Is Y2(B12H12)3 the main intermediate in the decomposition process of Y(BH4)3?

Article abstract of DOI:10.1039/c3cc41184b

Dodecaborates, i.e. the [B₁₂H₁₂]^2- containing species, are often observed as main intermediates in the hydrogen sorption cycle of metal borohydrides, hindering rehydrogenation. In the decomposition process of Y(BH₄

Full text of DOI:10.1039/c3cc41184b

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Is Y (B H ) the main intermediate in the
2
12 12 3
decomposition process of Y(BH ) ?†
4
3
Cite this: Chem. Commun., 2013,
4
9, 5234
a
a
b
c
c
Yigang Yan,* Arndt Remhof, Daniel Rentsch, Young-Su Lee, Young Whan Cho
Received 13th February 2013,
Accepted 11th April 2013
a
and Andreas Z u¨ ttel
DOI: 10.1039/c3cc41184b
www.rsc.org/chemcomm
2
À
Dodecaborates, i.e. the [B12
H
12
]
containing species, are often observed Fourier transform infrared spectroscopy during the decomposition
15
as main intermediates in the hydrogen sorption cycle of metal boro- of Y(BH ) . However, no further characterization was reported to
4
3
hydrides, hindering rehydrogenation. In the decomposition process of support the formation of Y (B H ) . Here, we demonstrate that
2
12 12 3
Y(BH
4
)
3
, yttrium octahydrotriborate, i.e. Y(B
3
H
8
)
3
, rather than the stable Y(BH
of Y
magnetic resonance (NMR). Besides, Y
(n is the valence of metal M), with by ball milling Y(BH with B and its stability was examined.
The dehydrogenation of Y(BH (prepared according to
for LiBH ) hydrogen densities, have been widely investigated for ref. 6) was carried out by temperature programmed desorption
4
)
3
decomposes via the formation of Y(B
are detected using solution state B nuclear
was synthesized
3 8 3
H ) , and only traces
1
1
2
Y (B12
H
12
)
3
, is formed as the main intermediate.
2 12 3
(B12H )
2
12 3
(B12H )
Metal borohydrides M(BH
high gravimetric (18.4 wt% for LiBH
4 n
)
4
)
3
2 6
H
À3
4
) and volumetric (102 kg m
4 3
)
4
1,2
hydrogen storage in recent years. The stability of M(BH ) was (TPD) under a constant flow of 1, 5 and 10 bar H , respectively.
4
n
2
reported to depend on the charge transfer from M to [BH ], and The samples dehydrogenated at 240 to 350 1C were dissolved in
4
1
1
thereby a correlation between the electronegativity (w
and the stability of the corresponding M(BH was established.
Applying this empirical rule to Y (w
= 1.2), a decomposition A main resonance at À30.8 ppm together with some additional
temperature of 500 K, well below those of alkaline and alkaline minor resonances is observed in the samples decomposed at 240,
earth metal borohydrides, is expected for Y(BH . Combined with 255 and 280 1C, while no dissolved intermediates are observed
a hydrogen content of 9.1 wt%, it makes Y(BH ) an attractive in the sample decomposed at 350 1C. The chemical shift of
p
) of metal M
2
D O and examined using B NMR. Fig. 1a shows the solution
3
11
1
4
)
n
4 3 2
state B( H) NMR spectra of Y(BH ) decomposed under 1 bar H .
p
4 3
)
4
3
candidate for solid hydrogen storage.
À30.8 ppm agrees well with that of octahydrotriborate (i.e. the
À 16–18 1 11
Experimentally, Y(BH ) was observed to release hydrogen [B H ] species) reported in the literature.
In the 2D H– B
4
3
3
8
4
–6
below 200 1C. The whole hydrogen release process occurs in HMQC NMR spectrum of Y(BH
4
)
3
after heating to 255 1C
À
multistep reactions with the formation of amorphous inter- (Fig. 1b), a single correlation signal of [B
3
H
8
]
centered at
1
11
mediates. Within the hydrogen sorption cycle, the reaction 0.2 ppm ( H) and À30.8 ppm ( B) is observed. This single
pathway and the intermediates involved determine the reversibility. resonance is explained by the fact that all three boron atoms are
2
À
For example, dodecaborates (i.e. the [B12
unwanted by-products and reduce the rehydrogenation perfor- hydrogen atoms. In addition, the minor resonance in Fig. 1a
mance, are often observed as intermediates in the decomposition at 19.6 ppm assigned to boric acid is attributed to the hydro-
process of alkaline and alkaline earth metal borohydrides.
Gennari reported the observation of the [B12
H ]
12
species), which are equivalent and that each one couples with all eight of the
1
6
7
–14
F. C. lysis product of the remaining Y(BH4)3 in D O. The minor
2
2À
H
12
]
species using resonances at À15.2, À16.1 and À17.3 ppm suggest the existence
2 2À 19
À
12
of [B12H ]
or its derivatives [B12
H
12Àn(OH)
n
]
(n = 1 to 4),
a
EMPA, Swiss Federal Laboratories for Materials Science and Technology,
Hydrogen & Energy, 8600 D u¨ bendorf, Switzerland. E-mail: yigang.yan@empa.ch;
Fax: +41 58 765 40 22; Tel: +41 58 765 40 82
and those at À8.4, À22.7 and À36.2 ppm are possibly related to
2À 20
14
the formation of [B10H ] .
b
c
The applied H
decomposition pathway of metal borohydrides.
2
external pressure is known to influence the
EMPA, Swiss Federal Laboratories for Materials Science and Technology,
Functional Polymers, 8600 D u¨ bendorf, Switzerland
1
3,21
Therefore,
High Temperature Energy Materials Research Center, Korea Institute of Science
and Technology, 136-791 Seoul, Republic of Korea
the experiment was repeated at 5 and 10 bar external H2
pressures. The decomposition products were dissolved in D O
2
†
Electronic supplementary information (ESI) available: Experimental details of
11
1
and the corresponding B( H) NMR spectra are shown in
Fig. S1 (ESI†). Y(B is also identified as the main phase in
Y(BH decomposed at 300 1C under both 5 and 10 bar, and no
11
1
11
1
sample preparation and B and H NMR measurements, solution state B( H)
NMR spectra of Y(BH decomposed under 5 and 10 bar H and hydrogen
. See DOI: 10.1039/
3
8 3
H )
4
)
3
2
4 3
)
desorption performance of the as-synthesized Y
c3cc41184b
2 12 3
(B12H )
dissolved intermediate species are observed in the samples
5
234 Chem. Commun., 2013, 49, 5234--5236
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1
1
Fig. 2
(B12
and 81 Hz were determined for [B12
B NMR spectra (128.38 MHz) recorded in DMSO-d
6
of the as-prepared
1
1
Y
2
12 3
H ) : (a) H decoupled and (b) H coupled. Coupling constants of 124, 124
2
À
2À
À
H
12
]
, [B10
H
10
]
and [BH ] , respectively.
4
11
1
2 4 3
Fig. 1 (a) Solution state (D O) B( H) NMR spectra (128.38 MHz) of Y(BH )
1
11
decomposed at 240 to 350 1C; (b) two dimensional H– B HMQC NMR spectrum
of Y(BH decomposed at 255 1C.
4 3
)
1
1
1
decomposed at 400 1C. As a consequence, the above observations Fig. 3
indicate that regardless of H external pressures, Y(BH
2
B( H) NMR spectra (128.38 MHz) recorded in D O of (a) the as-
2
)
4 3
decom- prepared Y
2
(B12
H
12
)
3
and (b) Y
2
(B12
H
12
)
3
after heating to 450 1C for 5 h.
3 8 3
poses via the formation of Y(B H ) as the main intermediate
according to eqn (1).
2
0
2À
10
of À15.2 ppm, and the [B10
H
]
species shows two resonances
at À30.2 and À0.8 ppm, respectively. The observation of boric acid
Y(BH
4
)
3
- 1/3Y(B
3
H
8
)
3
+ 2/3YH
3
+ H
2
(1)
at 19.6 ppm is attributed to the hydrolysis of the remaining
1
1
À
Y(BH ) in D O. Combining the B NMR results recorded in
4 3 2
both DMSO-d6 and D O, it was found that Y (B H ) was
2 2 12 12 3
successfully formed according to eqn (2). Furthermore, the
NMR spectra (Fig. 2) suggest that the [B12
main phase in the as-synthesized Y (B H ) , as shown in
Table 1.
To examine the stability of Y (B H ) , the as-synthesized
Y (B H ) was heated up to 450 1C under a constant flow of 1 bar
H . Only limited hydrogen (1.4 wt%) is released in this process, as
The formation of the [B
3
H
8
]
species has also been observed in
1
8
the decomposition process of Mg(BH
4
)
2
.
It was believed that
12 as the main
1
1
À
B
the [B
3
H
8
]
species further convert to MgB12
H
2À
H ]
12
species is the
intermediate via a B–H condensation process. In contrast, only
2
À
11
2
12 12 3
traces of [B H ]
were detected using B NMR in the
1
2
12
decomposition process of Y(BH ) . To rule out the possibility
4
3
2
12 12 3
that Y
NMR due to stability or solubility reasons, we synthesized
by ball milling of Y(BH in B at 150 1C according
2 12 3
(B12H ) might not be detected using solution state
2
12 12 3
2
Y (B12H )
2 12 3
4
)
3
2 6
H
shown in Fig. S2 (ESI†), compared to the theoretical hydrogen
capacity of 6.0 wt% of Y (B12H ) . After heating to 450 1C for 5 h,
2 12 3
to eqn (2).
2Y(BH
4
)
3
+ 15B
2
H
6
- Y
2
(B12
H
12
)
3
+ 39H
2
(2)
1
1
The solution state B NMR spectra of the as-synthesized
recorded in DMSO-d and D O are shown in
Fig. 2 and 3, respectively. The major resonance at À15.3 ppm The remaining YH
Table
(B12
1
Relative amounts of the boron species in the as-synthesized
11
Y
2
(B12
H
12
)
3
6
2
Y
2
H
12
)
3
, based on solution state B NMR recorded in DMSO-d
6
and D
is not taken into account. B NMR chemical shifts are
reported relative to the 1 M B(OH) aqueous solution at 19.6 ppm
2
O.
11
3
with a coupling constant (J ) of 124 Hz (Fig. 2) agrees with
3
BH
2
À
8,20
those of the [B H ] species reported in the literature.
In
1
2
12
DMSO-d
6
D
2
O
addition, the two resonances at À28.5 ppm (J of 124 Hz) and
BH
11
11
Compound
d
B/ppm
Mol%
d
B/ppm
Mol%
2
À
À0.4 ppm assigned to [B10
H
10
]
as a side product are
at À35.3 ppm
2
BH of 81 Hz). In the B NMR spectrum recorded in D O
Y
Y
2
(B12
(B10
H
H
12
)
)
3
3
À15.3
À28.5, À0.4
À35.2
—
60 Æ 6
8 Æ 1
32 Æ 3
—
À15.2
À30.0, À0.8
—
42 Æ 4
7 Æ 1
—
51 Æ 5
observed, together with the remaining Y(BH )
4 3
1
1
2
10
(
J
Y(BH
4 3
)
2À
(Fig. 3a), the [B12
H
12
]
species shows a typical chemical shift Boric acid
19.6
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2
À
the signal of the [B12
12
H ]
species still belongs to the main Notes and references
11 1
resonance in the solution (D
Fig. 3b), indicating its relatively high stability. The increase in
the resonances at À12.8, À14.0, À17.1, À18.0, À19.3, À20.3 and
2
O) state B( H) NMR spectrum
1
2
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À
species, by the reaction of [BH
4
]
with B
2
H
6
has been reported
6
7
8
9
2
2–24
for LiBH
4
and NaBH
4
.
We demonstrated that this method
is also applicable to Y(BH ) in the presence of B H yielding
4
3
2
6
Y (B H ) , a species which is stable up to at least 450 1C and
2
12 12 3
readily soluble in water and DMSO-d . For the widely discussed
6
borohydrides such as LiBH
4
, NaBH
4 4 2 4 2
, Mg(BH ) and Ca(BH ) ,
2
À
the [B12
mediates in the decomposition process.
borohydrides, Y(BH shows a different decomposition route.
The absence of the dissolved B–H species in Y(BH decom-
posed at 350 1C (Fig. 1, top curve) implies that the stable
12
H ]
species have been identified as the main inter-
1
1
1
1
1
7
–14
In contrast to these
4 3
)
4 3
)
2À
[
B H ] species does not play a major role as an intermediate
12 12
in the decomposition process. Instead, yttrium octahydrotri-
borate Y(B was formed as the main intermediate.
The [B
favorable for the rehydrogenation of [BH
3
8 3
H )
À
H ]
8
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À
4
]
under moderate
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via the formation of
Y(B H ) is not enough to meet the on-board storage target.
1
1
8
conditions in the case of Mg(BH
4 2
) .
1
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hydrogen release amount of Y(BH )
4 3
1
3
8 3
2
À
Considering only traces of the stable [B H ] species formed
1
2
12
in the decomposition process (under an external pressure of
bar H ), it can be anticipated that Y(BH can be further
2
2
1
2
4 3
)
developed as a reversible hydrogen storage material under
moderate conditions.
Financial support by a grant from Switzerland through the
Swiss Contribution to the enlarged European Union and by the
Korean Research Council is gratefully acknowledged.
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5
236 Chem. Commun., 2013, 49, 5234--5236
This journal is c The Royal Society of Chemistry 2013