Generation of hydrogen in the hydrolysis of NaBH4 using Ir(0) catalyst

Article abstract of DOI:10.14233/ajchem.2014.17820

This study reports the results of kinetic of hydrogen generation from the catalytic hydrolysis of sodium borohydride. Iridium(0) particles catalyst was stabilized by diethylene glycol. Catalyst iridium was characterized by field emission scanning electron

Full text of DOI:10.14233/ajchem.2014.17820

Asian Journal of Chemistry; Vol. 26, No. 23 (2014), 8181-8184
ASIAN JOURNAL OF CHEMISTRY
http://dx.doi.org/10.14233/ajchem.2014.17820
Generation of Hydrogen in the Hydrolysis of NaBH4 Using Ir(0) Catalyst
1,2,*
MECIT AKSU
1Department of Chemistry, King Abdulaziz University, 21589 Jeddah, Saudi Arabia
2Department of Chemistry, Duzce University, 81100 Duzce, Turkey
*Corresponding author: E-mail: mecitaksu@duzce.edu.tr
Received: 15 April 2014;
Accepted: 28 May 2014;
Published online: 15 November 2014;
AJC-16321
This study reports the results of kinetic of hydrogen generation from the catalytic hydrolysis of sodium borohydride. Iridium(0) particles
catalyst was stabilized by diethylene glycol. Catalyst iridium was characterized by field emission scanning electron microscopy and
energy dispersive X-ray spectroscopy. Hydrolysis of sodium borohydride was carried out both with and without stirring. Effect of stirring,
sodium hydroxide concentration and sodium borohydride concentration on hydrogen volume and yield of hydrogen generation was
investigated for optimization. It was found that rate of H2 generation decreased with increasing NaOH concentration while increased with
increasing concentration of NaBH4. Stirring has positive effect on rate of hydrogen generation.
Keywords: Iridium catalyst, Sodium borohydride, Hydrogen generation, Hydrolysis.
develop suitable hydrogen storage and releasing materials
INTRODUCTION
in the last few decades, the efficient storage and production
The synthesis of metal nanoparticles with controllable size
and size distribution is of great importance because of their
potential applications in many fields, including catalysis1.
Today, a challenging issue in the synthesis of metal nanopar-
ticles is the achievement of the compositionally well-defined
shape and size controllable nanoparticles as the catalytic
activity of metal nanoparticles is drastically influenced by their
size and shape, as they control the surface structure, electronic
and oxidation states. Therefore, the use of well defined metal
nanoparticle catalysts allows us to assess the nature of active
sites in the catalytic reaction, which is vital for the rational
design of catalysts. Recent progress in the fabrication
techniques has enabled the synthesis of metal nanoparticles
with precisely controlled size, shape and composition1-6.
Ruthenium(0) nanoparticles7,8, hydrogen phosphate stabilized
nickel(0) nanoparticles9, CoB nanoparticles10, PVP stabilized
nickel(0)11 and cobalt(0) nanoparticles12, Co2B nanoparticles13,
bimetallic PtxNi1-x nanoparticles14, Co-La-Zr-B quaternary
amorphous nanoalloy2, Fe-B nanoparticles3, surface-alloyed
Ni foam4, Ru-RuO2/C15, carbon nanosheets supported Zr/Co16,
Ni-Ru nanocomposite17, Co-Cu-B18 catalysts were tested in
the hydrolysis of sodium borohydride.
of hydrogen are still two key problems in the "Hydrogen
economy"19,20. Hydrogen storage techniques that are related
to liquid-phase chemical hydrogen storage materials, such as
aqueous NaBH4, H3NBH3, N2H4, N2H4BH3 and HCO2H, have
attracted considerable attention4,21
.
The concentrations of NaBH4 and NaOH exert important
influences on the practical performance of on-demand hydrogen
generation system. Higher NaBH4 concentration is highly wanted
for achieving high hydrogen capacity, but gets restricted by
the solubility limitation of NaBH4 itself and hydrolysis product
NaBO2 in water. Certain amount of NaOH is required for
stabilizing the NaBH4 fuel solution, but causes capacity loss
and may pose a problem for rapid hydrogen generation and
instant response for hydrogen generation requirement22. This
study is aimed to illustrate the optimization of hydrogen gene-
ration reaction from NaBH4 via Ir(0) stabilized by diethylene
glycol as catalyst and effect of concentration of NaOH, NaBH4,
stirring and unstirring of reaction media on rate of hydrogen
production yield will be illustrated.
EXPERIMENTAL
Catalyst preparation: All reagents were of analytic grade
and double-distilled water was used throughout the experi-
ments. Ir(0) was prepared by reducing its precursor with
NaBH4 solution. Borohydride reduction of metal ions has been
studied extensively and it is well-known that Ir is obtained
Hydrogen is a potential and economical clean energy
carrier stored in molecular (pressurized vessels, liquified H2
tanks), atomic (metal hydrides), or hydride forms (protide
compounds). Although there has been enormous efforts to

8182 Aksu
Asian J. Chem.
upon reduction of iridium salt (IrCl3). The aqueous reduction
process is thought to be as follows23.
Spectrum 1
BH4- + Ir3+ + 2H2O → Ir(s) + BO2– + 3H+ + 2.5H2↑ (1)
0.299 g (1 mmol) IrCl3 suspended in 50 mL diethylene
glycol (Digol) with vigorous stirring at 80 ºC adding 5 mL of
water. 0.076 g (2 mmol) NaBH4 was dissolved in 5 mL of
distilled water. Two solutions were mixed and heated to 230 ºC
rapidly. A strong dark colour was observed immediately after
mixing. The system was maintained under magnetic stirring
condition at 230 ºC for 2 h and subsequently the heating was
turned off.A very stable colloidal dispersion was formed with
this procedure. In order to isolate the resulting Ir(0) particles,
50 mL of acetone was added to the dispersion. A black solid
was formed by precipitation, which was separated by centrifu-
gation, washed several times with acetone and ethanol and
dried at room atmosphere.
Generation of hydrogen: The hydrolysis reaction (reaction
1) was conducted in a 50 mL capacity batch reactor with an
internal volume of 25 mL. In a typical experiment, 10 mL
0.5-4.0 wt. %. NaBH4 solution containing 0-6 wt. %. NaOH
was placed in the reactor fitted with an outlet tube (connecting
to a mass flow meter) for collecting evolved H2 gas. The outlet
tube exhaust was placed under an inverted, water-filled gas
burette which was situated in a water-filled vessel. 50 5 mg
Ir(0) catalysts was added into the solution under mild stirring.
The reactions were carried at 293 K and the reaction tempe-
rature was maintained within 1 K of its set point using an
external water jacket. The hydrogen generation rate (mL min-1
g-1, based on the weight of Ir catalyst used) was recorded by
the mass flow meter and the volume of generated H2 was
measured by the water displacement method23.
2
3
4
5
6
keV
7
8
9
10
Fig. 2. EDS patterns of Ir(0) catalyst
The hydrogen generation from the hydrolysis of NaBH4
with Ir(0) catalyst in the presence of NaOH and effects of
stirring on hydrogen generation were investigated.
Figs. 3 and 4 show the hydrogen generation volume from
NaBH4 with Ir(0) catalyst produced for both with stirred and
unstirred solutions. Fig. 5 shows the effect of NaOH concen-
tration on hydrogen generation from NaBH4. It can be seen
that hydrogen production from hydrolysis reaction of NaBH4
with Ir(0) catalyst reduced in the presence of NaOH in 20 min.
700
Unstirred
600
500
400
300
200
100
0
catalyst→
NABH4 + 2H2O
4H2↑ + NaBO2(aq)
(2)
The yield of hydrogen is defined as follows based on
reaction. 2.
0
200
400
600
800
1000
1200
1400
Time (s)
3, 7 %
1 %
1.50 %
1.90 %
2.40 %
3 %
0.51 %
Fig. 3. Effect of unstirring on the hydrogen generation measured using
0.5 to 3.7 % NaBH4 at 20 ºC, using 50 5 mg of Ir(0) catalyst
n
generated H2
χ =
× 100 %
4 × nNaBH
4
800
Stirred
700
RESULTS AND DISCUSSION
600
500
400
300
200
100
0
Field emission scanning electron microscopy (FE-SEM)
images obtained with JEOL JSM-7600F device and EDS patterns
were determined via Xmax large area SSD connected to it.
FE-SEM images are given in Fig. 1. It is seen from micro-
graphs that Ir(0) catalyst contains flaky particles that resulting
products agglomerate with varying sizes. It shows hetero-
geneous morphology. EDX patterns given in Fig. 2 indicate
that that pure Ir(0) particles are obtained.
0
200
400
600
800
1000
1200
1400
Time (s)
0.052 %
0.1 %
0.15 %
0.20 %
0.22 %
0.29 %
0.36 %
Fig. 4. Effect of stirring on the hydrogen generation measured using 0.5 to
3.6 % NaBH4 at 20 ºC, using 50 5 mg of Ir(0) catalyst
Effect of NaOH concentration: Sodium hydroxide is
usually added as a stabilizer into the NaBH4 solution to prevent
its self-hydrolysis. Fig. 5 illustrates the hydrogen generation
volumes with different NaOH concentration, i.e. 0, 2, 4 and 6
wt. %, in 2,7 wt. % NaBH4 solution with 50 5 mg of Ir(0)
catalyst at 20 ºC. As seen in Fig. 5, the hydrogen generation
Fig. 1. SEM images of Ir(0) catalyst

Vol. 26, No. 23 (2014)
Generation of Hydrogen in the Hydrolysis of NaBH4 Using Ir(0) Catalyst 8183
600
500
400
300
200
100
0
concentration of NaBO2 might be the reason for slow hydrogen
generation for higher NaBH4 concentrations. Interestingly, a
more concentrated NaBH4 solution does not necessarily
produce hydrogen faster16. As can be seen from Figs. 3 and 4
upon reaching a critical point, hydrogen generation decreases
gradually. Fig. 7 shows the influence of NaBH4 on yield of hydro-
gen generation reaction. As seen, while yield is higher at low
NaBH4 concentrations, it decreases gradually after 2.5 wt. %.
Approximately 90-99 % of NaBH4 conversion efficiency
could be achieved concentrations less than optimal concen-
0
200
400
600
800
1000
1200 1400
Time (s)
tration, however it decreases down to 67 % at higher concen-
trations of NaBH4 investigated in this work. However, the
alkalinity and viscosity of the hydrolysis system also increase
with the NaBH4 concentration, which prevents hydrogen
generation and leads to a decrease of the hydrogen generation
rate. Similar case is also observed by Niu et al.25 in optimizing
preparation of carbon supported cobalt catalyst for hydrogen
generation from NaBH4.
0 %
2 %
4 %
6 %
Fig. 5. Effect of NaOH concentration on the hydrogen generation measured
using 0 to 6 % NaOH and 0.27 g NaBH4 at 20 ºC, using 50 5 mg
of Ir(0) catalyst
rate is decreased with increasing NaOH concentration in NaBH4
solution with Ir(0) catalyst. Fig. 6 shows yield of hydrogen
generation volume versus NaOH concentration, in which
hydrogen generation yield decreased with increasing NaOH
concentration. Depending upon the results, it is decided to
perform all hydrogen generation tests with addition of 1 wt. %
NaOH to solutions. Therefore, experiments are performed by
placing the same amounts of the catalyst (0.050 g), NaOH
and NaBH4 dissolved in to the reactor to initiate the reaction.
The effect of NaOH concentration on the NaBH4 hydrolysis
rate is similar to Ru-RuO2/C catalysis but completely different
from Ni(0) catalyzed reaction5,15. As known, the presence of
NaOH concentration in NaBH4 solution is reduced activity of
water and viscosity of solution, thus decreasing the available
free water to react with NaBH4.
120
100
80
60
40
Unstirred
Stirred
20
0
100
90
80
70
60
50
40
30
20
10
0
0
1
2
NaBH4 (%)
3
4
Fig. 7. Effect of stirring and unstirring on yield in 20 min reaction at 20 ºC
Effect of stirring: The volumes of the generated hydrogen
gas under unstirred and stirred conditions in the presence of
Ir(0) catalysts are shown in Figs. 3 and 4 as a comparison.
Yields of hydrogen generation both in stirred and unstirred
conditions are given in Fig. 7. It is apparent from figures that
stirring has a significant effect on the overall reaction rate and
yield and thus supports the evidence of an external mass
transfer resistance for the catalytic hydrolysis of sodium
borohydride. Thus, the observed rate difference between
unstirred and stirred conditions could be attributed to the effect
of external mass transfer resistance only. Similar values were
obtained by Rakap et al.26 on study of hydrogen generation
from hydrolysis of ammonia-borane using Pd-PVB-TiO2 and
Co-Ni-P/Pd-TiO2 under stirred conditions.
0
1
2
3
4
5
6
7
NaOH concentration (wt. %)
Fig. 6. Effect of NaOH concentration on yield in 20 min reaction at 20 ºC
Effect of NaBH4 concentration: Figs. 3 and 4 shows the
influence of NaBH4 concentration on the hydrogen generation
rate for both stirred and unstirred conditions.Amount of 0.050
g of Ir(0) catalyst at 20 ºC and 1 % wt NaOH was added to the
solution. The hydrogen generation volumes with different
NaBH4 concentration, i.e. 0.5, 1.0, 1.5, 2, 2.5, 3 and 3.5 wt. %,
in 1 wt. % NaOH solution with 50 5 mg of Ir (0) catalyst at
20 ºC are illustrated. It is found that the hydrogen generation
rate increases with increasing NaBH4 after reaching optimal
concentration, decreases with increasing concentration of
NaBH4. Optimal concentration of NaBH4 is found near 2.5
wt. % As reaction 1. proceeds, NaBO2 eventually exceeds its
solubility limit and precipitates out of solution24. Increasing
Conclusion
In summary, Ir(0) catalyst stabilized by polyol method
was employed for hydrogen generation tests both in stirred
and unstirred conditions for the first time. Influence of NaOH
concentration was illustrated. Stirring has positive effect on
hydrogen generation and yield. Optimal concentration of NaOH
is 1 wt. % and optimal concentration of NaBH4 concentration
was found to be 1 to 2.5 wt. %.

8184 Aksu
Asian J. Chem.
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