the protective effect imparted
by ZIF-8 shell could continuously facilitate NRR. Benefiting from
Department of Physics, School of Science
Beijing Jiaotong University, Beijing 100044, China
Supporting information for this article is given via a link at the end of
the document.
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Angewandte Chemie International Edition
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COMMUNICATION
the hydrophobic and porous ZIF-8 shell, the competing HER can
be suppressed while the diffusion of chemical species is weak-
ened, thereby reinforcing NRR. Hence, NPG@ZIF-8 catalyst can
bring about remarkable improvement of the NRR efficiency. The
stability of this composite structure is further evaluated over 12-h
cycles to confirm its suitability for industrial ammonia production.
Notably, the synergistic effect of porous metal core and ZIF shell
can also be used to develop other electrocatalysts and motivate
the in-depth studies on water-sensitive electrocatalytic reactions.
at 1021.2 and 1044.1 eV, while Ag 3d exhibits no obvious bind-
ing energy peak, which reveals the bare amount of Ag in com-
posite (Figure S6). Thus, the Au:Zn atomic percentage is 24:76.
X-ray diffraction (XRD) pattern of NPG@ZIF-8 shows both sharp
diffraction peaks of Au and highly-crystalline ZIF-8, originating
from individual NPG and ZIF-8 (Figure 2b). XRD tests were also
taken on different NPG@ZIF-8 composites to confirm their pris-
tine ZIF-8 structures (Figure S7). Energy-dispersive X-ray spec-
troscopy (EDS) test was taken to confirm the elemental distribu-
A simple protocol was used to synthesize composite material. tion and composition for NPG and NPG@ZIF-8. EDS mappings
Primarily, the NPG core was fabricated using one-step method.[9]
Scanning electron microscopy (SEM) reveals that NPG particles
display spherical morphology but highly-roughened surface with
randomly interconnected curved ligaments (Figure 1ai). A high
yield is observed for monodisperse NPG with the particle size of
of NPG indicate the homogeneous distribution of Au with a trace
amount of Ag (1% atomic ratio, Figure S8). Elemental mappings
indicate the homogeneous and overlapping distributions of Au,
Ag, and Zn in NPG@ZIF-8, signifying the uniform coating of ZIF-
8 on NPG surface (Figures 2c–2f). EDS signals reveal an Au:Zn
atomic ratio of 19:81, and the Ag content is much less than 1%
(Figure S9). Moreover, inductively coupled plasma optical emis-
sion spectrometry (ICP-OES) tests show a decrease of Au mass
composition from 96% to 19% after ZIF-8 encapsulation, indicat-
ing the complete coverage of ZIF-8 on NPG. The above results
quantitatively confirm a successful generation of NPG core and
homogeneous ZIF-8 shell (Table S2). Herein, the trace amount
of Ag in NPG@ZIF-8 stems from AgCl with no catalytic activity.
The porosity for NPG@ZIF-8 was further evaluated by nitrogen
gas sorption, showing the type I isotherm. Compared with pris-
tine ZIF-8, all composites show the slight decrease of Brunauer-
Emmett-Teller (BET) surface areas due to the embedded NPG
particles (Figure S10a). However, NPG does not bring about the
variation of pore size distribution for ZIF-8 shells (Figure S10b),
because the size of NPG is too large to occupy the cavities and /
or channels of ZIF-8 crystals (11.6 Å).
(
340 ± 18) nm and ligament thickness of (37 ± 7) nm (Figure 1ai
[
19]
and S1). Subsequently, the encapsulation procedure of ZIF-8
was performed to obtain NPG@ZIF-8. SEM image indicates the
formation of ~0.5 ȝm wide dodecahedral crystals of intergrown
structures (Figure S2). Transmission electron microscopy (TEM)
image shows the entire encapsulation of ZIF-8 shell of ~200 nm
thickness on NPG cores (Figure 1aii). Via adjusting the concen-
tration of reactants, NPG-L (NPG varied in ligament) and NPG-P
(NPG varied in particle size) can be prepared, which are coated
with ZIF-8 to explore their effect on electrochemical performanc-
es (Figure S3, Table S1). Notably, further increasing the particle
size of NPG leads to the uneven coating of ZIF-8 particles (Fig-
ure S4) while further increasing the ligament size will bring about
the increase of particle size,[9] which thus are not studied in this
work. The ultraviolet-visible (UV-vis) absorption spectra for core
and core-shell composite structures reveal the vanishment of the
broad peak at 585 nm after coating the ZIF-8 shell (Figure 1b),
which is the characteristic localized surface plasmon resonance
band of NPG due to its random porous structure and nanoscale
Au ligaments. This result also confirms the complete encapsula-
tion of ZIF-8 shell on NPG.
Figure 2. (a) XPS at (i) Au 4f-Zn 3p and (ii) Zn 2p binding energy windows. (b)
XRD patterns of individual NPG, ZIF-8, and core-shell NPG@ZIF-8. (c) High-
angle annular dark field-scanning transmission electron microscope (HAADF-
TEM) image and the corresponding EDS elemental mappings of NPG@ZIF-8
indicating (d) Au, (e) Ag and (f) Zn elemental distributions. Scale bar: 100 nm.
Figure 1. (a) (i) SEM image of NPG cores. Scale bar: 500 nm (inset: 100 nm).
(
ii) TEM image of NPG@ZIF-8 composite. Scale bar: 1 ȝm (inset: 300 nm). (b)
Corresponding UV-vis absorption spectra.
X-ray photoelectron spectroscopy (XPS) was used to deter-
mine the surface composition in terms of elements and oxidation
states for NPG and NPG@ZIF-8. High-resolution scan for NPG
The electrocatalytic NRR performance was evaluated under
ambient condition using the neutral liquid-based electrochemical
setup (Figure S11). The generated ammonia can be detected by
spectrophotometric indophenol blue approach[21] and calculated
using ammonia calibration curve (Figure S12). UV-vis spectros-
copy tests of the electrolyte after 2-h reaction exhibit an intense
(Figure S5) indicates that it is mostly made of Au with a trace
amount of Ag (~3%). NPG@ZIF-8 was further studied by high-
resolution Au 4f-Zn 3p, Ag 3d and Zn 2p scans (Figure 2a). Au
4f-Zn 3p peak is resolved to four separated peaks, Au 4f7/2 (83.6
eV), Au 4f5/2 (87.6 eV), Zn 3p3/2 (88.7 eV), and Zn 3p1/2 (91.2 eV), absorption peak at 655 nm, which is characteristic of indophenol
[
20]
which are consistent with Au-Zn alloy. Zn 2p shows the peaks
2 4
blue. In contrast, a very low absorbance intensity for the Na SO
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Angewandte Chemie International Edition
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COMMUNICATION
solution of NPG@ZIF-8 reveals that almost no ammonia can be
produced, which thus excludes the possibility of contamination
for NPG@ZIF-8 catalyst before the electrocatalytic reaction. To
further explore the electrocatalytic performance of NPG@ZIF-8,
linear sweep voltammetry (LSV) curves were obtained in an Ar-
ZIF-8 (Figure S18). Hydrophobic electrode surface is confirmed
to effectively suppress HER and create abundant three-phase
contact points for reactants, which has been utilized in decreas-
2 2
ing H ratio in CO
electroreduction.[18] The higher energy barrier
of hydrogen desorption by hydrophobic electrode thus depress-
es HER.
or N
ies from –1.0 to –0.4 V vs RHE, a notable change is observed in
current density under N -saturated environment. It is expected
2
-saturated environment (Figure S13). As the potential var-
2
that the highest NRR activity could be achieved in this potential
range. When the potential moves to more positive values vs RHE,
there is no further change of the current density under Ar- or N -
2
saturated environment. Thus, the chronoamperometry (CA) tests
were taken to determine the yield rate of ammonia and Faradaic
efficiency at a series of applied potentials from –1.0 to –0.4 V vs
RHE.
2
Time-dependent current density curves in N -saturated elec-
trolyte indicate that the current density remains almost steady at
various potentials for 2 h and thus suggests a good durability of
composite catalyst NPG@ZIF-8 (Figure 3a). When electrolyzed
at –0.80 V vs RHE, the electrolyte after electrochemical NRR for
2
h shows the highest absorbance intensity (Figure 3b), indicat-
ing the greatest ammonia yield. Ammonia yield rate and Farada-
ic efficiency of electrolytes were further calculated (see Support-
ing Information). Significantly, the highest ammonia yield rate of
–
1
–2
(
28.7 ± 0.9) ȝgxh xcm is obtained at –0.8 V vs RHE, whereas
Figure 3. NRR electrocatalysis of NPG@ZIF-8 composite. (a) CA tests with
NPG@ZIF-8 electrode at various potentials from –1.0 to –0.4 V vs RHE. (b)
Corresponding UV-vis spectra of electrolytes colored with indophenol indicator.
the greatest Faradaic efficiency of 44% is achieved at –0.6 V vs
RHE (Figure 3c). Notably, this efficiency is superior to the known
NRR electrocatalysts in similar water electrolyte systems (Table
(
c) Ammonia yield rates and Faradaic efficiencies at various potentials. (d)
Time-dependent current density at the electrolysis of –0.6 V vs RHE for 12 h.
S3). The by-product of NRR (N
2
H
4
) was spectrophotometrically
evaluated after electrolysis. As a result, few N is detected
(1.05 ± 0.1) ȝgxh xcm ) at –0.6 V vs RHE, which illustrates an
excellent selectivity of 98% (Figures S14 and S15). Moreover, a
2-h electrolysis at a constant potential of –0.6 V vs RHE brings
[22]
2
H
4
–
1
–2
(
Due to the presence of N component in ZIF-8, it is necessary
to verify the N source of generated ammonia. LSV tests taken in
1
about a slight decrease in current density, proving the favorable
stability of NPG@ZIF-8 electrocatalyst (Figure 3d).
A series of contrast experiments were taken to evaluate the
enhancement of electrochemical NRR using NPG@ZIF-8. Since
ZIF-8 exhibits almost no electrochemical activity (Figure 4a), the
2
Ar- or N -saturated environment provide the qualitative evidence
to N source in ammonia product (Figure S13). Very few ammo-
nia yield rate in Ar-saturated environment clearly confirms that in
N
2
-saturated environment, the N source of ammonia production
is from nitrogen gas, instead of the N component in NPG@ZIF-8.
main source of catalytic sites should come from NPG. In addition, Further, a very low ammonia yield by ZIF-8 catalyst ((0.1 ± 0.4)
–
1
–2
the role of NPG was further demonstrated by the catalytic activi-
ties of AuNPs and the corresponding composite (AuNPs@ZIF-8), ZIF-8 shell. Further, 15N isotopic labelled test was taken to prove
ȝgxh xcm ) reveals the impossibility of ammonia generation by
[
24]
tested under –0.6 V vs RHE. Actually, NPG@ZIF-8 shows much
higher ammonia yield rate and Faradaic efficiency than those of
the N source of ammonia (Figure S19).
Firstly, the standard
14NH
Cl and 15NH
Cl samples were utilized for H NMR analysis,
1
4
4
–
1
–2
indicating a triplet coupling for 14NH
15NH +. 14 and 15
+
AuNPs ((2.5 ± 0.8) ȝgxh xcm , 2%) and AuNPs@ZIF-8 ((8.9 ±
4
and a doublet coupling for
were then used as the feeding gases for
–
1
–2
1
.8) ȝgxh xcm , 7%) (Figure 4a and S16). This should originate
4
N
2
N
2
from its higher density of electrocatalytic active sites due to the
porosity and ligament structure. Also, NPG@ZIF-8 shows signif-
NRR at –0.6 V vs RHE. Also, the corresponding spectra provide
undoubtedly evidences to support the ammonia production from
NRR electrocatalytic reaction, instead of the nitrogen component
in NPG@ZIF-8 catalyst.
The effect of material size was further explored by comparing
the cyclic voltammetry of different-sized NPG@ZIF-8 electrodes
(NPG@ZIF-8, NPG-L@ZIF-8, and NPG-P@ZIF-8). NPG@ZIF-8
shows the highest absorbance intensity and thus, generates the
most ammonia, as revealed by UV-vis absorption spectra of the
corresponding electrolytes colored by indophenol indicator (Fig-
ure S20a). In comparison, NPG-L@ZIF-8 and NPG-P@ZIF-8
–
1
–2
icantly better NRR performance ((22.0 ± 0.3) ȝgxh xcm , 44%)
–
1
–2
than that of NPG ((2.2 ± 0.5) ȝgxh xcm , 6%) under the same
testing conditions (Figure 4a). Notably, the LSV test under Ar-
saturated environment indicates higher current density of NPG
than NPG@ZIF-8, which indicates a stronger HER performance
without ZIF-8 capsulation (Figure S17). The results jointly verify
that ZIF-8 can effectively suppress HER. Moreover, the porosity
of ZIF-8 enables the local saturation of reactants[23] near catalyst
surfaces to reinforce NRR. And ZIF-8 coating is used as hydro-
phobic barrier for blocking the water entry for hydrogen evolution. bring about lower ammonia yield rate of (10.4 ± 1.5) and (9.8 ±
–
1
–2
The water contact angle for NPG or NPG@ZIF-8 functionalized
film is 34° or 114°, respectively, illustrating the hydrophobicity of
0.8) ȝgxh xcm and poorer Faradaic efficiency of 7% and 22%
(Figure S20b). In NPG-L@ZIF-8, due to the small-sized ligament
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Angewandte Chemie International Edition
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COMMUNICATION
with formless morphology, a portion of activity sites are blocked
and thus, the electrocatalytic activity is lowered, resulting in the
deterioration of NRR electrocatalysis. As for NPG-P@ZIF-8, the
smaller NPG particle size leads to less active sites, thus reced-
ing the catalytic performance of NRR. To evaluate the stability of
NPG@ZIF-8, CA tests were taken for 10 consecutive cycles (2 h
for each) and the morphology of catalyst was characterized after
the long-time electrocatalysis (Figure 4b and S21). NPG@ZIF-8
catalyst shows a minor change in ammonia yield rate and Fara-
daic efficiency after ten times of recycling test, which also keeps
the original morphology. The high ammonia yield and Faradaic
efficiency illustrate the rationality of our design on NPG@ZIF-8
composite and the expectation on its high stability and superior
electrocatalytic activity. With ease of reproducible synthesis, this
result will enable porous metal embedded ZIF/MOF composites
as a promising series of electrocatalysts for ammonia production.
Keywords: nanoporous gold • zeolitic imidazolate frameworks
(
ZIFs) • nitrogen fixation • ammonia synthesis • electrocatalysis
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This article is protected by copyright. All rights reserved.

Angewandte Chemie International Edition
10.1002/anie.201909770
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
This work presents a unique
core-shell structure compo-
site of nanoporous gold
embedded in ZIF-8 shell.
Owing to the high catalytic
activity of nanoporous gold
and hydrophobic porous
shell of ZIF-8, superior
enhancement of electro-
chemical nitrogen fixation
performance is achieved
with the highest Faradaic
efficiency of 44%.
Yijie Yang, Shu-Qi Wang,
Haoming Wen, Tao Ye, Jing
Chen, Cheng-Peng Li,* and
Miao Du*
Page No. – Page No.
Nanoporous Gold Embed-
ded ZIF Composite for
Enhanced Electrochemical
Nitrogen Fixation
This article is protected by copyright. All rights reserved.