One-step synthesis of magnetically recyclable Co@BN core-shell nanocatalysts for catalytic reduction of nitroarenes

Article abstract of DOI:10.1039/c7ra04907b

We present a facile one-step synthesis of chemically stable and magnetic Co@BN core-shell nanoparticles. We found that Co@BN nanoparticles acted as excellent catalysts with high stability and magnetic recyclability for reduction of nitroarenes to aminoarenes under mild reaction conditions. The sample 13.6 wt% Co@BN showed the best catalytic activity for reduction of 4-nitrophenol (4-NP). In addition, a significant synergistic effect of the h-BN support was observed during the catalytic reaction by effectively adsorbing/concentrating and ionizing the reactant nitroaromatics. Moreover, the kinetics of the catalytic reaction were investigated at different reaction temperatures, 4-nitrophenol concentrations, sodium borohydride concentrations, metal loadings and catalyst amounts. The activation energy of the catalytic reduction of 4-nitrophenol for 13.6 wt% Co@BN was determined to be 102.93 kJ mol^-1. This work provides a clear example of Co@BN core-shell nanoparticles as nanocatalysts.

Full text of DOI:10.1039/c7ra04907b

RSC Advances
View Article Online
View Journal | View Issue
PAPER
One-step synthesis of magnetically recyclable
Co@BN core–shell nanocatalysts for catalytic
reduction of nitroarenes†
Cite this: RSC Adv., 2017, 7, 35451
Man Du, Qiuwen Liu, Caijin Huang *a and Xiaoqing Qiu*b
a
a
We present a facile one-step synthesis of chemically stable and magnetic Co@BN core–shell nanoparticles.
We found that Co@BN nanoparticles acted as excellent catalysts with high stability and magnetic
recyclability for reduction of nitroarenes to aminoarenes under mild reaction conditions. The sample
13.6 wt% Co@BN showed the best catalytic activity for reduction of 4-nitrophenol (4-NP). In addition,
a significant synergistic effect of the h-BN support was observed during the catalytic reaction by
effectively adsorbing/concentrating and ionizing the reactant nitroaromatics. Moreover, the kinetics of
the catalytic reaction were investigated at different reaction temperatures, 4-nitrophenol concentrations,
sodium borohydride concentrations, metal loadings and catalyst amounts. The activation energy of the
Received 2nd May 2017
Accepted 11th July 2017
DOI: 10.1039/c7ra04907b
ꢀ1
catalytic reduction of 4-nitrophenol for 13.6 wt% Co@BN was determined to be 102.93 kJ mol . This
rsc.li/rsc-advances
work provides a clear example of Co@BN core–shell nanoparticles as nanocatalysts.
catalytic and chemical properties compared with those of their
1
. Introduction
10–13
individual components.
Consequently, the core–shell
In recent years, earth-abundant available transition metal structure has enabled a signicant enhancement in photo-
14–18
19
nanoparticles have attracted growing attention in diverse elds luminescence,
20
chemical and thermal stability,
and
1
2
3
of optics, magnetics and catalysis because of their excellent processibility. Moreover, the shell also plays an important role
properties. Among these metals, cobalt is particularly inter- in the core–shell structure to improve the photoluminescence
15
esting for its wide use as catalysts because of its loosely bonded quantum yield, exhibit high activity and selectivity in
4
–7
21
d electrons and the resulting high electrical conductivity.
homogenous catalytic reaction, and enhance higher bio-
22
Searching for larger surface-to-volume ratios and more active catalytic activity and temperature stability. Somorjai et al.
sites constitutes part of the driving force in developing nano- demonstrated when Pt core is coated with a silica shell, it
scale catalysts. However, for metal nanoparticles, they are possesses excellent thermal stability and high catalytic perfor-
inclined to aggregate in aqueous solution owing to surface mance for high-temperature reactions. Oelhafen et al. re-
hydrophobicity, resulting in eventual loss of catalytic activity. ported that the metallic Co core surround by oxide shell can
23
8
To solve this problem, extensive efforts have been devoted to avoid directly exposing to the air and have the property to
24
stabilizing metal nanoparticles with various capping agents or exchange the magnetic system. The use of reduced graphene
supports. For instance, Crooks and co-workers demonstrated oxide (rGO) as a shell protects the core Cu nanoparticle from
that metal nanoparticles can be effectively stabilized through oxidation and the as-prepared Cu@rGO nanoparticles exhibit
9
macromolecular organic ligands. However, organic ligands can excellent catalytic performance towards the reduction of 4-
25
26
not be long-term stable in air, oxidizing solution or at high nitrophenol. Encouragingly, as an analogue to graphene,
temperature. Another interesting stabilization strategy is to hexagonal boron nitride (h-BN) sheets have been used as a shell
encapsulate nanoparticles with another material to form a core– in our previous work to wrap Cu nanoparticles and the as-
shell hetero-structure. While core particles are wrapped with obtained Cu@BN core–shell nanoparticles possesses good
27
a shell, it causes changes in magnetic, electronic, optical, catalytic performance and high chemical stability.
Two-dimensional (2D) h-BN material has attracted great
interest owing to its nontoxicity, high surface area, appropriate
State Key Laboratory of Photocatalysis on Energy and Environment, College of chemical anchoring sites, chemical and thermal stability.
a
28–30
Chemistry, Fuzhou University, Fuzhou 350002, P. R. China. E-mail: cjhuang@fzu.
Moreover, h-BN has been found to be an excellent support for
functional materials to promote their (photo)-catalytic perfor-
edu.cn
b
College of Chemistry and Chemical Engineering, Central South University, Changsha,
31–33
mance.
Recently, Lei et al. showed that h-BN can serve as an
4
†
1
1083, China. E-mail: xq-qiu@csu.edu.cn
excellent adsorbent material because of its good adsorption
Electronic supplementary information (ESI) available. See DOI:
0.1039/c7ra04907b
30
performance of organic compounds. For heterogeneous
This journal is © The Royal Society of Chemistry 2017
RSC Adv., 2017, 7, 35451–35459 | 35451

View Article Online
RSC Advances
Paper
catalytic process, concentrating reactants (e.g. organic spectrometer. According to the corresponding cobalt contents,
compounds) in the vicinity of the catalyst is benecial for the samples were denoted as 0.94 wt% Co@BN, 8.3 wt%
improving catalytic reaction rate. In this regard, h-BN was used Co@BN, 13.6 wt% Co@BN and 20.0 wt% Co@BN, respectively.
in this work as a promising shell material to wrap/stabilize For contrast experiments, the sample cobalt without h-BN was
cobalt nanoparticles for catalyzing organic compounds.
prepared as the procedure mentioned above by placing 1.8 g
Amino-aromatics are widely employed in dyes, pharmaceu- cobalt nitrate in a tube furnace without adding boron oxide and
3
4–38
tical, polymers and herbicides.
For example, 4-chloroaniline urea.
36
plays a signicant role in the production of dyes. In addition,
4
-aminophenol (4-AP) is an important intermediate in the
2.3 Catalyst characterization
39,40
preparation of analgesic and antipyretic drugs,
and also
The crystal structure of the samples was recorded with an X-ray
powder diffractometer (XRD, Bruker, D8 Advance, Cu-Ka1
radiation). Fourier transformed infrared (FTIR) spectroscopy
was conducted on a Nicolet Magna 670 Fourier transform
infrared spectrometer at room temperature. Raman spectra
were recorded by Raman spectrometer (Renishaw inVia Raman
Microprobe) with an excitation source at 532 nm. Transmission
Electron Microscope (TEM) and selected-area electron diffrac-
tion (SAED) characterization were taken on TITAN G2 60–300
electron beam aberration corrected transmission electron
microscope under an acceleration voltage of 300 kV. Magnetic
hysteresis loops were measured on a Physical Property
Measurement System (PPMS-9T, Quantum Design, USA).
Nitrogen sorption measurements and pore size analysis were
performed on a Micromeritics ASAP 2020 HD88 system. The
used as a photographic developer, a corrosion inhibitor and an
41,42
antioxidant.
Meanwhile, nitro-aromatics are considered to
be over 500-fold more toxic than their corresponding aromatic
43
amines. Therefore, conversion of nitro-aromatics to amino-
aromatics is of great importance for the industrial production
of amino compounds and industrial waste water treatment of
nitro-compounds. The catalytic hydrogenation of nitro-
aromatics is an attractive conversion pathway and has been
widely studied by using noble-metal based catalysts including
44,45
Ag and Au.
In this paper, we report a new type of nano-
composite, composed of a magnetic metal core, Co, and an
insulating shell, hexagonal boron nitride (h-BN). The as-
prepared Co@BN core–shell nanoparticles were introduced as
catalysts for hydrogenation of nitroarenes (e.g. 4-nitrophenol, 4-
nitroaniline and 4-nitroanisole) to their corresponding amino-
arenes. The experimental results showed that the as-
synthesized Co@BN nanoparticles acted as excellent catalysts
with high durable stability for reduction of nitroarenes to
aminoarenes under mild reaction conditions. The core–shell
structure and magnetic property of Co core enabled easy sepa-
ꢁ
sample was degassed at 180 C for 8 h prior to the experiment.
The electron paramagnetic resonance (EPR) spectra were
observed by Bruker A300 spectrometer at room temperature.
2.4 Catalytic activity measurements
ration from the reaction mixture. Moreover, effects of reactant Considering the precision of the experiment, aqueous solutions
concentration, metal loading, catalyst amount and reaction of 4-nitrophenol and NaBH were freshly prepared prior to
temperature on reaction rate and activation energy were studied being used. Typically, 35 ml of the deionized water, 4 ml of
as well.
nitroarenes (e.g. 4-nitrophenol) solution (100 ppm), 3 ml of
4
ꢀ
1
4
NaBH solution (0.2 mol l ) and 10 mg of the catalyst were
added to a 100 ml beaker with stirring. Every 2 min, 1.5 ml of the
reaction mixture was injected into a 2 ml centrifuge tube
immediately through a pressure syringe lter which can remove
the catalyst from the reaction solution and end the reaction.
The ltrated solution was then measured by using a conven-
tional UV-visible spectrometer (Cary 50, VARIAN) using deion-
ized water as a blank solution.
2
. Experimental
2.1 Materials
Boron oxide, urea and cobalt(II) nitrate were used as the raw
materials in the preparation of Co@BN composites. Boron
oxide was purchased from Aladdin Company. Cobalt(II) nitrate
hexahydrate (Co(NO ) $6H O $ 47.8%) was used as cobalt
3
2
2
precursor. Urea, 4-nitrophenol and sodium borohydride
NaBH
(
4
) were obtained from Sinopharm Chemical Reagent 3. Results and discussion
Corporation. All other chemicals were of analytical grade and
used without further purication. Deionized water was used
throughout the study.
3.1 Structure characterization, morphology and magnetism
Fig. 1 displays the XRD patterns of the as-obtained h-BN and
Co@BN samples. The characteristic peaks of h-BN sample
ꢁ
located at 26.7 and 42 are assigned to the (002) and (100)
crystalline facets, respectively (JCPDS card no. 45-0896). The
2
.2 Catalyst preparation
ꢁ
The synthesis of Co@BN composites is indicated as follows: diffraction peaks of Co@BN at around 44.2, 51.5 and 75.8
different amounts of cobalt nitrate (0.09, 0.9, 1.8 and 2.7 g, correspond to the (111), (200) and (220) planes of the cubic
respectively), 3 g of boron oxide and 6 g of urea were mixed in phase cobalt, respectively (JCPDS card no. 15-0806). Moreover,
ꢁ
the mortar until it was ground into a ne powder. The resultant the intensity of the peak at 44.2 for cobalt increases with
powder was placed in a tube furnace with sintering temperature increasing amount of the cobalt content. For the Co@BN
ꢁ
at 1250 C for 5 h. The cobalt contents in Co@BN composites samples, the diffraction peaks of h-BN are similar to that of the
were detected by the Ultima2 ICP optical emission pure phase boron nitride, indicating that the crystal structure of
35452 | RSC Adv., 2017, 7, 35451–35459
This journal is © The Royal Society of Chemistry 2017

View Article Online
Paper
RSC Advances
gives the Raman spectra of the Co@BN samples with various
ꢀ
1
cobalt contents. The spectral position located at 1368 cm
belongs to the characteristics of the high frequency vibration
5
0–52
Raman peaks (E ) between boron nitride layers.
Besides, it
is noteworthy that there are no Raman signals from Co (472
), 507 cm (F2g) and 662 cm (A1g)) and CoO (1000–
2
g
3 4
O
ꢀ
1
ꢀ1
ꢀ1
53
cm (E
g
ꢀ
1
48
1
100 cm ). This indicates that the Co@BN samples do not
contain detectable cobalt oxide, which is in good accordance
with the results of XRD and FTIR analysis.
Fig. 2 shows the as-synthesized Co@BN nanoparticles with
a core–shell structure. For the sample 13.6 wt% Co@BN, it has
a particle size of ca. 100–200 nm in diameter and a shell of 30–
50 nm in thickness (Fig. 2a and b). Moreover, the high-
resolution TEM (HRTEM) image (Fig. 2c) displays the lattice
Fig. 1 XRD patterns of h-BN and the Co@BN samples with various Co fringes corresponding to the (002) plane of h-BN and the (111)
loadings.
plane of cubic metallic cobalt, respectively. Fig. 2d presents the
selected-area electron diffraction (SAED) patterns indexed to
(
111), (200), and (220) of cubic cobalt, which is in agreement
boron nitride has not changed when boron nitride and cobalt
are combined. No diffraction peaks originating from cobalt
oxides were observed under the detection limit and sensibility
of the XRD apparatus.
with the XRD results. Meanwhile, Fig. 2d also demonstrates that
the as-formed cobalt core is single crystalline. The nanoake
shell shows a polycrystalline SAED pattern corresponding to the
hexagonal BN phase.
54
55
FTIR was performed to identify functional groups existed in
The magnetization curve displays that Co@BN nanoparticles
ꢀ1
the samples (Fig. S1a†). The peaks located at 786 and 1387 cm
are attributed to the out-of-plane B–N–B bending vibration (A2u
and the in-plane B–N transverse stretching vibration (E1u),
56,57
are superparamagnetic
magnetization (M
at 300 K (Fig. S2a†). The saturation
ꢀ
1
)
s
) value was determined to be 21.27 emu g
from the hysteresis loops of Co@BN. We also found that
Co@BN nanoparticles can be easily dispersed in a post-reaction
solution (see catalytic activity test for details) and quickly
collected by a magnet (Fig. S2b†), which is benecial for its
recovery and reuse for application in heterogeneous catalysis.
4
6,47
respectively.
cobalt oxides, such as CoO and Co
a broad infrared band around 510 and 500 cm , respectively.
Dopant incorporation, defects, and lattice disorder in a host
There are no obvious peaks corresponding to
3 4
O
which are featured by
ꢀ
1
48
49
lattice can be detected by analyzing Raman spectra. Fig. S1b†
Fig. 2 (a, b) TEM and (c) HRTEM images of 13.6 wt% Co@BN. (d) The SAED pattern of 13.6 wt% Co@BN.
This journal is © The Royal Society of Chemistry 2017
RSC Adv., 2017, 7, 35451–35459 | 35453

View Article Online
RSC Advances
.2 Catalytic activity of Co@BN
Paper
3
The catalytic performance of the as-prepared samples was
evaluated through catalytic hydrogenation of nitroarenes (e.g. 4-
nitrophenol). The reduction of 4-nitrophenol by excess NaBH
4
follows the equation under alkaline conditions indicated as
follows:
(1)
This reaction can be easily monitored by UV-visible spec-
troscopy. Normally, 4-nitrophenol has a typical absorption peak
at 317 nm (Fig. S3†). However, aer adding NaBH solution,
4
a red shi of the maximum absorption peak occurs from
317 nm to 400 nm immediately and its color accordingly altered
from light yellow to deep yellow, which is attributed to the
58
formation of 4-nitrophenolate ions under alkaline conditions.
Therefore, we monitored the concentration of 4-nitrophenol
through the absorbance at 400 nm. Interestingly, when h-BN
was mixed with 4-nitrophenol, the absorbance at 400 nm had
a slight increase, suggesting that the support h-BN can function
as a weak base to ionize a part of 4-nitrophenol. Fig. 3a shows
the time course of catalytic reduction of 4-nitrophenol by
4
NaBH over 13.63 wt% Co@BN composites. Decrease in the
absorbance of the peak at 400 nm with reaction time means the
conversion of 4-nitrophenol. Moreover, two isosbestic points
are observed at 277 nm and 312 nm, suggesting that the 4-
nitrophenol were quickly converted to 4-aminophenol without
59
any byproducts. Complete conversion of 4-nitrophenol to 4-
aminophenol was achieved within 22 min. The yield and
selectivity were 96% and 99%, respectively. The total turnover
number (TON) of 13.63 wt% Co@BN was 5, which is higher than
some cobalt-based catalysts reported in the literature (Table
S1†). These results suggest that Co@BN nanoparticles exhibited
a good catalytic activity.
To further explore the catalytic behavior of the Co@BN
samples, comparative experiments and kinetics on the conver-
sion of 4-nitrophenol to 4-aminophenol were investigated.
According to Lambert–Beer's law, the ratio of the concentration
Fig. 3 (a) Catalytic reduction of 4-NP in the presence of NaBH
4
by
1
3.63 wt% Co@BN nanocatalysts. (b) Variation of C /C versus reaction
t
0
time for the reduction of 4-NP with different catalysts. (c) Rate
constants of the catalytic reduction reaction of 4-NP obtained with
of 4-nitrophenol at any time (C
-nitrophenol (C ), namely C /C
of the corresponding absorbance (A
time course of C /C for the as-prepared catalysts, which reects
the catalytic activity of the catalysts used. 0.94 wt% Co@BN, h-
BN and bare cobalt were only slightly active with a slow reduc-
tion of 4-nitrophenol. Obviously, 13.6 wt% Co@BN possesses
the highest catalytic activity among the samples. Moreover,
t
) and the initial concentration of
, can be determined by the ratio
/A ). Fig. 3b describes the
different catalysts through a plot of ln(C
t
/C
0
) versus time (T ¼ 288 K).
4
0
t
0
t
0
t
0
reaction time for the catalysts. As can be seen, the linear rela-
tionship between ln(C /C ) and the corresponding reaction time
t
0
conrms that pseudo-rst-order kinetics is available for this
reaction. According to the rst order kinetic equation: ln(C /C )
t
0
¼
ꢀkt, the rate constant k can be directly achieved by using
since the concentration of NaBH
that of 4-nitrophenol ([NaBH
]/[4-NP] ¼ 205 : 1), this reduction
reaction (eqn (1)) can be considered as a pseudo-rst-order
4
is generally much higher than
absolute values of the slope of the linear plots. It is clear that
among the samples, the catalyst 13.6 wt% Co@BN has the
4
ꢀ1
highest rate constant 0.20 min , and was chosen as a model
59
t 0
reaction. Fig. 3c represents plots of ln(C /C ) versus relevant
catalyst in this study.
35454 | RSC Adv., 2017, 7, 35451–35459
This journal is © The Royal Society of Chemistry 2017

View Article Online
Paper
RSC Advances
Furthermore, we also investigated catalytic reduction of
other nitroarenes including 4-nitroaniline, 4-nitro-
chlorobenzene, 4-nitromethylbenzene and 4-nitroanisole under
the same reaction condition. Fig. 4 gives conversion of these
nitroarenes within 30 min in the presence of excess NaBH
4
using the catalyst 13.6 wt% Co@BN. The conversion reaches
about 100% for 4-nitroaniline, 4-nitrophenol and 4-nitro-
anisole, while about 82 and 74% for 4-nitrochlorobenzene and
4-nitromethylbenzene, respectively. This is likely due to
substituent effect from different functional groups (e.g.
60–62
hydroxyl, methyl) for nitroarenes.
Among these reactants,
nitroarenes with electron-donating groups (e.g. p-OH, p-NH , p-
2
OCH
electron-withdrawing groups (e.g. p-Cl),
phobic 4-methylnitrobenzene poor water-solubility restricts its
3
) appears to be more easily reduced than those with
60–62
while for hydro-
61
reaction activity. The results also suggest that the Co@BN
nanocatalysts possess excellent catalytic activity for the hydro-
genation of nitroarenes with different substituent groups.
3.3 Kinetic study
For this reduction reaction (eqn (1)), the standard reduction
ꢀ
potential for 4-NP/4-AP and H
3
BO
3
/BH
4
are ꢀ0.76 and ꢀ1.33 V
58,63
versus normal hydrogen electrode (NHE), respectively.
That
is to say, this reaction is a thermodynamically feasible process
as indicated by eqn (1). Nevertheless, it is difficult for this
reaction to spontaneously proceed without catalysts (Fig. S4†)
58
Fig. 5 (a) Plot of ln(C
t
/C
0
) versus time for the reduction of 4-nitro-
due to a high activation energy. Fig. 5 presents kinetic study on
phenol over 13.63 wt% Co@BN at different temperatures. Reaction
conditions: 10 mg of the catalyst; 2 ml of NaBH (0.2 M) and 4 ml of 4-
nitrophenol (100 ppm). (b) The corresponding Arrhenius plot.
the reduction of 4-nitrophenol to 4-aminophenol catalyzed by
4
ꢁ
1
3.6 wt% Co@BN at various temperatures: 15, 20, 25 and 30 C.
t 0
The relevant linear relationships between ln(C /C ) and the
corresponding reaction time are shown in Fig. 5a. It is clear that
the reaction rate is quite sensitive to temperature and the value
of rate constant k increases with the temperature conducted.
In addition, according to the Arrhenius equation:
Ea
ꢀ
1
wt% Co@BN was determined to be 102.93 kJ mol from the
Arrhenius plot (Fig. 5b).
As seen in eqn (1), the concentration of sodium hydroxide
and 4-nitrophenol and catalyst amount play an important role
ln k ¼ ln A ꢀ
a
, the activation energy (E ) can be obtained
RT
from the slope on the basis of the linear tting of ln k versus on the catalytic reaction. Fig. 6a shows the effect of 4-nitro-
1
000/T. The activation energy of the reaction catalyzed by 13.6 phenol at initial concentrations of 50, 100 and 200 ppm on
reaction kinetics, while keeping the NaBH4 concentration
constant at 0.2 M and the amount of catalyst at 10 mg. The rate
constant k was found to decrease with increasing the concen-
tration of 4-nitrophenol. This kind of phenomenon seems
abnormal, Xia et al. explained that a high concentration of 4-
nitrophenol may cover almost the entire surface of the catalyst,
which limits electron transfer from NaBH to 4-nitrophenol and
4
64
thus hinders the catalytic action of NaBH4. The slope of the
line given in the insert of Fig. 6a, 0.98 z 1, suggests that the
reaction is rst order with respect to the 4-nitrophenol
65,66
concentration.
Fig. 6b presents the effect of the NaBH
4
concentration (0.1, 0.15, 0.2 and 0.3 M) on the reduction of 4-
nitrophenol. 4-Aminophenol generation rate is improved with
the increment of the concentration of NaBH . The plot of ln k vs.
4
ln[NaBH ] gives a straight line with a slope of 0.97 z 1, meaning
4
that the catalytic reaction is rst order with respect to the
NaBH concentration (Fig. 6b inset). Fig. 6c reveals an effect of
4
catalyst dosage on 4-aminophenol generation rate. We can see
that the 4-aminophenol generation rate increases with the
Fig.
4 Conversion of nitroaromatic compounds with 13.6 wt%
Co@BN under the same condition.
This journal is © The Royal Society of Chemistry 2017
RSC Adv., 2017, 7, 35451–35459 | 35455

View Article Online
RSC Advances
Paper
reaction of 4-nitrophenol in each catalysis recycle given in
Fig. S5.† Furthermore, we have detected the load of Co on the
recycled catalyst by inductive coupled plasma emission spec-
trometer (Table S2†). A slight weight loss in cobalt load in
Co@BN catalysts ranges from 13.6% (before catalysis) and
11.5% (aer ve catalysis cycles). Considering detecting devia-
tion (ꢂ2%) for this measurement, we believe that Co nano-
structures are relatively stable during the reduction reactions.
The recycle experiments showed that the as-prepared Co@BN
nanocatalyst has an excellent catalytic stability. Moreover, by
comparing the XRD patterns of the catalyst before and aer the
reaction (Fig. S6†), there is no obvious change in diffraction
peaks, conrming the structure stability of the catalyst during
the reaction.
3.5 Adsorption of h-BN
High surface area, thermal and chemical stability makes h-BN
30
become an ideal candidate as an absorbent material, which
is helpful for heterogeneous catalysis. In this regard, the
specic surface area and pore size distribution of 13.6 wt%
2
Co@BN were analyzed using N adsorption and desorption
isotherms, respectively (Fig. S7†). The Co@BN sample displayed
typical type IV adsorption isotherms with an H3-type hysteresis
67
loop. Co@BN had BET-specic surface areas of approximately
2
ꢀ1
4
6.6 m g . The pore size distribution (calculated via the BJH
method) shows that the pore size of Co@BN is centered at
around 13.2 nm (Fig. S7b†). This suggests a mesoporous
structure characteristic of the Co@BN. Meanwhile, the meso-
porous size distribution is a relatively wide which is benecial
for improving the adsorption of the reactants. Meanwhile, the
absorption performance of BN on 4-nitrophenol solutions with
consecration of 50, 100 and 200 ppm were explored. Fig. S8a†
describes the absorbance change of 4-nitrophenol (100 ppm) at
3
1
17 nm over time in the presence of h-BN. Aer dispersing
0 mg of h-BN powder into 4-nitrophenol solutions with stir-
ring, the absorbance at 317 nm quickly decreased to a much
lower constant value within 2 min. Moreover, for the 4-nitro-
phenol solutions of 50 ppm and 200 ppm, the absorption
Fig. 6 Plots of ln(C
different concentrations of (a) 4-NP, (b) NaBH
t
/C
0
) versus time for the reduction of 4-NP with
and (c) dosage of 13.6
wt% Co@BN. The inset of each figure shows the plot of 4-AP gener-
ation rate vs. the concentration of 4-NP, NaBH and cobalt (both in
4
4
logarithmic scale), respectively (see text for details).
amount of catalyst. Moreover, the slope of the plot of ln k vs.
ln(catalyst amount) is 0.98, which is approximately equal to 1
(Fig. 6c insert). Therefore, the reduction of 4-NP is rst order
with respect to the amount of Co@BN catalyst.
3.4 Stability and recycle of Co@BN
Stability or recycling ability is of great importance to the
performance of catalysts. The recycling experiment for 13.6 wt%
Co@BN was carried out by repeating the reduction process for
Fig. 7 Plots of C
t 0
/C versus time for the reduction of 4-NP catalyzed
by the as-prepared 13.6 wt% Co@BN catalyst ([4-NP] ¼ 100 ppm). The

ve cycles (Fig. 7). Successive UV-vis spectra for the reduction catalyst was repeatedly used for five cycles.
35456 | RSC Adv., 2017, 7, 35451–35459
This journal is © The Royal Society of Chemistry 2017

View Article Online
Paper
RSC Advances
approaches saturation in nearly 3 min and 1 min, respectively
(Fig. S8b and c†). Through quick absorption of 4-nitrophenol,
a high local concentration of 4-nitrophenol occurs on the
surface of h-BN and at the vicinity of the catalyst. This is
benecial to accelerate the reaction rate.
3.6 Approaching to the mechanism
To get more insights into the reaction process, the EPR
experiment was conducted at room temperature using a spin
trapper DMPO. Results obtained from spin trapping experi-
ments are displayed in Fig. S9.† It is clear that spectra (b) and
(
(
c) originate from the formation of Hc radical adducts
6
8
DMPO-H, aN ¼ 16.6 G, aH ¼ 22.5 G) for the solution con-
taining and (b) DMPO, NaBH and 4-nitrophenol and (c)
4
DMPO and NaBH
the solution containing DMPO, NaBH
4
, respectively. When adding the catalyst in
and 4-nitrophenol,
4
EPR spectrum becomes more complicated (spectrum (a)). By
analyzing spectrum (a), we found that not only H-DMPO
adducts but also HOOc free radicals formed in the process
6
9
of catalytic reaction. The formation of HOOc may be due to
a large amount of Hc radicals rapidly produced over the
catalyst and trapped by trace amount oxygen in the solution.
Besides, control experiments revealed that no Hc adduct
formed for 4-nitrophenol or catalysts with and without
DMPO (Fig. S9d–h†). According to the EPR results, since the
catalytic reaction takes place on the surface of metal, the
formation of the hydrogen spin adducts most likely come
from the abstraction of hydrogen from the cobalt surface by
a spin trap. This is similar to the formation of Au–H species
6
9
during the alcohol oxidation. These results thus strongly
suggest intermediate formation of Co–H species during 4-
nitrophenol reduction.
The kinetics of this reaction has been modeled on the basis
of a Langmuir–Hinshelwood mechanism, namely, both reac-
Fig. 8 (a) Proposed mechanism and (b) possible reaction pathways for
the 4-NP reduction by NaBH in the presence of a Co@BN catalyst.
4
70
tants need to be adsorbed on the surface prior to reaction. It
means the adsorption of reactants is of great importance for the
reaction. In this work, the strong absorption of 4-nitrophenol by
h-BN improves local concentration of reactants around the
catalyst, which is favorable for the catalytic reaction. This can be
regarded as a synergistic effect of the shell h-BN and the core
cobalt. On the basis of our results, we propose the following
4
. Conclusions
In this study, we present a facile one-step synthesis of Co@BN
core–shell nanostructures through high-temperature calcina-
tion of boron oxide, urea and cobalt(II) nitrate. The as-prepared
Co@BN nanoparticles exhibited high chemical stability and
good magnetic properties and were used as catalysts for the
reduction of nitroarenes. We found that Co@BN nanoparticles
showed high catalytic performance, durable stability and
magnetic recyclability for hydrogenation of nitroarenes to
possible mechanism for Co@BN catalyzing 4-nitrophenol
ꢀ
reduction in the presence of NaBH
4
(Fig. 8a). Firstly, BH
4
and
4
-nitrophenol are adsorbed on the catalysts surface. Then the
ꢀ
adsorbed BH4 transfers electrons to Co NPs to form Co–H
species. Secondly, the electrons are transferred from Co–H
species to the adsorbed 4-nitrophenol and thus reduction of 4-
nitrophenol to 4-aminophenol proceeds. According to the
possible reaction pathways for the catalytic reduction of
aromatic nitro compounds based on Haber's model,
hydrogenation route for 4-nitrophenol could be schematically
presented in Fig. 8b. This is a six-electron transfer process.
4
aminoarenes in the presence of excess NaBH . We proved that
the sample 13.6 wt% Co@BN exhibited the excellent catalytic
activity in hydrogenation of 4-nitrophenol, 4-nitroaniline, 4-
nitrochlorobenzene, 4-nitromethylbenzene and 4-nitroanisole.
In addition, a signicant synergistic effect of the support h-BN
was observed by effectively adsorbing the reactant nitro-
aromatics, leading to an obvious increase in its surface
concentration. Furthermore, we also demonstrated that the
catalytic reaction is rst order with respect to 4-nitrophenol
71,72
a direct
64
73
Finally, 4-aminophenol is desorbed from the surface of Co@BN
catalysts and dispersed in the reaction solution. To gain further
insight into the detailed mechanism, however, more experi-
mental and theoretical work is needed.
This journal is © The Royal Society of Chemistry 2017
RSC Adv., 2017, 7, 35451–35459 | 35457

View Article Online
RSC Advances
Paper
concentration, sodium borohydride concentration and catalyst 20 M. Danek, K. F. Jensen, C. B. Murray and M. G. Bawendi,
amount. The activation energy for reduction of 4-nitrophenol
Chem. Mater., 1996, 8, 173–180.
21 C. W. Chen, M. Q. Chen, T. Serizawa and M. Akashi, Adv.
Mater., 1998, 10, 1122–1126.
ꢀ1
for 13.6 wt% Co@BN was determined to be 102.93 kJ mol
.
This work gives evidence that Co@BN core–shell nanoparticles
possess easy recyclability, chemical stability and excellent 22 S. Phadtare, A. Kumar, V. P. Vinod, C. Dash, D. V. Palakar,
catalytic activity for surface catalytic reaction, and the design
concept used in the Co@BN core–shell catalyst can be extended
to other metal/nonmetal nanocomposites.
M. Rao, P. G. Shukla, S. Sivaram and M. Sastry, Chem.
Mater., 2003, 15, 1944–1949.
23 S. H. Joo, J. Y. Park, C. K. Tsung, Y. Yamada, P. Yang and
G. A. Somorjai, Nat. Mater., 2009, 8, 126–131.
2
4 H. G. Boyen, G. Kastle, K. Zurn, T. Herzog, F. Weigl,
P. Ziemann, O. Mayer, C. Jerome, M. Moller, J. P. Spatz,
M. G. Garnier and P. Oelhafen, Adv. Funct. Mater., 2003,
Conflict of interest
The authors declare no competing nancial interest.
13, 359–364.
2
2
5 L. Hang, Y. Zhao, H. Zhang, G. Liu, W. Cai, Y. Li and L. Qu,
Acta Mater., 2016, 105, 59–67.
6 A. Lopez-Bezanilla, J. Huang, H. Terrones and B. G. Sumpter,
Nano Lett., 2011, 11, 3267–3273.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (No. U1662112, 21273038, 21543002 and
2
2
7 C. Huang, Q. liu, W. Fan and X. Qiu, Sci. Rep., 2015, 5, 16736.
8 L. Song, L. Ci, H. Lu, P. B. Sorokin, C. Jin, J. Ni,
A. G. Kvashnin, D. G. Kvashnin, J. Lou, B. I. Yakobson and
P. M. Ajayan, Nano Lett., 2010, 10, 3209–3215.
11305091).
References
2
9 Z. Liu, Y. Gong, W. Zhou, L. Ma, J. Yu, J. C. Idrobo, J. Jung,
A. H. MacDonald, R. Vajtai, J. Lou and P. M. Ajayan, Nat.
Commun., 2013, 4, 2451.
1
Y. Bao, H. Calderon and K. M. Krishnan, J. Phys. Chem. C,
007, 111, 1941–1944.
2
2
O. Margeat, C. Amiens, B. Chaudret, P. Lecante and 30 W. Lei, D. Portehault, D. Liu, S. Qin and Y. Chen, Nat.
R. E. Beneld, Chem. Mater., 2005, 17, 107–111.
Commun., 2013, 4, 1777.
L. Zhou, Y. Guo and K. Hideo, AIChE J., 2014, 60, 2907–2917. 31 Q. Weng, Y. Ide, X. Wang, X. Wang, C. Zhang, X. Jiang,
3
4
5
6
F. Hebrard and P. Kalck, Chem. Rev., 2009, 109, 4272–4282.
G. Cahiez and A. Moyeux, Chem. Rev., 2010, 110, 1435–1462.
Y. Xue, P. Dai, K. Komaguchi, Y. Bando and D. Golberg,
Nano Energy, 2015, 16, 19–27.
S. Gao, Y. Lin, X. Jiao, Y. Sun, Q. Luo, W. Zhang, D. Li, J. Yang 32 X. Lv, J. Wang, Z. Yan, D. Jiang and J. Liu, J. Mol. Catal. A:
and Y. Xie, Nature, 2016, 529, 68–71. Chem., 2016, 418–419, 146–153.
A. P. Unnarkat, T. Sridhar, H. Wang, S. Mahajani and 33 M. Wang, M. Li, L. Xu, L. Wang, Z. Ju, G. Li and Y. Qian,
A. K. Suresh, AIChE J., 2016, 62, 4384–4402. Catal. Sci. Technol., 2011, 1, 1159.
H. Wang, Z. Dong and C. Na, ACS Sustainable Chem. Eng., 34 G. Yadav and P. Goel, Clean Technol. Environ. Policy, 2002, 4,
7
8
9
2013, 1, 746–752.
227–234.
Y. Niu and R. M. Crooks, C. R. Chim., 2003, 6, 1049–1059.
35 G. D. Yadav, Y. B. Jadhav and S. Sengupta, Chem. Eng. Sci.,
2003, 58, 2681–2689.
1
1
1
1
1
1
0 C. J. Zhong and M. M. Maye, Adv. Mater., 2001, 13, 1507–
1
511.
36 Y. Zhang, H. P. Tran, I. Hussain, Y. Zhong and S. Huang,
Chem. Eng. J., 2015, 279, 396–401.
37 Y. Yuan, H. Tao, J. Fan and L. Ma, Chem. Eng. J., 2015, 268,
38–46.
38 Y. Q. Zhang, X. Z. Du and W. L. Huang, Chin. Chem. Lett.,
2011, 22, 358–361.
39 T. R. Mandlimath and B. Gopal, J. Mol. Catal. A: Chem., 2011,
350, 9–15.
1 E. E. Carpenter, C. Sangregorio and C. J. O'Connor, IEEE
Trans. Magn., 1999, 35, 3496–3498.
2 N. S. Sobal, U. Ebels, H. M ¨o hwald and M. Giersig, J. Phys.
Chem. B, 2003, 107, 7351–7354.
3 N. S. Sobal, M. Hilgendorff, H. M ¨o hwald and M. Giersig,
Nano Lett., 2002, 2, 621–624.
4 M. A. Hines and P. Guyot-Sionnest, J. Phys. Chem. C, 1996,
1
00, 468–471.
40 K. L. Wu, X. W. Wei, X. M. Zhou, D. H. Wu, X. W. Liu, Y. Ye
and Q. Wang, J. Phys. Chem. C, 2011, 115, 16268–16274.
5 B. O. Dabbousi, J. Rodriguez-Viejo, F. V. Mikulec, J. R. Heine,
H. Mattoussi, R. Ober, K. F. Jensen and M. G. Bawendi, J. 41 P. Deka, R. C. Deka and P. Bharali, New J. Chem., 2014, 38,
Phys. Chem. B, 1997, 101, 9463–9475. 1789.
6 X. Peng, M. C. Schlamp, A. V. Kadavanich and 42 J. Li, C. Y. Liu and Y. Liu, J. Mater. Chem., 2012, 22, 8426.
1
1
1
1
A. P. Alivisatos, J. Am. Chem. Soc., 1997, 119, 7019–7029.
7 O. I. Mi ´c i ´c , B. B. Smith and A. J. Nozik, J. Phys. Chem. B, 2000,
43 B. A. Donlon, E. Razo-Flores, J. A. Field and G. Lettinga, Appl.
Environ. Microbiol., 1995, 61, 3889–3893.
1
04, 12149–12156.
8 Y. W. Cao and U. Banin, Angew. Chem., Int. Ed., 1999, 38,
692–3694.
44 M. Guo, J. He, Y. Li, S. Ma and X. Sun, J. Hazard. Mater., 2016,
310, 89–97.
45 S. M. Alshehri, T. Almuqati, N. Almuqati, E. Al-Farraj,
N. Alhokbany and T. Ahamad, Carbohydr. Polym., 2016,
151, 135–143.
3
9 M. A. Correa-Duarte, M. Giersig and L. M. Liz-Marzan, Chem.
Phys. Lett., 1998, 286, 497–501.
35458 | RSC Adv., 2017, 7, 35451–35459
This journal is © The Royal Society of Chemistry 2017

View Article Online
Paper
RSC Advances
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6 C. Huang, W. Ye, Q. Liu and X. Qiu, ACS Appl. Mater. 60 A. Noschese, A. Buonerba, P. Canton, S. Milione,
Interfaces, 2014, 6, 14469–14476. C. Capacchione and A. Grassi, J. Catal., 2016, 340, 30–40.
7 X. Qiu, Q. Liu, M. Song and C. Huang, J. Colloid Interface Sci., 61 H. S. Shin and S. Huh, ACS Appl. Mater. Interfaces, 2012, 4,
016, 477, 131–137. 6324–6331.
8 Y. Li, W. Qiu, F. Qin, H. Fang, V. G. Hadjiev, D. Litvinov and 62 A. A. Vernekar, S. Patil, C. Bhat and S. G. Tilve, RSC Adv.,
J. Bao, J. Phys. Chem. C, 2016, 120, 4511–4516. 2013, 3, 13243.
9 V. Gandhi, R. Ganesan, H. H. Abdulrahman Syedahamed 63 T. Yang, H. Y. Zou and C. Z. Huang, ACS Appl. Mater.
and M. Thaiyan, J. Phys. Chem. C, 2014, 118, 9715–9725. Interfaces, 2015, 7, 15447–15457.
0 Q. Yang, Y. Z. Chen, Z. U. Wang, Q. Xu and H. L. Jiang, Chem. 64 F. Xia, X. Xu, X. Li, L. Zhang, L. Zhang, H. Qiu, W. Wang,
2
Commun., 2015, 51, 10419–10422.
Y. Liu and J. Gao, Ind. Eng. Chem. Res., 2014, 53, 10576–
1 X. Chen, J. Chang, H. Yan and D. Xia, J. Phys. Chem. C, 2016,
10582.
¨
120, 28912–28916.
65 D. Sun, V. Mazumder, O. Metin and S. Sun, ACS Catal., 2012,
2 S. Wunder, F. Polzer, Y. Lu, Y. Mei and M. Ballauff, J. Phys.
Chem. C, 2010, 114, 8814–8820.
3 D. T. Dam and J. M. Lee, ACS Appl. Mater. Interfaces, 2014, 6,
2, 1290–1295.
66 E. K. Abo-Hamed, T. Pennycook, Y. Vaynzof,
C. Toprakcioglu, A. Koutsioubas and O. A. Scherman,
Small, 2014, 10, 3145–3152.
20729–20737.
4 L. Gao, R. Li, X. Sui, R. Li, C. Chen and Q. Chen, Environ. Sci. 67 H. Li, F. Yue, C. Yang, P. Xue, N. Li, Y. Zhang and J. Wang,
Technol., 2014, 48, 10191–10197. CrystEngComm, 2017, 19, 64–71.
5 X. Xia, J. Tu, Y. Zhang, X. Wang, C. Gu, X. Zhao and H. Fan, 68 K. Makino, M. M. Mossoba and P. Riesz, J. Phys. Chem., 1983,
ACS Nano, 2012, 6, 5531–5538. 87, 1369–1377.
6 K. Y. Andrew Lin and H. A. Chang, Chem. Eng. J., 2016, 296, 69 M. Conte, H. Miyamura, S. Kobayashi and V. Chechik, J. Am.
43–251. Chem. Soc., 2009, 131, 7189–7196.
7 L. Zhang, W. Wang, L. Zhou, M. Shang and S. Sun, Appl. 70 S. Wunder, F. Polzer, Y. Lu, Y. Mei and M. Ballauff, J. Phys.
Catal., B, 2009, 90, 458–462. Chem. C, 2010, 114, 8814–8820.
8 E. Seo, J. Kim, Y. Hong, Y. S. Kim, D. Lee and B.-S. Kim, J. 71 A. Corma, P. Concepcion and P. Serna, Angew. Chem., 2007,
Phys. Chem. C, 2013, 117, 11686–11693. 46, 7266–7269.
9 M. Nemanashi and R. Meijboom, Catal. Commun., 2016, 83, 72 F. Haber, Z. Elektrochem., 1898, 22, 506.
3–57. 73 S. Saha, A. Pal, S. Kundu, S. Basu and T. Pal, Langmuir, 2010,
6, 2885–2893.
2
5
2
This journal is © The Royal Society of Chemistry 2017
RSC Adv., 2017, 7, 35451–35459 | 35459