Nickel- and cobalt-based catalysts for hydrogen generation by hydrolysis of borohydride

Article abstract of DOI:10.1016/j.jallcom.2005.08.019

Borohydrides, a group of compounds with large hydrogen contents, can generate hydrogen readily by their hydrolysis reaction. In this study catalysts based on nickel and cobalt were developed to accelerate the hydrolysis reaction for hydrogen generation. Catalysts in four forms were tested for each metal: fine metal powder, metal salt, metal boride and Raney metal. It was found that the hydrolysis reaction was primary a zero-order reaction under the catalysis of these catalysts. Although cobalt showed higher catalytic activity than nickel in most cases, Raney Ni exhibited almost the same activity as the Raney Co. Among four chemical forms for nickel, Raney Ni demonstrated the best performance. Moreover, Raney catalysts made from alloys of nickel and cobalt showed even higher activity. The reaction on Raney metals was also characterized by a slowdown in hydrogen generation rate at low borohydride concentrations because of a mass transfer limitation.

Full text of DOI:10.1016/j.jallcom.2005.08.019

Journal of Alloys and Compounds 415 (2006) 288–293
Nickel- and cobalt-based catalysts for hydrogen generation
by hydrolysis of borohydride
a,∗
b
a
Bin Hong Liu , Zhou Peng Li , S. Suda
a
Department of Environment and Chemical Engineering, Kogakuin University, 2665-1 Nakano-cho, Hachioji,
Tokyo 192-0015, Japan
b
Department of Chemical and Biochemical Engineering, Zhejiang University,
Hangzhou 310027, PR China
Received 21 July 2005; received in revised form 9 August 2005; accepted 10 August 2005
Available online 19 September 2005
Abstract
Borohydrides, a group of compounds with large hydrogen contents, can generate hydrogen readily by their hydrolysis reaction. In this study
catalysts based on nickel and cobalt were developed to accelerate the hydrolysis reaction for hydrogen generation. Catalysts in four forms were
tested for each metal: fine metal powder, metal salt, metal boride and Raney metal. It was found that the hydrolysis reaction was primary a zero-order
reaction under the catalysis of these catalysts. Although cobalt showed higher catalytic activity than nickel in most cases, Raney Ni exhibited
almost the same activity as the Raney Co. Among four chemical forms for nickel, Raney Ni demonstrated the best performance. Moreover, Raney
catalysts made from alloys of nickel and cobalt showed even higher activity. The reaction on Raney metals was also characterized by a slowdown
in hydrogen generation rate at low borohydride concentrations because of a mass transfer limitation.
©
2005 Elsevier B.V. All rights reserved.
Keywords: Hydrogen; Borohydride; Hydrolysis; Catalyst; Raney metals
1
. Introduction
of fuel cells as other hydrogen storage methods are too bulky or
inconvenient [7,8].
With growing concerns for energy and environment prob-
Sodium borohydridecangenerate a large amountofhydrogen
by the catalyzed hydrolysis reaction as follows:
lems, fuel cells are attracting more and more attentions as clean
and efficient power generation devices. However, their commer-
cialization is still lagging behind, mainly due to their high cost
and a lack of hydrogen infrastructure. As the current technolo-
gies for hydrogen generation by reforming hydrocarbons are
still not satisfactory considering the response time and the pres-
ence of carbon monoxide in hydrogen, hydrogen storage and
supply is now more preferred by the industries, especially for
mobile applications. Hydrogen storages in high-pressure ves-
sels and in metal hydrides are being considered as two viable
ways. However, their storage densities are not sufficient for the
requirements. On this background, chemical hydrides such as
sodium borohydride emerged as convenient hydrogen sources
for on-board hydrogen generation [7–9]. Hydrogen generation
by borohydride is especially suitable for portable applications
NaBH4 + 2H2O = NaBO2 + 4H2
(1)
About 10.8 wt.% hydrogen can be obtained from the stoichio-
metric ratio of the reactants. In the early 50 s, Schlesinger et
al. [1] made a detailed study on the hydrolysis reaction of
sodium borohydride for hydrogen generation. They found that
the hydrolysis reaction proceeded at a very slow rate if the solu-
tion was unbuffered or in the absence of catalysts. However, the
reaction can be significantly accelerated by addition of acids
or some catalysts like metals and their salts. Many inorganic
and organic acids were found to be effective in accelerating
the hydrolysis reaction, however the reaction usually became
uncontrollable. On the other side, solid-state catalysts such as
transition metals and their salts were found to be effective and
hydrogen generation rate was easy to control. Also these cat-
alysts would be reusable. Thus, they were considered more
advantageous over acids in a practical sense [6]. Among them
∗
Corresponding author.
E-mail address: bq97002@ns.kogakuin.ac.jp (B.H. Liu).
0
925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2005.08.019

B.H. Liu et al. / Journal of Alloys and Compounds 415 (2006) 288–293
289
cobalt and nickel borides were the most investigated catalysts
1–6]. Salts like CoCl2 was reduced spontaneously by borohy-
cific surface areas of powders were measured on Shimadzu
FLOWSORB II 2300.
[
dride to form metal borides like Co2B that showed excellent
catalytic activity on the hydrolysis reaction. Recently, Amen-
dola et al. [7] reported that they employed Ru supported in ion
exchange resin beads as the catalyst for hydrogen generation of
sodium borohydride. Also Pt supported on LiCoO2 was devel-
oped [9]. In a previous paper we studied the catalytic properties
of an intermetallic compound Mg2Ni and its hydride [8].
In the present paper, we examined the properties and per-
formance of cobalt- and nickel-based catalysts in four different
forms in an effort to develop inexpensive but effective catalysts
for hydrogen generation from borohydride. Factors influencing
hydrogen generation rate such as fuel concentrations are also
surveyed.
3. Results and discussion
3.1. Ni and Co powder
Metal borides prepared by reducing metal salts with borohy-
dride solutions are the most studied catalysts for the borohydride
hydrolysis [1–6]. However, few have reported the catalytic prop-
erties of these metals prepared by other methods. In this study,
we first examined the catalytic properties of commercially avail-
able fine Ni and Co powders. Fig. 1 shows the relations of
generated hydrogen volume with time at different temperatures.
It clearly shows that the reaction was a zero-order reaction in
which the hydrogen generation rate was independent of the boro-
hydride concentration in the solution. The result of zero-order
kinetics is similar as the observations on metal borides [5,6].
By comparing the powder size and specific surface area of Ni
and Co power, it seems that Co showed higher catalytic activity
than Ni. According to the calculated rate constants at different
2
. Experimental details
2
.1. Preparation of catalysts
Commercially available fine nickel powder Inco type Ni210
(
0.5–1 ␮m) from Inco Ltd. and fine Co powder (1–2 ␮m) from
Soekawa Rikagaku Co., Japan were examined as the catalysts for
borohydride hydrolysis. The catalytic properties of metal salts
were measured by using hydrated metal salts like CoCl2·6H2O,
NiCl2·6H2O and NiF2·4H2O in solid states. Also metal borides
were prepared by adding alkaline borohydride solutions (boro-
hydride in 10 wt.% NaOH) into the solutions of CoCl2 and
NiCl2. The formed black precipitates were separated from the
solution, washed and dried in a vacuum desiccator at room tem-
perature. Raney Ni or Raney alloys were prepared as follows: the
precursor alloys Ni1−xCoxAl3 (x = 0, 0.05, 0.1, 0.25.0.5, 0.75,
1
.0) were prepared by arc-melting the pure constituent metals in
a water-cooled copper hearth in an argon atmosphere. The alloy
ingots were remelted three to four times to ensure homogeneous
compositions. The alloys were then pulverized to a size of less
than 75 ␮m and reacted with a 20 wt.% NaOH solution to leach
◦
the aluminum out at about 70 C for several hours and then kept
in the alkaline solution for use.
2
.2. Kinetics measurement of hydrogen generation
Sodium borohydride with a purity of 97% was from Rohm
and Haas Company, USA. No further purification was under-
taken. Borohydride solutions were prepared by dissolving
sodium borohydride into a 10 wt.% NaOH solution. The hydrol-
ysis reaction experiments were carried out in a three-neck flask
(
200 ml). The central neck was connected to a funnel (100 ml)
with a borohydride solution. Catalysts were initially put at the
bottom of the flask. The hydrolysis reaction was initiated by
opening the valve of the funnel and introducing the solution into
the flask. The volume of evolved hydrogen gas was measured by
a water displacement method or a gas flow meter and was trans-
formed to the value in standard temperature and pressure (STP).
The flask was immersed in a water bath to stabilize the temper-
ature. No stirring was employed in the flask. X-ray diffraction
Fig. 1. (a) Hydrogen generation kinetics at various temperatures under the catal-
ysis of Inco Ni210 powder (Ni: 0.5 g, solution: 0.2 g NaBH4 in 20 ml 10 wt.%
NaOH). (b)Relationofhydrogen generation kineticswiththe temperature forthe
Co powder catalyst (Co: 0.5 g, solution: 0.2 gNaBH4 in 20 ml 20 wt.% NaOH).
(XRD) analyses were carried out on a Rigaku RINT-1200. Spe-

2
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B.H. Liu et al. / Journal of Alloys and Compounds 415 (2006) 288–293
Table 1
Properties and performance of the catalysts
2
Catalyst
Powder size (␮m)
Specific surface area (m /g)
Hydrogen generation rate
Activation energy (kJ/mol)
(
ml/min/g catalyst)
Ni powder
Co powder
NiCl2·6H2O
NiF2·4H2O
CoCl2·6H2O
Ni2B
Co2B
Raney Ni
Raney Co
Raney Ni50Co50
0.5–1
1–2
Granule
Granule
Granule
1.98
1.15
19.5 (293 K)
126.2 (293 K)
9.8 (293 K)
34.7 (323 K)
570.0 (283 K)
18.3 (293 K)
468.3 (293 K)
228.5 (293 K)
267.5 (293 K)
648.2 (293 K)
−62.7
−41.9
−50.7
−53.7
−52.5
temperatures, the activation energies of Ni and Co are shown
in Table 1.
the formed metal borides. The overall hydrogen generation in
the presence of metal salts is simultaneously metal and acid cat-
alyzed.
3
.2. Cobalt and nickel salts and their reduction products
As described in Ref. [4], the reduction reaction of metal salts
proceeds as follows:
Cobalt salt CoCl2 and its reduction product Co2B from
CoCl2 + 2NaBH4 + 3H2O = 6.25H2 + 0.5Co2B
borohydride solution are well-known catalysts for borohydride
hydrolysis since Schlesinger et al. reported their excellent cat-
alytic activities [1]. In this work, we also examined the catalytic
properties of cobalt and nickel salts as well as their reduction
products from borohydride solution. Granules of the hydrated
salts were used in the present study. Fig. 2 shows the hydro-
gen generation rates when using CoCl2·6H2O, NiCl2·6H2O and
NiF2·4H2O. It was observed that the cobalt salt-induced violent
reaction and fast hydrogen generation even at lower tempera-
tures while the nickel salts only led to slow hydrogen release. In
all cases, black precipitates formed spontaneously when the salts
met with the borohydride solution. It was reported that the black
precipitates are metal–boron compounds. In the case of cobalt
and nickel, Co2B and Ni2B would be formed [1,3–5]. Accord-
ing to Kaufman and Sen [6], metal salt-catalyzed borohydride
hydrolysis is a successive reaction of a reduction of metal salts
by borohydride and then the hydrolysis under the catalysis of
+
2NaCl + 1.5HBO2
(2)
According to (2), 0.5 g CoCl2·6H2O containing 0.123 g Co will
need 0.159 g NaBH4 to finish the above reaction. As the solution
contained 0.2 g NaBH4 in this experiment, NaBH4 was in excess
for reaction (2). The mole ratio for NaBH4 and H2 in (2) will
be 1:3.1, rather than 1:4. However, we did not observe such
a large decrease in hydrogen volume. The released hydrogen
was still near the stoichiometric value in (1). The same result
was observed for the nickel salts. It suggests that the actual
reaction mechanism may be other than (2). More accurate and
detailed study is necessary to elucidate the reaction mechanism
considering the complicated nature of the system in which many
reactions take place in parallel.
Fig. 3 demonstrates the catalytic properties of metal borides.
Similar as the results shown in Fig. 2, the cobalt boride shows
Fig. 2. Catalytic performance of some cobalt and nickel salts (hydrated salts:
Fig. 3. Comparison of catalytic properties between cobalt and nickel boride
(cobalt or nickel boride: 0.5 g, solution: 0.2 g NaBH4 in 20 ml 10 wt.% NaOH).
0
.5 g, solution: 0.2 g NaBH4 in 20 ml 10 wt.% NaOH).

B.H. Liu et al. / Journal of Alloys and Compounds 415 (2006) 288–293
291
Fig. 4. XRD patterns for Ni powder and Raney Ni.
much higher catalytic activity than the nickel borides. It was
reported that the catalytic activity of nickel borides was only
a little inferior to that of cobalt boride [1,5,6]. The discrepancy
between our results and the others may originate from the prepa-
ration conditions of the metal borides. In our experiment, metal
borides were prepared by reacting salt solution with strong alka-
line solution of borohydrde (NaBH4 in 10 wt.% NaOH), while
other authors used solid NaBH4 powder [1] or NaBH4 dissolved
in water [6]. It was reported by Paul et al. [3] that the catalytic
activity of nickel boride was strongly dependent on the pH dur-
ing its formation, and nickel boride prepared in alkaline solution
showed much lower activity than the ones prepared in acidic or
neutral conditions. Therefore, it needs further investigation to
find out the differences in composition or surface structure for
nickel borides prepared in different pH conditions and to relate
them with catalytic activities.
Fig. 5. Hydrogen generation kinetics under the catalysis of Raney Ni and Raney
Co (Raney Ni or Co: 0.5 g, solution: 1 g NaBH4 in 100 ml 10 wt.% NaOH).
3.3.2. Raney Ni–Co alloys
Although Raney Ni and Raney Co already offered very high
catalytic activities, their alloys exhibited even better perfor-
mance. Fig. 6 shows the XRD patterns of the precursor alloys
Ni1−xCoxAl3. The hydrogen generation kinetics of the Raney
alloys is illustrated in Fig. 7. It can be seen that the alloys led
to faster hydrogen generation rates than the pure metals. It was
also found by Doren [10] that addition of some Fe in Raney Ni
improved the hydrolysis kinetics. This kind of phenomenon is
very interesting from a practical as well as academic point of
view. Further investigation is being carried out to understand
the mechanism.
3.4. Effects of NaBH4 and NaOH concentrations
3
3
.3. Raney catalysts
Hydrolysis kinetics was found to be not only dependent on
catalyst performance, but also influenced by other factors. Fig. 8
showstheeffectsofinitialNaBH4 concentrationonthehydrogen
generation kinetics under the catalysis of fine Ni powder Ni210.
Although the reaction showed the zero-order kinetics, change
of the initial concentration of NaBH4 was still found to slightly
affect the hydrogen generation rate. As the changes in generation
rate happened from the initial stage of the reaction, it may be
.3.1. Raney Ni and Co
Althoughsomepapers[1,6]mentionedthatRaneyNiisactive
for the hydrolysis reaction, there was no detailed report on their
catalytic properties. In this work, we measured the catalytic
properties of Raney metals based on cobalt and nickel. Fig. 4
shows the XRD patterns of the nickel powder and Raney Ni.
From the broadened peaks of Raney Ni, it can be deduced that
Raney Ni was of extremely fine structure. Fig. 5 shows hydrogen
generation kinetics using Raney Ni and Raney Co as the catalyst.
It is apparent that both Raney Ni and Raney Co have very good
catalytic activity. They showed excellent response with almost
no perceivable time delay. Fig. 5 also reveals that the hydroly-
sis reaction has also zero-order kinetics under the catalysis of
Raney metals. However, it was found that the reaction slowed
down around 90% completion and it took a much longer time
to finally finish the rest reaction. In this respect, Raney metals
showed different performance with the fine metal powders as
the latter offered constant hydrogen generation rates until 100%
completion.
The Raney Ni showed almost the same catalytic activity as the
Raney Co, though in other forms Co had much better catalytic
performance than Ni.
Fig. 6. XRD patterns of Ni1−xCoxAl3 (x = 0, 0.25, 0.5, 0.75, 1.0) alloys.

2
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B.H. Liu et al. / Journal of Alloys and Compounds 415 (2006) 288–293
Fig. 9. Hydrogen generation kinetics influenced by the initial borohydride con-
centration using Raney Ni–Co catalyst (Raney Ni95Co5: 0.5 g, temperature:
Fig. 7. Hydrogen generation kinetics under the catalysis of Raney Ni–Co alloys
(
Raney Ni100−xCox: 0.5 g, solution: 1 g NaBH4 in 100 ml 10 wt.% NaOH, tem-
293 K).
perature: 293 K).
duetosomeinfluenceonthecatalyst. Borohydrideconcentration
showed different effects on Raney metals. As demonstrated in
Fig. 9, hydrogen generation rate decreased when borohydride
concentration was lower than 0.2 wt.% NaBH4 while it was not
influenced at higher concentrations. As the reaction kinetics was
greatly enhanced on Raney metals, mass transfer became the
limiting factor at low borohydride concentrations. The very fine
surface structure may also increase the difficulty of mass transfer
as we found that at extremely low borohydride concentrations
the hydrogen generation rate was even lower than that on the Ni
powder.
The effect of NaOH concentration on reaction kinetics is
shown in Fig. 10. When NaOH concentration was less than
20 wt.%, there was no influence on hydrogen generation rate.
However, when NaOH concentration was increased to 30 wt.%,
hydrogen generation rate was affected. At initial stage, the
hydrogen generation rate was kept but began to divert from the
straight line. The decrease of hydrogen generation rate is sup-
posed to be caused by a decrease in water activity.
Fig. 10. Influence of NaOH concentration on hydrogen generation kinetics (Ni
powder catalyst: 0.5 g, solution: 0.2 g NaBH4 in 20 ml NaOH solution, temper-
ature: 293 K).
The experimental results summarized in Table 1 reveal that
it is possible for us to use non-precious metals like nickel and
cobalt to achieve very high hydrogen generation rate. Hydrogen
generation from borohydride is proven to be a practically viable
way of hydrogen supply for fuel cells. Among different forms for
the metals, Raney catalysts especially Ni with small additions
of Co exhibited high catalytic activity and thus may be very
promising as low cost catalysts for hydrogen generation from
borohydride.
4. Conclusions
Catalysts for the borohydride hydrolysis have been developed
based on transition metal cobalt and nickel. Four preparation
methods were employed for each metal. Commercially available
fine powders of cobalt and nickel showed considerable catalytic
activities and the hydrolysis reaction was zero-order kinetics.
Both cobalt salt CoCl2 and its reduction product from borohy-
dride solution-induced violent reaction. However, nickel salts
and their reduction products showed much slower kinetics. On
Fig. 8. Influence of initial borohydride concentration on hydrogen generation
kinetics using the Ni powder catalyst (Ni powder: 0.5 g, temperature: 293 K).

B.H. Liu et al. / Journal of Alloys and Compounds 415 (2006) 288–293
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the other hand, Raney Ni exhibited excellent catalytic activi-
[
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Moreover, Raney alloys of cobalt and nickel gave even higher
generation rates. The low cost and high performance made them
promising candidates as the catalysts for hydrogen generation
of borohydride.
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