Synthesis and application of a MOF-derived Ni@C catalyst by the guidance from an: In situ hot stage in TEM

Article abstract of DOI:10.1039/c7ra03162a

Metal-organic frameworks (MOFs) as a class of crystalline porous solids have attracted considerable attention due to their promising potential performance. MOFs have been recently proved to be ideal sacrificial templates for fabricating their respective derivatives by changing the thermal conditions. However, uncertainties still remain, and the direct observation of transition from MOF to metal nanoparticles (NPs) dispersed in carbon matrix is an important and crucial task for the development of MOF-derived materials. Here, transmission electron microscopy (TEM) combined with in situ hot stage technique was applied to directly observe the transition from MOF to metal NPs. Through in situ TEM experiment, the nanocrystals of Ni-ntca precursor (ntca = 1,4,5,8-naphthalenetetra carboxylic acid) are pyrolyzed under the temperature of 400, 500, or 600 °C to synthesize abundant Ni-NPs embedded in hierarchically porous carbon composites. Furthermore, the as-prepared samples show high catalytic activity and stability for the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) with NaBH₄ in aqueous conditions. More importantly, Ni@C-600, which has nickel contents of 72.8%, shorten the reduction time to 3.5 min with high conversion of nearly 100%. When the catalyst is applied to recycle after being separated from the reaction by an extern magnet, it still keeps high conversion of 92% after 8 cycles, addressing the high stability of the composites. It is believed that these results will further facilitate the exploration of the technique of the TEM combined with in situ hot stage as a powerful tool in the carbonization of MOFs to obtain MOF-derived materials with different applications.

Full text of DOI:10.1039/c7ra03162a

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Synthesis and application of a MOF-derived Ni@C
catalyst by the guidance from an in situ hot stage in
TEM†
Cite this: RSC Adv., 2017, 7, 26377
*
Dan Xu, Ying Pan, Mingyi Chen, Qinying Pan, Liangkui Zhu, Ming Xue, Daliang Zhang,
*
Qianrong Fang
and Shilun Qiu
Metal–organic frameworks (MOFs) as a class of crystalline porous solids have attracted considerable
attention due to their promising potential performance. MOFs have been recently proved to be ideal
sacrificial templates for fabricating their respective derivatives by changing the thermal conditions.
However, uncertainties still remain, and the direct observation of transition from MOF to metal
nanoparticles (NPs) dispersed in carbon matrix is an important and crucial task for the development of
MOF-derived materials. Here, transmission electron microscopy (TEM) combined with in situ hot stage
technique was applied to directly observe the transition from MOF to metal NPs. Through in situ TEM
experiment, the nanocrystals of Ni–ntca precursor (ntca ¼ 1,4,5,8-naphthalenetetra carboxylic acid) are
pyrolyzed under the temperature of 400, 500, or 600 ꢀC to synthesize abundant Ni-NPs embedded in
hierarchically porous carbon composites. Furthermore, the as-prepared samples show high catalytic
activity and stability for the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) with NaBH4 in
aqueous conditions. More importantly, Ni@C-600, which has nickel contents of 72.8%, shorten the
reduction time to 3.5 min with high conversion of nearly 100%. When the catalyst is applied to recycle
after being separated from the reaction by an extern magnet, it still keeps high conversion of 92% after 8
cycles, addressing the high stability of the composites. It is believed that these results will further
facilitate the exploration of the technique of the TEM combined with in situ hot stage as a powerful tool
in the carbonization of MOFs to obtain MOF-derived materials with different applications.
Received 17th March 2017
Accepted 3rd May 2017
DOI: 10.1039/c7ra03162a
rsc.li/rsc-advances
materials, nanostructured metal oxides and carbon–metal
hybrids have been quickly developed.14–17 Liu's group prepared
Introduction
carbon–cobalt hybrid materials through the thermolysis of the
Co–ZIF precursor at 750 C which has been explored as a good
Metal–organic frameworks (MOFs), also have been called
porous coordination polymers (PCPs), consist of metal ions or
metal-containing clusters in combination with a continuously
expanding library of multi-functional organic ligands.1–6 Their
fascinating feature of high porosity as well as large surface
areas, made them useful for applications in gas storage and
separation, ion exchange, catalysis, sensing, and electro-
chemical capacitance over the past two decades.7–12 Recently,
MOFs have been proved to be ideal sacricial templates for
fabricating their respective derivatives by changing the thermal
conditions in the different atmosphere.
In 2008, Xu's group synthesized nanoporous carbon mate-
rials by pyrolyzing MOF-5 and furfuryl alcohol (FA) for the rst
time. They opened the gate of carbonizing MOFs and developed
the applications of MOF-derived nanomaterials.13 Aer that, the
use of MOFs as templates to create nanoporous carbon
ꢀ
catalyst for oxygen reduction reaction (ORR).18 The porous
carbon-based nanomaterials prepared by this novel MOF-
templated route possessed many advantages such as large
internal surface area, high crystallinity, tailorable porous
structure and tuned in modication, and more and more
researchers focused on fabricating desired catalysts derived
from MOFs precursors.19,20
There were various MOF-templated carbon-based nano-
materials synthesized via a choice of suitable MOF templates,
pyrolysis atmosphere and temperature. Zhang's group
employed zinc–nickel–terephthalate frameworks as precursors
to prepare a carbon–nickel hybrid material. By adjusting the
ratio of Zn/Ni, the catalysts with densely populated metal sites
were applied in the converting of 4-nitrophenol.21 In addition,
the use of other metal-based MOFs (Fe, Co, Cu etc.)17,22–25 as
templates has promoted the development of the catalytic eld.
During the process of carbonization, the carbon generated from
the organic linkers coated metal NPs and further prevented
them from aggregating. Moreover, the in situ generation
State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin
University, Changchun, 130012, China. E-mail: sqiu@jlu.edu.cn
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c7ra03162a
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methods makes metal NPs more uniformly and stably dispersed temperatures with a heating rate of 5 ꢀC minꢁ1. Aer the pre-set
in porous carbon matrix compared with other method of temperature reached, the powders were annealed for 3 h. These
loading metal into carbon.
products were donated as Ni@C-X (X is considered as pre-set
Transmission electron microscopy (TEM) with high resolu- temperature).
tion transmission electron microscopy (HRTEM) and electron
diffraction (ED) was employed to characterize the microstruc-
Catalytic tests on reduction of 4-NP
ture and ultrastructure of materials.26–28 The TEM equipped
with a heating holder and a hot stage controller allows in situ
observations of phenomena, which accelerated the develop-
ment of metallurgical applications. In 1970, the hot stage TEM
was applied to study crystal nucleation in a lithia–silica glassy
system.29 In 2007, a in situ TEM experiment was used as
In the quartz cell, pipetting 2.05 mL of deionized water, fol-
lowed by the addition of 60 mL of 4-nitrophenol solution (5
mM), and the 0.65 mL of NaBH4 aqueous solution (0.2 M) was
added to the mixture and the color changed to deep yellow
solution. Then the catalyst suspension (50 mL, 6 mg mLꢁ1) was
added to the above mixture solution. During the reaction, the
reaction was taken at 1 min intervals and analyzed by UV/vis
spectroscopy, which were recorded in the scanning range of
250–500 nm at room temperature. For recycling, the catalyst
was recovered using an external magnetic eld for the next
cycle.
a
guideline to synthesize as-grown ordered CoPt nano-
particles.30 So far, in situ TEM is a powerful method for the
observation of micro structural phase changes,31 nucleation,32
growth and dissolution processes.33,34
Here, TEM combined with in situ hot stage technique was
applied to directly observe the transition from MOF to metal
NPs. Through in situ TEM experiment, the nanocrystals of Ni–
ntca precursor (ntca ¼ 1,4,5,8-naphthalenetetracarboxylic acid)
The in situ hot stage experiment
ꢀ
are pyrolyzed under the temperature of 400, 500, or 600 C to
A small amount of Ni–ntca was dispersed in ethanol, aer
ultrasonic treatment, dripped onto a copper grid supporting
holey carbon lm. A single crystal of Ni–ntca was selected to
observe the transition under beam irradiation. The TEM images
and Selected Area Electron Diffraction (SAED) patterns of Ni–
ntca were taken every 5 minutes. The parameters of this
experiment were listed in Table 1.
synthesize abundant Ni-NPs embedded in hierarchically porous
carbon composites. Furthermore, the as-prepared samples
show high catalytic activity and stability for the reduction of 4-
nitrophenol (4-NP) to 4-aminophenol (4-AP) with NaBH4 in
aqueous conditions. More importantly, Ni@C-600, which has
nickel contents of 72.8%, shorten the reduction time to 3.5 min
with high conversion of nearly 100%. When the catalyst is
applied to recycle aer being separated from the reaction by an
extern magnet, it still keeps high conversion of 92% aer 8 Characterization
cycles, addressing the high stability of the composites.
The X-ray diffraction patterns were recorded on a SHIMADZU
LabX XRD-6000 diffractometer with Cu-Ka radiation (l ¼
ꢀ
ꢀ
˚
1.5418 A) in the 2q range of 10–80 with a step size of 0.02 and
a time of 4 second per step. Raman spectra were measured on
an inVia microscopic confocal laser Raman spectrometer. We
investigated the morphology with a JEOS JSM-6510 and
microstructure of the particles with a JEM-2100 TEM system.
An in situ hot stage holder and heater control from GatanS-
martSet Model 901 was used in this study. Thermogravimetric
analysis (TGA) was performed using a SHIMADZU DTG-60
thermal analyzer system at a heating rate of 10 ꢀC minꢁ1
from 50 ꢀC to 600 ꢀC under dry air atmosphere with a ow
Experimental
Synthesis of 1,4,5,8-naphthalenetetracarboxylic acid (NTCA)
1,4,5,8-Naphthalenetetracarboxylic dianhydride (NTCDA,
1
mmol) was dissolved in a aqueous solution of NaOH under
stirring at 60–70 ꢀC. Aer hydrolyzing for 1 h, 1 M HCl was
added dropwise until a precipitate formed and pH was about 2–
3. Then the precipitate was ltered and dried at room
temperature.35
rate of 30 mL minꢁ1
. Nitrogen adsorption–desorption
Synthesis of Ni–ntca
measurements were measured with a Micro Meritics TriStar II
3020 surface area and pore size analyzer at 77 K. The absor-
Ni(NO3)2$6H2O (0.116 g, 0.4 mmol) was dissolved in a mixture
solvent of 1.4 mL of dimethylformamide (DMF), 1.4 mL of
ethanol, 1.4 mL of water. Then NTCA (0.015 g, 0.05 mmol) was
added into the above solvent under sonicating. The resulting
mixture was transferred into a Teon autoclave and maintained
at 100 ꢀC for 3 days. The grown powders were washed with DMF
and distilled water several times and dried at room tempera-
ture. The as-prepared powders were called as Ni–ntca.
bance of 4-NP was obtained on
spectrophotometer.
a
UV-2450 UV/vis
Table 1 The parameters of the beam irradiation experiment
Condition
Unit
High tension
Spot size
Current density
Exposure time
Magnication
200 kV
Synthesis of nickel@carbon hybrids
Spot 5
1.1–8.7 pA cm2
45 s M
In a typical synthesis, the precursor Ni–ntca were paved in
a ceramic boat and placed into a furnace. Carbonizing the
powder under a nitrogen ow and heating to different
50k
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playing a leading role in the process of in situ experiment, and
the following work was performed using TEM. The Ni–ntca was
irradiated under beam for a long time using TEM. The TEM
image and SAED patterns of the crystal were collected on time
during irradiation and contrasted with Fig. 1d, there was no
appearance of Ni NPs within 2 hours, addressing that beam
didn't work for the transition from MOFs to metal NPs, while
led to the collapse of MOFs structure (Fig. S3†). Following, these
thermal phenomena became a reference for us in preparing
Ni@C-X composite series by carbonizing the Ni–ntca at 300 ꢀC,
Results and discussion
The Ni–ntca crystals were synthesized by hydrothermal process.
It can be easily obtained by adding nickel nitrate and NTCA into
a mixture solvent, and then heated in a Teon autoclave for
72 h. The light brown crystal powders were analyzed by X-ray
diffraction (XRD) measurement (Fig. S1†). The relative intensi-
ties and peak positions in the XRD patterns were in good
agreement with the predicted diffraction patterns, revealing the
high crystalline of the synthesized Ni–ntca.36
ꢀ
ꢀ
400 C, 500 ꢀC, and 600 C under a nitrogen ow.
This moment, a hot stage was used for in situ experiments in
TEM to directly observe the changes of the Ni–ntca. Lots of TEM
images and the corresponding diffraction (ED) patterns of the
Ni–ntca particles were taken as elevating the carbonization
temperature in TEM (Fig. 1). The single crystal was displayed in
Fig. 1a, and the ED pattern of crystal showed us high degree of
crystallinity in Fig. 1b. Then, we raised the temperature in
100 ꢀC increments, and stopped 5 minutes at each temperature.
At a temperature of 300 ꢀC, the nanoparticles (black dots)
emerged in the carbon (Fig. 1c), which conrmed by the
appearance of diffraction rings (Fig. 1d). The corresponding
crystalline diffraction rings of (111) and (200) in the SAED
pattern indicated the borning of Ni NPs. From 400 ꢀC to 600 ꢀC,
the crystallization degree of the Ni NPs and the stability of the
carbon gradually enhanced (Fig. 1e–h). When the temperature
exceeded 600 ꢀC, the Ni NPs were unstable and came out of the
carbon to form large particles (Fig. 1h). When extending the
holding time of 5 minutes to 20 minutes, we found that the
results as shown in Fig. S2† were similar to those from 5
minutes. As it is well known the MOFs materials are sensitive to
the beam irradiation, we tried to explain that the beam was not
The XRD patterns of a series of samples aer carbonization
were showed in Fig. 2 and S4.† One low broad peak centered at
2q ¼ 25ꢀ assigned to amorphous carbon,37 the other three
Fig. 2 PXRD patterns of Ni@C-400, Ni@C-500, Ni@C-600 and the
simulated data of cubic phased Ni (JCPDS no. 04-0850).
Fig. 1 The hot stage TEM images at (a) room temperature, (b) Selected Area Electron Diffraction (SAED) pattern of Ni–ntca in [001] zone axe, (c)
300 ꢀC, (d) SAED pattern of (c), (e) 400 ꢀC, (f) 500 ꢀC, (g) 600 ꢀC, and (h) 700 ꢀC.
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prominent peaks near at 2q ¼ 44.3ꢀ, 51.6ꢀ and 76.2ꢀ, being well
assigned to the reections of the (111), (200) and (220) crystal-
line planes of cubic-phased Ni (JCPDS no. 04-0850), respectively,
ꢀ
were detected (Fig. 2). When pyrolyzing the Ni–ntca at 400 C,
500 ꢀC and 600 ꢀC in a nitrogen ow, the Ni–ntca have been
completely converted to amorphous carbon and cubic-phased
Ni NPs, and there was no peaks showing Ni been oxidized to
nickel monoxide (NiO). However, the peaks of Ni@C-300 were
different from the cubic-phased Ni and were proven to be not
pure Ni nanoparticles (Fig. S4†).
As seen in Fig. 3, a broad D band appeared at 1340 cmꢁ1
which ascribed to the disorder-induced mode, while a relatively
sharp G band appeared at 1575 cmꢁ1 which ascribed to the E2g
mode from the sp2 carbon. The relative ratios of D bands to G
bands (ID/IG) in Raman spectra, was 0.8, indicating the crystal-
lization degree of graphitic carbon. This ID/IG ratio was
comparable to those measured for GO and rGO (ID/IG ¼ ꢂ1),38
indicating that high quality 3D graphite carbon was formed.
Likewise, there was a 2D band at 2676 cmꢁ1
.
To investigate the samples aer carbonizing, we character-
ized the samples by TEM. From the TEM images (Fig. 4a–f),
because of nanoscale morphology of Ni–ntca, they were all
thermodynamically unstable and able to aggregate, and nally
appeared irregularly shapes. Aer carbonizing, the composites
still kept indenite shapes (Fig. 4a and b). While further
magnifying the edge at 300k, the Ni NP were uniformly depos-
ited in the carbon matrix, almost all of the particles were
wrapped by thin carbon, thus there was no reunion as well
(Fig. 4c and d). The distribution range of Ni NPs was mainly
distributed from 4 nm to 7 nm measured by TEM (Fig. S5†). To
investigate crystallization degree of metal Ni, we continued to
magnify the thin edge to 500k, and clearly observed metal lattice
fringe, indicating that each particle had a single crystalline
structure (Fig. 4e and f). The distance of the adjacent lattice
Fig. 4 TEM images of Ni@C-600 at 100k (a and b), 300k (c and d), and
500k ((e) and (f), inset: FFT-ED pattern).
fringes (marked by the white arrows) was calculated to be about with the results of Raman. The TEM images of Ni@C-400 and
0.203 nm, corresponding well to the d111 spacing of cubic Ni. Ni@C-500 were placed in Fig. S6.† The images of scanning
The corresponding fast Fourier transform (FFT) patterns were transmission electron microscope (STEM) were showed in
showed in the inset in Fig. 4e and f. Ni NPs were used as cata- Fig. S7,† further conrming the uniform distribution of Ni@C-
lysts to induce the graphitization of carbon during the X. From the TEM results, the synthetic Ni@C-X composites with
carbonization, thus three-dimensional graphite layers were a structure of thin 3D graphite carbon and plentiful Ni NPs will
readily produced around the metal Ni, which was in accordance enhance the catalytic activity. The resultant samples were all
composed of uniformly dispersed Ni NPs and carbon matrix.
To calculate the loading content of Ni NPs in the nanoporous
carbon, the thermo gravimetric analysis (TGA) was carried out
in the temperature range of 50–600 ꢀC under dry air atmosphere
at a heating rate of 10 C minꢁ1. From the TGA curves (Fig. 5),
ꢀ
the nickel loading amounts of three samples were about 68.6%,
72.1% and 72.9% for Ni@C-400, Ni@C-500, Ni@C-600, respec-
tively. Obviously, during carbonization, the metal contents
increased while the carbon was generally declined as elevating
the temperature. In the test of catalytic reduction of 4-nitro-
phenol (4-NP), the more Ni loading quality will acquire more
excellent catalytic performance.
The N2 adsorption/desorption isotherms of Ni@C-X were
measured to explore porous structure (Fig. 6). Three samples
exhibited typical type IV isotherms with a hysteresis loop,
showing the existence of mesoporous (Fig. 6a). The pore size
Fig. 3 Raman pattern of Ni@C-600.
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the N2 absorption isotherm of the Ni–ntca was also shown in
Fig. S8.† The BET surface area of the Ni–ntca was 100 m2 gꢁ1
and the pore size distribution showed it was microspores.
At the end, we measured the catalytic activity of Ni@C-X for
reduction of 4-NP. In recent years, the reduction of 4-nitro-
phenol (4-NP) to 4-aminophenol (4-AP) with NaBH4 in aqueous
solution has become such a model reaction that many groups
used to test catalytic performance. We can monitor accurately
through UV/vis spectroscopy. It was clearly that 4-NP had
a strong absorption peak at 400 nm in an aqueous solution of
NaBH4 and the absorption peak shied to 295 nm addressing
the formation of 4-aminophenol (4-AP). The color of 4-NP
solution was light yellow, with mixing NaBH4 into solution and
turned into light yellow, with adding the catalyst composite and
turned into colorless.39,40 The whole catalytic progress was
measured regularly by UV/vis spectroscopy. Fig. 7a displayed the
typical evolution of the UV/vis spectra with time for the sample.
Aer the 0.3 mg catalyst is added to solution, the reaction
immediately proceeded and we measured the absorbance curve
every 1 minute. The intensity of peaks of the nitro compound at
400 nm decreased sharply, while the aminophenol at 295 nm
increased. No other peak appeared, indicating no side reaction
had taken place. When the peak belong to the nitro compound
was no longer observed and the corresponding color changed
from yellow to colorless, demonstrating the catalytic reduction
of 4-NP completed. The reduction time of converting 4-NP to 4-
AP over Ni@C-600 was 3.5 min (Fig. 7c), which was faster than
Ni@C-400 of 6 min (Fig. 7a) and Ni@C-500 of 4 min (Fig. 7b).
According to the knowledge of physical chemistry principles,
the above catalytic reduction reactions followed the Langmuir–
Hinshelwood apparent rst order kinetics mode without
thinking about an amounts of NaBH4. The equation is described
as dCt/dt ¼ dAt/dt ¼ kappCtt ¼ kappAtt or ln(Ct/C0) ¼ ln(At/A0) ¼
Fig. 5 TGA curves of Ni@C-400 (black), Ni@C-500 (red), and Ni@C-
600 (blue).
kappt, Ct and At are the concentration and absorbance of 4-NP at
400 nm at time t, C0 and A0 are the initial concentration and
absorbance of 4-NP at 400 nm, and kapp is the apparent rate
constant.41 Fig. 7d showed the concentration of Ct (C at time t) to
its initial value C0 (Ct/C0) versus reaction time for the reduction of
4-NP over the catalyst of Ni@C-400, Ni@C-500 and Ni@C-600. It
can be found that the resultant catalysts were highly active for
catalyzing 4-NP reduction with nearly 100% efficiency. Fig. 7e
showed plots of ln(Ct/C0) versus reaction time. The respective
rate constant was calculated to be 22.5, 20.8 and 12.0 ꢃ 10ꢁ3 sꢁ1
.
The rate constant per gram of Ni@C-X (K) was then calculated,
the activity factors K were calculated to be 40.0, 69.3 and 75.0 sꢁ1
Fig. 6 (a) N2 absorption–desorption isotherms and (b) their corre-
sponding pore size distributions (N2 absorption–desorption isotherms gꢁ1 for Ni@C-400, Ni@C-500 and Ni@C-600, respectively.
for Ni@C-500 and Ni@C-600 are vertically offset by 40 and 80 cm3
Obviously, Ni@C-600 exhibited high catalytic activity compared
g
ꢁ1).
with Ni@C-500 and Ni@C-400, which beneted from its large
nickel loading amounts and high crystallinity with quick
accessibility in and out of the porous carbon matrix. The turn-
over frequency (TOF) was further evaluated, which was dened
as the number of moles of 4-NP reduced per mole of nickel per
hour (Table 2). The rate constant and TOF values of Ni@C-X were
much bigger than the previously reported Ni-NPC-600 and other
Ni-based catalysts21,39,42,43 (Table 2), which can be summarized to
following factors: (1) the smaller Ni NPs, which supplied the
catalytic reduction with a larger reaction area and more active
distribution curves (Fig. 6b) indicated that the mesoporous
sizes were centered at around 9.0 nm, the microporous sizes
were centered at about 1 nm, and the macroporous still existed
in the samples, however, the dominating porous structure was
mesopores. Brunauer–Emmett–Teller (BET) specic surface
area of 138 m2 gꢁ1, 142 m2 gꢁ1 and 139 m2 gꢁ1 were obtained
for Ni@C-400, Ni@C-500, Ni@C-600, respectively. Furthermore,
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Fig. 7 Successive UV-vis absorption spectra during the catalytic reduction of 4-NP using (a) Ni@C-400, (b) Ni@C-500, (c) Ni@C-600 as catalyst;
(d) Ct/C0 versus reaction time for the reduction of 4-NP and (e) plot of ln(Ct/C0) against the reaction time of 4-NP reduction over Ni@C-400
(black line), Ni@C-500 (red line), Ni@C-600 (blue line); (f) the reusability of Ni@C-600 catalyst.
Table 2 Textual properties, kinetic constants and turnover frequencies of the catalysts
Ni loading
(wt%)
Catalyst
SBET (m2 gꢁ1
)
kapp ꢃ 10ꢁ3 (sꢁ1
)
Kb (sꢁ1 gꢁ1
)
TOFc (hꢁ1
)
Reference
Ni@C-400
Ni@C-500
Ni@C-600
Ni–NPC-600
Ni0.1/NPC
Ni1.0/NPC
Ni/m-CN
Ni0.52Sb0.48/SBA-15
Ni/SiO2
68.6a
72.1a
72.9a
—
46.5
81.4
48.3
2.85
14.6
138
142
139
169
389
51
134
416
185
12.0
20.8
22.5
5.9
2.0
2.8
9.1
3.7
4.5
40.0
69.3
75.0
2.95
6.7
9.2
9.1
140
1.5
2.1
3.4
6.6
0.19
0.468
0.292
1.2
—
12.2
This work
This work
This work
34
21
21
42
43
39
a
Estimated by TGA. b The activity factors [K]: the rate constant per gram. c TOF (turnover frequency): moles of 4-NP converted per mole Ni per hour.
sites for reduction; (2) the larger loading content of Ni NPs with
more catalytic sites were benecial to enhanced the catalytic
Conclusions
performance; (3) the carbon surrounded Ni NPs were thinner
and had a 3D structure, making the 4-nitrophenolate ion rapidly
pass through and reach; (4) the easier hydrophilic characteristic
due to a little amounts of carbon, which made it convenient to
absorb borohydride anion on the surface of Ni NPs.
To evaluate the reusability of our catalyst, the successive
cycles of the catalytic reduction were carried out. As shown in
Fig. 7f, the catalyst can be successfully used aer being sepa-
rated from reaction by an extern magnet, and still kept high
activity and striking conversion of 92% aer 8 cycles. The large
conversion efficiency indicated the Ni NPs were stable and
protected by the wrapped carbon. The TEM images of Fig. S9†
told us there was no aggregation of Ni NPs, and structure and
morphology were well maintained, fully demonstrating the
good stability of the catalyst.
In conclusion, we selected a new Ni–ntca as the effective
precursor to construct composites structure of magnetic nickel
NPs embedded in hierarchically porous carbon matrix. The
pyrolysis temperature was determined with transform of
surface and the corresponding electron diffraction (ED) of the
crystal during directly observing the transition of a Ni–ntca to Ni
NPs. The product of 600 ꢀC (Ni@C-600) exhibited excellent
catalysis activity and remarkable durability for the reduction of
4-NP to 4-AP in mild condition. The nickel loading contents of
Ni@C-600 were as high as 72.8%, which shortened catalysis
reaction time to 3.5 min and possessed nearly 100% conversion.
Moreover, the catalyst still remained large conversion efficiency
of 92% aer 8 cycles. It is believed that these results will further
facilitate the exploration of the technique of the TEM combined
with in situ hot stage as a powerful tool in the carbonization of
MOFs to obtain derivations of MOFs with different applications.
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Acknowledgements
This work was supported by National Natural Science Founda-
tion of China (21390394, 21571079, 21621001, 21571076, and
21571078), “111” project (B07016), and Guangdong Science and
Technology Department Project (2013B090200052 and
2012D0501990028). Q. F. acknowledges the Thousand Talents
program (China).
25 F. Ke, J. F. Zhu, L. G. Qiu and X. Jiang, Chem. Commun., 2013,
49, 1267–1269.
26 Z. Liu, T. Ohsuna, O. Terasaki, M. A. Camblor, M. Diaz-
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