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i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 1 1 7 9 4e1 1 8 0 1
Avai lab le at www.sc iencedi rect .com
journa l homepage : www.e lsev ie r . com/ loca te /he
Nickel-palladium nanoparticle catalyzed hydrogen generationfrom hydrous hydrazine for chemical hydrogen storage
Sanjay Kumar Singh, Yasuo Iizuka, Qiang Xu*
National Institute of Advanced Industrial Science and Technology (AIST), Ikeda, Osaka, Japan
a r t i c l e i n f o
Article history:
Received 5 March 2011
Received in revised form
6 June 2011
Accepted 12 June 2011
Available online 13 July 2011
Keywords:
Ni-Pd nanocatalysts
Bimetallic
Hydrous hydrazine
Hydrogen generation
* Corresponding author. Tel.: þ81 72 751 956E-mail address: [email protected] (Q. Xu).
0360-3199/$ e see front matter Copyright ªdoi:10.1016/j.ijhydene.2011.06.069
a b s t r a c t
In this study, we report Ni-Pd bimetallic nanoparticle catalysts (nanocatalyst) (Ni1-xPdx)
synthesized by alloying Ni and Pd with varying Pd contents, which exhibit appreciably high
H2 selectivity (>80% at x ¼ 0.40) from the decomposition of hydrous hydrazine at mild
reaction condition with Ni0.60Pd0.40 nanocatalyst, whereas the corresponding monometallic
counterparts are either inactive (Pd nanoparticles) or poorly active (Ni nanoparticles
exhibit 33% H2 selectivity). In addition to powder X-ray diffraction (XRD), X-ray photo-
electron spectra (XPS) analysis and electron microscopy (TEM/SEM), the structural and
electronic characteristics of Ni-Pd nanocatalysts were investigated and established using
extended X-ray absorption fine structure (EXAFS) analysis. Unlike the high activity of Ni-Pd
nanocatalysts, Pd-M (M ¼ Fe, Co and Cu) bimetallic nanocatalysts exhibit poor catalytic
activity. These results imply that alloy composition of Ni-Pd nanocatalysts is critical, where
the co-existence of both the metals on the catalyst active surface and the formation of
inter-metallic Ni-Pd bond results in high catalytic performance for the decomposition of
hydrous hydrazine to hydrogen.
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
reserved.
1. Introduction storage materials, fuel cell electrocatalysis, catalytic reform-
Scientific and technological prospective of alloy or core-shell
nanostructures of bimetallic nanoparticles are of great
importance because of their interesting physical and chemical
properties, bringing into effect from the inter-metallic
combinations of different metals [1e3]. The hetero-metallic
bond formation with the introduction of a second metal
results from the inter-metallic charge transfer or orbital
hybridization of the metals. These electronic-structural
modifications drastically influence the catalytic performance
of the mixed-metal catalyst systems [4e9]. Various bimetallic
nanoparticles have been extensively investigated over past
decades for various important catalytic processes, such as
catalytic hydrogen generation from chemical hydrogen
2; fax: þ81 72 751 7942.
2011, Hydrogen Energy P
ing, oxidation-reduction organic reactions and so on [4e16].
Chemical hydrogen storagematerials are of particular interest
among scientific society due to their high hydrogen capacities,
which is one of the key requirements for developing
a hydrogen-based society [17e21]. However, no single mate-
rial investigated to date fulfills all the necessary storage and
transportation requirements, such as volumetric and gravi-
metric hydrogen capacities, handling pressure and tempera-
ture, recycling of byproduct, and so on [17e25]. Hydrous
hydrazine, such as hydrazine monohydrate (H2NNH2$H2O)
[26], a liquid having a hydrogen content available for hydrogen
release as high as 8.0 wt%, merits attention as a promising
hydrogen storage material due to its decomposition at mild
reaction conditions, easy recharging as a liquid and only
ublications, LLC. Published by Elsevier Ltd. All rights reserved.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 1 1 7 9 4e1 1 8 0 1 11795
production of nitrogen in addition to hydrogen by complete
decomposition via: H2NNH2 / N2 þ 2H2 (1) [27e35]. Because
nitrogen can be transformed to ammonia by the Haber-Bosch
process or an energy efficient electrolysis process and subse-
quently to hydrazine in a large scale [36e42], the key to exploit
the potentials of hydrazine as a hydrogen storage material is
the development of suitable catalysts that can avoid the
undesired reaction pathway (2): 3H2NNH2 / 4NH3 þ N2(g) (2).
Our recent explorations towards catalytic decomposition of
hydrous hydrazine to hydrogen with various mono- and
bimetallic nanocatalysts have shown that bimetallic alloy
nanocatalysts might possess catalytic performance superior
to their monometallic counterparts, which are either inactive
or poorly active for this reaction [27e30]. Among the various
bimetallic nanocatalysts studied for this reaction, we have
found that the catalytic performance of bimetallic nano-
catalysts can be significantly influenced by the composition
between the two constituent elements [28e30]. Herein we
have synthesized Ni-Pd bimetallic nanocatalysts by alloying
Ni and Pd with varying Pd contents, and characterized
extensively the electronic and structural properties of the Ni-
Pd nanocatalysts by extended X-ray absorption fine structure
(EXAFS) studies in combination with powder X-ray diffraction
(XRD), X-ray photoelectron spectra (XPS), and electron
microscopy (TEM/SEM). The catalytic activities of the
synthesized Ni-Pd nanocatalysts have been examined for the
decomposition of hydrous hydrazine to hydrogen at mild
reaction conditions. The synthesized Ni-Pd nanocatalysts
exhibit high catalytic performances with high H2 selectivities,
whereas the corresponding monometallic counterparts are
either inactive (Pd nanoparticles) or poorly active (Ni nano-
particles) under analogous reaction conditions.
2. Experimental method
2.1. Chemicals
Commercial chemicals were used as received for catalyst
preparation and hydrazine decomposition experiments.
Hydrazine monohydrate (H2NNH2$H2O, 99%), sodium boro-
hydride (NaBH4, 99%), hexadecyltrimethyl ammonium
bromide (CTAB, 95%), FeCl2$4H2O (95%) were obtained from
Sigma-Aldrich Co. K2PdCl4, CoCl2$6H2O (99.5%), NiCl2$6H2O
(99.9%) and CuCl2 (95%) were purchased from Wako pure
chemical Industries, Ltd.
2.2. Preparation of Ni1-xPdx (x ¼ 0.20e0.90)nanocatalysts
A series of Ni1-xPdx nanocatalysts (x ¼ 0.20e0.90) were
synthesized using a surfactant aided co-reduction method,
where x represents themolar portion of Pd. A typical synthetic
procedure for Ni0.60Pd0.40 is described here. An aqueous
suspension of NiCl2$6H2O (0.030 M), K2PdCl4 (0.020 M) and
CTAB (0.068 M), obtained by subsequent sonication and stir-
ring for 5 min, was reduced by NaBH4 (1.3 M). The content of
the flask was vigorously shaken to obtain the Ni0.60Pd0.40
nanocatalyst as a black suspension, which was then used for
the catalytic reaction. The concentration of NiCl2$6H2O and
K2PdCl4 used for the preparation of Ni1-xPdx (x ¼ 0.20e0.80)
were: 0.040 M and 0.010 M for Ni0.80Pd0.20; 0.035 M and 0.015 M
for Ni0.70Pd0.30; 0.032M and 0.017M for Ni0.65Pd0.35; 0.027M and
0.022 M for Ni0.55Pd0.45; 0.025 M and 0.025 M for Ni0.50Pd0.50;
0.020 M and 0.030 M for Ni0.40Pd0.60; 0.010 M and 0.040 M for
Ni0.20Pd0.80; 0.005 M and 0.045 M for Ni0.10Pd0.90, respectively.
2.3. Preparation of monometallic Ni and Pdnanocatalysts
An analogous synthetic procedure as used for the Ni-Pd
nanocatalyst was adapted for the preparation of mono-
metallic Ni and Pd nanocatalysts using only NiCl2$6H2O
(0.050 M) and K2PdCl4 (0.050 M), respectively.
2.4. Preparation of M0.60Pd0.40 (M ¼ Fe, Co, and Cu)nanocatalysts
A similar synthetic procedure as used for Ni0.60Pd0.40 was
adapted to synthesize M0.60Pd0.40 (M ¼ Fe, Co and Cu) nano-
catalysts using 0.030 M solutions of FeCl2$4H2O, CoCl2$6H2O,
and CuCl2, respectively, in place of NiCl2$6H2O.
2.5. Catalytic hydrazine decomposition experiments
Catalytic reactions were carried out following the previously
reported method [27e30].
2.6. Characterization of nanocatalysts
Nanocatalysts used for TEM, XPS, EXAFS and powder XRD
measurements were collected by centrifugation, washed with
water (5.0 mL, twice), ethanol (2.0 mL, twice) and acetone
(2.0 mL) and dried in vacuum at 323 K for 5 h. Powder X-ray
diffraction (XRD) studies were performed on a Rigaku RINT-
2000 X-ray diffractometer (Cu Ka). Observations by means of
transmission electron microscope (TEM, FEI TECNAI G2)
equipped with selected area electron diffraction (SAED) and
energy dispersed X-ray detector (EDS) were applied for the
detailed microstructure information. The TEM samples were
prepared by depositing a few droplets of the nanoparticle
suspension onto the copper grids coated by the amorphous
carbon, which were then dried under argon atmosphere. The
surface area measurements were performed by N2 adsorption
at liquid N2 temperature using automatic volumetric adsorp-
tion equipment (Belsorp II). XPS analysis was carried out on
a Shimadzu ESCA-3400 X-ray photoelectron spectrometer
using a Mg Ka source (10 kV, 10 mA). The Ar sputtering
experiments were carried out under the conditions of back-
ground vacuum 3.2 � 10�6 Pa, sputtering acceleration voltage
1 kV. Ni and Pd K-edge X-ray absorption near-edge structure
(XANES) as well as corresponding extended X-ray absorption
fine structure (EXAFS) measurements were taken in a trans-
mission mode at the room temperature at the beam line
BL14B2 at the Spring-8, Hyogo, Japan. The electron storage
ring was operated at 8 GeV. A double crystal Si(311) mono-
chromator was employed for energy selection. The incident
photon intensity was measured by an ion chamber filled with
80% N2-20% Ar gas mixture for Ni-K edge and 80% Ar-20% Kr
for Pd-K edge. The reference compounds used were Ni and Pd
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 1 1 7 9 4e1 1 8 0 111796
metal foils, NiO powder and PdO powder. The Fourier trans-
form (FT) of k3-weighted EXAFS were obtained using a range
over 1.8e18 A�1 by using Athena [43e46]. The curve-fitting for
the Fourier transformdatawere analyzed by Artemis, by using
the theoretical parameters based on FEFF [43e46].
3. Results and discussion
3.1. Synthesis and characterization of the nanocatalysts
Bimetallic Ni1-xPdx (x ¼ 0.20e0.90) nanocatalysts with various
compositions of Ni and Pd were synthesized via a surfactant
aided co-reduction synthetic process. NiCl2$6H2O and K2PdCl4were co-reduced in an aqueous solution using sodium boro-
hydride, a reductant, in the presence of hexadecyltrimethyl
ammonium bromide (CTAB), a surfactant, at room tempera-
ture. An orange suspension of the bimetallic salts (Ni2þ and
Pd2þ) quickly turned to a black suspension of Ni-Pd nano-
particles in thepresenceof the reductant.MonometallicNi and
Pd nanoparticles were prepared fromNiCl2$6H2O and K2PdCl4,
respectively, via an analogous procedure to that for the Ni-Pd
alloy nanocatalysts. Physical mixture of monometallic Ni and
Pd nanoparticles wasmade bymixing the separately prepared
single-component nanoparticles. The synthesized nano-
particles were fully characterized to investigate their struc-
tural and electronic properties using XRD, XPS and EXAFS
analyses. The structural properties of the representative
Ni0.60Pd0.40 nanocatalyst have been characterized by TEM. For
Fig. 1 e (a) TEM, (b) HAADF-STEM, (c) HRTEM (inset SAED) im
better understanding, the XPS and EXAFS data of the
Ni0.60Pd0.40 nanocatalyst have been compared with that of
monometallic counterparts (Ni and Pd nanoparticles) along
with other Ni-Pd nanocatalysts with different compositions
(e.g. Ni0.80Pd0.20 and Ni0.20Pd0.80 nanocatalysts).
TEM (Fig. 1a and Figure S1) and high angle annular dark
field scanning TEM (HAADF-STEM) images (Fig. 1b) of the
Ni0.60Pd0.40 nanocatalyst revealed the presence of irregularly
shaped nanoparticles with partial aggregation of nano-
particles. The high resolution TEM (HRTEM, Fig. 1c) and the
corresponding selected area electron diffraction (SAED, Fig. 1c
inset and Figure S1) patterns indicate the crystalline nature of
the Ni0.60Pd0.40 nanocatalyst. We have not observed any
evidences for separate Ni and Pd contrast in the HRTEM/
HAADF-STEM images of Ni-Pd nanoparticles, which sug-
gestes the existence of alloy state of Ni and Pd in the bimetallic
nanoparticles. Energy-dispersive X-ray analysis (EDS, Fig. 1d
and Figure S1) of the Ni0.60Pd0.40 nanocatalyst, collected at
multiple positions, exhibits the presence of both Ni and Pd
with an average atomic composition of 59% Ni and 41% Pd.
Powder X-ray diffraction (XRD, Figure S2) profile of the Ni-Pd
nanocatalysts, for the 2q range of 20�e90�, reveals the crys-
talline structures of the preparedNi-Pd nanocatalyst. The XRD
profile of Ni0.60Pd0.40 nanocatalyst shows typical face-centered
cubic ( fcc) diffraction peaks with 2q values of 40.3�, 45.6�, 68.2�
and 82.8� indexed to diffraction planes of (111), (200), (220) and
(311), respectively. The diffraction peaks (111) and (200) cor-
responding to 2q values of 40.3 and 45.6, respectively indicate
the formation of Ni-Pd alloy (JCPDS file No. 05-0681 (Pd) and
ages and (d) EDS spectrum of Ni0.60Pd0.40 nanocatalysts.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 1 1 7 9 4e1 1 8 0 1 11797
JCPDS file No. 04-0850(Ni)). No oxide or individual peaks of
pure Ni or Pd have been observed in the XRD profiles of the Ni-
Pd nanocatalysts. In contrast to the Ni0.60Pd0.40 nanocatalyst,
the XRD pattern (Figure S2) of the physical mixture of Ni and
Pd nanoparticles shows separate peaks of Ni and Pd. Consis-
tent with the XRD results, the characteristic signals for
metallic Ni[2p] and Pd [3d] can be observed in the X-ray
photoelectron spectra (XPS) for the bimetallic Ni0.60Pd0.40
nanocatalyst (Fig. 2), indicating the co-existence of both the
metals in the Ni-Pd nanocatalyst [47e49]. A thin oxide film
was observed on the surface of the Ni0.60Pd0.40 nanocatalyst,
presumably formed during the exposure of the sample to air,
however, it could be readily removed by Ar sputtering.
Meanwhile, signals with binding energies of 853.62 eV and
336.28 eV can be attributed to the Ni[2p3/2] core level of Ni0 and
the Pd[3d5/2] core level of Pd0, respectively, for the Ni0.60Pd0.40
nanocatalyst [49]. The shift in the Pd[3d5/2] levels to higher
binding energies for the bimetallic Ni0.60Pd0.40 nanocatalyst
relative to that for themonometallic Pd sample and the shift of
Ni[2p3/2] levels to lower energies relative to the monometallic
Ni sample are consistent with the alloy formation. Analogous
885 880 875 870 865 860 855 850
Ni 2p
Ni 2p
Ni 2p3/2Ni 2p
1/2
885 880 875 870 865 860 855 850
).
u.
a(
y
tis
ne
tn
I
Binding Energy (eV)
).u.
a(
yti
sn
et
nI
Binding Energy (eV)
346 344 342 340 338 336 334 332
Pd 3d
Pd 3d
Pd 3d5/2
Pd 3d3/2
346 344 342 340 338 336 334 332
).
u.
a(
y
ti
sn
et
nI
Binding Energy (eV)
).u.
a(
yti
sn
et
nI
Binding Energy (eV)
a
b
Fig. 2 e XPS patterns of Ni0.60Pd0.40 nanocatalysts showing
(a) Ni[2p] and (b) Pd [3d] core levels. (Insets show the XPS
patterns for the Ni[2p] and Pd [3d] core levels of (a) Ni and
(b) Pd nanoparticles, respectively).
characteristics of Ni[2p3/2] and Pd[3d5/2] bands have also been
observed for XPS spectra for the Ni0.20Pd0.80 and Ni0.80Pd0.20
nanocatalysts (Figure S3 and Figure S4). Ar sputtering for
186 min for the Ni-Pd nanocatalysts exhibits no significant
change in the relative intensities of the features due to Ni0 and
Pd0, which implies the presence of uniform alloy composition
for the Ni-Pd nanocatalysts. No Cl- and B- species are detected
in the XPS measurements for the Ni-Pd nanocatalysts. The
nitrogen adsorptionedesorption isotherms (Figure S5) of the
Ni0.60Pd0.40 nanocatalysts reveal the Brunauer-Emmett-Teller
(BET) surface area of 49.9 m2 g�1.
The XANES and derivative of XANES spectra for the Ni, Pd,
Ni0.80Pd0.20, Ni0.60Pd0.40 and Ni0.20Pd0.80 nanocatalysts, along
with those for reference materials, Ni foil, Pd foil, NiO and
PdO, are displayed in Fig. 3 and Figure S6. The XANES spec-
trum of Pd nanoparticles at Pd-K-edge shown in Fig. 3
resembles that of Pd metal foil, indicating the existence of
zero-valent Pd. The first derivative of XANES spectra for Pd
nanoparticles shown in Fig. 3 is also in good agreement with
the presence of zero-valent Pd [47,48,50e54]. Consistent with
the Pd nanoparticles, the XANES spectra of the Ni0.60Pd0.40
nanocatalyst also exhibit all characteristics of zero-valent Pd.
In addition, the shape of the edge spectra of the Ni0.60Pd0.40
nanocatalyst contrasts markedly to all the features of palla-
dium oxides, confirming the absence of palladium in oxidized
states. Moreover, the XANES and derivative of XANES spectra
for Ni0.80Pd0.20 and Ni0.20Pd0.80 nanocatalysts also resemble
with that of metallic Pd foil, indicating the presence of Pd in
zero-valent state [47,48]. At Ni K-edge, the XANES and deriv-
ative of XANES spectra (Figure S6) for Ni nanoparticles and Ni-
Pd alloys shows the presence of metallic nickel with oxides of
nickel. The thin film of oxides of nickel is presumably formed
during the sample preparation and is consistent with XPS
results (Figure S3 and Figure S4).
The Fourier transform of k3-weighted EXAFS spectra at Pd-
K-edge of the Ni0.60Pd0.40 nanocatalyst along with those for Pd
foil, Pd nanoparticles, Ni0.80Pd0.20 and Ni0.20Pd0.80 nano-
catalysts are shown in Fig. 4. The prominent peak between 2.0
24320 24340 24360 24380 24400 24420
0.0
0.2
0.4
0.6
0.8
1.0
1.2
(a)
(b)
(c)
(d)
(e)
(f)
E(eV)
).
u.
a(
ec
na
br
os
ba
de
zil
am
ro
N
24320 24340 24360 24380 24400 24420
-0.04
-0.02
0.00
0.02
0.04
0.06
0.08
E (eV)
Fig. 3 e XANES and first derivative of XANES (Inset) spectra
of (a) Pd foil, (b) Pd nanoparticles, (c) Ni0.20Pd0.80,
(d) Ni0.60Pd0.40, (e) Ni0.80Pd0.20 nanocatalysts, and (f) PdO at
the Pd K-edge.
0 1 2 3 4 5 6
0
10
20
30
40
(f)
(e)
(d)
(c)
(b)
(a)
k3
-w
eig
hted
F
ou
rie
r T
ran
sfo
rm
(a
.u
.)
R(Å)
Fig. 4 e The Fourier transform of k3-weighted EXAFS
spectra of (a) Pd foil, (b) Pd nanoparticles, (c) Ni0.20Pd0.80,
(d) Ni0.60Pd0.40, (e) Ni0.80Pd0.20 nanocatalysts, and (f) PdO at
the Pd K-edge.
0 50 100 150 200 250 300 350 400
0.0
0.5
1.0
1.5
2.0
2.5
3.0
(ix)
(i)(viii)
(vii)
(vi)
(v)
(iv)
(iii)
(ii)
n(H
2 +
N2) / n
(H
2N
NH
2)
Time (min)
0.00 0.20 0.40 0.60 0.80 1.00
0.0
0.5
1.0
1.5
2.0
2.5
3.0
n(H
2 +
N2) / n
(H
2N
NH
2)
x value in Ni1-x
Pdx nanocatalysts
a
b
Fig. 5 e (a) Hydrogen selectivities for decomposition of
hydrous hydrazine (0.5 M) catalyzed by Ni, Pd and Ni1-xPdx
(x [ 0.20e0.90) nanocatalysts at 323 K (catalyst/
H2NNH2 [ 1:10). (b) Time course plots for the
decomposition of hydrous hydrazine (0.5 M) catalyzed by
(i) Ni, (ii) Ni0.80Pd0.20, (iii) Ni0.70Pd0.30, (iv) Ni0.60Pd0.40,
(v) Ni0.50Pd0.50, (vi) Ni0.40Pd0.60, (vii) Ni0.20Pd0.80, (viii)
Ni0.10Pd0.90 and (ix) Pd nanocatalysts at 323 K (catalyst/
H2NNH2 [ 1:10).
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 1 1 7 9 4e1 1 8 0 111798
and 3.0 A for Pd foil can be assigned to PdePd metal bond,
which is determined to be 2.740 A in dimension as the best fit
parameter in curve-fitting analysis. With the increase in Ni/Pd
ratio in the examined Ni-Pd alloy nanocatalysts, the EXAFS
spectra show a progressive decrease in the height of main
peak assigned for PdePd bond with a gradual shift to lower
dimension from 2.740 A (Pd nanoparticles) to 2.733, 2.725 and
2.720 A, for the Ni0.20Pd0.80, Ni0.60Pd0.40 and Ni0.80Pd0.20 nano-
catalysts, respectively. This significant shift and successive
decrease of the peak intensity with the increase in the Ni
content in the Ni-Pd alloy nanocatalysts indicate the forma-
tion of Ni-Pd bonds in addition to the PdePd bonds and is
consistent with the alloy composition for the Ni-Pd nano-
catalysts [47,48]. The Fourier transform of k3-weighted EXAFS
spectrum (Figure S7) for the Ni nanoparticles at Ni K-edge
exhibits a broad peak with a maximum at w2.0 A, which can
be assigned to the NieNi bondwith reference to that for Ni-foil
[47]. However, the presence of a thin film of oxides of nickel
makes it difficult to evaluate the exact coordination number of
Ni. In contrast to the single broad peak for the Ni nanoparticle,
splitting of themain peak and the formation of a shoulder can
be observed in longer distance range with the increase in Pd
content in the case of Ni-Pd alloy nanocatalysts. For
Ni0.20Pd0.80 nanocatalyst the main peak, as observed for the Ni
nanoparticles, splits into two peaks and the intensity of the
peak at longer distance is higher than that for the NieNi peak.
The new peak observed for the bimetallic Ni-Pd nanocatalysts
with high Pd content can be attributed to the Ni-Pd bond
[47,48]. The progressive decrease in the intensity of NieNi
peak and the appearance of a new peak at longer distance
with an increase in Pd content in the Ni-Pd alloy nanocatalysts
clearly indicates the formation of Ni-Pd bonds in the Ni-Pd
nanocatalysts [47,48]. Although the Ni K-edge data are not
very resolved, the results obtained with Ni and Pd K-edge data
support the appearance of Ni-Pd bonds in Ni-Pd nanocatalysts
and therefore confirm the alloy formation for the Ni-Pd
nanocatalysts.
3.2. Catalytic performance of the Ni-Pd nanocatalysts
Catalytic performance of the Ni-Pd nanocatalysts has been
extensively studied for the decomposition of hydrous
hydrazine to hydrogen at mild reaction condition. Catalytic
hydrazine decomposition reactions are initiated with the
introduction of hydrazine monohydrate into the reactor
containing an aqueous suspension of catalysts kept at
a constant temperature of 323 K. To investigate the depen-
dence of hydrogen selectivity on the Ni/Pd ratio, Ni1-xPdx
nanocatalysts with a wide range of Pd content, x ¼ 0.20e0.90,
have been examined (Fig. 5a). For all the Ni-Pd nanocatalysts,
gas release is initiated with the addition of the hydrazine
monohydrate, and the amount of resulting gas is measured
volumetrically for the evaluation of selectivity towards
hydrogen. Among the range of x ¼ 0.20e0.90, the Ni0.60Pd0.40
nanocatalyst exhibits the highest hydrogen selectivity for
hydrazine decomposition. A release of 2.5 equivalents of
gases was observed in 190min (Fig. 5b), which corresponds to
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 1 1 7 9 4e1 1 8 0 1 11799
w82% selectivity for hydrogen and the overall decomposition
reaction of hydrous hydrazine into hydrogen and nitrogen at
room temperature in the presence of Ni0.60Pd0.40 nano-
catalysts can be described as: H2NNH2 / 0.88N2 þ 1.64H2 þ0.24NH3. However, under analogous conditions, the Ni
nanoparticles exhibit poor activity with 33% H2 selectivity at
323 K [55], whereas Pd nanoparticles are inactive. Further, it
is found that the Ni-Pd nanocatalysts with Pd contents in the
range of x ¼ 0.35e0.45 exhibit the highest value for H2
selectivity (79%e82%). Further increase in either Ni or Pd
content from Ni0.60Pd0.40 results in the decrease of H2 selec-
tivity. A sharp decrease in H2 selectivity to w35% was
observed at x ¼ 0.20, whereas H2 selectivity decreases to
w56% at x ¼ 0.50 and then to w33% at x ¼ 0.90.
It has been shown that the reaction temperature signifi-
cantly influence the catalytic activity of Ni-based nano-
catalysts for the decomposition of hydrous hydrazine to
hydrogen [55]. Temperature dependency of the catalytic
activity of Ni0.60Pd0.40 nanocatalysts for hydrazine decompo-
sition were examined (Fig. 6) by performing the catalytic
reactions at different reaction temperatures (298e343 K). In
contrast to the low hydrogen selectivity of w7% at 298 K,
a significant enhancement in the catalytic performance of
Ni0.60Pd0.40 nanocatalysts for hydrous hydrazine decomposi-
tion has been observed with an increase in the reaction
temperature. However, no further increase in H2 selectivity
was observed at higher temperatures >323 K, while the reac-
tion completion time is significantly reduced to 110 min at
343 K in contrast to 190 min at 323 K.
To further confirm that the presence of two metals on the
catalytic active sites is critical only in the form of alloy (elec-
tronically modified) and not as the separate metals, we have
examined the catalytic activity of the physical mixture of Ni
and Pd nanoparticles and found that the physical mixture of
Ni and Pd nanoparticles (Ni/Pd 60:40) exhibit poor activity in
contrast to the high catalytic performance of the Ni0.60Pd0.40
alloy nanocatalyst (Figure S8). These results imply that the
0 50 100 150 200 250 300 350 400 450 500
0.0
0.5
1.0
1.5
2.0
2.5
3.0
(iv) (iii)
(ii)
(i)
Time (min)
n(H
2 +
N2) / n
(H
2N
NH
2)
Fig. 6 e Time course plots for hydrogen generation from
H2NNH2$H2O (0.5 M) in aqueous solution in the presence of
the Ni0.60Pd0.40 nanocatalyst (catalyst/H2NNH2 [ 1:10) at
(i) 298, (ii) 313, (iii) 323 and (iv) 343 K.
electronically modified catalyst surface is crucial and the
presence of bimetallic phase as the active sites on the surface
is essentially required for obtaining high catalytic perfor-
mance for hydrogen generation from hydrous hydrazine
decomposition. Furthermore, in contrast to the high catalytic
performance of Ni-Pd nanocatalysts, the analogously
synthesized M0.60Pd0.40 (M ¼ Fe, Co and Cu) nanocatalysts
exhibit poor or no activity for hydrazine decomposition in
aqueous solution, indicating that alloying the Fe, Co and Cu
metals with Pd has no positive effects on the hydrogen
selectivity, in contrast with the drastically positive effect from
Ni (Fig. 7 and Figure S9).
Detailed structural and electronic analyses of the Ni-Pd
nanocatalysts confirm the alloy composition for the synthe-
sized Ni-Pd nanocatalysts. In general, alloy materials
have distinct interactions with the reactant molecules
in comparison with corresponding monometallic catalysts
[1e9,27e30,47,48,52e54]. The formation of heterometallic
bonds with strong metalemetal interactions might tune the
bonding pattern of the catalyst surface to the reactant mole-
cules and stabilize the possible reaction intermediates,
leading to improved catalytic activity and selectivity in
comparison with those of the corresponding monometallic
counterparts [47,48,50e54]. The catalytic performance of the
Ni0.60Pd0.40 nanocatalyst superior to the corresponding
monometallic counterparts, which are either inactive (Pd) or
poorly active (Ni), are due to the strong interaction between Ni
and Pd, which is well supported by XPS and EXAFS analysis. In
addition, the existence of Ni and Pd metals in an alloy state is
a key factor behind the observed high catalytic performance of
the Ni0.60Pd0.40 nanocatalysts. Since the parent monometallic
Ni and Pd nanoparticles show poor catalytic activity for the
hydrogen generation from hydrazine, the presence of both
metals, with inter-metallic Ni-Pd bonding, on the catalyst
active centers is vital for the activation of bonds in hydrazine
for hydrogen generation via the reaction pathway H2NNH2 /
N2 þ 2H2 (1) prior to pathway 3H2NNH2 / 4NH3 þ N2(g) (2).
Fe
Co
Ni
Cu
Pd
Fe
Pd
Co
Pd
Ni
Pd
Cu
. Pd
0
20
40
60
80
100
Selectivit
y f
or H
(%
)
Nanocatalysts
Fig. 7 e Comparative H2 selectivity plots for Fe, Co, Ni, Cu,
Pd and M0.60Pd0.40 (M [ Fe, Co, Ni and Cu) nanocatalysts by
catalytic decomposition of hydrous hydrazine to hydrogen
in aqueous solution (0.5 M) at 323 K (catalyst/
H2NNH2 [ 1:10).
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 1 1 7 9 4e1 1 8 0 111800
4. Conclusions
In conclusion, bimetallic Ni-Pd nanocatalysts Ni1-xPdx,
synthesized by alloying Ni and Pd, exhibit high hydrogen
selectivity (>80% at x ¼ 0.40) for the decomposition of hydra-
zine in aqueous solution to hydrogen at 323 K, whereas the
monometallic Ni and Pd counterparts are either poorly active
or inactive at analogous conditions for this reaction. There is
a significant correlation between the composition of Ni-Pd
nanocatalysts and hydrogen selectivity. EXAFS and XPS
analyses of the Ni0.60Pd0.40 nanocatalyst and its extensive
comparison with corresponding parent monometallic
components (Ni and Pd nanoparticles) and other Ni-Pd
nanocatalysts with different combinations infer a uniform
alloy composition with inter-metallic bonding which is
a crucial factor for the observed high catalytic performance of
the Ni0.60Pd0.40 nanocatalysts for hydrous hydrazine decom-
position to hydrogen.
Acknowledgements
We acknowledge the financial support from JSPS and AIST.
S.K.S. thanks JSPS for a postdoctoral fellowship.
Appendix. Supplementary material
Supplementary data associated with this article can be found,
in the online version, at doi:10.1016/j.ijhydene.2011.06.069.
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