Controlled synthesis of ultrafine surfactant-free NiPt nanocatalysts toward efficient and complete hydrogen generation from hydrazine borane at room temperature

Article abstract of DOI:10.1021/cs501329c

A sodium-hydroxide-assisted reduction approach has been developed to control the synthesis of ultrafine surfactant-free bimetallic NiPt nanoparticles (NPs) supported on nanoporous carbon, Maxsorb MSC-30. For the first time, this catalyst exerts exceedingly high catalytic activity for 100% selective conversion of hydrazine borane (HB) to hydrogen at room temperature. This remarkably facile and effective reduction approach provides a powerful entry into ultrafine alloy NPs to make full use of noble metals and achieve enhanced performance, where the compositions of the alloy can be widely adjusted and tailored. Moreover, the catalytic results open up new avenues in the effective application of HB as a promising hydrogen storage material. (Chemical Equation Presented).

Full text of DOI:10.1021/cs501329c

Research Article
pubs.acs.org/acscatalysis
Controlled Synthesis of Ultrafine Surfactant-Free NiPt Nanocatalysts
toward Efficient and Complete Hydrogen Generation from Hydrazine
Borane at Room Temperature
†
†
‡
,†
Qi-Long Zhu, Di-Chang Zhong, Umit B. Demirci, and Qiang Xu*
†National Institute of Advanced Industrial Science and Technology (AIST), 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan
‡IEM (Institut Europeen des Membranes), UMR 5635 (CNRS-ENSCM-UM2), Universite Montpellier 2, Place E. Bataillon,
F-34095, Montpellier, France
*
S Supporting Information
ABSTRACT: A sodium-hydroxide-assisted reduction approach has been
developed to control the synthesis of ultrafine surfactant-free bimetallic NiPt
nanoparticles (NPs) supported on nanoporous carbon, Maxsorb MSC-30. For the
first time, this catalyst exerts exceedingly high catalytic activity for 100% selective
conversion of hydrazine borane (HB) to hydrogen at room temperature. This
remarkably facile and effective reduction approach provides a powerful entry into
ultrafine alloy NPs to make full use of noble metals and achieve enhanced
performance, where the compositions of the alloy can be widely adjusted and
tailored. Moreover, the catalytic results open up new avenues in the effective
application of HB as a promising hydrogen storage material.
KEYWORDS: hydrazine borane, hydrogen evolution, ultrafine nanoparticles, bimetallic, heterogeneous catalysis
1
. INTRODUCTION
storage materials, represented by the sodium borohydride
NaBH ) and ammonia borane (NH BH ), have been
(
4
3
3
Ultrafine metal nanoparticles (NPs) with extremely narrow size
range have attracted particular interest due to their increased
surface area and the number of edge and corner atoms, which
4
extensively studied owing to the lightness of boron.
Hydrolyses of NaBH and NH BH have attracted considerable
4
3
3
1
attention because of their significant hydrogen contents (10.8
and 19.6 wt % H, respectively). Yet their technological
implementation suffers from low effective material-based
gravimetric hydrogen storage capacities (GHSC, 7.3 wt % in
NaBH −4H O, eq 1 and 5.9 wt % in NH BH −4H O, eq 2)
greatly improve the catalytic properties. However, as the
surface energy increases with decreasing particle size, the
syntheses of ultrafine nanoparticles remain a challenge in terms
of particle size and distribution control, and catalyst stability
under various operating conditions. In order to get well-
dispersed metallic particles and protect them from agglomer-
ation, organic surfactants have been widely used. For instance,
it has been reported that ultrafine and uniform alloy
4
2
3
3
2
for consideration in vehicle applications. Recently, hydrous
2
hydrazine (N H ·H O), a liquid at room temperature having
2
4
2
high hydrogen capacity (8.0 wt %), has been identified as a
potential liquid-phase chemical hydrogen storage material,
whereas the safety issues due to its volatility and toxicity need
nanoparticles can be achieved by using dendrimers and
oleylamines as stabilizing agents in the syntheses.
2a,e,f
Never-
5
to be solved for practical use.
theless, the exposed surface of nanocatalysts is often
contaminated by the chemical interactions between the
surfactant molecules and the catalyst surface in the process of
preparation and therefore decreases the catalytic activities. For
catalytic applications, general and facile methods that can easily
control the nucleation and growth of the uncapped metal NPs
with high uniformity, especially non-noble metal-based NPs, are
highly desirable.
On the other hand, despite decades of extensive efforts, the
search for effective on-board hydrogen storage materials with
the combination of high gravimetric/volumetric hydrogen
density and adequate kinetics remains one of the most
challenging barriers on the path to a “hydrogen economy”
Received: August 22, 2014
Revised: October 15, 2014
3
society. Since the early 2000s, boron-based chemical hydrogen
©
XXXX American Chemical Society
4261
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Research Article
Hydrazine borane (HB, N H BH ), a stable and safe solid at
room temperature having hydrogen content as high as 15.4 wt
to hydrogen via hydrolysis of BH and decomposition of N H
4
at room temperature.
2
4
3
3
2
%
, has high potential for use as a high-performance hydrogen
6
−8
storage material.
It can be prepared by mixing sodium
2. EXPERIMENTAL SECTION
borohydride and hydrazine hemisulfate at room temperature
2
.1. Chemicals and Materials. Hydrazine borane (HB)
6c
7a
and obtained at high purity (i.e., 99%) in our conditions.
A
was synthesized according to the reported literature. The
other chemicals were purchased and used without further
purification. Nickel chloride hexahydrate (NiCl ·6H O, Wako
Pure Chemical Industries, Ltd., 99%), potassium tetrachlor-
oplatinate (K PtCl , Wako Pure Chemical Industries, Ltd.,
strategy combining hydrolysis of the group BH and selective
3
decomposition of the moiety N H to N and H (eq 3a) might
2
4
2
2
2
2
7
be a promising approach for complete dehydrogenation of HB.
This corresponds to a theoretical GHSC of 10.0 wt % for the
2
4
system HB−3H O, which is much higher than those of
>98%), sodium borohydride (NaBH , Sigma-Aldrich, 99%),
2
4
NaBH −4H O and NH BH −4H O. With suitable catalysts, it
sodium hydroxide (NaOH, Chameleon Reagent, >98%),
Maxsorb MSC-30 (Nanoporous Carbon, Kansai Coke and
Chemicals Co. Ltd.), and cetyltrimethylammonium bromide
(CTAB, Sigma-Aldrich, 95%) were used as received. Deionized
water with a specific resistance of 18.2 MΩ·cm was obtained by
reverse osmosis followed by ion-exchange and filtration (RFD
4
2
3
3
2
is expected to readily hydrolyze the group BH in an aqueous
3
solution at room temperature, producing hydrazine N H , in
2
4
comparison with the formation of ammonia NH3 in the
hydrolysis of ammonia borane. Furthermore, the in situ formed
hydrazine can be selectively decomposed to N and H in the
2
2
2
50NB, Toyo Seisakusho Kaisha, Ltd., Japan).
.2. Syntheses of Catalysts. The NiPt nanoparticles
presence of appropriate catalysts, which makes HB more
attractive than AB. However, to maximize the efficacy of HB as
a hydrogen storage material, one must avoid the undesired
reaction pathway given by eq 3b. Recently, a few surfactant-
stabilized nanocatalysts have been synthesized to catalyze the
dehydrogenation of HB, but they display incomplete hydrogen
selectivity and slow kinetics even at elevated temperatures (50
2
supported by MSC-30 were synthesized by a facile sodium-
hydroxide-assisted reduction method. Typically, 100 mg of
MSC-30 carbon powder in a two-necked round-bottomed flask
was ultrasonically dispersed in 2.5 mL of water and
subsequently mixed with an aqueous solution of K PtCl
2
4
7a,8
and/or NiCl2 with desired concentrations. The resultant
aqueous suspension was further homogenized under sonication
°C).
Therefore, it is critical to develop new catalysts that
exhibit high catalytic performance to both hydrolysis of the
group BH and selective decomposition of the moiety N H
4
for 30 min. Then, 12 mg of NaBH dissolved in 1.0 mL of 3.0
4
3
2
M NaOH solution was added into the above obtained solution
with vigorous shaking, resulting in the generation of catalyst as
under moderate conditions for establishment of the hydrogen
storage system based on HB.
2+
2+
2+
a dark suspension. The molar ratios of Pt /(Pt + Ni ) were
changed with several values (0, 0.05, 0.15, 0.30, 0.40, 0.50. 0.70,
Owing to its inherent advantages such as accessible porosity,
large surface area, high chemical and thermal stability,
nanoporous carbon, Maxsorb MSC-30 (pore size, 2.1 nm;
2+
2+
1
.00), while the molar contents of (Pt + Ni ) added to 100
mg of MSC-30 matrix were kept at 0.10 mmol. The catalysts
with different Pt/Ni molar ratios were denoted as Ni1−xPtx/
MSC-30.
2
−1
BET surface area, ∼3000 m g ) has capabilities that make it
superior to organic surfactants as support for high-performance
9
nanocatalysts. Moreover, the anchoring of nanocatalysts on
For comparison, another catalyst with Ni/Pt molar ratio of
:4 was prepared using the same synthetic procedure but in the
MSC-30 could be capable of facilitating the electron transfer
and mass transport kinetics during the catalytic reaction
6
absence of NaOH, which was denoted as Ni Pt /MSC-
0.6 0.4
9
b
process. Herein, encouraged by the catalytic performance of
the NiPt system having shown hydrogen selectivity up to 100%
3
0_N.
.3. Instrumentation. Laboratory powder X-ray diffraction
2
5e,10
in the dehydrogenation of hydrous N H ,
we synthesized
2
4
patterns were collected for synthesized catalysts on a Mac
Science MXP3 V diffractometer with Ni-filtered Cu Kα
radiation (λ = 0.15406 nm; 40 kV, 20 mA). The surface area
MSC-30 supported NiPt nanocatalyst by using a facile one-pot
sodium-hydroxide-assisted reduction approach in the presence
of sodium hydroxide, which played a key role to achieve
ultrafine NiPt alloy NPs via generating intermediate nickel
hydroxide gel during the reduction process (Figure 1).
Surprisingly, the resultant surfactant-free nanocatalyst is the
first one exerting 100% hydrogen selectivity and exceedingly
high activity toward complete conversion of hydrazine borane
measurements were performed with N adsorption/desorption
2
isotherms at liquid nitrogen temperature (77 K) after
dehydration under vacuum at 120 °C for 12 h using automatic
volumetric adsorption equipment (Belsorp-max). X-ray photo-
electron spectroscopic (XPS) measurements were conducted
on a Shimadzu ESCA-3400 X-ray photoelectron spectrometer
using an Mg Kα source (10 kV, 10 mA). The argon sputtering
experiments were carried out under the conditions of
background vacuum of 3.2 × 10− Pa and sputtering
acceleration voltage of 2 kV and sputtering current of 10 mA.
Charging of the catalyst samples was corrected by setting the
binding energy of the adventitious carbon (C 1s) at 284.6 eV.
The TEM and HAADF-STEM images and the EDX spectra
were recorded on transmission electron microscope (TEM,
6
2
TECNAI G F20) with operating voltage at 200 kV equipped
with energy-dispersive X-ray (EDX) detector. Mass analysis of
the generated gases was performed using a Balzers Prisma QMS
2
00 mass spectrometer. UV−vis absorption spectra were
Figure 1. Schematic representation for the preparation of NiPt/MSC-
3
0 nanocatalyst via a sodium-hydroxide-assisted reduction approach
recorded on a Shimadzu UV-2550 spectrophotometer and
corrected by deionized water as background absorption.
and gas (H + N ) evolution from the dehydrogenation of HB.
2
2
4
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Figure 2. (a,b) TEM and (c) HAADF-STEM images of Ni Pt /MSC-30, (d) TEM image of Ni Pt /MSC-30_N, and size distribution
0.6 0.4
0.6 0.4
histograms of (e) Ni Pt /MSC-30 and (f) Ni Pt /MSC-30_N.
0.6 0.4
0.6 0.4
2
.4. Catalysis. The reaction apparatus for measuring the
catalytic dehydrogenation of HB was carried out for five runs at
30 °C.
7b
H /N evolution from HB is the same as previously reported.
2
2
Stability of the Catalysts. After the reaction, the catalyst
Typically, the two-necked round-bottomed flask (30 mL) with
the as-synthesized catalyst was placed in a water bath at 30 °C
under ambient atmosphere. A gas buret filled with water was
connected to the reaction flask to measure the volume of
released gas (lab temperature kept constant at 25 °C during
measurements). The reaction started under shaking when 1.0
mL of aqueous solution containing 1.0 mmol HB was injected
into the mixture using a syringe. The volume of the evolved
hydrogen gas was monitored by recording the displacement of
water in the gas buret. The reaction was completed when there
was no more gas generation. The molar ratios of (Ni + Pt)/HB
were theoretically fixed at 0.10 for all the catalytic reactions.
Durability of the Catalysts. For testing the durability of the
Ni Pt /MSC-30 was separated from the reaction solution by
0.6 0.4
centrifugation, which was washed three times with water, and
then dried under vacuum at room temperature for the PXRD
and TEM analyses.
2
.5. Calculation Methods for Kinetics. The metal
dispersion (D ) value is defined as the ratio of the number
M
of surface atoms to the total number of atoms. For the
supported metal catalyst, the metal dispersion was calculated on
the basis of the average particle diameter by using the
11
equation:
DM = 6
nsM
ρNd
P
catalyst Ni Pt /MSC-30, an aliquot of HB in 1.0 mL of 1.0 M
NaOH solution was subsequently added into the reaction flask
after the completion of the last run. This cycling test for the
0.6 0.4
Where n is the number of metal atoms on the surface per unit
s
19
−2
19
−2
area (1.25 × 10 m for Pt, and 1.54 × 10 m for Ni), M is
1
−
−1
the molecular weight (195.08 g mol for Pt, and 58.69 g mol
4
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Figure 3. Color changes of the aqueous solution of Ni2+ and Pt2+ during the reduction processes in the (a) presence and (b) absence of NaOH.
for Ni), ρ is the density (21.35 g cm for Pt, and 8.91 g cm−3
−
3
(Figure 2a−c and 2e). The corresponding selected area energy
dispersion (SAED) pattern indicates the crystalline nature of
the Pt NPs (inset in Figure 2b). When the NiPt NPs were
loaded without NaOH, a larger average particle size of 4.2 ± 0.6
nm was observed with some severely aggregated particles
(Figures 2d,f and S5). However, the X-ray diffraction (XRD)
and X-ray photoelectron spectroscopy (XPS) analyses show
that the two catalysts possess similar crystallinity and electronic
structures (Figures S2−S3). The XRD patterns of both
catalysts display primarily the characteristics of the face-
centered cubic (fcc) Pt with slight shift to higher angles with
respect to those of Pt/MSC-30, suggesting a lattice contraction
and alloy formation between Pt and Ni because platinum is
2
3
−1
for Ni), N is 6.023 × 10 mol , and d is the average particle
P
diameter measured by TEM. The average values of these
parameters were used to calculate the metal dispersion of NiPt
considering the molar ratio of Ni to Pt in the catalysts.
The kinetics of hydrogen generation in this work was
evaluated by using site-time- yield (STY) value, which is
defined as the number of molecules of a specified product made
12
per catalytic site per unit time. STY was calculated from the
equation:
5
P V /RT
atm gas
STY =
6
D nmetalt
M
13
known to alloy well with nickel. The presence of NiPt NPs
where Patm is the atmospheric pressure (101 325 Pa), Vgas is the
final volume of generated H /N gas, R is the universal gas
constant (8.3145 m Pa mol K ), T is the room temperature
298 K), DM is the metal dispersion, nmetal is the total mole
number of NiPt atoms in catalyst, and t is the completion time
was also verified by N sorption. In comparison to metal-free
2
2
2
MSC-30, appreciable decreases in the amount of N adsorption
2
3
−1 −1
for both catalysts were observed (Figure S1). In addition, the
larger decrease of surface area for Ni Pt /MSC-30 is
(
0
.6 0.4
attributed to the occupation of the dispersed size-matched
NiPt NPs in the mesopores (∼2.2 nm) of MSC-30. The
particles in the confined cavities of MSC-30 can be protected
from aggregation and thus may provide higher catalytic stability
and durability. However, due to the particle size larger than the
pore size of the carbon support, NiPt NPs in Ni Pt /MSC-
of the reaction.
3
. RESULTS AND DISCUSSION
The surfactant-free NiPt alloy NPs supported on MSC-30 were
synthesized by coreduction of nickel(II) chloride and
potassium hexachloroplatinum (II) in the presence of MSC-
0.6 0.4
30_N should be almost located on the surface of the MSC-30
framework.
3
0 using an alkaline solution of sodium borohydride as a
reductant. The resulting Ni1−xPt /MSC-30 was used for
Therefore, NaOH serves as an efficient dispersing agent to
control the size distribution during the growth of NiPt NPs.
For further making clear the role of NaOH in the catalyst
formation, we tried to gather insight into the changes of the
metal precursors during the reduction process by monitoring
the color changes. To eliminate the color disturbance of carbon,
only an aqueous solution of the metal precursors was used. As
x
catalytically releasing H from HB. Because the preparation of
2
NiPt catalysts was carried out in a reductive condition without
exposure to air, which then were used for the dehydrogenation
of HB immediately, the oxidation of nickel has been mostly
avoided. The catalyst with the Ni/Pt atomic ratio of 6:4
exhibited the highest activity (vide infra), and therefore,
Ni Pt /MSC-30 was chosen as the model catalyst to explore
2
+
shown in Figure 3, contrary to the fast reduction of Ni and
0.6 0.4
2
+
the effect of sodium hydroxide on the distribution of the
Pt by NaBH , upon the introduction of the NaOH and
4
2+
2+
nanoparticles. For comparison, Ni Pt /MSC-30_N was also
NaBH mixture to the aqueous Ni and Pt solution, the fast
0
.6 0.4
4
prepared in the absence of NaOH.
generation of Ni(OH) gel occurred preferentially prior to the
2
2+
The microstructures of Ni Pt /MSC-30 and Ni Pt /
reduction. The hydrated Pt ions would be simultaneously
converted into the chlorohydroxypallatium(II) species upon
the addition of NaOH, for example, PtCl(OH)(H O) or
0
.6 0.4
0.6 0.4
MSC-30_N catalysts were investigated with a full character-
ization. The transmission electron microscopy (TEM) and
high-angle annular dark-field scanning TEM (HAADF-STEM)
images of Ni Pt /MSC-30 show that the NiPt NPs are highly
2
2
−
PtCl (OH)(H O) , which could be stable for a long time (1−2
2
2
14
days). Subsequently, the hydroxide intermediate acted as
precursor and capping agent in the successive reduction to
prevent the resulting NiPt NPs from aggregation, leading to the
0.6 0.4
dispersed into the framework of MSC-30 with an average
particle size of 2.1 ± 0.3 nm based on TEM observations
4
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Figure 4. (a) Volume of the generated gas (H + N ) versus time and (b) Pt-content dependence of n(H + N )/n(HB) for the dehydrogenation of
2
2
2
2
HB over NiPt/MSC-30 with different Ni/Pt molar ratios prepared with NaOH (nmetal/nHB = 0.1, 30 °C).
a
Table 1. Catalytic Activities for Dehydrogenation of Hydrazine Borane Catalyzed by Different Heterogeneous Catalysts
H generation rate (L h− gmetal−1)
1
ref
catalyst
n(metal)/n(HB)
temp (°C)
n(H + N )/n(HB)
2
2
2
H2
Ni0.6Pt0.4/MSC-30
0.1
30
50
50
50
50
50
50
50
50
5.95 ± 0.05
5.80 ± 0.20
5.79 ± 0.05
4 ± 0.05
5.1 ± 0.05
4.9 ± 0.05
4.1
142
24
7
this work
Rh Ni/CTAB
4
0.1
7b
7a
8a
8a
8a
8b
8b
8b
Ni0.89Pt0.11/CTAB
Ni0.77Ru0.23/CTAB
Ni0.89Rh0.11/CTAB
Ni0.89Ir0.11/CTAB
0.14
0.15
0.16
0.14
0.013
0.013
0.013
very low
very low
very low
very low
very low
very low
RhCl precatalyst
3
RuCl precatalyst
3.3
3
IrCl precatalyst
3.3
3
a
The catalysts that can only release the hydrogen from the BH group of HB are not included.
3
formation of ultrafine NPs. In the process of reduction, the
first-seeded Pt atoms are catalytically active for hydrogen
generation by hydrolysis of sodium borohydride; this is
accompanied by the generation of the intermediate Pt−H
species, which act as a reducing agent for the reduction of
nickel hydroxide and thus lead to the formation of NiPt NPs, as
disperse the Pt species and prevent the aggregation of the final
metal NPs.
The catalytic activity of Ni1−xPt /MSC-30 prepared in the
x
presence of NaOH for HB dehydrogenation has been
investigated at room temperature with a constant molar ratio
of catalyst/HB = 1:10. It has been found that the catalytic
activity and selectivity to hydrogen strongly depend on the Ni/
Pt ratio (Figure 4). The dehydrogenation of HB was initiated
by the introduction of aqueous HB solution into the reaction
flask with vigorous shaking at room temperature. The released
gas was measured volumetrically, and its composition was
analyzed by mass spectrometry. Both monometallic Ni and Pt
nanocatalysts showed activity for hydrogen release by
hydrolysis of the BH3 group in HB only, whereas the
15
evidenced by the XPS measurement (Figure S3a). The
reduction of Ni2 and Pt also has been followed by UV−vis
spectroscopy. From the UV−vis spectra of the monitoring for
the reduction process of NiPt NPs at different reaction times
+
2+
(
Figure S14), it can be found that the intensity of the strong
2+
absorption of Pt species was maintained even after the
addition of the alkaline solution of sodium borohydride for 20
2+
s. The phenomena further suggest that the reduction of Ni
and Pt2 took place after generating the nickel hydroxide
+
Ni1−xPt /MSC-30 nanocatalysts with platinum contents in the
x
intermediate. Besides the formation of Ni(OH) species, the
range of 15−70 mol % exhibited high catalytic activities and
2
stabilization of NaBH in basic solution might also make the
100% hydrogen selectivity with 5.95 ± 0.05 equiv (H + N )
4
2
2
nucleation process more controllable. In contrast, the tentative
per HB released. This exceptional catalytic performance
demonstrates that the resulting synergistic effect between Ni
2
+
reduction of aqueous Ni by NaBH in alkaline conditions
4
only resulted in the light-green precipitation of Ni(OH) ; in
and Pt is required for the complete conversion of HB to H . A
detailed analysis of the results in Figure 4 shows that the
2
2
other words, Ni2 as Ni(OH) cannot be reduced by sodium
+
2
borohydride, as evidenced by the XRD, XPS and TEM
measurement (Figures S2, S3e, S6c,d, and S13). In addition,
the Ni/Pt molar ratio in the catalysts was also found to be
important for the particle size distribution. The TEM
measurement demonstrated that the NiPt NPs in catalysts
obtained with the Ni/Pt molar ratios more than 1:1 are highly
dispersed (Figure S7a−d). Although the Ni/Pt molar ratio was
decreased to less than 1:1, a larger NiPt particle size and
broader size distribution were observed (Figure S7e, f), which
may be due to the fact that the amount of Ni(OH)2
intermediate first generated was not sufficient to efficiently
catalytic activity of Ni1−xPt /MSC-30 is first improved with the
x
increasing content of Pt and reaches the maximum at 0.4. Then,
when the Pt content further increases, the tendency reverses.
These catalysts with 15−70 mol % Pt exhibit analogous time
course profiles with a process composed of two distinct steps
with different kinetics. The first step proceeds fast, where
slightly more than 3.0 equiv H evolves in less than 0.5 min,
2
corresponding to the hydrolysis of the BH group. The second
3
step, the decomposition of the N H moiety, is much slower,
2
4
which determines the time that is needed by the entire
dehydrogenation process of HB. It is observed that all the
4
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catalysts prepared by our method show much better activity
and selectivity compared to the previously reported analogous
7a
NiPt nanocatalysts stabilized by CTAB (Table 1).
The Ni Pt /MSC-30 catalyst was most active and
0
.6 0.4
efficiently and completely released gases in a stoichiometric
amount (5.95 ± 0.05 equiv) from HB in 4.5 min (nmetal/nHB
.1) at room temperature (Figure 4). The gases were identified
=
0
by mass spectrometry to be hydrogen and nitrogen with a H2/
N ratio of 5.0 (Figure S11), in agreement with the volumetric
2
observations of 100% selectivity for hydrogen. The average rate
−
1
−1
of H production was determined to be 142 L
h
gmetal −
,
2
H2
1
corresponding to a theoretical power density of 192 W h
−1
9b
gmetal for energy generation. Particularly noteworthy is that
after calibration of the size of the nanocatalyst (i.e., ca. 2.1 nm
for Ni Pt /MSC-30, corresponding to the metal dispersion
0.6 0.4
(
D ) of approximately 50%), the site-time yield of H was
M
2
−1
calculated to be 0.37 s at 30 °C, which is much higher than
those of the most active Rh Ni catalysts reported for this
reaction even at high temperatures (Table 1). Meanwhile,
Ni Pt /MSC-30 showed much higher H selectivity and is
Figure 5. Volume of the generated gas (H
2
+ N
2
2
) versus time for the
dehydrogenation of (a) HB and (b) N H ·H O over Ni Pt /MSC-
2
4
0.6 0.4
4
7b
3
0 (nmetal/nHB or N2H4·H2O = 0.1, 30 °C).
0.6 0.4
2
cheaper compared to Rh Ni (it contains only 40% Pt compared
4
to 80% Rh). It was found that there were some decreases in
activity over five cycles (Figure S12), which may be due to the
slight increase in the particle size of NiPt NPs during the
catalytic process (Figures S15, S16). Nevertheless, Ni Pt /
NiPt NPs was much lower than that of surfactant-free ones for
HB dehydrogenation (Figure S10). The observations further
confirm that the absence of any protective surfactants,
especially a hydrophobic surfactant shell around the metal
NPs in an aqueous reaction medium, significantly benefits the
access of reactants to the metal surface and therefore benefits
the catalytic activity.
0.6 0.4
MSC-30_N can only release 3 equiv of H ; even after adding
2
the same amount of NaOH as an important catalyst promoter
(vide infra), it still exhibited much lower catalytic activity than
that of Ni Pt /MSC-30 (Figure S8). It has been known that
0.6 0.4
the catalytic activity generally increases with the decrease in
metal NPs size, as smaller particles possess higher surface areas
Besides the good dispersion and the surfactant-free character
of the Ni0.6Pt0.4 NPs immobilized by MSC-30, the presence of
NaOH plays an important role in promoting the catalytic
performance. After washing away the NaOH present in the
synthesis process of Ni Pt /MSC-30, the selectivity and
1c,9b
available for reactants.
Reasonably, the decrease in the
particle size of NiPt NPs on MSC-30 due to the presence of
NaOH should be responsible for the exceedingly high activity
of Ni Pt /MSC-30. It is also concluded that the alkaline
0.6 0.4
0.6 0.4
activity of the catalyst decreased significantly with only 4.0
equiv of gases released even after 20 min (Figure S9). This
indicates that NaOH has indeed a galvanizing impact on the
selectivity and activity of the dehydrogenation of HB. Similarly,
for Ni Pt /MSC-30_N catalyst without NaOH, only 3.0
additive is extremely important for the catalyst preparation to
improvement of kinetics, which not only serves as an efficient
dispersing agent to control the particle size during the
formation of NiPt NPs in the present system but also plays a
crucial role as a catalyst promoter to improve the catalytic
performance.
0.6 0.4
equiv of gases was permitted to release, which originated only
from hydrolysis of the BH group, while the addition of NaOH
3
It should be noted that a lower catalytic activity was observed
over the Ni Pt /MSC-30 catalyst for the decomposition of
could obviously improve the catalytic performance of Ni Pt /
0
.6 0.4
0.6 0.4
MSC-30_N, further confirming the promotion effect of NaOH
Figure S8). To determine the effect of the amount of NaOH
N H ·H O than that for the dehydrogenation of HB under the
2
4
2
(
same condition (Figure 5). It took 6.5 min to completely
on the catalytic performance, we have synthesized MSC-30
supported Ni Pt NPs with various concentrations of NaOH
decompose the same amount of N H ·H O (1.0 mmol) as that
2
4
2
0.6 0.4
resulted from the second step of the HB dehydrogenation
(
0−4 M). It has been found that the selectivity and activity of
reaction. According to a previous work, the B(OH) formed in
3
7b
the catalysts for the dehydrogenation of HB increased with the
concentration of NaOH until the value reached 3M, after which
a further increase in the amount of NaOH showed little effect
on the dehydrogenation of HB (Figure 6). As HB is very stable
in the solution of MSC-30 and NaOH, NaOH only serves as a
catalyst promoter for the decomposition of the N H moiety in
the first step has no influence on the decomposition of N H .
2
4
Thus, the results reported here indicate that the dehydrogen-
ation of HB is not simply divided into two steps in this catalytic
reaction. A plausible explanation is that when the BH group of
3
HB is catalytically hydrolyzed by Ni Pt , concurrently, some
0
.6 0.4
of the resulting N H moieties directly interact with metal NPs
2
4
2
4
the second step of this reaction. The possible reason for the
effects of the alkaline additive may be understood as follows:
to generate N and H , thus promoting the kinetic properties of
2
2
HB dehydrogenation.
(
a) the addition of NaOH could decrease the concentration of
To determine the significant effect of the surfactant-free
nature of NiPt NPs on the catalytic activity, surfactant
cetyltrimethylammonium bromide (CTAB) supported
Ni Pt nanocatalyst (Ni Pt /CTAB) was prepared using
undesirable N H5 (N H5 + OH− ↔ N H + H O) and
+
+
2
2
2
4
2
promote the rate-determining deprotonation step (N
* + H*) along the decomposition process of N
and H ; (b) the alkaline solution makes the catalyst surface
H →
2
4
N
H
2
H to N
2 4 2
0.6 0.4
0.6 0.4
3
our recently reported procedure, which gave NiPt NPs with an
2
16
average size of ∼3.5 nm. Surprisingly, despite their similar
highly basic, which helps inhibit the formation of basic NH3
particle size, the catalytic activity of the surfactant-protected
and therefore benefits the high H selectivity.
2
4
266
dx.doi.org/10.1021/cs501329c | ACS Catal. 2014, 4, 4261−4268

ACS Catalysis
Research Article
Deshpande, P. A.; Rajamathi, M.; Madras, G.; Ravishankar, N. Chem.
Mater. 2011, 23, 2772−2780. (c) Zhu, Q.-L.; Li, J.; Xu, Q. J. Am.
Chem. Soc. 2013, 135, 10210−10213. (d) Kim, B. H.; Hackett, M. J.;
Park, J.; Hyeon, T. Chem. Mater. 2014, 26, 59−71. (e) Shang, L.; Bian,
T.; Zhang, B.; Zhang, D.; Wu, L.-Z.; Tung, C.-H.; Yin, Y.; Zhang, T.
Angew. Chem., Int. Ed. 2014, 53, 250−254.
(2) (a) Scott, R. W. J.; Wilson, O. M.; Oh, S.-K.; Kenik, E. A.;
Crooks, R. M. J. Am. Chem. Soc. 2004, 126, 15583−15591. (b) Ye, H.;
Crooks, R. M. J. Am. Chem. Soc. 2005, 127, 4930−4934. (c) Scott, R.
W.; Wilson, O. M.; Crooks, R. M. J. Phys. Chem. B 2005, 109, 692−
7
04. (d) Takahashi, M.; Imaoka, T.; Hongo, Y.; Yamamoto, K. Angew.
Chem., Int. Ed. 2013, 52, 7419−7421. (e) Zhang, S.; Metin, O.; Su, D.;
Sun, S. Angew. Chem., Int. Ed. 2013, 52, 3681−3684. (f) Yu, Y.; Yang,
W.; Sun, X.; Zhu, W.; Li, X.-Z.; Sellmyer, D. J.; Sun, S. Nano Lett.
2
014, 14, 2778−2782.
3) (a) Farha, O. K.; Spokoyny, A. M.; Mulfort, K. L.; Hawthorne, M.
F.; Mirkin, C. A.; Hupp, J. T. J. Am. Chem. Soc. 2007, 129, 12680−
2681. (b) Chen, B.; Zhao, X.; Putkham, A.; Hong, K.; Lobkovsky, E.
B.; Hurtado, E. J.; Fletcher, A. J.; Thomas, K. M. J. Am. Chem. Soc.
(
1
Figure 6. Volume of the generated gas (H + N ) versus time for the
2
2
dehydrogenation of HB over Ni Pt /MSC-30 prepared with
0
.6 0.4
2
008, 130, 6411−6423. (c) Fellay, C.; Dyson, P. J.; Laurenczy, G.
different concentrations of NaOH solution (1.0 mL) added (nmetal
nHB = 0.1, 30 °C).
/
Angew. Chem., Int. Ed. 2008, 47, 3966−3968. (d) Jiang, H.-L.; Singh, S.
K.; Yan, J.-M.; Zhang, X.-B.; Xu, Q. ChemSusChem 2010, 3, 541−549.
(
̈
e) Boddien, A.; Loges, B. r.; Gartner, F.; Torborg, C.; Fumino, K.;
4
. CONCLUSIONS
Junge, H.; Ludwig, R.; Beller, M. J. Am. Chem. Soc. 2010, 132, 8924−
8934. (f) Boddien, A.; Mellmann, D.; Gartner, F.; Jackstell, R.; Junge,
H.; Dyson, P. J.; Laurenczy, G.; Ludwig, R.; Beller, M. Science 2011,
In summary, for the first time, a surfactant-free NiPt bimetallic
nanocatalyst with good dispersion, prepared by using a sodium-
hydroxide-assisted reduction approach in the presence of MSC-
3
33, 1733−1736. (g) Bardhan, R.; Ruminski, A. M.; Brand, A.; Urban,
J. J. Energy Environ. Sci. 2011, 4, 4882−4895. (h) Jeon, K.-J.; Moon, H.
R.; Ruminski, A. M.; Jiang, B.; Kisielowski, C.; Bardhan, R.; Urban, J. J.
Nat. Mater. 2011, 10, 286−290.
30, exhibits remarkable catalytic activity for efficient and
complete dehydrogenation of hydrazine borane at room
temperature, wherein NaOH not only serves as an efficient
dispersing agent to control the particle size during the
formation of NiPt NPs but also plays an important role as a
catalyst promoter. The utilization of the sodium-hydroxide-
assisted reduction approach to obtain ultrafine alloy nano-
particles opens up new avenues for designing highly efficient
nanocatalysts. Meanwhile, the obtained catalyst is believed to
give a tremendous boost to the practical application of
hydrazine borane for chemical hydrogen storage with high
material-based GHSC of 10.0%.
(4) (a) Yan, J.-M.; Zhang, X.-B.; Han, S.; Shioyama, H.; Xu, Q.
Angew. Chem., Int. Ed. 2008, 47, 2287−2289. (b) Kim, S.-K.; Han, W.-
S.; Kim, T.-J.; Kim, T.-Y.; Nam, S. W.; Mitoraj, M.; Piekos, L.;
Michalak, A.; Hwang, S.-J.; Kang, S. O. J. Am. Chem. Soc. 2010, 132,
9
954−9955. (c) Metin, O. n.; Mazumder, V.; Ozkar, S.; Sun, S. J. Am.
Chem. Soc. 2010, 132, 1468−1469. (d) Jiang, H.-L.; Xu, Q. Catal.
Today 2011, 170, 56−63. (e) Hung, T.-F.; Kuo, H.-C.; Tsai, C.-W.;
Chen, H. M.; Liu, R.-S.; Weng, B.-J.; Lee, J.-F. J. Mater. Chem. 2011,
21, 11754−11759. (f) Sanyal, U.; Demirci, U. B.; Jagirdar, B. R.; Miele,
P. ChemSusChem 2011, 4, 1731−1739. (g) Neiner, D.; Karkamkar, A.;
Bowden, M.; Joon Choi, Y.; Luedtke, A.; Holladay, J.; Fisher, A.;
Szymczak, N.; Autrey, T. Energy Environ. Sci. 2011, 4, 4187−4193.
ASSOCIATED CONTENT
Supporting Information
The following files are available free of charge on the ACS
■
(
(
h) Yadav, M.; Xu, Q. Energy Environ. Sci. 2012, 5, 9698−9725.
i) Huang, Z.; Autrey, T. Energy Environ. Sci. 2012, 5, 9257−9268.
*
S
(
5) (a) Singh, S. K.; Xu, Q. J. Am. Chem. Soc. 2009, 131, 18032−
Publications website at DOI: 10.1021/cs501329c.
1
8033. (b) Singh, S. K.; Singh, A. K.; Aranishi, K.; Xu, Q. J. Am. Chem.
PXRD, BET, XPS, TEM, EDX for catalysts; results of
catalytic HB dehydrogenation; durability and stability
results of catalysts (PDF).
Soc. 2011, 133, 19638−19641. (c) He, L.; Huang, Y.; Wang, A.; Wang,
X.; Chen, X.; Delgado, J. J.; Zhang, T. Angew. Chem., Int. Ed. 2012, 51,
6191−6194. (d) Singh, S. K.; Xu, Q. Catal. Sci. Technol. 2013, 3,
1
889−1900. (e) He, L.; Huang, Y.; Wang, A.; Liu, Y.; Liu, X.; Chen,
X.; Delgado, J. J.; Wang, X.; Zhang, T. J. Catal. 2013, 298, 1−9.
6) (a) He, T.; Wu, H.; Wu, G.; Wang, J.; Zhou, W.; Xiong, Z.; Chen,
J.; Zhang, T.; Chen, P. Energy Environ. Sci. 2012, 5, 5686−5689.
b) Wu, H.; Zhou, W.; Pinkerton, F. E.; Udovic, T. J.; Yildirim, T.;
AUTHOR INFORMATION
Corresponding Author
E-mail: q.xu@aist.go.jp. Tel: +81-72-751-9562. Fax: +81-72-
51-9629.
■
(
*
(
7
Rush, J. J. Energy Environ. Sci. 2012, 5, 7531−7535. (c) Moury, R.;
Moussa, G.; Demirci, U. B.; Hannauer, J.; Bernard, S.; Petit, E.; van der
Lee, A.; Miele, P. Phys. Chem. Chem. Phys. 2012, 14, 1768−1777.
Notes
The authors declare no competing financial interest.
(7) (a) Hannauer, J.; Akdim, O.; Demirci, U. B.; Geantet, C.;
Herrmann, J.-M.; Miele, P.; Xu, Q. Energy Environ. Sci. 2011, 4, 3355−
ACKNOWLEDGMENTS
■
3
358. (b) Zhong, D.-C.; Aranishi, K.; Singh, A. K.; Demirci, U. B.; Xu,
The authors are thankful to the reviewers for valuable
suggestions, Dr. Takeyuki Uchida for TEM measurements,
and AIST and JSPS for financial support. Q.L.Z. thanks the
JSPS for a postdoctoral fellowship.
Q. Chem. Commun. 2012, 48, 11945−11947. (c) Li, C.; Dou, Y.; Liu,
J.; Chen, Y.; He, S.; Wei, M.; Evans, D. G.; Duan, X. Chem. Commun.
2
(
013, 49, 9992−9994.
8) (a) Cakanyıldırım, C.; Demirci, U. B.; Sener, T.; Xu, Q.; Miele, P.
Int. J. Hydrogen Energy 2012, 37, 9722−9729. (b) Hannauer, J.;
Demirci, U. B.; Geantet, C.; Herrmann, J.-M.; Miele, P. Int. J. Hydrogen
Energy 2012, 37, 10758−10767. (c) Sener, D.; Karahan, S.;
Zahmakiran, M.; Ozkar, S. Int. J. Hydrogen Energy 2012, 37, 5143−
REFERENCES
■
(
1) (a) Chen, X.; Wu, G.; Chen, J.; Chen, X.; Xie, Z.; Wang, X. J. Am.
Chem. Soc. 2011, 133, 3693−3695. (b) Kundu, P.; Nethravathi, C.;
4
267
dx.doi.org/10.1021/cs501329c | ACS Catal. 2014, 4, 4261−4268

ACS Catalysis
Research Article
5
4
(
151. (d) Karahan, S.; Zahmakiran, M.; Ozkar, S. Dalton Trans. 2012,
1, 4912−4918.
9) (a) Jeon, S.-i.; Park, H.-r.; Yeo, J.-g.; Yang, S.; Cho, C. H.; Han,
M. H.; Kim, D. K. Energy Environ. Sci. 2013, 6, 1471−1475. (b) Zhu,
Q.-L.; Tsumori, N.; Xu, Q. Chem. Sci. 2014, 5, 195−199.
(
10) (a) Aranishi, K.; Singh, A. K.; Xu, Q. ChemCatChem 2013, 5,
2248−2252. (b) Singh, A. K.; Xu, Q. ChemCatChem 2013, 5, 3000−
3004.
(
11) (a) Scholten, J. J. F.; Pijpers, A. P.; Hustings, A. M. L. Catal.
Rev.: Sci. Eng. 1985, 27, 151−206. (b) Carabineiro, S. A. C.; Machado,
B. F.; Bacsa, R. R.; Serp, P.; Drazic, G.; Faria, J. L.; Figueiredo, J. L. J.
Catal. 2010, 273, 191−198. (c) Long, J.; Liu, H.; Wu, S.; Liao, S.; Li,
Y. ACS Catal. 2013, 3, 647−654.
(12) Boudart, M. Chem. Rev. 1995, 95, 661−666.
(13) (a) Deivaraj, T. C.; Chen, W.; Lee, J. Y. J. Mater. Chem. 2003,
1
3, 2555−2560. (b) Lima, F. H. B.; Salgado, J. R. C.; Gonzalez, E. R.;
Ticianelli, E. A. J. Electrochem. Soc. 2007, 154, A369−A375.
c) Loukrakpam, R.; Luo, J.; He, T.; Chen, Y.; Xu, Z.; Njoki, P. N.;
(
Wanjala, B. N.; Fang, B.; Mott, D.; Yin, J.; Klar, J.; Powell, B.; Zhong,
C.-J. J. Phys. Chem. C 2011, 115, 1682−1694.
(14) Henglein, A. J. Phys. Chem. B 2000, 104, 6767−6772.
(15) Yan, J.-M.; Zhang, X.-B.; Akita, T.; Haruta, M.; Xu, Q. J. Am.
Chem. Soc. 2010, 132, 5326−5327.
16) Singh, A. K.; Xu, Q. Int. J. Hydrogen Energy 2014, 39, 9128−
134.
(
9
4
268
dx.doi.org/10.1021/cs501329c | ACS Catal. 2014, 4, 4261−4268