Ni1-xPtx (x = 0-0.12) hollow spheres as catalysts for hydrogen generation from ammonia borane

Article abstract of DOI:10.1021/ic061712e

In this paper, nest-like Ni_1-xPt_x (x = 0, 0.03, 0.06, 0.09, and 0.12) hollow spheres of submicrometer sizes have been prepared through a template-replacement route and investigated as catalysts for generating hydrogen from ammonia borane (NH₃BH₃). Experimental investigations have demonstrated that the obtained Ni _1-xPt_x alloy hollow spheres exhibit favorable catalytic activities for both the hydrolysis and the thermolysis of NH₃BH ₃. It was found that, in the presence of the Ni_0.88Pt _0.12 catalyst, the hydrolysis of NH₃BH₃ causes a quick release of H₂, while the thermal decomposition of NH ₃BH₃ occurs at lowered temperatures with increased mass loss. The present results indicate that NH₃BH₃ along with Ni_1-xPt_x alloy hollow spheres may find some applications for small-scale on-board hydrogen storage and supply.

Full text of DOI:10.1021/ic061712e

Inorg. Chem. 2007, 46, 788−794
Ni1-xPt (x
) 0−0.12) Hollow Spheres as Catalysts for Hydrogen
x
Generation from Ammonia Borane
Fangyi Cheng, Hua Ma, Yueming Li, and Jun Chen*
Institute of New Energy Material Chemistry, Nankai UniVersity, Tianjin 300071, People’s Republic
of China
Received September 10, 2006
In this paper, nest-like Ni
been prepared through a template-replacement route and investigated as catalysts for generating hydrogen from
ammonia borane (NH BH ). Experimental investigations have demonstrated that the obtained Ni Pt alloy hollow
spheres exhibit favorable catalytic activities for both the hydrolysis and the thermolysis of NH BH . It was found
that, in the presence of the Ni0.88Pt0.12 catalyst, the hydrolysis of NH BH causes a quick release of H , while the
thermal decomposition of NH BH occurs at lowered temperatures with increased mass loss. The present results
indicate that NH BH along with Ni
hydrogen storage and supply.
1-x x
Pt (x ) 0, 0.03, 0.06, 0.09, and 0.12) hollow spheres of submicrometer sizes have
3
3
1-x
x
3
3
3
3
2
3
3
3
3
1-x
Pt
x
alloy hollow spheres may find some applications for small-scale on-board
Introduction
borane compounds have also attracted much research interest
as a candidate for hydrogen storage material.
1
2-20
Hydrogen has been considered as one of the best alterna-
tive energy carriers to satisfy the increasing demand for an
efficient and clean energy supply because of its abundance,
high-energy density, and environmental friendliness. The
main technical challenge for the widespread application of
hydrogen is its real-time production and its safe and
convenient storage. As a result, various kinds of solid
materials for hydrogen storage have been investigated and
found to be promising. Typical examples are metal/complex
Although NH BH is isoelectronic with C H , it is a solid
3
3
2 6
rather than a gas at ambient temperature because of its strong
polarity and intermolecular interactions that are derived from
the different electronegativities of B and N atoms. As a solid
1
,2
material, NH
contains a high theoretical gravimetric capacity of 19.6 wt
of hydrogen. H can be released from NH BH through a
pyrolysis or hydrolysis route. The pyrolysis of NH BH has
3 3
BH is stable under ambient conditions and
3
%
2
3
3
4
3
3
hydrides, metal nitrides and imides, carbon materials,
5-11
(9) Str o¨ bel, R.; Garche, J.; Moseley, P. T.; J o¨ rissen, L.; Wolf, G. J. Power
Sources 2006, 159, 781.
10) (a) Rood, J. A.; Noll, B. C.; Henderson, K. W. Inorg. Chem. 2006,
inorganic nanostructures, and metal-organic frameworks.
Up to now, ammonia borane (NH BH ) and related amine
3
3
(
45, 5521. (b) Chen, B. L.; Ma, S. Q.; Inpata, E.; Lobkovsky, E. B.;
*
To whom correspondence should be addressed. Fax: +86-22-2350-
Yang, J. Inorg. Chem. 2006, 45, 5718. (c) Rosi, N. L.; Eckert, J.;
Eddaoudi, M.; Vodak, D. T.; Kim, J.; O’Keeffe, M.; Yaghi, O. M.
Science 2003, 300, 1127.
9
118. E-mail: chenabc@nankai.edu.cn.
1) Sperling, D.; DeLuchi, M. A. Annu. ReV. Energy 1989, 14, 375.
2) Schlapbach, L.; Z u¨ ttel, A. Nature 2001, 414, 353.
3) Ogden, J. M. Annu. ReV. Energy EnViron. 1999, 24, 227.
4) Z u¨ ttel, A. Mater. Today 2003, 6, 24.
5) (a) Grochala, W.; Edwards, P. P. Chem. ReV. 2004, 104, 1283. (b)
Holladay, J. D.; Wang, Y.; Jones, E. Chem. ReV. 2004, 104, 4767.
6) (a) Hu, J. J.; Wu, G. T.; Liu, Y. F.; Xiong, Z. T.; Chen, P.; Murata,
K.; Sakata, K.; Wolf, G. J. Phys. Chem. B 2006, 110, 14688. (b) Chen,
P.; Xiong, Z. T.; Luo, J. Z.; Lin, J. Y.; Tan, K. L. Nature 2002, 420,
(
(
(
(
(
(11) Seayad, A. M.; Antonelli, D. M. AdV. Mater. 2004, 16, 765.
(12) Bluhm, M. E.; Bradley, M. G.; Butterick, R.; Kusari, U.; Sneddon, L.
G. J. Am. Chem. Soc. 2006, 128, 7748.
(13) Chandra, M.; Xu, Q. J. Power Sources 2006, 156, 190.
(14) Gutowska, A.; Li, L.; Shin, Y.; Wang, C. M.; Li, X. S.; Linehan, J.
C.; Smith, R. S.; Kay, B. D.; Schmid, B.; Shaw, W.; Gutowski, M.;
Autrey, T. Angew. Chem., Int. Ed. 2005, 44, 3578.
(
(
(
(15) Baitalow, F.; Baumann, J.; Wolf, G.; Jaenicke-R o¨ âbler, K.; Leitner,
302.
G. Thermochim. Acta 2002, 391, 159.
7) (a) Wang, J.; Ebner, A. D.; Ritter, J. A. J. Am. Chem. Soc. 2006, 128,
(16) Wolf, G.; Baumann, J.; Baitalow, F.; Hoffmann, F. P. Thermochim.
Acta 2000, 343, 19.
(17) Sit, V.; Geanangel, R. A.; Wendlandt, W. W. Thermochim. Acta 1987,
113, 379.
(18) Kelly, H. C.; Marriott, V. B. Inorg. Chem. 1979, 18, 2875.
(19) Ryschkewitsch, G. E.; Birnbaum, E. R. Inorg. Chem. 1965, 4, 575.
(20) Denney, M. C.; Pons, V.; Hebden, T. J.; Heinekey, D. M.; Goldberg,
K. I. J. Am. Chem. Soc. 2006, 128, 12048.
5
949. (b) Sch u¨ th, F.; Bogdanovi c´ , B.; Felderhoff, M. Chem. Commun.
2004, 2249. (c) Chen, J.; Kuriyama, N.; Xu, Q.; Takeshita, H. T.;
Sakai, T. J. Phys. Chem. B 2001, 105, 11214.
8) (a) Lim, S. H.; Luo, J.; Zhong, Z.; Ji, W.; Lin, J. Inorg. Chem. 2005,
4
4, 4124. (b) Chen, J.; Li, S. L.; Tao, Z. L.; Shen, Y. T.; Cui, C. X.
J. Am. Chem. Soc. 2003, 125, 5284. (c) Chen, J.; Kuriyama, N.; Yuan,
H. T.; Takeshita, H. T.; Sakai, T. J. Am. Chem. Soc. 2001, 123, 11813.
788 Inorganic Chemistry, Vol. 46, No. 3, 2007
10.1021/ic061712e CCC: $37.00
© 2007 American Chemical Society
Published on Web 01/06/2007

x
Ni1-xPt Hollow Spheres as Catalysts for Hydrogen Generation
been widely investigated. Experimental and computational
results have revealed that ammonia borane decomposes upon
melting at 385 K with a hydrogen release of approximately
6.5 wt % of the initial mass, while the H
2
generation is
moderately exothermic with a reaction enthalpy of -21 kJ
-
1 15-17
mol .
molysis of NH
that the hydrogen release properties of NH
Recently, Autrey et al. investigated the ther-
BH within nanoporous silica, and they found
BH were
3
3
3
3
significantly enhanced because of the suppression of borazine
release, the modification of decomposition enthalpy, and the
lowered activation barrier.14 With respect to the hydrolysis
route, early literature has shown that NH
3 3
BH could undergo
uncatalyzed solvolysis and acid-catalyzed hydrolysis with
Figure 1. Illustration of the procedures for preparing Ni1-xPtx (x ) 0-0.12)
hollow spheres.
18,19
the formation of boric acid and H
2
.
Recently, Xu and
Chandra reported a high-performance hydrogen generation
system based on transition metal-catalyzed hydrolysis of
submicrometer-sized hollow spheres followed a PSA-assisted (PSA
)
poly(styrene-co-methacrylic acid)) template route (Figure 1),
NH
3
BH
3
at room temperature.13
24
which was similar to a previous report and was carried out with
the following procedures. First, PSA submicrometer-sized spheres
were prepared through the emulsifier-free emulsion copolymeri-
zation of styrene with methacrylic acid.25 Second, purified PSA
spheres were suspended in distilled water to form a white slurry,
which was used as the template for the deposition of Ni. A mixture
12-20
It can be seen from previous literature
that the use of
ammonia borane, especially as a source for an on-board and
small-scale hydrogen supply, seems to be possible and
interesting. However, two major hurdles must be overcome
before its use in practical applications: (1) to develop
efficient and economical catalysts for further improving the
kinetic and thermodynamic properties under moderate condi-
tions and (2) to realize the reversibility. For addressing the
former target, herein we report on a new kind of catalyst,
2
of the PSA slurry and a 0.4 M NiCl aqueous solution was stirred
for 30 min under argon atmosphere. Then, a freshly prepared NaBH
4
solution (0.012 M) was added slowly under stirring. The amount
of reducing agent was over 1.5 times that of the Ni2 valency to
ensure a complete reduction of the metal ions to the elemental state.
+
namely, nest-like Ni1-xPt
(x ) 0, 0.03, 0.06, 0.09, and 0.12)
x
2 6
Afterward, a certain amount of K PtCl (0.0012 M) was added
submicrometer-sized hollow spheres, which exhibit favorable
catalytic activities for both the hydrolysis and the thermolysis
of ammonia borane. The hollow alloy spheres possess some
prominent advantages over their bulk counterparts. First, the
hollow structural geometry endows them with beneficial
properties of low density, high surface areas, extra interior
reaction space, and a savings on material, as has been shown
dropwise to the above mixture according to the target molar ratio
of Ni:Pt. The replacement reaction resulted in the formation of
Ni1-xPt -PSA composite structure. Finally, the obtained core-shell
x
composites were dispersed in toluene to completely dissolve the
PSA core template. The final product was collected, washed with
water and ethanol several times, dried naturally, and calcined at
350 °C for 1 h in a flowing argon gas.
The chemical reactions involved in the preparation process can
be briefly expressed as
2
1-23
in the case of different hollow metallic nanospheres.
Second, the porous submicrometer-sized hollow spheres that
consist of firmly combined alloy nanoparticles can provide
a great quantity of catalytic sites and, meanwhile, avoid the
aggregation of small catalyst particles, which is a common
problem facing substrate-supported metal catalysts. Further-
more, the alloying of noble metal Pt with transition metal
Ni exhibits comparable catalytic activities to pure Pt and,
nevertheless, greatly reduces the catalyst cost. Therefore, we
Ni2+ + BH4 + 2H O f NiV + BO2 + 2H + 3H v (1)
-
-
+
2
2
2
-
-
(
1 + x)Ni + xPtCl6 f 2xNi2+ + Ni1-xPt + 6xCl
(2)
x
The phase structure of the as-synthesized samples was characterized
by powder X-ray diffraction (XRD) on a Rigaku D/max 2500 X-ray
diffractometer (Cu KR radiation, λ ) 1.541 78 Å). Scanning
electron microscope (SEM) images were taken on Philips XL-30
and JEOL JSM-6700F microscopes operated at the accelerating
voltages of 20 and 10 kV, respectively. Transmission electron
microscope (TEM) and high-resolution TEM (HRTEM) analysis
were performed on a Philips Tecnai F20 microscope equipped with
an energy dispersive X-ray (EDX) spectrometer using an accelerat-
ing voltage of 200 kV. Elemental composition was determined by
an inductively coupled plasma (ICP) emission spectroscopy (Ther-
mo Jarrell-Ash Corp). Specific surface areas were measured with
a Micromeritics ASAP 2010 Brunauer-Emmett-Teller (BET)
surface analysis instrument using nitrogen gas.
suggest that the Ni1-xPt
BH may find possible applications in small-scale hydrogen
storage and on-board supply, which are required in many
x 3
hollow spheres along with NH -
3
hydrogen-utilization technologies such as minitype H
cells.
2
fuel
Experimental Section
Synthesis and Characterization. All chemicals were of analyti-
cal grade and were used as purchased without further purification.
The synthesis of Ni1-xPt (x ) 0, 0.03, 0.06, 0.09, and 0.12)
x
Catalyst Study. The hydrolysis of ammonia borane was carried
out at controlled temperatures and concentrations. A weighed
catalyst (Ni1-xPt hollow spheres or commercial nickel powder)
x
(
21) (a) Vasquez, Y.; Sra, A. K.; Schaak, R. E. J. Am. Chem. Soc. 2005,
127, 12504. (b) Liang, H. P.; Zhang, H. M., Hu, J. S.; Guo, Y. G.;
Wan, L. J.; Bai, C. L. Angew. Chem., Int. Ed. 2004, 43, 1540.
22) Sun, Y.; Xia, Y. Science 2002, 298, 2176.
(
(
23) Kim, S. W.; Kim, M.; Lee, W. Y.; Tyeon, T. J. Am. Chem. Soc. 2002,
(24) Caruso, F.; Caruso, R. A.; M o¨ hwald, H. Science 1998, 282, 1111.
(25) Wang, P. H.; Pan, C. Y. Colloid Polym. Sci. 2000, 278, 581.
124, 7642.
Inorganic Chemistry, Vol. 46, No. 3, 2007 789

Cheng et al.
Figure 2. (a) SEM and (b) TEM images of the as-synthesized PSA
submicrometer-sized spheres.
Figure 3. (A) XRD patterns of the as-synthesized (a-e) Ni1-xPtx (x ) 0,
0.03, 0.06, 0.09, and 0.12) samples. Schematic depiction showing the (B)
top view and (C) side view of the fcc crystal structure of M(111) (M ) Pt
or Ni).
was placed in a three-necked glass container and mixed with an
aqueous NH
3
BH
3
(50 mL) solution under constant stirring. The
was measured by an inverted, water-filled,
generated amount of H
2
and graduated burette. At different stages of the hydrolysis reaction,
the resulting solutions were filtered, and the filtrates were collected
for nuclear magnetic resonance (NMR) analysis. 11B NMR spectra
were recorded on a JEOL JNM-AL400 spectrometer with an
operating frequency of 128.15 MHz. D
2
3 2 5 2
O and BF ‚(C H ) O were
used as a lock and an external reference, respectively. In the
thermolysis experiments, Ni0.88Pt0.12 hollow spheres were added to
a solution of NH
under vacuum to obtain a solid mixture with an approximately 1.8
wt % Pt loading. The thermal decomposition of NH BH was
monitored by a temperature-programmed desorption (TPD) system
TPD-43, BEL Japan, Inc.). In the TPD test, neat NH BH or a
mixture of NH BH and Ni0.88Pt0.12 catalyst were investigated under
a carrier gas of purified argon. The temperature was linearly
3 3
BH in methanol. This solution was then dried
3
3
(
3
3
3
3
-
1
increased from 25 to 250 °C at a rate of 2 °C min . Thermo-
gravimetry and differential thermogravimetry (TG and DTG; model
TG 209, Netzsch) measurements were performed under a nitrogen
-
1
atmosphere at a heating rate of 5 K min . Fourier transform
infrared (FTIR) spectra were obtained on a Bruker Tensor 27
-1
spectrometer in the wavelength range of 400-4000 cm on sample
pellets made with KBr.
Figure 4. Morphology and composition analysis of the as-synthesized
Ni0.88Pt0.12 hollow spheres: (a) typical SEM image, (b, c, d) TEM images
at different magnifications, and the corresponding (e) HRTEM image and
(f) EDX spectrum.
Results and Discussion
Synthesis and Characterization. Ni1-xPt
x
hollow spheres
sphere can be well indexed to (111), (200), and (220) crystal
planes of the face-centered cubic Ni (fcc Ni; JCPDS No.
were synthesized through a template route by a replacement
reaction, as shown in Figure 1. In our synthesis, PSA spheres
were prepared and employed as sacrificial templates for the
formation of hollow structures. The obtained PSA spheres
are well dispersed with uniform diameters of approximately
04-0850). All of the Ni1-xPt samples exhibit similar
x
diffraction patterns to that of pure nickel without any extra
super-lattice reflection, indicating the formation of a metallic
alloy. The replacement of Pt does not change the Ni-type
fcc structure (Figure 3b,c) in which Pt and Ni atoms are
distributed homogeneously and randomly at the Ni position.
In addition, the d(111) values calculated from 2θ data are
2
40 nm (Figure 2). Ni is first deposited on the surface of
2+
the PSA spheres when Ni is reduced by NaBH
The formation of Ni1-xPt is attained through the replacement
reaction between K PtCl and Ni (eq 2), where the driving
force comes from the large standard reduction potential gap
4
(eq 1).
x
2
6
0.204, 0.216, 0.218, 0.220, and 0.225 nm for Ni, Ni0.97Pt0.03,
Ni0.94Pt0.06, Ni0.91Pt0.09, and Ni0.88Pt0.12 samples, respectively.
2
+
between the Ni /Ni (-0.250 V vs standard hydrogen
The diffraction peaks of all the Ni1-xPt samples shift to
x
6-
electrode (SHE)) and the PtCl /Pt (0.735 V vs SHE) redox
lower angles compared to that of the pure Ni sample,
pairs. After the partial consumption of Ni, the resultant
demonstrating a lattice expanding due to the substitution of
the larger Pt atoms for the smaller Ni atoms.
The morphology and composition of the obtained product
were investigated by SEM, TEM, EDX, and ICP analyses.
Figure 4a,b show the typical SEM and TEM images of the
obtained Ni0.88Pt0.12 sample. Analogous morphologies are
Ni1-xPt
x
-PSA core-shell structures are dispersed in a
toluene solution to dissolve the PSA template cores, resulting
in the formation of Ni1-xPt hollow spheres.
Figure 3a shows the XRD patterns of the obtained Ni1-xPt
x ) 0, 0.03, 0.06, 0.09, and 0.12) samples after heating them
x
x
(
at 350 °C for 1 h under an argon atmosphere. Three
observed for the as-synthesized Ni1-xPt
x
(x ) 0.03, 0.06, and
characteristic peaks of the as-synthesized nickel hollow
0.09) samples (see Figure S1 in Supporting Information).
790 Inorganic Chemistry, Vol. 46, No. 3, 2007

x
Ni1-xPt Hollow Spheres as Catalysts for Hydrogen Generation
The notable different morphologies between Figure 2 and
Figure 4 confirm that hollow spheres are produced by
dissolving the polystyrene (PS) templates. Both complete and
broken hollow spheres are obtained. The break of some
hollow spheres may be caused by the removal of PS cores
and/or the dispersion treatment of the samples before SEM
and TEM measurements. The hollow spherical structure is
almost stable at temperatures lower than 350 °C; however,
more broken spheres can be found after heating the sample
at 400 °C for a longer time than 2 h. High-magnification
TEM images (Figure 4c,d) reveal that the wall thickness of
the hollow sphere is 20-40 nm and the porous shell consists
of smaller nanoparticles and nanowhiskers. These charac-
x
teristics endow the Ni1-xPt hollow spheres with high surface
areas. The corresponding HRTEM image (Figure 4e) of a
single nanowhisker displays an interlayer distance of 0.21
nm, which can be attributed to the separation between the
Figure 5. Hydrogen generation from the hydrolysis of aqueous NH3BH3
solution (50 mL, 0.5 wt %) containing various kinds of catalysts (12 mg)
at 20 °C.
neighboring [111] planes of Ni0.88Pt0.12
.
EDX data (Figure 4f) of many hollow spheres prove an
average elemental composition of Ni0.88Pt0.12; the presence
of C and Cu is derived from the copper mesh and carbon
film that support the sample during TEM measurement. It
has been reported that the reaction of borohydride reduction
of metal ions is quite complex and the product characteristic
In comparison, the release of hydrogen is greatly quickened
in the presence of Ni1-xPt (x ) 0.03, 0.06, 0.09, and 0.12)
x
hollow spheres, denoting their favorable catalytic activities.
The as-synthesized Ni0.88Pt0.12 hollow spheres are most
catalytically active, with which the hydrolysis reaction of
NH BH is completed in approximately 30 min. After the
3
3
2
6
depends on the experimental conditions and procedures.
completion of hydrogen release, the molar ratio of generated
H to the initial NH BH is close to 3.0, which corresponds
2
1,26c
26d
This reaction may yield metals,
metal borides, or a
2
3
3
mixture of them2 by varying reaction parameters such as
temperature, adding rate of reactants, and gas atmosphere.
Since boron cannot be detected by EDX measurement, ICP
elemental analysis of the synthesized samples has been
performed for further determining their composition. The
boron content of the bulk sample is about 0.5 wt %, and the
molar ratio of Ni and Pt determined by the ICP analysis is
consistent with the EDX result. For the sake of conciseness,
6b
to approximately 8.9 wt % of the reactants (i.e., NH BH3
3
and H O, excluding solvent water).
2
The catalytic performance of the as-synthesized Ni0.88Pt0.12
hollow spheres is comparable to that of the reported Pt black
1
3
in regard to hydrogen yield. However, the present catalyst
is superior to pure Pt with a higher H release rate in terms
2
of the same Pt loading. Considering the different price
between Pt and Ni, the alloy catalysts are more cost-effective.
Moreover, although the hydrolysis of NH BH can also be
x
we intend to omit B in the present Ni1-xPt catalysts though
the small amount of B may partially contribute to catalytic
activity.
3
3
1
8
catalyzed by acid, the related H release rate is relatively
2
low and depends intensively on the concentration of acid,
Catalyst Study. NH
solution under ambient conditions in the absence of a catalyst,
without observed H release. We have investigated the
catalytic activities of the synthesized Ni1-xPt (x ) 0, 0.03,
.06, 0.09, and 0.12) hollow spheres for hydrogen generation
by employing them as catalysts for the hydrolysis and
thermolysis of NH BH . At first, the catalytic activities of
commercial nickel powders (SEM image shown in Figure
S2, Supporting Information) and the obtained hollow spheres
have been tested and compared, with respect to the hydrolysis
3
BH
3
is stable in air and aqueous
which may cause unfavorable drawbacks in a practical
application. In contrast, the present Ni0.88Pt0.12 can be
effectively used in an NH BH aqueous solution (pH > 7),
since the hydrolysis reaction can be started by adding the
solid catalyst into the solution and ceased by separating them.
Furthermore, the as-synthesized alloy hollow sphere catalysts,
treated by filtrating and washing, can be repeatedly utilized
with no significant catalytic deactivation in the hydrolysis
experiment.
2
3
3
x
0
3
3
The rates of hydrogen generation at different temperatures
of aqueous NH
generated as a function of time for a 50 mL NH
3
BH
3
solution. Figure 5 plots the H
BH
2
amount
solution
are measured in the presence of Ni1-xPt hollow spheres (see
an example in Figure S3, Supporting Information). Through
x
3
3
with ∼12 mg of catalysts. Commercial nickel powders
exhibit almost no catalytic activity in the hydrolysis of NH
BH at room temperature, while the synthesized nickel
hollow spheres can accelerate the reaction to some extent.
the Arrhenius treatment of the temperature-dependent reac-
3
-
tion rates, the activation energy of the hydrolysis of NH -
3
-
1
3
BH is determined to be 70, 57, 46, 36, and 30 kJ mol for
3
Ni, Ni0.97Pt0.03, Ni0.94Pt0.06, Ni0.91Pt0.09, and Ni0.88Pt0.12, re-
spectively. The order of the activation energy of Ni1-xPt
x
(
26) (a) Ganem, B.; Osby, J. O. Chem. ReV. 1986, 86, 763. (b) Glavee, G.
alloy catalysts is consistent with that of their catalytic
activities. Hence, a noble element plays the key role in
catalyzing the hydrolysis of ammonia borane. In addition,
we have also investigated the concentration dependence of
N.; Klabunde, K. J.; Sorensen, C. M.; Hadjipanayis, G. C. Inorg. Chem.
1
993, 32, 474. (c) Park, K.; Choi, J.; Kwon, B.; Lee, S.; Sung, Y.;
Ha, H.; Hong, S.; Kim, H.; Wieckowski, A. J. Phys. Chem. B 2002,
06, 1869. (d) Dong, H.; Yang, H.; Ai, X.; Cha, C. Int. J. Hydrogen
Energy 2003, 28, 1095.
1
Inorganic Chemistry, Vol. 46, No. 3, 2007 791

Cheng et al.
Figure 6. Hydrogen release from aqueous NH3BH3 solution (50 mL) with
different concentrations in the presence of 20 mg of Ni0.88Pt0.12 hollow
spheres.
Figure 7. 11B NMR spectra of the aqueous solution measured at different
stages of the hydrolysis reaction: (a) freshly prepared NH3BH3 (0.5 wt %)
solution; after reaction for (b) 5 min and (c) 10 min in the presence of
Ni0.88Pt0.12 hollow spheres; and (d) after the completion of the reaction.
the hydrolysis reaction of an aqueous NH
containing Ni0.88Pt0.12 hollow spheres. As shown in Figure
, the concentration of NH BH has some slight influence
on the rate of hydrogen generation. The amount of H
3 3
BH solution
6
3
3
2
generated by Ni0.88Pt0.12 hollow spheres increases nearly
linearly with time before the hydrolysis reaction approaches
its end, indicating an example of quasi zero-order kinetics.
Hence, the synthesized Ni0.88Pt0.12 hollow spheres preserve
their catalytic activities even at a solution concentration of
5
wt %. The H
of the hydrolysis process when using a more concentrated
NH BH solution. This rate drop may be due to the blocking
of catalyst sites by the formed boracic precipitate. It should
also be noted that the dissolution of solid NH BH at room
temperature becomes difficult at concentrations higher than
wt %, which happens to be a hurdle in attaining a further
higher gravimetric density of H production. On the other
2
release rate slows down during the last period
3
3
3
3
5
2
hand, there is still much room left to further improve the
performance of hydrogen generation by optimizing the
related experimental parameters.
Figure 8. SEM image of the mixture of NH3BH3 and Ni0.88Pt0.12 hollow
spheres.
solutions analyzed exhibit pH values of 9.0, 9.4, 9.7, and
NMR analysis has been carried out to study the hydrolysis
process of ammonia borane. Figure 7 shows the 1 B NMR
spectra of the resultant solutions taken at different stages of
the reaction that is catalyzed by Ni0.88Pt0.12 hollow spheres.
1
1
1
0.0, respectively. The changes of single B resonance and
1
pH values are possibly attributed to an equilibrium process
and a rapid change between boracic compounds such as
2
-
13
2-
BO
2
, H
3
BO
3
, and other borate species. Given BO
2
as
3 3
The newly prepared NH BH solution is colorless with weak
the representative boracic product, the hydrolysis reaction
alkalinity and presents quadruplet peaks centered at δ )
of the NH
3
BH
3
aqueous solution containing Ni1-xPt
x
hollow
(3)
-
ture.
23.9 ppm (Figure 7a), in agreement with previous litera-
spheres can simply be described as follows:
2
7,28
After reaction for 5 min, the relative intensities of
the quadruplet peaks (Figure 7b) become lower, and a notable
single 11B peak is observed at δ ) 13.1 ppm, indicating the
+
-
NH BH + 2H O f NH + BO2 + 3H2v
3
3
2
4
break of B-N and B-H bonds in NH
3 3
BH and the formation
Since Ni0.88Pt0.12 hollow spheres exhibit the highest activi-
ties among the as-prepared Ni1-xPt (x ) 0, 0.03, 0.06, 0.09,
and 0.12) catalysts in the hydrolysis experiment, we further
evaluate their catalytic performance in the thermolysis of
ammonia borane. Figure 8 shows the SEM image of the
of a B-O bond. After hydrolysis for 10 min, the intensities
x
11
of the B quadruplet peaks further decrease, and the
downfield peak shifts to a higher position of 10.5 ppm
(
Figure 7c). After the completion of the reaction, the single
B peak shifts to 6.8 ppm, and the initial resonance signals
1
1
mixture of NH
Pt loading). The white arrows mark some of the preserved
hollow interiors while the white squares indicate the coating
of NH BH on the sphere surface. It can be seen from this
figure that solid NH BH and catalysts are well mixed with
3 3
BH and Ni0.88Pt0.12 hollow spheres (1.8 wt
centered at -23.9 ppm disappear (Figure 7d), revealing the
complete consumption of NH BH . Moreover, the four
%
3
3
3
3
(
(
27) Gaines, D. F.; Schaeffer, R. J. Am. Chem. Soc. 1964, 86, 1505.
28) Heitsch, C. W. Inorg. Chem. 1965, 4, 1019.
3
3
792 Inorganic Chemistry, Vol. 46, No. 3, 2007

x
Ni1-xPt Hollow Spheres as Catalysts for Hydrogen Generation
Figure 10. FTIR spectra of (a) unheated NH3BH3 and the samples obtained
after heating (b) neat NH3BH3 and (c) NH3BH3 mixed with Ni0.88Pt0.12 at
120 °C for 2 h under nitrogen atmosphere.
observed in a narrow temperature range centered at 100 °C.
Meanwhile, there is almost no mass loss at 180 °C for the
mixed sample, whereas neat NH BH loses weight up to a
3 3
temperature of 200 °C. Thus, TG, DTG, and TPD measure-
ments confirm that Ni0.88Pt0.12 catalysts result in an enhanced
3 3
thermal decomposition of NH BH , with a lowered heating
temperature and an increased rate of mass loss.
Two typical thermally decomposed samples, prepared by
heating NH
C for 2 h, have been characterized by IR, ICP, and XRD to
analyze the composition of the thermolysis product. Figure
10 shows the FTIR spectra of unheated NH BH and two
thermally decomposed samples. The corresponding data of
neat NH BH (Figure 10a) agree well with reported results:
the main absorbance bands centered at 3320-3200, 2335,
3 3
BH with and without Ni0.88Pt0.12 catalysts at 120
°
Figure 9. TG (red line) and DTG (black line) curves of the thermal
deposition of (a) neat NH3BH3 and (b) the mixture of NH3BH3 and Ni0.88-
Pt0.12 hollow spheres.
3
3
sufficient contact. By repeatedly impregnating a saturated
methanolic solution, NH BH can be engaged with Ni0.88-
3 3
Pt0.12 hollow spheres with a relatively low catalyst loading.
This may offer some advantages relative to the reported
3
3
-
1
1605, 1377, 1160 (1063), and 797 (727) cm are assigned
to the modes of N-H stretching, B-H stretching, N-H
bending, N-H out of plane, B-H torsion, and B-N
system of SBA-15 and NH
3 3 3 3
BH , where NH BH should be
2
9
loaded into the tiny channels/pores of the nanostructured
stretching, respectively. It can be seen from Figure 10b
that the relative absorbance intensities of B-H stretching,
B-H torsion, and N-H bending modes evidently decrease
1
4
silica scaffold. Furthermore, the mixture is facile to be
separated by dispersing it in alcohol to collect the undissolved
Ni0.88Pt0.12 solid; thus, the catalysts can be recovered and
repeatedly used.
3 3
compared to the data of unheated NH BH , indicating the
partial break of B-H and N-H bonds. In comparison, the
corresponding B-H modes and N-H bending mode almost
disappear in the spectrum of the thermally decomposed
We have performed TG, TPD (Figure S4, Supporting
Information), XRD (Figure S5, Supporting Information), and
FTIR measurements to study the catalyzed thermolysis of
sample obtained with catalysts (Figure 10c). Therefore, Ni0.88
Pt0.12 catalysts facilitate the dissociation of B-H and N-H
bonds during the decomposition of NH BH . According to
TG and FTIR measurements, we envisage that the catalyzed
thermolysis of NH BH leads to the formation of (BNH
at 120 °C, along with the release of H and other volatile
-
NH
thermal decomposition of NH
Pt0.12 hollow spheres at the same heating rate. The neat NH
BH
3 3
BH . Figure 9 shows the TG and DTG curves of the
3
BH
3
with and without Ni0.88
-
3
3
3
-
x n
)
3
sample delivers two distinguished steps of mass loss,
3
3
centered at 109 and 160 °C, respectively. The first step ends
at 127 °C with a mass loss of approximately 14%, which
2
boric compounds. ICP analysis of the obtained product
confirms an elemental composition of B:N:H ) 1:1:1.4.
Furthermore, XRD patterns (Figure S5) indicate that the
thermally decomposed product is a white noncrystalline solid,
probably corresponds to a hydrogen release of 2.2 mol H
mol NH BH with the formation of nonvolatile polymeric
aminoborane. Since the total mass loss exceeds the theoretical
hydrogen content of NH BH
(∼19.6 wt %), it is obvious
that the second step of NH BH decomposition generates
some other volatile products in addition to H , which may
include gaseous BNH species, diborane, and borazine.
With Ni0.88Pt0.12 catalysts, a large mass loss of 24.8% is
2
/
3
3
3 3
while NH BH is crystalline.
3
3
3
3
From the above investigations, the as-synthesized Ni1-xPtx
hollow spheres, especially Ni0.88Pt0.12 hollow spheres, exhibit
2
1
5
x
(29) Baumann, J.; Baitalow, F.; Wolf, G. Thermochim. Acta 2005, 430, 9.
Inorganic Chemistry, Vol. 46, No. 3, 2007 793

Cheng et al.
favorable catalytic activities in both the hydrolysis and the
thermolysis of NH BH . Some preliminary interpretations
are suggested as follows to understand their catalytic
performance. On one hand, the nest-like morphology and
0.06, 0.09, and 0.12) hollow spheres. The obtained hollow
spheres that have been prepared through a PSA-assisted
template route exhibit favorable catalytic activities in the
hydrolysis and thermolysis of ammonia borane with quick
generation of hydrogen. The reaction system consisting of
3
3
x
structure of the obtained Ni1-xPt hollow spheres are
beneficial factors for their high catalytic activities. Porous
surfaces and submicrometer sizes endow the catalysts with
high surface areas (e.g., 105 m g for Ni0.88Pt0.12) and large
surface-to-volume atomic ratio, which are undoubtedly
correlated with numerous catalytic sites. The open hollow
structure may provide both the inner and the outer surfaces
3 3 x
NH BH and the nest-like Ni1-xPt hollow spheres may find
some possible applications in the field of small hydrogen
fuel cells. Furthermore, the present synthesis route can be
extended to prepare other alloy hollow spheres. The syn-
thesized submicrometer-sized Ni1-xPt hollow spheres might
x
also be useful as catalysts for other reaction systems such
as hydrogenation and oxygen reduction.
2
-1
contact between catalysts and NH
of NH BH is determined to be a zero-order and heteroge-
neous reaction, the adsorption of NH BH on the catalyst
3 3
BH . Since the hydrolysis
3
3
3
3
Acknowledgment. This work was supported by the
National NSFC (20325102, 90406001, and 50631020) and
surface is an important process. Large quantities of active
sites would thus result in enhanced kinetics. On the other
hand, transition metals (Ir, Ru, and Rh) and their complexes
have been demonstrated to be appropriate catalysts for the
dehydrocoupling/dehydrogenation of amine borane or borane
973 Program (2005CB623607). Partial discussions with Drs.
Q. Xu and N. Jiang at National Institute of AIST are also
acknowledged.
adducts with hydrogen elimination and B-N bond forma-
tion,20,30
Supporting Information Available: Typical SEM and TEM
images and EDX spectra of Ni1-xPt (x ) 0.03, 0.06, 0.09), SEM
x
image of commercial Ni powders, hydrogen evolution graph, TPD
curve, and XRD patterns. This material is available free of charge
via the Internet at http://pubs.acs.org.
and transition metals such as Pt, Rh, and Pd can
1
3
catalyze the hydrolysis of NH
interaction between NH
3 3
BH .
It is possible that the
promotes the
BH . Definite
3 3 x
BH and Ni1-xPt
hydrolysis and thermal decomposition of NH
3
3
statements to the related catalytic mechanism as well as the
structure of the reaction product have not been obtained in
this work and hence still require further studies.
IC061712E
Conclusions
(30) (a) Jaska, C. A.; Temple, K.; Lough, A. J.; Manners, I. Chem. Commun.
2
001, 962. (b) Jaska, C. A.; Temple, K.; Lough, A. J.; Manners, I. J.
In summary, we have demonstrated the synthesis and
Am. Chem. Soc. 2003, 125, 9424. (c) Jaska, C. A.; Manners, I. J. Am.
Chem. Soc. 2004, 126, 9776.
catalytic study of submicrometer-sized Ni1-xPt (x ) 0, 0.03,
x
794 Inorganic Chemistry, Vol. 46, No. 3, 2007