A one-pot protocol for synthesis of non-noble metal-based core-shell nanoparticles under ambient conditions: Toward highly active and cost-effective catalysts for hydrolytic dehydrogenation of NH3BH3

Article abstract of DOI:10.1039/c1cc13989d

A one-pot synthesis of non-noble transition metal-based core-shell nanoparticles (NPs) has been developed under ambient conditions. The obtained Cu@M (M = Co, Fe, Ni) NPs exhibit superior catalytic activity for hydrolytic dehydrogenation of NH₃BH₃, compared to the alloy and monometallic counterparts.

Full text of DOI:10.1039/c1cc13989d

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COMMUNICATION
A one-pot protocol for synthesis of non-noble metal-based core–shell
nanoparticles under ambient conditions: toward highly active and
cost-effective catalysts for hydrolytic dehydrogenation of NH BH w
3
3
Hai-Long Jiang, Tomoki Akita and Qiang Xu*
Received 4th July 2011, Accepted 24th August 2011
DOI: 10.1039/c1cc13989d
A
one-pot synthesis of non-noble transition metal-based
to further improve kinetic properties under moderate condi-
8
tions is crucial for the practical application of this system.
core–shell nanoparticles (NPs) has been developed under ambient
conditions. The obtained Cu@M (M = Co, Fe, Ni) NPs exhibit
superior catalytic activity for hydrolytic dehydrogenation of
Herein, we present a facile one-step and general route for
in situ synthesis of a series of Cu@M (M = Co, Fe, Ni)
core–shell NPs under ambient conditions in very short time
3 3
NH BH , compared to the alloy and monometallic counterparts.
(within several to tens of minutes). In contrast to the mono-
Bimetallic core–shell nanoparticles (NPs) have attracted
considerable interest owing to their unique physical and
chemical properties and potential applications in contrast to
metallic and alloy counterparts, the in situ generated bimetallic
core–shell NPs have exhibited synergistic and superior catalytic
activity for hydrolytic dehydrogenation of ammonia borane.
In a typical synthesis of Cu@M (M = Co, Fe, Ni) core–shell
NPs, to the round-bottom flask containing ammonia borane, a
mixture of copper(II) chloride, cobalt(II) or nickel(II) chloride or
iron(II) sulfate aqueous solution, and polyvinylpyrrolidone K 30
1
,2
monometallic counterparts and alloys. Although there are
numerous reports on the preparation of core–shell NPs with
various compositions and their diverse applications, especially
in heterogeneous catalysis, almost all catalysts are noble
metal-based and their high cost and limited reserve greatly
inhibit the applications in an industrial scale. By contrast, low-
cost transition metal-based core–shell NPs have been rarely
reported. To the best of our knowledge, so far, only Co@Cu
2+
2+
(PVP) as a capping agent was introduced. The Cu and M
were reduced in sequence to produce core–shell structured NPs
2+
during the reduction process, in which Cu with high reduction
o
o
potentials (E Cu(II)/Cu(
I
)
= +0.159 eV vs. SHE; E Cu(
+0.520 eV vs. SHE) was first reduced by NH BH
generated Cu–H or/and subsequent M–H species with a strong
I
)/Cu
=
3
core–shell NPs were obtained based on a two-step reaction
3
3
. The
4a
4b
and very large Cu@Ni NPs (4100 nm) and Ni@Co NPs
460 nm) were recently synthesized with precise temperature
2+
(
reducing ability can further reduce M although they cannot be
controlling in a polyol-type reaction and under microwave
irradiation, respectively. There is no investigation on the
synergistic performance of both non-noble metal species in
these reports. Therefore, exploiting a facile and general strategy
to obtain low-cost transition metal-based bimetallic core–shell
NPs with synergistic/enhanced performances (such as, catalysis)
still remains undeveloped.
directly reduced by NH BH due to their lower reduction poten-
3
3
o
o
tials (E Co(II)/Co = À0.28 eV vs. SHE; E Fe(II)/Fe = À0.44 eV vs.
o
1a
SHE; E Ni(II)/Ni = À0.25 eV vs. SHE), during which the pre-
0
formed Cu NPs, serving as in situ seeds/core NPs, could induce
0
the successive growth of M as a shell to thus yield Cu@M
core–shell NPs. Such in situ reduction and one-pot synthetic
protocol for core–shell NPs is to take advantage of the differences
in the reduction potentials of core and shell metal salts, where the
On the other hand, hydrogen is a globally accepted clean
fuel. The development of effective hydrogen-storage materials
3 3
key point is to employ a suitable reducing agent. NH BH is a
is imperative but challenging to establish a future hydrogen
5
economy. Ammonia borane (AB, NH
desirable reducing agent based on its following merits: (1) with
weak reduction ability, it can only reduce the metal cations with
high reduction potential; (2) during the first metal reduction to be
3
BH ) has a hydrogen
3
content of 19.6 wt%, which exceeds that of gasoline and
makes it one of the most attractive candidates for chemical
core/seed NPs, the hydrolysis of NH BH catalyzed by these NPs
3 3
6
,7
hydrogen storage application. Hydrogen generation from
hydrolysis of NH BH is a promising alternative approach in
produces M–H species as a stronger reducing agent to further
reduce the second metal cation with lower reduction potential as
a shell.
3
3
addition to thermolysis. However, the search for suitable
catalysts that meet performance, cost and safe requirements
The reaction process can be readily monitored by the naked
eye due to the obvious color evolution in the solution (Fig. S1, ESIw).
2
+
2+
The Cu and M salts were mixed with PVP as a capping
agent in water. The solution color instantly changed to khaki
National Institute of Advanced Industrial Science and Technology
(AIST), Ikeda, Osaka 563-8577, Japan. E-mail: q.xu@aist.go.jp;
Fax: +81-72-751-9629; Tel: +81-72-751-9562
w Electronic supplementary information (ESI) available: Full sample
preparation and characterization data. See DOI: 10.1039/c1cc13989d
3 3
upon the addition of NH BH , indicating the reduction of
2
+
+
2+
Cu to Cu prior to the M reduction. Subsequently, the
color turned darker and darker along with plentiful H2
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Chem. Commun., 2011, 47, 10999–11001 10999

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+
generation, revealing the complete reduction of Cu /Cu
2+
2
+
and M . From the difference of the reduction sequence
explained above and the solution color evolution, we can
reasonably propose the core–shell nanostructure of the
obtained Cu-based bimetallic catalyst. The structure assumption
was supported by UV-vis and X-ray photoelectron spectro-
scopy (XPS) studies. As shown in Fig. 1, the peak at around
5
70 nm resulting from the surface plasmon resonance (SPR) of
Cu NPs disappears after introducing another metal (Co, Fe,
Ni) as the main component in shells. The XPS investigation
with Ar etching was conducted for the representative Cu@Co
NPs. Results show that Cu 2p3/2 and 2p1/2 peaks are almost
unobservable before Ar etching whereas the peaks appear and
increase after Ar etching. However, the metallic Co species can
be earlier detected before Ar etching. The slight peak shift of
Co 2p3/2 and 2p1/2 on the outer shell before Ar etching could
be ascribed to the surface oxidation in the Cu@Co sample
during the exposure to air.
The transmission electron microscopy (TEM) observation for
the representative Cu@Co NPs further verifies the core–shell
nanostructure and the core/shell is readily distinguishable
(Fig. 2). The Cu@Co NPs are found to be not monodisperse
but interconnected to afford a nanochain-like structure possibly
owing to the weak wrapping/protecting capability of PVP. The
energy-dispersive X-ray spectroscopy (EDS) and electron energy
loss spectroscopy (EELS) analyses for the selected points in the
high-angle annular dark-field scanning transmission electron
microscopy (HAADF-STEM) images clearly show that the white
rice-like NPs are Cu whereas the outer gray shell is composed by
Co. In addition, the lattice fringes of a Cu core indicate its
crystallinity whereas a Co shell could be amorphous from the
high resolution TEM (HRTEM) image (Fig. 2a), which are also
Fig. 2 Microstructure observation for representative Cu@Co NPs.
(
(
a) HRTEM and (b) TEM images. (c, d) HAADF-STEM images and
e) the corresponding EDS spectra for selected points 1 and 2 in (c), as
well as (f) the corresponding EELS analyses for selected points 1 and 2
in (d). The oxygen signal in (f) is ascribed to the partial oxidation of
particle surface.
approved by the absence of Co peaks but gradually increasing
Cu peaks along with a larger Cu/Co ratio in the powder XRD
patterns (Fig. S2, ESIw).
The obtained Cu@M (M = Co, Fe, Ni) NPs have been
employed as catalysts for hydrolytic dehydrogenation of
NH
chemical hydrogen storage material.
Fig. 3, Cu @M approximately shows the best activity among
all the catalysts with different Cu/M ratios although Cu @Ni
and Cu @Ni have similar results, revealing the optimal combi-
nation of Cu/M = 1/4. As a whole, the catalytic activity of a
Cu @Co catalyst is superior to Cu @Fe and Cu @Ni
3 3
BH , which has been demonstrated to be a promising
5d,8d,h
As displayed in
1
4
1
4
2
3
,
x
5Àx
x
5Àx
x
5Àx
which could be attributed to the more excellent activity of Co
NPs than that of Fe or Ni NPs toward hydrogen generation from
8
b,9
aqueous NH BH based on the previous reports.
In addition,
the monometallic and alloyed NP counterparts have also been
prepared with a conventional reducing agent NaBH . The resul-
tant Cu alloy NPs (Fig. 3d) and Cu/Co/Fe/Ni monometallic
NPs (Fig. S4, ESIw) display much lower catalytic activities than
those of the core–shell Cu @M NPs, which demonstrates the
synergistic effects between Cu and M metals in the core–shell
nanostructure. As a result, H generation over Cu @Co NPs is
3
3
4
1 4
M
1
4
2
1
4
the fastest among all the catalysts, with which the reaction is
completed in 10 min, comparable to or even better than that of
Fig. 1 (Top) UV-vis spectra of Cu and Cu@M (M = Co, Fe, Ni)
samples in aqueous PVP solution. (Bottom) XPS spectra of Cu 2p and
Co 2p for representative Cu@Co NPs before and after Ar etching for
2c
Au@Co NPs with an expensive Au core. Moreover, the
magnetism of all the Cu@M NPs benefits their separation from
the solution by simply using an external magnet (Fig. S5, ESIw).
60 min.
1
1000 Chem. Commun., 2011, 47, 10999–11001
This journal is c The Royal Society of Chemistry 2011

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aqueous solution
@Fe5Àx, (c)
(M = Co,
Fe, Ni) alloy catalysts under ambient conditions. The molar ratio of
À1
x
(0.26 mol L , 10 mL) over (a) Cu @Co5Àx, (b) Cu
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@Ni5Àx (x = 0.5, 1, 2, 3, 4) core–shell and (d) Cu M
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¨
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Chem. Commun., 2011, 47, 10999–11001 11001