Bimetallic nickel-iridium nanocatalysts for hydrogen generation by decomposition of hydrous hydrazine

Article abstract of DOI:10.1039/c0cc01879a

Alloying Ni with Ir leads to the formation of highly active catalysts for complete decomposition of hydrous hydrazine with 100% H₂ selectivity at room temperature. Use of surfactants enhances the activity by suppressing the agglomeration of nan

Full text of DOI:10.1039/c0cc01879a

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Bimetallic nickel-iridium nanocatalysts for hydrogen generation
by decomposition of hydrous hydrazinew
Sanjay Kumar Singh and Qiang Xu*
Received 14th June 2010, Accepted 14th July 2010
DOI: 10.1039/c0cc01879a
Alloying Ni with Ir leads to the formation of highly active
catalysts for complete decomposition of hydrous hydrazine with
and H
2 6
IrCl were mixed in the specified molar ratio in an
aqueous solution in presence of ionic surfactant hexadecyltri-
À
+
methyl ammonium salts (CTA X , where X = Br and
À
À
1
00% H
2
selectivity at room temperature. Use of surfactants
À
enhances the activity by suppressing the agglomeration of
nanoparticles, but does not affect the bimetallic compositions
of the nanoparticles.
Cl , hereafter denoted as CTAB and CTAC, respectively) as
stabilizing agents. The above aqueous suspension of metal
salts and surfactant were subsequently stirred and sonicated to
homogenize the content. A distinct color change from the
initial light yellow to a bright orange colored suspension was
observed during heating of the suspension (50 1C, 5 min,
Fig. 1). The bright orange color remains constant even
after prolonged heating and cooling to room temperature,
indicating the complexation of the surfactant and the metal
salts. Bimetallic NiIr nanoparticles can be obtained by
reducing the above suspension with the addition of a freshly
Developing bimetallic heterogeneous catalysts is crucial for
various chemical processes such as catalytic hydrogen genera-
tion from chemical hydrogen storage materials, fuel-cell
electrocatalysis, catalytic reforming, oxidation reduction
1
,2
reactions, and so on. Nanosized catalysts (nanocatalysts)
have largely been engaged for hydrogen generation from
various potential chemical hydrogen storage materials such
3
as ammonia borane. Hydrous hydrazine, such as hydrazine
4
prepared aqueous solution of NaBH . Ni0.95Ir0.05–B and
monohydrate (N
H
2 4
ÁH
2
O), merits attention as a potential
Ni0.95Ir0.05–C represents the NiIr nanocatalysts obtained in
the presence of CTAB and CTAC, respectively. A surfactant
free Ni0.95Ir0.05 nanocatalyst was also prepared analogously.
The observed color change during the complexation process
of the surfactant to bimetallic metal salts are in accordance
with the significant shift in UV-vis spectra as shown in Fig. 1.
UV-vis peaks, due to metal-to-ligand charge transfer (MLCT)
absorptions, at B439 nm and B495 nm observed for the metal
hydrogen storage material because of its high hydrogen
content (8.0 wt%) for hydrogen gas release, decomposition
at ambient conditions, easy recharging as a liquid, and only
production of nitrogen in addition to hydrogen by complete
4
decomposition:
H
2
NNH
2
- N
2
(g) + 2H
2
(g)
(1)
+
À
À
À
À
ion-surfactant suspensions of CTA X (X = Br and Cl )
are significantly red shifted to 444 nm (5 nm) and 503 nm
However, to maximise the usability of hydrazine as a hydrogen
storage material, one should avoid the undesired reaction
(
8 nm), respectively, upon heating, whereas no shift was
observed for surfactant free bimetallic salt solution
Fig. S1w). Moreover, addition of sodium borohydride to the
5
,6
pathway:
(
3H
2
NNH
2
- 4NH
3
+ N
2
(g)
(2)
metal ion-surfactant suspension immediately led to the generation
of NiIr nanoparticles as inferred by the immediate disappearance
of the prominent peaks in the corresponding UV-spectra.
Appearance of the characteristic signals for metallic Ni[2p]
and Ir[4f] in the XPS spectra of the NiIr nanoparticles
and develop low-cost catalysts.
In our continuous effort to develop highly active and
selective catalysts with low contents of noble metals for
4b,d
hydrogen generation via hydrazine decomposition,
herein
7
we report that bimetallic nanocatalysts formed by alloying
nickel with iridium (with Ir content as low as 5 mol%) exhibits
(Fig. S2w) indicates the co-existence of both the metals. The
1
2
00% H selectivity in decomposition of hydrous hydrazine at
ambient conditions, whereas the corresponding monometallic
catalysts are either inactive (Ni) or have poor selectivity
4
c
(
2
7% H selectivity for Ir). Also, we have found that stabilising
agents facilitate the formation of active catalysts, without
affecting the composition of the nanoparticles.
Nickel-iridium (NiIr) nanocatalysts with 95 mol% Ni and
5
mol% Ir (Ni0 Ir0.05) composition were synthesized by
.95
2
co-reduction of Ni and Ir metal ions. Briefly, the NiCl2
+
4+
Fig. 1 UV-vis absorption spectra after (a) the addition of aqueous
À
National Institute of Advanced Industrial Science and Technology
(AIST), Ikeda, Osaka, Japan. E-mail: q.xu@aist.go.jp;
Fax: + 81 727517942; Tel: + 81 727519652
w Electronic supplementary information (ESI) available: Experimental
details for catalyst preparation, characterisation and catalytic hydrous
hydrazine decomposition reaction. See DOI: 10.1039/c0cc01879a
2+
4+
solutions of the mixture of Ni and Ir salts to CTA
+
X (X = Br
(
(
A) and Cl (B)), (b) heating the solution (a) for 5 min at 50 1C
inset shows the color change observed for (a) and (b)), and (c)
reduction of the solution (b) by sodium borohydride for the preparation
of (A) Ni0.95Ir0.05–B and (B) Ni0.95Ir0.05–C nanoparticles, respectively.
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Ni0.95Ir0.05–B and Ni0.95Ir0.05–C nanocatalyst surfaces show a
thin oxide cover presumably formed during the exposure of
the sample to air, which can be readily removed by Ar
sputtering after about 6 min (Fig. S3w). Meanwhile, the
Ni[2p3/2] feature can be attributed to Ni at 851.32 eV, and
0
the Ir[4f7/2] feature can be attributed to Ir at 61.55 eV, which
a lattice expansion due to the substitution of larger Ir atoms
for the smaller Ni atoms.
Transmission electron microscopic (TEM), high resolution
TEM (HRTEM) and high angle annular dark field scanning
TEM (HAADF-STEM) images of the Ni0.95Ir0.05–B and
Ni0.95Ir0.05–C nanocatalysts reveal the presence of nano-
particles with an average particle size of B5 nm (Fig. 2). In
contrast, TEM and HAADF-STEM images reveal serious
agglomeration of surfactant free Ni0.95Ir0.05 nanocatalysts to
10–20 nm irregularly sized spheres (Fig. 2). The corresponding
selected area energy dispersion (SAED, Fig. 2) patterns
indicate the crystalline nature of these NiIr NPs. TEM
energy-dispersive X-ray spectroscopy (EDX, Fig. S7w) point
analysis of the randomly chosen NPs exhibits the uniform
composition of the NPs with an average Ni : Ir atomic ratio of
95 : 5 for Ni0.95Ir0.05–B and Ni0.95Ir0.05–C NPs. Analogous to
the surfactant stabilized Ni0.95Ir0.05–B and Ni0.95Ir0.05–C NPs,
an alloy composition with a Ni : Ir atomic ratio of 97 : 3
for surfactant free Ni0.95Ir0.05 NPs was exhibited by EDX
analysis. In combination with the UV-vis, XPS and TEM
results for Ni0.95Ir0.05–B and Ni0.95Ir0.05–C, and in comparison
with those for surfactant free Ni0.95Ir0.05 NPs, it is clear that the
surfactants (CTAB and CTAC) used in the present work do not
affect the compositions of the bimetallic nanoparticles, while
they stabilize the nanoparticles by suppressing agglomeration.
Catalytic performance of the Ni0.95Ir0.05–B and
Ni0.95Ir0.05–C nanocatalysts towards the hydrazine decompo-
sition reaction in aqueous solution for hydrogen generation
was examined and compared with that of surfactant free
Ni0.95Ir0.05 nanocatalyst. Reaction was initiated by introdu-
0
are in the same range for the Ni0.95Ir0.05–B and Ni0.95Ir0.05–C
nanocatalysts. After periodic sputtering for 0 to 186 min, the
0
0
relative intensities of the features due to the Ni and Ir remain
prominent, implying the presence of uniform composition of
the metallic Ni and Ir in the Ni0.95Ir0.05–B and Ni0.95Ir0.05–C
nanoparticles. The observed shift in the Ir[4f7/2] levels for the
Ni0.95Ir0.05–B and Ni0.95Ir0.05–C nanocatalysts to higher
binding energies relative to that for the monometallic Ir
sample and the Ni[2p3/2] levels shifted to lower energies
relative to the monometallic Ni sample are consistent with
7
the alloy formation. XPS measurements for surfactant free
Ni0.95Ir0.05 nanocatalyst with periodic Ar sputtering from 0 to
1
86 min (Fig. S2w) also depict the alloy composition with
metallic Ni and Ir. A trace of oxide cover was also observed on
the surface of the Ni0.95Ir0.05 NPs which can also be removed
by Ar sputtering of 6 min. No Cl- and B-species were detected
in the XPS measurements for any of the NiIr nanocatalysts
(Fig. S4w). Powder X-ray diffraction (XRD, Fig. S5 and S6w)
illustrates the crystalline nature of all the NiIr nanocatalysts,
where the major diffraction peaks between the 2y value of
4
01–501 can be indexed to the (111) plane of fcc Ni
PDF #65-0380), in accordance with the higher Ni content.
(
As expected, the diffraction peaks of all the NiIr nanocatalysts
shift to lower angles compared to those of Ni metal, indicating
cing N
2
H
4
ÁH
2
O into the reactor containing an aqueous
suspension of the nanocatalyst. Gaseous product released
during the reaction were measured volumetrically and the
selectivity towards hydrogen generation was evaluated on
the basis of equations (1) and (2). A release of 3.0 equivalents
of gases was observed in 390 min for Ni0.95Ir0.05–B and
4
40 min for Ni0.95Ir0.05–C (Fig. 3), which were identified by
mass spectrometry to be hydrogen and nitrogen with H /N
2
2
ratio of 2.0. The volumetric and mass spectrometric results are
in agreement with 100% selectivity for hydrogen and nitrogen
according to (1), indicating the complete decomposition of
hydrazine into hydrogen and nitrogen at room temperature. In
1
5
addition to the above results, the absence of N NMR signal
for ammonia in the solution after the decomposition of
hydrazine (Fig. S8w) supports the complete decomposition of
hydrazine to hydrogen and nitrogen. Ni0.90Ir0.10–B and
Ni0.90Ir0.10–C nanocatalysts also exhibits 100% H selectivity
2
for hydrous hydrazine decomposition (Fig. S9w).
Investigation of NiIr–B nanocatalysts with varying Ir
content (1 mol% B75 mol%) exhibits strong dependence of
H
2
selectivity on the NiIr compositions (Fig. S10 and S11w).
NiIr–B nanocatalysts with 5B10 mol% Ir contents show the
highest H selectivity (100%), whereas with an increase in Ir
content to >25 mol% in the NiIr–B nanocatalysts and a
decrease in Ir content to 1 mol% result in decrease of the H
2
2
selectivity to 85.5% and 76%, respectively. In contrast to the
surfactant stabilized NiIr nanocatalysts (Ni0.95Ir0.05–B and
Ni0.95Ir0.05–C), hydrazine decomposition reaction catalyzed
by the surfactant free Ni0.95Ir0.05 nanocatalysts could only be
Fig. 2 (a,c,e) HAADF-STEM (inset SAED) and (b,d,f) TEM (inset
HRTEM) images for (a,b) Ni0.95Ir0.05, (c,d) Ni0.95Ir0.05–B and (e,f)
Ni0.95Ir0.05–C nanoparticles. Scale bars for HRTEM are 5 nm.
6
546 | Chem. Commun., 2010, 46, 6545–6547
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molecule as well as tunes the stability of reaction intermediates
on the modified catalyst surface. Since Ni is inactive and Ir
shows poor H2 selectivity for hydrogen generation from
hydrazine, the presence of both metals on the catalyst surface
is crucial for the catalytic activity. The co-existence of Ni and
Ir on the catalyst surface results in the activation of hydrazine
bonds towards the reaction pathway (1) over pathway (2) for
the complete decomposition of hydrazine in aqueous solution
to hydrogen and nitrogen.
In summary, we report a surfactant stabilised highly active
bimetallic Ni0.95Ir0.05 alloy nanocatalysts prepared by alloying
Ir (5 mol%) with Ni (95 mol%), which exhibits 100% H
2
Fig. 3 (a,b) Time course plots and (c,d) the corresponding mass
selectivity for complete decomposition of hydrous hydrazine
at room temperature. Notably, the corresponding mono-
metallic counterpart has poor H selectivity (7% H selectivity,
spectral profiles of the gaseous products of the hydrous hydrazine
(0.5 M) decomposition over (a,c) Ni0.95Ir0.05–B and (b,d) Ni0.95Ir0.05–C
2
2
nanocatalysts (catalyst/substrate = 1/10 molar ratio) at 298 K.
Ir NPs) or is inactive (Ni NPs). Our efforts to search for a
high-performance catalyst system with low noble metal
content might facilitate the application of hydrous hydrazine
as a highly promising practical material for hydrogen storage.
Authors are grateful to AIST and JSPS for financial
support. S.K.S. thanks JSPS for a postdoctoral fellowship.
completed in 18 h, releasing 3.0 equivalents of gases
(
(
Fig. S12w). This is in accordance with electron microscopic
TEM), EDX and XPS results which indicated a serious
agglomeration of metal nanoparticles for surfactant free
Ni0.95Ir0.05 nanocatalysts, in contrast to the Ni0.95Ir0.05–B
and Ni0.95Ir0.05–C nanocatalysts. Further in support to the
above resuts, BET surface area measurements (Fig. S13w)
show that the surfactant free Ni0.95Ir0.05 nanocatalyst has a
Notes and references
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À1
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2
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À1
2
À1
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3
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1
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characteristic properties that are distinctly different from those
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2
(
1
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This journal is ꢀc The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 6545–6547 | 6547