Metal–Sulfur Linkages Achieved by Organic Tethering of Ruthenium Nanocrystals for Enhanced Electrochemical Nitrogen Reduction

Article abstract of DOI:10.1002/anie.202009435

Inspired by the metal–sulfur (M-S) linkages in the nitrogenase enzyme, here we show a surface modification strategy to modulate the electronic structure and improve the N₂ availability on a catalytic surface, which suppresses the hydrogen evolution reaction (HER) and improves the rate of NH₃ production. Ruthenium nanocrystals anchored on reduced graphene oxide (Ru/rGO) are modified with different aliphatic thiols to achieve M-S linkages. A high faradaic efficiency (11 %) with an improved NH₃ yield (50 μg h^?1 mg^?1) is achieved at ?0.1 V vs. RHE in acidic conditions by using dodecanethiol. DFT calculations reveal intermediate N₂ adsorption and desorption of the product is achieved by electronic structure modification along with the suppression of the HER by surface modification. The modified catalyst shows excellent stability and recyclability for NH₃ production, as confirmed by rigorous control experiments including ¹⁵N isotope labeling experiments.

Full text of DOI:10.1002/anie.202009435

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Accepted Article
Title: Metal-sulfur linkages achieved by organic tethering of Ru
nanocrystals for enhanced electrochemical nitrogen reduction
Authors: Muhammad Ibrar Ahmed, Chuangwei Liu, Yong Zhao,
Wenhao Ren, Xianjue Chen, Sheng Chen, and Chuan Zhao
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202009435
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10.1002/anie.202009435
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COMMUNICATION
Metal-sulfur linkages achieved by organic tethering of Ru
nanocrystals for enhanced electrochemical nitrogen reduction
Muhammad Ibrar Ahmed,[a]+ Chuangwei Liu,[b]+ Yong Zhao,[a] Wenhao Ren,[a] Xianjue Chen,[a] Sheng
Chen,[a] and Chuan Zhao*[a]
[a]
M.I. Ahmed [+], Dr. Y. Zhao, Dr. W. Ren, Dr. X. Chen, Dr. S. Chen, Prof. C. Zhao*
School of Chemistry,
University of New South Wales,
Sydney, 2052, Australia
chuan.zhao@unsw.edu.au
[b] Dr. C. Liu [+]
Department of Energy Conversion and Storage,
Technical University of Denmark,
Lyngby, 2800, Denmark
[+] These authors contributed equally to this work.
Abstract: Electrochemical nitrogen reduction (NRR) is a clean,
facile, and sustainable approach towards ammonia (NH3) production
at ambient conditions. However, the inert nature of nitrogen and
competitive hydrogen evolution reaction (HER) command it towards
sluggish kinetics and poor selectivity. Inspired by the metal-sulfur
(M-S) linkages in the nitrogenase enzyme, here we show a surface
modification strategy to modulate the electronic structure and
improve the N2 availability on the catalytic surface, which
suppresses the HER and synergistically improved the rate of NH3
production. Thus, ruthenium nanocrystals anchored on reduced
graphene oxide (Ru/rGO) are modified with different aliphatic thiols
to achieve M-S linkages. A high faradaic efficiency (11%) with an
improved NH3 yield (50 µg h-1 mg-1) is achieved at -0.1 V vs RHE in
acidic conditions by using dodecanethiol. DFT calculations reveal
intermediate N2 adsorption and desorption of the product is achieved
by electronic structure modification along with the suppression of the
HER by surface modification. The modified catalyst exerts excellent
stability and recyclability for NH3 production, as confirmed by
rigorous control experiments including the 15N isotope labeling
experiments.
of proton, and (ii) limiting the proton transfer rate. The activity of
proton in bulk electrolytes can generally be controlled by using
non-aqueous solvents or aprotic ionic liquid-based electrolytes.
However, their use at a large scale with high current densities is
limited due to the high viscosity, low conductivity, and mass
transport, as well as complex setup requirements for NRR.[13] In
this regard, surface modification by organic tethering is a facile
strategy that can induce hydrophobicity to limit the proton
transfer on the surface of the catalyst by creating a barrier that
resists the flow of proton without interruption to the flow of non-
polar moieties like N2 and CO2.[14–16] This results in the enhanced
availability of reactant moieties on the surface of the catalyst as
compared to the proton.
Nitrogenase in nature has evolved M-S linkages in the enzyme
for prebiotic N2 fixation to help retain low-valence metal localized
charge density sites[17,18] which can facilitate the N2 adsorption
and subsequent activation and polarization.[19] Moreover, the
proton-accepting ability of sulfur is also supporting the
hydrogenation steps in the NRR pathway. Inspired by
nitrogenase, here we show a Ru-based catalyst with Ru-S
linkages for NRR, achieved by facile surface modification of Ru
with aliphatic thiols. Thiols are known to form self-assembled
monolayers (SAM) on Ru surfaces.[20,21] The organic and
hydrophobic nature of the SAM limits proton transfer while
allowing N2 transport to the surface of the catalyst. Importantly,
the formation of Ru-S linkage also modifies the electronic
structure to facilitate the N2 adsorption, activation, and
polarization on the active site (Ru).[22] This enhanced interaction
with N2 improves the overall selectivity of NRR process by
suppression of HER. A cumulative effect of HER suppression,
electronic structure modulation, and enhanced N2 availability
results in enhanced overall NRR performance. Compared with
non-modified Ru nanocrystals anchored on reduced graphene
oxide (Ru/rGO), significantly enhanced FE of 11% and improved
rate of NH3 production (50 µg h-1 mg-1) is achieved at -0.1 V vs
RHE on Ru nanocrystals anchored on reduced graphene oxide
modified with dodecanethiol (Ru/rGO-C12). Also, control
samples of Ru/rGO modified with different alkyl chain lengths
including hexanethiol (Ru/rGO-C6), and octadecanethiol
(Ru/rGO-C18) are also evaluated for NRR. Rigorous analysis
including 15N isotope labeling experiments is employed for NRR
detection following the recently developed protocols.[23] Due to
its facile and versatile nature, this approach can be employed to
different other catalysts to ameliorate the NRR selectivity.
Ammonia (NH3) is an important commodity in the food and
chemical industry. Due to its high gravimetric hydrogen content
and energy density, it is also considered as a promising
hydrogen carrier.[1,2] Haber-Bosch (HB) process has been
producing NH3 for more than 100 years, which consumes energy
and emits carbon dioxide in vast amounts. Therefore, it is highly
desirable to develop a clean, facile, and sustainable alternative
to this process. Electrochemical nitrogen reduction reaction
(NRR) powered by renewable energy sources is potentially a
sustainable approach for nitrogen (N2) fixation. However, the
inert nature of N2 and intensive competition from hydrogen
evolution reaction (HER) impede its performance with sluggish
kinetics and low selectivity.[3–7] Ru-based transition metal
catalysts are active catalysts for NRR at ambient conditions that
can activate and polarize the inert N2 molecule by accepting and
donating electronic density.[8–11] However, they are limited by the
low selectivity and faradaic efficiency (FE) towards NH3 that
could be attributed to the high affinity of H-adsorption on the Ru
surface.
To overcome this selectivity challenge, Norskov et al.
proposed different strategies to limit proton and electron flow to
the catalyst surface,[12] such as (i) decreasing the concentration
1
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10.1002/anie.202009435
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Figure 2. a) CA curves in 0.05 M H2SO4, b) UV-visible colorimetric spectra at
different voltages stained by Indophenol blue method after 2h NRR in N2, c)
FE and rate of NH3 production and d, e) Control experiments evaluated by 1H-
NMR and Indophenol blue method for Ru/rGO-C12, f) comparison of FE and
rate of NH3 for different catalysts.
Figure 1. a) Schematic illustration of synthesis and modification of Ru/rGO
with aliphatic thiols, b) TEM image, c) HR-TEM image, inset c) FT pattern, and
TEM-EDS elemental mapping of Ru/rGO-C12.
Figure S6a reveals the XPS spectrum of Ru3d and C1s for
all samples. For Ru/rGO sample, peaks at 280.3 eV and 284.7
eV are characteristic of the split spin-orbit region of Ru3d and
can be ascribed to Ru3d5/2 and Ru3d3/2 component of metallic
Ru, respectively.[27] After modification, there is a clear decrease
in the intensity of Ru peaks that validates the surface coverage
with thiols and is more prominent with long-tail thiols (Figure S7).
Moreover, a peak shift of 0.1 to 0.3 eV to higher binding energy
for Ru is also observed in modified samples that could be
ascribed to the Ru-S thiolate bond with localized charge density
sites.[20] Such localized charge density sites can potentially act
as the adsorption sites for the N2. Moreover, corresponding C1s
peaks are present in all samples that can be attributed to the
chemical structure of rGO. However, due to the waxy nature of
octadecanethiol, the disappearance of the C-O peak in Ru/rGO-
C18 suggests the surface coverage with thiols. Furthermore,
Figure S6b shows the S2p region of the XPS spectra of modified
samples. An interaction of Ru-S is confirmed by the peak for
S2p3/2 at 162.6 eV that is assigned to Ru-S linkage.[26,28] It can
be inferred from these results that S atoms are mostly attached
to Ru NPs along with residual free thiols distributed on the
surface of rGO. Similarly, corresponding O1s and the survey
spectrum for all samples are shown in Figure S6c and S6d,
respectively. There is no evident peak of S in the survey
spectrum of the non-modified samples. Also, there is no N peak
at 400 eV that confirms well that our synthesis approach is clean
without any N interference. Inductively coupled plasma optical
emission spectroscopy (ICP-OES) analysis shown in Table S1
suggests the amount of Ru and S are approximately 5 and 3
wt% in modified samples, respectively.
A solvothermal approach is used for the development of
Ru/rGO by employing ethylene glycol and sodium oleate as
reducing and structure-directing agent (Figure 1a).[24] Surface
modification of these nanocrystals is done by stirring in 0.05 M
ethanolic solution of different aliphatic thiols for 24 h.
Transmission electron microscopy (TEM) image of Ru/rGO-C12
reveals a homogenous distribution of Ru nanocrystals on the
surface of rGO with a size of around 5-8 nm (Figure 1b).
Furthermore, crystalline nature of Ru nanocrystals is also
confirmed by high-resolution transmission electron microscopy
(HR-TEM) (Figure 1c), and fast Fourier transform analysis
indicating lattice fringes of 0.22 nm corresponding to the (101)
lattice plane of Ru, which is consistent with the XRD analysis
(Figure S5a). In addition, elemental mapping confirms the
incorporation of sulfur (S) after surface modification (also see
Figure S1 line spectrum). Similarly, Figures S2-S4 reveal the
morphological elucidation with the elemental mapping of control
samples.
Figure S5a shows the XRD patterns of all prepared
samples with broad peaks that reflect the small size of the Ru
nanocrystals. A peak at 23° is ascribed to the presence of rGO,
suggesting the reduction of graphene oxide during the
solvothermal process. Peaks at 38.4°, 42.3°, and 44.0° are the
characteristic peaks of the Ru hexagonal phase (JCPDS-006-
0663) that can be ascribed to the (100), (002) and (101) planes,
respectively. The overall decrease in the intensity of Ru peaks is
observed after the modification with different thiols due to the
amorphous nature of carbon. Fourier transform infrared
spectroscopy (FTIR) is conducted to investigate the interaction
of different thiols on the surface of the Ru nanocrystals (Figure
S5b). Characteristic peaks of rGO at 1586 and 1730 cm-1 can be
ascribed to the C=C and C=O linkages, respectively. The -CH
stretching peaks in the region between 2850-3000 cm-1 also
confirm the presence of long alkyl tails of thiols on the surface of
Ru/rGO as they are absent in the non-modified sample.[25] These
-CH peaks are more prominent in C18 molecules as compared
to C6 and C12 due to the long alkyl tail (Figure S5c). Raman
spectroscopy (Figure S5d) reveals an increase in the ID/IG ratio
after the transformation of graphite oxide (GO) to rGO in the
solvothermal process. However, the ID/IG ratio of the thiol-
modified samples is slightly larger which could be attributed to
the reduction in C=O moieties after surface coverage with thiols.
It is accepted that these defect sites are usually the attachment
sites for thiols in the form of thiolates.[26]
The electrocatalytic NRR performance is evaluated in 0.05
M
H2SO4 in
a
three-electrode, double compartment
electrochemical cell where two compartments are separated by
a glass frit (Figure S8). Ammonia quantification is established by
the Indophenol blue method (IB) and 1H NMR with respective
calibration curves after three independent sets of measurements
(Figure S9 and S10). Watt and Chrisp method is opted for the
estimation of hydrazine (N2H4) as a possible by-product (Figure
S11). Figure S12 reveals the linear sweep voltammograms of
modified and non-modified samples in the Ar atmosphere. A
clear decrease in current density is observed for the thiol-
modified samples, as compared to the non-modified samples,
due to the HER suppression as
a
result of surface
modification.[16] Chronoamperometric experiments (CA) were
conducted for 2 h in N2 and the respective electrolytes were
analyzed for NH3 by IB and 1H NMR methods. A comparative
LSV data in Ar and N2 atmosphere is indicative of NRR
2
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Figure 3. a, b) Recyclability of NRR performance and long-term stability of
Ru/rGO-C12 after NRR in 0.05 M H2SO4, c) FE, and rate of NH3 in 14N2 and
15N2 environment, d) NH3 quantification by 1H-NMR in 15N2 environment.
capability of this catalyst (Figure S13). CA curves for Ru/rGO-
C12 are shown in Figure 2a that reflects good stability and
durability of the catalyst surface. The initial decrease in the
current density is attributed to double-layer capacitance current
and the fast depletion of available reactants on the electrode
surface.[5] Figure 2b reveals the stained electrolytes by the IB
method after CA experiments on Ru/rGO-C12 surface at
different applied potentials. At 655 nm, the highest absorption
was achieved at -0.1 V vs RHE. A FE of 11% with a high rate of
NH3 production (50 µg h-1 mg-1) is achieved and confirmed by
three independent measurements (Figure 2c). This rate of NH3
decreases as expected at higher applied potentials due to the
competing HER, which is kinetically faster than NRR.[29] Control
experiments under open circuit potential in N2, and at -0.1 V in
Ar confirmed that there is no interference from the applied gas
impurity, environment, and the catalyst system itself. These
control experiments results are corroborated well with both
approaches as shown in Figures 2d and 2e. From the above
results, it can be concluded that the origin of produced NH3 is
from the reduction of applied N2 gas. There is no detectable Ru
leaching is observed after NRR evaluations in the electrolyte at
different voltages (Table S3).
The above results show Ru/rGO-C12 outperformed all other
control samples in NRR performance (Figure 2f). Importantly, all
modified samples produced NH3 with a higher rate as compared
to the non-modified sample except Ru/rGO-C18, suggesting the
formation of Ru-S linkage can promote NRR and suppress HER.
However, due to the short alkyl tails of hexanethiol, there is an
appreciable amount of HER that lowered the NRR as compared
to Ru/rGO-C12. Relatively low NRR performance observed on
Ru/rGO-C18 is ascribed to the dense coverage of thiol on the
catalyst surface that reduced the overall electrode conductivity
and available active sites. Modification with dodecanethiol
provided an optimum surface coverage of thiols that suppresses
HER along with the electronic structure modulation and
enhanced availability of reactants on the surface of the catalyst
by the selective allowance of N2 over proton.[30] This is further
elaborated by the double-layer capacitance measurements of all
samples by cyclic voltammetry (Figure S14). The
electrochemical surface area of the modified samples decreased
drastically as compared to the non-modified sample but the NRR
performance improved more than ten folds. Corresponding NRR
performance data for control samples is provided in Figure S15.
Also, no N2H4 was obtained after the 2h CA electrolysis
experiment at -0.1 V vs RHE confirming the excellent selectivity
of this catalyst (Figure S16). The NMR analysis of electrolyte
was also carried out after the 2h CA experiment in Ar at -0.1 V
vs RHE that shows no distinct peak of thiol present in the
electrolyte confirming negligible thiol leaching from the catalyst
Figure 4. a) The optimized atomic configuration b) Bader charge distribution of
*N2 on modified samples c) the Gibbs free energy profile of NRR on Ru/rGO-
C12. Black, purple, white, red, and yellow spheres represent C, Ru, H, O, and
S, respectively.
surface (Figure S17).
A recyclability experiment was conducted for Ru/rGO-C12
by using the same electrode for different cycles. After five cycles
of 2h-electrolysis, a stable FE was achieved around 11% (Figure
3a) with a small decrease in the rate of NH3 production that
could be attributed to the consumption of defects sites on the
rGO or the adsorption of NH3 on the catalyst surface.[9] Moreover,
a
stable current density was obtained during continuous
electrolysis for 12 h, indicating the stability and durability of this
catalyst (Figure 3b). The homogenous size distribution of Ru
nanocrystals is retained after 12 h electrolysis that validates the
role of surface modification in the stability and durability of
material (Figure S18). 15N2 isotope labeling experiment was
conducted similarly as 14N2, except a bubbling of Ar gas for 30
min before the 15N2 NRR experiments to confirm the origin of
produced NH3. Quantification of the produced NH3 by 14N2 and
15N2 was done by the respective calibration curves after
standardization with 14NH3 and 15NH4Cl (15N isotope-labeled
ammonium chloride). At -0.1 V vs RHE, a similar amount of NH3
is produced with 11% FE after NRR for 4h in 14N2 and 15N2
(Figure 3c) that further confirms the applied N2 gas is the main
source for NH3 formation. Corresponding NMR spectrum for the
15N2 isotope labeling experiment is shown in Figure 3d.
DFT calculations were performed to understand the origin
and mechanistic insights of the “modification effect” (ESI for
computational details). Figure 4a and S19 represent the
optimized atomic configuration of Ru/rGO modified with a thiol.
Also, S-CH2-CH3 (Ru/rGO-C6), S-CH2-CH2-CH3 (Ru/rGO-C12),
and S-CH2-CH2-CH2-CH3 (Ru/rGO-C18) were employed to
simplify calculation models. An optimized charge density of N2
with an increase in bond lengths from 1.10 to 1.15 Å is observed
in modified samples that can be attributed to the facile
adsorption, activation, and polarization of N2 molecule on Ru
due to the electronic structure modification by thiols (Figure 4b).
A decrease in N2 adsorption energy value is observed as the
carbon chain length of thiol increased which resulted in the
deviation of electronic density away from Ru due to attached
thiols and a corresponding loss of Bader charges from 0.53 to
0.85 eV is observed. Figure S20 represents optimized structures
and HER capability of Ru site directly attached to the carbon
chain. Figure 4c, S21, and S22 reveal the free energy NRR
profile by distal and alternating pathways for Ru-rGO-C12, Ru-
rGO-C6, and Ru-rGO-C18, respectively. Importantly, HER
becomes more energy-intensive as the tail length of thiol
increased. The 1st hydrogenation step for NRR requires the
same amount of energy for both pathways for all samples, but
later hydrogenation steps are showing variations. For Ru-rGO-
3
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COMMUNICATION
C12 and Ru-rGO-C18, the 2nd and 3rd hydrogenations require
lower energy for the distal pathway compared with the
alternating pathway making the distal pathway as the plausible
route for NRR. However, the 2nd hydrogenation in Ru-rGO-C6 for
the alternating pathway is more favorable as compared to the
distal pathway that can be ascribed to the enhanced N2
adsorption on the surface of Ru in this structure as evident from
the N2 adsorption energy values. Also, the 3rd hydrogenation
step for the alternating pathway is more energy-intensive, which
makes it less favorable for NH3 production. Although the Ru-
rGO-C6 has a relatively smaller ΔGmax, it also has low energy
requirements for HER in comparison with other modified
samples. The improved NRR performance on Ru-rGO-C12 can
be assigned to intermediate N2 adsorption and desorption
capabilities by electronic modifications along with suppression of
the competing HER reaction by surface modification.
In summary, we developed a facile organic tethering
strategy to suppress HER with an improved rate of NH3
production. Ru/rGO-C12 achieved an optimum surface coverage
of thiols that induces electronic structure modulation and
improved molecular confinement along with HER suppression. A
high FE of 11% with an improved NH3 yield (50 µg h-1 mg-1) is
achieved at -0.1 V vs RHE. Also, this modified catalyst exerts
excellent stability and recyclability that are confirmed by different
control experiments including 15N isotope labeling experiments.
It can be anticipated that M-S linkages will provide an
opportunity towards rational catalyst design strategy for NRR as
well as for other energy-related applications. Specifically,
inspired by nature’s nitrogenase, the development of M-S-C or
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10.1002/anie.202009435
Angewandte Chemie International Edition
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This work represents a facile organic tethering strategy to suppress HER along with electronic structure modulation on the surface of
Ru nanocrystals and a FE of 11% with a high rate of NH3 (50 µg h-1 mg-1) is achieved.
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This article is protected by copyright. All rights reserved.