Improvement of hydrogen desorption kinetics in the LiH-NH3 system by addition of KH

Article abstract of DOI:10.1039/c1cc15523g

It was found that, when a little amount of KH (5 mol%) was added in the LiH-NH₃ hydrogen storage system, the hydrogen desorption kinetics of this system at 100 °C was drastically improved by the KH "pseudo-catalytic" effect. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c1cc15523g

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Improvement of hydrogen desorption kinetics in the LiH–NH3 system by
addition of KHw
Yun-Lei Teng, Takayuki Ichikawa,* Hiroki Miyaoka and Yoshitsugu Kojima
Received 7th September 2011, Accepted 5th October 2011
DOI: 10.1039/c1cc15523g
It was found that, when a little amount of KH (5 mol%) was
added in the LiH–NH3 hydrogen storage system, the hydrogen
desorption kinetics of this system at 100 8C was drastically
improved by the KH ‘‘pseudo-catalytic’’ effect.
Recently, it was found that the reactivity of alkali metal
hydrides with NH3 is better in the order of Li o Na o K.2
Although the KH–NH3 system exhibits the best reactivity for
both the hydrogen desorption and absorption reactions, the
hydrogen capacity of the KH–NH3 system is much less than
that of the LiH–NH3 system. An improvement of the hydro-
gen storage kinetics for the LiH–NH3 system with higher H2
capacity is necessary. In addition, remarkable enhancement in
the kinetics of dehydrogenation was achieved by introducing
3 mol% KH into the Mg(NH2)2/2LiH system, in which the
KH plays important roles.7
The development of high performance hydrogen storage materials
is an important issue to establish suitable transportation
technologies for a clean and sustainable society. As compared
to approaches such as compression and liquefaction, solid-state
hydrogen storage, especially for light elements containing
systems, is able to realize high gravimetric and volumetric
hydrogen densities.1 Among them, ammonia (NH3) would be
recognized as one of the attractive hydrogen storage and
transportation materials because it has a high hydrogen
storage capacity of 17.8 mass% and is easily liquefied by
compression under 1.0 MPa pressure at room temperature.2–4
However, the practical application of NH3 itself as a hydrogen
storage material is limited, because a high temperature over
400 1C and a suitable catalyst are required to decompose NH3
into hydrogen (H2) and nitrogen (N2).5 Here, as promising
hydrogen storage materials, metal–N–H systems are studied
all over the world.6–8 In these systems, ammonia (NH3) is an
important intermediate for the elemental reaction described as
follows,8
In this study, the hydrogen desorption properties for the
reactions of the alkali metal hydrides (LiH, KH, and the
KH-added LiH) and NH3 systems (NH3: 0.5 MPa, NH3/MH =
1 mol/mol) at room temperature and 100 1C were systematically
investigated to design the system with better kinetic properties.
Fig. 1 shows the hydrogen generation profile of the reaction
between MH (KH, the KH-added LiH, and LiH) and NH3 at
room temperature and 100 1C. The reaction yields for KH, the
KH-added LiH, and LiH at room temperature for 60 minutes
were 89.9, 32.5, and 31.3%, respectively, indicating that the
reaction kinetics of KH was better than LiH. Actually, these
results are consistent with the previous reports.2 The KH-added
LiH + NH3 2 LiNH2 + H2.
(1)
This reaction can be regarded as a hydrogen storage system.2
The system has a high hydrogen capacity of 8.1 mass%Àa1nd a
suitable enthalpy change DH = 50 Æ 9 kJ mol H2 for
practical use.2 The hydrogen desorption reaction can proceed
even at room temperature due to hydrolysis-type exothermic
reaction. The reverse reaction from lithium amide (LiNH2)
into lithium hydride (LiH) and NH3 occurred below 300 1C by
removing NH3 under 0.5 MPa of H2 partial pressure flow
conditions.2 Thus, the lithium hydride (LiH) and NH3 system
should be recognized as a promising recyclable hydrogen
storage system.
1-3-1 Kagamiyama, Higashi-Hiroshima, 739-8530, Japan.
E-mail: tichi@hiroshima-u.ac.jp; Fax: +81-82-424-5744;
Tel: +81-82-424-5744
w Electronic supplementary information (ESI) available: X-Ray
diffraction profiles shown as Fig. S1. See DOI: 10.1039/c1cc15523g
Fig. 1 Hydrogen generation profile for reactions between MH (KH,
the KH-added LiH, and LiH) and NH3 at room temperature and 100 1C
for 60 minutes. Inset of the figure is the hydrogen generation profile for
reactions at 100 1C for 10, 30, and 60 minutes.
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 12227–12229 12227

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LiH showed a little faster reactivity than that of LiH at room
temperature due to superior reactivity of the addition of 5 mol%
KH. The reaction yields at 100 1C were 91.8%, 61.3%, and
45.4% for KH, the KH-added LiH, and LiH, respectively. In the
case of the KH-added LiH, if KH has no catalytic effect, the
expected yield should be calculated as 47.7% (=91.8% Â 0.05 +
45.4% Â 0.95). However, the reactivity was enhanced due to the
increase in temperature and it was about 30% and 13.6% higher
than that at room temperature and the calculated value, respec-
tively. The increase of yields of the KH-added LiH was significant,
compared with the variation of other samples.
reactions of the KH-added LiH with NH3, the diffraction peaks of
LiNH2 and KH were observed, however there were no diffraction
peaks of KNH2, indicating that the added KH was not consumed
during the reaction. On the other hand, compared with the just
ball-milled KH-added LiH (Fig. 2d), the ratio of diffraction
intensity of LiH to KH decreases largely after reaction with
NH3 at 100 1C for 60 minutes. This indicates that some of LiH
was consumed during the reaction to form amide.
In the previous reports, it has been proved that the reactivity
of KH with NH3 is much better than that of LiH.2 Thus, when
KH and LiH coexist in an NH3 atmosphere, KH would
preferentially react with NH3 by reaction (2).
The inset of Fig. 1 also shows the reaction yield of the three
kinds of hydride samples with NH3 at 100 1C as a function of
the reaction time. Although all the samples showed the
increases of the reaction yield with the time, the KH-added
LiH showed the larger increase than others. After the reaction
for 10 minutes, the reaction yield for the KH-added LiH was a
little more than that of LiH. However, the reaction yield of the
KH-added LiH reached more than 60%, which was closer to
that of KH even though the 95% sample corresponds to LiH.
From the results, it was confirmed that the reaction yield at
100 1C was clearly improved when a little amount of KH was
added in the LiH–NH3 hydrogen storage system.
KH + LiH + NH3 - KNH2 + LiH + H2
(2)
Here, the enthalpy change DH for the reaction of KNH2 and LÀiH1
to form LiNH2 and KH expressed by eqn (3) is À17.8 kJ mol H2
,
considering that the standard enthalpies of formation DH0 of the
compounds are À128.9 (KNH2), À90.5 (LiH), À57.7 (KH),
and À179.5 (LiNH2).9
KNH2 + LiH - KH + LiNH2
(3)
This DH is negative, suggesting that this reaction can easily proceed
in an exothermic manner. Actually, by ball-milling a mixture
of KNH2 and LiH with a 1/1 molar ratio for 2 hours under a
1.0 MPa H2 atmosphere, the diffraction peaks corresponding
to KH and LiNH2 were observed from the XRD measurement
(Fig. S1a, ESIw). The N–H stretching modes due to LiNH2 were
observed from the IR measurement (Fig. S2, ESIw). This result
indicates that reaction (3) proceeds under the mechanochemical
conditions. To confirm whether reaction (3) can proceed by
heating to 100 1C or not, the hand-milled KNH2 and LiH with
a 1/1 molar ratio was prepared and heated at 100 1C for
60 minutes. Fig. S1b and c (ESIw) show the XRD patterns after
hand milling and heat treatment. In the XRD pattern of the
hand-milled mixture, there are only the diffraction peaks of
KNH2 and LiH, suggesting that the reaction cannot proceed
by the hand-milling at the room temperature. After heating to
100 1C, KNH2 and LiH phases were almost changed to KH and
LiNH2 (see Fig. S1c, ESIw), indicating that the heating to at least
100 1C is necessary to induce reaction (3).
To understand the improving mechanism of hydrogen
desorption kinetics for the KH-added LiH–NH3 system, structural
changes due to the reaction were investigated. The XRD patterns
of the products formed in the reactions of MH (KH, the
KH-added LiH, and LiH) with NH3 at 100 1C for 60 minutes
are shown in Fig. 2. The XRD patterns of each hydride and amide
available in the database are shown as reference. After the reaction
with NH3, the diffraction peaks of the corresponding amide phase
were observed for both hydrides. For the product obtained by the
On the basis of the above experiments, the mechanism of the
improvement of hydrogen desorption kinetics for the
KH-added LiH–NH3 system was proposed as the following
reaction (eqn (4)).
KH + NH3 - KNH2 + H2
KNH2 + LiH - KH + LiNH2
1KH + 20LiH + 21NH3 - 1KNH2 + 20LiNH2 + 21H2
(4)
In the KH-added LiH–NH3 system, KH can immediately
react with NH3 to form KNH2 and release H2. Then, at
100 1C, the generated KNH2 reacts with LiH to form LiNH2
by the solid–solid reaction, resulting in recovery of KH.
Therefore, these two reactions can be regarded as cyclic
reaction, which would continue until LiH is exhausted. The
reason for the kinetic improvement of the hydrogen
desorption reaction on the LiH–NH3 system is that KH with
Fig. 2 XRD patterns of products by reactions of ball-milled
KH (a), LiH (b), and the KH-added LiH (c) with NH3 (0.5 MPa,
NH3/MH = 1 mol/mol) at 100 1C for 60 minutes and the XRD
patterns of the just ball-milled 5 mol% KH-added LiH (d).
c
12228 Chem. Commun., 2011, 47, 12227–12229
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KNH2 and release H2 under the heat-treatment conditions.
Then, under these conditions, the generated KNH2 reacts with
LiH to regenerate KH. These two reactions can be regarded as
cyclic reaction, which would continue until LiH is exhausted.
Therefore, a possible reason for the improvement of dehydro-
genation kinetics in the KH-added Li–Mg–N–H system is that
KH with superior reactivity to NH3 can accelerate the inter-
action between LiH and NH3 by the ‘‘pseudo-catalytic’’ effect
as mentioned above.
In conclusion, the hydrogen desorption kinetics of the
LiH–NH3 system was clearly improved by addition of a little
amount of KH and heating up to 100 1C. The reason for
the improvement is that KH with superior reactivity to NH3 can
play the role of a catalyst to immediately form LiNH2 as the
reaction product by solid–solid reaction indicated in eqn (3).
This work was supported by the project ‘‘Advanced Funda-
mental Research Project on Hydrogen Storage Materials’’ of
the New Energy and Industrial Technology Development
Organization (NEDO).
Notes and references
Fig. 3 XRD patterns of products by reactions of the 5 mol%
KH-added LiH with NH3 (0.5 MPa, NH3/MH = 1 mol/mol) under
different operations: (a) reacting at 100 1C for 60 minutes, (b) reacting
at 100 1C for 60 minutes and then at room temperature for 15 minutes.
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superior reactivity to NH3 can accelerate the formation of
LiNH2 by the ‘‘pseudo-catalytic’’ effect. To confirm this
mechanism, the following two experiments were performed.
After the reaction between the KH-added LiH and NH3 at
100 1C for 60 minutes, (1) the gas inside the vessel was
evacuated immediately, (2) gas evacuation was carried out
after decreasing the temperature to room temperature and
keeping for 15 minutes. The products obtained by two differ-
ent operations were identified by XRD. As shown in Fig. 3, for
the products in the first experiment, there were no diffraction
peaks corresponding to KNH2 (Fig. 3(a)). On the other hand,
obvious diffraction peaks of KNH2 were observed for
the products in the second experiment (Fig. 3(b)), where the
characteristic peaks were located at 30.2 and 28.81. These
results indicate that reaction (3) can proceed at 100 1C even
though it is stopped at room temperature. As a result, it is
expected that KNH2 generated by the reaction between KH
and NH3 remains at room temperature. Thus, the above
experimental facts are consistent with the proposed mechanism.
As shown in Fig. 3(b), new diffraction peaks at 12.81, 16.51,
and 33.41 are identified as the Li3K(NH2)4 ternary amide,
indicating that partially formed KNH2 reacted with LiNH2.
It is worthy to note that the ternary amide may also play
important roles as the intermediate state in the reaction
process, which should need further investigation.
By considering the present and previous studies, the following
remarks can be made on the mechanism about the improvement
of the dehydrogenation kinetics in the KH-added Li–Mg–N–H
system.7 It is well known that NH3 is an important intermedi-
ate for the dehydrogenation of the Li–Mg–N–H system.8 In
the KH-added Mg(NH2)2/2LiH system, KH can immediately
react with NH3 from the decomposition of Mg(NH2)2 to form
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 12227–12229 12229