First row transition metal ion-assisted ammonia-borane hydrolysis for hydrogen generation

Article abstract of DOI:10.1021/ic800805r

Ammonia-borane (AB) hydrolysis for the generation of hydrogen has been studied using first row transition metal ions, such as Co²⁺, Ni ²⁺, and Cu²⁺. In the cases of cobalt- and nickel-assisted AB hydrolysis, amorphous powd

Full text of DOI:10.1021/ic800805r

Inorg. Chem. 2008, 47, 7424-7429
First Row Transition Metal Ion-Assisted Ammonia-Borane Hydrolysis
for Hydrogen Generation
Suresh Babu Kalidindi, M. Indirani, and Balaji R. Jagirdar*
Department of Inorganic & Physical Chemistry, Indian Institute of Science,
Bangalore 560 012, India
Received May 3, 2008
Ammonia-borane (AB) hydrolysis for the generation of hydrogen has been studied using first row transition metal
ions, such as Co2+, Ni2+, and Cu2+. In the cases of cobalt- and nickel-assisted AB hydrolysis, amorphous powders
are formed that are highly catalytically active for hydrogen generation. Annealing of these amorphous powders
followed by powder X-ray diffraction measurements revealed the presence of Co(0) and Co2B and Ni(0) and Ni3B,
respectively. On the other hand, copper-assisted AB hydrolysis was catalyzed by in situ generated H+ and Cu(0)
nanoparticles. The reduction ability of AB for the realization of coinage metal nanoparticles from the respective
metal salts has also been studied. These reduction reactions were found to be facile, affording colloids of pure
metal nanoparticles. Nanoparticles prepared in this manner were characterized by UV-visible spectroscopy and
high-resolution electron microscopy.
Introduction
by the use of materials like nanoscaffolds,5 ionic liquids,6
and Ir metal complexes as catalysts.7 The catalytic hydrolysis
of AB has been studied using various transition metal
catalysts.8 Although rapid H2 release rates have been
achieved using various precious metal catalysts, there is a
general desire for first row, cheap, and abundant transition
metal catalysts with high stability. Recently, Xu and co-
workers reported AB hydrolysis for H2 generation catalyzed
by Fe nanoparticles.9 They found that the reduction of Fe2+
with NaBH4 in the presence of AB resulted in amorphous
Fe nanoparticles that were several times more active than
crystalline Fe nanoparticles. In our search for a more
abundant first-row transition-metal-based catalyst, we re-
cently investigated the activities of nanostructured Cu, Cu2O,
Hydrogen has been in the limelight for the past several
years from the standpoint of the so-called hydrogen economy.
It is considered to be the best alternative to hydrocarbon fuels
due primarily to its clean burning nature and environmental
friendliness and also high-energy content equal to 142 MJ/
kg.1 This is three times larger than that of liquid hydrocar-
bons. However, storing hydrogen in large quantities for on-
board applications remains a major hurdle for its widespread
usage.2 The low density of H2 makes it difficult to store in
compressed or liquefied form.
Ammonia-borane (H3N · BH3, AB) has been attracting a
great deal of attention as a hydrogen storage medium.3 It
contains 19.6 wt % of H2, which is greater than the U.S.
Department of Energy target (by 2015) of 9 wt % hydrogen
for a material to be practically applicable.4 It is a stable and
safer material to handle at room temperature. The release of
hydrogen from AB can be accomplished either by thermoly-
sis or a hydrolysis route. The thermolytic generation of
hydrogen from AB has been well studied. The decomposition
temperatures of AB for the release of H2 were brought down
(5) 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.
(6) Bluhm, M. E.; Bradley, M. G.; Butterick, R.; Kusari, U.; Sneddon,
L. G. J. Am. Chem. Soc. 2006, 128, 7748.
(7) Denney, M. C.; Pons, V.; Hebden, T. J.; Heinekey, D. M.; Goldberg,
K. I. J. Am. Chem. Soc. 2006, 128, 12048.
(8) (a) Cheng, F.; Ma, H.; Li, Chen, J. Inorg. Chem. 2007, 46, 788. (b)
Ramachandran, P. V.; Gagare, P. D. Inorg. Chem. 2007, 46, 7810.
(c) Clark, T. J.; Whittell, G. R.; Manners, I. Inorg. Chem. 2007, 46,
7522. (d) Chandra, M.; Xu, Q. J. Power Sources 2007, 168, 135. (e)
Keaton, R. J.; Blacquiere, J. M.; Baker, R. T. J. Am. Chem. Soc. 2007,
129, 1844.
* Author to whom correspondence should be addressed. E-mail:
jagirdar@ipc.iisc.ernet.in.
(1) Schlapbach, L.; Zuttel, A. Nature 2001, 414, 353.
(2) Berg, A. W. C. V.; Area´n, C. O. Chem. Commun. 2008, 668.
(3) Stephens, F. H.; Pons, V.; Baker, R. T. Dalton Trans. 2007, 2613.
(4) Marder, T. B. Angew. Chem., Int. Ed. 2007, 46, 8116.
(9) Yan, J.-M.; Zhang, X.-B.; Han, S.; Shioyama, H.; Xu, Q. Angew.
Chem., Int. Ed. 2008, 47, 1.
7424 Inorganic Chemistry, Vol. 47, No. 16, 2008
10.1021/ic800805r CCC: $40.75  2008 American Chemical Society
Published on Web 07/23/2008

Ammonia-Borane Hydrolysis for Hydrogen Generation
volumetric flasks. Then, a 50% AB solution was prepared in water
just before the addition. In a typical experiment, the ratio of metal
salt/AB/PVP was maintained at 1:5:10.
and Cu@Cu2O nanoparticles for AB hydrolysis and found
that Cu@Cu2O or Cu2O nanoparticles were more active than
Cu0 nanoparticles.10
A metal salt solution (0.001 g in water) and PVP were dissolved
in 30 mL of water, and then the solution was purged with argon
for about 1 h. An ammonia-borane solution was added dropwise
to this solution over a period of 30 min. The hydrogen evolution
was observed during the addition of AB. The resultant colloids were
wine red, yellow, and purple-red in color respectively in the cases
of Cu, Ag, and Au nanoparticles.
Herein, we studied the activities of various first row
transition metal ions toward AB hydrolysis for the release
of H2. The most interesting outcome of this work is that AB
can reduce metal ions upon hydrolysis. In the case of Co2+
and Ni2+, the metal powders that resulted were amorphous
in nature, whereas in the case of copper, it was crystalline.
The amorphous Co and Ni nanopowders were found to be
catalytically active toward AB hydrolysis. We have further
explored the reduction ability of AB by demonstrating the
generation of coinage metal nanoparticles.
Results and Discussion
Transition-metal-catalyzed NaBH4 hydrolysis using metal
ions and acids has been well studied by Kaufman and Sen.12
It was found that the overall production of hydrogen using
metal salts is both acid- as well as transition-metal-catalyzed.
This observation is in accordance with the mechanism
previously proposed by Holbrook and Twist that the reaction
proceeds through the intermediacy of a metal boride complex
and a metal hydride.13 Later, Klabunde and co-workers
studied the reactions of Fe2+,14 Co2+,15 Ni2+, and Cu2+,16
with NaBH4 in water and diglyme. The reduction of these
ions in water resulted in the formation of Fe, Co2B, Ni2B,
and Cu nanoparticles, respectively. The reactions were quite
vigorous, proceeding to completion in less than 2 min. The
reduction ability of NaBH4 was further explored by several
other groups for the preparation of Fe,17 Cu,18 Ag,19 and
Au20 nanoparticles.
With a view to realize cheap, stable, and abundant first-
row transition metal catalysts for hydrogen generation from
AB, we tested the activities of various metal ions. The most
active ions to bring about the hydrolysis of AB were found
to be Co2+, Ni2+, and Cu2+. The hydrolysis reactions were
carried out using 7, 15, 30, 50, 75, and 100 mol % of these
metal ions. In most cases, aqueous AB released stoichio-
metric amounts of hydrogen (H2/H3N · BH3 ) 3.0 mol)
according to the following equation (eq 1). Reaction comple-
tion was established using 11B NMR spectroscopy (see the
Supporting Information).
Experimental Section
Materials. Ammonia-borane was synthesized from NH4CO3
and NaBH4 using the procedure described by Hu et al.11 All of the
metal salts NiCl2 ·6H2O, CoCl2 ·6H2O, CuCl2 ·6H2O, CuSO4 ·2H2O,
AgNO3, and HAuCl4 ·3H2O were obtained from S. D. Fine
Chemicals Limited, India. Polyvinylpyrrolidone (PVP) was pur-
chased from Fluka.
Instrumentation. A Perkin-Elmer Lambda 35 UV/vis spectrom-
eter was used for recording the UV-visible spectra for the Cu,
Ag, and Au colloids. Transmission electron microscope (TEM)
bright field images, high-resolution TEM (HRTEM) images, and
selective area electron diffraction (SAED) patterns were obtained
using a TECNAI F30 transmission electron microscope. The TEM
samples were prepared on carbon-coated copper grids. The samples
were dried under a table lamp for more than 2 h after mounting.
The powder X-ray diffraction measurements were carried out using
a Philips powder X-ray diffractometer.
Ni2+-, Co2+-, and Cu2+-Assisted AB Hydrolysis. Ammonia-
borane (0.5 mmol, 16 mg) was dissolved in 20 mL of water in a
Schlenk flask. A fixed mole percent of the metal ion salt
(NiCl2 ·6H2O, CoCl2 ·6H2O, CuCl2 ·6H2O) was added to this AB
solution with stirring. Hydrogen evolution was monitored using a
gas burette, which was connected to Schlenk flask through a water
trap containing 50 mL of water. Time taken for the evolution of 1
mL of hydrogen was noted down for each milliliter. When hydrogen
evolution ceased, the water levels were adjusted to equal height
by moving the reservoir, and then the final reading was corrected
for the water vapor pressure. The hydrolysis reaction was carried
out using 7, 15, 30, 50, 75, and 100 mol % of the respective salts.
Ni, Co, and Cu Nanoparticle-Catalyzed AB Hydrolysis. The
final products obtained in the Ni2+-, Co2+-, and Cu2+-assisted AB
hydrolysis reaction using 30 mol % of metal salts were isolated by
filtration. The powders were washed with ethanol and dried under
a vacuum. The powder XRD analysis revealed that they were made
up of the respective metal and metal boride nanoparticles in the
cases of Ni2+ and Co2+ but Cu and Cu2O nanoparticles in the case
of Cu2+. The hydrolysis of AB (0.5 mmol) in 20 mL of water was
carried out using 30 mol % of these powder samples.
Ni2+, Cu2+, Co2+
H3N · BH3 + 2H2O
8 NH+4 + BO2- + 3H2 (1)
298 K
Co2+-Assisted Hydrolysis of AB to Generate H2. Hy-
drogen evolution from an AB solution assisted by Co2+
showed an induction period of nearly 1 h, depending on the
concentration of the starting cobalt salt used. Figure 1 shows
the hydrogen evolution curves for various mole percents of
(12) Kaufman, C. M.; Sen, B. J. Chem. Soc., Dalton Trans. 1985, 307.
(13) Holbrook, K. A.; Twist, P. J. J. Chem. Soc., Dalton Trans. 1971, 890.
(14) Glavee, G. N.; Klabunde, K. J.; Sorensen, C. M.; Hadjapanayis, G. C.
Inorg. Chem. 1995, 34, 28.
Preparation of Coinage Metal Nanoparticles. CuSO4 ·2H2O,
AgNO3, and HAuCl4 ·3H2O were used as the starting metal
precursors for the preparation of Cu, Ag, and Au nanoparticles,
respectively. PVP was used as a capping agent. Initially, 0.01 M
stock solutions of these metal salts were prepared in 100 mL
(15) Glavee, G. N.; Klabunde, K. J.; Sorensen, C. M.; Hadjapanayis, G. C.
Langmuir 1992, 8, 771.
(16) Glavee, G. N.; Klabunde, K. J.; Sorensen, C. M.; Hadjapanayis, G. C.
Langmuir 1994, 10, 4726.
(17) Huang, K.-C.; Ehrman, S. H. Langmuir 2007, 23, 1419.
(18) Kapoor, S.; Joshi, R.; Mukherjee, T. Chem. Phys. Lett. 2002, 354,
443.
(10) Kalidindi, S. B.; Sanyal, U.; Jagirdar, B. R. Phys. Chem. Chem. Phys.
2008, DOI: 10.1039/B805726E.
(19) Petit, C.; Lixon, P.; Pileni, M.-P. J. Phys. Chem. 1993, 97, 12974.
(20) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R.
J. Chem. Soc., Chem. Commun. 1994, 801.
(11) Hu, M. G.; van Paasschen, J. M.; Geanangel, R. A. J. Inorg. Nucl.
Chem. 1977, 39, 2147.
Inorganic Chemistry, Vol. 47, No. 16, 2008 7425

Kalidindi et al.
Figure 3. H2 generation from the hydrolysis of AB (0.5 mmol) in the
presence of various mole percents of NiCl2 ·6H2O in 20 mL of water.
Figure 1. H2 generation from the hydrolysis of AB (0.5 mmol) in the
presence of various mole percents of CoCl2 ·6H2O in 20 mL of water.
Figure 2. Powder XRD pattern of the black precipitate from the hydrolysis
of AB using 30 mol % CoCl2 ·6H2O (top). Black precipitate annealed at
500 °C for 12 h under argon in a sealed capillary tube (bottom).
Figure 4. Powder XRD pattern of the precipitate from the hydrolysis of
AB using 30 mol % NiCl2 ·6H2O (top). Precipitate annealed at 500 °C for
12 h under argon in a sealed capillary tube (bottom).
CoCl2 ·6H2O used. In each case, almost 3 mol of H2 evolution
was noted. In the presence of 30 and 50 mol % of the cobalt
salt, H2 evolution was complete in ca. 30 min, whereas it
took a much longer time, 45 and 36 min, when the starting
concentration of the Co2+ was 7 and 15 mol %, respectively.
When the hydrogen evolution ceased, black precipitates
settled at the bottom of the flask. This powder was isolated
under Ar in the case of the reaction with 30 mol % of Co2+
via filtration and was washed several times with ethanol and
dried under a vacuum. The powder X-ray diffractogram of
this sample was devoid of any sharp peaks, indicating its
amorphous nature. Therefore, the powder was annealed at
500 °C for 12 h under argon in a sealed capillary tube. The
powder pattern of the annealed sample exhibited peaks
corresponding to Co and Co2B (Figure 2). For the reactions
with 7, 15, and 30 mol % of cobalt salt, the supernatant
solutions were colorless. The evaporation of water from the
supernatant solution by heating it at ca. 100 °C for a few
hours gave a white crystalline solid that was found to be
B(OH)3 by FTIR spectroscopy. A concentration of 30 mol
% of the cobalt salt was found to be optimum for the
generation of 3 mol of H2 in 30 min.
Ni2+-Assisted Hydrolysis of AB to Generate H2. As in
the case of cobalt, Ni2+-assisted AB hydrolysis showed an
induction period of ca. 3 h. Before the onset of H2 evolution,
a black material was deposited on the reaction vessel. Figure
3 shows the H2 evolution curves by AB hydrolysis using
various mole percents of NiCl2 · 6H2O. The final black
precipitate that resulted was isolated in the case of the
reaction with 30 mol % of the nickel salt. The XRD pattern
of this powder exhibited very broad and weak peaks,
indicating its amorphous nature. Upon annealing the sample
at 500 °C for 12 h, the powder XRD pattern revealed sharp
peaks corresponding to both Ni and Ni3B (Figure 4). A thin
film of Ni was deposited on the walls of the reaction flask
Figure 5. H2 generation from the hydrolysis of AB (0.5 mmol) in the
presence of various mole percents of CuCl2 ·6H2O in 20 mL of water.
in the case of reactions with higher mole percents (<50 mol
%) of the nickel salt. In addition, the supernatant solutions
in these cases were pale green in color. Upon removal of
the water by evaporation, the solid residue that was obtained
contained both anhydrous NiCl2 and boric acid. This suggests
that some of the nickel salt remained unreacted in these cases.
On the other hand, reactions with 7, 15, and 30 mol % of
Ni2+ gave supernatant solutions that were colorless. A
workup of these solutions gave colorless solid products of
B(OH)3, as evidenced by FTIR spectroscopy. A total of 30
mol % of Ni2+ was found to be optimum, in which case, 3
mol of H2 were liberated in less than 35 min.
Cu2+-Assisted Hydrolysis of AB to Generate H2. Unlike
Co2+ and Ni2+, hydrogen evolution took place almost
instantaneously upon the addition of CuCl2 · 6H2O to an
aqueous solution of AB. Figure 5 shows the hydrogen
evolution curves for Cu2+-assisted AB hydrolysis. The
addition of 7, 15, 30, and 50 mol % of Cu2+ to an AB
solution resulted in a color change from blue to brown and
then to dark brown and finally to black, accompanied by
7426 Inorganic Chemistry, Vol. 47, No. 16, 2008

Ammonia-Borane Hydrolysis for Hydrogen Generation
Figure 7. Changes in pH during Co2+-, Ni2+-, and Cu2+-assisted (15 mol
% in each case) AB hydrolysis.
Figure 6. Powder XRD pattern of the precipitate from the hydrolysis of
AB using 30 mol % CuCl2 ·6H2O.
Table 1. Activities of Co2+, Ni2+, and Cu2+ Ions Towards Hydrogen
Generation via AB Hydrolysisa
CoCl2 ·6H2O
NiCl2 ·6H2O
CuCl2 ·6H2O
metal salt
mol %
TI.P.
TH2
TI.P.
TH2
TI.P.
TH2
7
78
60
72
57
56
59
45
36
29
20
19
16
170
140
112
137
92
125
57
34
28
28
35
2
2
∼1
∼1
<1
<1
168
112
51
9
11
6
15
30
50
75
100
162
a TI.P.: induction period in min. TH2: time taken for H2 evolution in min.
Figure 8. Hydrogen generation by the hydrolysis of AB (0.5 mmol) using
amorphous Co and Co2B; Ni and Ni3B and polycrystalline Cu nanoparticles
(metal/AB ) 0.3).
the evolution of 3 mol of H2. However, the use of 75 and
100 mol % of the copper salt afforded a dark brown
precipitate, accompanied by a less than stoichiometric amount
of H2, as per eq 1. The precipitate obtained in the reaction
with 30 mol % of the copper salt was isolated. Unlike the
cases of Co2+ and Ni2+, this powder sample was found to
be crystalline. The XRD pattern revealed the presence of
Cu and Cu2O (Figure 6). We noted that, at higher concentra-
tions of AB, Cu2+ was completely reduced to Cu(0). The
presence of Cu2O is due to the surface oxidation of Cu(0),
which could not be avoided.
Comparison of the Activities of Co2+, Ni2+, and Cu2+
Toward AB Hydrolysis. The activities of Co2+, Ni2+, and
Cu2+ ion-assisted hydrolysis of AB are summarized in Table
1. In the cases of Co2+- and Ni2+-assisted AB hydrolysis,
no clear trends in the induction period for the generation of
the catalytically active species were apparent. The induction
periods were roughly 1 h and 2-3 h respectively for Co2+
and Ni2+, no matter what the starting concentrations of the
respective metal salts used were. For all three cases, the time
taken for H2 liberation followed a regular trend: an increase
in the mole percent of the starting metal salt (up to 50 mol
%) led to faster H2 release. However, for concentrations >50
mol % of the metal salt, no trend was apparent in all three
cases, which is unclear at this time. Without taking into
account the induction period, it is apparent that Co2+ is more
active than Ni2+ and Cu2+ in the case of reactions using 7,
15, and 30 mol % of the respective salt. However, at higher
mole percents of the ions, we found that Cu2+ is more active
than Co2+ and Ni2+. In the case of Cu2+, ammonia-borane
hydrolysis was found to be fast initially, slowing down after
ca. 5 min. We also monitored the pH changes during the
hydrolysis reactions. While the pH remained within the
8.5-9.0 range in the case of Co2+ and Ni2+ systems, large
variations in the pH were noted in the case of the Cu2+
system. Figure 7 shows the pH changes for the 15 mol %
Cu2+-assisted AB hydrolysis. It is apparent that large
quantities of H+ production take place soon after the addition
of Cu2+ during the initial 2-3 min, which is responsible for
the initial rapid H2 evolution. Acid-catalyzed AB hydrolysis
reaction kinetics have been found to be quite fast.21 After
about 5 min of reaction time, the hydrogen evolution slowed
down, and the reaction was catalyzed by in situ generated
Cu nanoparticles. We recently reported that Cu@Cu2O and
Cu2O nanoparticles are active catalysts for the generation
of hydrogen from AB; however, Cu nanoparticles are not as
active.10 The recyclability of the copper catalyst obtained in
the present work was tested and found to be quite good; we
found no decrease in the activity for up to eight cycles.
Unlike the case of Cu2+, the pH remained in the range
8.5-9.0 (Figure 7) during the course of the reaction when
cobalt and nickel salts were employed. This suggests that
the mechanism is quite different for the Co2+- and Ni2+-
assisted AB hydrolysis in comparison to that of Cu2+. The
visual observation of H2 evolution that is initiated only after
the formation of amorphous black material in the cases of
Co2+- and Ni2+-assisted AB hydrolysis suggests that they
are the catalytically active species. In order to establish this
further, we isolated the black amorphous materials in both
cases and tested their activities for AB hydrolysis. Figure 8
shows the hydrogen evolution curves using the black
amorphous materials for AB hydrolysis. In both of these
cases, the activities are more or less the same, and so were
the recyclabilities. The activities of the catalysts remained
unchanged for up to eight cycles. The powder XRD pattern
of the black amorphous materials upon annealing evidenced
(21) Kelly, H. C.; Marriott, V. B. Inorg. Chem. 1979, 18, 2875.
Inorganic Chemistry, Vol. 47, No. 16, 2008 7427

Kalidindi et al.
the presence of Co and Co2B and Ni and Ni3B, respecti-
vely.
Thus, these are examples of first-row transition metal
catalysts that produce 3 mol of H2 within 30 min via the
hydrolysis of AB. On the other hand, copper nanoparticles
(protected from air) that were isolated from the Cu2+-assisted
AB hydrolysis reaction were found to effect slower H2
evolution compared to Cu2+, which is in accordance with
our postulate of initial acid catalysis.
Coinage Metal Nanoparticles Using Ammonia-
Borane. The observation that Cu2+ ions got reduced to Cu0
upon the hydrolysis of AB prompted us to explore further
the reducing ability of AB. Thus, we prepared Cu, Ag, and
Au nanoparticles from CuSO4 · 2H2O, AgNO3, and
HAuCl4 ·3H2O, respectively, using AB as the reducing agent.
A well-known, conventional reducing agent usually em-
ployed for nanoparticles synthesis, NaBH4 suffers from
certain drawbacks such as its instability in water, faster
reaction rates which can lead to uncontrollable particle sizes,
and the formation of water-soluble impurities like NaCl that
are very difficult to remove, especially when water is
employed as the reaction medium. Some of these difficulties
can be overcome by using AB as the reducing agent in place
of NaBH4. Ammonia-borane is stable toward hydrolysis in
aqueous solutions for at least 4 days. No water-soluble
impurity like the one obtained in NaBH4 reduction is
obtained. Recently, Manners and co-workers reported the
formation of 2 nm Rh nanoparticles when [Rh(µ-Cl)(1,5-
cod)]2 was treated with Me2NH · BH3.22
Figure 9. (a) UV-visible spectrum of Cu colloid. (b) TEM bright field
image of Cu colloid. (c) Histogram showing the particle size distribution.
(d) HRTEM image of Cu(0) nanoparticle. (e) SAED pattern of Cu(0)
nanoparticles.
The copper colloids obtained via reduction of Cu2+ to Cu0
using AB were wine red in color, exhibiting a surface
plasmon resonance (SPR) at 560 nm (Figure 9a). For copper
nanoparticles of 10 nm diameter, the UV-visible spectrum
is characterized by a broad SPR at 570 nm.23 Subtle
variations in the position of the SPR are due to the different
surfactants present (here, PVP) at the surface of the particles
and the dielectric properties of the solvent.24 The colloids
were found to be quite stable toward oxidation for at least 2
days, in the presence of excess AB. This is due to the
presence of a constant H2 atmosphere generated via very slow
hydrolysis of AB catalyzed by pure Cu0 nanoparticles. The
colloids were also stable toward precipitation of Cu nano-
particles for months. The TEM bright field image (Figure
9b) revealed the presence of 3-7 nm particles with an
average particle size of 4.3 nm (Figure 9c). The HRTEM
image (Figure 9d) showed lattice fringes corresponding to
the (111) plane of the face-centered-cubic (FCC) copper
phase. The SAED pattern (Figure 9e) shows a ring
(22) (a) Jaska, C. A.; Temple, K.; Lough, A. J.; Manners, I. J. Am. Chem.
Soc. 2003, 125, 9424. (b) Jaska, C. A.; Manners, I. J. Am. Chem. Soc.
2004, 126, 9776. (c) Fulton, J. L.; Linehan, J. C.; Autrey, T.;
Balasubramanian, M.; Chen, Y.; Szymczak, N. K. J. Am. Chem. Soc.
2007, 129, 11936.
Figure 10. (a) UV-visible spectrum of Ag colloid. (b) TEM bright field
image of Ag colloid. (c) Histogram showing the particle size distribution.
(d) HRTEM image of Ag(0) nanoparticle. (e) SAED pattern of Ag(0)
nanoparticles.
(23) Creighton, J. A.; Eadon, D. G. J. Chem. Soc., Faraday Trans. 1991,
87, 3881.
pattern: all four rings match with the lattice planes (111),
(200), (220), and (311) of the FCC copper phase. From these
findings, we conclude that these samples are comprised of
pure crystalline Cu0nanoparticles.
(24) (a) Huang, H. H.; Yan, F. Q.; Kek, Y. M.; Chew, C. H.; Xu, G. Q.;
Ji, W.; Oh, P. S.; Tang, S. H. Langmuir 1997, 13, 172. (b) Ziegler,
K.; Doty, R. C.; Johnston, K. P.; Korgel, B. A. J. Am. Chem. Soc.
2001, 123, 7797. (c) Rothe, J.; Hormes, J.; Bo¨nnemann, H.; Brijoux,
W.; Siepen, K. J. Am. Chem. Soc. 1998, 120, 6019.
7428 Inorganic Chemistry, Vol. 47, No. 16, 2008

Ammonia-Borane Hydrolysis for Hydrogen Generation
nanoparticles, we were successful in obtaining Au nanopar-
ticles via reduction of HAuCl4 · 3H2O. The resultant purple-
red gold colloids showed a broad SPR at 500 nm (Figure
11a.) The Au colloids were also found to be quite stable
toward precipitation for several months, similar to the Cu
and Ag systems. Unlike Cu and Ag, the Au nanoparticles
obtained by this method were found to be polydisperse, as
evidenced by the TEM BF image (Figure 11b) with particle
sizes ranging from 2 to 9 nm. The average particle size in
this case is 5.1 nm (Figure 11c). The HRTEM image (Figure
11d) and the SAED pattern (Figure 11e) further support the
presence of pure crystalline Au0 nanoparticles.
Conclusions
Several first row transition metal ions assist the hydrolysis
of AB under very mild conditions for the generation of H2.
Co2+ and Ni2+ assisted AB hydrolysis show some induction
period during which time catalytically active amorphous
species are formed that show exceptional activities. The
Cu2+-assisted hydrolysis of AB was catalyzed by both in
situ generated H+ and Cu(0) nanoparticles. Of the three cases,
the cobalt system was found to be more active than the nickel
and the copper systems, without taking into consideration
the induction period. The ammonia-borane reduction of
coinage metal salts resulted in the respective metal nano-
particles under very mild conditions. Our work is, therefore,
a demonstration of the deployment of cheap and abundant
and, at the same time, stable first row transition metal
catalysts for the generation of H2 from AB via hydrolysis.
Figure 11. (a) UV-visible spectrum of Au colloid. (b) TEM bright field
image of Au colloid. (c) Histogram showing the particle size distribution.
(d) HRTEM image of Au(0) nanoparticle. (e) SAED pattern of Au(0)
nanoparticles.
We also prepared yellow-colored silver colloids by the
reduction of AgNO3 using AB in the presence of PVP as
the surfactant. These samples exhibited an SPR at 383 nm
(Figure 10a). Silver nanoparticles of 10 nm diameter exhibit
the SPR at 390 nm.23 The blue shift of the SPR in our
samples could be attributed to the smaller particle size. These
colloids were found to be stable for months. The TEM bright
field (BF) image (Figure 10b) showed the presence of
spherical particles with an average diameter of 4.7 nm
(Figure 10c). The HRTEM image (Figure 10d) and the
SAED pattern (Figure 10e) further support the presence of
pure crystalline FCC Ag0 nanoparticles.
Acknowledgment. We are grateful to the Council of
Scientific and Industrial Research, India, for financial support.
We thank the I.I.Sc. Institute Nanoscience Initiative and the
Solid State and Structural Chemistry Unit, I.I.Sc., for
allowing us to access the microscopic facilities and the
powder X-ray diffractometer, respectively. S.B.K. thanks the
CSIR for a fellowship.
Supporting Information Available: 11B NMR spectrum show-
1
ing the formation of B(OH)3 and H NMR spectra showing the
production of H2 and the formation of NH4+ in the transition metal
ion-assisted hydrolysis of AB. This material is available free of
charge via the Internet at http://pubs.acs.org.
In a manner analogous to the reduction of the copper and
silver salts for the realization of the respective metal
IC800805R
Inorganic Chemistry, Vol. 47, No. 16, 2008 7429