Novel Biomass-Derived Fe3O4@Pd NPs as Efficient and Sustainable Nanocatalyst for Nitroarene Reduction in Aqueous Media

Article abstract of DOI:10.1007/s10562-019-02829-0

Abstract: A novel magnetically recyclable nitrogen-doped Fe₃O₄@Pd NPs was prepared from the biomass-based materials which was employed as carbon and nitrogen source. The as-prepared catalysts were fully characterized by a variety of physicochemical techniques and were exploited for nitroaromatic hydrogenation with broad scope and excellent chemoselectivity using molecular hydrogen as a reductant. The heterogeneous catalysts can be recovered easily and reused for at least eight recycling reactions without obviously loss of catalytic properties. In addition, using this protocol, the key intermediate of marketed drug Osimertinib could be synthesized easily. Graphical Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-019-02829-0

Catalysis Letters
https://doi.org/10.1007/s10562-019-02829-0
Novel Biomass‑Derived Fe O @Pd NPs as Efcient and Sustainable
3
4
Nanocatalyst ꢀor Nitroarene Reduction in Aqueous Media
1
2
2
2
2
Linwei Zhao · Kai Zheng · Jianying Tong · Jianzhong Jin · Chao Shen
Received: 13 February 2019 / Accepted: 14 May 2019
©
Springer Science+Business Media, LLC, part of Springer Nature 2019
Abstract
A novel magnetically recyclable nitrogen-doped Fe O @Pd NPs was prepared from the biomass-based materials which was
3
4
employed as carbon and nitrogen source. The as-prepared catalysts were fully characterized by a variety of physicochemi-
cal techniques and were exploited for nitroaromatic hydrogenation with broad scope and excellent chemoselectivity using
molecular hydrogen as a reductant. The heterogeneous catalysts can be recovered easily and reused for at least eight recycling
reactions without obviously loss of catalytic properties. In addition, using this protocol, the key intermediate of marketed
drug Osimertinib could be synthesized easily.
Graphical Abstract
Osimertinib
N
key intermediate
NO
2
NH
2
NH
2
N
N
+
H2
R
R
Fe
3
O
4
@NC/Pd (chitosan)
o
N
N
H
N
H
2
O, 80 C, 6 h
O
Wide substrate variety
High chemoselective
Easy separation and recycle
Good potential application
A core-shell nanostructured Pd@nitrogen-containing catalyst exhibited robust catalytic performance in the hydrogenation of nitroarene
Keywords Heterogeneous catalysis · Catalysis · Hydrogenation · Processes and reactions · Green chemistry · Catalysis
1
Introduction
Biomass-based materials have great potentials as functional
materials, which can replace traditional materials, reducing
the dependence on fossil resources and environmental pol-
lution [1]. Meanwhile, the biomass materials possess many
attracting characteristics, such as high speciꢀc surface area,
stability in most organic solvents, especially abundant func-
tional groups, which are capable of chelating with metal ions
or nanoparticles, thereby providing the required stability [2,
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s10562-019-02829-0) contains
supplementary material, which is available to authorized users.
*
Jianzhong Jin
hzjjz@163.com
3
]. Owing to their abundance in nature, renewability, biodeg-
*
Chao Shen
radability and non-toxicity, biomass materials are regarded
as an ideal alternative for the preparation of heterogeneous
catalysts [4]. Up to now, metal nanoparticles stabilized by
either inorganic or carbon-based materials are evolving into
an eꢁcient class of catalyst for various organic reactions
shenchaozju@163.com
1
2
College of Petroleum Chemical Industry, Changzhou
University, Changzhou 213164, China
College of Biology and Environmental Engineering,
Zhejiang Shuren University, Hangzhou 310015, China
Vol.:(0
1
123456789)
3

L. Zhao et al.
[
5–7]. For instance, various biomass carbon-based materials
2 Results and Discussion
heterogeneous metal catalysts were developed, particularly
magnetic carbon-based materials heterogeneous those noble
metals catalysts [8–10]. Notable progress in the development
of magnetic carbon-based heterogeneous materials and their
catalytic applications in organic synthesis has been reported
The schematic procedure for the preparation of the Fe O @
3
4
NC/Pd catalysts is shown in Scheme 1. For the synthesis of
spherical magnetic core/shell catalysts were pre-synthesized
in solution and subsequently carbonized in a hydrothermal
synthesis reactor. The Fe O cores were preserved during
[
11, 12]. In this respect, our group has developed various
3
4
biomass-based heterogeneous catalysts which can be used
in many types of catalytic reactions such as Suzuki–Miyaura
coupling, Heck reaction, Sonogashira reaction and C-S cou-
pling [13–16].
the hydrothermal synthesis and the glucosamine and chi-
tosan shells were transformed into carbon shells envelop-
ing the magnetic spheres, forming magnetic Fe O @NC
3
4
2
+
microspheres. Alternatively, the Pd ions with a positive
charge were adsorbed on the surface of Fe O @NC. Then,
Nitrogen doped carbon heterogeneous materials have
attracted much attention due to their special structure, good
properties, and potential applications [17–19]. The most
common of N-doped carbon heterogeneous materials is pay-
ing attention [20, 21]. To obtain N-doped carbon, our atten-
tion was drawn to inexpensive bio-available ligands with
amine-functionalized carbohydrate backbones glucosamine
and chitosan. Glucosamine is a widely used for the treat-
ment of osteoarthritis and can be easily modiꢀed by other
molecules because of its alterable functional groups [22].
Chitin is widely found in shrimp or crab shells, produced as
bio-waste in industrial and chitosan is a product of deacety-
lation of chitin, so it is plentiful [23]. Furthermore, chitosan
and glucosamine has become an attractive precursor for the
preparation of nitrogen-doped carbonaceous materials upon
hydrothermal treatment. So glucosamine and chitosan were
used to prepare a precursor with nitrogen-doped carbon and
other nitrogen-doped carbon materials. Considering these
characteristics of chitosan and glucosamine, the as-pre-
pared catalysts were carefully prepared and characterized
by a multitude of analytical techniques. These heterogene-
ous catalysts were successfully employed for the catalytic
hydrogenation of nitro compound using molecular hydrogen
in water. Aromatic amines are key building units for many
3
4
the adsorbed noble metal ions were reduced by ascorbic
acid. Here, Fe O @NC acted as “surfactant” to absorb the
3
4
2
+
Pd ions, which did not need other surfactants. The Pd
content was determined to be 5.1 wt% by ICP-OES. It was
worth noting that the Fe O @NC/Pd catalyst can be well
3
4
dispersed in water, which facilitates the substrate to contact
the active sites of the catalyst adequately for chemical reac-
tions in water.
These as-prepared catalysts have been characterized
by infrared analysis (FT-IR). In Fig. 1, the characteristic
−
1
absorption band of Fe–O is clearly observed at 673 cm
.
−
1
The presence of absorption bands at 1558 cm could be
−
1
corresponded to C=C stretching vibrations, 1730 cm and
−
1
3
386 cm are associated with the C = O and O–H vibra-
tions, which reꢂected the carbonization of chitosan or glu-
cosamine during the hydrothermal process. Besides the
−
1
peaks at 1130 cm could be attributed to the C–N, which
conꢀrmed the successful attachment of nitrogen-doped car-
bon on the surface of Fe O in Fig. 1b, c.
3
4
A magnetic catalyst should possess suꢁcient paramag-
netic properties for practical applications. Therefore, the
ꢀ
ne chemicals including pharmaceuticals, agrochemicals,
3.0
2.5
2.0
dyes and bulk chemicals [24, 25]. In this respect, catalytic
hydrogenation using molecular hydrogen represents the
most eꢁcient, environmentally benign, and straightforward
approach among the various methods [26]. Herein, we inves-
tigate the selective hydrogenation of nitroarenes exploring
the potential of biomass-based Fe O @NC/Pd catalysts.
a
b
3
4
1.5
1.0
0.5
0.0
c
4000
3500
3000
2500
2000
1500
1000
500
-1
Wavenumber (cm )
Fig. 1 FTIR spectra of (a) Fe O @C/Pd (glucose); (b) Fe O @NC/
3
4
3
4
Scheme 1 Synthesis of biomass-derived Fe O @NC/Pd catalyst
Pd (glucosamine); (c) Fe O @NC/Pd (chitosan)
3 4
3
4
1
3

Novel Biomass‑Derived Fe O @Pd NPs as Efcient and Sustainable Nanocatalyst ꢀor…
3
4
3
3
2
2
1
1
5
0
5
0
5
0
5
0
5
and the thickness of the shell carbon layer is about 30 nm. In
Fig. 3b, it can also be found that the surface of the Fe O @
c
3
4
NC(chitosan) sphere was decorated with 280 nm diameter
Pd nanoparticles. The TEM studies conꢀrmed the incorpo-
rated Pd NPs on the surface of Fe O @NC nanospheres and
b
3
4
also indicated that the catalyst has a core/shell nanostructure.
In addition, the results mean the carbonization does not dam-
age the core/shell structure.
a
-
-
-
-
-
-
-
10
15
20
25
30
35
After the characterization of the catalysts, the catalytic
activities of magnetic catalysts were examined in the reduc-
tion reaction. The hydrogenation of nitrobenzene 1a was
used as the probe reaction to explore the performance of
magnetic core/shell catalysts. Ethanol was selected as the
solvent and the optimized reaction conditions were 80 °C
-
20000
-10000
0
10000
20000
Field (Oe)
and 1 atm H . No conversion was observed if the reac-
2
Fig. 2 Hysteresis loops for the (a) Fe O @C/Pd (glucose); (b)
tion was performed in the presence of Fe O or Fe O @
3
4
3
4
3
4
Fe O @NC/Pd (glucosamine); (c) Fe O @NC/Pd (chitosan)
3
4
3
4
NC (Table 1, entries 1,2). To our delight, the hydrogena-
tion of nitrobenzene proceeded with good yield based on
starting material when Fe O @C/Pd (glucose), Fe O @
3
4
3
4
NC/Pd (glucosamine) and Fe O @NC/Pd (chitosan) were
3
4
used as catalyst (Table 1, entries 3–5). In comparison, the
best result was achieved with Fe O @NC/Pd (chitosan).
3
4
To increase the extent of conversion, other reaction condi-
tions such as solvent and temperature were screened in this
reaction. Then, water as good solvent was employed in this
reaction (Table 1, entries 5–8). Not only for the considera-
tions of environmental and eꢁciency issue, but also aniline
was obtained as the sole product without any detectable
by-product when water as the probe reaction solvent. As
expected, Fe O @NC/Pd (chitosan) has the best catalytic
Fig. 3 TEM images of a Fe O @NC/Pd (glucosamine); b Fe O @
3
4
3
4
NC/Pd (chitosan)
3
4
eꢃect in water (Table 1, entries 8, 9). Moreover, the eꢃect
of reaction temperature was also investigated, and 80 °C was
proved be optimal (Table 1, entries 10–15). We also attempt
to reduce the amount of catalyst, but the result was not really
perfect (Table 1, entries 15). Surprisely, the results shown
that using Fe O @NC/Pd catalyst the catalytic eꢃect was
magnetic hysteresis loops (M–H loops) (Fig. 2) of magnetic
catalysts were measured at room temperature (300 K) in
an applied magnetic ꢀeld up to 20,000 Oe. The isother-
mal magnetization curve displayed a rapid increase with
increasing applied magnetic ꢀeld due to superparamag-
netic relaxation and the Fe O @NC/Pd (glucosamine) and
3
4
better than the market for Pd/C under the same conditions
3
4
Fe O @NC/Pd(chitosan) saturation magnetization was 8.2
(Table 1, entries 16).
3
4
and 26.9 emu/g. Therefore, the magnetization of magnetic
catalyst was strong enough for magnetic separation by a per-
manent magne. The superparamagnetic behavior and high
saturation magnetization of Fe O @NC/Pd nanoparticles
The selective hydrogenation protocol of the Fe O @NC/
3
4
Pd (chitosan) catalyst was further extended to the conver-
sion of various substituted nitroarenes to the corresponding
amines to demonstrate the general applicability. Table 2 sum-
marizes the hydrogenation of various functional nitroarenes.
It could be seen that the Fe O @NC/Pd (chitosan) catalyst
3
4
could not only beneꢀt their dispersibility and redispersibil-
ity in the solution, but also guarantee the quick response of
these nanoparticles to external magnetic ꢀelds. Thus both
the superparamagnetic property and high magnetization of
the as prepared nanoparticles are ideal for use in practical
applications.
3
4
could smoothly transform each functional nitroarene fully,
and exhibited excellent selectivities in the hydrogenation to
the corresponding aromatic amines. In all cases, the desired
aromatic amines were obtained in excellent yields. Because,
the electronic features of the nitroarenes almost did not aꢃect
the reaction yield. It is observed the position of the substi-
The morphology of the hybrid nanoparticles shows a
core–shell feature with ꢀne particles deposited on the sur-
face, as shown in Fig. 3a. It was easily observed that the
diameter of the as-prepared Fe O @NC (glucosamine)
tute –OCH group showed no inꢂuence on the reduction of
3
nitroanisole (Table 2, 2b–2d). Three kinds of nitroanisole
3
4
nanoparticles with carbon shells is about 300 nm in size,
produced the corresponding aromatic amines yields were
1
3

L. Zhao et al.
Table 1 Optimization of reaction conditions
Table 2 Fe O @NC/Pd (chitosan) catalyzed hydrogenation of diꢃer-
3
4
ent nitroarenes
a
Entry Catalyst
Solvent
Temp (°C) Time (h) Yield (%)
c
d
2
a,99%
2b,98%
2c,98%
2d,97%
1
2
3
4
Fe O
EtOH
EtOH
80
80
80
80
6
6
6
6
–
–
85
91
3
4
Fe O @NC
3
4
b
c
Fe O @C/Pd EtOH
3
4
Fe O @NC/ EtOH
3
Pd
4
d
c,d
c d
95d
2e,98%
2f,98%
2j,96%
2n,95%
2r,98%
2g,96%
2k,96%
2h,96%
2l,95%
2p,93%
2t,96%
5
6
7
8
9
1
1
1
1
1
1
1
Fe O @NC/ EtOH
80
80
6
6
6
6
6
6
6
6
4
2
8
8
3
Pd
4
Fe O @NC/ MeOH
94d
88d
99d
95c
93d
69d
43d
84d
58d
68d,e
97
3
Pd
4
Fe O @NC/ Isopropanol 80
3
Pd
4
c e
c
c
c
2i,97%
Fe O @NC/ H O
80
80
70
60
rt
3
Pd
4
2
Fe O @NC/ H O
3
Pd
4
2
d
d
c
c
0
1
2
3
4
5
6
Fe O @NC/ H O
2m,96%
2o,96%
3
Pd
4
2
Fe O @NC/ H O
3
Pd
4
2
Fe O @NC/ H O
3
Pd
4
2
c
c,e
c,e
c
2
q,97%
2s,92%
Fe O @NC/ H O
80
80
80
80
3
Pd
4
2
The reaction conditions: nitroarenes 1a (1 mmol), H2 (1 atm),
0.5 mol% catalysts, 6 h and H O (3 mL) under air
2
Fe O @NC/ H O
3
Pd
4
2
a
HPLC yield
b
Fe O @NC/ H O
EtOH:H O=1:1 (v:v)
3
Pd
4
2
2
c
1
2 h
Pd/C
H O
d
2
24 h
The reaction conditions: nitrobenzene 1a (1 mmol), H (1 atm),
2
0
.5 mol% catalysts and solvent (3 mL) under air
a
HPLC yield
b
c
d
e
Fe O @C/Pd(glucose)
3
4
Fe O @NC/Pd (glucosamine)
3
4
Fe O @NC/Pd (chitosan)
3
4
0
.25 mol% catalysts
Scheme 2 Application of the catalyst in the synthesis of key interme-
not much diꢃerence. Nitroarenes bearing inert or less reac-
tive functional groups, such as alkyl, alkoxy and biphenyl
were hydrogenated giving the corresponding anilines in up
to quantitative yields (Table 2, 2b–2i). Halogen-substituted
nitroarenes were smoothly hydro-genated without any dehal-
ogenation process aꢃording the desired halogen-substituted
anilines in excellent yields (Table 2, 2j–2 m). In addi-
tion, other substituents including amino groups were well
diate of marketed drug Osimertinib
tolerated, too (Table 2, 2n–2o). Importantly, several reduc-
ible groups, like –CHO, –COCH and –COOH remained
3
totally unaꢃected under the reaction conditions (Table 2,
2p–2 s). Finally, heteroaromatic amine 2r was aꢃorded in
96% yield (Table 2, 2r).
1
3

Novel Biomass‑Derived Fe O @Pd NPs as Efcient and Sustainable Nanocatalyst ꢀor…
3
4
For practical applications of catalytic systems, the
hydrogenation protocol was employed to obtain the key
intermediate of marketed drug Osimertinib (Scheme 2).
The great signiꢀcance of key intermediate 4 lies in their
use as precursors for the synthesis of antitumor drugs Osi-
mertinib [27]. This transformation was accomplished with
excellent results in the presence of Fe O @NC/Pd (chi-
continuously stirred at 80 °C for 5 h. Nitrobenzene conver-
sion was kept the same after removal of the catalyst. All of
these results suggested that the Fe O @NC/Pd (chitosan)
3
4
was stable, recyclable, and showed no apparent loss of
catalyst eꢁciency.
3
4
tosan) catalyst in water at 80 °C for 6 h. Then the product
was isolated in 98% yield. More importantly, ICP-OES
studies detected no palladium contamination in the reac-
tion solvent (residual Pd content < 10 ppm).
3 Conclusions
A novel magnetically recyclable nitrogen-doped Fe O @Pd
3
4
NPs was prepared from the biomass-based materials which
Since stability, ease of separation, and reusability of
the catalyst are among the most important features of
heterogeneous catalysts, the recyclability of Fe O @NC/
was employed as carbon and nitrogen source. The Fe O @
3
4
NC-supported palladium catalysts were fully characterized and
exhibited superior catalytic activity and high chemoselectivity
for the hydrogenation of a variety of substituted nitroarenes
including those with reducible functional groups to the corre-
sponding aromaticamines in water. The Fe O @NC/Pd cata-
3
4
Pd(chitosan) was tested for the hydrogenation of nitroben-
zene in Fig. 4. In these recycling experiments, the activ-
ity of the catalyst and selectivity for aniline formation
remained excellent for the seventh run. However, a pro-
gressive loss of activity occurred and the aniline yield
remained at 90% after eight runs. Furthermore, hot ꢀllet-
ing leaching experiment was also conducted. After 1 h, the
magnetic Fe O @NC/Pd (chitosan) catalyst was collected
3
4
lyst can be recovered easily and reused for at least eight recy-
cling reactions. The catalytic system not only exhibits excellent
activity and selectivity but also represents a green and sustain-
able catalytic process, which facilitates facile operation, easy
separation, and catalyst recycle.
3
4
by an external magnet, and the clear liquid solution was
3.1 Experimental Section
3
.1.1 General Inꢀormation
The starting materials were commercially available and
were used without further puriꢀcation except solvents. All
of the reaction were monitored by TLC. The products were
isolated by column chromatography on silica gel (200–300
mesh) using petroleum ether (60–90 °C) and ethyl acetate.
Fourier-transform infrared (FTIR) spectra were measured with
a Bruker Tensor 27 FT-IR by using KBr pellets. Transmission
electron microscopy (TEM) images were obtained from a FEI
T20 microscope. The magnetic properties of the nanoparticles
were determined by a vibrating sample magnetometer (Lake
1
00
1
13
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
Shore 7304, USA). H and C NMR spectra were measured
with a Bruker Advance 400 spectrometer by using CDCl as
3
solvents and TMS as the internal standard.The Pd content in
the supported catalyst was determined by an Perkin-Elmer
Optima 2100 DV.
9
9
99
99
97
96
93
91
90
3
.1.2 Preparation oꢀ Fe O Nanoparticles
3 4
Fe O nanoparticles is synthesized by a solvothermal reaction
3
4
method. Typically, FeCl ·6H O (1.5 g), NaAC (2 g) and PVP
3
2
(
1 g) were dissolved in ethylene glycol (30 mL) under mag-
netic stirring. The resultant solution was transferred into a Tef-
lon lined stainless steel autoclave, sealed, and heated to 200 °C
for 12 h. After the reaction was complete, the black Fe O
0
2
4
6
8
R uns
3
4
nanoparticles were separated by using a permanent magnet and
Fig. 4 Recycling and reuse of Fe O @NC/Pd (chitosan) in the reduc-
3
4
tion reaction
1
3

L. Zhao et al.
washed several times with ethanol water. Finally, the resulting
product was dried under vacuum at 60 °C for 24 h.
The formed black suspension was ultrasonically mixed with
PdCl (35 mg) ethanol solution (3 mL) for 1 h, then an ascor-
2
bic acid ethanol solution (8 mL) was dropped into the above
mixture with vigorous stirring under 60 °C. After 2 h of
reduction, the products were separated by an external mag-
net and washed several times with water. The products were
dried in vacuum to obtain Fe O @C/Pd and Fe O @NC/Pd.
3
.1.3 Preparation oꢀ Fe O @C (glucose) Nanoparticles
3 4
Fe O @C was synthesized by in situ carbonization of glu-
3
4
cose in the presence of Fe O under hydrothermal condi-
3
4
3
4
3
4
tions. Fe O nanoparticles (100 mg) were dispersed in water
3
4
(
10 mL) containing glucose (1.6 g) by ultrasonication. Sub-
3.1.7 General Procedure ꢀor the Reduction Reactions
sequently, It was transferred into a Teꢂon lined stainless
steel autoclave, sealed, and heated at 180 °C for 10 h and
cooled down at room temperature. After the reaction was
complete, the resulting nanoparticles were collected by a
permanent magnet and washed with ethanol followed by
water. Finally, the black colored product was dried under
vacuum for 24 h to give Fe O @C nanoparticles.
Nitroarenes (1.0 mmol), H (1 atm), catalyst (0.5 mol%) and
2
solvent (3 mL) were added into a reaction ꢂask and stirred at
80 °C. The reaction progress was monitored by HPLC. After
completion of the reaction, the mixture was allowed to cool
to room temperature. Then the aqueous phase was extracted
with ethyl acetate for 3 times. Then the organic phases were
3
4
combined, dried over anhydrous MgSO , concentrated under
4
3
.1.4 Preparation oꢀ Fe O @NC (glucosamine)
vacuum and puriꢀed by column chromatography to aꢃord
3
4
Nanoparticles
the desired product.
Fe O @NC was synthesized by in situ carbonization of
3
4
glucosamine in the presence of Fe O under hydrothermal
3.1.8 General Procedure ꢀor Catalyst Recovery
3
4
conditions. Fe O nanoparticles (100 mg) were dispersed in
3
4
water (10 mL) containing glucosamine (1.6 g) by ultrasoni-
cation. Subsequently, It was transferred into a Teꢂon lined
stainless steel autoclave, sealed, and heated at 180 °C for
Nitrobenzene (1.0 mmol), H (1 atm), and Fe O @NC/Pd
2
3
4
(chitosan) (0.5 mol%) were mixed in H O (3 mL). The mix-
2
ture was stirred at 80 °C. After the reaction was complete,
the supported catalyst was separated by an external magnet
and washed with water (3×2 mL) and ethanol (3×2 mL),
then dried in vacuum and directly used in the next run.
1
0 h and cooled down at room temperature. After the reac-
tion was complete, the resulting nanoparticles were collected
by a permanent magnet and washed with ethanol followed
by water. Finally, the black colored product was dried under
vacuum for 24 h to give Fe O @NC nanoparticles.
Acknowledgements Financial support from Science and Technology
Plan of Zhejiang Province (No. 2017C31054, 2017C32011), Zhejiang
Provincial Natural Science Foundation of China (No. LY17B020005)
and the National Natural Science Foundation of China (No. 21302171).
We acknowledge the support of the Young and Middle-aged academic
team Project of Zhejiang Shuren University.
3
4
3
.1.5 Preparation oꢀ Fe O @NC (chitosan)Nanoparticles
3 4
Fe O @NC was synthesized by in situ carbonization of
3
4
chitosan in the presence of Fe O under hydrothermal
3
4
Compliance with Ethical Standards
conditions. Fe O nanoparticles (100 mg) were dispersed
3
4
in 3% HAC solution (50 mL) containing chitosan (0.25 g)
by ultrasonication. Then, the PH value is close to 8–9 with
ammonium hydroxide. Subsequently, It was transferred into
a Teꢂon lined stainless steel autoclave, sealed, and heated
at 180 °C for 10 h and cooled down at room temperature.
After the reaction was complete, the resulting nanoparticles
were collected by a permanent magnet and washed with
ethanol followed by water. Finally, the black colored prod-
uct was dried under vacuum for 24 h to give Fe O @NC
Conꢀlict oꢀ interest The authors declare that they have no conꢂict of
interest.
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3
4
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8
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