Fibrous nano-silica containing immobilized Ni@Au core-shell nanoparticles: A highly active and reusable catalyst for the reduction of 4-nitrophenol and 2-nitroaniline

Article abstract of DOI:10.1016/j.molcata.2014.08.002

A novel, dandelion-like fibrous nano-silica catalyst (Ni@Au/KCC-1) has been synthesized by modifying fibrous nano-silica (KCC-1) with Ni@Au core-shell nanoparticles (NPs). KCC-1 was prepared using a hydrothermal method and has a dandelion-like shape, high surface area, and easy accessibility; KCC-1 can also be functionalized with 3-mercaptopropyltriethoxysilane. The mercaptopropyl groups on the fibers act as robust anchors for the immobilization of Ni@Au NPs, thus preventing the aggregation of the Ni@Au NPs. We investigated the catalytic performance of the Ni@Au/KCC-1 nanocatalyst by reducing 4-nitrophenol to 4-aminophenol in the presence of NaBH₄ as a probe reaction. The resulting Ni@Au/KCC-1 nanocatalyst exhibited superior catalytic activity to Ni@Au NPs, which may be attributed to the high accessibility of the KCC-1 support material. To some extent, it also may be due to the poor aggregation of Ni@Au NPs on the KCC-1 nano-silica support. The Ni@Au/KCC-1 nanocatalyst also showed high catalytic activity when used to reduce 2-nitroaniline. It is noteworthy that using Ni cores to fabricate the active sites Ni@Au NPs resulted in a lower amount of Au needed than is typical, because most of the Au-NPs catalyzed reactions occur on the surfaces of the NPs. In addition, the Ni cores give the Ni@Au/KCC-1 nanocatalyst superparamagnetic properties that increase its ease of recovery by a powerful magnet, allowing for it to be reused. The abovementioned approach based on fibrous KCC-1 and Ni@Au NPs provided a useful platform for the fabrication of noble-metal-based nanocatalysts with easy accessibility and a low cost, which may allow for an efficient green alternative for various catalytic reductions.

Full text of DOI:10.1016/j.molcata.2014.08.002

Journal of Molecular Catalysis A: Chemical 395 (2014) 58–65
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Fibrous nano-silica containing immobilized Ni@Au core–shell
nanoparticles: A highly active and reusable catalyst for the
reduction of 4-nitrophenol and 2-nitroaniline
∗
∗
Xuanduong Le, Zhengping Dong , Wei Zhang, Xinlin Li, Jiantai Ma
College of Chemistry and Chemical Engineering, Gansu Provincial Engineering Laboratory for Chemical Catalysis, Lanzhou University,
Lanzhou 730000, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 17 April 2014
Received in revised form 29 July 2014
Accepted 3 August 2014
Available online 10 August 2014
A novel, dandelion-like fibrous nano-silica catalyst (Ni@Au/KCC-1) has been synthesized by modify-
ing fibrous nano-silica (KCC-1) with Ni@Au core–shell nanoparticles (NPs). KCC-1 was prepared using a
hydrothermal method and has a dandelion-like shape, high surface area, and easy accessibility; KCC-1
can also be functionalized with 3-mercaptopropyltriethoxysilane. The mercaptopropyl groups on the
fibers act as robust anchors for the immobilization of Ni@Au NPs, thus preventing the aggregation of the
Ni@Au NPs. We investigated the catalytic performance of the Ni@Au/KCC-1 nanocatalyst by reducing
Keywords:
4
1
-nitrophenol to 4-aminophenol in the presence of NaBH4 as a probe reaction. The resulting Ni@Au/KCC-
nanocatalyst exhibited superior catalytic activity to Ni@Au NPs, which may be attributed to the high
Fibrous nano-silica
Ni@Au core–shell nanoparticles
Reduction
accessibility of the KCC-1 support material. To some extent, it also may be due to the poor aggregation of
Ni@Au NPs on the KCC-1 nano-silica support. The Ni@Au/KCC-1 nanocatalyst also showed high catalytic
activity when used to reduce 2-nitroaniline. It is noteworthy that using Ni cores to fabricate the active sites
Ni@Au NPs resulted in a lower amount of Au needed than is typical, because most of the Au-NPs catalyzed
reactions occur on the surfaces of the NPs. In addition, the Ni cores give the Ni@Au/KCC-1 nanocatalyst
superparamagnetic properties that increase its ease of recovery by a powerful magnet, allowing for it
to be reused. The abovementioned approach based on fibrous KCC-1 and Ni@Au NPs provided a useful
platform for the fabrication of noble-metal-based nanocatalysts with easy accessibility and a low cost,
which may allow for an efficient green alternative for various catalytic reductions.
4
2
-Nitrophenol
-Nitroaniline
©
2014 Elsevier B.V. All rights reserved.
1
. Introduction
activity of AuNPs in these reactions strongly depends on the active
atoms at the NPs surface. As the particle sizes decrease, the surface
energies of the AuNPs increase, making them unstable, and result-
ing in a high tendency for inter-particle aggregation [19]. This can
reduce the catalytic efficiency of the catalysts. To overcome this
disadvantage, AuNPs are often immobilized onto solid supports
with mespores, including SBA-15 [20–22], Al O [23–25], MCM-41
[26,27], and SBA-16 [28]. When using one of these Au-supported
catalysts, the small AuNPs become dispersed mainly on the large,
internal surfaces and pores of the support. A consequence of this
is that the active sites inside the pores are not readily available,
which limits their applications. Because of this, catalyst supports
with good accessibility and high surface areas (that is, not inside
the pores), are highly desirable.
The use of noble metal nanoparticles (NPs) in catalysis has long
been a topic of research because of their strong catalytic activities
1–5]. Among the reported noble metal NPs used in catalysis, gold
[
NPs (AuNPs) are the most stable metal NPs, and have been exten-
sively studied because of their size-related properties, which can
be leveraged in various applications [6–10]. AuNPs are also widely
recognized as one of the most promising catalysts for a number
of reactions, including the reduction of nitrophenol [11–13], the
hydrodechlorination of 2,4-dichlorophenol [14], the oxidation of
CO [15,16], the aerobic selective oxidation of glucose to gluconate
2
3
[
17], and the C–C cross coupling reactions [18]. Because smaller
AuNPs have a higher surface-to-volume ratio, the high catalytic
On the other hand, for many reported noble metal NPs catalysts,
catalytic steps take place always at the surface of the NPs. Because
of this, the inner noble metal atoms of the NPs can be replaced by
other non-noble metals such as Fe, Co and Ni in order to reduce
∗ Corresponding authors. Tel.: +86 931 8912577; fax: +86 931 8912582.
E-mail addresses: dongzhp@lzu.edu.cn (Z. Dong), majiantai@lzu.edu.cn (J. Ma).
http://dx.doi.org/10.1016/j.molcata.2014.08.002
1
381-1169/© 2014 Elsevier B.V. All rights reserved.

X. Le et al. / Journal of Molecular Catalysis A: Chemical 395 (2014) 58–65
59
◦
the use of the noble metal without sacrificing the overall perfor-
mance of the catalyst [29]. In addition, the presence of Fe, Co and Ni
in hybrid, core–shell NPs possess excellent stability, solvent com-
patibility, catalytic activity, and magnetic separability. All of these
properties greatly enhance the potential application of these cata-
lysts. Despite these advantages, to date, noble metal NPs catalysts
that possess inactive cores, excellent catalytic activities, low-cost,
and efficient recoverable properties are very rare.
heated at 120 C for 5 h. After completion of the reaction, the mix-
ture was allowed to cool to room temperature and the silica formed
was isolated by centrifugation, washed with distilled water and
acetone, and vacuum drying overnight. The as-synthesized material
◦
was then calcined at 550 C for 6 h in air, obtained KCC-1.
2.3. Fabrication of the Ni@Au/KCC-1 nanocatalyst
Fibrous Nano-Silica (KCC-1) with high surface area and high
accessibility was first reported in the year 2010 [30]. The unique
properties of KCC-1 are useful for designing silica-supported cat-
alysts, and the accessibility of their active sites can also easily be
increased. In our previous work, we synthesized Ag/KCC-1 nanocat-
alyst which exhibited excellent catalytic activity for the reduction
of 4-nitrophenol (4-NP) and 2-nitroaniline (2-NA) using sodium
The Ni@Au core–shell NPs were prepared following the work
reported by Peng et al. [32]. The Ni@Au core–shell NPs were pre-
pared method as follow. The first step is to obtain Au precursor
which is crucial for the preparation of Ni@Au nanoparticles. In a typ-
ical synthesis, 1.364 g of TPP and 1 g of HAuCl ·4H O were dissolved
4
2
in 50 and 35 mL of ethanol, respectively, and then the obtained solu-
tions were mixed together with slight agitation. White precipitates
([(C H5) P]AuCl complex) were generated from this process and
borohydride (NaBH ) in water at room temperature owing to the
4
6
3
easy accessibility of the active sites [31].
Here, we report the preparation of a novel nanocatalyst based on
core–shell Ni@Au NPs modified dandelion-like fibrous nano-silica
were filtered and further washed several times with ethanol and
acetone and dried in vacuum.
In the next step, 0.5 mmol of Ni(acac)2 was added in 7 mL of
◦
(
Ni@Au/KCC-1). Using non-noble metal like Fe, Co, or Ni is ideal
oleylamine and then heated at 45 C. To aid the formation of Ni
choices for the material used in the core of the noble metal NPs
as all of them make the catalyst easy to recycle and reuse owing
to their superparamagnetic properties. We characterized the cat-
alytic activities of Ni@Au/KCC-1 in a model reaction (the reduction
of 4-NP). The Ni@Au/KCC-1 showed high catalytic activities dur-
ing the reduction of 4-NP as compared with not only Ni@Au NPs,
but also other Au catalysts reported recently by other researchers.
The Ni@Au/KCC-1 nanocatalyst also showed high catalytic activity
during the reduction of 2-NA. The robust activity of this catalyst is
attributed to its high accessibility and low likelihood of the aggrega-
tion of the Ni@Au NPs on the KCC-1 nano-silica support system. The
replacement of Ni@Au NP cores by Ni atoms could reduce the use of
Au without sacrificing the overall performance of the catalyst, and
may also increase the ease of recovery and resuse of the catalyst.
Thus, Ni@Au NPs immobilized on KCC-1 exhibited highly effective
catalytic property, showing promising potential for the catalytic
reduction of nitroaromatic compounds leading to their safe and
eco-friendly disposal.
nanoparticles, a small amount of HAuCl ·4H O (5.00 mg) that was
4
2
mixed with 0.2 mL of toluene was also added in the above solution.
This reaction generated Au-seeded Ni nanoparticles that served as
the core component of Ni@Au nanoparticles. The resulted solu-
◦
tion was heated to 195 C for 30 min. After that, the obtained
◦
solution was cooled to 155 C and a solution containing 0.1 g of
pre-made Au precursor in 1 mL of dichloromethane was quickly
injected in the pre-prepared oleylamine solution containing Ni
nanoparticles. After a short temperature drop (approximately to
◦
◦
120 C) due to injection, the reaction temperature was kept at
140 C for 30 min before the solution was cooled to room temper-
ature. The synthesized products were washed and separated by
repeated precipitation using the mixture of hexane and ethanol
to obtain black powder which could be redispersed in hexane and
dichloromethane for the purpose of further characterization.
The schematic describing the preparation of Ni@Au/KCC-1 is
shown in Scheme 1.
KCC-1 nano-silica was functionalized with MPTES prior to the
immobilization of Ni@Au NPs on its surface. KCC-1 (1 g) was
added to 50 mL of anhydrous toluene under ultrasonic treatment
for 10 min to disperse it homogeneously. Then, 0.3 g of MPTES
was added dropwise, and the mixture was refluxed for 10 h in
a N2 atmosphere. The mixture was then cooled to room tem-
perature and the resulting mercaptopropylfunctionalized KCC-1
2
. Experimental
2.1. Materials
Tetraethyl orthosilicate (TEOS), Nickel(II) acetylacetonate
(HS-KCC-1) was centrifuged; washed repeatedly with chloroform,
(
Ni(acac) ), Chloroauric acid (HAuCl ), tiphenylphosphine (TPP),
2
4
dichloromethane, and ethanol; and finally dried in vacuum.
oleyl amine (OAm), 4-NP, 2-NA, and 3-mercaptopropyltriethoxy-
silane (MPTES) were purchased from Sigma Aldrich. Cyclohex-
ane, pentanol, Cetylpyridinium bromide (CPB), and NaBH4 were
reagent-grade-quality and were purchased from Tianjing Guangfu
Chemical Company (Tianjin, China). All reagents and solvents used
for synthesis and measuring were used as supplied, with no modi-
fication.
HS-KCC-1 (0.4 g) was ultrasonically dispersed in 50 mL H O, and
2
0
.1 g of Ni@Au NPs was added. After being stirred and ultrasonically
dispersed for 1 h, Ni@Au/KCC-1 was obtained by centrifugation and
dried in a vacuum.
2.4. Catalytic reactions with Ni@Au NPs and Ni@Au/KCC-1
nanocatalyst
2.2. Preparation of KCC-1
The reduction of 4-NP by NaBH4 at room temperature was
chosen as a model reaction for comparison of the catalytic per-
formance of Ni@Au NPs and the Ni@Au/KCC-1 nanocatalyst. In
KCC-1 was prepared using the microwave-assisted hydro-
−
7
thermal technique, as previously reported in literature [30].
We also successfully synthesized KCC-1 using a hydrothermal
method without microwave-assistance. Firstly, TEOS (2.5 g,
short, 2.5 mL of aqueous 4-NP solution (0.12 mM, 3 × 10 mol) was
mixed with 0.5 mL of freshly prepared aqueous NaBH solution
4
−
2
(0.5 M, 2.5 × 10 mol), resulting in the formation of a deep yel-
low solution. Then, 30.0 ␮L of catalyst (Ni@Au NPs; 2 mg/mL) was
added to this resulting solution, and the reaction was allowed to
proceed until the solution became colorless.
−
1
0
(
0
.12 × 10 mol) was dissolved in a solution of cyclohexane
30 mL) and pentanol (1.5 mL). A stirred solution of CPB (1 g,
−
2
.26 × 10 mol) and urea (0.6 g, 0.01 mol) in water (30 mL) was
then added. Secondly, the above mixture was stirred for 30 min
at room temperature, and the resulting solution was placed in
a teflon-sealed hydrothermal reactor. The reaction mixture was
The catalytic reduction of 2-NA at room temperature was
also chosen as a model reaction for investigating the catalytic
activity of the Ni@Au/KCC-1 nanocatalyst. The procedure was

6
0
X. Le et al. / Journal of Molecular Catalysis A: Chemical 395 (2014) 58–65
Scheme 1. Synthesis of Ni@Au/KCC-1 nanocatalyst.
as follows: 30 ␮L of catalyst (10 mg/mL, Ni@Au NPs 2 mg/mL)
sites for the adsorption of Ni@Au NPs onto the nano-silica surface.
The results of TGA (Fig. S2) and the elemental analysis (Table S1)
showed that the content of the MPTES grafted onto the surface of
KCC-1 was about 0.2 mmol/g. The ICP-AES analysis we conducted
showed that the proportion of the KCC-1 surface covered with
Ni@Au NPs should be approximately 20%, which is similar to what
we calculated in our experiment (Table S1).
Fig. 1a shows a TEM image of Ni@Au NPs. The diameter of the
NPs is ranges from 8 to 18 nm, and the morphology of the NPs is
near-spherical. The TEM image also reveals that a large number
of Ni@Au NPs aggregated with each other. The selected area elec-
tron diffraction (SAED) pattern (Fig. 1a, inset) of these NPs exhibits
mixed diffraction rings that belong to fcc-Au and fcc-Ni. In addi-
tion, the HRTEM image (Fig. 1b) verified the core–shell structure.
The measured lattice fringes (0.238 nm) from the shell region corre-
spond to Au (1 1 1) planes, whereas those measured from the core
region (0.206 nm) are accordant to the Ni (1 1 1) planes, showing
clear differences between the core and shell.
in an aqueous suspension (2.8 mL H O) was mixed with 40 ␮L
2
−
2
−7
−
of (1.26 × 10 mol/L, 5.04 × 10 mol) of 2-NA aqueous solution.
4
Then, 0.13 mL (0.5 M, 0.65 × 10 mol) of NaBH4 solution was
injected rapidly while stirring. The color of the mixture vanished
gradually; complete colorlessness indicated complete reduction.
The progress of all reactions conducted here was monitored by
measuring the UV–Vis absorption spectra of the reaction mixture.
2.5. The recovery and reuse of the Ni@Au/KCC-1 nanocatalyst
Reduction reactions were amplified 20× to characterize the
reusability of the Ni@Au/KCC-1 nanocatalyst. The reaction conver-
sion was determined by GC. The catalyst was recovered using a
powerful magnet and then washed with water and ethanol and
dried in a vacuum at room temperature before being used in the
next catalytic run. The reduction of 4-NP and 2-NA catalytic pro-
cess was carried out within 10 min for each cycle. This procedure
was repeated ten times.
Fig. 1c (HAADF-STEM) revealed that Ni@Au nanoparticles have
a brighter contrast at the edge and darker contrast in the core. Thus
the core and shell have different chemical compositions was indi-
cated. Because of Au have a much higher atomic number than Ni, it
scatters electrons very intensively, therefore it was appeared bright
in the HAADF image. The STEM-EDS (Fig. 1d) on a single particle
clearly shown strong Au signals at the particle edge and strong Ni
signals in the particle center revealed a typical core–shell structure.
KCC-1 has high accessibility, which has been attributed to its
fibers [30,33]. Therefore, the unique properties of KCC-1 may be
useful for the design of silica-supported catalysts, for which the
accessibility of active sites can be increased significantly. The
morphology of KCC-1 is shown in Fig. 1e. In the figure, KCC-
1 microspheres with porous fibrous structure are uniform and
monodisperse, with an average diameter of about 300 nm (Fig. 1e).
The HRTEM image shown in Fig. 1e further clarifies that the distance
between the two fibers is about 22 nm (Fig. 1e, inset). The large dis-
tance between fibers and the high surface area of KCC-1 (Table S1)
make the Ni@Au NPs with diameters between 8 and 18 nm easily
loaded onto KCC-1 nano-silica. It also prevents the aggregation of
3
. Results and discussion
3.1. Catalysts characterization
Ni@Au NPs were prepared as decomposition [(C H5) P]AuCl
6
3
complexes on the surface of Ni NPs cores [32]. OAm was used as
both the solvent and surfactant. Using Ni(acac)2 as the starting
material, the Ni core was first generated by adding Au seeds to
◦
the reaction mixture at 45 C. Then, the [(C H5) P]AuCl complexes
6
3
◦
decomposed and the Au shell was formed at about 195 C [32].
For the preparation of Ni@Au/KCC-1 nanocatalyst, the as-prepared
KCC-1 nano-silica was functionalized with mercaptopropyl groups
so that the Ni@Au NPs could be easily anchored onto the KCC-1
fibers (Scheme 1). The properties of KCC-1 and HS-KCC-1, includ-
ing results of FTIR (Fig. S1), TGA (Fig. S2), Elemental analysis (Table
S1) and surface areas (Table S1) are provided in the Supporting
Information. The FTIR spectrum (Fig. S1) of HS-KCC-1 revealed
the presence of mercaptopropyl groups, which provided binding

X. Le et al. / Journal of Molecular Catalysis A: Chemical 395 (2014) 58–65
61
Fig. 1. (a) TEM image along with SAED pattern (inset) of Ni@Au NPs, (b) HRTEM image a single Ni@Au NP, (c) HAADF image showing the contrast between core (appear dark)
and shell (appear bright) component, (d) STEM-EDS line-scan analysis of a single Ni@Au nanoparticle, (e) TEM images of KCC-1 with HRTEM image of KCC-1 (inset), (f) TEM
image of Ni@Au/KCC-1.
the Ni@Au NPs and increases the accessibility of the active site of
the nanocatalyst Ni@Au/KCC-1. A TEM image of the Ni@Au/KCC-1
sample is shown in Fig. 1f. The Ni@Au NPs are remarkably modi-
fied between the fibers of the KCC-1 microspheres, and no obvious
aggregation is observed. When functionalized with MPTES, the
mercaptopropyl groups may act as robust anchors for the immobi-
lization of Ni@Au NPs. This process can also prevent the leaching
of Ni@Au NPs from their support.
We used the EDS spectrum collected from energy dispersive X-
ray analysis to determine the chemical composition of Ni@Au NPs.
The peaks of Ni and Au are shown in Fig. 2a. This figure gives the
Ni@Au NPs catalyst compositions (note that the Cu peak arises from
the copper grid in TEM analysis). XRD analysis also can provide
evidence for the composition of Ni@Au NPs. Fig. 2b reveals that
Ni@Au NPs exhibit sharp peaks of Au (1 1 1), Ni (1 1 1), Au (2 0 0),
Au (2 2 0) and Au (3 1 1) (JCPDS cards No. 04-0784), which also verify
the exists of fcc-Au and fcc-Ni. The wide hump in the range of 2ꢀ
◦
o
from 15 to 30 is typical for amorphous silica (for example, KCC-1).
When the Ni@Au NPs were immobilized on the KCC-1 nano-silica
surface, all of the characteristic, expected peaks of Ni@Au NPs were
present, as was the amorphous silica peak. These results indicated
that the immobilization of Ni@Au NPs was successful.
In order to examine the valence states of Au and Ni in the
Ni@Au/KCC-1, XPS analysis was carried out (Fig. 2c). The XPS ele-
mental survey scan of the surface of Ni@Au/KCC-1 microspheres
revealed that silicon, oxygen, gold, nickel, and carbon are present
in the samples. While the binding energy of the Ni 2p at 853 eV is
much weaker than those of Au 4d and Au 4f, this is mainly attributed
to the Ni atoms that were concentrated in the cores of Ni@Au NPs.
As shown in Fig. 2c, the Au 4d binding energies at 353 and 335 eV,

6
2
X. Le et al. / Journal of Molecular Catalysis A: Chemical 395 (2014) 58–65
Fig. 3. Room temperature magnetization curves of Ni@Au NPs and Ni@Au/KCC-1.
reaction solution with the use of a powerful magnet (Fig. 3, inset),
which makes the Ni@Au/KCC-1 nanocatalyst recoverable and able
to be used repeatedly.
3.2. Catalytic tests
We investigated the catalytic performance of the Ni@Au/KCC-
nanocatalyst by using the reduction of 4-NP into 4-AP in
1
the presence of NaBH4 as a probe reaction. The reaction was
easily monitored by UV–Vis spectroscopy because the reactant (4-
nitrophenolate anion) and product (4-AP) have different maximum
absorption peaks, i.e., ꢁmax = 400 nm (reactant) and 300 nm (prod-
uct) [34]. In comparison, Ni@Au NPs were also used for catalytic
reducing the 4-NP.
Initially, the 4-NP solution has a light yellow color, which turns
bright yellow upon the addition of NaBH . This colour change
4
is due to the formation of 4-nitrophenolate (Fig. 4a, inset). This
process is accompanied by a strong absorption peak of 317 nm
for 4-NP, which is remarkably red-shifted to 400 nm following
the reaction [35–37]. Even after adding superfluous NaBH , the
4
maximum absorption of 400 nm did not change over time, which
confirmed that the reduction did not proceed by aqueous NaBH
4
ꢀ
ꢁ
0
solution. The reduction of 4-NP
E
= −0.76 V by NaBH4
(
4-NP/4-AP)
ꢀ
ꢁ
E
= −1.33 V is thermodynamically feasible but kinet-
−
)
ically restricted in the absence of a catalyst [38].
When catalyst is introduced into the solution, the absorption
at 400 nm decreases and absorption at 300 nm increases gradually,
which indicates the replacement of 4-NP with 4-AP [39]. Visually,
the color of the reaction mixture changes from yellow to colorless
(
Scheme 2), also indicating the replacement of 4-NP with 4-AP.
As shown in Fig. 4b, when 6 × 10 mg of Ni@Au catalyst was
−
2
added to the reaction mixture, the reduction reaction completed
in about 12 min, while for the Ni@Au/KCC-1 nanocatalyst with
the same amount of Ni@Au NPs, the reduction reaction took only
Fig. 2. (a) EDS spectra of Ni@Au NPs, (b) XRD pattern of KCC-1, Ni@Au NPs and
Ni@Au/KCC-1, (c) XPS wide-scan spectrum of the Ni@Au/KCC-1.
1
0 min (Fig. 4c). This difference suggests that Ni@Au/KCC-1 has
and the Au 4f binding energies at 88 and 84 eV, are characteristic
of metallic Au.
higher catalytic activity than Ni@Au NPs. This is likely due to the
immobilization of the Ni@Au NPs onto the KCC-1 nano-silica, which
overcomes the issue of aggregation and enhances the catalytic
activity.
The concentration of NaBH4 is much higher than that of 4-NP;
thus, it can be considered essentially constant during the reaction.
This high concentration also protects the 4-AP from aerial oxidation
[40]. Therefore, we used pseudo first-order kinetics with respect to
Magnetization curves (Fig. 3) show the superparamagnetic
behavior of both the Ni@Au NPs and the Ni@Au/KCC-1 nanocata-
lyst. The magnetic saturation values of the Ni@Au NPs is 4.53 emu/g.
When Ni@Au NPs were functionalized on the KCC-1, the mag-
netic saturation values were reduced to only 0.9 emu/g. However,
the Ni@Au/KCC-1 nanocatalyst can still be easily separated from a

X. Le et al. / Journal of Molecular Catalysis A: Chemical 395 (2014) 58–65
63
Fig. 5. Plots of Ct /C0 (a) and ln(Ct /C0) (b) versus reaction time for the reduction of
-NP over Ni@Au NPs and Ni@Au/KCC-1.
4
Fig. 4. UV–Vis spectra of 4-NP before and after adding NaBH4 solution (a), and the
successive reduction of 4-NP to 4-AP over the prepared catalysts: (b) Ni@Au NPs
and (c) Ni@Au/KCC-1.
Scheme 2. The reduction of 4-NP and 2-NA on the surfaces of Ni@Au/KCC-1 catalyst.
4
-NP to calculate the catalytic rate. The reaction conversion was cal-
−
3
−3 −1
s for the Ni@Au NPs
culated from Ct/C (Fig. 5a), which was measured using the relative
intensity of UV–Vis absorbance (At/A ) at 400 nm. Ct is the concen-
tration of 4-NP at reaction time t; C0 is the initial concentration.
constant k to be 6.4 × 10 and 8.3 × 10
0
and Ni@Au/KCC-1 catalyzed reductions of 4-NP, respectively.
0
ꢀ
For a quantitative comparison, the activity parameter k = k/M
As shown in Fig. 5b, the linear correlation between ln(Ct/C ) and
reaction time demonstrates that the 4-NP reduction reaction is of
the first-order. Thus, the kinetic equation of this catalytic reaction
was introduced, which is the ratio of the rate constant k to the
weight of the active sites Au added (M) [35,38]. Using this equation,
the reaction rate constant per unit mass (MAu) was calculated to be
0
−
1
−1
for the reduction of 4-NP using Ni@Au/KCC-
is: ln(Ct/C ) = −kt, where k is the apparent first-order rate constant
307 and 237 s
g
0
−
1
(
s
), and t is the reaction time. We calculated the reaction rate
1 and Ni@Au NPs, respectively. The Ni@Au/KCC-1 nanocatalyst

6
4
X. Le et al. / Journal of Molecular Catalysis A: Chemical 395 (2014) 58–65
Table 1
The comparison of catalytic activities for the reduction of 4-NP with catalysts reported in literatures.
k (s 1)a
−
−1 −1
)b
k/MAu (s g
Catalyst
Catalyst used (mg)
Au content (%)
−
−
−3
3
3
Au/graphene [47]
Carbon@Au [48]
Ni@Au NPs (this work)
Ni@Au/KCC-1 (this work)
0.1
0.1
24
22.7
45
9
3.17 × 10
5.42 × 10
132
239
237
307
−
2
6 × 10
6.4 × 10
8.3 × 10
−
3
0.3
a
The reaction rate constant.
The reaction rate constant per total weight of Au in the used catalyst.
b
Fig. 7. The reusability of Ni@Au/KCC-1 nanocatalyst as a catalysts for the reduction
of 4-NP and 2-NA with NaBH4.
reaction rate constant per unit mass (MAu) was calculated to be
−
1
−1
.
1
74 s
g
The comparison of the catalytic activity of the Ni@Au/KCC-1
nanocatalyst with Ni@Au NPs and other reported Au NP-based cat-
alysts in the reduction of 4-NP are listed in Table 1. Our results
showed that the reaction rate constant per total weight of Au for
Ni@Au/KCC-1 is higher than that of the unsupported Ni@Au NPs.
−
1
−1
g
It is also higher than the Au/graphene catalyst (132 s
the carbon@Au core–shell nanofiber catalyst (239 s
) and
−1
g ) reported
−
1
previously [47,48]. The good catalytic activity of the Ni@Au/KCC-1
nanocatalyst owes to the ease of access of its active site, and the
low aggregation of the Ni@Au NPs on the KCC-1 support. In this sit-
−
Fig. 6. (a) UV–Vis spectra the successive reduction of 2-NA to o-PDA over
Ni@Au/KCC-1 catalyst, (b) Plots of Ct /C0 and ln(Ct /C0) versus reaction time for the
reduction of 2-NA to o-PDA over Ni@Au/KCC-1 nanocatalyst.
uation, both 4-NP and BH4 can be easily adsorbed on the Ni@Au
NPs surface, which allows the reduction reactions to start quickly
and finish rapidly.
One of the advantages of the Ni@Au/KCC-1 nanocatalyst that we
present here is that the recovery of the catalyst is easily achieved by
magnetic separation, because the Ni@Au NPs have magnetic cores.
This allows the NPs to be reused for the next catalytic run. As shown
in Fig. 7, the recovered Ni@Au/KCC-1 nanocatalyst exhibited almost
constant catalytic activity for at least ten successive cycles in both
the reduction of 4-NP and the reduction of 2-NA, with stable con-
versions above 97% within 10 min. This result suggests that the
Ni@Au/KCC-1 nanocatalyst possesses robust stability.
clearly had higher catalytic activity than Ni@Au NPs. This was likely
due to the ease of accessibility of the Ni@Au/KCC-1 nanocatalyst’s
active site, and the low amount of aggregation of Ni@Au NPs on the
KCC-1 nano-silica support.
Based on our above results, we also used the Ni@Au/KCC-1
nanocatalyst to perform the catalytic reduction of 2-NA. The reduc-
tion of 2-NA can also be easily monitored because the product
for the reduction reaction was only, the progress of the reaction
can be determined by measuring the change in UV–Vis absorbance
4. Conclusion
[
9,37,41–46]. As shown in Fig. 6a, 2-NA shows a typical absorbance
peak at 410 nm. When the Ni@Au/KCC-1 nanocatalyst was added,
the absorbance peak intensity decreased gradually as 2-NA was
reduced into o-PDA. As shown in Fig. 6b, the linear correlation
In conclusion, we have successfully developed a novel, fibrous
nano-silica catalyst called Ni@Au/KCC-1. This nanocatalyst, with
Ni@Au NPs as the active sites, can not only reduce the use of
Au without reducing catalytic activity, but can also confer super-
paramagnetic properties to the nanocatalyst, making the catalyst
easy to recover for reuse. The Ni@Au/KCC-1 nanocatalyst exhibited
between ln(Ct/C ) and the reaction time (t) confirms that the 2-
0
NA reduction reaction is a first-order reaction. The reaction rate
constant calculated from the linear slope was 4.7 × 10−
3
−1
s . The

X. Le et al. / Journal of Molecular Catalysis A: Chemical 395 (2014) 58–65
65
excellent catalytic activity in its reduction of 4-NP and 2-NA, which
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