Hollow Ni-Co-B amorphous alloy nanospheres: Facile fabrication via vesicle-assisted chemical reduction and their enhanced catalytic performances

Article abstract of DOI:10.1039/c4ta04533e

In this paper, we develop a simple vesicle-assisted chemical reduction approach for synthesizing hollow Ni-Co-B nanospheres. With various characterization techniques, the resulting Ni-Co-B nanospheres are identified as amorphous alloys with a hollow chamber. Coexistence of Ni^II and Co^II species plays a significant role in fabricating hollow nanospheric structures, because only solid nanoparticles can be obtained in the presence of a mono-metallic precursor. During liquid-phase hydrogenation of 2-ethyl-2-hexenaldehyde, hollow Ni-Co-B catalyst displays significant bi-site catalysis from bimetals and delivers much greater activity as well as better selectivity than associated with the dense Ni-Co-B catalyst. Additionally, this catalyst is also easily handled in liquid-phase reactions due to its lower density and magnetic property. The material design concept presented in this work opens a new avenue for the development of hollow non-noble metallic nanospheres and will draw more attention in the foreseeable future.

Full text of DOI:10.1039/c4ta04533e

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Materials Chemistry A
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Hollow Ni–Co–B amorphous alloy nanospheres:
facile fabrication via vesicle-assisted chemical
reduction and their enhanced catalytic
performances†
Cite this: DOI: 10.1039/c4ta04533e
*
W. Wei, Y. Zhao, S. C. Peng, H. Y. Zhang, Y. P. Bian, H. X. Li and H. Li
In this paper, we develop a simple vesicle-assisted chemical reduction approach for synthesizing hollow
Ni–Co–B nanospheres. With various characterization techniques, the resulting Ni–Co–B nanospheres
are identified as amorphous alloys with a hollow chamber. Coexistence of NiII and CoII species plays a
significant role in fabricating hollow nanospheric structures, because only solid nanoparticles can be
obtained in the presence of a mono-metallic precursor. During liquid-phase hydrogenation of 2-ethyl-
2-hexenaldehyde, hollow Ni–Co–B catalyst displays significant bi-site catalysis from bimetals and
delivers much greater activity as well as better selectivity than associated with the dense Ni–Co–B
catalyst. Additionally, this catalyst is also easily handled in liquid-phase reactions due to its lower density
and magnetic property. The material design concept presented in this work opens a new avenue for the
development of hollow non-noble metallic nanospheres and will draw more attention in the foreseeable
future.
Received 1st September 2014
Accepted 29th September 2014
DOI: 10.1039/c4ta04533e
www.rsc.org/MaterialsA
mechanisms of galvanic replacement have also been developed
to fabricate hollow metal structures,3 which are more facile and
Introduction
cost-effective since the templates could act as reducing agents
to produce metal shells and no additional steps are needed to
remove the templates. Nevertheless, the synthesis of hollow
metal NSs is limited mainly to noble metals when using this
synthesis strategy and thus their practical applications are
limited due to the high cost.4 From the viewpoint of practical
applications, exploring more simple approaches to preparing
hollow non-noble metal architectures is an important issue.
Recently, attention has turned to developing facile and effi-
cient so-templating methods for hollow metal structures.
In recent years, hollow structures have attracted immense
attention owing to their potential applications in adsorption,
microelectronics, photonics, as well as catalysis.1 In the domain
of catalysis, hollow metallic nanospheres (NSs) with permeable
shells represent a new class of efficient catalysts because they
offer some advantages over their dense counterparts in terms of
increased surface area, low density, easy recovery, self-sup-
porting capacity, cost reduction, and surface permeability.2
Generally, hollow structures in metals can be routinely fabri-
cated based on a hard templating approach, in which the “hard”
templates (e.g., polymer colloid beads and silica spheres) are
coated with metallic shell via a layer-by-layer technique and
then the templates are selectively removed.2a,b However, this
method is synthetically challenging, since multiple steps are
oen necessary for achieving hollow metal structures, compli-
cating the preparation process. Strategies based on the
Based on
a simple vesicle-assisted chemical reduction
approach, we successfully synthesized hollow Pd NSs,2g which
delivered much superior catalytic properties compared to dense
Pd nanoparticles (NPs) during liquid-phase phenol hydrogena-
tion. However, the problem resulting from the use of expensive
Pd metal has not been completely overcome despite the
subsequent synthesis of hollow Pd–Co2i and Pd–Fe2j NSs
through a similar method to economize on the catalyst cost.
Therefore, a key issue for the present study is to develop a facile
so-templating method for the synthesis of highly efficient
hollow non-noble metallic catalysts.
The Education Ministry Key Lab of Resource Chemistry and Shanghai Key Laboratory
of Rare Earth Functional Materials, Shanghai Normal University, Shanghai 200234, P.
R. China. E-mail: lihui@shnu.edu.cn; Fax: +86 21 6432 2272; Tel: +86 21 6432 3578
† Electronic supplementary information (ESI) available: TEM images of
Ni–Co–B-S, Ni–Co–B synthesized by the similar conditions used to synthesize
Ni–Co–B-H but without Bu4PBr, Ni–Co–B synthesized by the similar conditions
used to synthesize Ni–Co–B-H but without KCl, and Ni–Co–B-S and Ni–Co–B-H
aer 5 and 7 consecutive runs, respectively; DSC curves of Ni–Co–B-S and
Ni–Co–B-H; UV/vis spectra of aqueous solutions of NiCl2 + CoCl2, NiCl2 + CoCl2
It is well recognized that a binary alloy is expected to display
not only a combination of the properties associated with each of
the two distinct metals, but also new capabilities due to a
synergistic effect between the two metals.5 In the case of hydro-
genation of 2-ethyl-2-hexenaldehyde (EHEA) to 2-ethyl-1-hexanol
+
KCl, and NiCl2 + CoCl2 + KCl + Bu4PBr; reaction proles of EHEA
hydrogenation over Ni–B, Co–B, and Ni–Co–B-S. See DOI: 10.1039/c4ta04533e
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Material characterization
The bulk composition was analyzed by means of inductively
coupled plasma optical emission spectrometry (ICP-OES; Varian
VISTA-MPX). The amorphous structure was investigated by both
X-ray diffraction (XRD; Rigaku D/Max-RB with Cu Ka radiation)
and selective-area electronic diffraction (SAED; JEOL JEM2100).
The crystallization process was followed by differential scan-
ning calorimetry (DSC; Shimadzu DSC-60) under an N2 atmo-
sphere at a heating rate of 5 K minꢁ1. The shapes and
morphologies of catalysts were observed by both eld emission
scanning electron microscopy (FESEM; Hitachi S-4800) and
transmission electron microscopy (TEM; JEOL JEM2100). The
surface electronic states were determined by X-ray photoelec-
tron spectroscopy (XPS; ULVAC-PHI PHI5000 VersaProbe
system using Al Ka radiation), during which all samples were
pretreated in situ in a pure Ar atmosphere to avoid oxidation.
The binding energy (BE) values were calibrated by using C 1s ¼
284.6 eV as a reference. The active surface area (Sact) was
measured by hydrogen chemisorption with a Micromeritics
AutoChem II 2920 instrument assuming H/M(s) ¼ 1 and a
surface area of 6.5 ꢂ 10ꢁ20 m2 per M atom. The hydrogen
temperature-programmed desorption (H2-TPD) curve was
obtained with the same instrument in Ar ow at a ramping rate
Scheme 1 Possible reaction pathways for EHEA hydrogenation.
(EHO) (Scheme 1), a valuable synthetic alcohol,6 for instance,
highly dispersed Pd on Co–B amorphous alloy displayed signi-
cant synergy between sites, in which both metals play specic
roles with respect to the reactants.7 More specically, Pd prefers
the hydrogenation of C]C bonds rather than that of C]O
groups in EHEA molecules; while Co is selective to the hydroge-
nation of C]O bonds and its catalytic activity can be enhanced
by a synergetic effect from Pd. Furthermore, Ni is expected to be a
possible substitute of Pd for the hydrogenation of C]C bonds
because its strong affinity for the adsorption of C]C moieties in
a,b-unsaturated aldehydes has been proved in our previous
studies.8 In this research a simple and efficient method to
synthesize hollow Ni–Co–B amorphous alloy NSs through a
vesicle-assisted chemical reduction method is reported. During the
liquid-phase hydrogenation of EHEA to EHO, the hollow Ni–Co–B
catalyst displayed much superior performances compared to
reference monometallic Ni–B, Co–B, and dense Ni–Co–B cata-
lysts, providing great potential for applications in industry.
of 10 K minꢁ1
.
Activity test
In a typical experiment, liquid-phase hydrogenation of 2-ethyl-
2-hexenaldehyde was carried out in a 250 mL stainless steel
autoclave containing 0.3 g Ni–Co–B-H, 4 mL of 2-ethyl-2-hex-
enaldehyde, 45 mL of EtOH, and 1.0 MPa of H2 at 373 K. The
reaction mixture was sampled at intervals for product analysis
using a gas chromatograph equipped with an EC-WAX column
and a ame ionization detector. All results have been repro-
duced and the error was limited to within 5%. Preliminary
kinetic studies revealed the following: that there was a plateau
in the dependency of the reaction initial rate when the stirring
rate was above 750 rpm; and that the reaction initial rate varied
linearly with the catalyst amount from 0.1 to 0.5 g, indicating
that the stirring rate of 800 rpm was high enough such that the
hydrogenation rates were independent of mass transfer. Aer
cooling to room temperature at the end of the reaction, the
catalyst was separated by a magnet and washed with deionized
water and EtOH for further characterization and applications.
To test the catalyst durability, the used catalyst was separated
and washed thoroughly with deionized water and EtOH aer
each run of the reaction. Then, the catalyst was reused with a
fresh charge of substances for a subsequent recycle run under
the same reaction conditions.
Experimental section
Material preparation
All of the chemicals used in this research were of analytical
grade and were used without further purication.
Hollow Ni–Co–B amorphous alloy was synthesized by a
vesicle-assisted chemical reduction method. In a typical run of
catalyst synthesis, an aqueous solution composed of NiCl2
(5.0 mL, 0.10 M) and CoCl2 (5.0 mL, 0.10 M) was rst added into
an aqueous solution of Bu4PBr (100 mL, 0.010 M), and then 30 g
of KCl was added into the above solution to form a saturated
solution. Thereaer, an aqueous solution of KBH4 (5.0 mL, 0.50 M)
was added dropwise under vigorous stirring at 273 K. Aer the
reaction was complete, the black precipitate was washed free
from inorganic ions and soluble amphiphilic substances with
deionized water until pH ꢀ 7 was achieved, followed by three
washings with absolute alcohol (EtOH). Finally, the sample was
stored in EtOH until use. The as-prepared hollow Ni–Co–B
amorphous alloy was designated Ni–Co–B-H (hollow). The Ni
and Co contents in the Ni–Co samples were adjusted by varying
the molar ratio of Ni2+/Co2+ in the precursor solution. Without
addition of Co2+ or Ni2+ in the preparation solution, pure Ni or
Results and discussion
Structural characteristics
Co was also synthesized. For comparison, a solid Ni–Co–B Ni–Co–B-H was rst synthesized in an aqueous solution by
amorphous alloy was also prepared by the conventional using a Bu4P+/[MX4]2ꢁ (M ¼ Ni and Co) composite vesicle
method, that is, by direct reduction of Ni2+ and Co2+ with KBH4, stemming from the strong electrostatic interaction between
which was denoted as Ni–Co–B-S (solid).
Bu4P+ and [MX4]2ꢁ as precursors and structure-directing
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Fig.
2
XRD patterns of (a) the fresh Ni–Co–B-H, (b) the fresh
Ni–Co–B-S, and (c) the Ni–Co–B-H and (d) the Ni–Co–B-S after
being treated at 673 K for 2 h in N2 flow.
Fig. 1 (a) FESEM and (b) TEM images of Ni–Co–B-H. (c) TEM image of
a single Ni–Co–B-H hollow sphere. (d) EDS mapping of one sphere in
(b). The inset is the SAED pattern.
Table 1 Structural parameters of the as-prepared catalysts
Bulk composition Surface composition
SPt (m2 gꢁ1
)
template. The FESEM image in Fig. 1a reveals that Ni–Co–B-H
displays uniform spheres with an average diameter around 40
nm. Meanwhile, the TEM observation of Ni–Co–B-H (Fig. 1b)
shows clear contrast difference between the center and the
edge, indicating the interior of these NSs is hollow. The high-
magnication TEM image of an individual sphere (Fig. 1c)
reveals that the outer shell of Ni–Co–B-H is comprised of
uniform particles with an average size of 2 nm. These NPs
constitute a self-supported shell with wormhole-like mesopores
(marked with arrows), which should provide a pathway for high
diffusion. The pore size is roughly estimated to be about 2 nm.
The energy-dispersive spectrometry (EDS) mapping for a single
Ni–Co–B-H sphere (Fig. 1d) shows the distribution of Ni and Co
is very uniform throughout the whole shell of hollow NS. In
contrast to the hollow NSs, the Ni–Co–B (Fig. 1S†) obtained via
the conventional method in pure aqueous solution is present in
the form of solid NPs with extremely broad size distribution
(Ni–Co–B-S).
The inset SAED image in Fig. 1b displays successive
diffraction halos indicative of a typical amorphous structure.9
Furthermore, XRD patterns (Fig. 2) demonstrate that, similar to
the conventional Ni–Co–B-S, the fresh Ni–Co–B-H is present as a
typical amorphous structure indicated by a single broad peak
around 2q ¼ 45ꢃ.10 Heat treatment of the fresh Ni–Co–B samples
(at 673 K in N2 for 2 h) resulted in the appearance of several
diffraction peaks corresponding to metallic Ni and Co, and
crystalline Ni–B, Co–B and Ni–Co–B alloys. The appearance of
Ni–Co–B crystalline phases during the crystallization process
veried the formation of alloys between Ni, Co and B for the as-
prepared Ni–Co–B, as demonstrated by our previous studies.11
The heat-treated Ni–Co–B-H exhibited fewer crystalline phases
relative to the heat-treated Ni–Co–B-S, indicating the stronger
thermal stability against crystallization. This might be related to
the increased B content in Ni–Co–B-H compared to Ni–Co–B-S,12
as demonstrated by ICP-OES analysis (Table 1). The crystalli-
zation process was further investigated by DSC analysis. As
Catalyst
(atom%)
(atom%)
Ni–Co–B-S
Ni–Co–B-H Ni31Co31B38
Ni34Co33B33
Ni41Co30B29
Ni39Co28B33
16
30
shown in Fig. S2,† Ni–Co–B-H exhibits an exothermic peak at
740 K higher by at least 57 K than that for Ni–Co–B-S, further
conrming the enhanced thermal stability of the former. XPS
reveals that for all the Ni species in either Ni–Co–B-S or
Ni–Co–B-H, the core level of Ni 2p3/2 is at 852.7 eV (Fig. 3),
indicating that all Ni atoms are present in the metallic state.11,13
Meanwhile, only one peak with BE at ca. 778.2 eV is observed in
the Co 2p3/2 level, indicating that almost all the Co species are
present as the metallic state in both samples.11,14 However, the B
species are present as both elemental B and oxidized B, with B
1s BE values of around 188.1 and 192.9 eV. The B 1s BE of
elemental B in either Ni–Co–B-S or Ni–Co–B-H exceeds that of
pure B (187.1 eV)13 by 1.0 eV, further proving that the elemental
B is alloyed with the metallic Ni or Co. In alloys, partial electrons
may be transferred from B to Ni or Co, as well demonstrated
previously.11,13 Thus, the combined results from SAED, XRD,
Fig. 3 XPS spectra of (a) Ni–Co–B-S and (b) Ni–Co–B-H.
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DSC, and XPS characterizations clearly demonstrated the
formation of Ni–Co–B amorphous alloy through the present
preparation method. According to the calculation based on XPS
peak areas, the surface molar ratio of the alloying B to the
metallic Ni and Co (Table 1) in Ni–Co–B-H is 33/67, higher than
that in Ni–Co–B-S (29/71), implying that the present preparation
method has signicant inuence on the surface composition of
Ni–Co–B amorphous alloy, and Ni–Co–B-H is surface-enriched
with the alloying B relative to Ni–Co–B-S.
Fig. 4 TEM image of vesicles formed through a mixture of NiCl2/
CoCl2/KCl/Bu4PBr obtained by drop-depositing solutions on carbon
grids and staining with phosphotungstanic acid.
Formation process
The formation of hollow Ni–Co–B NSs is likely based on the
Bu4P+/[MX4]2ꢁ (M ¼ Ni and Co) composite vesicles, which
provide templates for the growth of hollow structures. The
vesicle-assisted chemical reduction process might consist of
two steps (Scheme 2). The rst step should be the formation of
spherical vesicle precursors induced by the electrostatic inter-
action between [MX4]2ꢁ and Bu4P+. The KCl and the Bu4PBr in
water may be dissociated and provide Clꢁ and Brꢁ ions. These
halide anions can coordinate with M2+ (M ¼ Ni and Co) to form
[MX4]2ꢁ. The coordination of Xꢁ to M2+ was evidenced by the
distinct red shi of the UV/visible absorption (Fig. S3†). The
strong electrostatic interaction between Bu4P+ and [MX4]2ꢁ
induces the rapid co-assembly into Bu4P+/[MX4]2ꢁ composite
vesicles. The formation of vesicles has been conrmed by the
TEM image (Fig. 4), which displays almost the same diameter
(45 nm) as the average interior-cavity size of the nal product
(see Fig. 1). The second step is the reduction of the conned
[MX4]2ꢁ ions with borohydride into Ni–Co–B clusters, which
grow to particles coating the vesicles and constructing a thin
shell surrounding the vesicle templates. Aer removing the
vesicle template by washing with water, hollow Ni–Co–B NSs are
obtained. The wormhole-like mesoporous structure of the outer
shell can be attributed to the pileup of Ni–Co–B NPs.
formation of irregular Ni–Co–B particles in the absence of KCl
while keeping other experimental conditions the same as those
of the typical preparation (Fig. S5†).
Furthermore, the Ni/Co molar ratio also signicantly inu-
ences the formation of hollow NSs. We point out that neither
hollow Ni–B nor hollow Co–B (ref. 14) was formed if mono-
metallic precursor was used (Fig. 5). Meanwhile, Zhang and Li15
had demonstrated that CoCl2 does not induce vesicle formation
in the presence of Bu4NBr. More interestingly, co-reduction of
mixed [NiX4]2ꢁ and [CoX4]2ꢁ with Ni/Co molar ratio of 1/1
appears to form hollow NSs in high yield (see Fig. 1b).
Increasing the Ni/Co molar ratio to 3/1 or decreasing it to 1/3 led
to the formation of a mixture of hollow and solid particles
(Fig. 6a and b). Almost solely solid samples were obtained with
further increasing or decreasing the Ni/Co molar ratio to 5/1 or
1/5 (Fig. 6c and d). Based on these results, we propose that the
formation of the hollow Ni–Co–B NSs is originated from the co-
reduction of [NiX4]2ꢁ and [CoX4]2ꢁ conned on the surface of
vesicles and the subsequent growth into a thin shell coating the
vesicles. Accordingly, the generation of [MX4]2ꢁ complexes has
both positive and negative inuences on the fabrication of
hollow NSs. On the one hand, the strong electrostatic interac-
tion between Bu4P+ and [MX4]2ꢁ can induce the formation of
Bu4P+/[MX4]2ꢁ composite vesicles, providing a structure-direct-
ing template. On the other hand, the generation of [MX4]2ꢁ
complexes is expected to decrease the reducibility of MII
species. Therefore, slower reduction of [NiX4]2ꢁ or [CoX4]2ꢁ
allows the vesicle phase to reorganize, and only large particles to
grow (Fig. 5). Indeed, the chemical preparation of M–B amor-
phous alloy is an autocatalytic reaction, i.e., the formation of
M–B can be catalyzed by itself.16 Extended X-ray absorption ne
Both Bu4PBr and KCl play a key role in fabricating hollow
Ni–Co–B NSs. Only solid Ni–Co–B NPs with a broad size range
were obtained without using the Bu4PBr template (Fig. S4†).
Because the coordination of Brꢁ with M2+ is notꢁstrong enough
to induce a reaction between [MBr4]2ꢁ and BH4 to occur in a
smooth manner, the strongly exothermic reaction leads to the
Scheme 2 Illustration of the formation process of hollow Ni–Co–B Fig. 5 TEM images of (a) Ni–B and (b) Co–B synthesized under the
nanospheres.
same conditions used to synthesize Ni–Co–B-H.
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Catalytic performances
The hollow chambers of Ni–Co–B-H are very stable, and can
be well preserved in water for several months. Additionally,
the shells of this material are thin (ꢀ3 nm) and porous, which
is expected to have little inuence on the diffusion of reaction
molecules in liquid-phase reaction. More importantly, these
hollow NSs can be readily dispersed in liquid-phase solution
due to their low density, and can be easily recovered from
a reaction system by a magnet due to their magnetic
property (Fig. 8). All these properties present opportunities
for this material to be used as a highly efficient catalyst
in liquid-phase reaction. The as-prepared catalysts were
subjected to liquid-phase EHEA hydrogenation to EHO for
evaluating their performances. In addition to the production
of target product (EHO), both 2-ethylhexanal (EHA) and
2-ethyl-2-hexenol (EHEO) were identied as by-products
(Scheme 1) under the present conditions, 1.0 MPa of H2 and
373 K.
Preliminary studies suggested that Ni–B prefers the
hydrogenation of C]C bonds rather than that of C]O bonds
in EHEA molecules (Fig. S6a†) due to its strong affinity for the
adsorption of the C]C group;8 while Co–B exhibits nearly the
same initial selectivity to EHO as to EHA, giving equivalent
chances for both C]O and C]C hydrogenation (Fig. S6b†).
Thanks to the synergistic effect of Ni and Co demonstrated by
EXAFS analysis,17 binary-metallic Ni–Co–B-S amorphous alloy
delivered much higher catalytic activity and selectivity
(Fig. S6c†) than the mono-metal amorphous alloys. Although
EHO can be obtained with high yield (>90%) over Ni–Co–B-S,
saturated aldehyde (EHA) could not be hydrogenated
completely to the nal product under the present conditions.
Compared with Ni–Co–B-S, Ni–Co–B-H displayed similar bi-
site catalysis from bimetals and evidently much improved
catalytic activity (Fig. 9a). The higher catalytic activity of
Ni–Co–B-H can be mainly attributed to its larger number of
active sites (see Table 1), deriving from the hollow structures
with mesopores. More specically, the mesopores are large
enough to allow the penetration of substances and it is
possible for the catalytic reaction to occur on the inner side of
the shell. On the other hand, the increased hydrogenation
ability for C]O bonds associated with Ni–Co–B-H (see
Fig. S6c† and 9a) can be partially due to its higher surface B
content relative to Ni–Co–B-S as conrmed by XPS. It is well
documented that surface B-enrichment results in higher
Fig. 6 TEM images of Ni–Co–B synthesized under the same condi-
tions used to synthesize Ni–Co–B-H but with different Ni/Co molar
ratios: (a) Ni/Co ¼ 3/1, (b) Ni/Co ¼ 1/3, (c) Ni/Co ¼ 5/1, (d) Ni/Co ¼ 1/5.
structure (EXAFS) analysis revealed that Ni and Co have a
synergistic effect to the structure of the bimetallic Ni–Co–B
amorphous alloys.17 Specically, co-existence of Ni and Co
strengthens the M–B interaction and increases the structural
disordering factors of Ni–Co–B. The effect of Co content in
Ni–Co–B on these parameters is represented in a volcano-shaped
curve, and the maximum values are achieved for Ni/Co molar
ratio of 1/1. As a result, co-reduction of a mixture containing the
same amount of [NiX4]2ꢁ and [CoX4]2ꢁ affords highly active
primary Ni–Co–B particles at the interface of vesicles. Subse-
quently, these primary particles agglomerate and coalesce
quickly, until a wall thickness is reached at which the structure
is mechanically stable. Besides the molar ratio of [NiX4]2ꢁ to
[CoX4]2ꢁ, both the concentration of borohydride aqueous
solution and the reduction temperature are also important for
obtaining well-dened hollow structured Ni–Co–B. As
conrmed by TEM images (Fig. 7), co-reduction of the mixed
[NiX4]2ꢁ and [CoX4]2ꢁ with low concentration of borohydride
(0.05 M) could not achieve evident hollow structures. It is
noteworthy that much lower reduction temperature (263 K)
produces completely dense particles. Accordingly, the inability
to achieve hollow structures should be due to the reduction rate
being too slow to form an integrated outer shell covering the
vesicles. The proposed mechanism can also explain why only
fast reduction is able to generate such a hollow metallic nano-
structure; only then can the hollow nanoarchitecture be
generated before the vesicle phase is able to rearrange.
Fig. 7 TEM images of Ni–Co–B synthesized under the same condi-
tions used to synthesize Ni–Co–B-H but (a) with low concentration Fig. 8 Illustration of the easy handling of Ni–Co–B-H catalyst in
(0.05 M) or (b) at low reduction temperature (263 K).
liquid-phase reaction.
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Acknowledgements
This work was supported by the National Natural Science
Foundation of China (21273149), PCSIRT (IRT1269), the
Program for New Century Excellent Talents in University (NCET-
11-1052), and the Shanghai Science & Technology and Educa-
tion Committee (11JC1408900, 12490502800, 10SG41,
12YZ084).
Notes and references
Fig. 9 (a) Reaction profile and (b) recycling test of EHEA hydrogena-
tion over Ni–Co–B-H. (-) EHEA, (,) EHO, (O) EHA, and (P) EHEO.
Reaction conditions: catalyst (0.3 g), EHEA (4 mL), EtOH (45 mL), T ¼
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373 K, PH ¼ 1.0 MPa, stirring rate ¼ 800 rpm. Each run was conducted
2
for 6 h in the recycling test.
electron density on metal active sites in M–B aꢁmorphous
alloys,12,14 which is favorable for the formation of H species18
and activation of the adsorbed C]O groups through
an electron back-donation from the dx2ꢁy2 orbital of metal to
the p*C]O antibonding orbital of the C]O bonds,19 leading to
the enhanced reactivity.
Besides its high efficiency, Ni–Co–B-H could be easily
separated from the reaction solution by a magnet and used
repeatedly for more than 7 times with only 7% decrease of
EHO yield in EHEA hydrogenation (Fig. 9b). However, the EHO
yield decreases by 62% over Ni–Co–B-S aer being used 5
times. ICP analysis revealed that the weight loss of Ni–Co–B-S
aer 5 consecutive runs is 27%, while no leaching of Ni or Co
could be detected for Ni–Co–B-H during repeated use. This
implies that the larger diameter of hollow NSs can reduce the
loss of catalyst during recycling tests. Additionally, TEM
images (Fig. S7†) reveal that the deactivation of Ni–Co–B-S can
also be due to a severe agglomeration of NPs. However, the
hollow chambers of Ni–Co–B-H are quite stable which can be
well preserved aer being used 7 times. According to this
observation, the high durability of Ni–Co–B-H can also be
attributed to its self-supporting capacity, which retards the
Ni–Co–B NPs from agglomeration during the hydrogenation
process.
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Conclusions
In summary, we developed a simple approach for synthesizing
hollow-structured Ni–Co–B amorphous alloy NSs. Based on
various characterizations, a vesicle-assisted chemical reduction
mechanism to form such a hollow structure is proposed. In
particular, our present method is facile and cost-effective. Thus,
this strategy can potentially be extended to other hollow
metallic NSs with different composition. The one-pot produc-
tion of EHO through EHEA hydrogenation highlights the role of
the signicant bi-site catalysis from bimetals. This might
provide a preferable method for the synthesis of other hollow
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