Hydrogen generation from hydrolysis of sodium borohydride using Ru(0) nanoclusters as catalyst

Article abstract of DOI:10.1016/j.jallcom.2004.10.084

Sodium borohydride is stable in aqueous alkaline solution, however, it hydrolyses in water to hydrogen gas in the presence of suitable catalyst. By this way hydrogen can be generated safely for the fuel cells. Generating H ₂ catalytically from

Full text of DOI:10.1016/j.jallcom.2004.10.084

Journal of Alloys and Compounds 404–406 (2005) 728–731
Hydrogen generation from hydrolysis of sodium borohydride
using Ru(0) nanoclusters as catalyst
∗
¨
S. Ozkar , M. Zahmakıran
Department of Chemistry, Middle East Technical University, 06531 Ankara, Turkey
Received 17 June 2004; received in revised form 14 October 2004; accepted 18 October 2004
Available online 14 July 2005
Abstract
Sodium borohydride is stable in aqueous alkaline solution, however, it hydrolyses in water to hydrogen gas in the presence of suitable
catalyst. By this way hydrogen can be generated safely for the fuel cells. Generating H2 catalytically from NaBH4 solutions has many
advantages: NaBH4 solutions are nonflammable, reaction products are environmentally benign, rate of H2 generation is easily controlled, the
reaction product NaBO2 can be recycled, H2 can be generated even at low temperatures. All of the catalysts that has been used in hydrolysis
of sodium borohydride are bulk metals and they act as heterogeneous catalysts. The limited surface area of the heterogeneous catalysts causes
lower catalytic activity as the activity of catalyst is directly related to its surface area. Thus, the use of metal nanoparticles with large surface
area provides potential route to increase the catalytic activity. Here, we report, for the first time, the use of ruthenium(0) nanoclusters as
catalyst in the hydrolysis of sodium borohydride liberating hydrogen gas. The ruthenium nanoparticles are generated from the reduction of
ruthenium(III) chloride by sodium borohydride in water and stabilized by specific ligand. The ruthenium(0) nanoclusters are found to be
highly active catalyst for the hydrolysis of sodium borohydride.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Sodium borohydride; Hydrolysis; Catalyst; Ruthenium nanoparticles
1. Introduction
gas. Among these chemical hydrides, sodium borohydride
(NaBH4) provides safe and practical mean of producing
hydrogen. Hydrolysis of NaBH4 produces hydrogen gas and
water-soluble sodium metaborate, NaBO2, in the presence
ofsuitable catalyst [3]. By this way hydrogen can be gener-
ated safely for the fuel cells.
In modern society, hydrogen is an important chemical
material which is utilized in large amounts in synthetic chem-
ical industries. In near future hydrogen is expected to become
an energy vector. Hydrogen is produced from various energy
sources, stored, transported and used in industries. Hence,
fuel cells using hydrogen as fuel have been under develop-
ment. Chemical energy released upon reaction of hydrogen
and oxygen is directly converted into electric energy in fuel
cell [1].
Ru(0)nanoclusters
NaBH4 + 2H2O −−−−−−−−−→ NaBO2 + 4H2,
ꢀH = 300 kJ
(1)
As new fueling concept, it can be suggested that the chem-
ical hydrides (NaBH4, KBH4, LiH, NaH, etc.) act as new
fuel source supplying hydrogen at normal temperature [2].
Chemical hydrides are very reactive toward hydrolysis in
water, which results in releasing large amount of hydrogen
Generating H2 catalytically from NaBH4 solutions has
many advantages: NaBH4 solutions are nonflammable, reac-
tion products are environmentally benign, rate of H2 gener-
ation is easily controlled, the reaction product NaBO2 can
be recycled, H2 can be generated even at low temperatures.
Such hydrolysis of sodium borohydride can be accelerated
by catalysts [4], by acid [5], or under elevated temperature
[6].
∗
Corresponding author. Tel.: +90 312 210 3212; fax: +90 312 210 12 80.
¨
E-mail address: sozkar@metu.edu.tr (S. Ozkar).
0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2004.10.084

¨
S. Ozkar, M. Zahmakıran / Journal of Alloys and Compounds 404–406 (2005) 728–731
729
All of the catalysts used in hydrolysis of sodium borohy-
dride so far are bulk metals and they act as heterogeneous
catalysts [7]. The limited surface area of the heterogeneous
catalysts causes to lower catalytic activity as the activity of
catalyst is directly related to its surface area. Thus, the use of
metal nanoparticles with large surface area provides potential
route to increase the catalytic activity [8]. Here, we report
for the first time the results of our study on the hydrogen
generation from the catalytic hydrolysis of sodium borohy-
dride using water-dispersible ruthenium(0) nanoclusters as
catalyst.
different sets of experiments were performed: in the first set
of experiments the concentration of NaBH4 was kept constant
at 150 mM, and the Ru(0) nanoclsuters concentration was
varied in the range of 0.2, 0.4, 0.6, 0.8, 1.0 and 1.4 mM. In the
second set of experiments, Ru(0) nanoclusters concentration
was held constant at 0.4 mM while the NaBH4 concentration
was varied to get various NaBH4/Ru(0) ratio in the range of
100, 200, and 500.
Finally, we performed the catalytic hydrolysis of NaBH4
in the presence of Ru(0) nanoclusters at constant NaBH4 and
Ru(0) concentrations at different temperatures 30, 35, 40, and
45 ◦C.
2. Experimental
4. Results and discussion
2.1. Preparation of Ru(0)—acetate stabilized
nanoclusters
Water-dispersible ruthenium(0) nanoclusters were pre-
pared from the reduction of ruthenium(III) chloride by
sodium borohydride in water and stabilized by acetate. These
Ru(0) nanoclusters were used, for the first time, as catalyst
in the hydrolysis of sodium borohydride liberating hydrogen
gas. The ruthenium(0) nanoclusters are found to be highly
active catalyst for the hydrolysis of sodium borohydride as
shown in Fig. 1, which plots the volume of H2 generated ver-
sus time during the catalytic hydrolysis of 150 mM NaBH4
solution in the presence of Ru(0) nanoparticles in different
concentrations at 25 ◦C. It is seen that Ru(0) nanoclusters
have high catalytic activity in the hydrolysis of NaBH4 even
at low concentrations and room temperature. The hydrogen
evolution starts immediately without any induction period
as we used preformed catalyst. The hydrogen evolution rate
remains constant until all the sodium borohydride react. The
hydrogen generation rate was determined from the linear por-
tion of the plot for each experiment with different Ru(0)
concentration. Fig. 3 shows the plot of hydrogen generation
rate versus Ru(0) concentration, both in logarithmic scale.
One obtainsstraight line, the slope of which is found to be
1.073. This indicates that the hydrolysis is first order with
respect to the concentration of Ru(0) nanoclusters catalyst.
Fig. 2 shows the change in concentration of NaBH4 with
time during the catalytic hydrolysis starting with three differ-
ent initial concentrations of sodium borohydride but constant
Ruthenium(III) chloride hydrate, sodium acetate (99%)
and sodium borohydride (98%) from Aldrich were used as
received. Deionized water was distilled by water purification
system. All glass ware and Teflon coated magnetic stir bars
were cleaned with acetone, followed by copious rinsing with
distilled water before drying in an oven.
Ru(0)nanoclusterswerepreparedbyfollowingthegeneral
procedure given in the literature [9]: 1.0 mL of 1.0 M aque-
ous sodium acetate solution was added to 10 mL of 2.0 mM
aqueous RuCl3 solution and 1.0 mL of 0.112 M of aqueous
NaBH4 solution was added to this solution dropwise under
vigorous stirring. Molar ratio of NaBH4 to RuCl3 greater than
5 was used to ensure complete reduction of Ru to its zero oxi-
dation state. Ru(0) nanoclusters obtained was dark brown in
color and very stable. No precipitation was observed even
after few days of storage.
2.2. Catalytic activity of Ru(0) nanoclusters in the
hydrolysis of sodium borohydride
The catalytic activity of Ru(0) nanoclusters was deter-
mined by measuring the amount of hydrogen generated from
the hydrolysis of sodium borohydride. In all experiments total
volume of solution was held constant at 50 mL, NaBH4 solu-
tion was thermostated to a preset temperature in the sealed
flask and given amount of catalyst, prepared in aqueous solu-
tion in separate flask, was added to the reaction flask and the
reaction was started while the solution was stirred vigorously.
Graduated glass column filled with water was connected to
the top outlet of the flask as gas burette. The volume of hydro-
gen gas evolved was measured by the displacement of water
level in the column.
3. Kinetic study
In order to establish the rate law for catalytic hydrolysis
of NaBH4 using water-dispersible Ru(0) nanoclusters, two
Fig. 1. The graph of volume of hydrogen (mL) vs. time (s) in different Ru(0)
concentrations in all sets [NaBH4] = 150 mM at 25 ◦C.

¨
730
S. Ozkar, M. Zahmakıran / Journal of Alloys and Compounds 404–406 (2005) 728–731
Table 1
Rate constants for the hydrolysis of sodium borohydride catalyzed by Ru(0)
nanoclusters starting with solution of 150 mM NaBH4 and 0.4 mM Ru(0)
nanoclusters at different temperatures
Temperature (◦C)
Rate constant (turnover frequency)
(mol NaBH4 [mol Ru(0)]−1 s−1
)
30
35
40
45
2.9
3.9
4.8
5.1
Fig. 2. The graph of NaBH4 concentration vs. time in three different
NaBH4/Ru(0) ratios at 25 ◦C ([Ru] = 0,4 mM in all three sets).
Fig. 4. The Arrhenius plot, ln k vs. the reciprocal absolute temperature 1/T,
in the temperature range is 30–45 ◦C.
Fig. 3. The graph of ln rate vs. ln [Ru] at 25 ◦C and 150 mM NaBH4.
Table 2
Turnover frequencies for the hydrolysis of sodium borohydride catalyzed by
Ru(0) nanoclusters starting with solution of 0.4 mM Ru(0) nanoclusters and
various concentration of NaBH4 (different NaBH4 /Ru(0) ratio) at 35 ◦C
Ru(0) concentration at 25 ◦C. It is seen that the concentra-
tion of sodium borohydride decreases linearly as the reaction
proceeds. Furthermore, the lines for all the three experiments
have the same slope, indicating that the catalytic hydrolysis
is zero order in substrate concentration. Thus, the rate law for
the catalytic hydrolysis of sodium borohydride can be given
as
NaBH4/Ru(0)
Turnover frequency (s−1
)
100
200
500
7.9
6.8
6.6
5. Conclusion
−4d[NaBH4]
dt
d[H2]
=
= k[Ru]
(2)
We have shown that water-dispersible ruthenium(0)
nanoparticles can be obtained by using acetate anion stabi-
lizer. These water-dispersible ruthenium(0) nanoclusters are
highly active catalyst in the hydrolysis of sodium borohy-
dride which has been used as an effective source for produc-
ing H2. Using ruthenium(0) nanoclusters catalyst provides
the lowest activation energy ever found for the hydrolysis
of sodium borohydride. Thus, the catalytic hydrolysis of
sodium borohydride can be performed at ambient temper-
ature and hydrogen gas can be generated at an appreciable
rate.
dt
The values of the rate constant k were determined at vari-
ous temperatures and listed in Table 1. Fig. 4 shows the Arrhe-
nius plot, ln k versus the reciprocal absolute temperature
(1/T). The slope of the straight line gives an activation energy
of 28.51 kJ/mol for the Ru(0) nanoclusters catalyzed hydrol-
ysis of sodium borohydride. This value compares favorably
with the activation energy found by Amendola [2a] at higher
NaBH4 and NaOH concentrations, and activation energies
found by Kaufman and Sen [10] for the same hydrolysis
but with different bulk metal catalysts; 75 kJ/mole for cobalt,
71 kJ/mol for nickel, and 63 kJ/mol for Raney nickel.
The catalytic efficiency of Ru(0) nanoclusters for the
hydrolysis of sodium borohydride is expressed by the
turnover frequency (N). The turnover frequencies were given
in Table 1 for different temperatures and Table 2 for different
NaBH4/Ru[0] ratios.
Acknowledgment
Partial support of this work by Turkish Academy of Sci-
ences and METU DPT OYP Program is gratefully acknowl-
edged.

¨
S. Ozkar, M. Zahmakıran / Journal of Alloys and Compounds 404–406 (2005) 728–731
731
References
(b) J.H. Kim, H. Lee, S.C. Han, H.S. Kim, M.S. Song, J.Y. Lee, Int. J.
Hydrogen Energy 29 (2004) 263–267;
(c) D. Hua, Y. Hanxi, A. Xinping, C. Chuansain, Int. J. Hydrogen
Energy 28 (2003) 1095–1100;
(d) Y. Kojima, K.I. Suzuki, K. Fukumoto, M. Sasaki, T. Yamamoto,
Y. Kawai, H. Hayashi, Int. J. Hydrogen Energy 27 (2002) 1029–
1034.
[1] M. Sossina Haile, Acta Mater. 51 (2003) 5981–6000.
[2] (a) S.C. Amendola, P. Onnerud, M.T. Kelly, P.J. Petillo, S.L. Sharp-
Goldman, M. Binder, J. Power Source 85 (2000) 186–189;
(b) S.C. Amendola, J.M. Janjua, N.C. Spencer, M.T. Kelly, P.J. Petillo,
S.L. Sharp-Goldman, M. Binder, Int. J. Hydrogen Energy 25 (2000)
969–975;
(c) J.Y. Lee, H.H. Lee, J.H. Lee, D.M. Kim, J.H. Kim, J. Electrochem.
Soc. 149 (5) (2002) A603–A606.
[3] H.I. Schlesinger, H.C. Brown, A.B. Finholt, J.R. Gilbreath, H.R. Hock-
stra, E.K. Hydo, J. Am. Chem. Soc. 75 (1953) 215.
[4] (a) A. Levy, J.B. Brown, C.J. Lyons, Ind. Eng. Chem. 52 (1960) 211;
(b) C.M. Kaufman, B. Sen, J. Chem. Soc. Dalton Trans. (1985) 307;
(c) H.C. Brown, C.A. Brown, J. Am. Chem. Soc. 84 (1962) 1493.
[5] B.D. James, M.G.H. Wallbridge, Prog. Inorg. Chem. 11 (1970) 99–231.
[6] R. Aicllo, J.H. Sharp, M.A. Matthews, Int. J. Hydrogen Energy 24
(1999) 1123–1130.
[8] (a) J.D. Aiken, R.G. Finke, J. Mol. Catal. A: Chem. 145 (1999) 1–44;
(b) A. Roucoux, J. Schulz, H. Patin, Chem Rev. 102 (2002) 3757–3778;
(c) J. Widegren, R.G. Finke, J. Mol. Catal. A: Chem. 198 (2003) 317–
341;
¨
(d) S. Ozkar, R.G. Finke, J. Am. Chem. Soc. 124 (2002) 5796–5810.
[9] J. Yang, C. Deivaraj, H.P. Too, J.Y. Lee Langmuir 20 (2005) 4241–
4245.
[10] C.M. Kaufman, B. Sen, J. Chem. Soc., Dalton Trans. (1985) 307.
[7] (a) I.I. Korobov, N.G. Mozgina, L.N. Blinova, Kinet. Catal. 48 (3)
(1995) 380–384;