Silica aerogel-supported cobalt nanocomposites as efficient catalysts toward hydrogen generation from aqueous ammonia borane

Article abstract of DOI:10.1039/c4ra14002h

In this study, a mesoporous silica aerogel-supported cobalt (Co/SAG) nanocomposite synthesized by a facile chemical reduction was used as an alternative catalyst for hydrogen generation from aqueous NH₃BH₃. The result showed that Co/SAG exhibited 41% higher hydrogen generation rate for the NH₃BH₃ hydrolysis than the ordered mesoporous silica (MCM-41)-supported Co catalysts (Co/MCM-41) prepared by the same reduction method. This is attributed to the fact that the Co nanoparticles were smaller (less than 5 nm in diameter) and better-deposited in SAG than MCM-41 as observed in the TEM micrographs. It is also found that the Co/SAG catalyst delivered superior turnover frequency (3013 ml H₂ min^-1 g_metal^-1) and activation energy (46.4 kJ mol^-1) than most of the Co-based catalysts reported. This study showed that the Co/SAG prepared is a potential non-precious catalyst for hydrogen generation from aqueous NH₃BH₃. This journal is

Full text of DOI:10.1039/c4ra14002h

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Silica aerogel-supported cobalt nanocomposites as
efficient catalysts toward hydrogen generation
from aqueous ammonia borane†
Cite this: RSC Adv., 2015, 5, 13985
Pin-Ju Yu,a Mu-Hsin Lee,a Hung-Ming Hsu,a Huei-Mei Tsaib and Yui Whei Chen-
a
*
Yang
In this study, a mesoporous silica aerogel-supported cobalt (Co/SAG) nanocomposite synthesized by a
facile chemical reduction was used as an alternative catalyst for hydrogen generation from aqueous
NH3BH3. The result showed that Co/SAG exhibited 41% higher hydrogen generation rate for the NH3BH3
hydrolysis than the ordered mesoporous silica (MCM-41)-supported Co catalysts (Co/MCM-41) prepared
by the same reduction method. This is attributed to the fact that the Co nanoparticles were smaller (less
than 5 nm in diameter) and better-deposited in SAG than MCM-41 as observed in the TEM micrographs.
It is also found that the Co/SAG catalyst delivered superior turnover frequency (3013 ml H2 minꢀ1
gmetalꢀ1) and activation energy (46.4 kJ molꢀ1) than most of the Co-based catalysts reported. This study
showed that the Co/SAG prepared is a potential non-precious catalyst for hydrogen generation from
aqueous NH3BH3.
Received 6th November 2014
Accepted 21st January 2015
DOI: 10.1039/c4ra14002h
www.rsc.org/advances
catalysts it can deliver 3 mol H2 per mol of NH3BH3 at room
temperature.13 The hydrolysis of ammonia borane can be
1. Introduction
expressed in equation:
With the increasing demand for a sustainable and clean energy
supply, hydrogen is considered to be one of the alternative
energy sources as compared to fossil fuels.1 Over the past
decades, hydrogen has been commonly stored in tanks as
compressed or liqueed form,2 on carbon materials,3–5
hydrogen-absorbing alloys,6,7 and metal–organic frameworks,8
but their volumetric and gravimetric efficiencies are still too low
to make them practically feasible. Therefore, searching for
reliable and efficient methods for hydrogen storage is essential
for the future hydrogen economy. Recently, ammonia borane
(NH3BH3) with nontoxic, stable and eco-friendly features has
been recognized as an attractive candidate for hydrogen storage
applications because of its gravimetric hydrogen capacity (19.6
wt%), high solubility and demonstrated stability in neutral
aqueous solutions.9 To generate hydrogen from ammonia
borane, several approaches, including solid-phase thermol-
ysis,10 catalytic dehydrogenation in non-aqueous solvents,11 and
hydrolysis,12 have been reported. Among them, the hydrolysis
appeared to be one of the best ways because with appropriate
+
ꢀ
(aq)
NH3BH3(s) + 2H2O(l) / NH4
+ BO2
+ 3H2(g)
(aq)
The precious metal-based catalysts such as Pt, Pd, Ru, and
Rh have been found to be effective for accelerating the hydro-
lysis of ammonia borane, but they are not suitable for practical
applications due to their limited resources and high cost.14–17
Therefore, numerous efforts have been made to develop the
efficient and economical non-noble metal-based catalysts18–25 in
element forms e.g. Fe, Co, Ni, Cu or supported forms on g-Al2O3,
SiO2, carbon, etc. supports. Silica aerogels are materials that can
comprise as high as 96% porosity, while the remaining 4% is a
network structure of silicon dioxide. Due to their unique char-
acteristics, such as high porosity, large surface area, low density
and low thermal conductivity, silica aerogels have been used as
advance materials in the applications of thermal insulation,
electrical batteries, nuclear waste storage, catalysis, acoustic
insulation, adsorbents, etc.26
In this study, mesoporous silica aerogel-supported Co
nanocomposite (Co/SAG) was synthesized by the facile chemical
reduction method for use as an alternative catalyst for hydro-
lysis of ammonia borane. For comparison, the ordered meso-
porous silica (MCM-41)-supported Co nanocomposite (Co/
MCM-41) was prepared by the same reduction method. The
crystalline structure, morphology, chemical composition and
porous characteristic of the catalysts prepared were
aDepartment of Chemistry, Center for Nanotechnology and Center for Biomedical
Technology, Chung Yuan Christian University, Chung-Li 32023, Taiwan, Republic of
China. E-mail: yuiwhei@cycu.edu.tw; Tel: +886-3-2653317
bMaterials and Electro-Optics Research Division, Electric Energy Section, Chung Shan
Institute of Science and Technology, Lung-Tan 32544, Taiwan, Republic of China
† Electronic supplementary information (ESI) available: EDX of (a) Co/SAG and (b)
Co/MCM-41. The plot of hydrogen generation rate versus the concentration of AB.
See DOI: 10.1039/c4ra14002h
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characterized by X-ray diffraction (XRD), transmission electron
microscopy (TEM), inductively coupled plasma-atomic emis-
sion spectrometer (ICP-AES) and nitrogen adsorption–desorp-
tion analyzer. The rates and activation energies for hydrolysis of
ammonia borane catalyzed by the as-prepared Co/SAG catalyst
at various temperatures and ammonia borane concentrations
were evaluated and compared with that by the Co/MCM-41
catalyst. To the best of our knowledge, it is the rst study to
employ mesoporous silica aerogels as supporting material to
synthesize Co-based catalysts and explore the inuence of
morphology on Co-based catalysts towards hydrolysis of
ammonia borane.
2. Experimental
Scheme 1 Illustration of Co-based catalysts synthesis by a facile
chemical reduction using NaBH4 as reducing agent.
2.1 Chemicals
All chemicals, including cetrimonium bromide (CTAB, 99%,
Acros Organics), tetraethoxysilane (TEOS, 98%, Acros Organics),
ammonium hydroxide (NH4OH, 99.8%, Pharmco), cobalt(II)
nitrate hexahydrate (Co(NO3)2$6H2O, 98%, Alfa Aesar), sodium
borohydride (NaBH4, 99%, Aldrich), ammonia borane (NH3BH3,
97%, Aldrich) were used as received.
homogeneously dispersed in the Co precursor solution by an
ultrasonic processor (Sonicator S-4000, Misonix, USA) under the
amplitude of 20% for 30 minutes (Step I). Aerward, 15 wt% of
the freshly prepared NaBH4 aqueous solution was slowly added
dropwise to reduce the Co2+ (Step II). The Co-based catalysts
separated from the black suspension by centrifuge (CN-830,
Hsiang-Tai Machinery Industry Co., Ltd., Taiwan) under 3000
rpm were nally freeze-dried under 15 Pa at 223 K for 1 day (Step
III). The amounts of Co deposited on Co/SAG and Co/MCM-41
were 24 wt% and 23 wt%, respectively, as determined by an
inductively coupled plasma-atomic emission spectrometer (ICP-
AES, ICAP 9000, Jarrell-A).
2.2 Preparation of mesoporous silica aerogel (SAG)
The SAG was prepared by the sol–gel process from the silicon
precursor, TEOS, using 1-butyl-3-methylimidazolium tetra-
uoroborate (BMIBF) as the template and solvent (methanol) as
reported.27 The processes are described as follows: BMIBF (3.0
g), methanol (1.9 g) and deionized water (3.4 g) were mixed in a
bottle then the designated stoichiometric ratio amount of TEOS
(5.8 g) was added. A homogeneous gel was formed by gelation at
room temperature within 1 h aer mixing and aged at ambient
temperature for 2 days in the open air. Then the entrapped ionic
liquid was Soxhlet-extracted with ethanol for 2 days. The SAG
was nally obtained by freeze-drying under 15 Pa at 223 K for 12
h with the freeze-dryer (Eyela FDU-1200, Tokyo Rikakikai,
Japan).
2.5 Characterizations
To identify the crystalline structure of the Co-based catalysts,
a powder X-ray diffractometer (PXRD, Bruker D2 PHASER)
˚
with a Cu target (l ¼ 1.541 A) excited at 30 kV and 10 mA was
conducted to record their corresponding PXRD patterns of
Co/SAG and Co/MCM-41 from 10ꢁ to 60ꢁ. The microstructure
of the Co/SAG and Co/MCM-41 were examined by the trans-
mission electron microscopy (TEM, JEOL Ltd., TEM-2010) at
200 kV. The pore volume, pore size distribution, and Bru-
nauer–Emmett–Teller (BET) surface area of the SAG, MCM-
41, Co/SAG and Co/MCM-41 powders were determined by a
nitrogen adsorption–desorption analyzer (Tristar 3000,
Micromeritics) at 77 K.
2.3 Preparation of ordered mesoporous silica (MCM-41)
The MCM-41 powder was synthesized according to the pub-
lished method from an alkaline solution containing CTAB,
TEOS, NH4OH as catalyst and deionized water.28 For a typical
procedure, CTAB was well dissolved into NH4OH aqueous
solution and then TEOS was successively added to the above
mixture. The precursor solution was continuously stirred at 363
K for 24 h, where the molar ratio of CTAB : TEOS : NH4-
OH : H2O was kept at 1.0 : 4.5 : 54 : 620. The resulting suspen-
sion was ltered and washed with deionized water at least three
times. To ensure the CTAB was completely removed, the dried
powders were nally calcined in an air atmosphere at 823 K for 6
hours, yielding the ordered MCM-41.
2.6 Hydrogen generation experiment
The hydrogen generation experiments were performed by using
the water displacement method.30 A weighed catalyst with
metal/NH3BH3 molar ratio ¼ 0.05–0.2 was rst placed in a twin-
necked round-bottom ask (25 ml) and the ask was then
sealed with an outlet tube for collecting the generated
hydrogen. An aqueous NH3BH3 solution (5 ml) with the desig-
nated concentration (0.33–5 wt%) was loaded into a ask, which
2.4 Synthesis of Co-based catalysts, Co/SAG and Co/MCM-41
To synthesize the Co-based catalysts, a facile chemical reduc- was immersed into a water bath to maintain the constant
tion using NaBH4 as reducing agent was utilized as illustrated in temperature. The outlet tube exhaust was placed under an
Scheme 1.29 A desired amount of SAG or MCM-41 was inverted, water-lled gas burette that was situated in a water-
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lled vessel to monitor the volume of the displaced water, which pattern in the mall angle range of MCM-41 showed three well-
corresponded to the volume of released hydrogen. For recycling resolved peaks, indexed as (100), (110), and (200) diffractions,
test, the used catalysts were collected and the hydrogen gener- conrming the hexagonal structure as reported.28 The Co-based
ation was repeated with the same procedures as described catalysts were also analyzed by the powder X-ray diffraction
above.
measurements and the corresponding patterns are depicted in
Fig. 1(b). Similar to that for SAG and MCM-41, the broad
peaks at the Bragg angles (2q) of ꢂ23.5ꢁ for both Co/SAG
and Co/MCM-41 are assigned to the SiO2 supports.27 Because
the (1 1 1) plane of the pure cubic phase of Co assigned by
JCPDS 15-0806 is at 2q ¼ ꢂ21.8ꢁ, it is reasonable to believe that
the Co (1 1 1) peak was overlapped with that of the SiO2, but is
difficult to determine the Co phase.29
The transmission electron microscopy (TEM) micrographs
of Co/SAG and Co/MCM-41 were measured for the micro-
structural investigation and are displayed in Fig. 2. The dark
spots on the silica supports are recognized as the reduced Co
particles deposited. It shows that the Co nanoparticles were
quite homogenously deposited among the mesopores of Co/
SAG with particle sizes less than 5 nm (Fig. 2(a)). On the
contrary, larger Co nanoparticles (red circled) with particle
sizes of 13–20 nm are observed on the surface of Co/MCM-41
in Fig. 2(b). The formation of the smaller Co nanoparticles in
Co/SAG is attributed to the network structure of SAG,
3. Results and discussion
3.1 Characterization of Co/SAG and Co/MCM-41
Aer the reduction process, both SAG and MCM-41 were
changed from white to black as shown in Scheme 1, implying
that the Co nanoparticles were successfully deposited on the
SiO2 supports via the chemical reduction. The loading amount
of Co on Co/SAG (24 wt%) determined by the ICP-AES
measurement was slightly higher than that on Co/MCM-41
(23 wt%), but both were quite close to the theoretical value
(25 wt%), revealing that the method used is an efficient reduc-
tion for preparation of the Co nanoparticles on the silica-
supports.
The powder X-ray diffraction patterns of the mesoporous
silicas, SAG and MCM-41, were measured and shown in
Fig. 1(a). No peak was found for SAG at small angle range,
indicating that SAG was an amorphous material. However, the
Fig. 1 PXRD patterns of (a) SAG, (b) MCM-41 and (c) Co/SAG and Co/MCM-41.
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Fig. 2 TEM micrographs of (a) Co/SAG and (b) Co/MCM-41, scale bar: 20 nm.
providing more sites for deposition of the reduced Co- isotherms with the BET theory and Barrett–Joyner–Halenda
nanoparticles and the interconnected mesoporous pores (BJH) method. As listed in Table 1, the surface area, pore
that made the Co-precursor easier to penetrate into the pores, volume, and pore diameter of SAG obtained was 621 m2 gꢀ1
,
increasing the opportunities for the reduced Co- 1.7 cm3 gꢀ1, and 10.1 nm, respectively, and that of MCM-41
nanoparticles to deposit not only on the outer surface but obtained was 798 m2 gꢀ1, 0.7 cm3 gꢀ1, and 2.9 nm, respec-
also the interior of the matrix, as illustrated in the inset of tively. As expected for aerogels, the porosity and the pore
Fig. 2(a). The results indicate that there were smaller Co diameter of SAG were much higher than that of MCM-41. This
nanoparticles and better distribution in Co/SAG than in Co/ supports that the Co-precursor solution can penetrate into
MCM-41. Therefore, with the same weight of Co, Co/SAG the mesopores of SAG more easily, providing more opportu-
was anticipated to provide more catalytic active-sites for the nity for the reduced Co particles to deposit not only the
hydrolysis of ammonia borane than Co/MCM-41. The EDX surface of SAG but also among the interior of the inter-
spectra in Fig. S1† indicated the signals assigned to Co, Si connected pores. On the contrary, due to the tiny pore size in
and O elements in Co/SAG and Co/MCM-41, conrming that MCM-41 the reduced Co nanoparticles were difficult to
the reduced Co nanoparticles were successfully deposited to deposit into its mesopores. Aer the Co deposition, the value
the SAG and MCM-41.
of surface area and pore volume, of Co/SAG was signicantly
Fig. 3 shows the adsorption–desorption isotherms of the changed to 449 m2 gꢀ1, and 1.4 cm3 gꢀ1, respectively, while
SAG, Co/SAG, MCM-41 and Co/MCM-41 powders. According to that of Co/MCM-41 was only slightly changed to 741 m2 gꢀ1
the IUPAC classication, all the samples exhibited type IV and 0.6 cm3 gꢀ1, respectively. The changes of the surface area
isotherms, indicating that they were all typical mesoporous and pore volume for Co/SAG and Co/MCM-41 conrm that
materials.
the Co nanoparticles were successfully deposited to SAG and
The shapes of the hysteresis loops for SAG (Fig. 3(a)) and MCM-41. The large decreases (27.7% decrease in the surface
Co/SAG (Fig. 3(b)) as well as that for MCM-41 (Fig. 3(c)) and area and 17.6% in pore volume) for Co/SAG support that the
Co/MCM-41 (Fig. 3(d)) were similar and occurred at the same deposition not only occurred on the surface but also among
pressure, respectively, implying that the main structures of pores of the SAG matrix. However, the less changes (7.1%
SAG and MCM-41 were remained before and aer the Co decrease in the surface area and 14.3% in pore volume) for
deposition. However, the hysteresis loops of SAG and Co/SAG Co/MCM-41 conrm that the Co deposition mainly
occurred at higher pressure than that of MCM-41 and Co/ occurred on the surface of MCM-41, forming larger particles
MCM-41, conrming that the mesopores were quite irreg- but less affecting the morphology. On the other hand,
ular and interconnected for SAG and Co/SAG, while that were because the pores in Co/SAG were much larger than that in
regular mesopores as expected for MCM-41 and Co/MCM-41, Co/MCM-41, it was also anticipated that the NH3BH3 solution
in which the former is known to have hexagonal structure. would be more easily diffuse in the pores of Co/SAG to react
The BET specic surface area, pore volume and average pore with the interior Co, leading to a more efficient hydrolysis to
diameters of these powders were evaluated from the generate hydrogen.
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Fig. 3 Nitrogen adsorption–desorption isotherms of (a) SAG, (b) Co/SAG, (c) MCM-41 and (d) Co/MCM-41.
Table 1 BET Surface area, pore size and pore volume of the samples
minutes in the presence of the Co/SAG and Co/MCM-41 cata-
lysts, respectively. As the M/AB ratio was increased from 0.05 to
0.2, the hydrogen release rate with Co/MCM-41 catalyst changed
from 3.9 to 11.1 ml minꢀ1, while that with Co/SAG catalyst
increased from 4.9 to 15.6 ml minꢀ1. For M/AB ¼ 0.2, the
hydrogen generation rate obtained with the Co/SAG catalyst was
about 41% higher than that with the Co/MCM-41 catalyst. The
enhancement is ascribed to the smaller particle size and better
distribution of the deposited Co nanoparticles in SAG than that
in MCM-41, providing more catalytic sites for hydrolysis of the
aqueous NH3BH3 as discussed above.
Surface area
Pore volume
Pore size
(nm)
Samples
(m2 gꢀ1
)
(cm3 gꢀ1
)
SAG
621
449
798
741
1.7
1.4
0.7
0.6
10.1
12.3
2.9
Co/SAG
MCM-41
Co/MCM-41
3.0
3.2 Hydrogen generation measurements
Fig. 5 illustrates the hydrogen generation under the catalysis
of the Co-based catalysts at M/AB ¼ 0.05 in the aqueous solution
with various AB concentrations at 303 K. Fig. 5(a) indicates that
the hydrogen release rates of the hydrolysis catalyzed by Co/SAG
from the 0.33, 1.0 and 5.0 wt% AB solutions are 5.0, 4.98, and
5.0 (ml H2 minꢀ1), respectively. This indicates that the hydrogen
release rate is basically independent of the NH3BH3 concen-
tration as shown in Fig. S2,† revealing a zero-order kinetic with
respect to the substrate concentration for the hydrolysis cata-
lyzed by Co/SAG as the study previously reported.17
Fig. 4 shows the hydrogen generation proles vs. time for the
aqueous NH3BH3 (0.33 wt%) with various molar ratio of metal
to NH3BH3 (M/AB) to evaluate the catalytic activities of the Co-
based catalysts.
For comparison, the proles for the AB solutions (0.33 wt%)
containing the support but no Co showed no release of H2 gas
(Fig. 4) as the references. As can be seen, basically no hydrogen
was released for the case without the metals, indicating that the
SAG and MCM-41 were not catalysts for the hydrogen genera-
tion reaction. As listed in Table 2, the 0.33 wt% AB solution with
M/AB ¼ 0.05, the hydrogen generation was nished in 8 and 10
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Fig. 4 Hydrogen generation from aqueous NH3BH3 (0.33 wt%) catalyzed by (a) Co/SAG and (b) Co/MCM-41 at 303 K under M/AB ratio from 0.05
to 0.2.
Table 2 Hydrogen generation (ml minꢀ1) from aqueous NH3BH3 (0.33
ꢀ1=3d½NH3BH3ꢃ d½H2ꢃ
¼
¼ k
dt
wt%) catalyzed by Co/SAG and Co/MCM-41 at 303 K under M/AB ratio
from 0.05 to 0.2
dt
M/AB
Fig. 6(b) shows the Arrhenius plot, ln(k) versus the reciprocal
absolute temperature.
Catalyst
0.05
0.2
The slope of the straight line gave the Ea value of 46.4 kJ
Co/MCM-41
Co/SAG
3.9
4.9
11.1 molꢀ1 for Co/SAG. For comparison the turnover frequency
15.6
(TOF) and activation energy (Ea) of different catalysts used for
the hydrolytic dehydrogenation of ammonia borane,18,31–41 are
listed in Table 3. As can be seen, the Ea value of Co/SAG
prepared in this study is lower than those of the most metal
oxide-supported non-precious catalysts reported, such as Co/g-
Al2O3,18 1.2% Co@M41S33 and Co–Ni–P/Pd–TiO2,40 implying
that the hydrolytic dehydrogenation of ammonia borane can be
easier taken place with Co/SAG as the catalyst. In addition, the
TOF value of Co/SAG is also higher than most of the Co-based
catalysts reported.35–38 The results demonstrate that Co/SAG is
able to function as an efficient non-precious catalyst towards
hydrogen generation from aqueous ammonia borane.
The hydrogen generation from the aqueous NH3BH3 (0.33
wt%) in the presence of Co/SAG under M/AB ¼ 0.05 at
different temperatures (303–323 K) is displayed in Fig. 6(a).
As expected, the hydrolysis reaction was quickened with
increasing temperature. To calculate its activation energy,
the rate law and Arrhenius equation for the catalytic hydro-
lysis of aqueous NH3BH3 are given as the following
equation.18
Fig. 5 Hydrolysis of various NH3BH3 concentration catalyzed by (a) Co/SAG and (b) Co/MCM-41 at 303 K with the catalyst amount kept
unchanged.
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Fig. 6 (a) Time plots of catalytic dehydrogenation of aqueous NH3BH3 (0.33 wt%) over Co/SAG from 303 K to 323 K at M/AB ¼ 0.05. (b) ln(k) vs. 1/
T plot calculated from (a).
The favorable hydrogen generation from aqueous NH3BH3
by the catalysis with the Co supported interconnected porous
SAG prepared in this study can be reasoned according to the
mechanism reported by J. Chen et al.32 The small Co nano-
particles deposited not only on the surface but among the
interconnected mesopores in the SAG network structure, as
shown in the TEM images above, provided more sites that could
form the transient Co–H bonds, which were required for the
hydrolysis of AB, consequently, increasing the opportunities for
the H2 generation.
3.3 Recyclability test
The recyclability of Co/SAG for hydrolysis of the aqueous
NH3BH3 (0.33 wt%) was evaluated at 303 K and the corre-
Fig. 7 Recyclability test of Co/SAG for hydrolysis of aqueous NH3BH3
(0.33 wt%) solution at 303 K where the M/AB value was 0.05.
sponding plots are shown in Fig. 7. It is seen that the rates of
hydrogen generation were not decayed from 1st to 3rd cycles.
Even aer ve cycles, the H2/NH3BH3 mole ratio of the dehy-
generation from aqueous NH3BH3. Similar cyclic phenomena
have also been reported from the other non-precious
drogenation of NH3BH3 still retained to 3.0, indicating that the
catalysts.23,42,43
Co/SAG catalyst is an efficient catalyst for the hydrogen
Table 3 Values of turnover frequency (TOF) and activation energy (Ea) in different catalysts used for the hydrolytic dehydrogenation of ammonia
borane
Catalysts
TOF (ml H2 minꢀ1 gꢀ1 of metal)
Ea (kJ molꢀ1
)
References
Co/g-Al2O3
Ru@SiO2
Co0.32@Pt0.68/C NPs
1.2% Co@M41S
Co–Cr–P–B
—
—
62.0
38.2
41.5
54.6
44.0
43.0
—
18
31
32
33
34
35
36
37
38
38
39
40
4874
4378
3325
2460
1150
1000
394
376
—
Co–Mo–B
Co–B/mesoporous silica
(Cu0.5Co0.5)2Al-Cat
Crystalline Co–B
Amorphous Co–B
Intrazeolite Co(0) nanoclusters
Co–Ni–P/Pd–TiO2
NiCo–Pt
—
46.2
47.5
56.0
54.9
45.7
46.4
—
—
3013
41
Co/SAG
This study
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¨
¨
15 O. Metin, ¸S. ¸Sahin and S. Ozkar, Int. J. Hydrogen Energy, 2009,
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In this study, the mesoporous silica aerogel-supported cobalt
(Co/SAG) nanocomposite was successfully synthesized using a
facile chemical reduction, employing sodium borohydride as
the reducing agent at room temperature. The TEM micrograph
revealed that Co nanoparticles with particle size less than 5 nm
were homogenously deposited to Co/SAG. Co/SAG exhibited
superior catalytic activity for hydrogen generation from the
aqueous ammonia borane at ambient condition than Co/MCM-
41 and exhibited a zero-order kinetics. The turnover frequenꢀcy1
(TOF) and activation energy (Ea) were 3013 ml H2 minꢀ1 gmetal
and 46.4 kJ molꢀ1, respectively. The TOF value was higher and
the Ea value was lower than the corresponding values of most of
the Co-based catalysts reported for hydrolysis of the aqueous
NH3BH3 solution. In addition, the Co/SAG catalyst was found
recyclable for dehydrogenation of aqueous AB solution with
retaining molar ratio of H2/NH3BH3 ¼ 3.0. These results indi-
cate that the as-prepared silica aerogel-supported cobalt nano-
composite is an efficient catalyst for hydrogen generation from
aqueous ammonia borane.
¨
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Acknowledgements
The authors would like to thank Chung Shan Institute of
Science and Technology (CSIST-808-V314) and Chung Yuan
Christian University of Taiwan (R.O.C.) for providing the
nancial support to this research work.
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13992 | RSC Adv., 2015, 5, 13985–13992
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