Hydrogen storage of metal nitride by a mechanochemical reaction

Article abstract of DOI:10.1039/b406392a

Metal imides (Li₂NH, CaNH), a metal amide (LiNH₂) and metal hydrides (LiH, CaH₂) were synthesized by ball milling of their respective metal nitrides (Li₃N, Ca₃N₂) in a H₂ atmosphere at 1 MPa and at room temperature.

Full text of DOI:10.1039/b406392a

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Hydrogen storage of metal nitride by a mechanochemical reaction
Yoshitsugu Kojima* and Yasuaki Kawai
Toyota Central R&D Labs., Inc., Nagakute-cho, Aichi-gun, Aichi 480-1192, Japan.
E-mail: kojima@mosk.tytlabs.co.jp; Fax: 181-561-63-6137; Tel: 181-561-63-5367
Received (in Cambridge, UK) 28th April 2004, Accepted 13th July 2004
First published as an Advance Article on the web 20th August 2004
Metal imides (Li2NH, CaNH), a metal amide (LiNH2) and
metal hydrides (LiH, CaH2) were synthesized by ball milling
of their respective metal nitrides (Li3N, Ca3N2) in a H2
atmosphere at 1 MPa and at room temperature.
Hydrogen (H2) can be stored in tanks as compressed1 or liquefied
H21 or by adsorption on carbon materials.1–6 It can also be stored
8,9
in H2-absorbing alloys7 or as a chemical hydride, such as NaBH4
or NaAlH4,10 as well as in an organic hydride, such as methyl-
cyclohexane or decalin.11 In recent years, attention has been given
to metal nitrides.12,13 Chen et al. reported that metal nitride (Li3N)
absorbed hydrogen at high temperature (468–528 K).12 For usable
H2 storage, H2 absorption/desorption at ambient temperature is
necessary and Ichikawa et al. demonstrated that a mixture of
LiNH2 and LiH prepared by a ball milling process lowered the
temperature of H2 desorption.13 Recently, it has been shown that
chemical reaction can occur at room temperature during mechano-
chemical processing.14,15
For the first time, we have demonstrated that metal imide, metal
amide and metal hydride can be synthesized using a mechano-
chemical reaction of a metal nitride (Li3N, Ca3N2) in a H2
atmosphere at a pressure of 1 MPa and at room temperature. The
starting materials for this mechanochemical reaction were lithium
nitride (Li3N) and calcium nitride (Ca3N2).{ Mechanochemical
reactions were performed in a planetary ball mill (Fritsch P-5). The
mill container (Cr–Mo steel pot with an internal volume of 300 mL)
was loaded with 5 g of metal nitride and 40 pieces of steel ball with
a diameter of 9.5 mm. The metal nitrides were milled at 400 rpm for
20 h in a H2 gas atmosphere at a pressure of 1 MPa at room
temperature (296 K).
Fig. 2 X-ray diffraction intensity curves of ball-milled Li3N in H2 and Li3N
together with the data of LiH (JCPDS file No. 09-0189), Li2NH (JCPDS
file No. 06-0417) and LiNH2 (JCPDS file No. 06-0418).
under vacuum (0.1 Pa) for 1 h.12 The N–H symmetric and
asymmetric stretching vibrations of LiNH2 occur at 3260 cm21 and
3315 cm21, respectively.17 Li2NH gives rise to two characteristic
bands at 3180 cm21 and 3250 cm21. We notice that the ball-milled
Li3N contains Li2NH and LiNH2 due to the presence of these
characteristic IR bands at 3180 cm21, 3260 cm21 and 3315 cm21
.
Thus, we believe that the ball-milled Li3N in H2 is partially
converted to Li2NH, LiNH2 and possibly LiH. The O–H stretching
vibration of LiOH is not in this region. This suggests that from the
XRD curve, the peak at about 33u is Li2O.
It has been reported that hydrogen storage in Li3N takes the
following reaction path.12
The products after ball milling were light brown, different from
the dark brown starting materials (Fig. 1). The X-ray diffraction
(XRD) intensity curves (Fig. 2) show the ball-milled Li3N is a
mixture which includes Li3N, Li2NH, LiNH2 and LiH.16 The
diffraction peaks of the ball-milled Li3N at 2h of 38.3u, 44.5u and
64.8u are raised from (111), (200) and (220) planes of LiH in which
its unit cell is cubic in shape.16 The unit cell of Li2NH is cubic in
shape16 and the unit cell of LiNH2 is tetragonal in shape.16 As the
XRD curve of the LiNH2 phase is quite similar to that of the
Li2NH phase, it is difficult to characterize the hydrogenating
reaction during ball milling only from the XRD curves. The
strongest peak of the ball milled Li3N in the XRD curve is at about
33u, which comes from either Li2O or LiOH. This contamination
might originate from small leaks in the pulverizing system. The
structure of the ball-milled Li3N was also investigated by infrared
spectroscopy. Fig. 3 shows the infrared absorption spectra of the
ball-milled Li3N, Li3N, LiNH2 (Sigma-Aldrich, purity: 95%) and
Li2NH. Li2NH was synthesized by decomposing LiNH2 at 673 K
Li3N 1 2H2 < LiNH2 1 2LiH
(1)
H2 of 10.4 wt% [2H2/Li3N 1 2H2] can be stored in this reaction.
The H2 content of the ball-milled Li3N determined using the
Hydrogen Analyzer was 5.0 wt% and half of the theoretical value.
The H2 storage involved the following two-step reaction path.12
Li3N 1 2H2 < Li2NH 1 LiH 1 H2 < LiNH2 1 2LiH (2)
We speculate that insufficient H2 was supplied in Li3N powder,
thus unreacted Li3N remained due to the decrease in the H2 content
during the ball milling.
Fig. 3 Infrared absorption spectra of ball-milled Li3N in H2 and Li3N
together with the data of Li2NH and LiNH2.
Fig. 1 Change of colour by mechanochemical reaction.
2 2 1 0
C h e m . C o m m u n . , 2 0 0 4 , 2 2 1 0 – 2 2 1 1
T h i s j o u r n a l i s ß T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

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process as follows. H2 is firstly absorbed on new particle surfaces
created by pulverization during initial milling. The absorbed H2
reacts with metal nitride to form metal imide, metal amide and
metal hydride under further high-energy ball impacts. Local tem-
perature rise, induced by ball impacts, may also contribute to the
hydrogenation reaction.
In conclusion, mixtures of metal imide, amide, hydride (Li2NH,
LiNH2, LiH or CaNH, CaH2) can be synthesized by a mechano-
chemical reaction of their respective metal nitride (Li3N, Ca3N2) in
a H2 atmosphere at room temperature. The H2 contents of the ball-
milled Li3N and Ca3N2 were 5.0 and 3.2 wt%, respectively.
The authors are greatly indebted to T. Noritake, Dr T. Hioki,
N. Ohba, M. Yamamoto and Dr G. J. Shafer of the Toyota
Central R&D Labs., Inc. for their help and discussions.
Fig. 4 X-ray diffraction intensity curves of ball-milled Ca3N2 in H2 and
Ca3N2 together with the data of CaH2 (JCPDS file No. 65-2384), Ca2NH
(JCPDS file No. 26-0308) and CaNH (JCPDS file No. 75-0430).
Notes and references
{ Li3N (Kojundo Chemical Laboratory Co., Ltd., Japan, molecular
weight: 34.82, density: 1.38 g cm23, purity: w 99%) and Ca3N2 (Sigma-
Aldrich, molecular weight: 148.25, density: 2.63 g cm23, purity: w 99%)
were used in this experiment. High purity H2 gas (w 99.99999%) was used
as the reaction atmosphere. A ThermoNicolet AVATAR 360 E.S.P. FT-
IR spectrometer with an ATR system was used for IR studies in the
wavenumber region from 3400 to 2900 cm21 in an inert atmosphere of N2.
X-ray diffraction intensity curves in an inert atmosphere (Ar) were
recorded with CuKa radiation (50 kV, 300 mA) filtered by
a
monochromator using Rigaku Rint-TTR. The apertures of the first,
second and third slits were 0.5, 0.5 and 0.15 mm, respectively. With the
Horiba EMGA-621 Hydrogen Analyzer, H2 contained in a specimen was
extracted by heating at 2273 K in an inert gas (Ar). After column
separation, H2 was quickly analysed with a thermal conductivity detector.
Fig. 5 Infrared absorption spectra of ball-milled Ca3N2 in H2 and Ca3N2.
The XRD intensity curves of ball-milled Ca3N2, CaH2 and
Ca2NH are shown in Fig. 4. Fig. 4 indicates that the ball-milled
Ca3N2 in H2 does not contain Ca3N2. We notice that the diffrac-
tion peaks of the ball-milled Ca3N2 at 2h of 27.9u, 30.1u, 31.8u, 41.5u
and 60.0u are raised from (011), (200), (111), (211), (202) planes of
CaH2, in which the unit cell of CaH2 is orthorhombic in shape.13
The unit cells of Ca2NH and CaNH are cubic in shape16 and their
XRD curves are similar to each other.
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The structure of the ball-milled Ca3N2 was also characterized by
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structure of CaNH generated from XRD indicates the presence of
an N–H bond, but the crystal structure generated by XRD shows
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milled Ca3N2 includes CaNH. The H2 content of the ball-milled
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mechanochemical reaction can be expressed by equation (3)
´
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Ca3N2 1 2H2 A CaH2 1 2CaNH
(3)
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where the theoretical H2 content obtained by equation (3) is
3.1 wt% [2H2/(CaH2 1 2CaNH)]. The H2 absorption during
milling suggests that the hydrogenation process is a two-step
C h e m . C o m m u n . , 2 0 0 4 , 2 2 1 0 – 2 2 1 1
2 2 1 1