NbN nanoparticles as additive for the high dehydrogenation properties of LiAlH4

Article abstract of DOI:10.1039/c3dt52313f

The effects of NbN nanoparticles synthesized via a simple urea glass route on the dehydrogenation properties of LiAlH₄ have been systematically investigated. The particle size of the as-synthesized NbN nanoparticles is determined to be about 10 nm. The surface configuration and dehydrogenation behaviors of the 2 mol% NbN-doped LiAlH₄ (2% NbN-LiAlH₄) system are also discussed. It is found that the 2% NbN-LiAlH₄ sample starts to decompose at about 95 °C and releases a total of 7.10 wt% hydrogen, which is 55 °C lower than that of as-milled LiAlH₄. The isothermal dehydrogenation kinetics shows that the 2% NbN-LiAlH₄ sample could release approximately 6.10 wt% hydrogen in 150 min at 130 °C, whereas as-received LiAlH₄ only releases about 0.63 wt% hydrogen under the same conditions, revealing that the enhancements arising upon adding NbN nanoparticles are almost 8-9 times that of as-milled LiAlH₄. The activation energy (E_a) is calculated to be 71.91 and 90.87 kJ mol^-1 for the first and second hydrogen desorption of the NbN-LiAlH₄ sample, a 38% and 32% reduction relative to as-received LiAlH₄, respectively. A detailed modeling study shows that the first dehydrogenation step can be sufficiently interpreted with the nucleation and growth in a one-dimensional model based on the first-order reaction. More interestingly, the dehydrogenated LiAlH₄ sample can recharge H₂ under a 5.5 MPa hydrogen pressure. An SEM image of the dehydrogenated 8% NbN-LiAlH₄ sample after HP-DSC under 5.5 MPa H ₂ shows that some nanorods appear. The Royal Society of Chemistry.

Full text of DOI:10.1039/c3dt52313f

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NbN nanoparticles as additive for the high
dehydrogenation properties of LiAlH4†
Cite this: Dalton Trans., 2014, 43,
1806
Li Li, Yanan Xu, Ying Wang, Yijing Wang,* Fangyuan Qiu, Cuihua An, Lifang Jiao and
Huatang Yuan
The effects of NbN nanoparticles synthesized via a simple “urea glass” route on the dehydrogenation pro-
perties of LiAlH4 have been systematically investigated. The particle size of the as-synthesized NbN nano-
particles is determined to be about 10 nm. The surface configuration and dehydrogenation behaviors of
the 2 mol% NbN-doped LiAlH4 (2% NbN–LiAlH4) system are also discussed. It is found that the 2% NbN–
LiAlH4 sample starts to decompose at about 95 °C and releases a total of 7.10 wt% hydrogen, which is
55 °C lower than that of as-milled LiAlH4. The isothermal dehydrogenation kinetics shows that the 2%
NbN–LiAlH4 sample could release approximately 6.10 wt% hydrogen in 150 min at 130 °C, whereas as-
received LiAlH4 only releases about 0.63 wt% hydrogen under the same conditions, revealing that the
enhancements arising upon adding NbN nanoparticles are almost 8–9 times that of as-milled LiAlH4. The
activation energy (Ea) is calculated to be 71.91 and 90.87 kJ mol−1 for the first and second hydrogen de-
sorption of the NbN–LiAlH4 sample, a 38% and 32% reduction relative to as-received LiAlH4, respectively.
A detailed modeling study shows that the first dehydrogenation step can be sufficiently interpreted with
the nucleation and growth in a one-dimensional model based on the first-order reaction. More interest-
ingly, the dehydrogenated LiAlH4 sample can recharge H2 under a 5.5 MPa hydrogen pressure. An SEM
image of the dehydrogenated 8% NbN–LiAlH4 sample after HP-DSC under 5.5 MPa H2 shows that some
nanorods appear.
Received 23rd August 2013,
Accepted 15th October 2013
DOI: 10.1039/c3dt52313f
www.rsc.org/dalton
candidates due to their high hydrogen storage capacity. In par-
ticular, the lightweight complex metal hydrides enhanced by
1. Introduction
Safe and efficient hydrogen storage technology is one of the transition metal (TM) compounds are promising solid-state
key technical barriers to the development of a kind of suitable hydrogen storage media for transportation applications since
on-board hydrogen storage material.1,2 At present, hydrogen the breakthrough discovery by Bogdanović et al. that Ti-doped
used for practical applications is stored by liquefaction and NaAlH4 can be reversibly stored and released.13
compression.3 However, the cryogenic technique and high
In recent years, lithium alanate (LiAlH4) is of particular
pressure storage of hydrogen require intensive energy and have interest for lightweight hydrogen storage due to its high hydro-
security issues. Hydrogen absorbent materials,4–6 such as zeo- gen storage capacity (7.9 wt% accessible) and relatively low
lites, carbon materials and metal/covalent organic frameworks decomposition temperature in the presence of suitable cata-
(MOFs/COFs), can store hydrogen at high pressures (∼9 MPa) lysts.9 In general, LiAlH4 releases hydrogen in the following
and low temperatures (∼77 K). In the past few decades, many stages (R1–R2) upon heating.
advanced materials, including conventional metal hydrides
LiAlH4ðsÞ ! LiAlH4ðlÞ
ðR1aÞ
ðR1bÞ
(MgH2),7 complex metal hydrides (MAlH4, MNH2 and MBH4,
M: alkali metal)8–10 and mixtures of these compounds,11,12
have been explored as potential on-board hydrogen storage
3LiAlH4ðlÞ ! Li3AlH6ðsÞ þ 2AlðsÞ þ 3H2ðgÞ
ð150–175 °CÞ ð5:3 wt% H2Þ
Li3AlH6ðsÞ ! 3LiHðsÞ þ AlðsÞ þ 1:5H2ðgÞ
ð180–220 °CÞ ð2:6 wt% H2Þ
ðR2Þ
Institute of New Energy Material Chemistry, Collaborative Innovation Center of
Chemical Science and Engineering (Tianjin), Key Laboratory of Advanced Energy
Materials Chemistry (MOE), Tianjin Key Lab of Metal and Molecule-based Material
However, a lot of obstacles, especially relatively low dehy-
drogenation kinetics and poor reversibility of LiAlH4, hinder
the practical applications for on-board hydrogen storage.
Numerous attempts have been made to overcome the barrier
Chemistry, Nankai University, Tianjin 300071, P. R. China.
E-mail: wangyj@nankai.edu.cn; Fax: +86 22 23503639; Tel: +86 22 23503639
†Electronic supplementary information (ESI) available. See DOI:
10.1039/c3dt52313f
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in recent years. Doping additives appears to effectively alter Kα radiation). The temperature programmed desorption (TPD)
the chemical character of LiAlH4, and therefore, improves the of H2 was performed using a home-made apparatus. About
dehydrogenation kinetics and reversibility. Interestingly, Nb- 70 mg of the sample was loaded in the reactor and heated in a
based compounds have gained considerable interest14,15 to 35 mL min−1 Ar flow at a ramping rate of 2 °C min−1. The
destabilize LiAlH4. For example, NbF5-doped LiAlH4 showed surface configuration of the samples was evaluated by scan-
approximately 7.0 wt% released at 188 °C.15 Isothermal de- ning electron microscopy (SEM, JEOL JSM-6700F Field Emis-
sorption results revealed an average dehydrogenation rate 5–6 sion), transmission electron microscopy (TEM) and high-
times faster than that of the as-received LiAlH4 sample. resolution transmission electron microscopy (HR-TEM) on a
Rafi-ud-din et al.14 found that Nb2O5-doped LiAlH4 showed an JEOL JEM-2100 TEM. Fourier transform infrared (FTIR) spectra
improvement in the onset dehydrogenation temperature, were collected at room temperature using an FTIR-650 spectro-
where approximately 6.9 wt% was released by 193 °C and the meter (Tianjin Gangdong) at a resolution of 4 cm−1. Hydrogen
sample could be rehydrogenated at 175 °C under 9.5 MPa. desorption was measured by an isothermal dehydrogenation
Meanwhile, previous investigations have revealed that nano- apparatus using a volumetric method. In the dehydrogenation
14,16–19
sized Nb2O5, Cr2O3, TiC, TiO2 and TiH2
particles can experiment, the sample was quickly heated to and kept at a
greatly enhance the dehydrogenation properties of LiAlH4. In given temperature. The weight loss percentage of the samples
our previous work, transition metal nitrides20,21 significantly was calculated according to the weight of the composite.
improved the hydrogen storage performances of LiAlH4 and Differential scanning calorimetry (DSC) measurements were
NaAlH4. Recently, NbN has become a material of interest in performed using a TA apparatus (DSC Q20P).
superconducting nanowire single-photon detectors (SNSPDs)22
with high thermal stability and chemical stability, and it can
also resist neutron irradiation. Thus, it is reasonable to deduce 2. Results and discussion
that NbN may be an efficient additive to catalyze the dehydro-
2.1. Characterization of the NbN nanoparticles
genation of LiAlH4.
The powder X-ray diffraction (XRD) pattern of the NbN nano-
In this work, considering the excellent catalytic effect of
particles synthesized via a simple “urea glass” route is illus-
Nb-based, transition metal nitrides and nanosized additives,
trated in Fig. 1a. All of the diffraction peaks could be perfectly
we first utilized NbN nanoparticles synthesized via a simple
indexed to cubic NbN (JCPDS card no. 65-5011; space group:
“urea glass” route as an additive and systematically investi-
gated the effects of NbN nanoparticles on the dehydrogenation
properties of LiAlH4.
ˉ
Fm3m), indicating the successful synthesis of NbN nanoparti-
cles. The lattice parameters are a = b = c = 4.390 Å, refined
from the diffraction pattern. Moreover, the initial particle size
of the NbN nanoparticles, estimated according to the Scherrer
equation, is determined to be about 10 nm.
2. Experimental section
Fig. 1b shows the field emission scanning electron
microscopy (FESEM) image of the as-prepared NbN. From the
NbN nanoparticles were prepared via a simple “urea glass”
highly magnified SEM image, it can be seen that the sample
route. In a typical synthesis, absolute ethyl alcohol was added
consists of uniform nanoparticles. However, agglomeration
to the NbCl5 powder. After that, a light pink but clear solution
may happen during the synthesis of NbN. The detailed
was obtained. Then, solid urea was slowly added to the solu-
tion with a specific urea : NbCl5 molar ratio (10 : 1). This dis-
persion was stirred until the urea completely solubilized and
the solution was completely clear. The gels were then put into
an oven and treated under an Ar flow at 800 °C for 3 h (heating
ramp: 1 °C min−1). In this case, a dark finely dispersed powder
was obtained.
LiAlH4 was purchased from Sigma-Aldrich Co. A mixture of
LiAlH4 and the as-synthesized NbN additive was prepared.
About 0.4 g of each mixture was charged into a stainless steel
vessel under an Ar atmosphere in a glove box and then
mechanically milled for 1 h at 450 rpm using a planetary ball
mill. After every 12 min of milling there was a 30 s pause and
the rotation was automatically reversed. For reference pur-
poses, an undoped LiAlH4 sample was also prepared in the
same manner. All the operations were carried out in a glove
box (Mikrouna Co., China) under a purified argon atmosphere
(H2O: <10 ppm; O2: <10 ppm).
Fig. 1 XRD pattern for the as-synthesized NbN sample (a), SEM image
The structural characteristics of the samples were identified
of the as-synthesized NbN sample (b) and TEM images of the as-
by powder X-ray diffraction (XRD, Rigaku D/Max PC2500, Cu synthesized NbN sample (c, d).
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structure of the NbN nanoparticles was further examined by after adding the NbN nanoparticles. The two endothermic
transmission electron microscopy (TEM)/high-resolution TEM events are attributed to the direct decomposition of solid
(HR-TEM). The TEM images (Fig. 1c and d) show that the size LiAlH4 to Li3AlH6 (R1b) and Li3AlH6 to Al/LiH (R2). It also indi-
of a single nanoparticle is approximately 10 nm. This is in cates that the NbN–LiAlH4 starts to decompose prior to its
good accord with the result according to the Scherrer equation. melting, viz., the melt of LiAlH4 is suppressed after adding the
The observed interplanar distances of 0.25 nm and 0.22 nm NbN nanoparticles. As shown in Fig. 2B, the onset dehydro-
correspond very well with the (111) and (200) crystal planes of genation temperatures for the NbN–LiAlH4 samples shift to
the NbN nanoparticles.
lower temperatures, indicating a remarkable decrease com-
pared with that of the as-received and as-milled LiAlH4. Fur-
thermore, the onset dehydrogenation temperatures (Tonset) and
dehydrogenation peak temperatures (Tpeak, (R1b) and (R2)) in
the DSC for the as-received LiAlH4, as-milled LiAlH4 and NbN–
LiAlH4 samples are listed in Table 1. However, an interesting
feature can be observed from Table 1. It is obvious from
Fig. S1 in the ESI† that the onset dehydrogenation temperature
2.2. Characterization of the NbN–LiAlH4 samples
2.2.1. Hydrogen desorption properties. In order to
compare the thermal decomposition performances of LiAlH4
with and without the NbN catalyst, the differential scanning
calorimetry (DSC) profiles of the as-received LiAlH4, as-milled
LiAlH4 and NbN–LiAlH4 samples within the temperature range
of 50–250 °C (heating ramp 2 °C min−1) are presented in
Fig. 2. As shown in Fig. 2A, there are two exothermic peaks
and two endothermic peaks in the DSC plot of the as-received
LiAlH4. The first exothermic peak at about 149 °C is attributed
to interactions between LiAlH4 and surface hydroxyl impuri-
ties. The first endothermic peak appears at 167.4 °C, corres-
ponding to the melting of LiAlH4 (R1a). Furthermore, the
second exothermic peak at 174.4 °C is attributed to the dehy-
drogenation step of liquid LiAlH4, viz., the first dehydrogena-
tion step (R1b). The second endothermic peak at 212.2 °C is
ascribed to the dehydrogenation of Li3AlH6 (R2). The obser-
vation of these four characteristic peaks is consistent with pre-
vious research.23,24 Compared with the as-received LiAlH4, the
dehydrogenation peak temperatures of the as-milled LiAlH4
slightly decrease by 7.8 and 14.8 °C for the first and second
dehydrogenation steps due to the activation by high energy
ball milling. Interestingly, in contrast with the DSC plots for
the as-received LiAlH4 and as-milled LiAlH4, those for the
NbN–LiAlH4 sample (Fig. 2B) are strikingly different, display-
ing two characteristic endothermic features in the DSC curves
(Tonset) and the first dehydrogenation peak temperature (Tpeak
,
(R1b)) reduce on increasing the amount of NbN nanoparticles.
When the NbN content increases to 4 mol%, the Tonset and
Tpeak (R1b) of 4% NbN–LiAlH4 are slightly elevated compared
to the 2% NbN–LiAlH4 sample. Then, they decrease when the
amount of NbN nanoparticles continues to increase, while the
second dehydrogenation peak temperature (Tpeak, (R2)) always
reduces on increasing the amount of NbN nanoparticles.
Fig. S2 in the ESI† shows the hydrogen desorption curves of
the 1% NbN–LiAlH4, 2% NbN–LiAlH4, 4% NbN–LiAlH4, 6%
NbN–LiAlH4 and 8% NbN–LiAlH4 samples after ball milling. It
is obvious from Fig. S2 in the ESI† that on increasing the NbN
doping amount, the dehydrogenation capacity decreases.
Therefore, through comprehensively considering the above
analyses, the 2% NbN–LiAlH4 sample exhibits optimal dehy-
drogenation properties, such as the onset dehydrogenation
temperature and the first dehydrogenation peak temperature.
In the following tests, we utilize 2 mol% as the optimal adding
content of the NbN nanopowders to analyze the catalytic
effects of the NbN nanopowders. For the 2% NbN–LiAlH4
sample, the first exothermic event begins to occur at about
95 °C, and the first exothermic peak appears at 138.4 °C.
In order to investigate the evolution of the NbN additive
before and after ball milling, the XRD patterns for the as-
milled LiAlH4 and x% NbN–LiAlH4 (x = 1, 2, 4, 6 and 8)
samples after ball-milling are presented in Fig. S3 in the ESI.†
It is clearly shown that there is both LiAlH4 and NbN in all the
Table 1 The onset dehydrogenation temperature and dehydrogenation
peak temperatures ((R1b) and (R2)) in the DSC for the as-received LiAlH4,
as-milled LiAlH4 and NbN–LiAlH4 samples
DSC/°C [heating ramp 2 °C min−1
]
Sample
Tonset
Tpeak [R1b]
Tpeak [R2]
As-received LiAlH4
As-milled LiAlH4
1% NbN–LiAlH4
2% NbN–LiAlH4
156.8
150.0
137.9
95.0
105.5
92.6
174.4
166.6
147.9
138.4
141.2
129.3
118.6
212.2
197.4
182.2
176.1
171.6
163.5
155.3
Fig. 2 DSC profiles of the as-received LiAlH4, as-milled LiAlH4, 1% 4% NbN–LiAlH4
6% NbN–LiAlH4
8% NbN–LiAlH4
NbN–LiAlH4, 2% NbN–LiAlH4, 4% NbN–LiAlH4, 6% NbN–LiAlH4 and 8%
NbN–LiAlH4 samples after ball milling (heating ramp 2 °C min−1).
90.3
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Fig. 3 Thermal desorption profiles of H2 for the as-milled LiAlH4 and 2%
NbN–LiAlH4 samples under an Ar atmosphere (heating ramp 2 °C min−1).
Fig. 4 FTIR spectra of the as-milled LiAlH4, 1% NbN–LiAlH4, 2% NbN–
LiAlH4, 4% NbN–LiAlH4, 6% NbN–LiAlH4 and 8% NbN–LiAlH4 samples
after ball milling.
NbN–LiAlH4 samples. The LiAlH4 diffraction peaks become
broader and weaker and the NbN diffraction peaks become
stronger and broader on increasing the NbN amount, indicat- LiAlH4 is stable under our high-energy ball milling conditions.
ing the increase in lattice strain and decrease of the grain size After doping NbN nanoparticles into the LiAlH4, it is clear that
of LiAlH4 and NbN. We certainly believe that it may be favour- there is an IR absorption peak at 1401 cm−1 25
,
revealing that a
able for the dehydrogenation of the NbN–LiAlH4 samples.
certain amount of LiAlH4 decomposes into Li3AlH6. However,
Fig. 3 shows the TPD curves of the as-milled LiAlH4 and 2% the Al–H stretching modes, ν3, of [AlH6]3− are weak, indicating
NbN–LiAlH4. It is obvious that the NbN nanoparticles signifi- the presence of a low amount of Li3AlH6, that is to say, the
cantly enhance the dehydrogenation properties of LiAlH4, decomposition of a low amount of LiAlH4 during ball milling.
resulting in a drastic reduction of the dehydrogenation temp- In addition, the intensity of the Al–H stretching mode, ν3, of
eratures ((R1b) and (R2)) in comparison with the as-milled [AlH6]3− increases on increasing the NbN content, suggesting
LiAlH4 sample. As shown in Fig. 3, the as-milled LiAlH4 starts that the decomposition reaction of LiAlH4 is enhanced. There-
to decompose at 150 °C. Approximately 3.5 and 2.8 wt% hydro- fore, we can conclude that the LiAlH4–NbN samples are highly
gen is released in the first and second dehydrogenation reac- reactive, and reaction (R1b) for 2% NbN–LiAlH4 may have par-
tions ((R1b) and (R2)). When doping NbN nanoparticles into tially occurred during the ball milling.
the LiAlH4, the onset dehydrogenation temperature (2% NbN–
2.2.2 Morphology of the samples. To investigate the micro-
LiAlH4) significantly decreases. The dehydrogenation tempera- structure of the NbN–LiAlH4 composites, scanning electron
ture of the first step (R1b) lowers to about 95.0 °C with a peak microscopy (SEM), transmission electron microscopy (TEM)
temperature at 138.4 °C. These features are almost consistent and energy dispersive spectrometer (EDS) elemental mappings
with the results of the DSC profiles (Fig. 2). The dehydrogena- (Fig. 5) were carried out. The SEM image of the as-received
tion temperature of the second step starts at ca. 169.1 °C, and LiAlH4 and NbN–LiAlH4 samples after ball milling are also
the dehydrogenation tends to reach equilibrium at ca. shown in Fig. S4 in the ESI.† The sample of the as-received
225.0 °C. The first dehydrogenation step (R1b) releases about LiAlH4 powder consists of large irregular-shaped particles
4.6 wt% hydrogen commencing at around 95.0 °C, with the (Fig. S4a, ESI†), and the particle size is larger than 5 μm. The
second dehydrogenation reaction (R2) exhibiting 2.5 wt% particle size of LiAlH4 significantly reduces after doping with
hydrogen. This reveals that NbN is an effective catalyst for the the NbN nanoparticles. It is also clearly demonstrated that on
dehydrogenation of commercial LiAlH4. However, it is obvious increasing the NbN content, there is an obvious size reduction
that there is a reduction in the dehydrogenation capacity after of the particle size of NbN–LiAlH4. For the 8% NbN–LiAlH4
doping with NbN nanoparticles. It may be the decomposition sample, the particle size is about 50–200 nm. The EDS elemen-
of LiAlH4 after adding the NbN catalyst and the 2% dilution tal mappings (Fig. 5b, c, e and f) indicate that the Nb and N
factor caused by the introduction of the NbN. This estimation elements of the NbN–LiAlH4 samples are uniformly distributed
can be further confirmed by the IR spectra shown in Fig. 4.
on the LiAlH4 matrix and do not suggest an obvious accumu-
For the as-milled LiAlH4, the distinct Al–H stretching lation in a particular location of the LiAlH4 matrix. In addition,
modes, ν3, of [AlH4]− observed at 1783 and 1640 cm−1 and the the Nb and N content of the 8% NbN–LiAlH4 sample is higher
Li–Al–H bending modes, ν4, of [AlH4]− observed at 886 and than the 2% NbN–LiAlH4 sample. This is consistent with the
710 cm−1 confirm that LiAlH4 does not decompose after ball- initial additive amount of NbN. Fig. 5g shows a TEM image of
milling. Thus, we can conclude that reaction (R1b) does not the 8% NbN–LiAlH4 sample. Fig. 5h shows the EDS spectra col-
occur for undoped LiAlH4 after ball milling and undoped lected at the positions indicated by the arrows in Fig. 5g. The
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Fig. 6 The thermal dehydrogenation characteristics of the as-milled
LiAlH4 sample at 130 °C and the 2% NbN–LiAlH4 sample at different
temperatures, respectively.
average dehydrogenation rate of 2% NbN–LiAlH4 is almost 8–9
times that of the as-milled LiAlH4.
In order to elucidate the catalytic mechanism of the NbN
nanoparticles, the XRD examinations for the as-milled LiAlH4
and the 2% NbN–LiAlH4 sample after dehydrogenation were
tested. The results are presented in Fig. 7. After dehydrogena-
tion of the 2% NbN–LiAlH4 sample at 90 °C, the LiAlH4 diffrac-
tion peaks nearly disappear and correspondingly, the Li3AlH6
and Al/LiH diffraction peaks appear. As the temperature is
elevated to 130 °C, LiAlH4 and Li3AlH6 are almost non-detect-
able, and the Al/LiH diffraction peaks are enhanced. In
addition, NbN can also be identified with relatively weak diff-
raction peaks in all the XRD profiles of the dehydrogenated
products. This phenomenon may be ascribed to the low
content of the additive and the nanocrystalline characteristics
of the NbN nanoparticles. It is also evident that no other Nb-
based phases are obtained from the XRD patterns in both the
as-milled and dehydrogenated samples. In addition, the
selected XRD patterns of the Al and the NbN are presented in
Fig. 7B. It is also clear that the main NbN diffraction peak
(111) remains without significant change to the position,
intensity and broadening during the whole synthesis and
dehydrogenation processes. From the above results, it can be
inferred that no reaction occurs between the NbN nanoparti-
cles and LiAlH4 during the whole synthesis and dehydrogena-
tion processes.
Fig. 5 SEM and SEM-EDS elemental mapping images of the NbN–
LiAlH4 samples: (a–c) 2% NbN–LiAlH4 sample, (d–f) 8% NbN–LiAlH4
sample, (g) TEM image of the 8% NbN–LiAlH4 sample; (h) EDS image of
the 8% NbN–LiAlH4 sample.
EDS result of the areas indicated by the arrows in Fig. 5h con-
firms that the particles consist of Nb, N, and Al, suggesting that
the black specks on the surface of LiAlH4 in Fig. 5g correspond
to NbN nanoparticles. That is to say, the NbN nanoparticles dis-
tribute on the LiAlH4 matrix, which confirms that surface modi-
fications have occurred after adding the NbN nanoparticles.
2.3. Analysis of the thermal decomposition characteristics
2.3.1 Thermal behaviours. To evaluate the catalytic effects
of the NbN nanoparticles on the dehydrogenation kinetics of
LiAlH4, the kinetic behaviors of the 2% NbN–LiAlH4 system
were investigated at different temperatures (Fig. 6). Obviously,
the addition of 2 mol% NbN dramatically decreases the temp-
erature of the hydrogen desorption from LiAlH4. The results
show that the 2% NbN–LiAlH4 composite can release 2.05,
4.57, 5.57 and 6.64 wt% hydrogen in 240 min at 100, 110, 120
and 130 °C, respectively. The 2% NbN–LiAlH4 sample releases
6.10 wt% of hydrogen in 150 min at 130 °C, which is 5.47 wt%
higher than that of the as-milled LiAlH4 sample under the
same conditions. The hydrogen release capacity of 2% NbN–
2.3.2 Kinetic properties. The apparent activation energy
(Ea) is determined with the Kissinger’s approach, as described
below, to quantitatively elucidate the kinetic improvement of
the 2% NbN–LiAlH4 sample.26
2
d lnðβ=Tm
dð1=TmÞ
Þ
Ea
R
¼
ðR4Þ
Here, β, Tm and R are the heating rate, the absolute temp-
LiAlH4 is around 7.10 wt%. The above results show that the erature for the maximum desorption rate and the gas constant,
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Fig. 8 (A) DSC profiles of 2% NbN–LiAlH4 (0.5, 2 and 5 °C min−1).
(B) Kissinger plots for the first and second dehydrogenation steps of 2%
NbN–LiAlH4 (the black Kissinger plots: the first-step dehydrogenation;
the red Kissinger plots: the second-step dehydrogenation).
Table 2 Comparison of Ea for LiAlH4 doped with different catalysts
calculated by the Kissinger method
First stage
Second stage
Catalyst
[kJ mol−1
]
[kJ mol−1
]
Reference
Free catalyst
K2TiF6
TiN
TiCl3·1/3AlC13
Nano NbN
116.2
78.2
87.46
89
133.0
90.8
92.87
103
30
30
31
32
71.91
90.87
Present work
Fig. 7 (A) XRD patterns for the as-milled LiAlH4 and 2% NbN–LiAlH4
samples after dehydrogenation. (B) XRD patterns of the enlarged part of
Al (111) and NbN (111) under different temperatures.
respective analytical equation can be obtained. For the analysis
of the nucleation and growth processes, a classical equation,
referred to as the JMA equation, was derived simultaneously by
Avarmi and Johnson and Mehl.29 The expression is:
respectively. In this work, Tm was obtained using DSC
measurements (Fig. 8A) at several heating ramps (0.5, 2 and
5 °C min−1). Fig. 8B plots the dependence of ln(β/Tm2) on 1/Tm.
Clearly, a good linear relationship exists between ln(β/Tm2) and
1/Tm. By fitting, the value of Ea is calculated to be 71.91 and
90.87 kJ mol−1 for the first and second hydrogen desorption of
the NbN–LiAlH4 sample, which are 44.29 kJ mol−1 and
42.13 kJ mol−1 lower than those of the as-received LiAlH4
(116.2 kJ mol−1 and 133.0 kJ mol−1) and previous reports.27,28
For comparison, the activation energies related to the
decomposition reactions of pure and doped LiAlH4 are recapi-
tulated in Table 2. The comparison implies the superiority of
NbN nanoparticles on improving the dehydrogenation kinetics
of commercial LiAlH4.
ln½ꢀlnð1 ꢀ αðtÞÞꢁ ¼ n lnðtÞ þ n lnðkÞ
ðR5Þ
in which α(t) is the fraction already reacted at time t, n is the
Avrami exponent and k is the rate constant.
The dehydrogenation rate, k, can be obtained by fitting the
first dehydrogenation data with the JMA equation at 110 °C,
120 °C and 130 °C, the results of which are shown in Table 3.
It is found that the values of the dehydrogenation rates, k, of
these NbN-doped LiAlH4 are enlarged on increasing the
dehydrogenation temperatures, which is coincident with the
Table 3 Kinetic data for the first dehydrogenation step of 2 mol%
To further understand the dehydrogenation kinetics mech-
anism, the activation energy (Ea) for the first dehydrogenation
step of the NbN-doped LiAlH4 system is also determined by
the JMA model and Arrhenius equation after fitting the first
dehydrogenation data. The kinetic rate-limiting step can be
deduced if a good fit of the experimental data with the
NbN-doped LiAlH4 (R1b)
Temperature (°C)
−lnk
k (×10−3 s−1
)
110
120
130
8.837
8.090
7.704
1.453
3.066
4.510
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dehydrogenation kinetics curves in Fig. 6. In addition, the line-
arity of plotting ln[−ln(1 − α(t))] against ln(t) is achieved for
3. Conclusion
each curve at 110 °C, 120 °C and 130 °C (Fig. S5, ESI†), indicat- In summary, the dehydrogenation properties of LiAlH4 are
ing that the first dehydrogenation step can be sufficiently remarkably improved by doping with novel NbN nanoparticles.
interpreted with the nucleation and growth in a one-dimen- It can be concluded that NbN nanoparticles are a promising
sional model based on the first-order reaction. Furthermore, additive for the as-received LiAlH4 in that it produces a signifi-
the calculated Ea value for NbN-doped LiAlH4 is shown in cantly reduced dehydrogenation temperature and dramatically
typical Arrhenius plots in Fig. S6 in the ESI.† As expected, the enhanced dehydrogenation kinetics. The onset desorption
activation energy value of Ea for the first dehydrogenation step temperature of the 2% NbN–LiAlH4 sample is lowered to about
is 72.63 kJ mol−1, which is coincident with the activation 95 °C with a little decrease in the dehydrogenation capacity,
energy according to the DSC data in Fig. 8.
where approximately 7.10 wt% is released at 130 °C. The
isothermal dehydrogenation results at 130 °C reveal that the
kinetics of the dehydrogenation of 2% NbN–LiAlH4 is
almost 8–9 times that of the as-milled LiAlH4. The Ea values of
the NbN–LiAlH4 sample decrease to 71.91 and 90.87 kJ mol−1
after doping with NbN nanoparticles, resulting in decline rates
of 38% and 32%. More interestingly, the dehydrogenated
LiAlH4 sample can recharge H2 under a 5.5 MPa hydrogen
pressure. From the above analyses, it is reasonable to conclude
that NbN nanoparticles are an effective additive for signifi-
cantly enhancing the dehydrogenation properties of as-
received LiAlH4.
2.4 Analysis of the absorption behavior
The rehydriding characteristics of the dehydrogenated NbN–
LiAlH4 sample was preliminarily investigated to explore the
reversibility of the system. Fig. S7 in the ESI† shows the IR
spectra of the 8% NbN–LiAlH4 sample after a high pressure-
differential scanning calorimetry (HP-DSC) cycle. Interestingly,
the distinct active infrared vibration of the Al–H stretching
modes of Li3AlH6 appears at 1400 cm−1, confirming that dehy-
drogenated LiAlH4 can recharge H2 under a 5.5 MPa hydrogen
pressure. Meanwhile, we only observe a very weak peak at
about 1400 cm−1 for dehydrogenated LiAlH4 under a 4.5 MPa
hydrogen pressure, suggesting that it also may recharge H2
under a 4.5 MPa hydrogen pressure. That is, a higher hydrogen
pressure is necessary to further improve the hydrogenation
properties of the NbN–LiAlH4 system. This phenomenon
further confirms that the NbN nanoparticles play a critical role
in the reversibility of the NbN–LiAlH4 system. More interest-
ingly, an SEM image of the dehydrogenated 8% NbN–LiAlH4
sample after HP-DSC under 5.5 MPa H2 is shown in Fig. 9,
indicating that some nanorods appear. The lengths and the
diameters of those nanorods are about 50–100 nm and
20–30 nm, respectively. Further investigations on the dehydro-
Acknowledgements
This work was financially supported by the MOST projects
(2010CB631303, 2012AA051901), the NSFC (51071087,
51171083), the 111 Project (B12015), the Research Fund for the
Doctoral Program of Higher Education of China
(20120031110001) and the Natural Science Foundation of
Tianjin (11JCYBJC07700).
Notes and references
genation/hydrogenation
properties
and
microstructure
1 L. Schlapbach and A. Züttel, Nature, 2001, 414, 353.
2 B. Sakintuna, F. Darkrimb and M. Hirscherc,
Int. J. Hydrogen Energy, 2007, 32, 1121.
changes of the NbN–LiAlH4 system are underway.
3 U. Eberle, M. Felderhoff and F. Schuth, Angew. Chem., Int.
Ed., 2009, 48, 6608.
4 S. H. Jhung, J. W. Yoon, J. S. Lee and J. S. Chang,
Chem.–Eur. J., 2007, 13, 6502.
5 S. S. Han, H. Furukawa, O. M. Yaghi and W. A. Goddard III,
J. Am. Chem. Soc., 2008, 130, 11580.
6 S. B. Kalidindi, H. Oh, M. Hirscher, D. Esken, C. Wiktor,
S. Turner, G. V. Tendeloo and R. A. Fischer, Chem.–Eur. J.,
2012, 18, 10848.
7 P. Sirsch, F. N. Che, J. T. Titah and G. S. McGrady, Chem.–
Eur. J., 2012, 18, 9476.
8 R. J. Less, H. R. Simmonds, S. B. J. Dane and D. S. Wright,
Dalton Trans., 2013, 42, 6337.
9 S.-I. Orimo, Y. Nakamori, J. R. Eliseo, A. Züttel and
C. M. Jensen, Chem. Rev., 2007, 107, 4111.
10 Y. F. Zhou, Y. F. Liu, Y. Zhang, M. X. Gao and H. G. Pan,
Dalton Trans., 2012, 41, 10980.
Fig. 9 SEM image of the 8% NbN–LiAlH4 sample after HP-DSC under
5.5 MPa H2.
1812 | Dalton Trans., 2014, 43, 1806–1813
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View Article Online
Dalton Transactions
Paper
11 Y. T. Li, F. Fang, Y. Song, Y. S. Li, D. L. Sun, S. Y. Zheng, 22 M. Ziegler, L. Fritzsch, J. Day, S. Linzen, S. Anders,
L. A. Bendersky, Q. A. Zhang, L. Z. Ouyang and M. Zhu,
Dalton Trans., 2013, 42, 1810.
J. Toussaint and H.-G. Meyer, Supercond. Sci. Technol.,
2013, 26, 025008.
12 Y. F. Liu, J. J. Hu, G. T. Wu, Z. T. Xiong and P. Chen, J. Phys. 23 F. Q. Zhai, P. Li, A. Z. Sun, S. Wu, Q. Wan, W. N. Zhang,
Chem. C, 2007, 111, 19161.
13 B. Bogdanović and M. Schwickardi, J. Alloys Compd., 1997,
253–254, 1.
14 Rafi-ud-din, X. H. Qu, P. Li, Z. Lin and A. Mashkoor,
J. Phys. Chem. C, 2011, 115, 13088.
Y. L. Li, L. Q. Cui and X. H. Qu, J. Phys. Chem. C, 2012, 116,
11939.
24 J. R. A. Fernandez, F. Aguey-Zinsou, M. Elsaesser, X. Z. Ma,
M. Dornheim, T. Klassen and R. Bormann, Int. J. Hydrogen
Energy, 2007, 32, 1033.
15 M. Ismail, Y. Zhao, X. B. Yu and S. X. Dou, Int. J. Hydrogen 25 J. Block and A. P. Gray, Inorg. Chem., 1965, 4, 304.
Energy, 2010, 35, 2361. 26 H. E. Kissinger, Anal. Chem., 1957, 29, 1702.
16 Rafi-ud-din, Z. Lin, P. Li and X. H. Qu, J. Alloys Compd, 27 L. Li, F. Y. Qiu, Y. J. Wang, Y. N. Xu, C. H. An, G. Liu,
2010, 508, 119.
L. F. Jiao and H. T. Yuan, Int. J. Hydrogen Energy, 2013, 38,
3695.
28 A. Andreasen, J. Alloys Compd., 2006, 419, 40.
17 M. Ismail, Y. Zhao, X. B. Yu, I. P. Nevirkovets and S. X. Dou,
Int. J. Hydrogen Energy, 2011, 36, 8327.
18 S.-S. Liu, Z.-B. Li, C.-L. Jiao, X.-L. Si, L.-N. Yang, J. Zhang, 29 C. Giordano, C. Erpen, W. Yao, B. Milke and M. Antonietti,
H.-Y. Zhou, F.-L. Huang, Z. Gabelica, C. Schick, L.-X. Sun
and F. Xu, Int. J. Hydrogen Energy, 2013, 38, 2770.
19 B. Bogdanovic, M. Felderhoff, S. Kaskel, A. Pommerin,
K. Schlichte and F. Schuth, Adv. Mater., 2003, 15, 1012.
Chem. Mater., 2009, 21, 5136.
30 Z. B. Li, S. S. Liu, X. L. Si, J. Zhang, C. L. Jiao, S. Wang,
S. Liu, Y.-J. Zou, L. X. Sun and F. Xu, Int. J. Hydrogen Energy,
2012, 37, 3261.
20 L. Li, F. Y. Qiu, Y. J. Wang, G. Liu, Y. N. Xu, C. H. An, 31 L. Li, F. Y. Qiu, Y. J. Wang, Y. N. Xu, C. H. An, G. Liu,
Y. P. Wang, L. F. Jiao and H. T. Yuan, J. Mater. Chem., 2012,
22, 13782.
L. F. Jiao and H. T. Yuan, Int. J. Hydrogen Energy, 2013, 38,
3695.
21 L. Li, F. Y. Qiu, Y. J. Wang, Y. N. Xu, C. H. An, G. Liu, L. F. Jiao 32 J. Chen, N. Kuriyama, Q. Xu, H. T. Takeshita and T. Sakai,
and H. T. Yuan, Int. J. Hydrogen Energy, 2013, 38, 3695.
J. Phys. Chem. B, 2001, 105, 11214.
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 1806–1813 | 1813