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

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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: q.xu@aist.go.jp (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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