In Situ Confined Growth Based on a Self-Templating Reduction Strategy of Highly Dispersed Ni Nanoparticles in Hierarchical Yolk–Shell Fe@SiO2 Structures as Efficient Catalysts

Article abstract of DOI:10.1002/asia.201601196

Ni-based magnetic catalysts exhibit moderate activity, low cost, and magnetic reusability in hydrogenation reactions. However, Ni nanoparticles anchored on magnetic supports commonly suffer from undesirable agglomeration during catalytic reactions due to

Full text of DOI:10.1002/asia.201601196

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Accepted Article
Title: In-situ confined growth of highly dispersed Ni nanoparticles into
hierarchically yolk-shell Fe@SiO2 structures as efficient catalysts
based on a self-templating reduction strategy
Authors: Jiao Jiao, Haiyan Wang, Wanchun Guo, Ruifei Li, Kesong
Tian, Zhaopeng Xu, Yin Jia, Yuehao Wu, and Ling Cao
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In-situ confined growth of highly dispersed Ni nanoparticles into
hierarchically yolk-shell Fe@SiO2 structures as efficient catalysts
based on a self-templating reduction strategy
Jiao Jiao,[a] Haiyan Wang,*[a] Wanchun Guo,*[a] Ruifei Li,[a] Kesong Tian,[a] Zhaopeng Xu,[b] Yin Jia, [a]
Yuehao Wu,[a] and Ling Cao[a]
Dedication ((optional))
Abstract: Ni-based magnetic catalysts exhibited moderate activity,
low cost, and magnetic reusability for hydrogenation reactions.
However, Ni nanoparticles anchored on magnetic supports suffer
commonly from undesirable agglomeration during catalytic reactions
due to the relatively weak affinity of magnetic support to Ni
nanoparticles. A hierarchically yolk-shell Fe@SiO2/Ni catalyst with
an inner movable Fe core and an ultrathin SiO2/Ni nanosheets-
assembled shell was synthesized via a self-templating reduction
strategy with the hierarchically yolk-shell Fe3O4@nickel silicate
nanocomposite as the precursor. Spatial confinement of highly
dispersed Ni nanoparticles of the mean size of 4 nm into ultrathin
SiO2 nanosheets of the 2.6-nm thickness could not only prevent their
agglomeration during catalytic transformations but also expose
abundant active Ni sites accessible to the reactants. Moreover, the
large inner cavities and interlayer space among the assembled
ultrathin SiO2/Ni nanosheets provide suitable mesoporous channels
for the diffusion of the reactants toward active sites. As expected,
the Fe@SiO2/Ni catalyst displays high activity, high stability and
organic chemicals.[1-5] Supported noble-metal nanoparticles such
as Au, Pd, Pt and their alloys have long been dominant in
catalytic reduction of nitro compounds due to their outstanding
activity and selectivity,[6-7] however, their dwindling supply and
high cost force human being to focus on non-noble metal
catalysts (e.g. Cu, Fe, Co, Ni and their alloys) as their
alternatives.[8-12]
Up to now, supported nickel nanocatalysts, such as Ni-
stabilized Zirconia,[13] Ni-MCM-41,[14-16] have been one of the
most potential alternatives for the hydrogenation of nitroaromatic
substrates because of their low cost and moderate activity. In
particular, magnetically supported nickel nanocatalysts exhibit
facile magnetic separation to recover them as well as high
dispersion of Ni nanoparticles onto magnetic supports to
maximize their activity. Up to now, various magnetic Ni catalysts
such as Fe3O4-Ni,[17-18] Ni-NiFe2O4/carbon nanofiber,[19] and
Fe3O4-SiO2-P4VP-Ni[20] have been synthesized. However, Ni
nanoparticles anchored onto magnetic support may suffer
magnetic recoverability for the reduction of nitroaromatic compounds. commonly from undesirable deactivation, attributing mainly to
In particular, the Ni-based catalyst maintained the conversion of 4-
Nitroamine of over 98% and preserved the initial yolk-shell structure
without any obvious aggregation of Ni nanoparticles after over ten
catalytic cycles, confirming the high structural stability of the Ni-
based catalyst.
particle agglomeration and even loss during catalytic
hydrogenation involving the relatively weak affinity of support to
Ni nanoparticles and/or local destruction of support. Therefore, it
is still an issue to engineer highly efficient, magnetically
recoverable Ni-based nanocatalysts with greatly isolated sites
and robust stability.
On the basis of the spatially confined stability,[21]
encapsulation of Ni nanoparticles in an individually isolated form
into porous supports, such as carbon materials,[22-23] hollow SiO2
capsules,[24-25] the inner wall of Al2O3 nanotubes,[26] has turned
out to be an effective strategy to enhance their stability, because
these porous structures could isolate catalytically active Ni sites
to a large extent, and dramatically depress aggregation and loss
of Ni nanoparticles. Moreover, yolk-shell nanostructures, with an
inner movable magnetic core or multiple inner movable
magnetic cores and an outer hollow shell, have been proven to
be good supports for metal nanoparticles due to their facile
magnetic recoverability and high specific surface area.[27-28]
Introduction
Efficient reduction of nitroaromatic compounds to aminoaromatic
compounds over metal-based catalysts has drawn great
attention, attributing to efficient conversion of nitro-enriched
pollutants from chemical industries to their amino-enriched
counterparts, one of important medicine intermediates and
[a]
J. Jiao, Prof. H. Wang, W. Guo, R. Li, K. Tian, Y. Jia, Y. Wu, L. Cao,
Key Laboratory of Applied Chemistry of Hebei Province
College of Environmental and Chemical Engineering
Yanshan University
Herein, we describe
a
novel hierarchically yolk-shell
Fe@SiO2/Ni structure containing an inner movable Fe core and
a hollow shell assembled by ultrathin SiO2 nanosheets, into
each nanosheet which multiple in-situ generated Ni
nanoparticles in a highly isolated state are embedded via a
facile in-situ self-templating reduction strategy. Core-shell
Fe3O4@SiO2 microspheres are converted to hierarchically yolk-
shell Fe3O4@nickel silicate nanoarchitectures with an inner
Qinhuangdao 066004 (P. R. China)
E-mail: hywangysu@sina.cn, wc-g@ysu.edu.cn
Prof. Z. Xu
[b]
Key Laboratory for Special Fiber and Fiber Sensor of Hebei
Province
School of Information Science and Engineering
Yanshan University
Qinhuangdao 066004 (P. R. China)
movable Fe3O4 core and
assembled shell through simultaneous etching of SiO2 shell and
a nickel silicate nanosheets-
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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deposition of ultrathin nickel silicate nanosheets, followed by in-
situ transformation of this yolk-shell structure to the final
yolk/shell Fe@SiO2/Ni structure through H2 reduction.
Confinement of in-situ generated Ni nanoparticles into ultrathin
SiO2 nanosheets could preserve their highly isolated state
against their agglomeration during catalytic transformations.
Moreover, ultrathin SiO2 nanosheets could make highly
dispersed Ni nanoparticles into them to exposure abundant
active sites highly accessible to reactive species due to
nanosheets with the average thickness of ca. 2.6 nm less than 4
nm, the mean size of Ni particles, and nanosheets-assembled
hollow shell with high specific surface area and large pore
volume could make substitute molecules to diffuse towards Ni
nanoparticles through the large inner cavities and mesoporous
interlayer channels among the assembled ultrathin SiO2/Ni
nanosheets, thereby preserving their intrinsic catalytic activity.
As expected, the Ni-based catalyst exhibits not only facile
recoverability from strong magnetic response of Fe core, but
also high activity and stable recyclability, for the reduction of
nitroaromatic compounds including 4-nitroaniline, 2-nitroaniline,
4-nitrophenol, 2-nitrophenol, and 2-Amino-5-nitrophenol.
template and a precursor was converted to a hierarchically
hollow shell assembled by ultrathin nickel silicate nanosheets in
the alkaline environment, forming hierarchically yolk-shell
Fe3O4@nickel silicate nanocomposites. Alkaline condition,
supplied by the gradual hydrolysis of urea in the one-pot
hydrothermal process, causes the gradual dissolution of SiO2
accompanied by the controlled release of silicate anions, which
could further react with Ni2+ at the interface between SiO2 shell
and the solution to form ultrathin nickel silicate nanosheets.
Accompanied by the continual hydrolysis of urea, the SiO2 shell
was completely dissolved and in-situ transformed to hollow
structure assembled by ultrathin nickel silicate nanosheets.[31-34]
Finally, the hierarchical Fe3O4@Ni3Si2O5(OH)4 nanocomposites
with the yolk-shell structure were in-situ reduced by H2 to the
hierarchical Fe@SiO2/Ni nanocomposites with the yolk-shell
structure. In the reduction process, the inner movable Fe3O4
cores were reduced to Fe nanoparticles, and simultaneously the
outer hierarchical Ni3Si2O5(OH)4 shell was reduced to the
hierarchical SiO2/Ni shell with the in-situ growth of Ni
nanoparticles into ultrathin SiO2 nanosheets. The spatial
confinement of highly dispersed Ni nanoparticles into ultrathin
SiO2 nanosheets not only enhanced the stability of Ni
nanoparticles and but also achieved efficient exposure of active
sites to reactive molecules. Meanwhile, the hierarchically hollow
structure assembled by ultrathin nanosheets provides high
specific surface area and suitable channels of interlayer space
for the diffusion of the reactants towards active sites. The unique
hollow nanostructure as well as the movable Fe core endows
the hierarchically yolk-shell Fe@SiO2/Ni nanocomposites with
highly activity, robust durability and magnetic recoverability.
Results and Discussion
The synthetic procedure of the hierarchically yolk-shell
Fe@SiO2/Ni nanostructure is shown in scheme 1. Firstly, core-
shell Fe3O4@SiO2 microspheres were constructed by
a
solvothermal route and a subsequent sol-gel method according
to our previous reports.[29-30] Secondly, SiO2 shell as both a
Scheme
1.
Synthetic
illustration
for
the
hierarchically
yolk-shell
Fe@SiO2/Ni
nanocomposites.
Characterization of the hierarchically yolk-shell Fe@SiO2/Ni
nanocomposites
According to TEM image in Figure S1a, the PAA-modified Fe3O4
nanoclusters are highly uniform with an average size of
approximately 220 nm. As shown in Figure 1 a, Fe3O4@SiO2
nanocomposites have a uniform size and well-defined core-shell
structure with the SiO2 shell thickness of about 40 nm (Figure 1a
inset). The Fe3O4@Si3Ni2O5(OH)4 nanocomposites display
obviously the homogeneous particle size (Figure 1b) and the
hierarchically yolk-shell structure with an inner movable Fe3O4
core and an outer hierarchical nickel silicate shell assembled by
numerous ultrathin nanosheets (Figure 1b inset), generated from
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simultaneous dissolution of SiO2 shell and assembled deposition
of nickel silicate nanosheets at the solid/liquid interface. The
Fe@SiO2/Ni nanocomposites, the reduced products of the
also provide suitable interlayer channels for the reactants toward
abundant active sites. The fact that the size of Ni nanoparticles
is smaller than that of the ultrathin nanosheets confining Ni
nanoparticles is essential to efficient exposure of active sites.
The mapping images of a single Fe@SiO2/Ni nanocomposite
with yolk-shell structure (Figure 1g, h, i, j) confirm that Fe core is
confined inside hollow SiO2 shell and Ni nanoparticles are highly
dispersed into outer SiO2 shell assembeld by ultrathin
nanosheets, in combined with its corrspondign TEM image in
Figure 1f. Moreover, Ni content of up to 20.36 wt% determined
by ICP-AES further demonstrates the high loading of well
dispersed Ni nanoparticles in the Fe@SiO2/Ni nanocomposites,
which endows the Ni-based catalyst with fast kinetic rate for
specific chemical reactions, allowing its potential industrial
application.
Fe3O4@Si3Ni2O5(OH)4
nanocomposites,
exhibit
similar
hierarchical and yolk-shell structure with their precursors (Figure
1c). The inner movable Fe3O4 core and the outer hierarchical
nickel silicate shell were reduced to the movable Fe core and
the hierarchical SiO2/Ni shell, respectively. As shown in Figure
S1b and Figure 1c inset, the inner Fe cores show typically
hollow structure assembled loosedly by the Fe nanoparticles.
During the reduction process of the inner Fe3O4 core, hollow
SiO2 shell assembled by ultrathin sheets suppresses greatly the
diffussion of outer H2 molecules towards the inner Fe3O4 core,
thereby making the inward disffusion rate of hyrogen species far
lower than the outward disffusion rate of Fe species and finally
accheiving the conversion of Fe3O4 nanoclusters to Fe core with
hollow structure, reffered to the nanoscale Kirkendall effect in
the previsous report.[35] The highly dispersed Ni nanopariticles of
about 4 nm are spatially confined into ultrathin SiO2 nanosheets
with the thickness of approximately 2.6 nm (Figure 1d and 1e),
which not only prevent Ni nanoparticles from agglomeration but
Figure 1. SEM images of a) core-shell Fe3O4@SiO2 nanocomposites, b) the hierarchically yolk-shell Fe3O4@Ni3Si2O5(OH)4 nanocomposites and (c) the
hierarchically yolk-shell Fe@SiO2/Ni nanocomposites (the insets correspond to their TEM images); d-e) high-magnification TEM images of the Fe@SiO2/Ni
nanocomposites; f) TEM image of a single Fe@SiO2/Ni nanocomposite with yolk-shell structure and its corresponding elemental maps including the g) Si, h) O, i)
Fe
and
j)
Ni
elements.
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XRD analysis was also utilized to further identify the structure
of the samples. Compared with the XRD spectrum of PAA-
modified Fe3O4 nanoclusters (Figure 2a), that of Fe3O4@nickel
silicate nanocomposites show diffraction peaks of Fe3O4 and
nickel silicate. As depicted in Figure 2b, the peaks at 2θ = 30.0,
35.3, 43.0, 53.6, 57.0, and 62.6° are assigned to the (220), (311),
(400), (422), (511), and (400) planes of fcc Fe3O4 phase
(JCPDS card No. 19-0629), respectively. In addition, the
diffraction peaks at 2θ = 33.4, 59.9° are ascribed to (200) and
(060) planes of Si3Ni2O5(OH)4 (JCPDS card no. 49-1859).[31] The
XRD spectrum of the reduced product of Fe3O4@Si3Ni2O5(OH)4
nanocomposites (Figure 2c) is different from that of their
precursors. The newly emerged diffraction peaks at 2θ = 44.6,
64.9, 82.1, 44.6, 51.5, and 76.3o correspond to the (110), (200)
and (211) planes of bcc-Fe(0) phase (JCPDS card No. 65-
4899)[36-37] and (111), (200), and (220) planes of Ni(0) phase
(JCPDS card No. 04-0850),[19] respectively, which confirms that
P/P0=0.99 for the hierarchically yolk-shell Fe@SiO2/Ni structure
could be attributed to their large inner cavities and interlayer
mesoporous channels among the assembled ultrathin SiO2/Ni
nanosheets, in favor of the diffusion of the reactive molecules
toward active sites to enhance their catalytic activity.
the
hierarchically
yolk-shell
Fe3O4@Si3Ni2O5(OH)4
nanocomposites were successfully reduced to the hierarchically
yolk-shell Fe@SiO2/Ni nanocomposites.
Figure 3. N2 adsorption/desorption isotherms of a) Fe3O4@nickel silicate
nanocomposites and b) Fe@SiO2/Ni nanocomposites. The insets depict their
corresponding pore size distributions.
Figure 2. XRD patterns of a) PAA-modified Fe3O4 nanoclusters, b)
Fe3O4@nickel silicate nanocomposites, and c) Fe@SiO2/Ni nanocomposites.
The magnetic hysteresis loops of PAA modified Fe3O4
nanoclusters, Fe3O4@Ni3Si2O5(OH)4 nanocomposites, and
Fe@SiO2/Ni nanocomposites (Figure 4) provide another
evidence for the structural evolution in the entire synthetic
process of Fe@SiO2/Ni nanocomposites. Compared to PAA-
Nitrogen
adsorption/desorption
isotherms
and
the
corresponding pore size distributions of Fe3O4@nickel silicate
nanocomposites and Fe@SiO2/Ni nanocomposites are shown in
Figure 3. Compared to Fe3O4@Ni3Si2O5(OH)4 nanocomposites,
Fe@SiO2/Ni
nanocomposites
display
a
similar
N2
modified
nanocomposites
characteristic accompanied by
Fe3O4
nanoclusters,
Fe3O4@Ni3Si2O5(OH)4
superparamagnetic
specific saturation
adsorption/desorption isotherm identified as a IV type with a type
H3 hysteresis loop,[38] associated with capillary condensation
occurring in mesoporous channels, which correspond to
interlayer space among the assembled ultrathin SiO2/Ni
nanosheets. Their uniform pore size distribution with the size
centered at 3.7 nm (Figure 3b inset) confirms the mesoporous
feature. In addition, both a high BET specific surface area of
118.12 m2 g-1 and a high pore volume of 0.37 cm3/g at
show similar
a
a
magnetization decrease from 59.5 to 29.8 emu g-1, attributing to
the generation of the hollow nickel silicate shell assembled by
ultrathin nanosheets as nonmagnetic material. The magnetic
hysteresis loop of Fe@SiO2/Ni nanocomposites exhibits
ferromagnetic feature with
a small magnetic remanence,
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attributing mainly to the transformation of Fe3O4 nanoclusters
into Fe nanoparticles in the in-situ reduction process, in
consistence with TEM and XRD results. The specific saturation
magnetization of up to 91.2 emu g-1 endows the Ni-based
catalyst with the rapid recoverability using a magnet after the
catalytic transformations.
considered as a pseudo-first-order reaction. Figure 5 c shows
the plot of ln(Ct/C0) against the reaction time t, where Ct and C0
represent the concentrations of 4-NA at time t and 0 min,
respectively, calculated according to the peak areas of 4-NA
absorbance centered at 381 nm. The apparent reaction rate
constant (k) and the turnover frequency (TOF) of the
Fe@SiO2/Ni catalyst could be calculated to 0.266 min-1 and
179.83 h-1, respectively, according to the equation, ln(Ct/C0) = -kt,
and the conversion at 90%. High activity of the Ni-based catalyst
could be attributed to both the exposure of abundant active sites
of 4-nm Ni nanoparticles embedded into 2.6-nm ultrathin SiO2
nanosheets and suitable mesoporous channels among ultrathin
nanosheets for the reactive molecules. Meanwhile, the high
reduction rate of 4-NA using a trace Ni-based catalyst could be
attribted to high loading of highly dispersed Ni nanoparitcles into
ultrathin SiO2 nanosheets, in favor of further large-scale
utilization. In order to evaluate the stability of the Ni-based
catalyst, after each catalytic cycle the catalyst was magnetically
recovered and utilized to the next catalytic cycle. The conversion
of 4-NA to 1,4-benzenediamine still maintains over 98% after
over ten cycles (Figure 5d), and the recoverable catalyst after
over ten cycles preserve the initial cystal structure (Figure S6)
and yolk-shell structure and the highly dispersed distribution of
Ni nanoparticles into ultrathin SiO2 nanosheets without any
obvious aggregation, according to TEM images in Figure 6a and
6b, which are attributing to the spatial confinement of Ni
nanoparticles into ultrathin nanosheets against their
agglomeration during catalytic reactions. The HRTEM image in
Figure 6c shows that the size of the dispersed Ni nanopariticles
increases slightly from about 4 nm to about 5 nm after catalytic
cycles, possibly due to the movement of an extremely small
amount of Ni species in the form of single atoms or
subnanometer clusters onto SiO2 nanosheets towards Ni
nanoparticles during catalytic transformations. Compared with
Fe cores in the fresh catalysts (Figure S1b), those in the
recovered Fe@SiO2/Ni nanocomposites (Figure 6a) show
obvious change in the size and morphology after catalytic cycles,
probably due to breakage of the inner hollow Fe core and
reaggregation of Fe nanoparticles in SiO2 shell resulted from the
serious collision between the inner hollow Fe core and the outer
SiO2 shell in catalytic transformations. In spite of the breakage of
the inner hollow Fe core during catalytic reduction, the exsitance
of the outer SiO2 shell ensures reaggregation of Fe
nanoparticles confined into SiO2 shell against leakage of inner
Fe nanoparticles, thereby preserving magnetic recoverability of
Figure 4. Magnetic hysteresis loops of a) PAA modified Fe3O4 nanoclusters, b)
Fe3O4@nickel silicate nanocomposites, and c) Fe@SiO2/Ni nanocomposites.
Magnetically recoverable catalysis of the Fe@SiO2/Ni
nanocomposites for the reduction of nitroaromatic
compounds
The reduction of nitroaromatic compounds to aminoaromatic
compounds by NaBH4 is utilized to evaluate the activity of the
Ni-based catalyst. With respect to the reduction of 4-NA (Figure
5), the mixture of a 4-NA alcoholic solution and an aqueous
NaBH4 solultion revealed unchanged color and unchanged UV-
vis absorption spectrum without any catalyst after 24 h (Figure
5a). When a trace amount of the hierarchically yolk-shell
Fe@SiO2/Ni catalyst was added into the above mixture, the
color of the reaction mixture evolved from yellow to colorless in
18 min, and the time-dependent UV-vis spectra of the reaction
solution exhibited a gradual decrease of the absorption at 381
nm and a simultaneous increase of the absorption at 239 nm
(Figure 5b), indicating the gradual reduction of 4-NA to 1, 4-
[39-40]
benzenediamine with prolonged time.
On the basis of the
the
catalyst
after
catalytic
cyles.
reaction condition that the concentration of NaBH4 is 400 times
higher than that of 4-NA, the catalytic reduction of 4-NA could be
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Figure 5. UV/Vis spectra of a) a 4-NA alcoholic solution diluted in H2O and the mixture of a 4-NA alcoholic solution and a NaBH4 aqueous solution, b) 4-NA
catalyzed by Fe@SiO2/Ni nanocomposite with prolonged time, c) plot of ln (Ct/C0) against reaction time (t) for the catalytic reduction of 4-NA, d) Conversions of 4-
NA catalyzed by Fe@SiO2/Ni nanocomposite in ten cycles.
Figure 6. TEM images of the recovered yolk-shell Fe@SiO2/Ni nanocomposites for catalytic reduction of 4-NA after ten successive cycles.
The catalytic performances of Fe@SiO2/Ni nanocomposites
for the reduction of other nitroaromatic compounds were also
investigated as shown in Figure S2-S5 and Table 1. With repect
to the reduction of nitroaromatic compounds, all their
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conversions are over 97% in 36 minites. The kinetic reaction
rate constants of the reduction of 2-NA, 4-NP, 2-NP and 2-
amino-5-nitropheol were determined to 0.217, 0.119, 0.090 and
0.100 min-1, respectively. In addition, the TOFs of the reduction
of 2-NA, 4-NP, 2-NP and 2-amino-5-nitropheol catalyzed by
Fe@SiO2/Ni nanocomposites were determined to 146.70, 80.45,
60.84 and 67.59 h-1, respectively. The aforementioned results
confirm that Fe@SiO2/Ni nanocomposites exhibit excellent
activities and fast kinetic reaction rates for the reduction of the
nitroaromatic compounds, allowing futher industrial processes.
nm ultrathin SiO2 nanosheets not only could prevent Ni
nanoparticles from aggregation and even loss during catalytic
reactions but also make the Ni-based catalyst expose abundant
active sites accessible to the reactants. Moreover, their large
inner cavities and interlayer space among the assembled
ultrathin SiO2/Ni nanosheets provide suitable mesoporous
channels for the diffusion of the reactants towards active sites.
Therefore, the Ni-based catalyst exhibits high activity, recyclable
stability as well as magnetic recoverability with an aid of the
inner movable Fe core for the reduction of the nitroaromatic
compounds. Our ongoing studies are the construction of other
hierarchically yolk-shell nanocatalysts such as Fe@SiO2/Cu and
Fe@SiO2/Co based on our self-templating reduction strategy
and the expansion of the Fe@SiO2/Ni catalyst into other
important hydrogenation reactions.
Table 1. Conversions, kinetic constants (k) and TOFs of the reduction of
nitroaromatic compounds including 4-NA, 2-NA, 4-NP, 2-NP, and 2-Amino-5-
nitrophenol catalyzed by Fe@SiO2/Ni nanocomposites.
Reactants
4-NA
Time / min
Conversion / %
K / min-1
0.266
0.217
0.119
0.090
TOF / h-1
179.83
146.70
80.45
Experimental Section
18
15
36
36
>99
98
Materials
2-NA
4-NP
99
Ferric chloride hexahydrate (FeCl3·6H2O, 98%), nickel nitrate
hexahydrate (Ni(NO3)2·6H2O), 2-Nitroaniline (2-NA), 4-Nitroaniline (4-NA),
2-Nitrophenol (2-NP), and 4-Nitrophenol (4-NP) were purchased from
Aladdin. Anhydrous sodium acetate and ethylene glycol were supplied
from Guangzhou Guanghua Chemical Reagent Co., Ltd. Sodium
borohydride (NaBH4) and anhydrous ethanol were acquired from Tianjin
Guangfu Fine Chemical Research Institute. Poly (acrylic acid) (PAA;
Mr=1800), 2-Amino-5-nitrophenol, Tetraethyl orthosilicate (TEOS)
(>99%), ammonium hydroxide aqueous solution (28 wt%) and urea were
acquired from Sigma–Aldrich, Alfa Aesar, Sinopharm Chemical Reagent
Co., Ltd., Xilong Chemical Co., Ltd., and Shenyang Xinxing Reagent Co.,
Ltd., respectively. Deionized water was also used in the entire
experiments.
2-NP
97
60.84
2-Amino-5-
nitrophenol
36
98
0.100
67.59
In order to confirm the active sites in the catalytic reactions，
the effect of Fe nanoparticles on the reduction of the
nitroaromatic compounds was further investigated. Fe
nanoparticles were synthesized via the same in-situ reduction
procedure to synthesis of Fe@SiO2/Ni nanocomposites with only
alternation that the PAA-modified Fe3O4 nanoclusters were used
to replace the Fe3O4@nickel silicate nanocomposites. TEM
image in Figure S7 shows aggregates of Fe nanoparticles with
the average size of about 230 nm. Time-dependent UV spectra
of the reduction of nitroaromatic compounds in the presence of
Fe nanoparticles were depicted in Figure S8 and the
corresponding conversions ware listed in Table S1. According to
Table S1, the conversions of nitroaromatic compounds in the
presence of Fe nanoparticles are far less than those in the
presence of Fe@SiO2/Ni nanocomposites for the same reaction
time. In addition, hollow SiO2/Ni shells block greatly inner Fe
cores accessible to the reactant molecules. Therefore, the high
conversions of nitroaromatic compounds catalyzed by
Fe@SiO2/Ni nanocomposites could be mainly attributed to the
highly dispersed Ni nanoparticles with the small size.
Synthesis of core-shell Fe3O4@SiO2 microspheres
PAA-modified Fe3O4 nanoclusters were synthesized according to our
previous report.[29] Typically, FeCl3·6H2O (1.08 g) was dissolved in
ethylene glycol (40 mL), followed by the addition of PAA (0.108 g) and
anhydrous sodium acetate (9.0 g). The mixture was magnetically stirred
for 40 min to form
a yellow homogeneous solution. After being
transferred into an autoclave with 50-mL capacity, the above solution
was heated at 200 oC for 12 h. The as-synthesized product was
magnetically separated, and washed with deionized water and absolute
ethanol three times, respectively, and then dried in a vacuum for 8 h.
Core-shell Fe3O4@SiO2 microspheres were fabricated according to
our previous reports.[30] Typically, PAA-modified Fe3O4 nanoclusters (0.1
g) were dispersed into a mixed solvent of deionized water and anhydrous
ethanol (100 mL) with the volume ratio of 1:4. After adding NH3·H 2O (5
mL, 28 wt%), the dispersion was mechanically stirred at 500 rpm for 10
min at 25 oC, followed by the dripped addition of TEOS (0.5 mL) while
stirring. After vigorously stirring at 300 rpm 6 h, the resulting Fe3O4@SiO2
microspheres were magnetically separated, washed by ethanol four
times, and then dried in vacuum at 60 oC for 12 h.
Conclusions
In conclusion, the hierarchically yolk-shell Fe@SiO2/Ni
nanocomposites have been successfully synthesized based on
a self-templating reduction strategy using the hierarchically yolk-
shell Fe3O4@nickel silicate nanocomposites as the precursors.
With respect to the Fe@SiO2/Ni catalyst, spatial confinement of
4-nm Ni nanoparticles into hollow SiO2 shell assembled by 2.6-
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10.1002/asia.201601196
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Synthesis of the hierarchically yolk-shell Fe3O4@nickel silicate
nanocomposites
desorption branch of the isotherms. Magnetic properties of the samples
were measured with a Lake Shore-7407 vibrating sample magnetometer.
The contents of Ni species and Fe species in Fe@SiO2/Ni
nanocomposites were determined using an AGILENT-725 inductively
coupled plasma atomic emission spectrometer (ICP-AES). The time-
dependent absorption spectra of nitroaromatic compounds during
The Fe3O4@nickel silicate nanocomposites were synthesized combining
an improved protocol.[31] Typically, to the aqueous suspension (40 mL)
containing Fe3O4@SiO2 (0.1 g) were added urea (1 g) and of
Ni(NO3)2·6H2O (0.18 g) under vigorous ultrasonication, and then the
homogenous mixture was transferred to a Teflon-lined stainless-steel
autoclave. After reacting at 105 oC for 12 h, the autoclave was cooled to
room temperature. Finally, the product was magnetically separated,
rinsed with deionized water and anhydrous ethanol several times, and
then dried at 60 oC for 12 h.
catalytic reduction were obtained with
spectrophotometer.
a
Shimadzu UV 2550
Acknowledgements
This study has been supported by National Natural Science
Foundation of China (51503178), Natural Science Foundation of
Hebei Province (E2015203114), China Postdoctoral Science
Foundation (2015M571278), Postdoctoral Science Foundation
of Hebei Province (B2014003009), Science and Technology
Project of Higher Education in Hebei Province (QN2016001),
Independent Research Project of Yanshan University for Young
Teachers (14LGB019), and the Doctor Foundation of Yanshan
University (B789).
Synthesis of the hierarchically yolk-shell Fe@SiO2/Ni nanocomposites
The Fe3O4@ nickel silicate nanocomposites placed in a quartz boat were
heated to 550 oC at a rate of 10 oC/min in Argon atmosphere and
reduced at 550 oC for 3 h in H2 current. After being cooled to room
temperature in Argon atmosphere, the final products were collected.
Catalytic reduction of nitroaromatic compounds by the hierarchically yolk-
shell Fe@SiO2/Ni nanocomposites
Keywords:
Ni nanoparticles •
self-templating reduction
Catalytic performances of Fe@SiO2/Ni nanocomposites were
investigated by the reduction of nitroaromatic compounds including 4-NA,
•
yolk-shell • nanostructures • supported catalysts
2-NA, 4-NP, 2-NP and 2-Amino-5-nitrophenol using NaBH4 as
a
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Entry for the Table of Contents (Please choose one layout)
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Jiao Jiao,[a] Haiyan Wang,*[a] Wanchun
Guo,*[a] Ruifei Li,[a] Kesong Tian,[a]
Zhaopeng Xu,[b] Yuehao Wu,[a] and Ling
Cao[a]
Page No. – Page No.
In-situ confined growth of highly
dispersed Ni nanoparticles into
hierarchically yolk-shell Fe@SiO2
structures as efficient catalysts
based on a self-templating reduction
strategy
A hierarchically yolk-shell Fe@SiO2/Ni catalyst with an inner movable Fe core and
an ultrathin SiO2/Ni nanosheets-assembled shell was synthesized via a self-
templating reduction strategy. Spatial confinement of 4-nm Ni nanopariticles into
ultrathin SiO2 nanosheets of the 2.6-nm thickness could prevent their agglomeration
during catalytic reductions and expose abundant active Ni sites available to the
reactants, thereby showing high activity and stability.
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