Breaking the Passivation: Sodium Borohydride Synthesis by Reacting Hydrated Borax with Aluminum

Article abstract of DOI:10.1002/chem.202100552

A significant obstacle in the large-scale applications of sodium borohydride (NaBH₄) for hydrogen storage is its high cost. Herein, we report a new method to synthesize NaBH₄ by ball milling hydrated sodium tetraborate (Na₂B₄O₇ ? 10H₂O) with low-cost Al or Al₈₈Si₁₂, instead of Na, Mg or Ca. An effective strategy is developed to facilitate mass transfer during the reaction by introducing NaH to enable the formation of NaAlO₂ instead of dense Al₂O₃ on Al surface, and by using Si as a milling additive to prevent agglomeration and also break up passivation layers. Another advantage of this process is that hydrogen in Na₂B₄O₇ ? 10H₂O serves as a hydrogen source for NaBH₄ generation. Considering the low cost of the starting materials and simplicity in operation, our studies demonstrate the potential of producing NaBH₄ in a more economical way than the commercial process.

Full text of DOI:10.1002/chem.202100552

Accepted Article
Accepted Article
Title: Breaking the Passivation: Sodium Borohydride Synthesis by
Reacting Hydrated Borax with Aluminium
Authors: Yongyang Zhu, Hao Zhong, Hui Wang, Liuzhang Ouyang,
Jiangwen Liu, Zhenguo Huang, and Min Zhu
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To be cited as: Chem. Eur. J. 10.1002/chem.202100552
Link to VoR: https://doi.org/10.1002/chem.202100552
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Breaking the Passivation: Sodium Borohydride Synthesis by
Reacting Hydrated Borax with Aluminium
Yongyang Zhu,[a] Hao Zhong,[a] Hui Wang,[a] Liuzhang Ouyang,*[a, b] Jiangwen Liu,[a] Zhenguo Huang,*[c]
and Min Zhu[a]
[a]
Y. Zhu, Dr. H. Zhong, Prof. H. Wang, Prof. L. Ouyang, Prof. J. Liu, Prof. M. Zhu
School of Materials Science and Engineering, Guangdong Provincial Key Laboratory of Advanced Energy Storage Materials
South China University of Technology
Guangzhou, 510641, People's Republic of China
E-mail: meouyang@scut.edu.cn
[b]
[c]
Prof. L. Ouyang
China-Australia Joint Laboratory for Energy & Environmental Materials
Key Laboratory of Fuel Cell Technology of Guangdong Province
Guangzhou, 510641, People's Republic of China
Prof. Z. Huang
School of Civil and Environmental Engineering
University of Technology Sydney
Sydney, NSW, 2007, Australia
E-mail: zhenguo.huang@uts.edu.au
Supporting information for this article is given via a link at the end of the document.
Abstract: A significant obstacle in the large-scale applications of
sodium borohydride (NaBH4) for hydrogen storage is its high cost.
Herein, we report a new method to synthesize NaBH4 by ball milling
hydrated sodium tetraborate (Na2B4O7·10H2O) with low-cost Al or
Al88Si12, instead of Na, Mg or Ca. An effective strategy is developed
to facilitate mass transfer during the reaction by introducing NaH to
enable the formation of NaAlO2 instead of dense Al2O3 on Al surface,
and by using Si as a milling additive to prevent agglomeration and
also break up passivation layers. Another advantage of this process
is that hydrogen in Na2B4O7·10H2O serves as a hydrogen source for
NaBH4 generation. Considering the low cost of the starting materials
and simplicity in operation, our studies demonstrate the potential of
producing NaBH4 in a more economical way than the commercial
process.
anhydrous sodium tetraborate (Na2B4O7) obtained from naturally
abundant borax [6]. In those reactions, strong reducing agents
such as alkaline metal or metal hydride are typically employed to
convert B-O to B-H. High pressure hydrogen gas is often
needed for the formation of B-H. One traditional method, Bayer
process, involves heating up Na2B4O7, sodium (Na) and silicon
dioxide (SiO2) at high temperature (700°C ) under high hydrogen
pressure (reaction 1): [6a]
Na2B4O7+16Na+8H2+7SiO2 → 4NaBH4+7Na2SiO3 (1)
The consumption of a considerable amount of Na and high
pressure H2 at high temperature leads to the high price of
NaBH4. Therefore, it is necessary to develop alternative
methods to reduce the cost. Non-Na based reducing agents
[5a]
such as magnesium hydride (MgH2)
(CaH2)
and calcium hydride
[5b]
have been used to synthesize NaBH4 from NaBO2.
For example, NaBH4 was successfully obtained via annealing
MgH2 and NaBO2 at 550°C under 7 MPa H2.[5a] Due to the high
cost of MgH2, Mg was used as a substitute but the yield was
only 10%, which is due to the agglomeration of Mg powders and
consequently reduced reaction activity.[5a] To improve the yield,
Introduction
Transition from fossil fuels to renewables as primary energy
sources is important for solving energy and environmental
problems and realizing sustainable future.[1] In this regard,
hydrogen (H2) is considered as a suitable candidate for an
energy carrier in view of its high energy density, and generating
electrical and thermal power with zero greenhouse gas emission.
However, the lack of safe and efficient means to store large
amounts of hydrogen is a major barrier to wide deployment of
hydrogen. Sodium borohydride (NaBH4) as a hydrogen source
various additives such as Si, Ni, Fe and Co were introduced to
[5a]
this system to help with either the dispersion of Mg
activation of H2 molecules
or
5h]. In addition, Mg was also
[5c, 5d,
used to react with Na2B4O7 under high-pressure H2 (2.5 MPa)
via thermochemical method (550°C ) to prepare NaBH4.[6b]
The methods mentioned above are energy inefficient and
special safety measures are required when handling highly
corrosive/flammable chemicals at high temperatures and under
high H2 pressure. The H2 is mainly generated from fossil fuels,
which is not environmentally friendly and also adds raw
materials cost. Alternatively, NaBH4 has been successfully
synthesized through a mechano-chemical reaction at room
temperature. MgH2 was used as the reducing agent to react with
[2]
[3]
for direct fuel cell (DFC) or as a hydrogen storage material
has attracted great attention owing to its high hydrogen storage
density (10.8 wt%) [4]. However, its high cost poses a significant
obstacle in large-scale applications. Thus, developing cost-
effective NaBH4 synthesis becomes highly desirable.
Previously, significant efforts have been mainly devoted to
synthesizing NaBH4 from anhydrous sodium metaborate
[5e-g]
[6c, 6d]
NaBO2
or Na2B4O7
facilitated by high-energy ball
milling with the maximum NaBH4 yield of 76% and 78%,
respectively. The low temperature is certainly an operational
[5]
(NaBO2) recycled from the hydrolytic product of NaBH4
or
1
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Figure 1. (a) Yields of NaBH4 obtained via ball milling Na2B4O7·10H2O and Al with and without additives for 20 h (not including the pauses). (b) Yields of NaBH4,
(c) XRD patterns and (d) FTIR spectra of products obtained via ball milling NaH and Na2B4O7·10H2O in different molar ratios (Na2B4O7·10H2O and Al were fixed at
a 1:14.16 molar ratio) for 20 h. (e) XPS Al 2p spectra of commercial Al and the products obtained via milling NaH and Na2B4O7·10H2O in different molar ratios for
20 h.
[7]
advantage but MgH2 is still expensive. In addition, the
approaches mentioned above rely on anhydrous NaBO2 and
Na2B4O7, which requires dehydration of NaBO2·xH2O (real spent
fuel) at over 350°C
or Na2B4O7·10H2O (borax mineral) at
approximately 600°C [8]. Dehydration causes additional energy
consumption and therefore raises up the price of NaBH4.
2
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Recently, we reported that NaBH4 could be synthesized by ball-
milling NaBO2·xH2O (x=2, 4) or CO2 treated hydrolytic product
with Mg with high yields (above 68.55%).[9] It should be noted
that little attention has been paid to naturally abundant borax
(hydrated Na2B4O7).
Na2B4O7·10H2O systems leads to the formation of NaBH4 and
MgO. We speculate that Al2O3 film forms on the surface of Al
during ball milling. Unlike relatively loose MgO, dense Al2O3
[11]
blocks hydrogen from reaching Al underneath the oxide film,
thus restricting the formation of NaBH4. The other reason could
be that hydrogen dissociation on Al has high dissociation barrier
(>1 eV) than that on Mg (0.92 eV).[12]
As the 3th most abundant elements in the earth crust, Al is
more abundant and readily available than Mg (8th), Ca (5th) and
Na (6th). Al is more stable than Mg, Ca and Na in air, rendering it
convenient for transport and storage. More importantly, Al can
be oxidized into a higher valance state (Al3+) and would convert
more Na2B4O7 or NaBO2 into NaBH4. Besides, Al has higher
Moh’s hardness (MH) of 2.75 than Na (MH:0.4), Ca (MH: 1.75),
and Mg (MH: 2.5), which could result in better ball-milling
efficiency. Based upon all these reasons, a large-scale NaBH4
production using low-cost Al as a reducing agent is highly
desired. In this study, efforts have been devoted to NaBH4
synthesis by ball milling Al or Al88Si12 with Na2B4O7·10H2O in the
presence of a sodium-containing compound as a Na supplement
to achieve an atomic ratio of 1:1 for Na:B, as in NaBH4. There
are a few advantages associated with hydrated borax. First, H+
in the crystal water in Na2B4O7·10H2O acts as a hydrogen
source during the reaction, which reduces the demand for an
external H2 sources and therefore lowers the cost. Moreover,
dehydration of Na2B4O7·10H2O is avoided, which improves the
overall energy efficiency and decreases the cost. Based on
experimental results, the reaction mechanism was also
discussed.
To confirm the reaction of Al with NaH-Na2B4O7·10H2O
mixture, the products were characterized by FTIR (Figure 1d)
and XPS (Figure 1e). In the raw Al powders, XPS spectrum
reveals the coexistence of Al0 (Al 2p at 72.1 eV) and Al3+ (Al 2p
at 74.7 eV and 73.9 eV corresponding to the octahedral and
tetrahedral coordination state of Al3+, respectively), [13] indicating
the formation of Al2O3 due to slight oxidation of Al. With the
increase in NaH loading, the XPS peaks of Al0 and the
octahedrally coordinated Al3+ disappears (Figure 1e(2-4)) and
[13]
only tetrahedrally coordinated Al3+ remains,
which indicate
the conversion of Al0 to Al3+. Since Al3+ largely adopts
tetrahedral coordination, the oxide is unlikely to be Al2O3. From
Figure 1d, IR band at 811 cm-1 is associated with O-O triangular
species and the bands at 617 and 558 cm-1 can be assigned to
[14]
¯
the vibrations of Al-O bond
in AlO2 . Based upon the XPS
and IR results and the atomic ratio between Na and Al in the
starting materials, it is hypothesized that NaAlO2 is formed
during ball milling. NaAlO2 appears to be amorphous since
related XRD peaks are absent from Figure1c. The intensities of
the characteristic FTIR bands of NaAlO2 increase when more
NaH is used, indicating that NaH facilitates the formation of
NaAlO2. Different from Al2O3, NaAlO2 is relative loose and
porous, which allows for efficient mass transfer required for the
NaBH4 formation during ball milling.[11, 15]
Results and Discussion
Choice of Na-containing compounds as a Na supplement
Analysis of the NaBH4 formation
Na-containing compounds were used to compensate the Na
insufficiency because the atomic ratio of Na:B in Na2B4O7·10H2O
is 1:2, while it is 1:1 in NaBH4. Attempts were first made to
synthesize NaBH4 by reacting Al with Na2B4O7·10H2O and
different Na-containing compounds (Figure S1). In experiments
with NaOH, Na2CO3 and no additives, FTIR peaks
To investigate the reaction mechanism, mixtures of
Na2B4O7·10H2O, NaH, and Al were milled for different periods.
Na2[BO2(OH)] appeared after milling for 10 min and then rapidly
disappeared upon further milling, which may result from the
combination of reaction and amorphization during ball milling
(Figure 2a). H2 can be detected during this period (Figure S2(2)).
The formation of NaBH4 is confirmed by the appearance of IR
bands at 1125 and 2200-2400 cm-1 that are associated with B-H
corresponding to typical IR bands (1125, 2200-2400 cm-1) of
[9a, 9b, 10]
NaBH4 are almost invisible (Figure S1),
indicative of low
yields (below 1.5%) (Figure 1a). In contrast, NaH effectively
facilitates the formation of NaBH4. The vibration and deformation
IR bands of B-H at 2200-2400 cm-1 and 1125 cm-1, respectively,
are highly visible in the FTIR spectra (Figure S1).[9a, 9b, 10] To
improve the yield, different amounts of NaH were added. For 20
h milling, the yield increases with increasing molar ratio of
NaH:Na2B4O7·10H2O, from 33.3% for 4:1 to 35.3% for 6:1
(Figure 1b). The intensities of the XRD peaks of NaBH4 increase
when more NaH was used, while those of Fe2B and FeAl2
decreases significantly (Figure 1c). These indicate that NaH can
reduce Fe peeling off the ball mill reactor and improve the
synthesis. This is supported by EDS results (Table S1) which
reveal decreasing amounts of Fe in the ball milled powders
when more NaH was used. Since Fe2B is a reaction product
between Fe and NaBH4, these results indicate less NaBH4
decomposition with the presence of NaH.[9b] As a result, NaH
addition is favorable for the formation of NaBH4.
¯
bond (Figure 2b(1)) and the NMR resonance of BH4 at -42.1
9b, 10, 16]
ppm (Figure 2c(2)) after milling for 30 min.[9a,
The
intensity of the XRD peaks of Al decreases with milling time. As
the ball milling time increased to 4 h, the FTIR bands of NaAlO2
became more visible. The XRD peaks of NaBH4 were observed
only after 7 h milling, which could be due to small crystallite size
of NaBH4 and also the lower quantities of NaBH4 in 0.5-4 h
milled products. With further ball milling, the characteristic FTIR
bands and XRD peaks of NaBH4 become stronger (Figure S3).
Throughout the whole process, the diffraction peaks of NaAlO2
are invisible. NaAlO2 appears to be amorphous, as previously
discussed in section 3.1. H2 was detected as one of the by-
products of the ball-milling reaction (Figure S2 (5)). The overall
reaction can be assumed as follows:
4Na2B4O7·10H2O + 42NaH + 34Al → 16NaBH4 + 34NaAlO2 +
29H2 (2)
Reaction (2) is thermodynamically favorable (ΔGo298k= -929.08
kJ mol-1 NaBH4).
The yield is still lower than those using Mg as the reducing
[9b, 9c]
agent. According to our previous studies,
ball milling Mg-
Na2B4O7·10H2O, Mg-NaOH-Na2B4O7·10H2O, and Mg-Na2CO3-
3
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Figure 2. (a) XRD patterns and (b) FTIR spectra of the products obtained after milling Na2B4O7·10H2O, NaH, and Al mixtures (in a 1:6:14.16 molar ratio) for
different durations; (c) Solid-state 11B NMR spectra of Na2B4O7·10H2O and products obtained after milling for different durations.
To further understand the process, the as-milled mixtures
were also characterized using solid-state 11B NMR. The
resonance at around -13.6 ppm is assigned to the special
and short lifetimes.[18] Third, O2- in Na2[BO2H] bonds with Al
releasing electrons to the absorbed H, generating Al-O and H¯.
H¯ combines with B to form NaBOH2, and another Na+ in
¯
intermediate “[BOH2]¯” after ball milling of 30 min and 2 h (Figure
Na2[BO2H] combines with AlO2 forming NaAlO2. Finally, B in
16]
2c).[9b,
Based upon the results, it can be inferred that the
NaBOH2, as a Lewis acid, accepts H¯ to replace O2- forming
NaBH4. This process is similar to what happens during the
synthesis of NaBH4 by reacting sodium borate with Mg and
H2.[5h]
formation of NaBH4 takes place according to the following steps.
First, crystal water in Na2B4O7·10H2O firstly reacts with NaH
forming NaOH and H2. The hydrogen gas molecules are
absorbed on Al surfaces and dissociate into H. NaOH then
reacts with borax to form Na2[BO2(OH)] which plays an important
role in NaAlO2 formation. This is in agreement with a previous
study where Na2[BO2(OH)] was obtained from by heating
Na2B4O7 and NaOH.[17] Second, since B-O bond with bond
length of 1.35 Å in Na2[BO2(OH)] are stronger than B-OH (1.439
Å), B-OH preferentially breaks and combines with H¯
(from the dissociation of H2 on Al surface) to form Na2[BO2H]
which was not observed in experiments owing to its high activity
Using Al88Si12 for the synthesis
Based on the report that the yield of NaBH4 can be
improved through introducing Si as a milling aid,[10b, 19] we tried
an Al-Si alloy expecting that its hardness and brittleness can
improve milling efficiency and therefore facilitate the reaction. Al-
rich Al88Si12 eutectic alloy was chosen to react with NaH and
Na2B4O7·10H2O. In comparison, a mixture of Al and Si powders
4
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Figure 3. (a) XRD patterns and (b) Yields of NaBH4 via ball milling the mixtures for 20 h at 1200 CPM: Na2B4O7·10H2O, NaH, and Al in a 1:6:14.16 molar ratio;
Na2B4O7·10H2O, NaH, Al, and Si in a 1:6:14.16:1.92 molar ratio; Na2B4O7·10H2O, NaH, and Al88Si12 in a 1:6:0.161 molar ratio; (c) Impact of Al88Si12 loading on the
yields of NaBH4 and (d) XRD patterns of the products obtained after milling Al88Si12 and Na2B4O7·10H2O in different molar ratios (Na2B4O7·10H2O and NaH were
fixed at a 1:6 molar ratio) for 20 h.
with a molar ratio of 88:12 was also tried. For both cases, the
intensity of XRD peaks of FeAl2 and FeB2 decreases compared
with the experiment without Si (Figure 3a), indicating reduced
decomposition of NaBH4. The yield increased and then
plateaued when more Al88Si12 was used, and reached the
maximum value of 39.7% for the ratio of 0.161:1 (Al88Si12:
Na2B4O7·10H2O) (Figure 3b and 3c). The Al/Si mixture also
displays similar high yield. The improvement is likely due to
suppressed decomposition of NaBH4 and also increased
effective contact between the reactants.
and then decreased, a trend similar to previous studies.[5e, 6c, 10a]
Better contact among the reactants and more effective reaction
are expected with the increase in Al loading. However,
excessive amount of Al decreases effective contact and
therefore lower the ball-milling efficiency. Na2B4O7·10H2O was
replaced by Na2B4O7·5H2O to synthesize NaBH4 in the same
way, but the yields are much lower (Figure 4b). This is likely due
to more crystal water available as the hydrogen source in
Na2B4O7·10H2O than in Na2B4O7·5H2O.
To further improve the yield,
a milling additive was
introduced. Kojima et al. [5a] found that the introduction of Si to
NaBO2-Mg-H2 system significantly increased the yield of NaBH4.
As discussed in the previous section, Al88Si12 improves the yield
to an extent similar to that of Al/Si mixture. Considering that
Al88Si12 is slightly more expensive than Al, only Si was
introduced as an additive. The yield increases with increasing
the molar ratio of Si:Na2B4O7·10H2O, reaching the maximum of
Yield of NaBH4
As known from previous studies the loadings of reducing
agent has an important influence on the NaBH4 yield.[5b, 5e, 5g, 6c, 9a,
9b, 10]
The yield was optimized by varying the molar ratio of
Al:Na2B4O7·10H2O (Figure 4a). With the increase in the loading
of Al, the yield of NaBH4 increased to the highest value (35.3%)
61.9% for
a
ratio of 13.51:1(Figure 4c), higher than
5
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Figure 4. (a) Yields of NaBH4 obtained after ball milling Al and Na2B4O7·10H2O in different molar ratios (Na2B4O7·10H2O:NaH was fixed at a 1:6 molar ratio) for 20
h; (b) Yields of NaBH4 obtained after ball milling NaH and Na2B4O7·xH2O (x=5, 10) in different molar ratios for 20 h (Na2B4O7·5H2O:Al in a 1:10 molar ratio;
Na2B4O7·10H2O:Al in a 1:14.16 molar ratio); (c) Yields of NaBH4 and (d) XRD patterns of the products obtained after milling Si and Na2B4O7·10H2O in different
molar ratios (Na2B4O7·10H2O:NaH:Al was fixed at a 1:6:14.16 molar ratio) for 20 h.
NaBO2-Mg-H2, NaBO2·2H2O-Mg-H2, Na2B4O7-Mg-H2, and
Na2B4O7-MgH2 systems reported previously (10%, 12.3%,
45.6%, and 43% respectively).[5a, 6b, 6c, 20] For 20 h ball-milling,
the yield of 61.3% with Si is much higher than 35.3% without Si
additive (Figure 4c). The enhancement can be attributed to the
brittle and hard feature of Si that can help to break up
passivation layers. The formation of FeAl2 and FeB2 decreases
with increasing loading of Si, which indicates suppressed
decomposition of NaBH4 (Figure 4d). We note that the highest
yield of this system is still lower than those of Na2B4O7-NaOH-
Herein, we report a low-cost method to produce NaBH4 via
ball milling mixtures of Al (or Al+Si, or Al88Si12)-Na2B4O7·10H2O-
NaH. Commercially available Na2B4O7·10H2O serves as the
source of B and also provides certain amount of Na and H. NaH
plays a crucial role in improving the reaction by favoring the
formation of loose NaAlO2 instead of dense Al2O3 which restricts
mass transfer. The yield of NaBH4 increased from 35.5% to
61.9% after introducing Si, which acts as a grinding aid creating
fresh surfaces. Si in the by-product can be extracted and reused.
Compared with the commercial process, this mechanochemical
method has several advantages such as requiring no external
heat and no H2 supply, and employing low-cost Na2B4O7·10H2O
and Al. With further improvement in yield, this method can be an
alternative to the existing commercial NaBH4 synthesis process.
MgH2 (64%) [6c], Na2B4O7-Na2O2-MgH2 (67%) [6c], and NaBO2-
[5f, 5g]
MgH2 (70/71%)
where expensive MgH2 is used. This could
be primarily attributed to the higher reactivity of MgH2 than Al.
Conclusion
6
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Experimental Section
Acknowledgements
Mechano-chemical synthesis of NaBH4
This work was financially supported by the National
Natural
Science Foundation of China Projects (Nos. 51771075 and No.
52001125) and Foundation for Innovative Research Groups of
the National Natural Science Foundation of China (No.
NSFC51621001). Z.H. acknowledges support under the
Australian Research Council‘s Discovery Projects funding
scheme (project number DP170101773).
Na2B4O7·10H2O (ACS-grade, Aladdin), Na2B4O7·5H2O (purity: ≥99.5%,
Chron Chemicals), Al (purity: ≥99%, Aladdin), Al88Si12 alloy (purity: 99%,
Aike Reagent), Si (powder, 1 µm, Desheng Alloy Materials Co., Ltd),
NaOH (purity: ≥99%, Aladdin), Na2CO3 (purity: ≥99.8%, General-
Reagent), and NaH (purity: 90%, Sigma-Aldrich) were used as received
for the mechano-chemical synthesis of NaBH4. All the raw materials
except Na2B4O7·xH2O (x=5, 10) are stored in an argon glove box
(MIKROUNA, China) with <0.1 ppm H2O and O2. 1 g of reactants were
put in an 80 mL jar with 50 g of two different sized steel balls. Effects of
ball milling duration and atomic ratios among the start materials on the
yield were studied. In a shaker mill, impact is the most dominant force,
which benefits the mechanochemical reactions. This is in contrast to a
planetary mill which generates friction, shear, and impacts.[21] Besides,
shaker mills are generally capable of operating with higher rotation
speeds. As a result, the shaker ball mill could supply higher energy to
efficiently pulverize particles to allow better mechanochemical reactions.
Shaker mills have been found to be more effective in promoting the
NaBH4 formation, as compared with planetary mills.[5e] All reactions were
conducted in a shaker mill (QM-3C, Nanjing, China) at 1200 cycles per
min (CPM) at room temperature. To prevent overheating, the ball milling
process was conducted by alternating 30 min of milling and 30 min of
rest. Note that the ball-milling times mentioned in articles do not include
the pauses.
Keywords: aluminum • borax • borohydride • hydrogen •
hydrogen storage
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Anhydrous ethylenediamine (C2H8N2, Sigma-Aldrich, purity: 99%) was
used to extract NaBH4 from the milled products under an Ar atmosphere.
The filtrate was collected after passing through polytetrafluoroethylene
membrane (0.45 μm) and then dried in a freeze dryer, where the solvent
vapor (C2H8N2) was captured by a cold trap (-54°C ). The amount of
resultant NaBH4 was determined by iodimetric analysis [15]. H2SO4 (GR),
sodium thiosulfate solution (Na2S2O3, 0.1000 mol L-1), potassium iodide
(KI, purity: ≥99%), potassium iodate (KIO3, AR-grade), and starch
indicator (≥99%) were used for iodimetric analysis. All these chemicals
were purchased from Aladdin and used without further purification. The
yield of NaBH4 was calculated according to formula: [5g]
Yield = [(obtained mass NaBH4)/(theoretical mass NaBH4)]×100% (3)
where the theoretical mass NaBH4 is calculated based upon the
assumption that 1 mole Na2B4O7·xH2O (x=5,10) can produce 4 moles
NaBH4.
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8
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Entry for the Table of Contents
Insert graphic for Table of Contents here.
We present a cost-effective way to produce NaBH4 with high yield by ball milling hydrated borax (Na2B4O7·10H2O) with cheap Al or
Al88Si12 at ambient condition. An effective strategy was developed to facilitate mass transfer during the reaction by introducing NaH to
enable the formation of loose NaAlO2 instead of dense Al2O3 on Al surface, and by using Si as a milling additive that prevents
agglomeration and also breaks up passivation layers.
Yongyang Zhu, Hao Zhong, Hui Wang, Liuzhang Ouyang*, Jiangwen Liu, Zhenguo Huang*, and Min Zhu
9
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