Noble-metal-free bimetallic nanoparticle-catalyzed selective hydrogen generation from hydrous hydrazine for chemical hydrogen storage

Article abstract of DOI:10.1021/ja208475y

Noble-metal-free nickel-iron alloy nanoparticles exhibit excellent catalytic performance for the complete decomposition of hydrous hydrazine, for which the NiFe nanocatalyst, with equimolar compositions of Ni and Fe, shows 100% hydrogen selectivity in basic solution (0.5 M NaOH) at 343 K. The development of low-cost and high-performance catalysts may encourage the effective application of hydrous hydrazine as a promising hydrogen storage material.

Full text of DOI:10.1021/ja208475y

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Noble-Metal-Free Bimetallic Nanoparticle-Catalyzed Selective
Hydrogen Generation from Hydrous Hydrazine for Chemical
Hydrogen Storage
Sanjay Kumar Singh,†,‡ Ashish Kumar Singh,‡ Kengo Aranishi, and Qiang Xu*
National Institute of Advanced Industrial Science and Technology (AIST), Ikeda, Osaka 563-8577, Japan
S Supporting Information
b
incomplete and undesired decomposition to ammonia by the
ABSTRACT: Noble-metal-free nickelÀiron alloy nanopar-
ticles exhibit excellent catalytic performance for the com-
plete decomposition of hydrous hydrazine, for which the
NiFe nanocatalyst, with equimolar compositions of Ni and
Fe, shows 100% hydrogen selectivity in basic solution (0.5
M NaOH) at 343 K. The development of low-cost and high-
performance catalysts may encourage the effective applica-
tion of hydrous hydrazine as a promising hydrogen storage
material.
reaction 3H2NNH2 f N2 + 4NH3 (pathway 2) must be avoided.
Studies have shown a critical dependency of the decomposition
of hydrazine via pathway 1 or 2 on the catalyst used and the
reaction conditions.8,10
Our continuous research activities aiming toward the devel-
opment of high-performance catalysts for hydrogen generation
from hydrous hydrazine under moderate conditions have shown
that bimetallic alloy combinations based on nickel and a noble
metal (Rh, Pt, or Ir) can lead to complete decomposition of
hydrous hydrazine to generate hydrogen selectively.8 However,
despite the high catalytic performance of the bimetallic nano-
particle catalysts (nanocatalysts) investigated to date and our
efforts to reduce their noble metal content, the development of a
completely noble-metal-free catalyst remains challenging but is
crucial for promoting the potential application of hydrous hydrazine
as a hydrogen storage material. Herein we report high-performance
noble-metal-free bimetallic NiÀFe nanoparticle catalysts for
complete and selective decomposition of hydrous hydrazine to
hydrogen under moderate conditions.
Bimetallic NiÀFe nanocatalysts were prepared using a surfac-
tant-aided coreduction process (Scheme 1). To an aqueous
solution of nickel(II) chloride and ferrous(II) sulfate was added
an aqueous solution of sodium borohydride in the presence of
hexadecyltrimethylammonium bromide (CTAB). Ni3Fe, NiFe,
and NiFe3 represent the NiÀFe nanocatalysts prepared with
varying compositions of Ni and Fe, with Ni/Fe molar ratios of
3:1, 1:1 and 1:3, respectively. X-ray photoelectron spectroscopy
(XPS) spectra (Figures S1ÀS3)11 of the NiÀFe nanocatalysts
exhibit the coexistence of metallic Ni and Fe.12 In the XPS
spectrum of the NiFe nanocatalyst, a thin oxidized layer formed
during exposure of the sample to air appears with binding
energies for Ni 2p3/2 at 857.9 eV and for Fe 2p3/2 at 712.9 eV; this
layer can be readily removed by Ar sputtering (Figure S1).11
Meanwhile, the Ni 2p3/2 and 2p1/2 features with binding energies
of 852.7 and 870.2 eV, respectively, can be attributed to Ni0, and
the Fe 2p3/2 and 2p1/2 features with binding energies of 706.7
and 720.1 eV, respectively, can be attributed to Fe0. Furthermore,
with periodic Ar sputtering up to 186 min, the relative intensities
of the peaks due to Ni0 and Fe0 remain prominent, implying the
presence of uniform compositions of Ni and Fe in the NiFe
nanocatalyst. Similar XPS patterns have also been observed for
the Ni3Fe and NiFe3 nanocatalysts (Figures S2 and S3).11 As a
result of exposure of the NiÀFe nanocatalysts, oxides of iron and
anosized bimetallic heterogeneous catalysts, especially
N
those based on nonprecious, low-cost, and abundant metals,
are highly demanded for various interdisciplinary chemical
processes such as hydrogenation/dehydrogenation, fuel-cell
electrocatalysis, catalytic reforming, ammonia decomposition,
and so on.1,2 In view of the futuristic aspect of the use of
hydrogen as a clean fuel and the hurdles in its safe and efficient
storage, various advanced research approaches for the develop-
ment of new materials that can store and deliver hydrogen at
acceptable rates have been discovered.3À5 Unfortunately, even
after several decades of exploration, no single material investi-
gated to date meets all of the necessary transportation require-
ments, such as volumetric and gravimetric hydrogen capacities,
handling pressure and temperature, recycling of byproducts,
and so on.3À5 Recent developments in this direction have
suggested hydrous hydrazine, such as hydrazine monohydrate
(H2NNH2 H2O), which is a liquid over a wide range of
3
temperature (213À392 K),6 as a promising material for hydro-
gen storage.7,8 This material contains a hydrogen content avail-
able for hydrogen generation as high as 8.0 wt %, which could be
released by complete decomposition of hydrazine to hydrogen
and nitrogen via the reaction H2NNH2 f N2 + 2H2 (pathway 1),
and possesses the distinct advantages of easy recharging and the
availability of the current infrastructure of liquid fuels for
recharging. Furthermore, besides hydrogen, the only production
is nitrogen, which does not need on-board collection for
recycling. Moreover, nitrogen, being the most abundant gas in
air, can be transformed to ammonia (by the HaberÀBosch
process, homogeneous catalytic processes, or an electrolytic
process) and then to hydrazine on a large scale or perhaps
transformed directly to hydrazine by means of an electrolytic
process similar to that for ammonia synthesis.9 However, to exert
the hydrogen storage properties of hydrazine efficiently, its
Received: September 8, 2011
Published: November 09, 2011
r
2011 American Chemical Society
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Scheme 1. NiFe Nanocatalyst Preparation and Hydrazine
Decomposition
Figure 2. (a) Cross-sectional compositional line profiles of Ni and Fe in
NiFe nanoparticles recorded along the line shown in the HAADF-
STEM image (inset). (b) HRTEM image and the corresponding SAED
pattern (inset) for NiFe nanoparticles.
Figure 1. (a, c) TEM images and (b, d) corresponding HRTEM images
of NiFe nanoparticles.
nickel were formed on the catalyst surface. Since iron is more
sensitive to air than nickel, the XPS profiles of NiFe3 nanocata-
lysts show more oxide layers. Powder X-ray diffraction (PXRD)
measurements for the NiÀFe nanocatalysts illustrate the major
diffraction peaks over a 2θ range of 40À50°, which can be
indexed to the fcc (111) plane (Figure S4).11 The diffraction
peak for the NiFe nanocatalyst between the fcc diffraction peaks
of nickel and iron with a lattice constant of 3.589 Å indicates the
homogeneous distribution of Ni and Fe atoms in the NiFe
nanocatalyst.
Figure 3. Comparison of H2 selectivities in the decomposition of
hydrous hydrazine (0.5 M) to hydrogen in the presence of Ni, Ni3Fe,
NiFe, NiFe3, and Fe nanocatalysts (catalyst/H2NNH2 = 1:10) with
NaOH (0.5 M) at (a) 298 and (b) 343 K.
Transmission electron microscopy (TEM) images reveal an
average particle size of 10 nm for the NiFe nanoparticles (Figure 1
and Figure S5).11 The high-resolution TEM (HRTEM) images
(Figures 1b,d and 2b) and corresponding selected-area electron
diffraction (SAED) pattern (Figure 2b) indicate the crystalline
nature of the NiFe nanoparticles. Energy-dispersive X-ray spec-
troscopy (EDS) point analyses of randomly chosen nanoparticles
exhibit the presence of both Ni and Fe (Figure S7).11
We further characterized the NiFe nanoparticles by high-angle
annular dark-field scanning transmission electron microscopy
(HAADF-STEM) and EDS line scanning analyses (Figure 2a).
The compositional line profiles of Ni and Fe on the NiFe nano-
particles indicate the uniform alloy composition of NiFe nano-
particles without significant segregation of each component.
For the NiFe3 catalyst, TEM and EDS measurements showed
that the Ni/Fe ratio is constant at randomly selected points,
indicating its uniform alloy composition (Figure S8).11
The catalytic performance of the nanocatalysts was evaluated on
the basis of the volumetrically measured amount of gases released
during the reaction. It was recently reported that the catalytic
performance of Ni and other related bimetallic nanocatalysts for
the decomposition of hydrous hydrazine can be significantly
improved with the elevation of the reaction temperature from
room temperature to 323 K or higher.8a It was also found that the
enhancement was not limited to the acceleration of reaction
kinetics but was also extensively observed in the selectivity for the
decomposition of hydrous hydrazine to hydrogen.
Investigations of the catalytic performance of NiÀFe nanoca-
talysts with various Ni/Fe molar ratios for the decomposition of
hydrous hydrazine at 343 K suggested a significant dependence
of the hydrogen selectivity on the composition of the nanocata-
lyst. Although the NiFe nanocatalyst exhibited the highest
catalytic performance among the examined NiÀFe nanocata-
lysts, only 81% hydrogen selectivity was achieved (Figure S9).11
Surprisingly, it was found that the addition of NaOH significantly
enhanced the H2 selectivity (Figure S9).11 The NiFe nanocata-
lyst released gases in a stoichiometric amount (3.0 equiv) from
Catalytic decomposition of hydrous hydrazine in the presence
of the as-prepared NiÀFe nanocatalysts was initiated by the
introduction of hydrazine monohydrate into the reaction flask,
which was vigorously shaken at a constant reaction temperature.
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COMMUNICATION
of hydrous hydrazine is critically influenced by the components
of the bimetallic nanocatalyst (Figure S16).11 In contrast to the
significant enhancement of the catalytic activity of the bimetallic
NiFe nanoparticle catalyst, the replacement of Fe with Co in NiCo
nanoparticle catalysts results in only ∼18% hydrogen selectivity,
whereas NiCu nanoparticles exhibit ∼15% hydrogen selectivity for
the decomposition of hydrous hydrazine under analogous reaction
conditions. Other bimetallic nanoparticle catalysts obtained by
alloying different non-noble metals (e.g., CoCu) exhibited ∼15%
hydrogen selectivity, whereas the CoFe and FeCu nanoparticle
catalysts were found to be catalytically inactive for this reaction
under our reaction conditions.
The above results strongly support the conclusion that the
catalyst performance is substantially governed by its structure
and composition resulting from the introduction of a second
component. Considering the poor catalytic performance of the
monometallic counterparts, where Fe is inactive and Ni shows
low activity for the catalytic decomposition of hydrous hydrazine,
the interaction between Ni and Fe in the NiFe alloy, in addition
to the effects of temperature and alkaline additives, results in the
significant enhancement of the hydrogen selectivity for hydra-
zine decomposition. The observations make it reasonable to pro-
pose that the alloying of Ni and Fe leads to a modification of the
catalyst surface through incorporation of considerable interme-
tallic electronic interactions, and as a result, the presence of both
Ni and Fe on the catalyst surface significantly tunes the interac-
tions between the catalyst surface and the hydrazine bonds as
well as the stability of the reaction intermediates on the catalyst
surface. Consequently, the coexistence of Ni and Fe on the
catalyst surface results in the activation of hydrazine preferen-
tially by pathway 1 over pathway 2 for its complete decomposi-
tion to hydrogen and nitrogen.
Taking into consideration the previously reported NiÀRh,
NiÀPt, and NiÀIr catalysts,8 alloying Ni and the second metal, as
a common feature, at an atomic level is the key to obtaining
improved catalytic performance. It should be noted that the
combination of Ni and another noble metal with an appropriate
Ni/M ratio (M = Rh, Pt, Ni) can achieve 100% H2 selectivity at
room temperature without the help of NaOH. However, while
the NiÀFe catalyst requires a higher temperature (343 K) and
the coexistence of NaOH to reach 100% H2 selectivity, the
present NiFe catalyst is free of noble metals, which is of high
importance from the viewpoint of practical use.
Figure 4. Time-course plots for the decomposition of hydrous hydra-
zine (0.5 M) to hydrogen in the presence of (a) Fe, (b) Ni, and (c) NiFe
nanocatalysts (catalyst/H2NNH2 = 1:10) with NaOH (0.5 M) at 343 K.
the decomposition of hydrous hydrazine in 190 min with NaOH
(0.5 M) at 343 K (Figures 3 and 4 and Figure S9), corresponding
to complete decomposition of hydrous hydrazine to hydrogen
with 100% selectivity via pathway 1. In addition, the NiFe catalyst
exhibited high stability (Figure S10).11 The possible reason for
the effects of the alkaline additive might be understood as
follows: pathway 2 gives the basic product NH3; the addition
of NaOH may make the catalyst surface highly basic, which may
be unfavorable for the formation of basic NH3 and therefore for
pathway 2. In contrast to the NiFe nanocatalyst, 89% hydrogen
selectivity was observed for the Ni3Fe nanocatalyst, whereas the
NiFe3 nanocatalyst exhibited 71% hydrogen selectivity for the
decomposition of hydrous hydrazine at 343 K (Figure 3 and
Figure S11).11 Notably, addition of weaker bases (e.g., NH3,
CH3COONa) had no effect on the catalytic performance of the
NiFe catalysts (Figures S12 and S13).11 Improvements in the
catalytic performance due to addition of NaOH were also
observed for Ni45Pt55 and Ni50Ir50 catalysts; the addition of
NaOH (0.5 M) resulted in increases in the H2 selectivity from 61
to 86% for Ni45Pt55 and 7 to 95% for Ni50Ir50 at 298 K (Figure
S14),11 indicating that the presence of alkaline additives is
commonly beneficial in promoting pathway 1.
None of the NiÀFe nanoparticle catalysts were found to be
active for this reaction when examined at 298 K with or without
NaOH, indicating the crucial role of temperature in the decom-
position reaction (Figure 3). Fe nanoparticles prepared under
analogous conditions exhibit no activity for this reaction at 298 K
or at elevated temperatures. Ni nanoparticles, which are also
inactive at room temperature for the decomposition of hydrous
hydrazine, can show an enhancement in the activity with an
increase in the reaction temperature.7,8 An investigation of the
role of temperature on the NiFe nanocatalyst showed enhanced
activity and H2 selectivity for decomposition of hydrazine to
hydrogen at elevated temperatures. Although the NiFe catalyst
was inactive at 298 K, the activity was enhanced along with the
enhancement in H2 selectivity to 80% at 323 K and 100% at 343 K
in the presence of NaOH (0.5 M) (Figure S15).
In conclusion, we have presented here the development of a
highly efficient and low-cost catalyst of nonprecious nickel and
iron nanoparticles in alloy form, which achieves 100% hydrogen
selectivity from hydrous hydrazine decomposition in an alkaline
solution at 343 K. The present finding demonstrates the im-
portance of combining different non-noble metals to develop
active metal nanocatalysts whose distinct surface properties
enable them to outperform their parent metals. These low-cost,
high-performance catalysts may strongly encourage the effective
application of hydrous hydrazine as a promising hydrogen
storage material.
The enhanced catalytic activities observed for NiÀFe alloy
nanoparticles are of critical importance in view of the fact that
their corresponding mononuclear counterparts are either inac-
tive(Fenanoparticles) orless active(Ninanoparticles) (Figure4).
Investigations of the catalytic activities under analogous conditions
for other bimetallic nanoparticle catalysts prepared using different
non-noble metal combinations revealed that the decomposition
’ ASSOCIATED CONTENT
S
Supporting Information. Details of NiÀFe nanocata-
b
lyst preparation and characterization by PXRD, scanning elec-
tron microscopy, TEM, and XPS and the catalytic hydrazine
decomposition experiments. This material is available free of
charge via the Internet at http://pubs.acs.org.
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’ AUTHOR INFORMATION
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Present Addresses
†Institute of Inorganic Chemistry, Karlsruhe Institute of Tech-
nology (KIT), Geb. 30.45, Engesserstr. 15, D-76131 Karlsruhe,
Germany.
Author Contributions
‡These authors contributed equally.
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’ ACKNOWLEDGMENT
The authors thank the reviewers for valuable suggestions and
AIST and JSPS for financial support. S.K.S. thanks JSPS for a
postdoctoral fellowship.
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