Significantly improved dehydrogenation of LiBH4·NH3 assisted by Al2O3 nanoscaffolds
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Transcript of Significantly improved dehydrogenation of LiBH4·NH3 assisted by Al2O3 nanoscaffolds
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 5 8 1 7e5 8 2 4
Available online at w
journal homepage: www.elsevier .com/locate/he
Significantly improved dehydrogenation of LiBH4$NH3
assisted by Al2O3 nanoscaffolds
Xinyi Chen a, Wanyu Cai b, Yanhui Guo a, Xuebin Yu a,*aDepartment of Materials, Fudan University, 220 Handan Road, Shanghai 200433, PR Chinab Shaanxi Rock New Materials Co., Ltd., PR China
a r t i c l e i n f o
Article history:
Received 28 October 2011
Received in revised form
28 December 2011
Accepted 29 December 2011
Available online 24 January 2012
Keywords:
Hydrogen storage
Ammine lithium borohydride
Aluminum oxide nanoscaffolds
* Corresponding author. Tel.: þ86 21 5566458E-mail address: [email protected]
0360-3199/$ e see front matter Copyright ªdoi:10.1016/j.ijhydene.2011.12.162
a b s t r a c t
Enhanced dehydrogenation properties for ammine lithium borohydride (LiBH4$NH3) melt-
infiltrated into Al2O3 nanoscaffolds are reported. X-ray diffraction measurements verified
the formation of intermediate phase of amorphous state during heating the composites at
65 �C. Subsequently, it was revealed by combination of gravimetric and volumetric
measurements that a hydrogen desorption capacity of 12.8 wt.%, accounting for 91 mol%
of the total amount of the released gas at 230 �C, was achieved for the LiBH4$NH3/Al2O3
composite with a mass ratio of 1:4, while in the pristine LiBH4$NH3 merely trace amount
of H2 was detected at this temperature. Moreover, Fourier transform infrared spectra and11B nuclear magnetic resonance spectra were combined to clarify the facilitated recom-
bination of NH3 groups and BH�14 anions in the composites. As a consequence, the
mechanisms for the promoted dehydrogenation in the composites were reasonably
deduced as twofold, firstly, the nanosize effects of the loaded LiBH4$NH3 on the dehy-
drogenation properties in the presence of the oxide nanoscaffolds, which serve as the
highly dispersing support for the loaded materials, and assist the formation of the
amorphous phase during heating; secondly, the impact of Al2O3 nanoscaffolds on the
dehydrogenation of the loaded materials, via promotion of the recombination between
BH and NH groups.
Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
reserved.
1. Introduction have attracted great attention because of their high hydrogen
Despite tremendous efforts to resolve the present energy and
environmental problems by using hydrogen as energy carrier,
hydrogen storage is still the “bottleneck” that holds up the
realization of hydrogen economy [1]. BeNeH complex system,
consisting of hydrogen-enriched [NH] and [BH] groups, have
been regarded as promising candidates for on-board hydrogen
storage due to their high gravimetric and volumetric capac-
ities [2e5]. Recently, a new class of BeNeH system of ammine
metal borohydrides, M(BH4)m$nNH3 (M ¼ Li, Mg, Ca, Al, Y),
1.(X. Yu).2012, Hydrogen Energy P
capacity and low decomposition temperature [6e13].
However, some of these materials mainly desorb NH3 when
heating, resulting from theweak coordination strength of NH3
to the metal cation, e.g. ammine lithium borohydride
(LiBH4$NH3) [8,12,13]. To overcome this drawback, various
methods have been explored, including addition of several
metal chlorides or metal hydrides [12,13], to effectively
stabilize the ammonia and promote the recombination of the
NH/HB bond, thus promoting the dehydrogenation of
LiBH4$NH3.
ublications, LLC. Published by Elsevier Ltd. All rights reserved.
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i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 5 8 1 7e5 8 2 45818
Alternatively, minimizing the particle size of LiBH4$NH3
within nanoscales may also serve as effective means for
improving the dehydrogenation behavior, inspired by
a number of previous works [14e20]. A pioneering work con-
cerning nanoconfined ammonia borane in mesoporous silica
was published in 2005 [16], subsequently, rapid progress has
been made in various hydrogen storage systems (e.g., MgH2
[21], NaAlH4 [22,23], LiBH4 [24,25], and Mg(BH4) [26], etc.).
Among these research efforts many were directed to using
carbon related materials as nano-templates [27e30], since
they are the most common eligible candidates satisfying the
following requirements: light, allowing high loadings of the
materials, relatively inert toward the active materials.
Recently, we have also demonstrated that carbon nanotube is
an effective template for improving the dehydrogenation
behaviors of LiBH4$NH3, which reduces the onset dehydroge-
nation of the incorporated LiBH4$NH3 to temperatures below
100 �C [31].
On the other hand, starting from the early report of LiBH4
destabilized by SiO2 powder that showed dramatically
improved dehydrogenation behaviors [32], numerous transi-
tionmetal oxides have been demonstrated to effectively favor
the dehydrogenation of hydrides [33,34]. In light of these
successful demonstrations of both kinetic and thermody-
namic enhancements achieved via modification with oxides,
it is expected to select nanostructured oxides to confine
hydrides, on account of two main factors related to the
nanoengineering strategy: particle size effects and support
effects. As a successful example, the suppression of NH3
emission and the promoted dehydrogenation have been
realized in the LiBH4$NH3 confined in nanoporous silicon
dioxide [35].
In this study, confining LiBH4$NH3 into another oxide
nanoframeworks Al2O3 via melting infiltration is investigated
[12,13]. Our results may provide some insights into the
nanosize effect of the oxide nanoframeworks supported
hydrogen storage materials.
2. Experimental
LiBH4 (95%) was purchased from SigmaeAldrich andwas used
as received. Aluminum oxide (high surface area) was
purchased from Strem Chemicals, Inc., which has featured
the amorphous crystallite size with mean aggregate size of
5 mm. The purity of the ammonia used in the experiment is
approximately 99%.
2.1. Preparation of LiBH4$NH3 in ammonia atmosphere
LiBH4$NH3 was synthesized according to the method of
a previous report [13]. Briefly, a reactor was loaded with
a definite amount of LiBH4 and evacuated with a vacuum
pump. 1 atm NH3 flow was then introduced until the solid
LiBH4 became a viscous liquid due to the formation of
Li(NH3)xBH4 (x z 2). Then, the flow of NH3 was discontinued,
and the complex was exposed to vacuum to eliminate the
excess NH3 until x decreased back to 1 according to the weight
measurement. The white sticky solid obtained was confirmed
to be the LiBH4$NH3.
2.2. Preparation of the nanocomposites
As-prepared LiBH4$NH3 and Al2O3 nanopowders were manu-
ally ground to obtain a uniform mixture in a glove box under
an inert atmosphere (<5 ppm O2 and H2O). The mixed
composite was loaded into a glass bottle with a limited
capacity of 2 ml, and the bottle was sealed to prevent leakage
of NH3. Then, the sealed sample was heated and kept at 65 �Cfor 0.5 h. Themolten LiBH4$NH3 was infused into and surface-
deposited onto the Al2O3 nanoparticles, which led to the
formation of the nanocomposites (referred to as the
LiBH4$NH3/Al2O3 composites in the present study).
2.3. Structural characterization
Powder X-ray diffraction (powder XRD; Rigaku D/
Max2200VPC, Cu-Ka source, l ¼ 1.5418 A) measurements were
conducted to confirm the crystalline phase. Samples were
mounted on a Si single crystal in a glove box, and an amor-
phous polymer tape was used to cover the surface of the
powders to avoid oxidation during the XRDmeasurement. The
diffraction patterns were analyzed using the MDI Jade 5.0
software package (Materials Data Inc., Livermore, CA). The
BrunauereEmmetteTeller (BET) surface area, average pore
diameters, and N2 adsorption/desorption isotherms of the
Al2O3 and of the composites were tested by a TriStar 3000
surface area and porosimetry analyzer.
Fourier transform infrared (FT-IR) spectra of the samples
were recorded using a FT-IR spectrometer (FTIR-650). The
transmission mode was adopted.
Solid-state 11B nuclear magnetic resonance (NMR)
measurements were conducted with a Bruker Avance
300 MHz spectrometer using a Doty cross-polarization magic
angle spinning (CP-MAS) probe with no probe background. All
of those solid samples were spun at 5 kHz, using 4 mm ZrO2
rotors filled up in purified argon atmosphere glove boxes. The
NMR shifts (d) are reported in parts per million (ppm), exter-
nally referenced to H3BO3 at 0 ppm for 11B nuclei. A 0.55 ms
single-pulse excitationwas employed,with repetition times of
1.5 s.
2.4. Dehydrogenation property measurements
Gravimetric measurements of gas desorption were performed
by thermogravimetric analysis (TGA, TA Instruments STD 600)
connected to a mass spectrometer (MS, Hiden HPR20) using
a heating rate of 5 �C min�1 under 1 atm argon and a carrier
flow 25 rate of 200 cm3 min�1. Typical sample quantities were
5e10 mg.
3. Results and discussions
The XRD patterns of LiBH4$NH3/Al2O3 composites with mass
ratios of 1:2 and 1:4 are shown in Fig. 1, as compared with that
of the as-prepared LiBH4$NH3, whose characteristic peak
distribution agrees well with the reported data [12]. Charac-
teristic peaks assigned to LiBH4$NH3 are presented in both
mixtures after mere hand milling (Fig. 1b and c), but are more
broadened in the 1:4 ratio sample than in the 1:2 ratio one,
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Fig. 1 e XRD patterns for: LiBH4$NH3 (a); hand milled
LiBH4$NH3/Al2O3 (mass ratio of 1:2) (b), and the sample
after being heated to 65 �C (d); hand milled LiBH4$NH3/
Al2O3 (mass ratio of 1:4) (c), and the sample after being
heated to 65 �C (e). The peaks marked with “#” and “[” are
assigned to Al2O3 and LiBH4$NH3, respectively.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 5 8 1 7e5 8 2 4 5819
which indicates that hand milling leads to poorer crystallinity
in the 1:4 sample than in the 1:2 one. After keeping the
mixtures at 65 �C for 0.5 h, the diffraction peaks of LiBH4$NH3
are disappeared for both samples (Fig. 1d and e), demon-
strating either much refined crystallite size or amorphous
state of the loaded LiBH4$NH3.
BrunauereEmmetteTeller (BET) measurements were per-
formed to determine the incorporation of LiBH4$NH3 within
the Al2O3 nanoparticles for the 1:4 ratio sample. The pore size
distributions of the nanocomposites before and after heat
treatment at 65 �C and of the Al2O3 nanoparticles, determined
from the N2 desorption of the materials using the Bar-
retteJoynereHalenda (BJH) model are displayed in Fig. 2,
where N2 ad-/desorption curves of the hand-milled sample
Fig. 2 e N2 adsorptionedesorption isotherms and average
pore size distributions at 77.35 K for Al2O3 scaffolds, hand
milled LiBH4$NH3/Al2O3 with a mass ratio of 1:4 before and
after heat treatment at 65 �C for 1 h.
are displayed in the inset. The obtained BET specific surface
areas and related information for those materials are listed in
Table 1. In the case of the plain Al2O3, the N2 ad-desorption
isotherm curves (shown in the inset of Fig. 2) have the
profiles that are typical of porous adsorbents, the specific
surface area, pore volume and pore size are 1152.9 m2 g�1,
2.4 cm3 g�1, and 6.2 nm, respectively. However, a tremendous
reduction in the pore volume compared to the plain Al2O3, is
observed for the composite after hand milling, indicative of
part melting during milling for the loaded LiBH4$NH3, and the
resulting infiltration, which explains the broadened diffrac-
tion peaks observed in the XRD results (Fig. 1). However,
further reduction in the pore volume is absent after heating
the sample to 65 �C for half an hour, indicating that the
micromorphology and distribution of the molten LiBH4$NH3
remains substantially unaltered through heating. Given the
similar behaviors of the N2 desorption curves for the two
composites, heating the sample at 65 �C seems offer little help
to the impregnation, as compared to the CNTs enhanced
sample [31]. But nevertheless itmay be helpful for the uniform
dispersion of LiBH4$NH3 on the Al2O3 surface to promote the
interfacial contact. It should be mentioned that the pore
volume of the Al2O3 is reduced substantially after melt-
infiltration of LiBH4$NH3 whereas the average pore diameter
does not change remarkably. From Fig. 2 we can see that the
distribution of the pore diameter in the raw Al2O3 is in a wider
range of 2e9 nm. However, after melt-infiltration of
LiBH4$NH3, the range of pore diameters narrows to 3e7 nm.
Therefore, the disappearance of the poreswith pore diameters
below 3 nm,which results froma complete filling (or blockage)
of the pores by LiBH4$NH3, may contribute to the unchanged
average pore diameter of the loaded sample compared with
that of the raw Al2O3.
Decomposition properties of the LiBH4$NH3/Al2O3 samples
with different mass ratios (1:1, 1:2 and 1:4) from room
temperature to 280 �C were quantitatively examined using
ammonia eliminating volumetric measurements (AETPD) [36],
as shown in Fig. 3. While the pristine LiBH4$NH3 desorbs only
a trail amount ofH2 after beingheated to 280 �C, the LiBH4$NH3/
Al2O3 composites exhibit improved dehydrogenation behav-
iors, verified by the onset dehydrogenation temperatures at as
low as 65 �C.Moreover, the total hydrogen released amounts at
280 �C increase for the samples in the order of: the pristine
LiBH4$NH3 < LiBH4$NH3/Al2O3 (1:1) sample < LiBH4$NH3/Al2O3
(1:2) sample< LiBH4$NH3/Al2O3 (1:4) sample. It is noted that not
only is the low temperature dehydrogenation of the loaded
LiBH4$NH3 promoted by the Al2O3 nanoscaffolds, but the
extent of the early dehydrogenation also increases with the
mass fraction of Al2O3. There are two dehydrogenation events
that are clearly exhibited for the 1:2 and 1:1 samples, however,
a significant decline in the second stage is achieved for the 1:4
sample, to the extent that, there is no conspicuous second
stage of dehydrogenation at accelerated rate. It should be
mentioned that the pristine LiBH4$NH3 would lose NH3 in the
early heating process, which results in a large fraction of LiBH4
reformed in the complex during heating as demonstrated in
the literature [35]. Therefore, the second branch of the H2
evolution for the 1:1 and 1:2 composites in Fig. 3 can be
attributed to the reaction between the Al2O3 and the regen-
erated LiBH4, as confirmed by the dehydrogenation results of
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Table 1 e BrunauereEmmetteTeller (BET) measurement results for Al2O3 and LiBH4$NH3/Al2O3 composites with a massratio of 1:4.
Tested samples BET surface area/m2 g�1 Pore volume/cm3 g�1 Average pore diameter/nm
Al2O3 nanoparticles 1152.9517 2.438152 6.2054
Hand milled LiBH4$NH3/Al2O3 (1:4) (b) 342.4124 0.623001 5.9178
Sample (b) kept at 65 �C for 0.5 h 302.1835 0.581485 6.1210
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Al2O3/LiBH4 (Fig. 3d). Moreover, it is surmised that the occur-
rence of the reaction would be hindered with the increasing
addition of Al2O3, which can be explained by the fact thatmore
addition of nano-templates in the composites could more
effectively decrease the onset temperature of the reaction and
suppress the emission of NH3, due to the nanosize effect on
decreasing the regeneration of LiBH4.
Fig. 4 presents the synchronous TG-MS results for the
LiBH4$NH3/Al2O3 (mass ratio of 1:4) composite compared with
the pure LiBH4$NH3. No NH3 signal was detected by MS at
around 65 �C for the pristine and loaded LiBH4$NH3, indicating
that the loss of ammonia in the molten LiBH4$NH3 during the
sample preparation can be disregarded. Furthermore, no gas
impurities other than H2 and NH3 were detected throughout
the heating process for both samples, implying the availability
of the AETPD results above. There is only one step of decom-
position with its peak at around 115 �C for the LiBH4$NH3/
Al2O3 composite, duringwhich a large amount of H2 is evolved
accompanied with a small amount of NH3. In contrast, the
decomposition of LiBH4$NH3 proceeds in two separate stages
during the whole heating process, with nearly 40 wt.% mass
loss almost which is totally attributed to NH3 released at
temperatures below 250 �C, whereas the main hydrogen
evolution occurs only at temperatures above 400 �C. Further-more, the total mass loss of 23 wt.% at around 230 �C obtained
from the TG data for the 1:4 sample, in association with the
Fig. 3 e AETPD results for LiBH4$NH3/Al2O3 composites
with different mass ratios of 1:1 (c), 1:2 (b) and 1:4 (a). All
the samples were pre-heated at 65 �C for 0.5 h in argon
atmosphere. For comparison, the desorption profiles of
hand milled LiBH4/Al2O3(d) with a mass ratio of 1:4, and
that of LiBH4$NH3 (e), are also presented. The ramp rate is
5 �C minL1. The volume of desorbed gas is normalized to
the amount of pure hydrides.
AETPD results, indicates a hydrogen desorption capacity of
12.8 wt.% that represents 91 mol% of the total released gas
from the loaded LiBH4$NH3. While in the pristine LiBH4$NH3
merely trace amount of H2 was detected at this temperature.
The significantly suppressed NH3 release, as well as the shift
of the main dehydrogenation to lower temperature region for
the 1:4 sample, unambiguously point out the pronounced
modification effects of Al2O3 on the decomposing route of
LiBH4$NH3. The dehydrogenation properties significantly
surpass the properties reported for LiBH4$NH3 with hydrides
or chlorides as additives, and are similar to the one assisted by
nano-SiO2 [12,13,35], in terms of decreased dehydrogenation
temperature and increased hydrogen desorption capacity.
Since, presently, even assuming that all of the hydrogen
capacity in BH�14 anions is desorbed at temperatures below
230 �C, the calculated weight loss of 10.3 wt.% is still lower
than the experimental value of 12.8 wt.% at 230 �C as observed
in the TG-MSmeasurements. That is to say, the conversion of
NH3 to H2 desorption correlates with the dehydrogenation
enhancement. However, the underlyingmechanisms remains
unclear as to the roles served by Al2O3 in decreasing the
dehydrogenation temperature, shifting from above 400 �C in
the pristine LiBH4$NH3 to mere around 100 �C in the
composite, of which the understanding relies on more
powerful characterization method, such as XRD, FT-IR and
NMR spectra.
With the aim of elucidating the phase transformation
during dehydrogenation of the LiBH4$NH3/Al2O3 composites
(mass ratios of 1:2 and 1:4, respectively), XRD examinationwas
Fig. 4 e TG-MS results for pure LiBH4$NH3 and for the
LiBH4$NH3/Al2O3 (mass ratio of 1:4) composite. The
composite was heated from room temperature to 400 �Cwith a heating rate of 5 �CminL1, while the pure LiBH4$NH3
has been heated to 600 �C. The weight loss of the
composite is normalized to the amount of pure LiBH4$NH3.
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performed on the samples at different stages. It is noted that
characteristic peaks of LiBH4$NH3 remain intact for the pris-
tine LiBH4$NH3 after being heated to 100 �C, while the amor-
phous state is exhibited in the XRD patterns of the composites
at the same stage (Fig. 1d and e), which is a strong indication of
the influence of Al2O3 on the early decomposition of loaded
LiBH4$NH3 even at temperatures below 100 �C. On comparing
the XRD patterns of the post-heated composites and the pure
LiBH4$NH3 (Fig. 5aed), no parallel can be drawn between them
after heating to different stages. Pure LiBH4$NH3 mainly
desorbs NH3 and H2 in separate steps at a heating rate of
5 �C min�1 [12,13], resulting in the regeneration of part of the
LiBH4 at 400 �C (Fig. 5c). On the other hand, the loaded
LiBH4$NH3 in the 1:2 and 1:4 samples illustrates an alternative
reaction pathway upon heating, via the generation of the
nanosized or the amorphous phases, and the concomitantly
promoted dehydrogenation. Thus, the Al2O3 apparently
thwarts the transformation of the pristine LiBH4$NH3 to LiBH4,
as occurred in the case of the pristine LiBH4$NH3. However, the
broad diffraction features of the end products (Fig. 5e and f)
indicate that their crystalline order is poor, which frustrates
our intentions of deciphering the reaction path.
In contrast to the XRD analysis, which is limited to the
characterization of crystal materials, IR and NMR techniques
may provide powerful evidences on materials featuring
nanocrystalline or amorphous phases via characterization of
the vibrations of the chemical bonds. Fig. 6 presents the FT-IR
spectra of the LiBH4$NH3/Al2O3 compositeswithmass ratios of
1:4 and 1:2 in different states (Fig. 6a). As comparisons, the
spectra of as-prepared LiBH4$NH3 under the same conditions
are also presented (Fig. 6b). Obviously, the tridentate stretch-
ing modes of BeH bonds (2200e2400 cm�1) have almost dis-
appeared in the 1:4 sample after being heated to 250 �C ((iii) in
Fig. 6a), illustrating the substantial depletion of the BH�14
anions, which is also indicative of the nearly completion of the
Fig. 5 e XRD patterns for: pristine LiBH4$NH3 before (a) and
after being heated to 100 �C (b), 250 �C (c), and 400 �C (d),
respectively; hand milled LiBH4$NH3/Al2O3 (mass ratio of
1:2) being heated to 200 �C (e), hand milled LiBH4$NH3/
Al2O3 (mass ratio of 1:4) being heated to 200 �C (f). The
peaks marked with “#”, “[”, and “&”are assigned to Al2O3,
LiBH4$NH3 and LiBH4 respectively.
Fig. 6 e FT-IR spectra for (a) loaded LiBH4$NH3/Al2O3 sample
with a mass ratio of 1:4 at room temperature (i) and after
being heated to 100 �C (ii) and 250 �C (iii); and for the loaded
LiBH4$NH3/Al2O3 sample with a mass ratio of 1:2 at room
temperature (iv) and after being heated to 250 �C (v) and
400 �C (vi). (b) FT-IR spectra for the as-prepared LiBH4$NH3
at room temperature (i) and after being heated to 100 �C (ii),
250 �C (iii) and 400 �C (iv).
dehydrogenation at this stage, in accordance with the AETPD
and TG-MS results. However, there are still strong BeH bonds
present in the 1:2 sample after being heated to the same
temperature (v in Fig. 6a). In contrast, both the BeH stretching
and the BeH bending modes of the as-prepared LiBH4$NH3
remain intact during the whole heating process, signifying
that they are fairly insensitive to the heat treatment, which is
also consistent with the MS results and the literary reports
[12,13]. Thus, the comparisons of the FT-IR spectra clearly
indicate the pronounced effects of increasing addition of
Al2O3 on the low temperature consumption of the BH�14 anions
in the loaded LiBH4$NH3. Meanwhile, the broad BeN stretch-
ing modes centered at 1450 cm�1 is appeared in the spectra of
the 1:4 sample after being heated to 100 �C, indicating that the
loaded LiBH4$NH3 has undergone a phase transition to a BeN
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related substance. However, the same transformation does
not occur in the pristine LiBH4$NH3 upon heating, insteadwith
the early deprivation of the NH group as evidenced by the
elimination of NeH bonds at temperatures above 100 �C. Thisis due to the weak coordination bond N: / Liþ in LiBH4$NH3
[12,13], which easily breaks with the absence of Al2O3
templates to release NH3 before 250 �C, as is evident from the
MS results.
Solid-state 11B NMR measurements (Fig. 7) provided valu-
able hints in understanding the evolution of composite
components during the whole heating process. The two
overlapped strong broad line shapes in the range of þ20 to
�20 ppmcorresponding to a BN3 and/or BN2 environment for B
species [37,38], are presented in the reaction end products of
the composite. Whereas only a weak peak located at
d ¼ 12.8 ppm corresponding to the BeN environment [37,38] is
observed for the heated LiBH4$NH3, together with a much
conspicuous resonance at d ¼ �39.6 ppm that is assigned to
BH�14 anions [12]. This signals the weak combination between
[BH] and [NH] groups for the pristine LiBH4$NH3. Thus a brief
comparison of the 11B NMR results well illustrates the
enhanced combination of BH�14 anions and NH3 groups for the
Al2O3 assisted sample compared to the pristine LiBH4$NH3,
since the latter has merely trace amount of BeN composite
formed in the heated material at 300 �C, even in NH3 atmo-
sphere. Therefore, the fact that Al2O3 enhances combination
between [BH] and [NH] groups to form BeN polymer at low
temperatures, through covalently stabilizing the [NH] groups
that were weakly coordinated on Li cation in the pristine
LiBH4$NH3, is explicitly put forward by the NMR results.
In the present study, Al2O3 has been selected as the oxide
nanoframeworksmodel as the basis of further comprehensive
survey of varying oxides usages on supporting LiBH4$NH3. A
previous study on the composite of LiBH4$NH3 incorporated
into CNTs indicated that the H2 release peaked at tempera-
tures as high as 280 �C [31]. Thus the substantial dehydroge-
nation with a peak temperature at as low as 115 �C for the
Fig. 7 e 11B MAS NMR spectra of: LiBH4$NH3/Al2O3
composite with a mass ratio of 1:4 after being heated to
250 �C in argon atmosphere, and pristine LiBH4$NH3 after
being heated to 300 �C in 1 atm ammonia atmosphere. The
heating rate of both samples was 5 �C minL1.
LiBH4$NH3/Al2O3 composites in the present study relies on
more factors than just the effect of nanoconfinement. In
another previous study on LiBH4$NH3 incorporated into
nanoporous SiO2, major hydrogen evolution also occurred at
around 115 �C, on account of the promotion of the dehydro-
genation of LiBH4$NH3 by SiO2, thanks to the oxide enhanced
combination of LiBH4 and NH3 groups [35]. Thus the facilitated
recombination of BH�14 anions and the NH3 groups in the
loaded LiBH4$NH3 in the present study may also attribute to
the presence of Al2O3.
In the present study, the weak LieN coordination and
strong BeH bonds in the pristine LiBH4$NH3, which is
responsible for its deammoniation rather than dehydrogena-
tion during heating, has been modified by Al2O3, leading to
improved dehydrogenation behaviors of the composites.
Based upon the structural examinations above, it is surmised
that the underlying driving force of the promoted hydrogen
desorption at low temperatures can be attributed to the
recombination betweenNH3 and BH�14 anions, which occurs in
the early stage of hydrogen evolution, evidenced by the
presence of BeN bonds upon dehydrogenation at 100 �C in the
FT-IR spectra. The presence of BeN bonds not only leads to
the early combination of H(N) and H(B) to form H2, but also
stabilizes the NH groups via the establishment of BeN and/or
likely LieN bonds. In concluding, the combined observation of
amorphous phase formation and the BeN polymer formation
upon heating, in demonstration of the superior dehydroge-
nation route for the composites than for the pristine
LiBH4$NH3, are revealed to be twofold. First is the nanosize
effects of the loaded LiBH4$NH3, in the presence of the oxide
nanoscaffolds that serve as the highly dispersing support, and
contribute to the formation of amorphous phases during
heating. Moreover, Al2O3 also assists the combination of BH�14
anions and NH3 groups in the loaded LiBH4$NH3 through
formation of the BeN polymer in the early stage of heating,
and facilitates the recombination of the [BH] and [NH] groups
in the later heating process. As the main dehydrogenation of
the studied system is based on an exothermal combination of
BH and NH, direct reversibility of this system is thermody-
namically impossible. However, a chemical regeneration
route may be possible as demonstrated in AB system recently
[39]. Our future work will focus on this issue.
4. Conclusion
In the present study, LiBH4$NH3/Al2O3 composites have been
prepared by melt-infiltration technique, which demonstrate
significantly modified decomposition properties as compared
to the pristine LiBH4$NH3. It revealed that 2.5 mol H2 has been
evolved per mol LiBH4$NH3 by 230 �C, which accounts for
91mol%of the total amountof the releasedgasand12.8wt.%of
the loaded LiBH4$NH3, while in the pristine LiBH4$NH3 merely
trace amount of H2 has been detected at this temperature.
Furthermore, the dehydrogenation properties of the nano-
scaffolded LiBH4$NH3 significantly surpass that of the mate-
rials with hydrides or chlorides as additives, evidenced by the
decreased dehydrogenation temperature and increased
hydrogendesorption capacity. XRDmeasurements verified the
formation of amorphous state of the loaded materials after
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 5 8 1 7e5 8 2 4 5823
being heated at 65 �C. FT-IR and 11B NMR spectra were
combined to demonstrate the enhanced recombination of NH3
groups andBH�14 anions in the composites. The impact ofAl2O3
on thedehydrogenation canbeattributed to two factors: (1) the
effects of the oxide nanoscaffolds, which not only serve as the
highly dispersing support for the loaded LiBH4$NH3, but also
assist the formationof amorphousBeNphasesduringheating;
(2) the influenceof theAl2O3 on the combinationof theNH3and
BH�14 anion, through formation of the strong BeN bonds.
Acknowledgment
This work was partially supported by the Ministry of Science
and Technology of China (2010CB631302), the National
Natural Science Foundation of China (Grant No. 51071047), the
PhD Programs Foundation of Ministry of Education of China
(20090071110053) and Science and Technology Commission of
Shanghai Municipality (11JC1400700, 11520701100).
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