Nitrogen reduction through confined electro-catalysis with carbon nanotube inserted metal-organic frameworks

Article abstract of DOI:10.1039/d0ta11797h

Carbon-based nanomaterials are widely used in electro-catalysis because of their low cost, high conductivity and stability. However, their application towards selective electrochemical reduction of nitrogen to ammonia suffers from low activity and faradaic efficiency (FE). Here, we report a confined electrocatalysis strategy for enhanced ammonia production and FE in the electrochemical nitrogen reduction reaction (eNRR), by the construction of a carbon nanotube (CNT or NCNT) inserted porous metal-organic framework (MOF). The CNT/NCNT serves as the catalytic center and ensures an efficient pathway for electron conduction that is essential to electrocatalysis, while the general relative hydrophobicity within the MOF enriches the local concentration of N2 near the catalyst active sites, and more importantly suppresses the hydrogen evolution reaction (HER) to facilitate the FE. Among the systematically screened MOF and carbon nanotubes, NCNT@MIL-101(Fe) demonstrates the highest activity of 607.35 μg h-1 mgNCNT-1 and CNT@MIL-101(Fe) achieves the best FE of 37.28%. The significantly improved NRR performance of CNT@MOFs and NCNT@MOFs demonstrates the successful employment of confined catalysis in electrochemical reactions, which provides an alternative strategy for catalyst design in nitrogen fixation. This journal is

Full text of DOI:10.1039/d0ta11797h

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Zhenhai Wen et al.
An electrochemically neutralized energy-assisted low-cost
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Nitrogen Reduction through Confined Electro-catalysis with
Carbon Nanotube Inserted Metal Organic Frameworks†
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
a
a
a
a
Yang Lv , Yiqi Wang , Miao Yang , Zhangyan Mu , Shengtang Liu , Weiping Ding , Mengning Ding*
DOI: 10.1039/x0xx00000x
3 2
Carbon-based nanomaterials are widely used in electro-catalysis approach for alternative NH production from nitrogen (N
).5
because of their low cost, high conductivity and stability. However, Substantial research efforts have been dedicated to the
their application towards selective electrochemical reduction of electrochemical nitrogen reduction reaction (eNRR) under ambient
nitrogen to ammonia suffers from low activity and faradaic conditions, as an attractive to the energy-intensive Haber-Bosch
6
-9
efficiency (FE). Here, we report a confined electrocatalysis strategy process.
Nevertheless, the reaction rate of current eNRR
for enhanced ammonia production and FE in electrochemical approaches have been relatively low, and the destruction of
nitrogen reduction reaction (eNRR), by the construction of a carbon nitrogen triple bond requires high over potential, at which strong
nanotubes (CNTs or NCNTs) inserted porous metal organic hydrogen evolution reaction (HER) occurs as a competition, further
1
0
framework (MOF). The CNT/NCNT serves as the catalytic center and reducing the faradaic efficiency (FE) of eNRR. To this end, research
ensures an efficient pathway for electron conduction that is attempts have been made to suppress HER by modulating the
1
1
essential to electrocatalysis, while the general hydrophobic within catalysts and electrolyte selection. Integration of porous structure
the MOF enriches the local concentration of N near the catalyst on/over the active site, which includes metal organic frameworks
active sites, and more importantly suppresses hydrogen evolution (MOFs), zeolitic imidazolate frameworks (ZIFs), covalent organic
2
1
2-15
16-19
reaction (HER) to facilitate the FE. Over the systematically screened frameworks (COFs)
or other porous structures,
can
MOF and carbon nanotubes, NCNT@MIL-101(Fe) demonstrates the significantly improve the concentration of reactive species, or
-1
-1
highest activity of 607. 35 g h mg NCNT and CNT@MIL-101(Fe) facilitate the chemical conversion process through confined
2
0-22
achieves the best FE of 37.28%. The significantly improved NRR catalysis.
performance of CNT@MOFs and NCNT@MOFs demonstrate the pores from porous carbon materials provide a great number of
successful employment of confined catalysis in the electrochemical exposed active sites and fast mass transfer for N adsorption and
reactions, which provides an alternative strategy for the catalyst N≡N cleavage. The specific surface area of MOFs reflect the
Specific for eNRR, high surface area and abundant
2
2
3
design in nitrogen fixation.
number of reaction channels, and there was a pore enrichment
effect in the porous materials such as MOF, ZIF, COF and etc.,
where the pore structure and interior groups played a characteristic
.
2
4
adsorption effect on small gas molecules. On the other hand,
successful application of specific catalyst/support interface to
Introduction
25
suppress HER demonstrates another key factor for efficient eNRR.
Nitrogen fixation to ammonia (NH
3
) or other nitrogen containing Suppressing HER is also achieved via porous ZIF-8 coating on the
compounds has been a most important chemical process that builds Au/Ag electrode surface, where hydrophobic ZIF-8 framework
1
,2
up the foundation of life and agriculture. At present, the Haber- effectively limited the entry of water molecules and greatly
2
6,27
Bosch process is still predominantly used in industrial ammonia suppressed HER, leading to a high selectivity of eNRR.
The
production, which however requires harsh conditions of high relative hydrophobic environment can further enhance the
o
28
temperature (400~600 C) and high pressure (20~40 MPa), with low adsorption of N
2
.
In addition, utilization of the Le Chatelier's
3
,4
NH
Therefore, it is urgent to find a sustainable and energy-efficient efficiency by shifting the chemical equilibrium with low pressure.
To date, most effective catalytic systems for eNRR are based on
3 2
yield of 10~20% and high CO emission of about 3%.
principle has also been demonstrated, achieving better eNRR
2
9
2
7
30
31
noble metals such as Au, Pb, and Ru, while catalysts based on
non-noble metals have also been reported but with relatively lower
Carbon-based materials generally adsorb and activate
on their structural defects or sharp texture.35,36 In addition,
electron-deficient B dopants on 2D graphene could facilitate
a. Key Lab of Mesoscopic Chemistry, School of Chemistry and Chemical engineering,
Nanjing University, Nanjing 210023, China. Email: mding@nju.edu.cn
Electronic Supplementary Information (ESI) available: Experimental section,
characterization, including one PDF with 28 supporting figures and 3 supporting
tables. See DOI: 10.1039/x0xx00000x
3
2-34
activities.
†
N
2
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chemisorption of negatively polarized N atoms and promoted N-N diffusion and electrical transport facilitation, using CNT or N-doped
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cleavage during eNRR.37 Even though the FE and NH
DOI: 10.1039/D0TA11797H
yield of CNT inserted MOFs (CNT@MOFs and NCNT@MOFs) as a proof-of-
3
carbon-based materials are relatively lower than noble metal-based concept demonstration (Scheme 1). In a typical CNT@MOF hybrid
catalysts, it is worth exploring due to its low-cost and potential nanostructure, the CNT serves as the catalytic center and more
durability. To this end, if
microenvironment is constructed over the surface of high electrocatalysis, and the relative hydrophobic MOF ensures local N
conductivity carbon material, it can concurrently enrich N thus enrichment, the inhibition of water entrance (thus the suppression
keeping a high local nitrogen concentration around catalytic sites, of HER competition), and facilitation of the desorption/diffusion of
prevent the entrance of H O molecules thereby suppressing HER, NH product (polar molecule). The CNT@MOFs materials can be
and provide efficient electron transport pathway that are necessary obtained from a universal and facile hydrothermal synthesis,
for the electrochemical process. leading to catalyst tunability according to the controlled adsorption
a
porous and hydrophobic importantly ensures an efficient electron conduction pathway for
2
2
2
3
Herein, we present a combined catalytic approach that includes and hydrophobic capacity from combination of different CNTs and
the synergic effects of confined electrocatalysis, selective chemical MOFs entities.
Scheme 1. Nitrogen reduction enhancement through 1) nitrogen enrichment; 2) hydrophobic HER suppression and 3) efficient conducting
network of CNTs via carbon nanotube inserted MOFs.
may be due to the limited crystal growth in the presence of
Results and discussions
CNTs/NCNTs. Nonetheless, introduction of CNTs/NCNTs did not
alter the crystalline phase of CAU-17, therefore retain the porous
characteristic and adsorption capacities of CNT@MOFs and
NCNT@MOFs. Similar conclusion can also be found in
CNT/NCNT@UIO-66, CNT/NCNT@BIT-58, and CNT/NCNT@MIL-
General preparations of CNT@MOFs and NCNT@MOFs were
achieved through the one-step hydrothermal synthesis of MOFs in
the presence of pre-suspended CNTs or NCNTs. The experimental
details are schematically illustrated (Fig. S1†) and can also be found
in Methods section.38-41 Fig. 1a-f shows the SEM and TEM images of
typical CNT@MOF structure, such as UIO-66, CNT@UIO-66 and
NCNT@UIO-66, revealing that the CNTs or NCNTs were successfully
inserted into corresponding MOF materials. The neighboring
crystals are connected by CNTs/NCNTs to ensure the network
conductivity. Other samples of CNT/NCNT inserted BIT-58, CAU-17,
and MIL-101(Fe) have also be achieved (Fig. S2-S4†).
Powder XRD was carried out to determine the crystalline
structure and composition of the CNT@MOFs. As shown in Fig. 1g,
the XRD of CNT@MOFs and NCNT@MOFs are consistent with the
corresponding MOFs crystalline structure with no other
heteropeaks. In certain cases such as CNT@CAU-17 and
NCNT@CAU-17, the overall crystallinity of CAU-17 is not high, which
1
01(Fe), which further indicates that this synthetic methodology
can be universally applied to a broad range of CNT and MOF
combinations.
BET was employed to probe the specific surface areas (SSA) of
MOFs after the insertion of CNTs, which also measures the ability
for nitrogen enrichment. As shown in Fig. 2a&2d, the SSA of UIO-66
2
-1
is 958.3 m g . Although the SSA is decreased in CNT@UIO-66 (395.8
2
-1
2 -1
m g ) and NCNT@UIO-66 (361.1 m g ) after hybridization, it is a
significant improvement as compared to bare CNTs/NCNTs (127.6
2
-1
2 -1
m g and 56.5 m g ). Similar results were also observed in BIT-58,
CAU-17 and MIL-101(Fe) systems (Fig. 2b and Fig. S5 & Table S1†),
2
which provide the general basis towards N enrichment. Water
contact angle is an important index to evaluate the overall
2
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hydrophobicity of materials, which can also reflect the CV measurements under Ar.42,43 As shown in Fig. 2g&2h, the Cdl
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DOI: 10.1039/D0TA11797H
microenvironment within the porous MOF structure and hence the value of CNT/NCNT@MIL-101(Fe) are calculated to be 0.105/0.091
-2
ability to suppress HER over NRR. Fig. 2e&2i. and Fig. S6† show the mF cm , which are much compared to CNT/NCNTs (2.472/4.06 mF
-2
systematically tested water contact angles of CNT/NCNT@UIO-66, cm ), demonstrating the lower electrocatalytic activities of
CNT/NCNT@BIT-58, CNT/NCNT@CAU-17 and CNT/NCNT@MIL- CNT/NCNT@MIL-101(Fe) for HER. In addition, the electrocatalytic
o
o
o
o
1
2
01(Fe)
(34.1±0.8 /32.9±2.4 ,
43.3±2.5 /48.6±3.5 , activities of other CNT@MOFs for HER are basically consistent with
o
o
o
o
9.3±0.3 /35.4±2.2 and 57.7±4.6 /71.7±2.8 , respectively) (Fig. 2c trend of water contact angles (Fig. S7†). These results confirm that
o
and Fig. S7†) are all larger than that of CNT/NCNT(22±0.4 / the relathydrophobic capacities of CNT/NCNT@MOFs were
o
2
0.5±2.1 ). Moreover, the current densities of CNT/NCNT@MOFs generally improved compared with pristine CNTs/NCNTs and pure
are much lower compared to that of CNT/NCNTs under Ar, which MOFs, which serve as an important base for the inhibition of HER
suggested that the HER was inhibited and the degrees of and improved FE for NRR. It should also be mentioned that the
suppression are consistent with the relative hydrophobicity (Fig. relative hydrophobic local environment could also presumably
2
3
c&2f). To verify the effect of relative hydrophobic environment on facilitate the desorption of NH (with a dipole moment itself) and
suppressed HER, the electrochemical surface areas (ECSA) were their consequent diffusion into the solution.
obtained by extracting the double layer capacitance (Cdl) from the
Fig. 1. Scanning Electron Microscopy (SEM) images of (a) UIO-66, (c) CNT@UIO-66, and (e) NCNT@UIO-66. Transmission Electron
Microscopy (TEM) images of (b) UIO-66, (d) CNT@UIO-66, and (f) NCNT@UIO-66. (g) XRD patterns of MOF systems. Scale bars are 1 m in
(a), 200 nm in (b), 3 m in (c) and (e), and 100 nm in (d) and (f).
To evaluate the electrocatalytic NRR activities, the as-prepared CNT/NCNT@MOF catalysts were then systematically investigated.
CNT/NCNT@MOFs catalysts were tested in 0.05 M H
saturated high-purity N (≧99.999 %), using saturated high-purity absorption spectra detected by the indophenol blue method (Fig.
Ar (≧99.999 %) as background control. We first tested pristine CNTs S8†) indicate that the enhanced electrocatalytic N reduction can
and NCNTs, and obtained the highest NH yield rate and FE of 1.54 be achieved by CNT/NCNT@CAU-17 at selected potentials. As
g h mg cat./3.16 g h mg cat. and 1.77%/1.87%, respectively. As shown in Fig. 3c&3g, NH yield rates of CNT/NCNT@CAU-17
2 4
SO with Specifically for CNT/NCNT@CAU-17 (Fig. 3b&3f), the UV-vis
2
2
4
5
3
-
1
-1
-1
-1

3
-
1
-1
-1
-1
shown in Fig. 3a&3e and Fig. S10&S11†, the eNRR activity of NCNTs reached to 11.92±0.083 g h mg cat./13.30±0.01 g h mg cat. at -
is slightly better than that of CNTs, probably due to the N vacancies 0.45 V, with FEs of 31.27±0.29%/19.90±0.25% at -0.4 V/-0.45 V.
that facilitate
N
2
adsorption or the stabilization of N-H These results are 18 and 11 times higher than that of pristine CNTs
4
4
intermediates via hydrogen bonding. The eNRR activities of all and NCNTs, and the significantly improved eNRR performance is
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comparable
with
other
state-of-the-art
carbon-based g h-1 mg-1cat. at -0.55 V/-0.6 V, with FEs of 15.14%/18.13% at -
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DOI: 10.1039/D0TA11797H
electrocatalysts (Table S2†), but based on a simple strategy of 0.55V, which are 8.55/9.7 times higher than that of CNT/NCNT;
confined electrocatalysis without the introduction of additional CNT/NCNT@BIT-58 achieved the NH
-
1
-
3
yield rates of 4.135 g h mg
cat./8.108 g h mg cat. at -0.45 V, and FEs of 12.4%/15.03% at -
yield and FE 0.45V, 7.06/8.03 times higher than that of CNT/NCNT; for
1
-1
-1
catalytic sites. Furthermore, CNT/NCNT@CAU-17 manifest
considerable stabilities, with only small change in NH
during consecutive recycling between -saturated and Ar- CNT/NCNT@MIL-101(Fe), the NH
saturated electrolytes for 12 hrs (Fig. 3d&3h. and Fig. S12†).
Control experiments using Ar and N -saturated post electrolytes 0.45V, which are 21.06/13.45 times higher than that of CNT/NCNT
under -0.45 V and open circuit potential demonstrate evident (Fig. S13-S18†). Moreover, the NH yield rates of NCNT@MOFs are
accumulation of generated ammonia and ignorable accumulation of in general slightly higher than those of CNT@MOFs (as shown in Fig.
contaminated ammonia in the tests (Fig. S12†). 3k-I and Fig. S20†), which are consistent with the data of
To further confirm the universality of the strategy, we have CNT/NCNT, indicating that MOFs mainly plays the role of enriching
conducted systematic and detailed investigations on and relative hydrophobic suppression of HER in the reaction. To
CNT/NCNT@UIO-66, CNT/NCNT@BIT-58 and CNT/NCNT@MIL- verify the stabilities of the CNT@MOFs, we tested the XRD, SEM
01(Fe), the best UV-Vis absorption spectra of CNT/NCNT@MOFs and TEM of CNT/MOFs after electrolysis of 6 cycles. As shown in Fig.
3
-1
-
N
2
3
yield rates are 5.514 g h mg
cat./6.97 g h mg cat. at -0.45 V, and FEs are 37.28%/25.15% at -
1
-1
-1
2
3
N
2
1
are demonstrated in Fig. 3j and Fig. S19†) and the results clearly S21 and S22†, the crystalline structures of all the CNT@MOFs
show that the variety of assembled CNT/NCNT@MOFs with remained, with only a little change of morphology (such as the edge
different combination of building blocks all possess sufficient eNRR smoothness), thus confirming the satisfying stability of the
performance (activities and FEs) with good stabilities. NH
rates of CNT/NCNT@UIO-66 reached to 3.811 g h mg cat./6.081
3
yield CNT@MOFs.
-1
-1
2
Fig. 2. (a), (d) N adsorption-desorption isotherms of CNT/NCNT and UIO-66 system (as a representative MOF, for others see supporting
information). (b) Specific surface areas of CNT/NCNT@MOFs. (e) Water Contact angles plot of CNT/NCNT@MOFs. (c), (f) LSV curves of
CNT/NCNT and CNT/NCNT@MOFs under Ar. (g), (h) Capacitive current densities derived from CV curves against scan rates for CNT/NCNTs
and CNT/NCNT@MOFs. (i) Water contact angle images of CNT/NCNT and CNT/NCNT@MIL-101(Fe) (as a representative MOF, for others see
supporting information).
4
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In order to further clarify the rule of MOFs coating in the NRR 4a&4b, we can clearly observe that the NH yield rate is positively
enhancement in the CNT/NCNT@MOFs, the NRR performance of correlated with SSA and the contact angle. In particular, in Fig. 4a,
pure MOFs have been further evaluated. Date exhibited in Fig. 3i we found that when the SSAs of CNT and CNT@CAU-17 were
and Fig. S23-26† indicate that the pure MOFs generally have no similar, CNT@CAU-17 with larger contact angle showed higher NH
3
NRR activities, except for CAU-17 (yield rate of 3.98 g h-1 mg cat. yield rate, which also confirmed the inhibition of hydrophobicity to
and FE of 11.2%), which could be a result of unsaturated CAU-17 HER, thus promoting the eNRR; when the contact angles of NCNT
that contributed Bi sites for additional NRR catalysis. Mechanistic and NCNT@CAU-17 were similar, NCNT@CAU-17 with larger SSA
-1
aspects of eNRR over CNT/NCNT@MOFs and the role of MOFs have got a higher NH
been investigated in detail by analyzing the BET and water contact enrichment on eNRR. In general, the NH
angle results and their correlations to the electrocatalytic determined by SSA (see Fig. S27†), while the FE is greatly affected
behaviours. Interestingly, the NH yield rates from by hydrophobicity, as shown in Fig. 4c&4d. It is worth noticing that
3
yield rate, which proved that the positive effect of
N
2
3
yield rate is mainly
3
CNT/NCNT@MOFs samples were normalized to the amount of there is an irregular eNRR
actual active carbon (i.e., the mass of CNT/NCNT), as shown in Fig.
Fig. 3. (a), (e) UV-Vis absorption spectra of the electrolyte stained with indophenol blue indicator after electrolysis at selected potentials of
CNT/NCNTs (b), (f) UV-Vis absorption spectra of the electrolyte stained with indophenol blue indicator after electrolysis at selected
potentials of CNT/NCNT@CAU-17 (as a representative MOF, for others see supporting information). (c), (g) NH
CNT/NCNT@CAU-17 at selected potentials. Error bars denote the s.d. of the yield rates and faradaic efficiency calculated from three
independent samples. (d), (h) NH yield rates and FEs of CNT/NCNT@CAU-17 after each cycle at -0.45 V vs. RHE. (i), (j) The highest UV-Vis
3
yield rates and FEs of
3
absorption spectra of each pure MOFs and CNT@MOFs. (k) NH
CNT@MOFs.
3
yield rate of CNT@MOFs at selected potentials. (l) Corresponding FEs of
activity of CNT/NCNT@CAU-17 which is not restricted by hydrophobicity or SSA. This is probably due to the Bi sites exposed by the
unsaturated CAU-17 system, which has been identified as an excellent catalyst in the eNRR.4
6-48
This assumption agrees with the unusual
results of CAU-17 system in XRD, BET and contact angle. In addition, no discernible hydrazine was detected by the Watt and Chrisp method
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(
see Supplementary Fig. S9†) in this study (see Fig. S28-29†).46 Overall, this obvious correlation between the eNRR performance of
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DOI: 10.1039/D0TA11797H
2
CNT/NCNT@MOFs with the SSA and hydrophobicity of MOF coatings confirms our strategy. In short, SSA determines the N adsorption
capacity of CNT/NCNT@MOFs, thus changing the local N
2
concentration on the CNT surface, while the contact angle regulates the
hydrophobicity of CNT/NCNT@MOFs, resulting in different degrees maximum NH
3
yield. As shown in Fig. S30† that two peaks with a
of inhibition on HER, which results in a great improvement of the coupling constant of 72.9 Hz are detected in the baseline-
1
activity of eNRR (see Table S3†). This result also reveals the subtracted H NMR spectra, which is consistent with heteronuclear
1
15
15
4+
potential in the combination of our confined electrocatalysis coupling constant between H and N of NH , thus proving that
strategy with more active sites. To further elucidate the NH
3
origin, all ammonia product are from N
2
gas.
1
5
we performed electrolysis by using
2
N on NCNT@CAU-17 with the
3
Fig. 4. (a), (b) The optimal NH yield rate of CNT/NCNT@MOFs normalized to the amount of CNTs/NCNTs over different water contact
angles and specific surface areas. (c), (d) The optimal FEs of CNT/NCNT@MOFs over different water contact angles and specific surface
areas.
for the design and construction of complex electrocatalytic system
Conclusions
at mesoscopic level towards the efficient electrochemical nitrogen
fixation under ambient conditions.
CNT/NCNT have been successfully inserted by a variety of MOF
crystalline, resulting in a series of CNT/NCNT@MOF hybrid catalysts
with significantly enhanced performance towards electrochemical
nitrogen fixation. Systematic characterizations and mechanistic
investigations reveal that this catalytic system benefits from the
synergic combination of enrichment of nitrogen concentration,
suppression of HER competition via hydrophobic local environment,
and the facilitation of electrical transport along the CNTs network.
Authors contribution
Y. L. conceived the idea and wrote the paper. M. Y. contributed to
the material characterization. Z. M. participated in the drawing of
graphical abstract. S. L. and W. D. discussed the results and
commented on the manuscript. M. D. projected the concept,
reviewed and edited the manuscript.
As a result, accelerated NH
achieved. This strategy provides an effective and universal strategy
3
yield rates and improved FEs have been
6
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1 D. H. Deng, X. Q. Chen, L. Yu, Wu, Q. F. Liu, Y. LiVuie,wHA.rtXic.leYOannlinge,
Conflicts of interest
The authors have no conflicts of interest to report.
H. F. Tian, Y. F. Hu, P. P. Du, R. Si, J. H. Wang, X. J. Cui, H. B. Li,
DOI: 10.1039/D0TA11797H
J. P. Xiao, T. Xu, J. Deng, F. Yang, P. N. Duchesne, P. Zhang, J.
J. Zhou, L. T. Sun, J. Q. Li, X. L. Pan and X. H. Bao, Sci. Adv.,
2
015, 1, e1500462.
2
2
2 C. H. Turner, J. K. Johnson and K. E. Gubbins, J. Chem. Phys.,
Acknowledgements
This work was financially supported by the Fundamental Research
Funds for the Central Universities in China (020514380224 and
2
001, 114, 1851-1859.
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