Single-atom dispersed Co-N-C catalyst: Structure identification and performance for hydrogenative coupling of nitroarenes

Article abstract of DOI:10.1039/c6sc02105k

Co-N-C catalysts are promising candidates for substituting platinum in electrocatalysis and organic transformations. The heterogeneity of the Co species resulting from high-temperature pyrolysis, however, encumbers the structural identification of active sites. Herein, we report a self-supporting Co-N-C catalyst wherein cobalt is dispersed exclusively as single atoms. By using sub-?ngstr?m-resolution HAADF-STEM in combination with XAFS and DFT calculation, the exact structure of the Co-N-C is identified to be CoN₄C₈-1-2O₂, where the Co center atom is coordinated with four pyridinic N atoms in the graphitic layer, while two oxygen molecules are weakly adsorbed on Co atoms in perpendicular to the Co-N₄ plane. This single-atom dispersed Co-N-C catalyst presents excellent performance for the chemoselective hydrogenation of nitroarenes to produce azo compounds under mild reaction conditions.

Full text of DOI:10.1039/c6sc02105k

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Journal name
EDGE ARTICLE
Single-atom dispersed Co-N-C catalyst: structure identification
and performance for hydrogenative coupling of nitroarenes
Received 00th January 20xx,
Accepted 00th January 20xx
Wengang Liu,a,b, ‡ Leilei Zhang,a, ‡ Wensheng Yan,c Xiaoyan Liu,a Xiaofeng Yang,a Shu Miao,a
Wentao Wang,a Aiqin Wang,*,a Tao Zhang *,a
DOI: 10.1039/x0xx00000x
Co-N-C catalysts are promising candidates for substituting platinum in electrocatalysis and organic transformations. The
heterogeneity of the Co species resulting from high-temperature pyrolysis, however, encumbers the structure
identification of active sites. Herein, we report a self-supporting Co-N-C catalyst wherein cobalt is dispersed exclusively as
single atoms. By using subångström-resolution HAADF-STEM in combination with XAFS and DFT calculation, the exact
structure of the Co-N-C is identified to be CoN4C8-1-2O2, where Co center atom is coordinated with four pyridinic N atoms
in the graphitic layer while two oxygen molecules are weakly adsorbed on Co atoms in perpendicular to the Co-N4 plane.
This single-atom dispersed Co-N-C catalyst presents excellent performance for the chemoselective hydrogenation of
nitroarenes to produce azo compounds under mild reaction conditions.
www.rsc.org/
and secondary alcohols.8 By doing the control experiment of acid
leaching and the extensive characterizations of the catalysts before
and after acid treatment, we proposed that the single Co atoms
Introduction
M-N-C (M refers to transition metals, typically Fe and Co), a type of
metal and nitrogen bi-modified carbon materials which are usually
evolved from pyrolysis of metal complexes with macrocyclic N4
ligands, has been widely investigated in electrocatalysis as a
potential substitute of platinum.1-7 Owing to their capability of
activating both hydrogen and oxygen, this class of noble-metal-free
bonded with N within graphitic layers were catalytically active sites
whereas those particles of Co or CoOx were merely spectators.
Nevertheless, this claim is still to be proved unequivocally given
that multi-species of cobalt co-exist in the catalyst. On the other
hand, the heterogeneity of such catalysts, not only complicates the
understanding of catalytic mechanism, but also greatly decreases
the atomic efficiency, and even results in undesirable side-reactions.
To tackle these problems, it is highly desirable to synthesize a
single-atom M-N-C catalyst wherein M is exclusively dispersed as
single atoms by bonding with N atoms.
In fact, single-atom catalyst (SAC) is emerging as a new frontier
in heterogeneous catalysis;21-25 in particular, various supported
noble metal SACs have shown great potentials in a plethora of
chemoselective hydrogenation reactions attributed to the uniform
and isolated active sites, e.g., chemoselective hydrogenation of
nitroarenes to anilines,24 selective hydrogenation of acetylene to
ethylene,26 and 1,3-butadiene to 1-butylene.27, 28 However, there is
no report on the successful application of noble-metal-free SACs to
any chemoselective hydrogenation reactions due to the difficulty in
preparing such SACs. Very recently, a report on Co SAC supported
on N-doped graphene was published, which exhibited superior
activity as an electrocatalyst for hydrogen evolution reaction
(HER).29 Unfortunately, a clear image of the exact structure of the
active site is still lacking.
catalysts has recently been explored for a variety of hydrogenation
8-13
and oxidation reactions in organic synthesis,
such as reduction
of nitroarenes to anilines,9, 10 esterification of alcohols,11, 12 and
oxidation of ethylbenzene.13 One key issue associated with the
catalysis of Co(Fe)-N-C materials is the identification of active sites.3,
14-19
As most of the catalysts prepared by pyrolysis at high
o
temperatures (600-900 C) are composed of nanoparticles ranging
from a few to tens of nanometers, both exposed and encapsulated,
as well as of dispersed single atoms that are invisible with normal
electron microscopy techniques, the unambiguous identification of
active sites is still a pending challenge. In literature reports, big
CoOx nanoparticles,3 encapsulated Fe or FeOx nanoparticles,15, 20 as
well as invisible FeNx were all respectively assumed as the active
sites,4, 16 without compelling evidences. Very recently, we prepared
Co-N-C catalyst on a mesoporous carbon and investigated its
catalytic capability for aerobic oxidative cross-coupling of primary
a.State Key Laboratory of Catalysis, iChEM (Collaborative Innovation Center of
Chemistry for Energy Materials), Dalian Institute of Chemical Physics, Chinese
Academy of Sciences, 457 ZhongShan Road, Dalian, 116023, China, E-mail:
aqwang@dicp.ac.cn; taozhang@dicp.ac.cn.
Herein, we report a successful synthesis of a self-supporting
Co-N-C catalyst with single-atom dispersion by using a support-
sacrificed approach. The unrivalled uniformity of Co species
provides a good entry to the identification of active sites without
ambiguity. By using subångström-resolution high-angle annular
dark field aberration-corrected scanning transmission electron
b.University of Chinese Academy of Sciences, Beijing, 100049, China.
c. National Synchrotron Radiation Laboratory, University of Science and Technology
of China, Hefei, 230029, China.
‡
These authors contributed equally to this work
† Electronic Supplementary Information (ESI) available: Experimental details,
catalyst characterization and tests. See DOI: 10.1039/x0xx00000x
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microscopy (HAADF-STEM), X-ray absorption spectroscopy (XAS), support material could be easily removed by acid leaching, which
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DOI: 10.1039/C6SC02105K
and density functional theory (DFT) calculation, we provide resulted in the self-supporting Co-N-C material. The nitrogen
compelling evidence that the exact structure of the active site is sorption isotherms of the Co-N-C catalyst showed a typical type IV
CoN4C8-1-2O2 where Co center atom is coordinated with four isotherm with a hysteresis loop, indicating a mesoporous structure
pyridinic N atoms in the graphitic layer while two oxygen molecules (Fig. S2). The BET surface area was 679.9 m2 g-1. Such a favorable
are weakly adsorbed on Co atoms in perpendicular to the Co-N4 textural structure is desired for catalytic applications.23, 25, 30
plane. To the best of our knowledge, this is the first report on the
Characterization of Co dispersion. The dispersion of the Co-N-C
identification of Co-N-C structure. This single-atom Co-N-C catalyst
catalyst was determined with both XRD and electron microscopy
exhibited excellent activity and selectivity for the hydrogenative
techniques. First, XRD patterns (Fig. S3) did not show any peaks
coupling of nitroarenes to synthesize aromatic azo compounds. A
assignable to cobalt metal or its compounds, suggesting that the Co
variety of reducible functionalities were well tolerated in the
species in the sample are either highly dispersed or amorphous.
catalytic system, and the corresponding azo products could be
Then, normal electron microscopy techniques including SEM, STEM
obtained with good to excellent yields.
and HRTEM were employed to examine the dispersion of Co species.
In good agreement with the XRD result, no any cobalt-containing
nanoparticles but graphitic layers were observed in our efforts of
examining many different areas of the sample (Fig. 1 A and B, and
more images see Fig. S4-S5), implying that cobalt species must be
Results and discussion
Preparation of the self-supporting Co-N-C catalyst. The self-
supporting Co-N-C catalyst with single-atom dispersion was
highly dispersed as tiny clusters or single atoms that are
prepared by using a support-sacrificial approach. Briefly, a
undetectable or invisible by XRD, SEM and HRTEM techniques. At
Co(phen)2(OAc)2 complex precursor was first supported on Mg(OH)2
the meanwhile, elemental mapping result (Fig. S6) indicates that
o
and then the mixture was submitted to pyrolysis at 700 C in N2 for
the signals of Co, N and C are completely superimposed on each
2 h, after which the MgO support was removed by treating the
other at least in the nanoscale, suggesting that Co might be bonded
material with nitric acid. Elemental analysis of the as-prepared Co-
with N or C. In order to provide information at an atomic scale, we
N-C catalyst revealed a Co loading of 3.6 wt% while the Mg residue
then used subångström-resolution HAADF-STEM technique to
almost negligible (0.09 wt%). In comparison with the commonly
inspect the Co-N-C catalyst. To our surprise, a large number of
used carbon supports, the employment of Mg(OH)2 presented
uniformly dispersed Co single atoms were clearly observed (Fig. 1C,
prominent advantages of preventing the aggregation of cobalt (Fig.
D); examination of different regions revealed that no clusters or
S1), which could be attributed to the moderate interaction of
small particles were present in the vicinity of the single atoms
Mg(OH)2 with the Co species as well as its inertness towards the
(more images see Fig. S7). Assuming that all the Co components in
reaction with Co during high temperature pyrolysis process, both of
the sample are dispersed as single atoms, we calculated the density
of Co atoms to be approximately 0.55 Co atoms nm-2 based on the
them facilitated the dispersion of Co species. Furthermore, the
BET surface area and the loading of cobalt. Such a high density was
rarely obtained in supported precious metal SACs. 17, 19, 24
Structure identification of Co-N-C. Having this single-atom
dispersed Co-N-C catalyst in hand, we are able to identify the
structure associated with Co atoms. It has been well established
that XAS is a powerful technique to determine the chemical state
and coordination environment of centre atoms in the sample.31, 32
Fig. 2A shows the XANES spectra at Co K edge of the Co-N-C sample
as well as of several reference samples. In comparison with both
Co(II) phthalocyanine (green line) and Co (II) porphyrin (red line)
complexes featured with a pre-edge peak at 7714-7716 eV which
was regarded as a fingerprint of Co-N4 square-planar structures, the
absence of pre-edge peak in the Co-N-C catalyst (blue line)
34
suggested that it was not a planar structure.33,
Likewise, the
catalyst precursor Co(phen)2(OAc)2 was also lack of pre-edge peak
due to the tetragonal-like structure of Co(II) phenanthroline. A
comparison of E0 value (the first inflection point) showed that the
Co-N-C sample had almost the same E0 value (7720 eV) as its
precursor (Table S1), which was assigned to Co2+, whereas the
reference Co foil and Co3O4 had the E0 value of 7709 eV and 7727
eV, respectively. Further examination of Co-N-C samples which
were obtained at different annealing temperatures showed that the
pyrolysis at 900 oC resulted in the formation of metallic Co while
that at 500-700 oC always yielded Co2+ (Table S1).
Fig. 1 SEM (A), HRTEM (B), and HAADF-STEM (C, D) images of Co-N-
C catalyst. The white dots in (C, D) are Co single atoms.
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Co. Such a different structure may also account for their different
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reactivities in both electrocatalysis DaOnI:d10.10c3h9e/mC6oSsCe0le2c1t0i5vKe
transformations of organic molecules.2, 4, 5, 35
The above CoN4C8-1-2O2 structure was further corroborated
with EXAFS analysis. Fig. 3A shows the Fourier-transformed k2-
weighted EXAFS spectra at Co K edge. In contrast to the reference
samples Co foil and Co3O4, our Co-N-C catalyst did not present
prominent peak at the positions of either Co-Co or Co-O
coordinations, supporting the conclusion that it does not contain
either metallic Co or CoOx species. The EXAFS data were further
fitted with the model in Figure 2B in r-space (Fig. 3A), k-space (Fig.
3B), and q-space (Fig. S10) in the ranges of Δr (1.1-3.5 Å) and Δk
(3.0-12.0 Å-1), respectively, and the fitted results are in good
agreement with the experimental ones, corroborating the structure
of CoN4C8-1-2O2. Moreover, the oscillation of Co-N is the most
intense one, indicating that the Co-N is the nearest shell close to
the center and contributes the most to the total oscillations. The
best-fit result of the EXAFS data is summarized in Table 1, showing
the Co-N shell with a coordination number (CN) of 3.7 at a distance
Fig. 2 (A) The normalized XANES spectra at Co K-edge of different
samples. (B) Comparison between the K-edge XANES experimental
spectrum of Co-N-C (solid red line) and the theoretical spectrum
(black dotted line) calculated with the inset structure.
To determine the structure of the Co-N-C sample, we then
used DFT to establish various architecture models based on Co(II)
porphyrin with or without considering coordination with oxygen
atoms at axial direction (Table S2). Firstly, we calculated the XANES
spectra of the reference sample Co porphyrin to verify the validity
of our calculation method. As shown in Fig. S8, the calculated result
was in good agreement with the experimental spectrum of the Co
porphyrin, validating our calculation method. Then, with those Co-
N-C models, we calculated XANES (dotted lines in Fig. S9) and found
that the calculation results for CoN4C8-1-2O2 could best reproduce
the main features of the experimental spectrum, as shown in Fig. 2B.
In this structure, the Co center atom is coordinated with four
pyridinic N atoms, while the whole structure is deformed from a
plane. In perpendicular to the Co-N4 plane, two oxygen atoms are
weakly adsorbed on Co center. In comparison with the Fe-N-C
structure (FeN4C12O2) reported by Zitolo et al.,34 the present Co-N-C
catalyst adopted a quite different structure, which may result from
the intrinsic difference in the electronic properties between Fe and
Fig.3 (A) The k2-weighted Fourier transform spectra of experimental
and fitting Co-N-C catalyst as well as Co foil and Co3O4 reference
samples. (B) The contributions of different paths including Co-N
(blue line), Co-O (pink line) and Co-C (green and navy blue lines) in
k-space for Co-N-C sample.
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Table 1. EXAFS data fitting results of Co-N-C catalyst.
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DOI: 10.1039/C6SC02105K
Sample
Shell
N
R (Å)
σ2×102
(Å2)
0.7
1.6
1.8
ΔE0
r-factor
(%)
0.29
(eV)
-7.9
-7.9
-7.9
-7.9
Co-N-C
Co-N
Co-O
Co-C1
Co-C2
3.7
1.7
4.0
4.2
1.88
2.08
2.76
3.21
1.8
N, the coordination number for the absorber-backscatterer pair. R,
the average absorber−backscatterer distance. σ2, the Debye−Waller
factor. ΔE0, the inner potential correction. The accuracies of the
above parameters were estimated as N, ±20%; R, ±1%; σ2, ±20%;
ΔE0, ±20%. The data range used for data fitting in k-space (Δk) and
r-space (Δr) are 3.0-12.0 Å-1 and 1.1-3.5 Å, respectively.
of 1.88 Å, the Co-O shell with a CN of 1.7 at a distance of 2.08 Å,
and the Co-C shell with a CN of 4.0 and 4.2 at a distance of 2.76 Å
and 3.21 Å, respectively. This result is found to be consistent, within
an acceptable error, with the structural parameters of CoN4C8-1-
2O2 determined by DFT and XANES calculations (Table S2).
XPS characterization. The chemical states of Co and N in the sample
were also examined with XPS. The N 1s spectrum (Fig. 4A) could be
deconvoluted into two peaks centered at the binding energy of
400.8 and 399.1 eV, which were assigned to graphitic and pyridinic
N, respectively.2, 36 The formation of graphitic N was due to the
graphitization of the precursor 1,10-phenanthroline that was not
coordinated with Co cations, while the predominance of pyridinic N
(accounting for 57.5
% in total N species) resulted from
coordination with cobalt, according to the structure in Fig. 2B. In
agreement with the implications of XPS of N 1s, the XPS spectrum
of Co 2p3/2 is characteristic of Co2+ species(Fig. 4 B), as indicated by
the binding energy of 780.7 eV and the presence of a satellite peak
(green line).37 The predominance of Co2+ was also supported by
electron paramagnetic resonance (EPR) experiment (Fig. S11).38 This
Co2+ species, strongly coordinated with four N atoms in the
graphitic layer, is robust enough to resist against either aggregation
at high temperatures or leaching by acid treatment. In addition to N
and Co, there were also C (79.26 at.%), O (11.36 at.%), and the
residue Mg (0.06 at.%) detected by XPS on the catalyst surface
(Table S3, Fig. S12). It should be highlighted that the atomic
concentration of Co determined by XPS was 0.7 at%, corresponding
to a weight percentage of 3.5 wt%, which was very close to the ICP-
AES analysis result (3.6 wt%), demonstrating the homogeneous
dispersion of Co throughout the whole material. The O1s spectrum
was deconvoluted into two peaks at 531.5 and 533.1 eV, which
were ascribed to ketonic C=O groups and C−O groups, respectively
(Fig. S12). The absence of Co-O peak in the O1s was probably due to
the removal of weakly adsorbed oxygen molecules when the
sample was brought into high vacuum for recording the XPS spectra.
Fig. 4 (A) N 1s and (B) Co 2p XPS spectra of Co-N-C catalyst.
0.7 mol% Co, 0.2 equivalent of NaOH), the isolated yield of azo
reached 97%. The kinetic results (Fig. 5) showed that at the
beginning 40 min nitrobenzene was reduced to azoxybenzene
which was then further reduced to azobenzene in the following 50
min and aniline with extended duration. Clearly, azoxy was an
intermediate for the azo product, consistent with the earlier
proposed nitro-nitroso-hydroxylamine condensation pathway (Fig.
S13).40, 47 In this tandem reaction the Co-N-C catalyst was believed
to catalyze the hydrogenation steps whereas a small amount of
alkali (NaOH) was required to catalyze the condensation step
between nitrosobenzene and hydroxylamine. As the condensation
step was too fast to be observed, the hydrogenation step appeared
as a rate-determining step. It was also found that the reaction rate
was first-order dependence on H2 pressure (Fig. S14), suggesting
that hydrogen dissociation on the Co-N-C active sites proceed
smoothly.48 The calculation of the turnover frequency (TOF) based
on the initial conversion of nitrobenzene per Co atom showed that
the Co-N-C catalyst afforded a TOF of 35.9 h-1, which was only
slightly lower than that over precious metals (e.g., TOF over
Au/Mg4Al was 41.8 h-1)44, demonstrating the potential of the Co-N-
C material to replace expensive noble metal catalysts in chemical
transformations.
Catalytic performance. The well-defined and atomically dispersed
Co-N-C catalyst was evaluated in the hydrogenation of nitroarenes
to azo compounds, an important reaction in the fields of dyes and
pharmaceuticals.39-46 Investigation of the reaction conditions (Table
S4) showed that both the activity and selectivity were significantly
affected by the reaction temperature and time, H2 pressure, solvent,
o
and additional base. Under the optimized reaction conditions (80 C,
3 MPa H2, 1.5 h, tert butyl alcohol as the solvent, catalyst loading of
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Table 2. Substrate scope of the hydrogenation reactioVni.ew Article Online
DOI: 10.1039/C6SC02105K
For detailed reaction conditions, refer to the supporting
information. Yields refer to isolated products.
Fig. 5 Substrate and products concentration profiles as a function
of the reaction time. Reaction conditions: Co-N-C catalyst: 0.7 mol%; was strongly coordinated with four pyridinic nitrogen atoms within
NaOH (0.2 equiv.); solvent: t-BuOH (2 ml); 80 oC; 3 MPa H2.
graphitic layers, and oxygen atoms were weakly adsorbed on the Co
atoms in perpendicular to the Co-N4 plane. Such a unique structure
showed excellent activity, chemoselectivity and stability for the
synthesis of aromatic azo compounds through hydrogenative
coupling of nitroarenes, thus providing new opportunities of using
earth-abundant elements to replace scarce and high-cost noble
metal catalysts in a variety of chemical transformations. More
extensively, with the identification of the catalytically active
structure, design and synthesis of the better performance M-N-C
catalysts will be expected in future, facilitating the real replacement
of platinum in both thermo- and electro-catalysis in practice.
What is of more interest is the unprecedented selectivity of
Co-N-C catalyst for diverse nitroarene substrates, particularly those
bearing reducible groups. As shown in Table 2, both the electron-
rich (Table 2, entries 2-6) and electron-deficient (Table 2, entries 7-
12) nitroarenes were converted quickly and selectively into the
corresponding azo products. Moreover, for the more challenging
substrates 3-nitrostyrene (Table 2, entry 6), the catalyst could
reduce the nitro groups without any detectable concurrent
reduction of the unsaturated alkenes. Both the control reaction and
the ATR-IR (Attenuated Total Reflection-Infrared) spectroscopy
results (Table S5 and Fig. S15) revealed that the alkene group could
not be adsorbed on the Co-N-C moieties at all, which should
account for the extraordinarily high selectivity. Interestingly,
halogenated nitroarenes (Table 2, entries 8-12) can also be
excellent and useful substrates for selective hydrogenative coupling
reactions. Both mono and dihalogenated nitroarenes were all
quickly converted into corresponding chloro, bromo, or
dihalogenated azo compounds without other by-products. Notably,
for aryl iodides (Table 2, entry 12) that are prone to undergo
dehalogenation on many precious catalysts, the yield of iodo azo
compound on Co-N-C catalysts reached 94% without
dehalogenation. Furthermore, the catalyst could be easily
recovered from the reaction system by filtration and could be
reused for at least 5 times without change in selectivity or yield,
provided that the reaction time was suitably extended (Table S6).
HAADF-STEM images showed that the Co single atoms were still
uniformly dispersed throughout the sample after reuse and no
aggregation was observed (Fig. S16), demonstrating the high
stability of the Co-N-C catalyst.
Acknowledgements
The authors are grateful to the National Science Foundation of
China (21176235, 21203182, 21202163, 21303194, 21373206,
21476227, 21522608 and 21503219) and the Strategic Priority
Research Program of the Chinese Academy of Sciences
(XDB17020100). The foundation from the Youth Innovation
Promotion Association CAS (2014163) and the Hundred Talents
Program of DICP are also acknowledged. We also thank the BL 14W
beamline at the Shanghai Synchrotron Radiation Facility (SSRF) for
the XAFS experiments and Dr. Hongxian Han for his help in EPR
analysis.
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In summary, we have successfully synthesized self-supporting
atomically dispersed Co-N-C catalysts. The exact structure of the
catalyst was identified to be CoN4C8-1-2O2 where Co single atom
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