Bimetallic Ni?Pt nanocatalysts for selective decomposition of hydrazine in aqueous solution to hydrogen at room temperature for chemical hydrogen storage

Article abstract of DOI:10.1021/ic1007654

The design and development of alloy catalysts has been a focus of intense interest because suitable selection and control over the composition of alloy catalysts can result in greatly improved activity and selectivity, while it is unusual that the combination of two inactive metals gives a highly active catalyst. In this work, we found that Ni?Pt bimetallic nanocatalysts, with a platinum content as low as 7 mol %, prepared by the coreduction of the corresponding metal chlorides, exhibit excellent catalytic activity to the decomposition of hydrous hydrazine, producing hydrogen with a 100% selectivity at room temperature, whereas the corresponding single-component nickel and platinum counterparts are inactive for this reaction. These results provide new possibilities for searching heterometallic catalysts. The present catalyst with low noble-metal content promotes the practical use of the hydrogen-storage system based on the catalytic complete decomposition of hydrazine in aqueous solution at ambient conditions.

Full text of DOI:10.1021/ic1007654

6148 Inorg. Chem. 2010, 49, 6148–6152
DOI: 10.1021/ic1007654
Bimetallic Ni-Pt Nanocatalysts for Selective Decomposition of Hydrazine
in Aqueous Solution to Hydrogen at Room Temperature for Chemical
Hydrogen Storage
Sanjay K. Singh and Qiang Xu*
National Institute of Advanced Industrial Science and Technology (AIST), Ikeda, Osaka 563-8577, Japan
Received April 21, 2010
The design and development of alloy catalysts has been a focus of intense interest because suitable selection and
control over the composition of alloy catalysts can result in greatly improved activity and selectivity, while it is unusual
that the combination of two inactive metals gives a highly active catalyst. In this work, we found that Ni-Pt bimetallic
nanocatalysts, with a platinum content as low as 7 mol %, prepared by the coreduction of the corresponding metal
chlorides, exhibit excellent catalytic activity to the decomposition of hydrous hydrazine, producing hydrogen with a
100% selectivity at room temperature, whereas the corresponding single-component nickel and platinum counterparts
are inactive for this reaction. These results provide new possibilities for searching heterometallic catalysts. The present
catalyst with low noble-metal content promotes the practical use of the hydrogen-storage system based on the catalytic
complete decomposition of hydrazine in aqueous solution at ambient conditions.
Introduction
for applying it safely. Recent studies have shown that
hydrous hydrazine could be a potential hydrogen-storage
material via the complete decomposition reaction (1) at room
temperature.15,16
New materials that can solve challenging key hurdles for
safe and efficient hydrogen storage are of paramount
importance.1-6 Anhydrous hydrazine, a liquid at room tem-
perature with a hydrogen content as high as 12.5 wt %,7
could be a competent candidate for hydrogen storage;8-14
however, its incompatibility with metals is a major hurdle
catalyst
H2NNH2
s
H2O, rt
N ðgÞ þ 2H ðgÞ
ð1Þ
f
2
2
Hydrazine monohydrate, H2NNH2 H2O, a liquid at room
3
temperature, has a hydrogen-storage capacity (the hydrogen
content available for hydrogen generation) as high as 8.0 wt
%, meeting the DOE target for hydrogen-storage materials,6
and is safe in handling.7 Notably, the only production of
nitrogen in addition to hydrogen by the complete decom-
position of hydrazine as well as easy recharge with the current
available infrastructure of liquid fuel recharge gives high
advantages of this material for hydrogen storage.15,16 Because
nitrogen can be transformed to ammonia by the Haber-
Bosch process17 and subsequently to hydrazine on a large
scale,18,19 the key to exploiting hydrazine as a hydrogen-
storage material is the development of suitable catalysts
that can avoid the undesired reaction pathway (2), and the
catalysts should be economically viable.
*To whom correspondence should be addressed. E-mail: q.xu@aist.go.jp.
(1) Graetz, J. Chem. Soc. Rev. 2009, 38, 73.
(2) Hamilton, C. W.; Baker, R. T.; Staubitz, A.; Manners, I. Chem. Soc.
Rev. 2009, 38, 279.
(3) Xiong, Z.; Yong, C. K.; Wu, G.; Chen, P.; Shaw, W.; Karkamkar, A.;
Autrey, T.; Jones, M. O.; Johnson, S. R.; Edwards, P. P.; David, W. I. F. Nat.
Mater. 2008, 7, 138.
(4) Chandra, M.; Xu, Q. J. Power Sources 2006, 156, 190.
(5) Jiang, H.-L.; Singh, S. K.; Yan, J.-M.; Zhang, X.-B.; Xu, Q.
ChemSusChem 2010, 3, 541.
(6) U.S. DOE. Hydrogen, Fuel Cells & Infrastructure Technologies
Program Multi-Year Research, Development, and Demonstration Plan,
Hydrogen Storage Technical Plan, 2007, http://www1.eere.energy.gov/
hydrogenandfuelcells/mypp/.
(7) Schmidt, E. W. Hydrazine and its Derivatives: Preparation, Properties,
Applications, 2nd ed.; John Wiley & Sons: New York, 1984.
(8) Santos, J. B. O.; Valenc-a, G. P.; Rodrigues, J. A. J. J. Catal. 2002, 210, 1.
(9) Prasad, J.; Gland, J. L. Langmuir 1991, 7, 722.
(10) Armstrong, W. E.; Ryland, L. B.; Voge, H. H. U.S. Patent 4,124,538,
1978.
3H2NNH2 f 4NH3 þ N2ðgÞ
ð2Þ
(11) Zheng, M.; Cheng, R.; Chen, X.; Li, N.; Li, L.; Wang, X.; Zhang, T.
Int. J. Hydrogen Energy 2005, 30, 1081.
(12) Chen, X.; Zhang, T.; Zheng, M.; Wu, Z.; Wu, W.; Li, C. J. Catal.
2004, 224, 473.
(13) Alberas, D. J.; Kiss, J.; Liu, Z.-M.; White, J. M. Surf. Sci. 1992,
278, 51.
(14) Al-Haydari, Y. K.; Saleh, J. M.; Matloob, M. H. J. Phys. Chem.
1985, 89, 3286.
(15) Singh, S. K.; Xu, Q. J. Am. Chem. Soc. 2009, 131, 18032.
(16) Singh, S. K.; Zhang, X.-B.; Xu, Q. J. Am. Chem. Soc. 2009, 131, 9894.
(17) Smil, V. Enriching the Earth: Fritz Haber, Carl Bosch and the
Transformation of World Food Production; MIT Press: Cambridge, MA, 2001.
(18) Hayashi, H. Catal. Rev.-Sci. Eng. 1990, 32, 229.
(19) Hayashi, H. Res. Chem. Intermed. 1998, 24, 183.
r
pubs.acs.org/IC
Published on Web 06/02/2010
2010 American Chemical Society

Article
Inorganic Chemistry, Vol. 49, No. 13, 2010 6149
Recently, we have found that alloying nickel, which itself is
inactive for this reaction, with rhodium in a 1:4 (Ni/Rh)
molar ratio drastically enhances the selectivity to 100% for
the generation of hydrogen by the complete decomposition of
hydrazine in aqueous solution at room temperature.15 Here,
we report that the Ni-Pt bimetallic alloy nanocatalyst, with a
platinum content of 7-31 mol %, exhibits excellent catalytic
activity to the decomposition of hydrazine in aqueous solu-
tion, producing hydrogen with 100% selectivity at room tem-
perature in contrast to the corresponding single-component
nickel and platinum counterparts, which are inactive for this
reaction. It is noteworthy that the content of the noble metal
(platinum) can be lowered to a value as low as 7 mol % in
comparison to the value of 80 mol % for rhodium, which is
more expensive than platinum, in the Rh4Ni catalyst.
(0.036 mmol) for Ni0.83Pt0.17, 0.036 g (0.152 mmol) and 0.019 g
(0.046 mmol) for Ni0.77Pt0.23, 0.029 g (0.122 mmol) and 0.023 g
(0.055 mmol) for Ni0.69Pt0.31, 0.022 g (0.093 mmol) and 0.027 g
(0.065 mmol) for Ni0.59Pt0.41, 0.014 g (0.059 mmol) and 0.030 g
(0.072 mmol) for Ni0.45Pt0.55, and 0.007 g (0.029 mmol) and
0.034 g (0.082 mmol) for Ni0.26Pt0.74
Preparation of Monometallic Nickel and Platinum Nanocata-
lysts. A synthetic procedure analogous to that for the Ni0.77Pt0.23
.
-
nanocatalyst was adapted, using only NiCl2 6H2O (0.072 g, 0.304
3
mmol) and K2PtCl4 (0.038 g, 0.092 mmol) respectively for the
preparation of monometallic nickel and platinum nanocatalysts.
Preparation of Ni@Pt Core-shell Nanocatalysts. A 2.5 mL
aqueous solution of NiCl2 6H2O (0.036 g, 0.152 mmol) and
3
CTAB (0.105 g, 0.288 mmol) was added to a 1.5 mL of aqueous
solution of NaBH4 (0.020 g, 0.526 mmol), which was then vigor-
ously stirred and shaked to obtain a black suspension. The
nickel nanoparticles were collected by centrifugation (15000 rpm,
20 min, 298 K) and dried for 8 h at 363 K. The obtained nickel
nanoparticles were dispersed in a 4.0 mL aqueous solution of
K2PtCl4 (0.019 g, 0.046 mmol) by sonication, and the resulting
suspension was kept at room temperature for 8 h. The Ni@Pt
core-shell nanoparticles thus obtained were collected by centri-
fugation (15 000 rpm, 20 min, 298 K) and dried for 8 h at 363 K.
Preparation of Nickel and Platinum Physical Mixture Nano-
catalysts. Separately synthesized monometallic nickel and pla-
tinum nanoparticles were mixed together in an equimolar ratio
and dispersed in 4.0 mL of distilled water before being used for
the catalytic reaction.
Experimental Section
General Considerations. Commercial chemicals were used as
received for catalyst preparation and hydrazine decomposition
experiments. Hydrazine monohydrate (H2NNH2 H2O, 99%),
sodium borohydride (NaBH4, 99%), hexadecyltrimethylammo-
3
nium bromide (CTAB, 95%), and FeCl2 4H2O (95%) were
3
obtained from Aldrich. K2PtCl4, CoCl2 6H2O (99.5%), NiCl2
3
3
6H2O (99.9%), CuCl2 (95%), and NiO were purchased from
Wako. PtO2 xH2O (78.8%) was purchased from Mitsuwa
3
Chemicals. Mass analysis of the generated gases was performed
using a Balzers Prisma QMS 200 mass spectrometer. Powder
X-ray diffraction (XRD) studies were performed on a Rigaku
RINT-2000 X-ray diffractometer (Cu KR). Scanning electron
microscopy (SEM; Hitachi S-5000) and transmission electron
microscopy (TEM; FEI TECNAI G2) with selected area elec-
tron diffraction (SAED) and energy-dispersive X-ray detection
(EDS) were applied for detailed microstructural information.
The SEM and TEM samples were prepared by depositing a few
droplets of the nanoparticle suspension onto the amorphous
carbon-coated copper grids, which were dried under an argon
atmosphere. Surface area measurements were performed by
dinitrogen adsorption at liquid-nitrogen temperature using
automatic volumetric adsorption equipment (Belsorp II). 15N
NMR spectra were recorded on a JEOL JNM-AL400 spectro-
meter at an operating frequency of 40.40 MHz. Liquid samples
were contained in 5.0-mm-o.d. sample tubes, in which coaxial
inserts containing CD3CN (15N, δ -134.00 ppm) as an external
reference and a lock were placed. X-ray photoelectron micro-
scopy (XPS) analysis was carried out on a Shimadzu ESCA-
3400 X-ray photoelectron spectrometer using a Mg KR source
(10 kV, 10 mA). Argon sputtering experiments were carried out
under the conditions of background vacuum = 3.2 ꢀ 10-6 Pa
and sputtering acceleration voltage=1 kV. The atomic compo-
sition of Ni0.93Pt0.07 was analyzed by means of an inductively
coupled plasma (ICP) spectrometer (Rigaku, CIROS-120EOP).
Preparation of Ni1-xPtx (x = 0.03-0.74) Nanocatalysts. A
series of Ni1-xPtx nanocatalysts (x = 0.03-0.74) were synthe-
sized using a surfactant-aided coreduction method, where x
represents the molar portion of platinum. A typical synthetic
procedure for Ni0.93Pt0.07 is described here. To a 2.5 mL aqueous
Preparation of M0.77Pt0.23 (M = Fe, Co, and Cu) Nanocata-
lysts. A synthetic procedure similar to that for Ni0.77Pt0.23
was adapted to synthesize M0.77Pt0.23 (M = Fe, Co, and Cu)
nanocatalysts using FeCl2 4H2O (0.031 g, 0.156 mmol), CoCl2
3
3
6H2O (0.036 g, 0.152 mmol), and CuCl2 (0.020 g, 0.150 mmol),
respectively, in place of NiCl2 6H2O.
3
Catalytic Hydrazine Decomposition Experiments. Catalytic
reactions were carried out at room temperature using a two-
neck, round-bottomed flask, with one of the flask openings
connected to a gas buret and another used for the introduction
of hydrazine monohydrate. The catalytic decomposition reac-
tion of hydrazine for the release of hydrogen (along with
nitrogen) was initiated by stirring the mixture of hydrazine
monohydrate (0.1 mL, 1.97 mmol), which was introduced by
using a syringe to the reaction flask containing a 4.0 mL aqueous
suspension of 0.017 g of nanocatalysts (prepared as described
above). The gases released during the reaction were passed
through a trap containing 1.0 M hydrochloric acid to ensure
absorption of ammonia, if produced, of which the volume was
monitored using the gas buret. For preparation of the samples
for mass spectrometry (MS) and 15N NMR measurements, no
trap containing hydrochloric acid was used. Catalyst stability
experiments were conducted over the same catalyst by adding an
additional equivalent amount of N2H4 H2O to the reaction
3
vessel after completion of the previous catalytic run.
Characterization of Nanocatalysts. After the hydrazine decom-
position reaction, the suspension was centrifuged (15 000 rpm,
10 min, 298 K) to separate the solution and nanocatalyst, which
was washed twice with 5.0 mL of water and ethanol, dried at 373 K
for 8 h, and then used for SEM, TEM, XPS, and powder XRD
measurements.
suspension of NiCl2 6H2O (0.058 g, 0.245 mmol), K2PtCl4
3
(0.008 g, 0.019 mmol), and CTAB (0.105 g, 0.288 mmol),
obtained by subsequent sonication and stirring for 5 min, was
added dropwise a 1.5 mL aqueous solution of NaBH4 (0.020 g,
0.526 mmol). The contents of the flask is vigorously shaken for
2 min, resulting in the generation of a Ni0.93Pt0.07 nanocatalyst
as a black suspension, which was used for the catalytic react-
Results and Discussion
A facile surfactant-aided coreduction synthetic approach
was adopted for the preparation of the bimetallic Ni1-xPtx
(x=0.03-0.74) nanocatalysts with various compositions of
nickel and platinum, which were generated as a black suspen-
ion. The amounts of NiCl2 6H2O and K2PtCl4 used for the
3
sion by the coreduction of an aqueous solution of NiCl2
6H2O and K2PtCl4 using an aqueous solution of sodium boro-
hydride as the reducing agent in the presence of surfactant
preparation of Ni1-xPtx were 0.065 g (0.274 mmol) and 0.004 g
(0.010 mmol) for Ni0.97Pt0.03, 0.050 g (0.211 mmol) and 0.011 g
(0.027 mmol) for Ni0.89Pt0.11, 0.043 g (0.181 mmol) and 0.015 g
3

6150 Inorganic Chemistry, Vol. 49, No. 13, 2010
Singh and Xu
Figure 1. Selectivities and TOFs for hydrogen generation by the decom-
position of hydrazine in aqueous solution (0.5 M) catalyzed by nickel,
platinum, and Ni1-xPtx (x=0.03-0.74) nanocatalysts at room tempera-
Figure 2. Time course plots for hydrogen generation by the decomposi-
tion of hydrazine in aqueous solution (0.5 M) catalyzed by nickel, plati-
num, Ni0.93Pt0.07, Ni@Pt core-shell, NiO, PtO2, and a physical mixture
of monometallic nickel and platinum nanoparticles (catalyst = 0.017 g;
ture (catalyst=0.017 g; N2H4 H2O=0.1 mL). TOFs were calculated on
3
the basis of the data at 50% completion of the catalytic hydrazine
decomposition reaction.
N2H4 H2O=0.1 mL).
3
CTAB. Monometallic nickel and platinum nanoparticles were
prepared from NiCl2 6H2O and K2PtCl4, respectively, via an
3
analogous procedure. A physical mixture of monometallic
nickel and platinum nanoparticles was made by mixing the
separately prepared single-component nanoparticles. The cat-
alytic hydrazine decomposition reaction was initiated by in-
troducing hydrazine monohydrate into the reactor containing
an aqueous suspension of catalysts. For all of the Ni-Pt nano-
catalysts, gas release was initiated immediately with the addi-
tion of hydrazine monohydrate, and the amount of resulting
gas was measured volumetrically for evaluation of the selec-
tivity toward hydrogen while the gas compositions were identi-
fied by MS.
To investigate the dependence of the hydrogen selectiv-
ity on the Ni/Pt ratio, Ni1-xPtx nanocatalysts with a wide
range of platinum contents, 3-74 mol %, were examined
(Figure 1). It is found that the Ni-Pt nanocatalysts with
platinum contents in the range of 7-31 mol % exhibit 100%
hydrogen selectivity. In contrast to the slight decrease in the
hydrogen selectivity to 81% with a decrease in the platinum
content to 3 mol %, a sharp decrease in the hydrogen selec-
tivity to 28% with an increase in the platinum content to
74 mol % was observed. To our surprise, single-component
nickel and platinum nanoparticles, synthesized under analo-
gous reaction conditions, are catalytically inactive to the
hydrazine decomposition in aqueous solution.16 Turnover
frequencies (TOFs; h-1) for the catalytic decomposition
reaction of hydrazine in aqueous solution over Ni-Pt nano-
catalysts (platinum content 3-74 mol %) have been calcu-
lated and plotted in Figure 1, which indicated that TOFs of
the catalysts vary in the range of 1.5 (Ni0.26Pt0.74) to 7.9 h-1
(Ni0.69Pt0.31).
The Ni1-xPtx nanocatalysts with 7-31 mol % platinum
contents exhibit analogous time course profiles, releasing
3.0 equiv of gases (Figure S1 in the Supporting Information,
SI).20 For Ni0.93Pt0.07, a release of 3.0 equiv of gases was
observed in 190 min (Figure 2), which were identified by
MS to be hydrogen and nitrogen with a H2/N2 ratio of 2.0
(Figure 3). The volumetric and MS results are in agreement
with 100% selectivity for hydrogen and nitrogen according to
Figure 3. MS profile for the gases from the decomposition reaction of
hydrazine in aqueous solution (0.5 M) over the Ni0.93Pt0.07 nanocatalyst
(catalyst = 0.017 g, N2H4 H2O = 0.1 mL) detected by a mass spectro-
meter under an argon atmosphere at room temperature.
3
eq 1, indicating the complete decomposition of hydrazine in
aqueous solution into hydrogen and nitrogen at room tem-
perature. Furthermore, it is found that the hydrogen selec-
tivity is independent of the concentration of hydrazine
(Figure S2 in the SI).20 In addition to the above results, the
absence of a 15N NMR signal for ammonia in the solution
after the decomposition of hydrazine (Figure S3 in the SI)
also supports the complete decomposition of hydrazine to
hydrogen and nitrogen.20 In addition, we have examined the
activity and selectivity of Ni0.93Pt0.07 for the decomposition
of hydrazine in aqueous solution during repeated use, and no
significant changes were observed for five runs (Figure S4 in
the SI).
In contrast to the high activity observed for the Ni-Pt
alloy nanocatalyst, the physical mixture of the monometallic
nickel and platinum nanoparticles is catalytically inactive,
whereas the Ni@Pt core-shell nanoparticles exhibit poor
activity via reaction pathway (1), with which only 0.82 equiv
of gases is released, corresponding to ∼18% hydrogen
selectivity, indicating the involvement of the bimetallic phase
(20) See the Supporting Information for details.

Article
Inorganic Chemistry, Vol. 49, No. 13, 2010 6151
Figure 5. Powder XRD patterns of (a) nickel, (b) Ni0.97Pt0.03, (c) Ni0.93
Pt0.07, (d) Ni0.89Pt0.11, (e) Ni0.83Pt0.17, (f) Ni0.77Pt0.23, (g) Ni0.69Pt0.31
(h) Ni0.59Pt0.41, (i) Ni0.45Pt0.55, (j) Ni0.26Pt0.74, and (k) platinum nano-
catalysts.
-
,
Figure 4. (a and e) TEM images with corresponding HRTEM images
(insets), (b and f) EDS spectra (copper signals in EDS are from copper
grids), and (c and g) HAADF-STEM images with corresponding (d and h)
cross-sectional compositional profiles of Ni0.93Pt0.07 and Ni0.77Pt0.23
nanocatalysts after being used for the catalytic reaction, respectively.
˚
pressed lattice for Ni0.77Pt0.23 (a=3.820 A). The 2θ values for
Ni0.77Pt0.23 at 40.88°, 47.20°, 69.64°, and 83.48° can be
indexed to diffraction planes of (111), (200), (220), and
(311), respectively, corresponding to those of platinum
(PDF 65-2868). The Ni-Pt nanocatalysts with lower plati-
num contents (e17 mol %) show diffraction peaks similar to
those of nickel (PDF 65-0380), with only a visible prominent
peak corresponding to that of the (111) plane for nickel
(Figure 5). As expected, the diffraction peaks of all of the Ni-
Pt nanocatalysts shift to lower angle compared to that of
nickel nanoparticles, clearly indicating the alloy formation
where the lattice expansion occurred due to substitution of
larger platinum atoms for the smaller nickel atoms. The con-
sistency in shift of diffraction peaks to lower angle compared
to nickel nanoparticles with an increase in the nickel content
excludes any possible segregation of alloy particles, nickel
nanoparticles, or platinum particles on the surface.21-23 The
XRD profile (Figure S8 inthe SI) of the Ni@Pt nanoparticles
also shows fcc diffraction peaks for the platinum shell with a
as active species on the catalyst surface and that the synergic
interaction between nickel and platinum is necessary for the
catalytic activity (Figure 2). Contribution from the nickel or
platinum oxide to the observed activity of the Ni-Pt nano-
catalyst has been excluded as we have found that NiO, PtO2,
and a mixture of NiO and PtO2 are inactive for the decom-
position of hydrazine in aqueous solution under analogous
reaction conditions. Moreover, the analogously synthesized
M0.77Pt0.23 (M=Fe, Co, and Cu) nanocatalysts exhibit poor
or no activity for hydrazine decomposition in aqueous
solution, indicating that alloying of the iron, cobalt, and
copper metals with platinum has no positive effects on the
hydrogen selectivity, in contrast with the drastically positive
effect from nickel (Figure S5 in the SI).20
The TEM images of the Ni0.93Pt0.07 (Figure 4a) and
Ni0.77Pt0.23 (Figure 4e) catalysts show the nanoparticle ag-
glomerated. The high-resolution TEM (HRTEM; Figure 4b,f)
and the corresponding SAED (Figure S6 in the SI) patterns
indicate the crystalline nature of these Ni-Pt nanoparticles.20
TEM and EDS (Figure 4b,g) point analysis of the randomly
chosen nanoparticles exhibits the presence of both nickel and
platinum with approximate Ni/Pt atomic ratios of 91:9 and
76:24, respectively, for Ni0.93Pt0.07 and Ni0.77Pt0.23. The Ni/Pt
atomic ratio of the Ni0.93Pt0.07 catalyst has been confirmed by
ICP measurements (91.4:8.6 Ni/Pt). The uniform composi-
tion of the bimetallic Ni-Pt nanocatalyst was further con-
firmed by a line-scan EDS analysis (Figure 4d,h) across the
nanoparticles of Ni0.93Pt0.07 and Ni0.77Pt0.23, which shows no
segregation of platinum or nickel in the particles. Further-
more, no significant change in the Ni-Pt nanoparticle
morphology or composition was observed during the cata-
lytic reaction (Figures 4 and S7 in the SI). The TEM and
HRTEM images (Figure S7 in the SI) of the Ni0.93Pt0.07
nanoparticles before the catalytic reaction show an average
particle size of ∼5 nm. The EDS point analysis of various
nanoparticles chosen randomly exhibits the presence of both
nickel and platinum on the same spot with an average atomic
composition of 92:8. Crystalline structures of the prepared
Ni-Pt nanocatalysts were analyzed using powder XRD
(Figure 5).20 The XRD profiles of the Ni-Pt nanocatalysts
with g23 mol % platinum show face-centered cubic (fcc)
diffraction peaks similar to that of platinum, with a com-
20
˚
lattice parameter of 3.893 A.
XPS spectra for the bimetallic Ni1-xPtx nanocatalysts
exhibit characteristic signals for Ni0 and Pt0 (Figure S9 in
the SI),20 indicating the coexistence of both metals in the Ni-
Pt nanocatalysts. The observed thin oxide cover on the sur-
face was presumably formed during exposure of the sample
to air and was readily removed by argon sputtering of about
6 min (Figures S9 and S10 in the SI). The signals correspond-
ing to metallic nickel and platinum for the Ni 2p3/2 and Pt 4f7/2
levels in Ni0.93Pt0.07 and Ni0.77Pt0.23 nanocatalysts appear
at 851.72 (Ni0) and 71.70 eV (Pt0) and at 851.32 (Ni0) and
71.55 eV (Pt0), respectively. The Pt 4f7/2 levels for the bim-
etallic Ni-Pt nanocatalysts are shifted to higher bind-
ing energies relative to that for the monometallic platinum
sample and the Ni 2p3/2 levels are shifted to lower energies
relative to the monometallic nickel sample (Table S1 in the
SI).24 The electronic compositions and remarkable shifts in
(21) Liu, R.; Ley, K. L.; Pu, C.; Fan, O.; Leyarovska, N.; Segre, C.;
Smotkin, E. S. Bifunctional Pt-Ru-Os ternary alloys;improved Pt-based
anode for direct methanol fuel cells. In Electrode Processes VI; Wieckowski.
A., Itaya, K., Eds.; The Electrochemical Society: Pennington, NJ, 1996; pp 341.
(22) Deivaraj, T. C.; Chen., W.; Lee, J. Y. J. Mater. Chem. 2003, 13, 2555.
(23) Mathiyarasu, J.; Remona, A. M.; Mani, A.; Phani, K. L. N.;
Yegnaraman, V. J. Solid State Electrochem. 2004, 8, 968.

6152 Inorganic Chemistry, Vol. 49, No. 13, 2010
Singh and Xu
the relative binding energies for the Ni-Pt nanocatalysts are
consistent with the alloy formation. Furthermore, the uni-
formity of the Ni-Pt alloy nanoparticles is confirmed by a
nearly constant Ni/Pt ratio from the surface to the core in the
XPS spectra with argon sputtering, in accordance with EDS
and TEM observations. No chlorine and boron species were
detected in the XPS measurements. In addition, the nitrogen
adsorption-desorption isotherms (Figure S11 in the SI) of
the Ni-Pt nanocatalysts reveal that the Brunauer-Emmett-
Teller (BET) surface areas are 28.7, 51.0, 33.5, 28.2, and
reported that the third-body effectand thed-band center shift
effect account for the observed results.35,36 The present
observation of the Ni-Pt alloy nanocatalysts for hydrazine
decomposition in aqueous solution exhibits that the bime-
tallic Ni-Pt nanocatalyst surface has characteristic proper-
ties that are distinctly different from those of either platinum
or nickel monometallic nanoparticles. Furthermore, consid-
ering the uniform composition of the Ni-Pt catalyst, it is
reasonable to understand that the alloying of nickel and
platinum leads to a modification of the catalyst surface by
incorporating considerable intermetallic electronic interac-
tion35-38 and thereby controls the interaction of the catalyst
surface with the hydrazine molecule as well as tunes the
stability of the reaction intermediates on the modified cata-
lyst surface.8,9 Because neither the nickel nor the platinum
nanoparticles show catalytic activity in the hydrogen genera-
tion from hydrazine, the presence of both metals on the
catalyst surface is vital for the catalytic activity. This implies
that catalysis requires the existence of two different metal
sites on the surface. The coexistence of nickel and platinum
on the catalyst surface results in the activation of bonds in
hydrazine toward the reaction pathway (1) over pathway (2)
for the complete decomposition of hydrazine in aqueous
solution to hydrogen and nitrogen.
56.0 m2 g-1, respectively, for the Ni0.69Pt0.31, Ni0.77Pt0.23
Ni0.83Pt0.17, Ni0.89Pt0.11, and Ni0.93Pt0.07 nanocatalysts.
,
Because the electronic and structural properties of the
catalyst surfaces are closelyrelated to their catalytic activities,
the precise modification of the catalyst surfaces by the
introduction of a second component or change of the mor-
phology could facilitate tuning of the catalytic perfor-
mance.25-30 Generally, alloy materials have distinct interac-
tions with the reactant molecules in comparison with the
corresponding monometallic catalysts. The formation of
heterometallic bonds with strong metal-metal interactions
might tune the bonding pattern of the catalyst surface to the
reactant molecules and stabilize the possible reaction inter-
mediates, leading to improved catalytic activity and selec-
tivity in comparison with those of the corresponding
monometallic catalysts.29-34 The observed high activity
and 100% hydrogen selectivity over the Ni-Pt nanocatalysts
prepared from the respective inactive monometallic counter-
parts exhibit a striking feature that the strong interaction
between different metals may lead not only to quantitative
improvement but also to a qualitative leap of the catalytic
performance. It has been reported that, for a palladium-rich
PdxPt1-x alloy, the electronic structure and interaction be-
tween the catalyst surface and substrates can be finely tuned
by alloying with various amounts of platinum.35,36 It was
Conclusions
In summary, we demonstrated that the alloying of nickel
and platinum makes it possible to achieve 100% selectivity
for the decomposition of hydrazine in aqueous solution to
hydrogen at room temperature, whereas the monometallic
nickel and platinum counterparts are inactive to this reaction.
The bimetallic Ni-Pt nanocatalysts with the platinum con-
tent as low as 7 mol % (Ni0.93Pt0.07) present a step toward a
high-performance catalyst system, which will open a door to
exploiting hydrous hydrazine as a highly promising practical
material for hydrogen storage.
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Acknowledgment. We acknowledge the reviewers for
their valuable suggestions and JSPS for financial support.
S.K.S. thanks JSPS for a postdoctoral fellowship.
Supporting Information Available: Results for SEM, TEM,
XPS, BET, and powder XRD of the nanocatalysts and catalytic
hydrazine decomposition experiments in aqueous solution. This
material is available free of charge via the Internet at http://
pubs.acs.org.
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