Sodium borohydride formation when Mg reacts with hydrous sodium borates under hydrogen

Article abstract of DOI:10.1016/j.jallcom.2008.09.057

In this work, we explored the possibility of NaBH₄ synthesis when Mg reacted with hydrous sodium borates under hydrogen. It was found that Mg could react with the water in molten hydrous borates to form MgO and release hydrogen, which can be us

Full text of DOI:10.1016/j.jallcom.2008.09.057

Journal of Alloys and Compounds 476 (2009) L16–L20
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Letter
Sodium borohydride formation when Mg reacts with hydrous sodium borates
under hydrogen
∗
Bin Hong Liu, Zhou Peng Li , Jing Ke Zhu
College of Materials Science and Chemical Engineering, Zhejiang University, Hangzhou 310027, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 July 2008
Received in revised form
In this work, we explored the possibility of NaBH4 synthesis when Mg reacted with hydrous sodium
borates under hydrogen. It was found that Mg could react with the water in molten hydrous borates to
form MgO and release hydrogen, which can be used for NaBH4 synthesis. Then remained Mg would react
with formed anhydrous borate to form NaBH4. NaBO2 conversion rate was decreased with increase of the
crystalline water content in the hydrous sodium borates. Based on experimental results, a mechanism of
NaBH4 formation when Mg reacted with hydrous sodium borates was suggested.
10 September 2008
Accepted 11 September 2008
Available online 31 October 2008
©
2008 Elsevier B.V. All rights reserved.
Keywords:
Sodium borohydride formation
Hydrous sodium borate
Crystalline water
Anhydrous sodium borate
NaBO2 conversion rate
1
. Introduction
It is considered that spent solutions after NaBH4 hydrolysis can
be used as the raw material for NaBH4 recovery. Anhydrous NaBO2
can be prepared by evaporation to remove water in the spent solu-
tion. However, it is considered that some hydrous NaBO2 would
remain when preparing anhydrous NaBO2 in large scale. The anhy-
drous NaBO is deliquescent and easy to form hydrous NaBO when
exposing to air. It is considered that the crystalline water in the
borates would influence the NaBH4 synthesis. However, it is not
clear that the effect of crystalline water in hydrous borates on
NaBH4 synthesis.
Recently, sodium borohydride (NaBH ) has been attracted more
4
attentions as a hydrogen storage material for hydrogen genera-
tion [1–8] or as a fuel for the Direct Borohydride Fuel Cell (DBFC)
due to its high hydrogen storage capacity [9–18]. It is consid-
ered that NaBH4 recovery technology is an important issue when
NaBH4 is used as a hydrogen storage material because the NaBH4
production cost is too high to be a hydrogen source right now
2
2
[
15,19,20]. Recently, several NaBH4 synthesis methods have been
developed such as ball-milling synthesis [21–23] and thermal syn-
thesis [24–29]. Ball-milling synthesis is a convenient method by
which borohydrides are formed through mechano-chemical reac-
tions of borates with metal hydrides at room temperature. Thermal
synthesis is a mass production method by which borohydrides
are formed through chemical reactions of borates with metals and
hydrogen by heating up to around 600 C. For example, Mg reacts
with NaBO2 to form NaBH4 and MgO under hydrogen atmosphere
through following reaction:
In this work, we explored the possibility of NaBH4 synthesis
when Mg reacted with hydrous sodium borates under hydrogen
atmosphere. Based on experimental results, a mechanism of NaBH4
formation was suggested and discussed.
2. Experimental details
◦
Mg powder (purity: 99.9%, <200 mesh), hydrous sodium borate NaBO2·4H2O
(
purity: 98%) and hydrogen (purity: 99.99%) were used as raw materials. The diagram
of test apparatus for borohydride synthesis is as described in Ref. [26]. The confir-
mation of sodium borohydride formation was conducted by XRD analysis (RINT
2
Mg + NaBO + 2H = NaBH + 2MgO,
1000, Cu K ). Sample morphology observation and element distribution mapping
were conducted by scanning electron microscopy and electron probe microanaly-
sis (EPMA) analyses (EPMA-8705). NaBO2 conversion rates were both determined
by Sievert method through calculation of hydrogen consumption amount and by
iodimetric analysis.
2
2
4
␣
ꢀG = −342 kJ/mol NaBH4
(1)
Hydrous borates with different contents of crystalline water were prepared
by heating NaBO2·4H2O at different temperatures according to the result of dif-
ferential scanning calorimetry (DSC) analysis (Simadzu DSC-60) as shown in
Fig. 1. NaBO2·2H2O was obtained by heating NaBO2·4H2O at 90 C for 12 h and
∗ Corresponding author. Tel.: +86 571 87951977; fax: +86 571 87943149.
E-mail address: zhoupengli@zju.edu.cn (Z.P. Li).
◦
0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2008.09.057

B.H. Liu et al. / Journal of Alloys and Compounds 476 (2009) L16–L20
L17
Fig. 1. DSC analysis of NaBO2·4H2O during heating.
Fig. 3. XRD analyses of extracted samples and their remnants after Mg reacted with
anhydrous borate or hydrous borates under hydrogen.
Fig. 4. Influence of crystalline water content in hydrous borates on hydrogen con-
sumption behavior when Mg reacted with NaBO2·xH2O (x = 0, 0.5, 2) under hydrogen
atmosphere.
Fig. 2. XRD confirmation of NaBO2·2H2O and NaBO2·0.5H2O after heating
◦
◦
NaBO2·4H2O at 90 C and 200 C respectively.
NaBO2·0.5H2O was obtained by heating NaBO2·4H2O at 200 ◦C for 12 h. Due to that
dehydration peak of NaBO2·2H2O was close to that of NaBO2·H2O, it was difficult to
obtain pure NaBO2·H2O by dehydrating NaBO2·2H2O. Therefore, we investigated the
NaBH4 formation behavior when Mg reacted with NaBO2·2H2O and NaBO2·0.5H2O
respectively. The sample confirmations were done by XRD analysis as shown in Fig. 2.
Raw materials for sodium borohydride synthesis were well mixed in a glove box,
and then put the mixture into a stainless steel reactor in which a thermocouple was
placed in the reaction bed. The mole ratio of Mg to NaBO2 in hydrous or anhydrous
sodium borates was fixed to 2:1 or 4:1. The reactor was degassed during heating to
Fig. 5. XRD analysis of the sample after 2 mole of Mg reacted with 1 mole of
NaBO2·2H2O.
◦
◦
5
3
10 C at a heating rate of 40 C/min before introducing hydrogen (initial pressure:
◦
◦
.0 MPa). After that, the reactor was heated to 600 C at a heating rate of 5 C/min
◦
and held at 600 C under hydrogen atmosphere.
with NaBO ·2H O through following supposed reaction:
2
2
3
. Results and discussion
4Mg + NaBO2·2H2O = NaBH4 + 4MgO,
G = −991 kJ/mol NaBH4
ꢀ
(2)
3.1. NaBH4 formation
We calculated NaBO2 conversion rates through Sievert method
according to the gaseous hydrogen consumption of reaction (1)
and measured by iodimetric measurement as shown in Table 1.
It was found that there was some NaBH4 formed when 4 mole of
Theoretically speaking, hydrogen in crystalline water of hydrous
sodium borates can be directly used as a hydrogen source for NaBH4
synthesis in thermodynamics. It was thought that Mg would react
Table 1
NaBO2 conversion rates when Mg reacted with hydrous or anhydrous sodium borates.
NaBO2
NaBO2·0.5H2O
NaBO2·2H2O
NaBO2·4H2O
Sievert method
Iodimetric analysis
Sievert method
Iodimetric analysis
Sievert method
Iodimetric analysis
Sievert method
Iodimetric analysis
2
4
Mg
Mg
61.0%
–
63.5%
–
26.1%
–
28.8%
–
0
9.2%
0
–
0
–
0
12.3%

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B.H. Liu et al. / Journal of Alloys and Compounds 476 (2009) L16–L20
◦
Fig. 6. (a) SEM pictures of mixtures of Mg with anhydrous NaBO2 or hydrous sodium borate after heating to 510 C. (b) Mg and Na distribution in mixtures of Mg with
◦
anhydrous NaBO2 or hydrous sodium borate after heating to 510 C.

B.H. Liu et al. / Journal of Alloys and Compounds 476 (2009) L16–L20
L19
Fig. 7. SEM pictures of the mixtures after Mg reacted with anhydrous sodium borate or hydrous sodium borate under hydrogen.
Mg reacted with 1 mole of NaBO ·2H O though the NaBO conver-
Fig. 4 shows the influence of crystalline water content in hydrous
sodium borates on the hydrogen consumption behavior when Mg
reacted NaBO ·xH O (x = 0, 0.5, 2) under hydrogen. It was found that
2
2
2
sion rate was lower than that of the reaction (1), but no NaBH4 was
formed when 4 mole of Mg reacted with 1 mole of NaBO ·4H O.
2
2
2
2
In order to confirm the NaBH formation, we extracted the prod-
the more the crystalline water content in hydrous sodium borates,
the higher the hydrogen pressure would go up. It reconfirmed that
hydrogen was generated during heating the mixture of Mg and
hydrous sodium borates.
4
uct by anhydrous ethylenediamine with a purity of 99%. Hardened
filter paper was used to separate the extraction solution from rem-
nants. White powders were obtained by evaporating the extraction
solution under 0.01 MPa at room temperature. The obtained pow-
ders were subjected to XRD analysis for qualitative identification
of the borohydride formation. The XRD peaks clearly demonstrate
the existence of NaBH4 when 4 mole of Mg reacted with 1 mole of
NaBO ·2H O as shown in Fig. 3. It can be seen that the remnants
From Fig. 4, it also can be seen that the hydrogen consump-
tion rate (hydrogen pressure drop vs. time) was decreased with
increasing the crystalline water content in the hydrous sodium
borates. It is known that the more the crystalline water content
in the hydrous sodium borates, the more MgO would be formed
according to reaction (3). The thick layer of MgO on the Mg sur-
faces would significantly impede NaBH4 formation from reaction
(1). As a result, NaBO2 conversion rate and kinetic properties of
NaBH4 formation were decreased.
2
2
contained MgO, NaBO , some unreacted Mg and NaBO ·2H O.
2
2
2
According to reaction (2), NaBH4 formation would have noth-
ing to do with the hydrogen pressure. However, from experimental
results as shown in Fig. 4, it can be seen that hydrogen pressure
increased first, and then decreased during reaction. It indicated
that Mg would firstly react with NaBO ·2H O to produce NaBO
2
2
2
3.2. NaBH4 formation mechanism
and hydrogen by following reaction:
From these experimental results and discussion above, it can be
supposed that NaBH4 would be formed by following steps when
Mg reacted with hydrous borates:
2
Mg + NaBO ·2H O = 2MgO + NaBO + 2H ,
2
2
2
2
ꢀG = −324.4 kJ/mol H2
(3)
Then remained Mg would react with NaBO2 and hydrogen to
form NaBH4 by reaction (1). As a result, NaBH4 was formed when
mole of Mg reacted with 1 mole of NaBO ·2H O.
(
(
1) hydrous borates melted and coated on surfaces of Mg particles
◦
when heating to 510 C;
4
2
2
2) Mg reacted with H O in the molten borates to form anhydrous
2
In order to confirm the suggested NaBH4 formation hypothe-
sis when Mg reacted with hydrated sodium borate, we tried to use
borates and release hydrogen;
(3) MgO was formed on surfaces of the Mg particles;
2
mole of Mg to react with 1 mole of NaBO ·2H O under hydrogen. It
2
2
(
4) after all H O in the molten borates was consumed, remained Mg
2
was found that hydrogen pressure increased but no hydrogen pres-
sure drop occurred as shown in Fig. 4. The obtained sample was
subjected to XRD analysis. It was found that no NaBH4 was formed
as shown in Fig. 5, but X-ray diffraction peaks clearly showed the
existence of formed MgO. Through iodimetric measurement, no
NaBH4 formation was confirmed also.
reacted with anhydrous borate and hydrogen to form NaBH4.
It is interesting to note that the crystalline water in hydrous
sodium borates can be used as a hydrogen source for NaBH4 syn-
thesis. It is important for simplification of NaBH4 recovery process
and cost reduction of NaBH4 production.
The morphology and EPMA mapping after heating the mixture
◦
(
Mg + NaBO ·2H O) to 510 C is shown in Fig. 6(a) and (b) respec-
2
2
tively. It can be seen that molten hydrous borates were generated.
Fig. 7 shows the morphologic observation after Mg reacted with
hydrous or anhydrous sodium borate under hydrogen. It was found
many small pores remained in the molten substance, but no molten
substance was formed when Mg reacted with anhydrous borate. It
was considered that these pores were brought by hydrogen evolu-
tion when Mg reacted with H O in the molten NaBO ·2H O.
4. Conclusions
Mg can react with anhydrous borate to form NaBH . Mg would
4
react with crystalline water in hydrous sodium borates to form
anhydrous borate and release hydrogen, which can be used for
NaBH4 synthesis. Then remained Mg would react with formed
anhydrous borate to form NaBH . NaBO2 conversion rate was
2
2
2
4

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B.H. Liu et al. / Journal of Alloys and Compounds 476 (2009) L16–L20
decreased with increase of the crystalline water content in the
hydrous sodium borates.
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This work is financially supported by Hi-tech Research and
Development Program of China (863), grant nos. 2006AA05Z120
and 2007AA05Z144, Doctoral fund from Education ministry of
China (20070335003) and Fund from Science & Technology min-
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