Nickel-palladium nanoparticle catalyzed hydrogen generation from hydrous hydrazine for chemical hydrogen storage

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Nickel-palladium nanoparticle catalyzed hydrogen generation from 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

article info

abstract

Article history:

In this study, we report Ni-Pd bimetallic nanoparticle catalysts (nanocatalyst) (Ni1-xPdx)

Received 5 March 2011

synthesized by alloying Ni and Pd with varying Pd contents, which exhibit appreciably high

Received in revised form

H2 selectivity (>80% at x ¼ 0.40) from the decomposition of hydrous hydrazine at mild

6 June 2011

reaction condition with Ni0.60Pd0.40 nanocatalyst, whereas the corresponding monometallic

Accepted 12 June 2011

counterparts are either inactive (Pd nanoparticles) or poorly active (Ni nanoparticles

Available online 13 July 2011

exhibit 33% H2 selectivity). In addition to powder X-ray diffraction (XRD), X-ray photoelectron spectra (XPS) analysis and electron microscopy (TEM/SEM), the structural and

Keywords:

electronic characteristics of Ni-Pd nanocatalysts were investigated and established using

Ni-Pd nanocatalysts

extended X-ray absorption fine structure (EXAFS) analysis. Unlike the high activity of Ni-Pd

Bimetallic

nanocatalysts, Pd-M (M ¼ Fe, Co and Cu) bimetallic nanocatalysts exhibit poor catalytic

Hydrous hydrazine

activity. These results imply that alloy composition of Ni-Pd nanocatalysts is critical, where

Hydrogen generation

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

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

storage materials, fuel cell electrocatalysis, catalytic reforming, oxidation-reduction organic reactions and so on [4e16]. Chemical hydrogen storage materials 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 material investigated to date fulfills all the necessary storage and transportation requirements, such as volumetric and gravimetric hydrogen capacities, handling pressure and temperature, 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

* Corresponding author. Tel.: þ81 72 751 9562; fax: þ81 72 751 7942. E-mail address: [email protected] (Q. Xu). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.06.069

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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 subsequently 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 nanocatalysts 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 NiPd 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 nanoparticles) 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 borohydride (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 the molar 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 stirring 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

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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; 0.032 M and 0.017 M for Ni0.65Pd0.35; 0.027 M 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 Pd nanocatalysts An analogous synthetic procedure as used for the Ni-Pd nanocatalyst was adapted for the preparation of monometallic 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) nanocatalysts 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 RINT2000 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 adsorption 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 background vacuum 3.2  106 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 transmission 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) monochromator 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

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metal foils, NiO powder and PdO powder. The Fourier transform (FT) of k3-weighted EXAFS were obtained using a range ˚ 1 by using Athena [43e46]. The curve-fitting for over 1.8e18 A the Fourier transform data were 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 K2PdCl4 were co-reduced in an aqueous solution using sodium borohydride, a reductant, in the presence of hexadecyltrimethyl ammonium bromide (CTAB), a surfactant, at room temperature. An orange suspension of the bimetallic salts (Ni2þ and Pd2þ) quickly turned to a black suspension of Ni-Pd nanoparticles in the presence of the reductant. Monometallic Ni and Pd nanoparticles were prepared from NiCl2$6H2O and K2PdCl4, respectively, via an analogous procedure to that for the Ni-Pd alloy nanocatalysts. Physical mixture of monometallic Ni and Pd nanoparticles was made by mixing the separately prepared single-component nanoparticles. The synthesized nanoparticles were fully characterized to investigate their structural 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

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 nanoparticles. 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 suggestes 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 crystalline structures of the prepared Ni-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) corresponding to 2q values of 40.3 and 45.6, respectively indicate the formation of Ni-Pd alloy (JCPDS file No. 05-0681 (Pd) and

Fig. 1 e (a) TEM, (b) HAADF-STEM, (c) HRTEM (inset SAED) images and (d) EDS spectrum of Ni0.60Pd0.40 nanocatalysts.

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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 NiPd 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. Consistent 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 the monometallic 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

a

Ni 2p

Intensity (a.u.)

Intensity (a.u.)

Ni 2p

885 880 875 870 865 860 855 850 Binding Energy (eV)

885

880

875

Ni 2p3/2

Ni 2p1/2

870

865

860

855

850

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 g1. 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 spectrum 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 palladium 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 derivative of XANES spectra (Figure S6) for Ni nanoparticles and NiPd 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 PdK-edge of the Ni0.60Pd0.40 nanocatalyst along with those for Pd foil, Pd nanoparticles, Ni0.80Pd0.20 and Ni0.20Pd0.80 nanocatalysts are shown in Fig. 4. The prominent peak between 2.0

Binding Energy (eV)

b

1.2

Normalized absorbance (a.u.)

Intensity (a.u.)

Intensity (a.u.)

Pd 3d Pd 3d

Pd 3d5/2

346 344 342 340 338 336 334 332 Binding Energy (eV)

Pd 3d3/2

1.0

0.8

(a) (b) (c) (d) (e) (f) 0.08

0.6

0.06 0.04 0.02

0.4 0.00 -0.02

0.2

-0.04 24320

24340

24360

24380

24400

24420

E (eV)

0.0

346

344

342

340

338

336

334

332

Binding Energy (eV)

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).

24320

24340

24360

24380

24400

24420

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.

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3.2. (a)

30

(b) 20

(c) (d) (f) (e)

10

3

k -weighted Fourier Transform (a.u.)

40

0 0

1

2

3

4

5

6

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.

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 dependence 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 190 min (Fig. 5b), which corresponds to

a

3.0

n(H2 + N2) / n(H2NNH2)

2.5

2.0

1.5

1.0

0.5

0.0 0.00

0.20

0.40

0.60

0.80

1.00

x value in Ni1-xPdx nanocatalysts

b

3.0

2.5

n(H2 + N2) / n(H2NNH2)

˚ for Pd foil can be assigned to PdePd metal bond, and 3.0 A ˚ in dimension as the best fit which is determined to be 2.740 A 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 ˚ (Pd nanoparticles) to 2.733, 2.725 and dimension from 2.740 A ˚ 2.720 A, for the Ni0.20Pd0.80, Ni0.60Pd0.40 and Ni0.80Pd0.20 nanocatalysts, 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 formation of Ni-Pd bonds in addition to the PdePd bonds and is consistent with the alloy composition for the Ni-Pd nanocatalysts [47,48]. The Fourier transform of k3-weighted EXAFS spectrum (Figure S7) for the Ni nanoparticles at Ni K-edge ˚ , which can exhibits a broad peak with a maximum at w2.0 A be assigned to the NieNi bond with 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 the main 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.

(iv) (iii)

2.0

(v) (vi)

1.5

(vii) (ii) (i)

(viii)

1.0

0.5

(ix)

0.0 0

50

100

150

200

250

300

350

400

Time (min)

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).

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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 nanocatalysts can be described as: H2NNH2 / 0.88N2 þ 1.64H2 þ 0.24NH3. 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 selectivity. 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 significantly influence the catalytic activity of Ni-based nanocatalysts for the decomposition of hydrous hydrazine to hydrogen [55]. Temperature dependency of the catalytic activity of Ni0.60Pd0.40 nanocatalysts for hydrazine decomposition 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 decomposition 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 reaction 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 (electronically 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

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 performance 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 synthesized 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 molecules 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).

100

3.0

Selectivity for H (%)

n(H2 + N2) / n(H2NNH2)

80

(iii)

(iv)

2.5

2.0

(ii) 1.5

60

40

20

1.0

(i)

0.5

0

300

350

400

450

500

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.

Pd

250

Time (min)

.

200

Cu

150

Pd

100

Ni

50

Pd

Pd

0

Co

Fe

Pd

Cu

Ni

Co

Fe

0.0

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).

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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 hydrazine 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 decomposition 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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