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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: yuxuebin@fudan.edu.cn

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.

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,

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

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

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 45820

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.

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 5821

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

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 45822

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

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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