Preparation and catalytic activities of Au/Co bimetallic nanoparticles for hydrogen generation from NaBH4 solution

Article abstract of DOI:10.1166/jnn.2015.9159

A series of poly(N -vinyl-2-pyrrolidone)-protected Au/Co bimetallic nanoparticles (BNPs) were prepared by a simple route based on dropwise addition of NaBH₄ . Their structures, particle sizes, and chemical compositions were characterized by Ultraviolet-visible spectrophotometry, X-ray photoelectron spectroscopy, Transmission electron microscopy and High-resolution transmission electron microscopy, and their catalytic activity for the hydrogen generation from hydrolysis of an alkaline NaBH₄ solution was examined. As-prepared alloy-structured Au/Co BNPs had an average size between 2.8 and 3.6 nm and showed a higher catalytic activity for the hydrogen generation than the corresponding Au and Co monometallic nanoparticles (MNPs). Of all the MNPs and BNPs, Au₂₀Co₈₀ BNPs exhibited the highest catalytic activity, and a hydrogen generation rate of 480 mol-H₂ ? h -¹ ? mol-M-¹ was achieved. The high catalytic activity of the BNPs can be ascribed to the formation of negatively charged Au atoms and positively charged Co atoms as a result of the electronic charge transfer effects in the BNPs.

Full text of DOI:10.1166/jnn.2015.9159

Article
Journal of
Nanoscience and Nanotechnology
Vol. 15, 2770–2776, 2015
Copyright © 2015 American Scientific Publishers
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Printed in the United States of America
Preparation and Catalytic Activities of Au/Co
Bimetallic Nanoparticles for Hydrogen
Generation from NaBH4 Solution
Xiaofeng Wang1, Zili Huang1, Lilin Lu2, Haijun Zhang3ꢀ4ꢀ∗, Yingnan Cao3ꢀ4,
Yajun Gu3ꢀ4, Zhong Cheng5, and Shaowei Zhang4
1Hubei Key Laboratory for Efficient Utilization and Agglomeration of Metallurgical Mineral Resources,
Wuhan University of Science and Technology, Wuhan 430081, China
2College of Chemical Engineering and Technology, Wuhan University of
Science and Technology, Wuhan 430081, China
3College of Materials and Metallurgy, Wuhan University of Science and Technology,
Wuhan, Hubei Province 430081, China
4State Key Laboratory Breeding Base of Refractories and Ceramics, Wuhan University of
Science and Technology, Wuhan 430081, China
5College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, China
A sDereielisveorfepdoblyy(NP-uvibnlyisl-h2i-npgyrrToelicdhonneo)l-opgroytetoct:eSdtaAtue/CUonbivimerestaitlylicofnaNneowpaYrtoicrlkesa(tBBNinPgsh) awmerteonpre-
IP: 128.226.37.5 On: Sat, 06 Dec 2014 14:03:50
pared by a simple route based on dropwise addition of NaBH4. Their structures, particle sizes, and
Copyright: American Scientific Publishers
chemical compositions were characterized by Ultraviolet-visible spectrophotometry, X-ray photo-
electron spectroscopy, Transmission electron microscopy and High-resolution transmission electron
microscopy, and their catalytic activity for the hydrogen generation from hydrolysis of an alkaline
NaBH4 solution was examined. As-prepared alloy-structured Au/Co BNPs had an average size
between 2.8 and 3.6 nm and showed a higher catalytic activity for the hydrogen generation than the
corresponding Au and Co monometallic nanoparticles (MNPs). Of all the MNPs and BNPs, Au20Co80
BNPs exhibited the highest catalytic activity, and a hydrogen generation rate of 480 mol-H2 ·h−1 ·mol-
M−1 was achieved. The high catalytic activity of the BNPs can be ascribed to the formation of
negatively charged Au atoms and positively charged Co atoms as a result of the electronic charge
transfer effects in the BNPs.
Keywords: Au/Co, Bimetallic Nanoparticles, Catalytic Activity, Hydrogen Generation, NaBH4,
Hydrolysis.
1. INTRODUCTION
the presence of a suitable catalyst, releasing hydrogen gas
along with the formation of water-soluble sodium metab-
orate, NaBO2.
Hydrogen (H2) is a globally accepted clean fuel.1ꢀ2
Recently, much attention has been paid to the hydroly-
sis of chemical hydrides (NaBH4, KBH4, LiBH4, etc.),3–8
which as a hydrogen source shows a number of advantages
in portable applications, including, nonflammability and
stability in air, easy control of hydrogen generation rate,
recyclability of the side products, and a high H2 storage
capacity. Among these chemical hydrides, sodium borohy-
dride (NaBH4) is the most feasible for safe and practical
production of hydrogen for fuel cells.9 It hydrolyzes in
Some non-noble transition metals, in particular, Co,
were reported to exhibit good activities for hydrogen gen-
eration from NaBH4.10ꢀ11 For example, hydrogen could
be released from an aqueous solution of NaBH4 within
2.5 min (molar ratio of Co/NaBH4 = 0ꢁ0056) when a flu-
orinated cobalt was as a catalyst.12 A Co–B thin film cat-
alyst prepared by the pulsed laser deposition technique
was about 2.5 times more efficient in hydrogen generation
than a Co powder catalyst.13 The activity of a Cr-doped
Co–B powder catalyst was about 4 times higher than that
∗Author to whom correspondence should be addressed.
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1533-4880/2015/15/2770/007
doi:10.1166/jnn.2015.9159

Wang et al.
Preparation and Catalytic Activities of Au/Co BNPs for H2 Generation from NaBH4 Solution
of a pure Co–B catalyst in hydrogen generation from the
NaBH4 hydrolysis.14
units to the total metal ions) in a 250 mL two-neck flask
ꢀ
and then stirred at 0 C for 30 min. After that, an aque-
Gold (Au)15ꢀ16 has long been known to be catalyt-
ically less active than other transition metals. How-
ever, its nanoparticles supported on metal oxides such
as TiO2 exhibited an extraordinary high activity for low-
temperature catalytic combustion, partial oxidation of
hydrocarbons, hydrogenation of unsaturated hydrocarbons,
and reduction of nitrogen oxides.17 Currently, a great deal
of attention is being paid to gold-containing bimetallic
nanoparticle (BNP) catalysts because of their high cat-
alytic activities at low-temperatures for a series of impor-
tant reactions,18ꢀ19 such as oxidation of CO and H2,20ꢀ21
reduction of NOx,22 epoxidation of C3H6,23 selective glu-
cose oxidation24–25 and combustion of methane.26 Never-
theless, to our knowledge, there has been no reported work
on the catalytic activity of Au-containing BNPs for hydro-
gen generation from NaBH4.
ous solution of NaBH4 (10 mL, 16.5 mM, 0 C; RNaBH
ꢀ
4
is defined as the molar ratio of NaBH4 to the total metal
ions) was added at a rate of one drop in every three sec-
onds under vigorous stirring for about 10 min, and then
ꢀ
the solutions were stirred for another 1 h at 0 C.
2.3. Characterization of Nanoparticles
Ultraviolet and visible light (UV-Vis) absorption spec-
tra were recorded over 200–800 nm with a Shimadzu
2550 recording spectrophotometer using a quartz cell with
an optical path length of 10 mm. Transmission electron
microscopy (TEM) images were taken at the accelerated
voltage of 80 kV using an FEI Tecnai G2 50-S-TWIN
TEM. The specimens were obtained by dropping one or
two dropplets of the Au/Co colloidal ethanol solution onto
a copper microgrid covered with a thin amorphous car-
bon film and followed by evaporating the ethanol in air
at room temperature. For each sample, generally at least
200 particles from different parts of the grid were selected
to evaluate their mean diameter and size distribution. High-
resolution TEM (HR-TEM) images were taken at the
accelerated voltage of 200 kV using a JEM-2100F Field
Emission High-resolution TEM. XPS measurement was
performed using a Quantum 2000 spectrometer with Al
Kꢁ radiation. Binding energies (BE) were normalized by
In the present work, Au/Co BNPs were synthesized
and characterized and their catalytic activities for hydro-
gen generation from the hydrolysis of an alkaline NaBH4
solution examined in detail. In addition, density func-
tional theory (DFT) calculations were carried out, assisting
understanding the catalytic effects of Au/Co BNPs. The
results showed that the negatively charged Au atoms due
to electron donation from the neighboring Co atoms in
the Au/Co BNPs had acted as catalytically active sites for
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the hydrogen generation from the hydrolysis of an alka-
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the C(1s) BE of adventitious carbon contamination taken
as 284.6 eV, and the analyses of Au were based on Au4f5/2
and Au4f7/2 peaks.
line NaBH4 solution, and the presence of the negatively
Copyright: American Scientific Publishers
charged Au atoms was also supported by X-ray photo-
electron spectroscopy (XPS) examinations and the DFT
calculations.
2.4. Catalytic Activity of Au/Co BNPs for the
Hydrolysis of Alkaline NaBH4
2. EXPERIMENTAL DETAILS
2.1. Raw Materials
The catalytic activity of Au/Co BNPs was evaluated based
on the volume of hydrogen gas generated from hydrolysis
of an alkaline NaBH4 solution. Experiment was carried out
in a two-necked round-bottom flask with one opening con-
nected to a gas burette and the other to an addition funnel
with a pressure-equalization arm. In all tests, the reaction
was started by vigorous stirring the mixture of Au/Co BNP
catalyst and the alkaline NaBH4 solution (50 mL, 30 mM
and pH 12.) in the molar ratio of 0.05. Thꢀe temperature for
activity evolution was maintained at 30 C using a water
bath, and the volume of the generated hydrogen gas was
measured by the displacement level of water in a burette.
Hydrogen tetrachloroaurate (III) tetrahydrate (HAuCl4 ·
4H2O, 99.9%), cobalt (II) chloride hexahydrate
(CoCl2 · 6H2O, 99%), sodium borohydride (NaBH4,
96.0%), poly(N -vinyl-2-pyrrolidone) (PVP, K30, average
molecular-weight of about 30,000), and sodium hydroxide
(NaOH, 96.0%) purchased from Sinopharm Ltd., were
used directly as the main starting materials without further
purification. All glassware and Teflon-coated magnetic
stirring bars were cleaned with aqua regia, followed by
repeated rinsing with pure water.
2.5. Density Functional Theory (DFT) Calculation
Electronic structures of the M55 clusters were calculated
using the Amsterdam Density Functional (ADF) program
package developed by Baerends and coworkers. In these
calculations, the Triple-ꢂ plus Polarization basis (TZP)
sets and the relativistic effects with Zero Order Regu-
lar Approximation (ZORA) were employed for all the
elements. The exchange-correlation energy of electrons
was described in the generalized gradient approximation
2.2. Preparation of PVP-Protected Au/Co BNPs
A series of Au/Co BNPs stabilized by PVP were prepared
by dropwise addition of NaBH4. For example, Au20Co80
BNPs (Hereafter, the subscripts indicate the synthetic feed-
ing ratio of the two metals) were prepared as follows:
aqueous solutions of HAuCl4 · 4H2O (10 mL, 0.66 mM)
and CoCl2 ·6H2O (40 mL, 0.66 mM) were combined with
an aqueous PVP solution (50 mL, 66 mM in monomer
unit; RPVP is defined as the molar ratio of PVP in monomer
J. Nanosci. Nanotechnol. 15, 2770–2776, 2015
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Preparation and Catalytic Activities of Au/Co BNPs for H2 Generation from NaBH4 Solution
Wang et al.
(GGA) with the functional parameterization of PBE.
The criterion of the self-consistent convergence of total
energy was set as ≥ 0ꢀ001 eV/atom. The atomic charges
were taken from the Multipole Derived Charge analysis
(MDC–q).
BNPs with various compositions. The size distribution
analysis based on the TEM images gives the average diam-
eter of 3ꢀ0 1ꢀ3 nm for Au, 3ꢀ1 1ꢀ1 nm for Au90Co10,
3ꢀ0 0ꢀ9 nm for Au80Co20, 2ꢀ8 1ꢀ1 nm for Au50Co50,
3ꢀ6 0ꢀ9 nm for Au20Co80, and 3ꢀ0 1ꢀ0 nm for Au10Co90
NPs. All the NPs were spherical and well-isolated, with
a standard deviation of about 30% in the diameter. In the
case of Co MNPs, large average diameters up to sev-
eral tens nanometers were determined based on TEM
(not shown). It can be deduced that as-prepared Au/Co
nanoparticles were composed of the bimetallic nanoparti-
cles based on the following reasons: (1) large particles up
to tens nanometers seen in the case of Co NPs were not
present in the as-prepared Au/Co bimetallic samples, and
(2) the particle size distribution of Au/Co BNPs was much
narrower than that of the physical mixture of Au and Co
MNPs.
In order to further confirm the formation of alloyed
structures in the as-prepared Au/Co BNPs, lattice fringes
analysis based on HR-TEM images of the Au20Co80 col-
loidal dispersions was also carried out. As revealed by
Figure 3, the particles possessed crystalline structures.
The interplanar distances of four individual randomly-
chosen particles were measured as 0.218 nm (particle-1),
0.194 nm (particle-2), 0.232 nm (particle-3), and 0.231 nm
(particle-4), respectively, as labeled in Figure 3. These lat-
3. RESULTS AND DISCUSSION
To examine the effect of compositions of Au/Co BNPs
on their catalytic activities, a series of BNPs with vari-
ous atomic ratios of Au/Co were prepared using dropwise
addition of NaBH4. It is well known that UV-Vis absorp-
tion spectra are strongly dependant on the size, composi-
tion and dielectric properties of nanoparticles as well as
their local environment. Figure 1 shows UV–Vis spectra
of Au/Co BNP aqueous dispersions with various composi-
tions. The spectrum of Au NP aqueous dispersion exhibits
a plasmon absorbance peak around 520 nm, which is con-
sistent with the result previously reported,27 whereas the
spectrum of Co NP aqueous dispersion exhibits a fea-
tureless and monotonically increasing absorbance toward
the high energy range. The UV-Vis spectra in Figure 1
also show the suppression of the Au plasmon peaks with
increasing the Co content, which suggests the co-existence
of Co atoms and Au atoms on the surfaces of Au/Co BNPs.
When the molar ratio of Au/Co is 20/80, the 520-nm sur-
face plasmon peak from the Au atoms finally disappears,
tice fringes do not match with any reflection plane dis-
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indicating the presence of many Co atoms on the surfaces
IP: 128.226.37.5 On: Sat, 0ta6ncDeesco2f 0p1u4re1A4:u03o:r50Co shown in Table I, which rules
of the BNPs. The UV-Vis spectra exclude the existence of
Copyright: American Scientific Publishers
out the formation of individual Au and Co NPs in the as-
Cocore/Aushell BNPs and Aucore/Coshell BNPs in the present
Au/Co BNPs. The reasons are as follows. If the former
were formed, there would be a plasmon band observed at
about 520 nm for all the BNPs due to the Au surface plas-
mon resonance. On the other hand, if the latter (i.e., Co
deposited onto Au NPs) were present, there would be no
plasmon resonance peaks observed in the UV-Vis spectra
of all the BNPs.
prepared Au/Co nanoparticles. Nevertheless, it should be
noted that the interplanar distances of 0.218 nm, 0.232 nm
and 0.231 nm, lie between the interplanar spacing of Co
(111) (0.205 nm) and that of Au (111) (0.236 nm), sug-
gesting that an alloyed structure was formed in the par-
ticles and the interplanar spacing can be assigned to the
(111) plane of the alloy-structured Au/Co. Similarly, the
interplanar distances, 0.194 nm, observed from particle-
2, can be assigned to the (200) plane of the alloy-
structured Au/Co (Table II). Moreover, it should be pointed
that, except particle 1, the measured interplanar distances
are not in consistent with the theoretical values of bulk
Au20Co80 alloy as listed in Table II. This result and the var-
ious interplanar distances of the (111) plane observed from
different particles indicate that the as-prepared Au20Co80
alloy BNPs did not possess an even composition. In other
words, not all of the as-prepared individual Au20Co80
BNPs have the chemical composition of Au/Co = 20/80.
In a word, based on the results of UV-Vis, TEM images
and lattice spacing of HR-TEM images, it can be rea-
sonably deduced that alloy-structured Au/Co BNPs were
formed.
Figure 2 presents a representative set of TEM images
and size distribution histograms of as-prepared Au/Co
2.0
Au
Au/Co = 90/10
Au/Co = 80/20
Decreasing Au
1.5
contents
Au/Co = 50/50
Au/Co = 20/80
Au/Co = 10/90
Co
1.0
0.5
0.0
200
300
400
500
600
700
800
The catalytic properties of as-prepared Au/Co BNPs
were studied using the hydrogen generation from hydrol-
ysis of an alkaline NaBH4 solution as a working tool. The
activity varies with the composition of the Au/Co BNPs
Wavelength/nm
Figure 1. UV-Vis spectra of colloidal dispersions of Au and Co MNPs,
Au/Co BNPs with various contents of Co.
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Wang et al.
Preparation and Catalytic Activities of Au/Co BNPs for H2 Generation from NaBH4 Solution
30
25
20
15
10
5
30
D
av = 3.0 nm
Dav = 3.1 nm
s = 1.1 nm
20
10
0
s = 1.3 nm
0
0 1 2 3 4 5 6 7 8 9 10
Diameter/nm
0
1 2 3 4 5 6 7 8 9 10
Diameter/nm
(a) Au
(b) Au90Co10
40
30
20
10
0
40
Dav = 2.8 nm
s = 1.1 nm
Dav = 3.0 nm
s = 0.9 nm
30
20
10
0
0 1 2 3 4 5 6 7 8 9 10
Diameter/nm
0 1 2 3 4 5 6 7 8 9 10
Diameter/nm
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Copyright: American Scientific Publishers
(c) Au80Co20
(d) Au50Co50
30
20
10
0
25
20
15
10
5
D
av = 3.6 nm
Dav = 3.0 nm
s = 1.0 nm
s = 0.9 nm
0
0 1 2 3 4 5 6 7 8 9 10
Diameter/nm
0 1 2 3 4 5 6 7 8 9 10
Diameter/nm
(e) Au20Co80
(f) Au10Co90
Figure 2. TEM micrographs and size distribution histograms of Au, Au90Co10, Au80Co20, Au50Co50, Au20Co80 and Au10Co90 nanoparticles.
J. Nanosci. Nanotechnol. 15, 2770–2776, 2015
2773

Preparation and Catalytic Activities of Au/Co BNPs for H2 Generation from NaBH4 Solution
Wang et al.
500
Particle-2
400
300
200
100
0
Particle-1
Particle-3
Particle-4
0
20
40
60
80
100
CAu/molar %
Figure 4. Effect of chemical composition on hydrogen generation from
the hydrolysis of alkaline NaBH4 solution over as-prepared Au/Co BNPs.
Figure 3. HR-TEM image of as-prepared Au20Co80 BNPs.
responsible for the high catalytic activities of several types
of BNPs.29ꢀ30 In the case of the Au/Co BNPs, the ioniza-
tion potentials of Au and Co are 9.22 and 7.86 eV, respec-
tively. The electronic charges could transfer from Co atoms
to Au atoms, leading to an increase in the electron density
on the Au atoms. Consequently, the negatively charged
Au atoms and positively charged Co atoms could act as
catalytically active sites for hydrogen generation from the
hydrolysis of an alkaline NaBH4 solution.
as shown in Figure 4. All the BNPs showed a higher cat-
alytic activity than pure Au or Co MNPs. The highest cat-
alytic activity, 480 mol-H2 · h−1 · mol-M−1, was achieved
in the case of Au20Co80 BNPs. It is more than 3.5 times
higher than that of Au or Co BNPs (130 and 77 mol-
H2 · h−1 · mol-M−1, respectively for Au and Co MNPs).
Even the activities for hydrogen generation of the prepared
Au/Co BNPs were normalized by the specific surface
areas, the Au20Co80 BNPs still show the highest activity
(results not shown here). The higher catalytic activity of
the Au/Co BNPs than that of MNPs is attributable to the
effects of electronic charge transfer between the Au and
Co atoms. Such effects were considered previously to be
To prove the electronic charge transfer effects and the
presence of the negatively charged Au atoms and pos-
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itively charged Co atoms in the BNPs, the electronic
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properties of the Au/Co BNPs were initially investigated
Copyright: American Scientific Publishers
using XPS. PVP-protected Au20Co80 BNPs prepared with
a low content of PVP (RPVP = 5) were examined by high-
resolution XPS using monochromated Al Kꢁ electron radi-
ation. As shown in Figure 5, the electron apparent BEs of
Au4f5/2 and Au4f7/2 in the Au20Co80 BNPs were 87.05 eV
and 83.19 eV, respectively. The electron apparent BE of
Au4f5/2 was about 0.58 eV lower than that of bulk Au4f5/2
(87.63 eV), and a similar result was also observed for
Au4f7/2 (about 0.76 eV lower than that of bulk Au4f7/2 of
83.95 eV). The negative shifts of the Au4f5/2 and Au4f7/2
Table I. Theoretical lattice spacing of bulk Au and Co with various
reflection planes.
Lattice distances, nm
Reflection planes
Au
Co
(111)
(200)
(220)
(311)
(222)
0.236
0.204
0.144
0.123
0.118
0.205
0.177
0.120
0.107
0.102
Notes: (Au: JCPDS 89-3697; Co: JCPDS 15-0806).
Au4f7/2
83.19 eV
Table II. Lattice spacing and indexed reflection planes of Au20Co80
BNPs determined by HR-TEM.
Au4f5/2
87.05 eV
Measured
lattice
Au20Co80 distances
Indexed reflection
planes of
Au/Co alloy
BNPs
Theoretical
lattice distance
of bulk
Au20Co80 alloy28
BNPs
(nm)
Remarks
Particle-1
0.218
In the range of
Au(111) and Co(111)
In the range of
Au(200) and Co(200)
In the range of
Au(111) and Co(111)
In the range of
Au(111) and Co(111)
(111)
(200)
(111)
(111)
0.212 nm for
(111) plane∗
0.184 nm for
(200) plane∗
0.212 nm for
(111) plane∗
0.212 nm for
(111) plane∗
Particle-2
Particle-3
Particle-4
0.194
0.232
0.231
80
82
84
86
88
90
Binding Energy/eV
Figure 5. Au XPS core level spectrum recorded from Au20Co80 BNPs
synthesized with a low content of PVP (RPVP = 5).
Notes: ∗Calculated based on the lattice parameters of Au20Co80 alloy.
2774
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Wang et al.
Preparation and Catalytic Activities of Au/Co BNPs for H2 Generation from NaBH4 Solution
(a) Au12Co43
(b) Au24Co31
Figure 6. DFT calculations of electronic structures of Au12Co43 and Au24Co31BNPs (yellow, Au; and blue, Co).
suggest that the Au atoms in the BNPs were indeed neg-
atively charged, and the Co atoms did donate electrons to
the Au atoms.
To further confirm the electron donation from Co atoms
to Au atoms, DFT calculations of a M55 model NPs of
Au12Co43 and Au24Co31 (the subscripts stand for the num-
ber of atoms in the BNPs) were preformed to assist under-
and a plausible mechanism involving adsorption of BH4−
onto the NPs to form an activated complex which might
be the rate-determining step, followed by H2 production
through attack by a H2O molecule can be used to describe
the process. In the case of catalysis by the present BNPs,
the positive charged Co atoms may favor the absorption of
BH−4 , and the negatively charged Au atoms would trans-
standing the electronic states of Co and Au atoms in the
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BNPs (Fig. 6). The most important finding from these cal-
Thus, we predict that the enhancement of the catalytic
Copyright: American Scientific Publishers
culations is that Au atoms are indeed negatively charged,
activity for hydrogen production from NaBH4 by present
Au/Co BNPs is due to the synergetic effect between the
charged atoms and BH4−/H2O, even though we have no
strict evidence on it.
whereas the Co atoms positively charged due to the elec-
tronic charge transfer from them to Au atoms. The DFT
calculation results are consistent with those of XPS, and
both confirm the presence of the electronic charge transfer
effect in the BNPs. It is considered that both the nega-
tively charged Au atoms and positively charged Co atoms
acted as catalytic active sites for hydrogen generation from
hydrolysis of NaBH4. The possible electronic charge trans-
fer effects in Au/Co BNPs are illustrated in Figure 7. At
this stage, the detailed catalytic mechanism of the BNPs
for hydrogen generation from hydrolysis of NaBH4 is still
not very clear, work along this line is now in progress.
In the presence of catalysts, NaBH4 can generate hydrogen
rapidly in the following way:
4. CONCLUSIONS
A series of Au/Co alloy BNPs with average sizes of 2.8–
3.6 nm were prepared by dropwise addition of NaBH4. The
as-prepared Au/Co BNPs showed higher catalytic activi-
ties for hydrolysis of NaBH4 than corresponding Au and
Co MNPs, which could be ascribed to the presence of
negatively charged Au atoms and positively charged Co
atoms. The catalytic activity of Au20Co80 BNPs for hydro-
gen generation from the hydrolysis of an alkaline NaBH4
solution is 480 mol-H2 ·h−1 ·mol-M−1 under the conditions
NaBH4ꢀsꢁ+2H2Oꢀlꢁ C−at→alyst NaBO2ꢀlꢁ+4H2ꢀgꢁ ↑ (1)
ꢀ
of pH = 12 and 30 C.
–1
+1
Acknowledgments: This work was financially sup-
ported by National Natural Science Foundation of China
(General program, 51272188).
NaBH4 + 2H2O = NaBO2 + 4H2
↑
δ+
δ–
-
δ+
e–
δ+
e
Co
Co
Au
Co
e–
Au
Co
e–
Co
e–
>
e–
e–
e–
δ+
e–
δ+
Co Co
Au Co
References and Notes
1. P. Chen, Z. Xiong, J. Luo, J. Lin, and K. L. Tan, Nature 420, 302
(2002).
7.86 eV
Ei/eV: 9.22 eV
δ+ δ+ δ–
Figure 7. Schematic illustration of electronic charge transfer effects in
Au20Co80 BNPs (Ei: Ionization energy).
2. J. M. Yan, X. B. Zhang, S. Han, H. Shioyama, and Q. Xu, Inorg.
Chem. 48, 7389 (2009).
J. Nanosci. Nanotechnol. 15, 2770–2776, 2015
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Preparation and Catalytic Activities of Au/Co BNPs for H2 Generation from NaBH4 Solution
Wang et al.
3. S. C. Amendola, S. L. Sharp-Goldman, M. S. Janjua, N. C. Spencer,
M. T. Kelly, P. J. Petillo, and M. Binder, Int. J. Hydrogen Ener.
25, 969 (2000).
16. R. Nirmala, H. M. Park, D. Kalpana, H. S. Kang, R. Navamathavan,
Y. S. Lee, and H. Y. Kim, J. Biomed. Nanotechnol. 7, 342
(2011).
4. Y. Kojima, K. I. Suzuki, K. Fukumoto, M. Sasaki, T. Yamamoto,
Y. Kawai, and H. Hayashi, Int. J. Hydrogen Ener. 27, 1029 (2002).
5. H. I. Schlesinger, H. C. Brown, A. E. Finholt, J. R. Gilbreath, H. R.
Hoekstra, and E. K. Hyde, J. Am. Chem. Soc. 75, 215 (1953).
6. S. C. Amendola, S. L. Sharp-Goldman, M. S. Janjua, M. T. Kelly,
P. J. Petillo, and M. Binder, J. Power Sources 85, 186 (2000).
7. Y. Kojima, Y. Kawai, M. Kimbara, H. Nakanishi, and S. Matsumoto,
Int. J. Hydrogen Ener. 29, 1213 (2004).
8. M. Chandra and Q. Xu, J. Power Sources 159, 855 (2006).
9. S. Özkar and M. Zahmakıran, J. Alloys Compd. 404–406, 728
(2005).
10. J. Lee, K. Y. Kong, C. R. Jung, E. Cho, S. P. Yoon, J. Han, T. G.
Lee, and S. W. Nam, Catal. Today 120, 305 (2007).
11. M. Rakap and S. Özkar, Appl. Catal. B 91, 21 (2009).
12. S. Suda, Y. M. Sun, B. H. Liu, Y. Zhou, S. Morimitsu, K. Arai,
N. Tsukamoto, M. Uchida, Y. Candra, and Z. P. Li, Appl. Phys. A
72, 209 (2001).
17. M. Haruta, Catal. Today 36, 153 (1997).
18. M. Shkir, S. Aarya, R. Singh, M. Arora, G. Bhagavannarayana, and
T. D. Senguttuvan, Nanosci. Nanotechnol. Lett. 4, 405 (2012).
19. X. J. Bai, T. Cao, and C. B. Cao, Sci. Adv. Mater. 4, 1007
(2012).
20. H. Huber, D. McIntosh, and G. A. Ozin, Inorg. Chem. 16, 975
(1977).
21. M. Haruta, N. Yamada, T. Kobayashi, and S. Iijima, J. Catal.
115, 301 (1989).
22. A. Ueda and M. Haruta, Gold Bull. 32, 3 (1999).
23. B. S. Uphade, Y. Yamada, T. Akita, T. Nakamura, and M. Haruta,
Appl. Catal. A 215, 137 (2001).
24. H. J. Zhang and N. Toshima, Catal. Sci. Technol. 3, 268 (2013).
25. N. Toshima and H. J. Zhang, Macromol. Symp. 317–318, 149 (2012).
26. R. J. H. Grisel, P. J. Kooyman, and B. E. Nieuwenhuys, J. Catal.
191, 430 (2000).
27. H. Tsunoyama, N. Ichikuni, H. Sakurai, and T. Tsukuda, J. Am.
Chem. Soc. 131, 7086 (2009).
28. H. Okamoto, T. B. Massatski, M. Hasebe, and T. Nishizawa, Bulletin
of Alloy Phase Diagram 6, 449 (1985).
29. H. J. Zhang and N. Toshima, Appl. Catal. A 447–448, 81 (2012).
30. G. W. Zhan, J. L. Huang, M. M. Du, I. Abdul-Rauf, Y. Ma, and
Q. B. Li, Mater. Lett. 65, 2990 (2011).
13. N. Patel, G. Guella, A. Kale, A. Miotello, B. Patton, C. Zanchetta,
L. Mirenghi, and P. Rotolo, Appl. Catal. A 323, 18 (2007).
14. R. Fernandes, N. Patel, and A. Miotello, Appl. Catal. B 92, 68
(2009).
15. J. J. Niu, V. Presser, C. J. Karwacki, and Y. Gogotsi, Mater. Express
1, 259 (2011).
Received: 21 September 2013. Accepted: 4 November 2013.
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