Synthesis of Ni/Au/Co trimetallic nanoparticles and their catalytic activity for hydrogen generation from alkaline sodium borohydride aqueous solution

Article abstract of DOI:10.1039/c5ra01892g

A series of poly (N-vinyl-2-pyrrolidone) stabilized colloidal Ni/Au/Co trimetallic nanoparticles (TNPs) were synthesized by co-reduction of their corresponding metal precursors via dropwise addition of NaBH₄. Ultraviolet-visible spectrophotometry (UV-Vis), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), powder X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HR-TEM) combined with energy dispersive spectrometry (EDS) were used to characterize the morphology, crystalline structure and electron distribution of the as-prepared TNPs. The effects of metal composition on the size distribution and hydrogen generation from catalytic hydrolysis of alkaline NaBH₄ aqueous solutions were also investigated. The results indicated that the as-prepared alloy-structured Ni₄₅Au₄₅Co₁₀ TNPs showed a maximum catalytic activity of 1170 mol-H₂ per h per mol-M, which is several times higher than that of the as-prepared Au, Ni or Co monometallic nanoparticles (MNPs), or Au₅₀Ni₅₀, Au₅₀Co₅₀ or Ni₅₀Co₅₀ bimetallic nanoparticles (BNPs). The enhanced catalytic activity of the TNPs compared with the MNPs and BNPs could be attributed to the presence of electron charge transfer effects among Au, Ni and Co atoms, which is supported by the results of X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculation. The apparent activation energy of the as-prepared Au₄₅Ni₄₅Co₁₀ TNPs in hydrolysis of NaBH₄ aqueous solution was determined as 18.8 kJ mol^-1.

Full text of DOI:10.1039/c5ra01892g

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Synthesis of Ni/Au/Co trimetallic nanoparticles and
their catalytic activity for hydrogen generation
from alkaline sodium borohydride aqueous
solution†
Cite this: RSC Adv., 2015, 5, 34364
a
a
a
b
c
Chengpeng Jiao, Zili Huang, Xiaofeng Wang, Haijun Zhang,* Lilin Lu
b
and Shaowei Zhang
A series of poly (N-vinyl-2-pyrrolidone) stabilized colloidal Ni/Au/Co trimetallic nanoparticles (TNPs) were
synthesized by co-reduction of their corresponding metal precursors via dropwise addition of NaBH
4
.
Ultraviolet-visible spectrophotometry (UV-Vis), X-ray photoelectron spectroscopy (XPS), transmission
electron microscopy (TEM), powder X-ray diffraction (XRD) and high-resolution transmission electron
microscopy (HR-TEM) combined with energy dispersive spectrometry (EDS) were used to characterize the
morphology, crystalline structure and electron distribution of the as-prepared TNPs. The effects of metal
composition on the size distribution and hydrogen generation from catalytic hydrolysis of alkaline NaBH
aqueous solutions were also investigated. The results indicated that the as-prepared alloy-structured
Ni45Au45Co10 TNPs showed a maximum catalytic activity of 1170 mol-H per h per mol-M, which is several
times higher than that of the as-prepared Au, Ni or Co monometallic nanoparticles (MNPs), or Au50Ni50
4
2
,
Au50Co50 or Ni50Co50 bimetallic nanoparticles (BNPs). The enhanced catalytic activity of the TNPs compared
with the MNPs and BNPs could be attributed to the presence of electron charge transfer effects among Au,
Ni and Co atoms, which is supported by the results of X-ray photoelectron spectroscopy (XPS) and density
functional theory (DFT) calculation. The apparent activation energy of the as-prepared Au45Ni45Co10 TNPs in
Received 30th January 2015
Accepted 7th April 2015
DOI: 10.1039/c5ra01892g
ꢀ1
www.rsc.org/advances
hydrolysis of NaBH aqueous solution was determined as 18.8 kJ mol .
4
7
of byproduct NaBO to borohydride, and the lower hydrogen
2
1
. Introduction
8
releasing temperature.
9,10
11,12
13–15
Over the past decade, chemical hydrogen storage materials have
Noble metal nanoparticles (NPs) such as Pt, Pd,
Ru
16–18
attracted a great deal of attention since they can release a large and Rh
are conventionally used as catalysts for hydrogen
amount of hydrogen gas at ambient temperature and thus act as generation. However, the relatively high cost and the limited
1–5
a new source for producing hydrogen. Compared with other reservation of the precious metals hindered their further indus-
chemical hydrogen storage materials such as ammonia borane trial applications. Recently, cost-effective transition metal NPs,
(
NH
3
BH
3
) and hydrazine hydrate (N
) is more competitive because of several advan- noble metal catalysts for hydrogen generation.
2 4 2
H $H
O), sodium borohy- such as Fe, Co and Ni, have been developed as alternatives to
19–21
dride (NaBH
4
For example,
22
tages such as easier control of the hydrogen generation rate and Busca, et al. reported that cubic structured cobalt NPs could be
6
hydrogen purity, the safer production process, the recyclability used as active and selective catalyst for ethanol steam reforming
23
¨
reaction, and OZdemir, et al. found a high catalytic performance
of Co/B catalysts in hydrogen generation from hydrolysis of
4
NaBH solution. In addition, many researches discussed the
Hubei Key Laboratory for Efficient Utilization and Agglomeration of Metallurgical effects of preparation condition, support's type and protective
a
Mineral Resources, Wuhan University of Science and Technology, Wuhan 430081, agents on morphologies and catalytic activities of Co NPs in
China
24–26
hydrogen generation.
b
The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science
Bi- and tri-metallic nanoparticles (BNPs and TNPs) were
reported to be more active in chemical reactions than their
counterparts of monometallic nanoparticles (MNPs) because of
and Technology, Mail box No. 121, No.947 Heping Road, Wuhan 430081, Hubei
Province, China. E-mail: zhanghaijun@wust.edu.cn; Fax: +86-27-68862829; Tel:
+
86-27-68862829
c
College of Chemical Engineering and Technology, Wuhan University of Science and the so-called lattice strain effects, geometric effects and elec-
27,28
Technology, Wuhan 430081, China
tronic charge transfer effects.
It was also demonstrated that
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c5ra01892g
alloying of noble metals with other non-noble transition metals
1
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not only led to the enhanced catalytic performance but also subscript denote the feeding ratio of the corresponding metal
29
reduced the overall cost. For instance, according to Jiang ions) were prepared as follows: aqueous solutions of
30
et al., Ag /Co BNP catalysts exhibited a ve times greater NiCl $6H O (25 mL, 0.66 mM), HAuCl $4H O (5 mL, 0.66 mM)
1
0
90
2
2
4
2
catalytic activity than Co catalysts in hydrolysis of NaBH solu- and CoCl $6H O (20 mL, 0.66 mM) were added to an aqueous
4
2
2
31
tion. Kargupta, et al. reported a highly active and stable gra- PVP solution (50 mL, 66 mM in monomer unit; RPVP ¼ 100, RPVP
phene supported bimetallic Co/Pt nanohybrid for hydrogen is dened as the molar ratio of PVP in monomer units to the
3
2
generation from NaBH
4
solution. And our group found that total metal ions) in a 250 mL two-neck ask and then stirred for
ꢂ
the catalytic activity of alloy-structured Au50Ni50 BNPs in 30 min at 0 C. This was followed by addition of an aqueous
ꢂ
hydrogen generation from the hydrolysis of NaBH
4
was several solution of NaBH
4
(10 mL, 16.5 mM, 0 C; RNaBH4 ¼ 5, RNaBH4 is
times higher than that of corresponding Au and Ni MNPs.
dened as the molar ratio of NaBH to the total metal ions) at a
4
Although much less work has been done to date on the rate of one drop per 3 seconds to the mixed solution under
synthesis and catalytic activity of TNPs comparing with the case vigorous stirring. And nally the colloidal dispersions of Ni/Au/
ꢂ
of BNPs, it should be pointed that the studies on the former Co TNPs were obtained aer further 1 h stirring at 0 C.
have been increasing rapidly, especially, during the past few
33
years. For example, Chan et al. found that Pt/Fe/Ni trimetallic
nanocatalysts with ultralow Pt loading showed a much better
2.3 Characterization of nanoparticles
oxygen reduction reaction (ORR) activity in fuel cells than a pure Ultraviolet and visible light (UV-Vis) absorption spectra were
Pt electrode catalyst. On the other hand, Au Pt Pd TNPs recorded over 200–800 nm using a Shimadzu 2550 recording
6
0
10
30
were reported to show a better catalytic activity for the glucose spectrophotometer equipped with a quartz cell with an optical
34
oxidation than their counterparts of MNPs and BNPs.
path length of 10 mm. Transmission electron microscopy (TEM)
Furthermore, core–shell structured-Pd/Co@Pt TNPs were found images were taken at the accelerated voltage of 80 kV using an
2
to show a greatly improved electrocatalytic performance in the FEI Tecnai G 50-S-TWIN TEM. The samples were obtained by
35
ORR compared to commercial Pt/C catalyst. Ni/Ag/Pd and dropping one or two droplets of the prepared colloidal ethanol
Ni40Au15Pd45 TNPs were reported to be catalytically effective, solution onto a copper microgrid covered with a thin amor-
achieving nearly 100% selectivity in the dehydrogenation phous carbon lm and followed by evaporating the water in air
36,37
38
of formic acid.
In addition, Basu et al. revealed that at room temperature. For each sample, generally at least 200
Ir Pt Sn /C electro-catalysts exhibited higher activity towards particles from different locations on the grid were selected to
5
20 15
ethanol oxidation than the corresponding BNPs.
evaluate the mean diameter. High-resolution transmission
Herein, we report a facile one-pot approach to the prepara- electron microscopy (HR-TEM) images were taken at the accel-
tion of PVP (poly (N-vinyl-2-pyrrolidone)) stabilized colloidal Ni/ erated voltage of 200 kV using a JEM-2100F Field Emission
Au/Co TNPs catalysts using NaBH
rst time. The effects of atomic ratios on the size distribution of powder TNPs was carried out on D/MAX-RB RU-200B rotation
and catalytic activity in hydrogen generation from alkaline anode high power X-ray diffractometer (Cu Ka radiation at a
4
as a reduction agent for the High-resolution TEM. Powder X-ray diffraction (XRD) analysis

ꢂ
ꢀ1
NaBH solutions were also examined. It was demonstrated that scanning rate of 0.026 s , room temperature). All the image
4
as-prepared alloy-structured Ni Au Co TNPs with an average analyses were processed by using the iPP soware package. X-
4
5
45
10
diameter of 1.9 ꢁ 0.6 nm showed much greater catalytic activity ray photoelectron spectroscopy (XPS) measurement was per-
in hydrogen generation than the corresponding MNPs and formed using a Quantum 2000 spectrometer with Al Ka radia-
BNPs. The corresponding apparent activation energy of tion. Binding energies (BE) were normalized by taking the C (1s)
Ni45Au45Co10 TNPs for hydrogen generation was also deter- BE of adventitious carbon contamination as 284.6 eV, and the
mined by using the Arrhenius method.
analyses on Au were based on Au 4f5/2 and Au 4f7/2 peaks.
2. Experimental section
2.4 Catalytic activity of Ni/Au/Co TNPs
2
.1 Raw materials
Hydrogen tetrachloroaurate tetrahydrate (HAuCl
9.9%), cobalt(II) chloride hexahydrate (CoCl $6H O, 99%),
nickel(II) chloride hexahydrate (NiCl $6H O, 99.0%), sodium
The catalytic activity of as-prepared colloidal NPs was evaluated
based on the volume of hydrogen generated from hydrolysis of
4
2
$4H O,
an alkaline NaBH solution. Experiments were carried out in a
4
9
2
2
two-necked round-bottom ask with one opening connected to
a gas burette and the other to an addition funnel with a
pressure-equalization arm. The volume of H generated was
2
2
2
borohydride (NaBH , 96.0%), PVP (K30, average molecular
4
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 purication.
measured by the displacement level of water in a burette at
room temperature, and the temperature for the activity evolu-
ꢂ
tion was maintained at 30 C using a water bath. All the catalytic
experiments were carried out aer vigorous stirring of the
quantied colloidal catalysts for 10 min to remove the residual
2.2 Preparation of PVP-stabilized Ni/Au/Co TNPs
PVP-stabilized Ni/Au/Co TNPs were prepared by adding NaBH4 H in the solution and to minimize the temperature difference
2
dropwise to a mixture of the corresponding metal precursors. between the colloidal catalyst and the water bath. In all catalytic
For example, Ni50Au10Co40 TNPs (hereaer, the numbers in the activity tests, the molar ratio of metals in the colloidal catalyst
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4
mixture (40 mL) to the NaBH in alkaline solution (10 mL, 30
mM, pH 12) was controlled at 0.05.
In the reusability evaluation of the as-prepared TNPs, the
catalyst was re-used immediately in a new batch aer the
4
previous test. A new basic NaBH aqueous solution (10 mL, 30
mM and pH 12) was added into the ask to regenerate
hydrogen. Such cycle was repeated 7 times under ambient
ꢂ
atmosphere at 30 C.
2
.5 Density functional theory (DFT) calculation
Fig. 1 UV-Vis spectra of colloidal dispersions of Au, Co, Ni, Ni50Au50
Ni50Au30Co20 and Ni50Au10Co40 NPs ([M ] ¼ 0.66 mM, RNaBH4 ¼ 5,
,
First-principles calculations were performed with the density
n+
3
39
functional theory (DFT) using the Dmol program. All electron RPVP ¼ 100).
relativistic core treatment and doubled numerical basis set with
polarization functions, along with the Perdew–Burke–Ernzerhof
40
(
PBE) generalized gradient approximation (GGA) exchange-
correlation functions were used to perform full optimization
of the investigated Ni25Au Co24 model clusters without
prepared colloidal TNP catalysts are alloy-structured since the
suppression of the Au plasmon peaks was evidently observed
with Au content decreasing.
6
symmetry constraint. Spin-restricted self-consistent-eld (SCF)
calculations were carried out with SCF convergence criterion set
to the root-mean-square change in the electronic density less
TEM images were taken to characterize the average size and
size distributions of the prepared NPs, a representative set of
TEM images along with size distribution histograms are shown
in Fig. 2. The average particle sizes of Au, Ni Au , Co Au ,
ꢀ
5
than of 10 eV. The convergence criteria applied for geometry
optimization were enforced to 0.00002 au for energy, 0.004 au
5
0
50
50
50
Ni Au Co and Ni Au Co NPs are 3.0 ꢁ 1.3 nm, 2.7 ꢁ 0.8
5
0
30
20
50
10
40
ꢀ
1
˚
A
˚
for force, and 0.005 A for maximum displacement. Mulliken
nm, 2.8 ꢁ 1.1 nm, 1.5 ꢁ 0.4 nm and 2.1 ꢁ 0.5 nm, respectively. It
can be concluded that the average sizes of all the evenly
dispersed spherical TNPs with a relatively narrower size distri-
bution are smaller than those of the prepared Au MNPs, and
Ni50Au50 and Co50Au50 BNPs. In the cases of Co and Ni MNPs,
our results (TEM not shown here) revealed that it is difficult to
prepare them with a very small size by using the present NaBH4
reduction method. As a result, Co and Ni NPs with an average
population analysis was performed to investigate the atomic
charges distribution.
3. Results and discussion
3
.1 Structure and catalytic activity of Ni/Au/Co TNPs
Our previous results indicated that alloy-structured Ni Au
50
5
0
BNPs prepared by dropwise addition of NaBH
times higher catalytic activity for hydrogen generation from the results conrmed that TNPs rather than physical mixtures of
hydrolysis of NaBH
aqueous solution than corresponding Au Au, Ni and Co MNPs or mixtures of MNPs and BNPs were
and Ni MNPs. Nevertheless, it is still far from practical obtained.
4
showed several size of about several tens nanometers were obtained. These
4
32
application considering the relatively high Au content (50
To further investigate the structure of the as-prepared
atom%) in such Au-containing catalysts and their not-high- TNPs, crystallographic lattice fringe analysis based on HR-
enough catalytic activity (800 mol-H per h per mol-M). Thus, TEM images of Ni50Au10Co40 was also carried out. As shown
2
partial substitution of Au by Co to prepare Ni/Au/Co TNPs was in Fig. 3, for particle-1 and particle-3, the measured average

rstly investigated in this work by keeping the Ni content at 50 interplanar distances of 0.217 nm and 0.218 nm did not
atom% and varying the Co content from 20 to 40 atom%. Au, Ni, match with any face in pure Au, Ni or Co NPs. However, the
Co, Ni50Au50, Ni50Au30Co20 and Ni50Au10Co40 NPs were prepared measured average interplanar distances of the TNPs were
by dropwise addition of NaBH
4
under identical conditions of between the interplanar distances of (111) face of Ni (0.1992
n+
[
M ] ¼ 0.66 mM, RNaBH4 ¼ 5, and RPVP ¼ 100. The UV-Vis nm; Ni: JCPDS 88-2326), Co (0.2047 nm; Co: JCPDS 15-0806)
spectra of as-prepared NPs are shown in Fig. 1. The spectrum and Au (0.2355 nm; Au: JCPDS 89-3697). Similarly, the inter-
of Au MNPs exhibits a plasmon absorbance peak around 520 planar distances of 0.133 nm and 0.185 nm for particle-2 and
41–44
nm which is consistent with those reported previously.
On particle-4 lie between the lattice spacing of (220) and (200)
the other hand, the spectrum of aqueous dispersed Ni and Co face of Ni (0.1220 nm for 220 face and 0.1725 nm for 200 face;
MNPs displays a featureless and monotonically increasing Ni: JCPDS 88-2326), Co (0.1253 nm for 220 face and 0.1772 nm
absorbance toward shorter wavelength. In the case of the for 200 face; Co: JCPDS 15-0806) and Au (0.1442 nm for 220
prepared TNPs, the overall absorbance strength dampens with face and 0.2040 nm for 200 face; Au: JCPDS 89-3697) respec-
increasing the Co contents from 20 to 40 atom%, suggesting the tively. These results further conrmed that alloy-structured
different compositions of the TNPs. And the obvious differences TNPs were formed and the formation of individual Au, Co
between the spectra of the Ni Au Co and Ni Au Co TNPs and Ni MNPs in the as-prepared NPs can be ruled out. XRD
5
0
30
20
50
10
40
and those of the corresponding MNPs and BNPs indicate that was also used to characterize the structure of the prepared
the composition of the formed TNPs were different from that of TNPs (Fig. S1†). It showed that 2 diffraction peaks at 2q ¼
ꢂ
ꢂ
MNPs or BNPs. It can be reasonably deduced that the as- 38.42 and 44.68 can be observed in the XRD spectrum of the
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Fig. 2 TEM micrographs and size distribution histograms of Au, Ni50Au50, Co50Au50, Ni50Au30Co20 and Ni50Au10Co40 NPs ([M ] ¼ 0.66 mM,
R
NaBH4 ¼ 5, RPVP ¼ 100).
Ni45Au45Co10 TNPs (Fig. S1†). Further analysis proved that the
The catalytic activities of the as-prepared Au, Ni, Co,
ꢂ
ꢂ
peaks centered at 38.42 and 44.68 are not well in consistent Ni50Au50, Co50Au50, Ni50Co50, Ni50Au30Co20 and Ni50Au10Co40
ꢂ
with the (111) and (200) face of the bulk Au (38.179 and NPs in hydrogen generation from alkaline NaBH
4
solution were
ꢂ
4
4.375 , respectively; Au: JCPDS 89-3697). It seems that they evaluated and compared (shown in Fig. 4). Although the
are located among the corresponding diffraction peak of the prepared TNPs did not show higher activity for hydrogen
three corresponding monometallic nanoclusters (Table S1†), generation (about 790 mol-H per h per mol-M) than Ni Au
50
2
50
2
this result is also in agreement with the results of the HRTEM (about 800 mol H /(h mol-M)), however, it should be pointed
in Fig. 3.
out that the prepared Ni50Au10Co40 and Ni50Au30Co20 TNPs are
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Fig. 5 UV-Vis spectra of colloidal dispersions of Ni(50ꢀx/2)Au(50ꢀx/2)Co
x ¼ 10, 20, 30, 40) TNPs ([M ] ¼ 0.66 mM, RNaBH4 ¼ 5, RPVP ¼ 100).
x
n+
(
n+
nm and 1.9 ꢁ 0.6 nm, respectively (Fig. 6). The catalytic activi-
Fig. 3 HR-TEM image of as-prepared Ni50Au10Co40 TNPs ([M ] ¼
.66 mM, RNaBH4 ¼ 5, RPVP ¼ 100).
0
ties of the TNPs were also evaluated for the hydrolysis of alka-
line NaBH
Ni45Au45Co10 TNPs showed the highest catalytic activity of 1170
mol-H per h per mol-M (Fig. 7) which was about 1.5, 8, 10 and
4 times greater than that of Ni Au BNPs, Au, Ni and Co
4
aqueous solution. The results indicated that the
2
1
5
0
50
MNPs respectively (Fig. 4 and Table S2†), and at least 2.5 times
greater than that of the physically mixed catalyst of Au and Ni
32
MNPs in the molar ratio of 50/50. These results suggested once
again that the prepared NPs were Ni/Au/Co TNPs, not physical
mixtures of the corresponding MNPs and/or BNPs.
The reusability of the Ni45Au45Co10 TNPs for hydrogen
generation from alkaline NaBH was also investigated. The
4
catalyst was re-used immediately in a new batch aer the
previous test. The catalytic activity of Ni45Au Co TNPs
45
10
sharply decreased aer use in the rst run, and about 60%
and 25% of the initial catalytic activity were maintained aer
the rst and seventh cycles, as shown in Fig. S2 and S3.† The
deactivation of the Ni45Au45Co10 TNPs can be ascribed to the
agglomeration of the TNPs aer the catalytic reaction, and
the agglomeration can be clearly observed from the TEM
images shown in Fig. S4† which demonstrated that the
originally well dispersed colloidal Ni Au Co TNPs were
Fig. 4 Comparison of catalytic activity of Au, Ni, Co, Ni50Au50
,
n+
Co50Au50, Ni50Co50, Ni50Au30Co20 and Ni50Au10Co40 NPs ([M ] ¼
0
.66 mM, RNaBH4 ¼ 5, RPVP ¼ 100).
much cheaper catalysts for hydrogen generation from alkaline
NaBH solution than Ni50Au50 BNPs taking the Au% into
4
4
5
45
10
consideration.
got together aer seven catalytic runs. Although the reus-
ability of the prepared TNPs is still far from satisfying, it
should be pointed out that, to the best of our knowledge, this
has been the rst report to date on the preparation and
catalytic activity of Ni/Au/Co TNPs for hydrogen generation
It is well known that the catalytic activity of NPs is deter-
mined by its structure and composition. To get a thorough
understanding of the effect of composition on the particle size
and catalytic activity of the Ni/Au/Co TNPs in hydrogen gener-
ation, another series of Ni(
Au
Co (x ¼ 10, 20, 30, 40)
50ꢀx/2)
(50ꢀx/2)
x
4
from hydrolysis of NaBH .
TNPs were also synthesized by varying the Co content while
xing the Au : Ni (atom%) at 1 : 1 (this was because our previous
results showed that the Ni50Au50 NPs possessed the highest
Moreover, the effect of temperature on the catalytic
activity of the as-prepared TNPs was also investigated using
Ni45Au45Co10 and Ni50Au10Co40 as example catalysts. The cata-
lytic properties were enhanced with increasing the reaction

32
catalytic activity in all the prepared Ni/Au BNPs ). The corre-
sponding UV-Vis spectra of the prepared TNPs (Fig. 5) show that
the spectra are similar to those of the Ni50Au30Co20 and
Ni Au Co TNPs synthesized earlier. The suppression of the
temperature. And the apparent activation energy (E
PVP-protected Ni45Au45Co10 TNPs in hydrogen generation from
the hydrolysis of alkaline NaBH solution was calculated by
a
) of the
5
0
10
40
4
characteristic absorption peak of Au NPs at 520 nm with the
reducing of Au content indicated again that alloy-structured Ni/
Au/Co TNPs were formed, which is in agreement with the results
in Fig. 1–4.
The average particle sizes based on TEM images of
Ni30Au30Co40, Ni35Au35Co30, Ni40Au40Co20 and Ni45Au45Co10
TNPs were determined as 3.1 ꢁ 0.6 nm, 3.1 ꢁ 0.9 nm, 2.5 ꢁ 0.6
using the Arrhenius method (Fig. 8). The slope of the linear plot
between the natural logarithm of the temperature-dependent
rate constant and the inverse of temperature is E /R, where R
a
is the universal gas constant. In terms of the Arrhenius plots
within the temperatures range from 303 to 318 K, E
lated to be 18.8 kJ mol for Ni45Au45Co10 and 91.6 kJ mol for
a
was calcu-
ꢀ1
ꢀ1
a
Ni50Au10Co40 TNPs (Fig. 8 and S5†). For comparison, the E values
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Fig. 6 TEM micrographs and size distribution histograms of Ni30Au30Co40, Ni35Au35Co30, Ni40Au40Co20 and Ni45Au45Co10 TNPs ([M ] ¼ 0.66
mM, RNaBH4 ¼ 5, RPVP ¼ 100).
Fig. 8 Linear fit of ln k to 1/T of Ni45Au45Co10 catalyst for hydrogen
generation from NaBH .
4
Fig. 7 Comparison of catalytic activity of Ni(50ꢀx/2)Au(50ꢀx/2)Co
0, 20, 30, 40) TNPs ([M ] ¼ 0.66 mM, RNaBH4 ¼ 5, RPVP ¼ 100).
x
(x ¼
n+
1
3.2 Correlation between catalytic activity of the TNPs and
the electronic structure
3
2
45
30
46
of Ni Au , Co/Cu/a-Al O , Ni/Fe–B, Ru/montmorillonite,
5
0
50
2 3
47
48
49
The higher catalytic activity of the Ni/Au/Co TNPs than that of
the corresponding MNPs and BNPs could be attributed to the
effects of electronic charge transfer between Ni, Au and Co
atoms. Such effects were conrmed previously to be responsible
Co/Ru–B, Co–P and Au/Co–Ti catalysts were reported to be
ꢀ
1
ꢀ1
ꢀ1
ꢀ1
3
0.3 kJ mol , 52 kJ mol , 57 kJ mol , 54.7 ꢁ 1 kJ mol , 48.8
ꢀ1 ꢀ1 ꢀ1
kJ mol , 48.1 kJ mol and 37.9 kJ mol respectively. These
results suggested that the Ni45Au45Co10 TNPs prepared in this
50–52
for the high catalytic activities of several types of BNPs
and
4
work were excellent catalysts for the hydrolysis of NaBH .
34
TNPs. It can be reasonable deduced that there exist at least
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investigated by using high-resolution XPS with monochromated
Al K electron radiation. The Ni Au Co TNPs stabilized by
a
50
10
40
low content of PVP (RPVP ¼ 5) were prepared for the character-
ization. XPS results (shown in Fig. S6†) showed that the electron
apparent BE of Au 4f7/2 was 83.85 eV for the Ni50Au10Co40 TNPs,
this is about 0.10 eV lower than that in bulk Au (83.95 eV). This
result provided a direct evidence for the presence of the nega-
tively charged Au atoms in the TNPs. As for Ni and Co atoms, the
Fig. 9 Schematic illustration of electronic charge transfer effects in electron BE values of Ni 2p3/2 and Co 2p3/2 were 855.80 eV and
Ni/Au/Co TNPs.
2+
7
81.05 eV, respectively, suggesting the formation of Ni and
2
+
2+
2+
Co ions. The formation of Ni and Co ions can be attributed
to the oxidation of the TNPs during the XPS measurement. Even
so, it can be still considered that Ni, Au and Co atoms in the
TNPs play dominant roles in the catalytic activity for hydrogen
generation from NaBH
tion of the TNPs was carried out in a N
4
solution considering that the prepara-
atmosphere, and the
2
TNPs in all the catalytic activity tests were protected by a suffi-
cient amount of PVP (RPVP ¼ 100).
The electronic charge transfer effects and the existence of
negatively charged Au and Co atoms, and positively charged Ni
atoms in the synthesized TNPs were further conrmed by the
DFT calculations of M55 model NPs with the composition of
6
Ni25Au Co24 (whose composition is similar to that of the
Ni50Au10Co40 TNPs). As seen from Fig. 10, the Au and Co atoms
in the M55 model are indeed negatively charged, whereas the Ni
atoms are positively charged. The DFT calculation results
suggest again the presence of electronic charge transfer effects
among Au, Ni and Co atoms in the TNPs.
6
Fig. 10 DFT calculations of electronic structure of Ni25Au Co24 TNPs
(
Ni atoms: the large light blue balls locate at top sites; Au atoms: the
large yellow balls locate at face sites; and Co atoms: the small dark
blue balls locate at edge sites).
4
. Conclusions
three types of charge transfer effects in the prepared Ni/Au/Co
TNPs due to the differences in the ionization energy values
A series of PVP-protected Ni50Au
x
Co50ꢀx and Ni(50ꢀx/2)Au(50ꢀx/2)Co
x
(
x ¼ 10, 20, 30, 40) TNPs with an alloyed structure were synthesized
(Ni: 7.63 eV; Au: 9.22 eV; and Co: 7.86 eV): (1) charge transfer
by co-reduction of corresponding metal precursors via dropwise
from individual Co atoms to Au atoms; (2) charge transfer from
individual Ni atoms to Au atoms; and (3) electron donation
from Ni to Co and then to Au atoms. The possible electronic
charge transfer routes in Ni/Au/Co TNPs are illustrated in Fig. 9.
The negatively charged Au and Co atoms, and positively charged
Ni atoms resulted from the electron charge transfer effects can
act as the catalytic active sites for the hydrogen generation from
addition of NaBH solution. Among the corresponding MNPs,
BNPs and TNPs, as-prepared Ni Au Co TNPs exhibited the
highest catalytic activity with a value of 1170 mol-H
mol-M in hydrogen generation from hydrolysis of NaBH
This could be attributed to the presence of negatively charged
Au and Co atoms and positively charged Ni atoms, which was
conrmed by both XPS and DFT calculations. The apparent
activation energy of the prepared Ni45Au45Co10 TNPs in
4
4
5
45
10
2
per h per
4
.
alkaline NaBH aqueous solution. In can be considered that the
4
negatively charged Au and Co atoms may facilitate the breaking
of the H–O bonds in H O molecules, in which the two hydrogen
2
atoms are in one-positive-valence. On the other hand, the
positively-charged Ni atoms may promote the breaking of the
hydrolysis of NaBH aqueous solution was determined as
4
ꢀ
1
18.8 kJ mol .
4
B–H chemical bonds in NaBH molecules. And then a hydrogen
molecule is formed via combination of the two hydrogen atoms Acknowledgements
produced from H O and NaBH molecules, respectively. We
consider that the enhanced catalytic activity of the TNPs could
be resulted from the synergetic effect between the charged Ni,
4 2
Au and Co atoms and NaBH /H O molecule.
To conrm the electronic charge transfer effects and the
presence of the negatively charged Au and Co atoms and posi-
tively charged Ni atoms in the TNPs, the electronic character-
istics of the surface atoms of Ni50Au10Co40 TNPs were
2
4
This work was nancially supported by National Natural
Science Foundation of China (General program, 51272188,
51472184, 51472185), State Basic Research Development
Program of China (973 Program, 2014CB660802), Natural
Science Foundation of Hubei Province, China (Contract no.
2
013CFA086) and Foreign cooperation projects in Science and
Technology of Hubei Province, China (Contract no.
013BHE002).
2
34370 | RSC Adv., 2015, 5, 34364–34371
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6 Y. H. Chen and C. Y. Pan, Int. J. Hydrogen Energy, 2014, 39(4),
References
1648.
1
A. Hamaed, T. K. A. Hoang, G. Moula, R. Aroca, 27 H. J. Zhang, T. Watanabe, M. Okumura, M. Haruta and
M. L. Trudeau and D. M. Antonelli, J. Am. Chem. Soc., 2011,
N. Toshima, Nat. Mater., 2012, 11(1), 49.
33(39), 15434.
R. Moury, U. B. Demirci, V. Ban, Y. Filinchuk, T. Ichikawa,
1
28 H. J. Zhang, T. Watanabe, M. Okumura, M. Haruta and
2
N. Toshima, J. Catal., 2013, 305, 7.
L. Zeng, K. Goshome and P. Miele, Progr. Aero. Sci., 2013, 29 J. Du, F. Y. Cheng, M. Si, J. Liang, Z. L. Tao and J. Chen, Int. J.
0, 45.
Hydrogen Energy, 2013, 38(14), 5768.
C. Y. Duan, H. Wang, X. M. Ou, F. Li and X. H. Zhang, ACS 30 L. H. Ai, X. M. Liu and J. Jiang, J. Power Sources, 2014, 257,
Appl. Mater. Interfaces, 2014, 6(12), 9742.
213.
G. Cipriani, V. Di-Dio, F. Genduso, D. La-Cascia, R. Liga, 31 S. Saha, V. Basak, A. Dasgupta, S. Ganguly, D. Banerjee and
6
3
4
R. Miceli and G. Ricco-Galluzzo, Int. J. Hydrogen Energy,
014, 39(16), 8482.
K. P. Brooks, T. A. Semelsberger, K. L. Simmons and B. van-
Hassel, J. Power Sources, 2014, 268, 950.
P. J. R. Pinto, T. Sousa, V. R. Fernandes, A. M. F. R. Pinto and
C. M. Rangel, Int. J. Electr. Power. Energ. Syst., 2013, 49, 57.
T. Ou, A. Barbucci, P. Carpanese, S. Congiu and M. Panizza,
Int. J. Hydrogen Energy, 2014, 39(21), 11094.
K. Kargupta, Int. J. Hydrogen Energy, 2014, 39(22), 11566.
32 X. F. Wang, S. R. Sun, Z. L. Huang, H. J. Zhang and
S. W. Zhang, Int. J. Hydrogen Energy, 2014, 39(2), 905.
33 B. S. Li and S. H. Chan, Int. J. Hydrogen Energy, 2013, 38(8),
3338.
34 H. J. Zhang, L. L. Lu, Y. L. Cao, S. Du, Z. Cheng and
S. W. Zhang, Mater. Res. Bull., 2014, 49, 393.
35 M. A. Matin, J. H. Jang and Y. U. Kwon, Int. J. Hydrogen
Energy, 2014, 39(8), 3710.
2
5
6
7
8
9
J. H. Wee, K. Y. Lee and S. H. Kim, Fuel Process. Technol.,
2006, 87(9), 811.
36 M. Yurderi, A. Bulut, M. Zahmakiran and M. Kaya, Appl.
Catal., B, 2014, 160–161(1), 514.
L. He, Y. Q. Huang, A. Q. Wang, Y. Liu, X. Y. Liu, X. W. Chen,
J. J. Delgado, X. D. Wang and T. Zhang, J. Catal., 2013, 298, 1. 37 Z. L. Wang, Y. Ping, J. M. Yan, H. J. Wang and Q. Jiang, Int. J.
0 C. Lucarelli, S. Albonetti, A. Vaccari, C. Resini, G. Taillades,
Hydrogen Energy, 2014, 39(10), 4850.
J. Roziere, K. E. Liew, A. Ohnesorge, C. Wolff, I. Gabellini 38 J. Tayal, B. Rawat and S. Basu, Int. J. Hydrogen Energy, 2011,
and D. Wails, Catal. Today, 2011, 175(1), 504.
36(22), 14884.
1 Y. Tong, X. F. Lu, W. N. Sun, G. D. Nie, L. Yang and C. Wang, 39 B. Delley, J. Chem. Phys., 2000, 113(18), 7756.
1
1
1
1
1
1
1
J. Power Sources, 2014, 261, 221.
2 C. Q. Hu, J. K. Pulleri, S. W. Ting and K. Y. Chan, Int. J.
Hydrogen Energy, 2014, 39(1), 381.
3 C. Crisafulli, S. Scir `e , M. Salanitri, R. Zito and S. Calamia, Int.
J. Hydrogen Energy, 2011, 36(6), 3817.
4 X. J. Li, G. Y. Fan and C. M. Zeng, Int. J. Hydrogen Energy,
40 J. P. Perdew, K. Burke and M. Ernzerhof, Phys. Rev. Lett.,
1996, 77, 3865.
41 X. Wang, X. J. Kong, Y. Yu and H. Zhang, J. Phys. Chem. C,
2007, 111(10), 3836.
42 R. Salvati, A. Longo, G. Carotenuto, S. De-Nicola, G. P. Pepe,
L. Nicolais and A. Barone, Appl. Surf. Sci., 2005, 248(1–4), 28.
43 H. L. Yue, Y. J. Hu, H. G. Huang, S. Jiang and B. Tu,
Spectrochim. Acta, Part A, 2014, 130, 402.
2014, 39(27), 14927.
5 Y. H. Li, Q. Zhang, N. W. Zhang, L. H. Zhu, J. B. Zheng and
B. H. Chen, Int. J. Hydrogen Energy, 2013, 38(30), 13360.
6 F. Gunnarsso, M. Z. Granlund, M. Englund, J. Dawody,
L. J. Pettersson and H. H ¨a relind, Appl. Catal., B, 2015, 162,
44 X. F. Wang, Z. L. Huang, L. L. Lu, H. J. Zhang, Y. L. Cao,
Y. J. Gu, Z. Cheng and S. W. Zhang, J. Nanosci.
Nanotechnol., 2014, 14, 1.
5
83.
45 R. Chamoun, U. B. Demirci, Y. Zaatar, A. Khoury and
P. Miele, Int. J. Hydrogen Energy, 2010, 35(13), 6583.
1
7 Y. Zhang, D. A. J. M. Ligthart, X. Y. Quek, L. Gao and
E. J. M. Hensen, Int. J. Hydrogen Energy, 2014, 39(22), 11537. 46 S. G. Peng, X. J. Fan, J. Zhang and F. Y. Wang, Appl. Catal., B,
8 M. Rakap, Appl. Catal., B, 2015, 163, 129.
2013, 140–141, 115.
9 J. Zhu, R. Li, W. L. Niu, Y. J. Wu and X. L. Gou, Int. J. Hydrogen 47 W. L. Wang, Y. C. Zhao, D. H. Chen, X. Wang, X. L. Peng and
Energy, 2013, 38(25), 10864.
J. N. Tian, Int. J. Hydrogen Energy, 2014, 39(28), 16202.
0 L. J. Jin, H. H. Si, J. B. Zhang, P. Lin, Z. Y. Hu, B. Qiu and 48 X. W. Zhang, J. Z. Zhao, F. Y. Cheng, J. Liang, Z. L. Tao and
1
1
2
2
2
2
2
2
H. Q. Hu, Int. J. Hydrogen Energy, 2013, 38(25), 10373.
1 L. H. Ai, X. Y. Gao and J. Jiang, J. Power Sources, 2014, 257, 49 L.
J. Chen, Int. J. Hydrogen Energy, 2010, 35(15), 8363.
A.
Tama ˇs auskait ˙e -Tama ˇs i u¯ nait ˙e , Jagminien ˙e ,
ˇ
˙
ˇ
ˇ
213.
A. Bal ˇc i u¯ nait ˙e , A. Zabielait ˙e , A. Zieliene, L. Naruskevicius,
J. Vai ˇc i u¯ nien ˙e , A. Selskis, R. Ju ˇs k ˙e nas and E. Norkus, Int. J.
Hydrogen Energy, 2013, 38(33), 14232.
2 G. Garbarino, P. Riani, M. A. Lucchini, F. Canepa, S. Kawale
and G. Busca, Int. J. Hydrogen Energy, 2013, 38(1), 82.
¨
50 H. J. Zhang, J. Okuni and N. Toshima, J. Colloid Interface Sci.,
2011, 354(1), 131.
3 F. Baydaroglu, E. Ozdemir and A. Hasimoglu, Int. J. Hydrogen
Energy, 2014, 39(3), 1516.
4 Y. C. Luo, T. H. Liu, Y. Hung, X. Y. Liu and C. Y. Mou, Int. J. 51 H. J. Zhang and N. Toshima, J. Colloid Interface Sci., 2013,
Hydrogen Energy, 2013, 38(18), 7280.
394, 166.
5 S. Gupta, N. Patel, R. Fernandes, D. C. Kothari and 52 H. J. Zhang, N. Toshima, K. Takasaki and M. Okumura, J.
A. Miotello, Int. J. Hydrogen Energy, 2013, 38(34), 14685.
Alloys Compd., 2014, 586, 462.
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RSC Adv., 2015, 5, 34364–34371 | 34371