Full text of DOI:10.1007/s003390100785

Appl. Phys. A 72, 209–212 (2001) / Digital Object Identifier (DOI) 10.1007/s003390100785
Applied Physics A
Materials
Science & Processing
Catalytic generation of hydrogen by applying
fluorinated-metal hydrides as catalysts
∗
S. Suda , Y.-M. Sun, B.-H. Liu, Y. Zhou, S. Morimitsu, K. Arai, N. Tsukamoto, M. Uchida, Y. Candra, Z.-P. Li
Chemical Energy Laboratory, Department of Environmental and Chemical Engineering, Kogakuin University,
2
665-1, Nakano-machi, Hachioji-shi, Tokyo 192-0015, Japan
Received: 13 November 2000/Accepted: 14 November 2000/Published online: 9 February 2001 –  Springer-Verlag 2001
Abstract. Metal–hydrogen complexes such as NaAlH4,
KBH4, and NaBH4 are known as high H-content materials.
The highly reactive natures of these materials against moist
air and water can be easily stabilized in aqueous KOH and
NaOH solutions. Accordingly, it is required to develop cat-
alysts suitable for generating hydrogen from the stabilized
metal–hydrogen complexes in alkaline solutions.
FC cars with regard to H-generation rates under severe oper-
ating conditions, energy density per weight and volume, and
other technical problems as well as their cost-effectiveness.
On the other hand, sodium aluminum hydrides have
been developed as H-storage material by applying their gas–
solid decomposition reactions. Ti-catalyzed NaAlH4 and
Na3AlH6 have been studied by Bogdanovic [4] and the cyclic
◦
This work is aimed at developing catalysts that can gen-
erate hydrogen from such solutions with considerably high
kinetics under moderate temperature and pressure conditions.
We have found that Mg2Ni, a typical high-temperature
hydriding alloy, exhibits excellent functions as a catalyst
H-capacity up to 200 C is reported as 4.2 wt. %. Ti-doped
and Ti/Zr-doped NaAlH4 have been extensively studied by
Jensen and others [5, 6]. H-capacity is reported as 4.0 wt. %
◦
by an advanced Ti-doped NaAlH4 at 100 C, and 4.5 wt. % at
◦
100 C by Ti/Zr-doped NaAlH4. The theoretical H-capacity
−
for the hydrolysis of BH -ion-containing solutions. The
obtained by 3-stepwise reactions is 7.4 wt. % under wide tem-
4
◦
◦
fluorination-treatment (F-treatment) effects on granular par-
ticles of Mg2Ni and Mg2NiH4 are reported in this paper.
perature ranges between 50 C and 250 C. A series of recent
research activities performed by Jensen and others was re-
ported elsewhere [7]. Reproduction, namely, recombination
of NaAlH4 reactions from “used fuel” after decomposition
to generate hydrogen has been developed by Gross and
others [8]. Cyclic durability, capacity, and kinetic should be
icluded in the future studies for engineering applications of
this material as suggested by Sandrock and others [9].
An alkaline metal–hydrogencomplex compound, NaBH4,
has been studied as a high H-storage material by many re-
searchers [10–14]. In [15–19], the engineering feasibility of
NaBH4 as the H-generation resource has been suggested.
A US patent was published in 1975 for recycling “used fuel”
as borates to NaBH4 [20].
Metal hydrides that contain more than 3 wt. % have been
targeted in the WE-NET Project of MITI, Japan [1]. An in-
ternational cooperative project under IEA Task-12 has been
conducted to develop hydrogen storage materials that store
more than 5 wt. % [2]. An academic group organized by about
5
0 universities and research institutes has been conducting
a project named “New Protium Function” for developing high
H-capacity metal hydrides since 1998 for four years, funded
by the Ministry of Education, Science, Sports and Culture [3].
Despite countless efforts in the past aimed at developing high
H-capacity materials, the level targeted earlier has not been
reached.
The basic reaction of NaBH4 is listed as NaBH4 +2H2O
→
4H2 +NaBO2 as can be seen in many chemical hand-
books and textbooks in which H-generation is accounted as
0.8 mass %. However, it does not happen in practice from
1
the experimental fact that the concentration of NaBH4 given
as the stoichiometric relation, exceeds the solubility limit in
an aqueous solution.
These materials have been attracting strong interest for
use in the fuelling systems for fuel-cell (FC) cars such as
liquid hydrogen, high-pressure hydrogen, metal hydrides,
methanol, gasoline, and carbons. However, the hydrogen-
storage (H-storage) system applicable to FC cars has not been
established. Moreover, H-storage systems as have been de-
veloped, do not necessarily fulfil the practical requirements of
In alkaline-stabilized solutions, materials such as KBH4
−
and NaBH4 exist as the form of BH -complex ions. It is not
4
reasonable to compute the H-content of this material simply
as 10.8 mass %, which seems to be extreme high compared to
other conventional H-containing materials. It is necessary to
take account of the solubility limit, density, and viscosity as
well as the formation of crystalline materials.
∗
Corresponding author. (E-mail: bt73093@ns.kogakuin.ac.jp)

2
10
The present work is aimed at developing a new H-
Table 1. Fluorinated metals and Mg2Ni used for catalytic hydrolysis
storage system based on the metal–hydrogen complex ions
such as BH4 in an aqueous KOH or NaOH solution. It is
−
Fluorinated
catalyst
Specific surface area
Weight
(g)
w
Surface area
2
2
(m /g)
a
(m )
a×w
our final goal to establish an H-supply system based on
the oxidation/reduction process by using metal oxides such
as KBO2 (potassium metaborate), NaBO2 (sodium metabo-
rate), and NaB4O7 (sodium tetraborate), which can be ob-
tained easily as natural resouces in the form of crystalline
materials.
Ni
Co
Cu
Fe
Ru
Zn
Mn
Ti
Mg
2.65
4.22
2.45
0.96
14.57
1.14
1.03
0.19
0.46
6.50
0.72
0.45
0.78
2.00
0.13
1.68
1.85
10.05
4.15
0.29
1.91
1.91
1.91
1.91
1.91
1.91
1.91
1.91
1.91
1.91
In this paper, the catalytic function of Mg2Ni treated by
an F-treatment and the reaction kinetics during hydrogen gen-
−
eration from BH4 complex ion-containing KOH and NaOH
solutions are reported.
Mg2Ni
1
Experimental results
−
A series of metals, Mg, Mg2Ni, MgH2, and Mg2NiH4
used as catalysts were fluorinated by an F-treatment pro-
cedure. F-solution was prepared by mixing 0.6 ml of HF
and 6 g of KF in 1 l of water. F-treatment was performed
for 5 g of each of the metals and Mg2Ni in 200 ml of
the solubility limit of BH -complex ion solution is illus-
4
trated as a function of its concentration under several alkaline
concentration conditions. The samples used as catalysts are
listed in Table 1. All samples used in this experiment were
2
weighted to have the same surface area (= 1.91 m /g) in
HF/KF solution by agitating at 500 rpm for 30 min at
order to evaluate the hydrolysis kinetics under the same con-
ditions. H-generated by the catalytic hydrolysis was measured
by a P–v–T method at room temperature. Reaction kinetics
during H-generation was determined under isothermal con-
ditions by the amount of H-generated as a function of time
elapsed.
◦
3
0 C. The samples after F-treatment were washed by wa-
ter several times and were dried under evacuated con-
dition after removing water by a centrifuge [21]. The
F-treatment is applied in order to remove oxides, to form
fluoride, and to create hydride layers at the extreme sur-
face of granular particles. The F-treatment reported by
Sun [22] is found to be (a few tens of times) more effect-
ive for generating a larger specific surface than the untreated
particle.
The experimental results on the untreated and F-treated
Mg2Ni are shown in Fig. 2. The kinetics of the hydrogenated
Mg2Ni after F-treatment (F-treated Mg2NiH4) is shown in
Fig. 3. Catalytic hydrolysis by several metals is illustrated
in Fig. 4 for comparison. F-treated Mg2Ni was hydrogenated
in a conventional Sievert apparatus to prepare the F-treated
Mg2NiH4. From an XRD analysis, Mg2NiH4 peaks were con-
firmed with peaks of MgF2.
−
The metal–hydrogen complex ion, BH , was prepared
4
as 10 wt. % KOH and NaOH solutions, individually. The
concentration of those complex ions was varied up to the sol-
◦
ubility limits in those alkaline solutions at 25 C. In Fig. 1,
−
Fig. 1. Solubility of BH4 in NaOH solution
Fig. 2. H-generated by F-treated- and untreated Mg2Ni

2
11
hydride phase is formed at the extreme surface of particles
during F-treatment [24]. The hydride phase in Mg2Ni, differ-
ently from other metal hydrides, forms hydride phase gradu-
ally from the surface towards the center of the particle [25].
The depth of hydride phase is dependent on the particle size
as well as on the hydrogen pressure and time elapsed during
hydrogenation process.
The fluoride layer is known as MgF2 both in pure-Mg and
Mg2Ni particle surfaces. These fluoride layers were reported
to improve the hydriding kinetics considerably under lower
temperature and pressure conditions [24, 26–28]. However,
pure-Mg did not exhibit a trace of catalytic function in this
experiment even after F-treatment as can be seen in Fig. 5.
The hydride layer formed during F-treatment at the ex-
treme surface of Mg2Ni was found to work as the active site
for hydrolysis. The Mg2Ni particle at the extreme surface is
estimated to disproportionate to xMgF2 and Mg2−xNiH4 dur-
−
ing F-treatment and to provide NiH -enriched, S. Suda phase
4
−
combining with Mg non-stoichiometrically. The NiH -en-
4
riched phase should act as the active catalytic site for hydrol-
ysis judging from those experimental data. Hydrogen existing
Fig. 3. H-generated by hydrogenated Mg2Ni after F-treatment (F-Mg2NiH4)
−
as the negative ion in NiH4 does not participate in the hy-
−
drolysis. It is the authors’ understanding that the NiH acts as
4
catalyst but not as H-donor in hydrolysis.
Noreus and others have extensively studied bonding
−
state and strength of NiH4 in Mg2NiH4 [29–34]. The high
temperature-level requirement for the releasing of hydrogen
from Mg2NiH4 is explained well by the tight bonding be-
tween Ni and H.
Meanwhile, hydrogenation of F-treated Mg2Ni did not
exhibit noticeable catalytic functions when compared with
the F-treated/unhydrided Mg2Ni. It is explained by the
stoichiometric disproportionation of Mg2−xNi to Mg2NiH4
and Nio as illustrated in the following equation: 2Mg2−xNi
o
o
+
2(2− x)H2 → (2− x)Mg2NiH4 + xNi . Metallic Ni is es-
timated to exhibit catalytic functions that are comparable with
5
00
00
4
Fig. 4. Rates of H-generation by various F-treated metals and Mg2Ni
300
2
Discussions
200
Catalytic effects of F-treated Mg2Ni were found apparently
more effective than those of untreated Mg2Ni. Between
F-treated Mg2Ni and F-treated Mg2NiH4, there was little dif-
ference in the rates of H-generation. From these experiments,
hydrogenation after F-treatment of Mg2Ni to form Mg2NiH4
was demonstrated to be not effective in the improvement of
catalytic hydrolysis of F-treated Mg2Ni.
In the previous works summarized in [23], it has been
proved that the surface oxides are removed by F-treatment
to produce the fluoride layer, which exhibits extremely high
affinity to H-uptakes. In addition, it has been known that the
Mg Ni
2
100
0
Mg
0
10
20
30
40
50
60
Time [min.]
Fig. 5. Catalytic effects of pure Mg and Mg2Ni in hydrolysis

2
12
the role of pure Ni and F-treated Ni. The NiH4−-complex ion
that combined stoichiometrically with Mg does not work as
the catalyst as shown in Fig. 2.
It is, however, preferable from practical engineering
and cost-effectiveness viewpoints that no further chemical
processing such as hydrogenation is required other than
F-treatment process.
6. R.A. Zidan, S. Takara, A.G. Hee, C.M. Jensen: J. Alloys Comp. 285,
19 (1999)
1
7
. B. Lewandowski, T. Seidl, S. Takara, D. Sun, C.M. Jensen: Proc. Int.
Symp. Metal-Hydrogen Systems (Noosa, Queensland, Australia 2000)
p. 288
8. K.J. Gross, C. Jensen, S. Takara, G.J. Thomas: Proc. Int. Symp. Metal-
Hydrogen Systems (Noosa, Queensland, Australia 2000) p. 287
9
. G. Sandrock, K. Gross, G.J. Thomas, C. Jensen, S. Takara: Proc. Int.
Symp. Metal-Hydrogen Systems (Noosa, Queensland, Australia 2000)
p. 278
1
0. H.I. Schlesinger, H.C. Brown, A.B. Finholt, J.R. Gilbreath, H.R. Hock-
stra, E.K. Hydo: J. Am. Chem. Soc. 75, 215 (1953)
3
Conclusions
1
1
1. A. Levy, J.B. Brown, C.J. Lyons: Ind. Eng. Chem. 52, 211 (1960)
2. H.C. Brown, C.A. Brown: J. Am. Chem. Soc. 84, 1493 (1962)
Catalytic functions of pure Mg and Mg2Ni were evaluated
in accordance with the studies on the fluorination effects on
13. J.A. Gardiner, J.W. Collat: J. Am. Chem. Soc. 87, 1692 (1965)
−
1
4. K.A. Holbrook, P.J. Twist: “Hydrolysis of the Brohydride Ion Cat-
alyzed by Motal-Boron Alloys”, J. Chem. Soc. (A), 890 (1971)
5. M.M. Krccvoy, R.W. Jacobson: Ventron Alembic 15, 2 (1979)
6. C.M. Kaufinan, B. Sen: “Hydrogen Generation by Hydrolysis of
Sodium Tetrahydroborate: Effects of Acids and Transition Metals and
Their Salts”, J. Chem. Soc., Dalton Trans. 307 (1985)
catalytic hydrolysis of BH -complex ion solutions.
4
F-treatment effects on Mg2Ni were found prominent for
improving the H-generation kinetics. The increased spe-
cific area by F-treatment enhanced the catalytic function of
Mg2Ni.
1
1
1
1
7. V.C.Y. Kong, F.R. Foulkes, D.W. Kirk, J.T. Hinatsu: Int. J. Hydrogen
Energy 24, 665 (1999)
8. S.C. Amendola, S.L. Sharp-Goldman, M. Saleem Janjua, N.C. Spencer,
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(2000)
Hydrogenation effects on the F-treated Mg2Ni, i.e. F−Mg2
NiH4, were not observed clearly in this work.
Hydrogenation during F-treatment was estimated to re-
sult in the disproportionation of Mg2Ni to xMgF2 and
−
1
2
9. S.C. Amendola, M. Binder, M.T. Kelly, P.J. Petillo, S.L. Sharp-
Goldman: “ A Novel Catalytic Process for generating Hydrogen Gas
from Aqueous borohydride Solutions”, Adv. in Hydrogen Energy 36
Mg2−xNiH4 at the extreme surface. And the NiH -enriched
4
surface is considered to contribute to the improved kinetics.
Disproportionation effects on the catalytic activity should be
subjected to further detailed studies.
(2000)
0. H.B.H. Cooper, Electrolytic Process for the Production of Alkali Metal
Borohydrides, US Patent 3734842 (22 May 1973); C.H. Hale, Pro-
duction of Metal Borohydrides and Organic Onium Borohydrides, US
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US Patent 5804329 (8 September 1998)
Acknowledgements. This work has been funded by NEDO (The New En-
ergy & Industrial Technology Development Organization) for “The Devel-
opment of Metal–Hydrogen Complex Solutions as the H-Storage System”.
It has also been supported in part by grant-in-aid for Scientific Research
on Priority Area A of the “New Protium Function” from the Ministry of
Education, Science, Sports and Culture. The authors wish to express their
special thanks to these two governmental organizations for their funding
and support.
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2
3. Z.-P. Li, Y.-M. Sun, B.-H. Liu, X.-P. Gao, S. Suda: Mater. Res. Soc.
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