Cobalt in N-doped carbon matrix catalyst for chemoselective hydrogenation of nitroarenes

Article abstract of DOI:10.1016/j.apcata.2019.04.033

Anilines as important intermediates for both organic synthesis and industrial manufactory are densely substituted with a variety of functional moieties, and the transformation of nitroarenes into corresponding anilines requires catalytically selective hydrogenation catalyst. Herein, we describe a simple pyrolysis strategy to prepare cobalt catalysts in nitrogen-doped carbon matrix applied in the selective hydrogenation of nitroarenes with molecular hydrogen. The Co/NC catalysts are obtained through thermal treatment of mixed precursors of cobalt phthalocyanine and melamine. The surface-modified Co particles with Co₃O₄ and CoN_x sites are surrounded by N-doped carbon layers according to a series of structural characterization results. These Co/NC catalysts are capable of efficiently selective hydrogenation of nitrobenzene and various substituted nitroarenes into corresponding anilines under relatively mild reaction conditions. The optimal catalytic hydrogenation performance is contributed to the fast rate of H₂ dissociated activation on the CoN_x active sites and the facile adsorption of the reactant substances, which is verified by the isotopic H₂-D₂ exchange experiments, reactant adsorption and the ORR reaction tests. Furthermore, the heterogeneous Co/NC catalyst is highly stable without the Co leaching and deactivation issues during the recycling reaction runs.

Full text of DOI:10.1016/j.apcata.2019.04.033

Accepted Manuscript
Title: Cobalt in N-doped carbon matrix catalyst for
chemoselective hydrogenation of nitroarenes
Authors: Yihu Dai, Chunyang Jiang, Min Xu, Bo Bian, Di Lu,
Yanhui Yang
PII:
S0926-860X(19)30192-9
DOI:
Reference:
https://doi.org/10.1016/j.apcata.2019.04.033
APCATA 17065
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Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
19 February 2019
18 April 2019
29 April 2019
Please cite this article as: Dai Y, Jiang C, Xu M, Bian B, Lu D, Yang Y, Cobalt in N-
doped carbon matrix catalyst for chemoselective hydrogenation of nitroarenes, Applied
Catalysis A, General (2019), https://doi.org/10.1016/j.apcata.2019.04.033
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Cobalt in N-Doped Carbon Matrix Catalyst for
Chemoselective Hydrogenation of Nitroarenes
Yihu Dai*, Chunyang Jiang, Min Xu, Bo Bian, Di Lu, Yanhui Yang*
Institute of Advanced Synthesis, School of Chemistry and Molecular Engineering,
Nanjing Tech University, Nanjing 211816, China
Corresponding authors:
E-mail: ias_yhdai@njtech.edu.cn (Y. Dai)
E-mail: yhyang@njtech.edu.cn (Y. Yang)
Graphical abstract
Highlights:
 High moiety-tolerance ability and reuse stability can be obtained on
Co/NC catalyst.
 Co/NC catalyst has composite structure of Co@Co3O4&CoNx and
N-doped C layers.
1

 H2-D2 exchange results suggest that fast H2 dissociation enhance the
hydrogenation activity.
 Reactant adsorption tests indicate that facile adsorption promotes the
hydrogenation reaction.
Abstract:
Anilines as important intermediates for both organic synthesis and industrial
manufactory are densely substituted with a variety of functional moieties, and the
transformation of nitroarenes into corresponding anilines requires catalytically
selective hydrogenation catalyst. Herein, we describe a simple pyrolysis strategy to
prepare cobalt catalysts in nitrogen-doped carbon matrix applied in the selective
hydrogenation of nitroarenes with molecular hydrogen. The Co/NC catalysts are
obtained through thermal treatment of mixed precursors of cobalt phthalocyanine and
melamine. The surface-modified Co particles with Co3O4 and CoNx sites are
surrounded by N-doped carbon layers according to a series of structural
characterization results. These Co/NC catalysts are capable of efficiently selective
hydrogenation of nitrobenzene and various substituted nitroarenes into corresponding
anilines under relatively mild reaction conditions. The optimal catalytic hydrogenation
performance is contributed to the fast rate of H2 dissociated activation on the CoNx
active sites and the facile adsorption of the reactant substances, which is verified by the
isotopic H2-D2 exchange experiments, reactant adsorption and the ORR reaction tests.
2

Furthermore, the heterogeneous Co/NC catalyst is highly stable without the Co leaching
and deactivation issues during the recycling reaction runs.
Keywords:
nitroarenes, anilines, selective hydrogenation, cobalt catalyst, N-doping carbon
3

1. Introduction
The reduction of nitroarenes to their substituted anilines is of great significance in
organic synthesis and industrial production of a variety of high-value intermediates
including dyes, pharmaceuticals and agrochemicals [1-3]. Functional anilines can be
obtained by using stoichiometric reducing agents, e.g., Na2S2O4 [4,5], N2H4 [6,7],
NaBH4 [8,9] and NH3BH3 [10,11], nonetheless, these reduction processes lead to
serious environmental pollution issues. Molecular hydrogen is thus a more attractive
and cleaner reductant, and accordingly the catalytic hydrogenation using hydrogen is
of great interests for both academic research and industrial productions [12-14].
Heterogeneous catalysts containing supported noble metal nanoparticles (NPs) are
industrially employed in the direct hydrogenation of nitroarenes [15-19]. Considering
the fact that several valuable anilines are densely substituted with easily reduced or
eliminated functional groups, especially ketones, aldehydes, alkenes, alkynes or
halogens, a high catalytic chemoselectivity is essential for an ideal applicable
hydrogenation catalyst. The superiority of the selective catalyst in the context of
moiety-tolerance will allow us to replace the complicated multi-step synthesis
involving group-protecting, separation and purification processes [20]. The
modification of certain additives on the Pt catalyst has demonstrated to be one of
effective one-step solutions for nitroarenes hydrogenation reaction [21]. Corma’s group
has also reported excellent Au-based catalysts to achieve highly selective
hydrogenation of nitro group in the presence of a number of reducible substituent
groups [22]. Nonetheless, the practical nitroarenes hydrogenation processes are still
4

suffering the chemoselectivity issue and the promising catalyst has to be less expensive,
highly active without compromising selectivity.
In addition to the noble metal catalysts, tremendous efforts have attempted to
develop cost-effective hydrogenation catalysts based on first-row transition metals [23-
26]. Beller’s group has reported the nanoscale cobalt oxides-based catalysts that were
capable of highly selective hydrogenation of nitro group for more than 80 nitroarenes
[27-30]. The catalysts were composed of the active cobalt oxide particles surrounded
by a nitrogen-doped carbon layer, and such unique structure can be furnished by a
pyrolytic deposition of metal-phenanthroline complexes on carbon support. Although
these Co catalysts supported on N-doped carbon sheets displayed the superior
selectivity towards hydrogenation of nitro group, some debate upon the structure and
the catalytic effects Co-relevant active sites still remains: (i) metallic Co0 or oxidized
CoOx, (ii) accessible or inaccessible Co sites, and (iii) catalytic mechanism of active
CoNx species. Hence, to better rationalize the relation between the catalytic activity and
the status of Co sites in Co-catalyzed nitroarene hydrogenation reactions, precise
synthesis method to gain well-controlled structure should be further explored.
Controllable pyrolysis can afford the selective metal/NC catalysts by employing
different metal salts (i.e. acetate, nitrate, etc.), nitrogen precursors (i.e. urea, melamine,
1,10-phenanthroline, etc.) and structure directing agents in certain processing steps [31-
38]. A variety of carbon materials have been also employed as the catalyst supports,
including active carbon (AC), carbon nanotubes (CNTs) and carbon black (CB) [39,40].
Another proven synthesis strategy is the carbonization of metal-organic-frameworks
5

(MOFs) consisting of both metals and N species under high-temperature [10,23,41-44].
Delicate modification on the MOFs can optimize the homogeneous dispersion of metal
sites and porous structure. For instance, the specific coordination interaction provided
by the amine groups at the MOFs skeleton assists the formation of isolated single-atom
Ru sites supported on N-doped porous carbon [45].
Herein, we report a simple method to synthesize Co/NC catalysts via the pyrolysis
of the mixture of cobalt phthalocyanine and melamine with different molar ratios. The
structure of Co-relevant sites in N-doped carbon matrix is carefully investigated with
various characterizations. The Co/NC catalysts are further applied on the
chemoselective hydrogenation of nitroarenes to their substituted anilines, and their
catalytic activity and selectivity as well as reusability are evaluated under a relatively
mild condition. We also study the H2 dissociation and reactant adsorption on active Co
sites to understand the catalytic effects of the Co/NC catalysts.
2. Experimental
2.1. Materials
Melamine (99%) was obtained from Aladdin Reagent Corp. (Shanghai, China).
Cobalt phthalocyanine (CoPc, 99%), nitrobenzene (99.5%), aniline (99.9%), and a
series of derivatives were purchased from Sigma-Aldrich (Shanghai, China). The
nitroarenes includes 2-chloro-1-fluoro-4-nitrobenzene (98%), 1,3,5-trichloro-2-
nitrobenzene (95%), 1,2-dichloro-4-fluoro-5-nitrobenzene (95%), 1-bromo-4,5-
dimethoxy-2-nitrobenzene (99%), 4-nitrobenzonitrile (97%), 1-doto-3-nitrobenzene
(99%), 2-nitro-4-(trifluoromethyl)benzaldehyde (99%), 4-nitrobenzaldehyde (98%), 2-
6

nitro-4-(trifluoromethyl)benzonitrile (97%), 1,3,5-trimethyl-2-nitrobenzene (97%), 1-
nitronaphthalene (97%). N2 (99.999%), H2 (99.999%) and He (99.999%) were supplied
by Nanjing Special Gas Factory (Nanjing, China). Pure D2 (99.999%, deuteration
degree >99.8%) was purchased from Suzhou Changyou Gas Co., Ltd. (Suzhou, China).
2.2. Catalyst preparation
A certain amount (0.15-0.45 g) of CoPc and 5 g of melamine were thoroughly
milled and the formed blue powder was transferred to a crucible. The first-stage
o
pyrolysis procedure was performed in a muffle furnace at 300 C for 1 h and then at
550 oC for 2 h with a heating rate of 2 oC/min. After natural cooling to room temperature,
the resulting solid was fully ground. The second-stage pyrolysis was performed in a
o
tubular furnace at 900 oC for 2 h with a heating rate of 3 C/min in 50 mL/min of N2.
The powder sample was finally obtained after cooling, which was named after Co/NC-
x (x=0.15 to 0.45 depending on the adding amount of CoPc precursor). The reference
sample ‘NC’ was also prepared through same procedure without mixing CoPc.
2.3. Catalyst characterization
The Co contents in the catalysts were measured by inductively coupled plasma
optical emission spectrometry (ICP-OES, Avio 200, PerkinElmer). Powder X-ray
diffraction (XRD) patterns were recorded on a Bruker AXS D8 Advance diffractometer
using Cu Kα radiation (λ=0.154 nm) at 40 kV and 40 mA. The N2 adsorption-desorption
measurements were performed at 77 K on a Quantachrome Autosorb-iQ3 adsorption
apparatus to obtain the Brunauer-Emmett-Teller (BET) specific surface areas. Prior to
measurement, the sample was degassed under vacuum at 473 K overnight.
7

Transmission electron microscopy (TEM) was performed by using a JEOL 2100F
instrument operating at 200 kV. X-ray photoelectron spectra (XPS) were recorded on a
VG Scientific ESCALAB Mark II spectrometer. All binding energies were referenced
to the peak of C 1s core level at 284.6 eV. The Raman measurements were carried out
on a Horiba HR Evolution spectrometer with 532 nm excitation laser.
2.4. Catalytic hydrogenation of nitroarenes
The catalytic hydrogenation reaction of nitroarenes was carried out in a 50 mL
Hastelloy batch reactor with a Teflon liner (NS-50-C276, Anhui Kemi Machinery
Technology Co., Ltd.). Catalyst (30 mg), nitrobenzene or functionalized nitroarenes (2
mmol), and ethanol solvent (10 mL) were added into the reactor. The residual air in the
reactor was removed by pressurizing and releasing H2 gas several times. The reaction
was performed at 343 K under 10-30 bar of H2. The stirring speed was set at 700 rpm
for all the reaction tests to eliminate the external mass transfer limitation. After the
reaction was completed, the autoclave was cooled to room temperature and the H2 was
released. The catalyst powder was removed by 1400 rpm centrifugation and all the
products in the solution were analyzed using a gas chromatograph (GC, Agilent, 7890B)
equipped with a flame ionization detector (FID) and a CP-WAX 52 CB column (30 m
× 0.25 mm × 0.25 μm), and tetradecane was the internal standard. The byproducts were
identified by a gas chromatography-mass spectrometry (GC-MS, Agilent, 6890N-
5973). The carbon balance was >99% in all the hydrogenation reactions. For recycling
tests, the used catalyst after each reaction run was hot filtrated and separated, washed
with ethanol several times, and then dried overnight for the next run.
8

2.5. H2-D2 exchange experiment
H2-D2 exchange experiments were performed in a fixed-bed reactor and the
feeding gas consisted of 4 mL/min of H2, 4 mL/min of D2 and 18 mL/ min of N2. A 15
mg portion of catalyst was loaded in the reactor. The exchange products of H2, HD and
D2 were analyzed with a QGA mass spectrometer (MS, Hiden Analytical Ltd.). Prior to
the H2-D2 exchange experiment, the sample was pretreated in N2 at 120 oC for 30 min.
After cooling to 70 oC, the gas was switched to the reactant gas composition to start the
H2-D2 exchange reaction with MS signal recording.
2.6. Adsorption experiment
The adsorption amounts of nitrobenzene reactant and aniline product on various
Co/NC catalysts were measured by adding the sample (20 mg) into 5 mL of ethanol
solution containing 20 μmol of the adsorbate. The suspension stirring was lasted for 40
min at room temperature and then the solid sample was filtered by centrifugation. The
amount of the adsorbate remained in the supernate was measured with GC to determine
the adsorption amount.
2.7. Electrochemical measurement
In consideration of the oxygen reduction reaction (ORR) reaction might provide
the structural information of the active CoNx sites in the Co/NC catalysts, the
electrochemical activity measurement towards the ORR reaction was performed in a
three-electrode cell using 0.1 M KOH electrolyte, a Pt sheet as counter electrode and a
Ag/AgCl electrode as reference electrode. A rotating ring-disk electrode (Autolab,
Metrohm) equipped with a glassy carbon disk (0.5 cm in diameter) was used as working
9

electrode. Co/NC catalyst (5 mg) was dispersed in 2.5 mL of solution (0.1 vol.% Nafion
in ethanol and water) to obtain a final dispersion of the catalyst via sonication for 6 h.
The dispersion (10 μL) was pipetted on the newly polished disk electrode to prepare
the working electrode. Linear sweep voltammetry (LSV) was measured in O2 saturated
o
0.1 M KOH solution at 25 C with a scanning rate of 20 mV/s and a rotation rate of
2000 rpm.
3. Results and discussion
3.1. Structural characterizations
The synthesis of Co/NC catalysts is achieved by simple pyrolysis treatment of
starting materials of CoPc and melamine with different mixture ratios. According to the
ICP-OES results, a series of as-prepared Co/NC catalysts have similar loading amount
of Co species in the range of 27.8-30.2 wt.%, as listed in Table 1, indicating that
structural evolution of the catalyst is centered on the formation of Co-based
microstructure. It can be indirectly supported by the fact that the ‘NC’ sample has a
remarkable low yield via same preparation process without mixing CoPc additive.
Melamine determines the formation of C3N4-like sheet morphology during thermal
decomposition, therefore, the production of Co/N-doped carbon sheet hybrids can
indeed arise from thermolysis of mixed melamine and CoPc. The crystal phases are
further characterized by the powder XRD patterns in Fig. 1A-E. The diffraction peak
at 26.2o suggests that graphitic carbon forms during the pyrolysis. The diffraction peaks
at 44.4o, 51.6o and 76.0o are assigned to metallic Co0 species [46]. Neither peaks of
Co3O4 phase nor peaks of CoO phase are observed on the XRD patterns. The specific
10

surface areas of the catalysts are determined by the N2 adsorption isotherms, as
presented in Fig. 1F-J and Table 1. Relatively larger surface areas are obtained with 203
and 250 m2/g for Co/NC-0.30 and Co/NC-0.35 catalysts, respectively, whereas a low
value of 53.6 m2/g is observed on Co/NC-0.15 catalyst. The TEM images in Fig. 2A-E
display that the metallic Co0 particles are surrounded by typical bamboo-like
nanofilaments, suggesting that the carbonization of melamine and the ligands in CoPc
leads to the deposition of thin carbon layers at high treatment temperature. The
statistical analyses of Co particle sizes in Fig. 2F-J demonstrate that the average size of
Co particles varies with rising the amount of CoPc precursor addition. The average
particle sizes of Co/NC-0.30, 0.35 and 0.45 catalysts are approximately 12-14 nm,
which are dramatically smaller than that of Co/NC-0.15 catalyst (40 nm), as listed in
Table 1.
The XPS spectroscopy is employed to reveal the electronic structure of N and Co
species in the Co/NC catalysts. The N 1s core level spectra of the Co/NC catalysts in
Fig. 3A-E can be deconvoluted into four types of N species, which are attributed to
pyridinic (398.6 eV), pyrrolic (400.1 eV), graphitic (401.1 eV) and oxidative N species
(402.8 eV) according to their binding energies [6,47]. The pyridine-type N species with
the lowest binding energy are bound to a metal ion, while the pyrrole-type N species
contribute two electrons to the carbon matrix [48,49]. Fig. 3F-J display the Co 2p 3/2
core level spectra that can be fitted by four components, i.e., metallic Co0 (778.6 eV),
oxidative Con+ (779.8 eV), N-bonded Co species (781.2 eV) and satellite peak (785.6
eV) [50]. The quantitative results are summarized in Table 1 and the distributions of
11

these co-existed Co or N species are distinctly different for various Co/NC catalysts.
Similar proportions of metallic Co0 are observed with approximately 20% for all the
catalysts. In particular, for two relative surface species of CoNx and pyridinic N
coordinated with Co atom, their distributions in the Co/NC catalysts exhibit volcano-
type curves, as shown in Fig. 4. The maximal contents of both CoNx and pyridinic N
species are obtained on the Co/NC-0.30 catalyst. Either reducing or increasing the
added amount of CoPc during catalyst synthesis causes lower contents of these two
surface species. The existence of oxidative Con+ species on the catalyst surface is
further confirmed by the typical characteristic peaks of Co3O4 at 194, 469, 513, 617
and 675 cm-1 on Raman spectra in Fig. 5 [51]. The Raman signals of defect-derived D-
band at 1345 cm-1 and graphite structure-derived G-band at 1585 cm-1 also indicate that
the catalyst surface consists of the defective graphitic carbon species [52]. All of these
observations manifest that the composite structure of CoNx-Co3O4 covered Co NPs and
N-doped carbon layers have been prepared via the simple pyrolysis procedure.
3.2. Catalytic hydrogenation of nitrobenzene to aniline over Co/NC catalysts
The catalytic one-pot hydrogenation of nitrobenzene with H2 is carried out on
these Co/NC catalysts. In order to clearly understand their catalytic performances,
several control reaction tests are firstly conducted. As shown in Table 2, the
nitrobenzene hydrogenation into aniline cannot proceed in the absence of the catalyst
(Entry 1) or H2 atmosphere (Entry 2). In addition, no nitrobenzene conversion is
observed over the starting material of CoPc (Entry 3 in Table 2) and the ‘NC’ catalyst
(Entry 4 in Table 2) prepared by pyrolysis of melamine via the same procedure. Highly
12

selective hydrogenation of nitrobenzene to aniline without any detectable byproducts
can be achieved on these Co/NC catalysts (Entries 5-11 in Table 2). These control
experiment results demonstrate that neither pure CoPc complex with Co-N
coordination sites nor non-Co-site ‘NC’sample is capable of the hydrogenation of nitro
group. The results also suggest that only the Co/N-doped carbon composite structure
that is generated via high-temperature pyrolysis process can provide the catalytically
active sites to accomplish the nitrobenzene-to-aniline transformation. Fig. 6 reveals a
volcano-type curve for the relevance between catalytic hydrogenation activity
(conversion of nitrobenzene) and adding amount of CoPc precursor during synthesis
process of the Co/NC catalysts. The Co/NC-0.15 catalyst displays an unsatisfactory
productivity of 6.7 mmol gCo-1 h-1 (Entry 5 in Table 2), however, under similarly high
selectivity towards aniline as >99%, the Co/NC-0.30 and Co/NC-0.35 catalysts increase
the activities by nearly eight times, reaching 53.8 and 51.8 mmol gCo-1 h-1 (Entries 8-9
in Table 2), respectively.
The catalytic activity of the Co/NC catalysts in nitrobenzene hydrogenation
apparently depends on the pressure of feeding H2 gas, e.g., a higher H2 pressure
promotes the reaction rate. The conversion of nitrobenzene increases from 41.0% to
-1
94.3% and the corresponding productivity increases from 23.4 to 53.8 mmol gCo h-1
(Entry 8 and Entries 12-13, Table 2) when the H2 pressure raises from 10 to 30 bar.
These results determine that all the hydrogenation reaction tests are performed with a
fixed H2 pressure at 30 bar. In considering that H2 dissociation over the active metal-
Nx sites should be one of the most critical affect factors for Co/NC-catalyzed
13

hydrogenation of nitrobenzene [53], the activity of the Co/NC catalysts towards H2-D2
exchange is measured. The H2-D2 exchange experiments are proceeded at 343 K that is
the same with the temperature in nitrobenzene hydrogenation reaction, and the
generation rates of gaseous HD product are recorded with an online MS. After co-
feeding of 1:1 H2 and D2 (N2 as balance gas), the steady state of HD generation with
the maximum rate can be reached within different time durations, as shown in Fig. 7A.
Fig. 7B summarizes the acceleration degrees (relative to the slope in Fig. 7A) and the
attainable maximal H2-D2 exchange rates over the Co/NC catalysts. The obviously
lower capability of H2 dissociation for Co/NC-0.15 catalyst is possibly due to its larger
average size of Co particles and a lower amount of CoNx sites on the surface. Both
Co/NC-0.30 and Co/NC-0.35 catalysts can rapidly reach the steady state within shorter
time, compared to Co/NC-0.25 and Co/NC-0.45 catalysts. The rates of the H2-D2
exchange for the Co/NC catalysts are in sequence of Co/NC-0.30 > Co/NC-0.35 >
Co/NC-0.45 > Co/NC-0.25 >> Co/NC-0.15, which is in line with the catalytic
hydrogenation performances. The sequence in the H2-D2 exchange rates is apparently
in accord with the order in the content of CoNx surface species for all Co/NC catalysts.
Furthermore, relatively larger surface areas (>200 m2/g) and smaller particle sizes
(approximately 12 nm) for Co/NC-0.30 and Co/NC-0.35 catalysts lead to more
catalytically active CoNx sites exposed on the catalyst surface, thus accelerating the H2
dissociation process and entire hydrogenation reaction cycle.
In addition to the dissociative activation of H2 molecule, the adsorption of
nitrobenzene as the reactant and aniline as the product on these Co/NC catalyst surface
14

is also essential [54]. The adsorption amounts of nitrobenzene and aniline are measured
on these Co/NC catalysts and the results are presented in Fig. 8. The small adsorption
amounts of nitrobenzene are observed on Co/NC-0.15 and Co/NC-0.25 catalysts, which
are 72.3 and 181 μmol/g, respectively, as shown in Fig. 8A. Apparently large adsorption
amounts of nitrobenzene (>300 μmol/g) are obtained on Co/NC-0.30, 0.35 and 0.45
catalysts. Acomparable level and a similar varying tendency for the adsorption amounts
of aniline product are observed on these Co/NC catalysts (Fig. 8B). These results
clearly suggest that enhanced adsorption for the reaction substrates contributes
significantly to the catalytic hydrogenation reaction of nitrobenzene to aniline over the
Co/NC catalysts. Combining of fast H2 dissociation rate and high reactant adsorption
ability strongly benefits the Co/NC-0.30 catalyst, affording satisfactory catalytic
properties in the hydrogenation reaction.
It has been claimed that the coordinated CoNx site rather than metallic Co or
oxidative CoOx is catalytically active towards the selective hydrogenation of nitro
group in the functionalized arenes. Fortunately, the investigation on the CoNx sites in
the Co/NC catalysts might be accomplished by evaluating the activity in the
electrochemically catalytic ORR reaction, since the isolated CoNx site has also been
reported as the active constituent for the ORR reaction [55-59]. The ORR reactions
over the Co/NC catalysts are performed in O2-saturated 0.1 M KOH electrolyte solution
with a rotating disk electrode (2000 rpm of rotating rate). The linear sweep voltammetry
(LSV) results in Fig. 9A indicate that Co/NC-0.30 catalyst shows the best ORR activity
in term of the most positive half-wave potential (E1/2) and the highest limiting diffusion
15

current density. The E1/2 of Co/NC-0.30 catalyst is 0.815 V, which is 1.1, 1.4 and 18.5
mV more positive compared with that of Co/NC-0.35, Co/NC-0.25 and Co/NC-0.15
catalysts, respectively. Fig. 9B shows that the calculated current density of the optimal
Co/NC-0.30 catalyst with 5.0 mA cm-2 is at the top of a volcano-type curve that is
perfectly corresponding to the distribution pattern of the CoNx surface species for the
Co/NC catalysts. These results demonstrate that more active CoNx sites incorporated in
the Co/NC catalysts can markedly facilitate not only the ORR reaction but also the
nitrobenzene hydrogenation reaction. The catalytic performance comparison between
two kinds of reactions can further explain the structure-activity relevance of Co/NC-
type catalysts as expected. Therefore, surface CoNx sites-enriched Co/NC-0.30 catalyst
reveals a superior catalytic activity in both ORR reaction and nitrobenzene
hydrogenation.
3.3. Reusability of Co/NC catalyst in nitrobenzene hydrogenation
The stability and reusability of the Co/NC catalyst in hydrogenation of
nitrobenzene to aniline are also inspected under the studied reaction conditions. As
presented in Fig. 10, the optimal Co/NC-0.30 catalyst displays >99% nitrobenzene
conversion after 4 h, and such high activity can be maintained in the first three reaction
runs. When half of the catalyst is used in the hydrogenation reaction, the nitrobenzene
conversion is just reduced to approximately 50%, which can still be kept in further two
reaction runs. The leaching of Co species into the solution generally causes the
deactivation issue for Co catalysts, however, in the case of the Co/NC catalyst system,
no Co species are detected in the spent reaction solution with ICP-OES examinations.
16

The structure of the spent Co/NC-0.30 catalyst after five-run reactions is
characterized with XRD and Raman spectra. The XRD pattern in Fig. 11A reveals that
no perceptible structural change is observed. The composite of graphitic carbon and
metallic Co particles is still kept during the hydrogenation reactions. The Raman
spectrum in Fig. 11B shows the vibration signals of Co3O4 and carbon phases that are
coexisted in the spent Co/NC-0.30 catalyst, suggesting that the oxidative Co3O4 species
on the surface of Co particles surrounded by the N-doped carbon layers are stable in
reductive reaction conditions.
3.4. Catalytic hydrogenation of nitroarenes to substituted anilines over Co/NC-
catalysts
Highly selective production of the industrially relevant anilines requires a
generally applicable catalyst material. A wide variety of substituted nitroarenes, which
are containing different kinds of groups such as halogen, aldehyde and nitrile, are
employed to explore the catalytic hydrogenation selectivity of the optimal Co/NC-0.30
catalyst. The identical reaction conditions are used and the catalytic performances are
summarized in Table 3. The halogen (iodo-, chloro- and fluoro-) moiety-substituted
nitrobenzenes can be efficiently and selectively transformed into corresponding
anilines (Entries 1-4 in Table 3). Although the dehalogenation process facilely occurs
under the reaction conditions, the process can be completely suppressed in the presence
of the Co/NC catalyst regardless of the number and the position of halogen moieties.
The tolerance of bromo-group is also reflected by the high catalytic selectivity in the
hydrogenation of the mixed-substituted nitroarene (Entry 5 in Table 3). For those
17

substances substituted with easily reducible moieties including methoxy-, nitrile- and
formyl-moiety, the Co/NC-0.30 catalyst is still able to selectively activate and reduce
the nitro-moiety with satisfactory activity (Entries 5-7 in Table 3). In addition, the
activity and selectivity in catalytic hydrogenation cannot be effected by the presence of
electron-deficient substituent (i.e. trifluoromethyl-group, Entries 8-9 in Table 3).
However, the steric hindrance effect will cause the decline of the hydrogenation activity
but not influence the high selectivity (Entry 10 in Table 3). Furthermore, Co/NC-0.30
catalyst can also efficiently achieve selective hydrogenation of the nitro-group located
in the naphthalene ring (Entry 11 in Table 3). The above reaction results demonstrate
that the Co in N-doped carbon matrix catalyst owns excellent catalytic activity and
chemoselectivity for the direct hydrogenation of nitroarenes with H2.
4. Conclusions
This work presents the Co in N-doped carbon matrix catalysts for selective
hydrogenation of nitroarenes into corresponding anilines. The active Co/NC catalysts
are prepared by a simple pyrolysis method with mixed CoPc complex and melamine as
the Co, N and C precursors. The characterization results demonstrate that the catalysts
are compositing of the oxidative Co3O4 and CoNx-modified Co particles encapsulated
by the N-doped carbon matrix. The optimal Co/NC catalyst posesses a relatively higher
amount of active CoNx sites, a smaller Co average particle size and a larger specific
surface area, which can be regulated by the mixture ratio of two precursors. The isotope
exchange, substance adsorption studies indicate that the excellent abilities of H2
dissociation on the CoNx active sites and reactant molecular adsorption can promote
18

the catalytic performance in nitro-group hydrogenation. Furthermore, the Co/NC
catalyst can tolerate numerous moieties that are easily reducible or readily removable,
and can also be used repeatedly without obvious activity loss.
Acknowledgments
We are grateful for the financial support from the National Natural Science
Foundation of China (21802070) and the support from the Jiangsu Provincial
Department of Education and Natural Science Foundation (17KJB150021 and
BK20170986). We also appreciate the support from Nanjing Technology Innovation
Fund for Overseas Talents.
References
[1] R.S. Downing, P.J. Kunkeler, H. van Bekkum, Catal. Today 37 (1997) 121-136.
[2] R.A. Sheldon, H. van Bekkum, Fine Chemicals through Heterogeneous Catalysis,
Wiley-VCH, Weinheim, 2001.
[3] P. Serna, A. Corma, ACS Catal. 5 (2015) 7114-7121.
[4] R.A. Scheuerman, D. Tumelty, Tetrahedron Lett. 41 (2000) 6531-6535.
[5] D.L. Yang, D. Fokas, J.Z. Li, L.B. Yu, C.M. Baldino, Synthesis (2005) 47-56.
[6] X.L. Cui, Q.L. Zhang, M. Tian, Z.P. Dong, New J. Chem. 41 (2017) 10165-10173.
[7] X.L. Cui, X. Zhou, Z.P. Dong, Catal. Commun. 107 (2018) 57-61.
[8] S. Karami, B. Zeynizadeh, Z. Shokri, Cellulose 25 (2018) 3295-3305.
[9] F. Yang, C. Chi, C.X. Wang, Y. Wang, Y.F. Li, Green Chem. 18 (2016) 4254-4262.
[10] X. Ma, Y.X. Zhou, H. Liu, Y. Li, H.L. Jiang, Chem. Commun. 52 (2016) 7719-
7722.
[11] J.-L. Sun, Y.-Z. Chen, B.-D. Ge, J.-H. Li, G.-M. Wang, ACS Appl. Mater.
Interfaces (2018).
19

[12] J.G. de Vries, C.J. Elsevier, The Handbook of Homogeneous Hydrogenation,
Wiley-VCH, Weinheim, 2007.
[13] P.G. Andersson, I.J. Munslow, Modern Reduction Methods, Wiley-VCH,
Weinheim, 2008.
[14] Y.H. Dai, X. Gao, X.F. Chu, C.Y. Jiang, Y. Yao, Z. Guo, C.M. Zhou, C. Wang, H.M.
Wang, Y.H. Yang, J. Catal. 364 (2018) 192-203.
[15] F.A. Harraz, S.E. El-Hout, H.M. Killa, I.A. Ibrahim, J. Catal. 286 (2012) 184-192.
[16] R.F. Nie, J.H. Wang, L.N. Wang, Y. Qin, P. Chen, Z.Y. Hou, Carbon 50 (2012)
586-596.
[17] C.H. Campos, M. Jofre, C.C. Torres, B. Pawelec, J.L.G. Fierro, P. Reyes, Appl.
Catal. A-Gen. 482 (2014) 127-136.
[18] F. Cardenas-Lizana, S. Gomez-Quero, M.A. Keane, ChemSusChem 1 (2008) 215-
221.
[19] F. Cardenas-Lizana, S. Gomez-Quero, M.A. Keane, Catal. Commun. 9 (2008) 475-
481.
[20] H.U. Blaser, H. Steiner, M. Studer, ChemCatChem 1 (2009) 210-221.
[21] E.H. Boymans, P.T. Witte, D. Vogt, Catal. Sci. Technol. 5 (2015) 176-183.
[22] A. Corma, P. Serna, Science 313 (2006) 332-334.
[23] X. Sun, A.I. Olivos-Suarez, D. Osadchii, M.J.V. Romero, F. Kapteijn, J. Gascon,
J. Catal. 357 (2018) 20-28.
[24] Z.Z. Wei, J. Wang, S.J. Mao, D.F. Su, H.Y. Jin, Y.H. Wang, F. Xu, H.R. Li, Y. Wang,
ACS Catal. 5 (2015) 4783-4789.
[25] R.M. Bullock, Science 342 (2013) 1054-1055.
[26] K. Junge, B. Wendt, N. Shaikh, M. Beller, Chem. Commun. 46 (2010) 1769-1771.
[27] R.V. Jagadeesh, D. Banerjee, P.B. Arockiam, H. Junge, K. Junge, M.M. Pohl, J.
20

Radnik, A. Bruckner, M. Beller, Green Chem. 17 (2015) 898-902.
[28] G. Wienhofer, I. Sorribes, A. Boddien, F. Westerhaus, K. Junge, H. Junge, R.
Llusar, M. Beller, J. Am. Chem. Soc. 133 (2011) 12875-12879.
[29] F.A. Westerhaus, R.V. Jagadeesh, G. Wienhofer, M.M. Pohl, J. Radnik, A.E.
Surkus, J. Rabeah, K. Junge, H. Junge, M. Nielsen, A. Bruckner, M. Beller, Nat. Chem.
5 (2013) 537-543.
[30] R.V. Jagadeesh, A.E. Surkus, H. Junge, M.M. Pohl, J. Radnik, J. Rabeah, H.M.
Huan, V. Schunemann, A. Bruckner, M. Beller, Science 342 (2013) 1073-1076.
[31] L.C. Liu, P. Concepcion, A. Corma, J. Catal. 340 (2016) 1-9.
[32] J. Wang, D.F. Gao, G.X. Wang, S. Miao, H.H. Wu, J.Y. Li, X.H. Bao, J. Mater.
Chem. A 2 (2014) 20067-20074.
[33] R.V. Jagadeesh, T. Stemmler, A.E. Surkus, M. Bauer, M.M. Pohl, J. Radnik, K.
Junge, H. Junge, A. Bruckner, M. Beller, Nat. Protoc. 10 (2015) 916-926.
[34] J.Y. Cao, X.D. Jia, M.W. Guo, Y.Y. Du, J. Xu, Z.D. Chen, Sustain. Energ. Fuels 2
(2018) 169-174.
[35] Y.Y. Sun, X.W. Li, Z.S. Cai, H.Z. Bai, G.P. Tang, Z.Y. Hou, Catal. Sci. Technol. 8
(2018) 4858-4863.
[36] H.G. Huang, X.G. Wang, Y. Sheng, C.J. Chen, X.J. Zou, X.F. Shang, X.G. Lu,
RSC Adv. 8 (2018) 8898-8909.
[37] P. Ren, C. Sun, Y. Shi, P. Song, Y. Yang, Y. Li, J. Mater. Chem. C 7 (2019) 1934-
1947.
[38] X. Shi, Y. Yang, L. Wang, Y. Li, J. Phys. Chem. C 123 (2019) 4007-4021.
[39] T. Stemmler, A.-E. Surkus, M.-M. Pohl, K. Junge, M. Beller, ChemSusChem 7
(2014) 3012-3016.
[40] H. Jin, J. Wang, D. Su, Z. Wei, Z. Pang, Y. Wang, J. Am. Chem. Soc. 137 (2015)
21

2688-2694.
[41] X. Sun, A.I. Olivos-Suarez, L. Oar-Arteta, E. Rozhko, D. Osadchii, A. Bavykina,
F. Kapteijn, J. Gascon, ChemCatChem 9 (2017) 1854-1862.
[42] T. Song, P. Ren, Y.A. Duan, Z.Z. Wang, X.F. Chen, Y. Yang, Green Chem. 20 (2018)
10.
[43] B. Tang, W.C. Song, E.C. Yang, X.J. Zhao, RSC Adv. 7 (2017) 1531-1539.
[44] P. Zhou, C. Yu, L. Jiang, K. Lv, Z. Zhang, J. Catal. 352 (2017) 264-273.
[45] X. Wang, W.X. Chen, L. Zhang, T. Yao, W. Liu, Y. Lin, H.X. Ju, J.C. Dong, L.R.
Zheng, W.S. Yan, X.S. Zheng, Z.J. Li, X.Q. Wang, J. Yang, D.S. He, Y. Wang, Z.X.
Deng, Y.E. Wu, Y.D. Li, J. Am. Chem. Soc. 139 (2017) 9419-9422.
[46] J. Tang, R.R. Salunkhe, J. Liu, N.L. Torad, M. Imura, S. Furukawa, Y. Yamauchi,
J. Am. Chem. Soc. 137 (2015) 1572-1580.
[47] Z. Li, M. Shao, L. Zhou, R. Zhang, C. Zhang, M. Wei, D.G. Evans, X. Duan, Adv.
Mater. 28 (2016) 2337-2344.
[48] J.R. Pels, F. Kapteijn, J.A. Moulijn, Q. Zhu, K.M. Thomas, Carbon 33 (1995)
1641-1653.
[49] J. Casanovas, J.M. Ricart, J. Rubio, F. Illas, J.M. Jiménez-Mateos, J. Am. Chem.
Soc. 118 (1996) 8071-8076.
[50] J. Deng, H.J. Song, M.S. Cui, Y.P. Du, Y. Fu, ChemSusChem 7 (2014) 3334-3340.
[51] S.L. Xiong, C.Z. Yuan, M.F. Zhang, B.J. Xi, Y.T. Qian, Chem.-Eur. J. 15 (2009)
5320-5326.
[52] M.M. Lucchese, F. Stavale, E.H.M. Ferreira, C. Vilani, M.V.O. Moutinho, R.B.
Capaz, C.A. Achete, A. Jorio, Carbon 48 (2010) 1592-1597.
[53] L.C. Liu, F. Gao, P. Concepcion, A. Corma, J. Catal. 350 (2017) 218-225.
[54] X.Y. Wan, C.M. Zhou, J.S. Chen, W.P. Deng, Q.H. Zhang, Y.H. Yang, Y. Wang,
22

ACS Catal. 4 (2014) 2175-2185.
[55] T. He, X.J. Wang, H.Q. Wu, H. Xue, P. Xue, J. Ma, M.L. Tan, S.H. He, R.J. Shen,
L.Z. Yi, Y. Zhang, J. Xiang, ACS Appl. Mater. Interfaces 9 (2017) 22490-22501.
[56] Y.Z. Chen, C.M. Wang, Z.Y. Wu, Y.J. Xiong, Q. Xu, S.H. Yu, H.L. Jiang, Adv.
Mater. 27 (2015) 5010-5016.
[57] X.L. Zhang, Z.X. Yang, Z.S. Lu, W.C. Wang, Carbon 130 (2018) 112-119.
[58] H.T. Yu, Y.C. Li, X.H. Li, L.Z. Fan, S.H. Yang, Chem. Eur. J. 20 (2014) 3457-
3462.
[59] T. He, H. Xue, X.J. Wang, S.H. He, Y.L. Lei, Y.Y. Zhang, R.J. Shen, Y. Zhang, J.
Xiang, Nanoscale 9 (2017) 8341-8348.
23

Fig. 1. XRD patterns (A-E) and nitrogen adsorption-desorption isotherms (F-J) of
various Co/NC catalysts.
24

Fig. 2. TEM images and the particle size distributions of the Co/NC catalysts.
25

Fig. 3. XPS spectra of N 1s (A-E) and Co 2p 3/2 (F-J) of the Co/NC catalysts.
26

Fig. 4. Summarized distributions of surface CoNx and pyridinic N species for various
Co/NC catalysts based on the XPS results.
27

Fig. 5. Raman spectra of the Co/NC catalysts.
28

Fig. 6. Catalytic performances of various Co/NC catalysts in hydrogenation of
nitrobenzene to aniline. Reaction conditions: 30 mg of catalyst, 2 mmol of
nitrobenzene, 10 mL of ethanol, 30 bar of H2, 343 K, and 4 h.
29