Selective nitrogen reduction to ammonia on iron porphyrin-based single-site metal-organic frameworks

Article abstract of DOI:10.1039/d0ta08741f

Constructing efficient catalysts for N₂reduction into value added ammonia under ambient conditions is a considerable challenge. Herein, well-defined single-site metal-organic frameworks (MOFs, M-TCPP; M = Fe, Co, or Zn) were constructed and evaluated as electrocatalysts for N₂reduction. The prepared Fe-TCPP exhibited prominent performance with a high NH₃yield of 44.77 μg h^?1mg_cat.^?1and a faradaic efficiency of 16.23%, superior to that of all the reported molecular and MOF catalysts. The superior performance was ascribed to the highly effective N₂activation at the Fe site, and benefited from the overall reaction thermodynamics advantage in the key reaction step of *NNH formation. This study gives an understanding of the intrinsic activity of well-defined catalysts in the electrocatalytic N₂reduction, and provides atomic-level insights into the rational design and engineering of highly active catalysts for artificial N₂fixation.

Full text of DOI:10.1039/d0ta08741f

Journal of
Materials Chemistry A
View Article Online
View Journal | View Issue
COMMUNICATION
Selective nitrogen reduction to ammonia on iron
porphyrin-based single-site metal–organic
frameworks†
Cite this: J. Mater. Chem. A, 2021, 9,
4673
ab
Meiyu Cong,‡ab Xuyang Chen,‡b Kai Xia,‡a Xin Ding,
Linlin Zhang,a Yu Jin,b
Received 5th September 2020
Accepted 31st January 2021
*
bc
a
*
*
Yan Gao
and Lixue Zhang
DOI: 10.1039/d0ta08741f
rsc.li/materials-a
Constructing efficient catalysts for N2 reduction into value added the presence of H2O under ambient conditions, has garnered
ammonia under ambient conditions is a considerable challenge. great interest as an alternative approach for the productionꢀo1f
Herein, well-defined single-site metal–organic frameworks (MOFs, NH3.10–21 However, owing to the high bond energy (941 kJ mol
)
M–TCPP; M ¼ Fe, Co, or Zn) were constructed and evaluated as of N^N and the occurrence of the competitive hydrogen
electrocatalysts for N2 reduction. The prepared Fe–TCPP exhibited evolution reaction (HER), high-activity and selective catalysts
prominent performance with a high NH3 yield of 44.77 mg hꢀ1 mgcat.
are essential.22–31
ꢀ1
and a faradaic efficiency of 16.23%, superior to that of all the reported
During the past few years, single-atom/single-site catalysts
molecular and MOF catalysts. The superior performance was ascribed with both heterogeneous and isostructural features have
to the highly effective N2 activation at the Fe site, and benefited from attracted much attention owing to their outstanding activity
the overall reaction thermodynamics advantage in the key reaction and efficient use of metals.32–37 However, their widespread
step of *NNH formation. This study gives an understanding of the application has been impeded by their tedious synthesis,
intrinsic activity of well-defined catalysts in the electrocatalytic N2 inhomogeneous structure, and low metal loading. As a potential
reduction, and provides atomic-level insights into the rational design consideration, single-site MOFs are a class of crystalline porous
and engineering of highly active catalysts for artificial N2 fixation.
materials with fascinating structures and desirable properties
such as permanent porosity, high surface area, and uniform
open cavities.38–40 The availability of various building blocks
consisting of metals and organic linkers makes it possible to
construct MOFs with unique properties for diverse applications.
The retention of the microporous structure in solvents is also
essential for gas-phase catalysis,41 especially suitable for the
NRR owing to the low solubility of N2 in electrolyte.42 These
desirable features thus make single-site MOFs excellent NRR
catalyst candidates; however, they have rarely been applied to
the NRR owing to their poor adsorption and activation ability
toward N2, and the thermodynamically strong energy barrier
(410 kJ molꢀ1) of N2 for the rst cleavage step.43,44
Ammonia (NH3), an important industrial raw material with
a large production of more than 750 000 tons per year, is widely
used in fertilizers, refrigerants, and other chemicals.1,2 NH3 also
shows great potential as a clean energy carrier owing to its high
energy density and clean combustion products, as well as the
abundant availability of raw material N2 in the atmosphere.3,4
Industrial-scale NH3 production mainly relies on the energy-
intensive Haber–Bosch process that necessitates high temper-
atures (300–500 ꢁC) and high pressures (15–25 MPa),
accounting for ꢂ2% of the global energy consumption and
1.6% of CO2 emission.5–9 Developing alternative environmen-
tally friendly N2 xation processes is thus desirable from both
energy and environmental perspectives. Recently, the electro-
chemical N2 reduction reaction (NRR), which is performed in
Inspired by the vital role of Fe in natural nitrogenase
enzymes, well-dened single-site Fe–TCPP MOFs [TCPP ¼
tetrakis(4-carboxyphenyl)porphyrin] with homogeneous cata-
lytic sites were rationally designed and evaluated for the elec-
trochemical NRR. These single-site Fe–TCPP MOFs showed
ꢀ1
high NRR performance. An NH3 yield of 44.77 mg hꢀ1 mgcat.
aCollege of Chemistry and Chemical Engineering, Qingdao University, Qingdao
266071, Shandong, PR China
bKey Laboratory of Fine Chemicals, Dalian University of Technology (DUT), Dalian
and a faradaic efficiency (FE) of 16.23% were obtained for Fe–
TCPP MOFs, superior to those of all the reported molecular
catalysts and MOF catalysts for the NRR (Table S1†).43–48 The
adsorption, activation and catalytic processes were studied by
means of experiments and density functional theory (DFT)
calculations. The selective adsorption and activation properties
116024, PR China. E-mail: dr.gaoyan@dlut.edu.cn
cNingbo Institute of Dalian University of Technology, Ningbo, 315000, PR China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ta08741f
‡ These authors contributed equally to this work.
This journal is © The Royal Society of Chemistry 2021
J. Mater. Chem. A, 2021, 9, 4673–4678 | 4673

View Article Online
Journal of Materials Chemistry A
Communication
Scheme 1 Schematic synthesis of the Fe–TCPP MOFs.
of the single-site MOFs were further investigated through
molecular strategies. We made an all-important attempt to
prepare single-site MOFs with molecular catalyst units, rather
than with node-sites as catalytic centers for articial N2 xation,
and provided atomic-level insights into the catalytic mechanism
for molecule based NRR electrocatalysts.44,47–49
Single-site Fe–TCPP was fabricated via molecular strategies
with mononuclear porphyrin (FeTCPP) molecules and
Zn(NO3)2, as shown in Scheme 1. The crystallinity of the
prepared catalyst was investigated by X-ray diffraction (XRD).
The XRD patterns of Fe–TCPP shown in Fig. S1† featured peaks
at 5.3ꢁ, 7.5ꢁ, 8.7ꢁ,12.0ꢁ and 19.5ꢁ that were ascribed to the (100),
(110), (002), (210), and (004) planes, respectively.50 The chemical
composition and binding states of the catalysts were investi-
gated by X-ray photoelectron spectroscopy (XPS, Fig. S2–S4†).
The full spectral scan shows that O, C, N, Zn and Fe elements
coexisted in Fe–TCPP. The peaks at 713.4 and 726.2 eV accom-
panied by their satellite peaks were attributed to Fe 2p3/2 and Fe
2p1/2, respectively. The sharp and symmetrical peaks centered at
1044.9 and 1022.0 eV were, respectively, ascribed to Zn 2p1/2 and
Zn 2p3/2. The Brunauer–Emmett–Teller (BET) surface area of Fe–
TCPP was calculated to be 77.22 m2 gꢀ1, with a pore size of ca.
2 nm (Fig. S5†). Fourier transform infrared (FTIR) spectroscopy
was used to further study the chemical composition of the
prepared catalysts (Fig. S6†). The broad band at ꢂ3405 cmꢀ1
was assigned to the vibrations of O–H from the carboxyl groups
of Fe–TCPP. The sharp band at ꢂ1604 cmꢀ1 was assigned to the
vibrations of C]C in the porphyrin ring and the sharp band at
ꢂ1401 cmꢀ1 was assigned to the vibrations of C–N. The bands
in the range of 1000–1011 cmꢀ1 and 690–801 cmꢀ1 were,
respectively, assigned to the rocking and bending vibrations of
C–H from the pyrrole and benzene rings. More structural
insights were gained and the coordination environment of iron
in Fe–TCPP was identied by X-ray absorption ne structure
(XAFS) analysis. As shown in the Fe K-edge X-ray absorption
near-edge structure (XANES) spectra (Fig. 1A), the absorption
edge suggested that the Fe species in Fe–TCPP was in a high
oxidation state, similar to that in Fe2O3. The EXAFS results show
that there is no rst-shell Fe–Fe contribution for the Fe–TCPP
sample. The Fourier transforms of the extended X-ray absorp-
tion ne structure (FT-EXAFS) spectra of Fe–TCPP exhibited
Fig. 1 (A) Fe K-edge XANES spectra of Fe–TCPP, Fe2O3, and Fe foil, (B)
Fourier transforms of the k3-weighted Fe K-edge of EXAFS spectra of
Fe–TCPP, Fe2O3, and Fe foil; (C) TEM images, (D) HRTEM images and
SAED pattern, and (E) EDS mapping images of Fe–TCPP, showing the
presence of elements Fe, C, and Zn. (F) AFM image and the corre-
sponding line profile (inset) of Fe–TCPP.
˚
a main peak at about 1.50 A, ascribed to Fe–N (Fig. 1B and S7–
˚
S9†). Notably, this FT amplitude at about 2.40 A should be
assigned to the Fe–C distance.51 Therefore, the Fe elements in
Fe–TCPP can be categorized into single-site Fe.
The morphology and microstructure of Fe–TCPP were
investigated by transmission electron microscopy (TEM) and
atomic force microscopy (AFM). TEM images indicated that the
obtained Fe–TCPP MOFs had a nanosheet structure due to the
planar structure of the porphyrin molecule, and the character-
istic spacing of 1.64 nm was ascribed to the (100) lattice plane
(Fig. 1C and D).50 The black particles in Fig. 1C were ascribed to
the possible formation of ZnO or Zn(OH)2, which formed during
the formation of Fe–TCPP MOFs from Fe–TCPP molecules.
Their presence would make no difference in the tests for eval-
uating the acid labile nature. Energy-dispersive X-ray spectros-
copy (EDS) mapping revealed that Fe, C, and Zn elements were
homogeneously distributed in the Fe–TCPP catalyst (Fig. 1E).
The AFM image showed that the thickness of the Fe–TCPP MOF
nanosheets was less than 4 nm, and the ultrathin structure
should be benecial to enhance the performance by increasing
the surface area and boosting mass transfer. Collectively, the
XRD, XPS, FTIR, and HRTEM data conrmed the successful
synthesis of ultrathin single-site Fe–TCPP. Then, single-site Co–
TCPP and Zn–TCPP MOFs were synthesized through the same
strategies for comparison studies.
The electrochemical N2 reduction was performed in an H-
type cell with 0.1 M HCl as an electrolyte, and Ag/AgCl as
4674 | J. Mater. Chem. A, 2021, 9, 4673–4678
This journal is © The Royal Society of Chemistry 2021

View Article Online
Communication
Journal of Materials Chemistry A
a FE of 16.23% (Fig. 2E) and a high NH3 yield of 44.77 mg hꢀ1
ꢀ1
mgcat.
(Fig. 2F). The NH3 yield and FE increased as the
potential shied from ꢀ0.1 to ꢀ0.3 V due to the slow multi-
electron transfer and decreased as the potential shied from
ꢀ0.3 to ꢀ0.5 V because of the increasing occurrence of the
competing HER on the surface of the catalyst (Fig. 2E and F).
The presence of N2H4 as a potential by-product aer NRR
electrolysis was assessed using a method developed by Watt and
Chrisp51 (Fig. S12†), and the results indicate that N2H4 is absent
and Fe–TCPP is highly selective toward NH3 production. The
source of the NH3 generated during the reaction was conrmed
in the comparison experiments performed at the same potential
of ꢀ0.3 V vs. RHE (Fig. S13–S16†). NH3 was not obtained when
the reaction was conducted in the presence of Fe–TCPP under
open-circuit conditions or in an Ar-saturated electrolyte, con-
rming that NH3 was not derived from NH3 pollution or the
ꢀ
reduction of NO3 in the electrolyte. In addition, the source of
NH3 was further assessed by isotope labeling experiments
+
(Fig. S17†). 14NH4 was detected aer reaction in a 14N2-satu-
rated electrolyte, while 15NH4+ was detected as a double peak in
1
the H NMR spectrum aer reaction in a 15N2-saturated elec-
trolyte. The results conrm that NH3 is derived from the elec-
ꢀ
trochemical reduction of N2 rather than NH3 pollution or NO3
reduction. Since NH4+ was quantied by both UV-vis absorption
spectroscopy and ion chromatography, we could also conrm
that the possible existence of Zn2+ in the electrolyte from
porphyrin would not inuence the NH4+ detection in principle.
In order to investigate the selective adsorption and activation
properties of the ultrathin single-site MOFs, the prepared Co–
TCPP and Zn–TCPP were also evaluated as NRR catalysts
(Fig. S18–S24†). As shown in Fig. 2E and S25,† the NRR activity
decreased in the order of Fe–TCPP (NH3 yield: 44.77 mg hꢀ1
mgcat.ꢀ1; FE: 16.23%) > Co–TCPP (NH3 yield: 28.3 mg hꢀ1
mgcat.ꢀ1; FE: 11.58%) > Zn–TCPP (NH3 yield: 19.59 mg hꢀ1
mgcat.ꢀ1; FE: 6.37%) > carbon paper (CP) (NH3 yield: 8.62 mg hꢀ1
mgcat.ꢀ1; FE: 4.52%) in N2-saturated electrolyte, indicating that
the active metal centre in the MOF catalysts played a vital role
during the electrocatalytic process. Successive cycling and time-
dependent tests were carried out and the results are shown in
Fig. S25–S28.† A slight decrease was observed in NH3 yield but
with unchanged faradaic efficiency. ICP-MS of the Fe–TCPP
molecule and electrolyte aer a long-term test was carried out
and the results are shown in Fig. S29.† Fe–TCPP molecules and
derivatives could be found in the electrolyte aer the long-term
test. So, the decrease of NH3 yield should be ascribed to the
possible damage of Fe–TCPP in an acid environment.
Fig. 2 (A) LSV curves of Fe–TCPP measured in Ar- and N2-saturated
electrolytes. (B) Variations in the current density as a function of
reaction time for the N2RR performed over the Fe–TCPP catalyst
under different applied potentials (inset: the photograph of the elec-
trolyte treated using an indophenol blue method). (C) UV-vis absor-
bance spectra and (D) ion chromatograms of Fe–TCPP measured at
different potentials after electrolysis for 2 h. (E) FE and (F) NH3 yield of
the NRR over Fe–TCPP, Co–TCPP, and Zn–TCPP as a function of
applied potentials.
a reference electrode in the cathode chamber. The counter
electrode, a graphite plate, was placed in the anode chamber.
Before the tests, N2 was puried with 3 M H2SO4. The pretreated
N2 was then bubbled into the cathode chamber rather than
being fed with a continuous ow. Fig. 2A presents the linear
sweep voltammetry (LSV) polarization curves of Fe–TCPP in Ar-
and N2-saturated electrolytes, in which the geometric current
density (j) is plotted against the potential (vs. RHE). As shown,
the faradaic range started from ꢀ0.05 V and extended to ꢀ0.5 V
vs. RHE. No N2 reduction occurred below or above the faradaic
ranges due to the slow reaction kinetics or because the strong
HER competitive reaction prevailed. Within the faradaic region,
the current density in N2-saturated electrolyte was higher than
that in Ar-saturated electrolyte, due to the contribution of the
NRR. Hence, the NRR performance was assessed in the faradaic
region. The chronoamperometric curves of Fe–TCPP acquired at
different potentials are shown in Fig. 2B. The current densities
increased with the increase of external potential. The extent of
NH3 production (NH3 yield and FE) was assessed by UV-vis
absorption spectroscopy (Fig. 2B) and ion chromatography
(Fig. 2C), and the corresponding calibration curves are shown in
Fig. S10 and S11.† The maximum NH3 yield and FE were
recorded at ꢀ0.3 V vs. RHE aer a 2 h electrocatalytic reaction,
which is consistent with the LSV results. Fe–TCPP displayed
DFT calculations were performed to better elucidate the
activation mechanism of the MOF-catalyzed NRR. The opti-
mized N2 binding site on Fe–TCPP, Co–TCPP, and Zn–TCPP is
shown in Fig. 3A–C, respectively, along with the bond lengths
for N–N and Fe(Co, Zn)–N and the adsorption free energy of N2
(DGb(N2)). The distance between the metal center and the
˚
˚
proximal N atom (Np) from N2 is 1.728 A in Fe–TCPP, 1.945 A in
˚
Co–TCPP, and 2.976 A in Zn–TCPP. Additionally, the Wiberg
Bond Index (WBI) of Fe–Np is 1.00, which is higher than the
average value (0.60) of typical Fe–N single bonds in Fe–TCPP.
This difference can be ascribed to the presence of a single dative
This journal is © The Royal Society of Chemistry 2021
J. Mater. Chem. A, 2021, 9, 4673–4678 | 4675

View Article Online
Journal of Materials Chemistry A
Communication
Fig. 3 Optimized structures of N2 adsorption onto the metal center of (A) Fe–TCPP, (B) Co–TCPP, and (C) Zn–TCPP. C, H, O, Fe, Co, and Zn are
colored grey, white, red, purple, orange, and cyan, respectively. (D) Highest occupied molecular orbital (HOMO) and (E) HOMOꢀ1 that form p
interactions between Fe and N2. (F) DOS and OPDOS of N2–Fe–TCPP. (G) HOMO and (H) HOMOꢀ1 that form p interactions between Co and N2.
(I) DOS and OPDOS of N2–Co–TCPP.
Np/Fe bond and p back-bonding of the d electrons from the Fe unfavorable spatial structure for p back-donation. These results
atom to the p* antibonding orbitals of N2. In Co–TCPP, the WBI indicate that N2 can be more effectively activated by Fe–TCPP
of Co–Np is 0.54, which is comparable to the average value (0.53) than by Co–TCPP, and Zn–TCPP is not viable for N2 activation.
in Co–TCPP, corresponding to the s single Co–Np bond. In Zn–
The NRR kinetic pathway was further investigated using the
˚
TCPP, the longer Zn–Np bond (2.976 A) and smaller WBI value Gibbs free energy (DG) diagrams. The most favorable catalytic
(0.04) when compared with those of typical s single Zn–N bonds NRR process with the free energy change for each elementary
˚
(average bond length: 2.053 A, average WBI: 0.32) conrm the step is shown in Fig. 4 and S30–S33.† The free energy of
absence of a chemical bond between Zn and Np atoms. The adsorption of N2 is stronger on Fe–TCPP (0.17 eV), followed by
˚
N^N bond of the free N2 molecule in the gas phase is ꢂ1.117 A, Co–TCPP (0.22 eV) and Zn–TCPP (0.32 eV). For all three M–TCPP
˚
which is slightly elongated in Fe–TCPP (1.140 A) and Co–TCPP catalysts, the N2RR tended to proceed through an alternative
˚
(1.127 A) but comparable in Zn–TCPP. From the electronic hydrogenation mechanism to produce NH3. N2 is initially
structure, the inertness of Zn–TCPP toward N2 capture is most adsorbed on a metal site, and subsequently hydrogenated by
likely due to the fully occupied 3d10 orbital. In contrast, in Fe– a proton to form *N–NH. Subsequently, Fe–TCPP tended to
TCPP and Co–TCPP, the difference between Fe–Np and Co–Np follow the distal pathway, and hydrogen reacted with the
can be explained from their molecular orbitals (Fig. 3D, E, G distal N atom to form NNH*2. NH3 subsequently detached
and H). p back-bonding is mostly absent between Co and Np from*NNH3, following the interaction of the proton with the
atoms in the weak, relatively long Co–Np bond. The weak Co–Np pre-hydrogenated N site in NNH*2, leaving the remaining N*
bond was ascribed to the participation of electrons in the atom on the Fe site. This remaining N* was subsequently
antibonding orbital of Co–Np, as indicated by the peak of the hydrogenated to produce a second NH3 via a similar path
OPDOS curve near the Fermi level (dashed line) in Fig. 3I. The involving a reaction with three protons coupled with electrons.
electron-lled antibonding orbital largely reduced the strength For Co–TCPP and Zn–TCPP, the proton would react with the Np
of the Co–Np bond, resulting in a longer CO–N bond and an atom to form *NH–NH. *NH–NH2 is subsequently generated
4676 | J. Mater. Chem. A, 2021, 9, 4673–4678
This journal is © The Royal Society of Chemistry 2021

View Article Online
Communication
Journal of Materials Chemistry A
Conflicts of interest
There are no conicts to declare.
Acknowledgements
This work was supported by the National Natural Science Foun-
dation of China (Grant No.: 21908120 and 21875030), the Natural
Science Foundation of Shandong Province (Grant No.:
ZR2018BB037), the Youth Innovation Team Project of Shandong
Provincial Education Department (No. 2019KJC023) and the
Project of Qingdao Applied Basic Research Programs of Science
and Technology (Grant No.: 18-2-2-10-jch and 18-2-2-35-jch).
Fig. 4 Free energy diagrams of the favorable NRR pathway over M–
TCPP catalysts (M ¼ Fe, Co, Zn).
Notes and references
1 F. Jiao and B. Xu, Adv. Mater., 2019, 31, e1805173.
2 D. R. MacFarlane, J. Choi, B. H. R. Suryanto, R. Jalili,
M. Chatti, L. M. Azofra and A. N. Simonov, Adv. Mater.,
2019, 1904804.
3 Q. Wang, Y. Lei, D. Wang and Y. Li, Energy Environ. Sci., 2019,
12, 1730–1750.
4 X. Zhu, S. Mou, Q. Peng, Q. Liu, Y. Luo, G. Chen, S. Gao and
X. Sun, J. Mater. Chem. A, 2020, 8, 1545–1556.
5 X. Cui, C. Tang and Q. Zhang, Adv. Energy Mater., 2018, 8,
1800369.
6 X. Guo, H. Du, F. Qu and J. Li, J. Mater. Chem. A, 2019, 7,
3531–3543.
7 W. Guo, K. Zhang, Z. Liang, R. Zou and Q. Xu, Chem. Soc.
Rev., 2019, 48, 5658–5716.
8 Y. Li, Y. Li, B. Wang, Y. Luo, D. Yang, P. Tong, J. Zhao, L. Luo,
Y. Zhou, S. Chen, F. Cheng and J. Qu, Nat. Chem., 2013, 5,
320–326.
9 Y. Chen, L. Liu, Y. Peng, P. Chen, Y. Luo and J. Qu, J. Am.
Chem. Soc., 2011, 133, 1147–1149.
10 N. Cao, Z. Chen, K. Zang, J. Xu, J. Zhong, J. Luo, X. Xu and
G. Zheng, Nat. Commun., 2019, 10, 2877.
11 Y. Zhang, W. Qiu, Y. Ma, Y. Luo, Z. Tian, G. Cui, F. Xie,
L. Chen, T. Li and X. Sun, ACS Catal., 2018, 8, 8540–8544.
12 W. Qiu, X. Y. Xie, J. Qiu, W. H. Fang, R. Liang, X. Ren, X. Ji,
G. Cui, A. M. Asiri, G. Cui, B. Tang and X. Sun, Nat. Commun.,
2018, 9, 3485.
13 X. Qu, L. Shen, Y. Mao, J. Lin, Y. Li, G. Li, Y. Zhang, Y. Jiang
and S. Sun, ACS Appl. Mater. Interfaces, 2019, 11, 31869–
31877.
14 Y. Guo, Z. Yao, B. J. J. Timmer, X. Sheng, L. Fan, Y. Li,
F. Zhang and L. Sun, Nano Energy, 2019, 62, 282–288.
15 X. Liu, H. Jang, P. Li, J. Wang, Q. Qin, M. G. Kim, G. Li and
J. Cho, Angew. Chem., Int. Ed., 2019, 58, 13329–13334.
16 W. Tong, B. Huang, P. Wang, L. Li, Q. Shao and X. Huang,
Angew. Chem., Int. Ed., 2020, 59, 2649–2653.
17 J. Wang, B. Huang, Y. Ji, M. Sun, T. Wu, R. Yin, X. Zhu, Y. Li,
Q. Shao and X. Huang, Adv. Mater., 2020, e1907112.
18 Y. Zhai, Z. Zhu, C. Zhu, K. Chen, X. Zhang, J. Tang and
J. Chen, Mater. Today, 2020, 38, 99–113.
upon reaction with another proton at the distal N atom. Then,
*NH2–NH2 is formed upon reaction with another proton at the
proximal N atom in a similar manner, and NH3 is formed due to
the reaction of the distal N site in *NH2–NH2, leaving *NH2 on
the Co/Zn site. This residual *NH2 is hydrogenated to give
a second NH3 via a similar path. Subsequently, the proton reacts
with the Np atom to form *NH–NH. *NH–NH2 is subsequently
generated upon reaction with another proton at the distal N
atom. Then, *NH2–NH2 is formed upon reaction with another
proton at the proximal N atom in a similar manner, and NH3 is
formed due to the reaction of the distal N site in *NH2–NH2,
leaving *NH2 on the Fe site. This residual *NH2 is hydrogenated
to give a second NH3 via a similar path. The different pathway
was ascribed to the enhanced steric hindrance of Fe sites due to
the effective N2 activation and shorter Fe–Np bond. The
potential-determining step corresponds to the rst hydrogena-
tion step from adsorbed N2 (*N–N) to NNH (*N–NH), and the
limiting potential for this step is calculated to be 1.20 eV for Fe–
TCPP, 1.99 eV for Co–TCPP, and 2.48 eV for Zn–TCPP. Hence,
the NRR activity decreases in the order of Fe–TCPP > Co–TCPP >
Zn–TCPP, which is consistent with the experimental results. N2
adsorption on the surface of the catalyst is essential for the
subsequent hydrogenation steps. Upon chemisorption of N2 on
the catalyst, N2 activation can proceed efficiently and the energy
barrier of the subsequent steps is reduced due to the enhanced
NRR performance.
In summary, well-dened single-site Fe–TCPP MOFs were
proposed and evaluated as NRR catalysts by combining exper-
iments and DFT calculations. A high NH3 yield of 44.77 mg hꢀ1
mgcat.ꢀ1 and a FE of 16.23% were obtained for Fe–TCPP MOFs,
superior to those of all the reported molecular and MOF cata-
lysts. The optimum activity of Fe–TCPP for NH3 formation was
further conrmed by DFT calculations, which veried the
effective N2 activation at the Fe site and the favorable overall
reaction thermodynamics on Fe–TCPP. Our study provides an
understanding of the intrinsic activity of well-dened catalysts
in the electrocatalytic N2 reduction, and provides atomic-level
insights into the rational design and engineering of highly
active molecular catalysts for articial N2 xation.
19 J. Han, Z. Liu, Y. Ma, G. Cui, F. Xie, F. Wang, Y. Wu, S. Gao,
Y. Xu and X. Sun, Nano Energy, 2018, 52, 264–270.
This journal is © The Royal Society of Chemistry 2021
J. Mater. Chem. A, 2021, 9, 4673–4678 | 4677

View Article Online
Journal of Materials Chemistry A
Communication
20 T. Xu, D. Ma, C. Li, Q. Liu, S. Lu, A. M. Asiri, C. Yang and 35 Q. Li, W. Chen, H. Xiao, Y. Gong, Z. Li, L. Zheng, X. Zheng,
X. Sun, Chem. Commun., 2020, 56, 3673–3676.
21 Y. Liu, X. Kong, X. Guo, Q. Li, J. Ke, R. Wang, Q. Li, Z. Geng
and J. Zeng, ACS Catal., 2019, 10, 1077–1085.
W. Yan, W. C. Cheong, R. Shen, N. Fu, L. Gu, Z. Zhuang,
C. Chen, D. Wang, Q. Peng, J. Li and Y. Li, Adv. Mater.,
2018, 30, e1800588.
22 X. R. Chen, Y. T. O. Guo, X. C. Du, Y. S. Zeng, J. W. Chu, 36 Z. Geng, Y. Liu, X. Kong, P. Li, K. Li, Z. Liu, J. Du, M. Shu,
C. H. Gong, J. W. Huang, C. Fan, X. F. Wang and J. Xiong,
R. Si and J. Zeng, Adv. Mater., 2018, e1803498.
Adv. Energy Mater., 2020, 10, 1903172.
37 W. Zhao, G. Li and Z. Tang, Nano Today, 2019, 27, 178–197.
23 H. Zou, W. Rong, B. Long, Y. Ji and L. Duan, ACS Catal., 2019, 38 H. Sun, Y. Lian, C. Yang, L. Xiong, P. Qi, Q. Mu, X. Zhao,
9, 10649–10655.
J. Guo, Z. Deng and Y. Peng, Energy Environ. Sci., 2018, 11,
2363–2371.
24 Y. Li, Q. Zhang, C. Li, H.-N. Fan, W.-B. Luo, H.-K. Liu and
S.-X. Dou, J. Mater. Chem. A, 2019, 7, 22242–22247.
25 L. Xia, X. Wu, Y. Wang, Z. Niu, Q. Liu, T. Li, X. Shi, A. M. Asiri
and X. Sun, Small Methods, 2018, 3, 1800251.
26 Y. P. Liu, Y. B. Li, H. Zhang and K. Chu, Inorg. Chem., 2019,
58, 10424–10431.
39 S. Zhao, Y. Wang, J. Dong, C.-T. He, H. Yin, P. An, K. Zhao,
X. Zhang, C. Gao, L. Zhang, J. Lv, J. Wang, J. Zhang,
A. M. Khattak, N. A. Khan, Z. Wei, J. Zhang, S. Liu,
H. Zhao and Z. Tang, Nat. Energy, 2016, 1, 16184.
40 X. F. Lu, L. F. Gu, J. W. Wang, J. X. Wu, P. Q. Liao and G. R. Li,
Adv. Mater., 2017, 29, 1604437.
27 R. B. Zhao, C. W. Liu, X. X. Zhang, X. J. Zhu, P. P. Wei, L. Ji,
Y. B. Guo, S. Y. Gao, Y. L. Luo, Z. M. Wang and X. P. Sun, J. 41 Q. Cui, G. Qin, W. Wang, K. R. Geethalakshmi, A. Du and
Mater. Chem. A, 2020, 8, 77–81. Q. Sun, J. Mater. Chem. A, 2019, 7, 14510–14518.
28 F. Wang, X. Lv, X. Zhu, J. Du, S. Lu, A. A. Alshehri, 42 H. Cheng, P. Cui, F. Wang, L. X. Ding and H. Wang, Angew.
K. A. Alzahrani, B. Zheng and X. Sun, Chem. Commun.,
2020, 56, 2107–2110.
29 M. Kuang, Y. Wang, W. Fang, H. Tan, M. Chen, J. Yao, C. Liu,
J. Xu, K. Zhou and Q. Yan, Adv. Mater., 2020, 32, e2002189.
Chem., Int. Ed., 2019, 58, 15541–15547.
43 W. Xiong, X. Cheng, T. Wang, Y. Luo, J. Feng, S. Lu,
A. M. Asiri, W. Li, Z. Jiang and X. Sun, Nano Res., 2020, 13,
1008–1012.
¨
30 F. Lu, S. Zhao, R. Guo, J. He, X. Peng, H. Bao, J. Fu, L. Han, 44 W. Li, W. Fang, C. Wu, K. N. Dinh, H. Ren, L. Zhao, C. Liu
G. Qi, J. Luo, X. Tang and X. Liu, Nano Energy, 2019, 61, 420–
427.
31 Y. Wang, X. Cui, J. Zhao, G. Jia, L. Gu, Q. Zhang, L. Meng,
and Q. Yan, J. Mater. Chem. A, 2020, 8, 3658–3666.
45 F. Xu, L. Zhang, X. Ding, M. Cong, Y. Jin, L. Chen and Y. Gao,
Chem. Commun., 2019, 55, 14111–14114.
Z. Shi, L. Zheng, C. Wang, Z. Zhang and W. Zheng, ACS 46 C. He, Z.-Y. Wu, L. Zhao, M. Ming, Y. Zhang, Y. Yi and
Catal., 2018, 9, 336–344. J.-S. Hu, ACS Catal., 2019, 9, 7311–7317.
32 L. Jiao, G. Wan, R. Zhang, H. Zhou, S. H. Yu and H. L. Jiang, 47 L. Zhao, X. Kuang, C. Chen, X. Sun, Z. Wang and Q. Wei,
Angew. Chem., Int. Ed., 2018, 57, 8525–8529. Chem. Commun., 2019, 55, 10170–10173.
33 K. Liu, X. Zhao, G. Ren, T. Yang, Y. Ren, A. F. Lee, Y. Su, 48 X. Zhao, F. Yin, N. Liu, G. Li, T. Fan and B. Chen, J. Mater.
X. Pan, J. Zhang, Z. Chen, J. Yang, X. Liu, T. Zhou, W. Xi, Sci., 2017, 52, 10175–10185.
J. Luo, C. Zeng, H. Matsumoto, W. Liu, Q. Jiang, K. Wilson, 49 R. Zhang, L. Jiao, W. Yang, G. Wan and H.-L. Jiang, J. Mater.
A. Wang, B. Qiao, W. Li and T. Zhang, Nat. Commun., 2020,
11, 1263.
34 X. J. Cui, H. B. Li, Y. Wang, Y. L. Hu, L. Hua, H. Y. Li,
X. W. Han, Q. F. Liu, F. Yang, L. M. He, X. Q. Chen,
Chem. A, 2019, 7, 26371–26377.
50 M. Zhao, Y. Wang, Q. Ma, Y. Huang, X. Zhang, J. Ping,
Z. Zhang, Q. Lu, Y. Yu, H. Xu, Y. Zhao and H. Zhang, Adv.
Mater., 2015, 27, 7372–7378.
Q. Y. Li, J. P. Xiao, D. H. Deng and X. H. Bao, Chem, 2018, 51 Y. Wang, X. Cui, J. Zhao, G. Jia, L. Gu, Q. Zhang, L. Meng,
4, 1902–1910.
Z. Shi, L. Zheng, C. Wang, Z. Zhang and W. Zheng, ACS
Catal., 2018, 9, 336–344.
4678 | J. Mater. Chem. A, 2021, 9, 4673–4678
This journal is © The Royal Society of Chemistry 2021