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The interactions of Li3FeN2 with H2 and NH3
Peikun Wang a,b, Jianping Guo a,**, Zhitao Xiong a, Guotao Wu a,
Junhu Wang a, Ping Chen a,c,d,*
a
Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China
University of Chinese Academy of Sciences, Beijing 100049, China
c
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian
116023, China
d
Collaborative Innovation Center of Chemistry for Energy Materials, Dalian 116023, China
b
article info
abstract
Article history:
Received 13 March 2016
Li3FeN2 was synthesized by calcinating equivalent Li3N and Fe under N2 atmosphere and
€ ssbauer
700 C. The interactions of Li3FeN2 with H2 and NH3 were studied by using XRD, Mo
Received in revised form
spectroscopy, temperature-programmed reaction and isothermal (de)hydrogenation. Our
10 May 2016
results show that the hydrogenation of Li3FeN2 gives rise to Fe metal rather than FeN
Accepted 11 May 2016
following the reaction of Li3FeN2 þ 2.5H2 ¼ 2LiNH2 þ LiH þ Fe. Whereas heating the mixture
Available online 13 June 2016
of 2LiNH2/LiH/Fe results in a two-step dehydrogenation that partially regenerates Li3FeN2.
Keywords:
and an amorphous FeN-containing compound. At higher temperatures Li3FeN2 out-
Li3FeN2
performs Fe2N in catalyzing ammonia decomposition.
Furthermore, Li3FeN2 can react easily with ammonia at room temperature, forming LiNH2
Hydrogen storage
€ ssbauer spectroscopy
Mo
© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Ammonia decomposition
Introduction
Hydrogen has been regarded as a clean and carbon-neutral
energy carrier [1,2], and considered to be one of the most
promising alternative candidates to fossil fuels [3e5]. However, the storage of H2 physically in liquid or gaseous
form is very difficult and energy-intensive due to the requirements of low temperature or high pressure [6]. Extensive and intensive research efforts have been devoted to
the development of hydrogen storage solid state materials,
including metal hydrides [7,8], carbon-based sorbents [9,10],
metal organic frameworks [11,12], complex and chemical
hydrides [13]. Among these, lithium nitride (Li3N) is a
promising hydrogen storage material, because it can in
principle reversibly store up to 11.4 wt% hydrogen according
to the reactions 1e3 [14]:
Li3N þ H2 ¼ Li2NH þ LiH DH ¼ 116 kJ/mol
(1)
Li2NH þ H2 ¼ LiNH2þLiH DH ¼ 45 kJ/mol
(2)
Overall reaction: Li3N þ 2H2 ¼ LiNH2 þ 2LiH DH ¼ 161 kJ/mol
(3)
Thermodynamic analyses show that the hydrogen
desorption from Li2NH and LiH (the reverse for reaction 1) is
highly endothermic with an enthalpy change of 116 kJ/mol-H2.
Therefore, the operation temperature at 1 bar of equilibrium
H2 desorption pressure should be above 450 C, which is too
* Corresponding author. Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China.
** Corresponding author.
E-mail address: [email protected] (P. Chen).
http://dx.doi.org/10.1016/j.ijhydene.2016.05.108
0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
14172
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 e n e r g y 4 1 ( 2 0 1 6 ) 1 4 1 7 1 e1 4 1 7 7
high for practical applications. Hence, only ca. 6.5 wt% of
hydrogen can be reversibly stored theoretically under moderate condition following the reaction 2.
To improve the hydrogen storage properties of Li3N, some
other elements such as Mg, Ca, Al, etc. have been incorporated
to the LieNeH system, leading to the development of LieMgeNeH [15e18], LieCaeNeH [15,19] and LieAleNeH [15,20]
systems. Xiong et al. studied the hydrogen storage properties of Li3AlN2 and found that Li3AlN2 can reversibly store
about 5.2 wt% H2, with ca. 30 kJ/mol-H2 drop in the heat of
reaction as compared with the neat Li3N. Such a thermodynamic improvement by the addition of Al is due to the relatively stable dehydrogenated product e Li3AlN2. The concept
of compositional alteration of materials, by stabilizing the
dehydrogenated products or reactants, has been demonstrated to be an effective approach to optimize the thermodynamic properties of hydrogen storage materials [21].
Recently, Langmi and He et al. reported that the addition
of transition metal elements (Fe [22], V and Mn [23]) can
also affect the thermodynamic properties of the LieNeH
by forming a ternary nitride of Li3FeN2, Li7VN4 and Li7MnN4,
respectively. Specifically, iron shares several common
properties with Al, such as the valence state (þ3), similar
mononitride forms (FeN and AlN), and ternary nitride
forms (Li3FeN2 and Li3AlN2). Hence, a similar equation for
hydrogen storage in Li3FeN2 as that of Li3AlN2 can be expected as shown in reactions 4 and 5. Thermodynamic analyses show that the enthalpies of the reaction 4 and 5 are
50.1 kJ/mol-H2 and 76 kJ/mol-H2, respectively. Both are lower
than the enthalpy of LieNeH system, which is 81 kJ/mol-H2.
Deduced from the reaction 5, Li3FeN2 can reversibly store
3.8 wt% H2.
AlN þ LiNH2 þ 2LiH ¼ Li3AlN2 þ 2H2 DH ¼ 100.2 kJ/mol
(4)
FeN þ LiNH2 þ 2LiH ¼ Li3FeN2 þ 2H2 DH ¼ 152 kJ/mol
(5)
Li3FeN2 has been extensively investigated as a promising
anode material in lithium secondary batteries [24,25]. It was
first synthesized by Frankenburger et al. by reacting Li3N
powder with elemental iron in a nitrogen atmosphere [26]. Its
crystal structure was determined by Gudat et al. as a fluorite
superstructure (space group Ibam) [27]. Magnetic ordering
phenomenon of Li3FeN2 was observed below 10 K, the calculated magnetic moment of 1.7 mb indicates a low spin state of
the Fe3þ ion [27]. Besides, Li3FeN2 has been proposed as a potential half-metallic material for possible application in
spintronics and spin injection by electronic structure calculations [28].
However, Langmi et al. results show that Li3FeN2 can uptake 2.7 wt.% hydrogen, of which only about 1.5 wt.% is
reversible. The presence of excess Li3N in the as-prepared
Li3FeN2 likely affected the hydrogen storage properties of
Li3FeN2. In addition, the reaction pathway as well as the
chemical states of iron during the hydrogenation and dehydrogenation process are worthy of investigation. More
recently, a highly active composite catalyst system made of
Li2NH and transition metal (nitride) for NH3 decomposition
was developed by us [29]. The formation of Li3FeN2 intermediate phase was observed in the catalytic process. Thus, the
objective of this contribution is to investigate the interaction
of Li3FeN2 with hydrogen and ammonia.
Herein, Li3FeN2 was prepared by calcinating equivalent
Li3N and iron under N2 atmosphere, and its yield was determined by combining the weight gain and quantitative analysis
€ ssbauer spectra. XRD, FTIR and Mo
€ ssbauer spectroscope
of Mo
were employed to study the phase and compositional changes
in the hydrogenation, dehydrogenation and ammoniation
processes. Our results show that the hydrogenation of Li3FeN2
gives rise to Fe metal rather than FeN following the reaction of
Li3FeN2 þ 2.5H2 ¼ 2LiNH2 þ LiH þ Fe. Whereas heating the
mixture of 2LiNH2/LiH/Fe results in a two-step dehydrogenation that partially regenerates Li3FeN2. Furthermore, Li3FeN2
can react easily with ammonia at room temperature, forming
LiNH2 and an amorphous FeN-containing compound. At
higher temperatures Li3FeN2 outperforms Fe2N in catalyzing
ammonia decomposition.
Experimental
Li3N (Alfa Aesar, 99.4%) was used as received. Iron powder
(Sigma Aldrich, 99.9%, 200 mesh) was pretreated by using a
Retsch PM400 type planetary ball mill at 150 rpm for 20 h (the
mass ratio of ball to sample is about 500:1) to reduce the
particle size.
Li3N and the pretreated iron powder (total mass: 700 mg)
were mixed in a molar ratio of 1:1 in an agate mortar for
10 min, and then transferred to a stainless steel reactor.
After evacuation, the reactor was filled with N2 of 150
psi, heated to and held at 700 C for 4 h. Hydrogenation experiments were performed on an automatic Sieverts-type
apparatus (Advanced Materials Co.). Li3FeN2 was pretreated
by using a high energy shaker mill (SPEX 8000) for 15 min
before testing. Under a 10 bar hydrogen pressure, the Li3FeN2
sample was heated from 20 C to 200 C at a ramping rate of
0.5 C/min, and then held at 200 C until no observable
pressure drop.
Temperature-programmed decomposition (TPD) of the
hydrogenated Li3FeN2 product was conducted on a homemade reactor-Mass Spectrometer (MS) combined system.
The sample was purged with Ar and heated to 450 C at a
ramping rate of 2 C/min.
Ammoniation of Li3FeN2 was performed in a homemade
stainless steel reactor. 30 mg of pretreated Li3FeN2 sample was
loaded in the reactor. The reactor was then filled with liquid
NH3 at room temperature. The gaseous product was analyzed
by MS and the solid products were collected and characterized
€ ssbauer spectroscope.
by XRD and Mo
Ammonia decomposition experiment was performed on a
continuous flow quartz reactor. 30 mg pretreated Li3FeN2
sample was put in the central section of the reactor and was
tested under a flow of pure NH3 (gas flow rate ¼ 30 ml/min),
and the temperature was raised at a ramping rate of 2 K min1.
The gas composition was analyzed using an on-line gas
chromatograph.
X-ray powder diffraction (XRD) patterns were recorded on a
PANalytical X'pert diffractometer with Cu Ka radiation at a
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 e n e r g y 4 1 ( 2 0 1 6 ) 1 4 1 7 1 e1 4 1 7 7
setting of 40 kV and 40 mA. Fourier Transform Infrared
Spectrometry (FTIR) measurements were performed on a
Perkin Elmer FTIR-3000 unit in DRIFT mode. Data was accumulated for 32 scans and the scanning resolution was 4 cm1.
57
€ ssbauer spectra of the samples were recorded using
Fe Mo
a Topologic 500 A spectrometer and a proportional counter at
room temperature. 57Co(Rh) moving in a constant acceleration
mode was used as radioactive source. All of the spectral analyses were conducted assuming a Lorentzian line shape for
€ ssbauer
computer folding and fitting. Accordingly, 57Fe Mo
spectral parameters such as the isomer shift, the electric
quadrupole splitting, the full linewidth at half maximum and
the relative spectral area of different components on the absorption patterns were obtained. The values were quoted
relative to a-Fe at room temperature.
All the materials handlings were performed in a glove box
filled with purified argon to keep a low water vapor concentration (lower than 0.1 ppm) and a low oxygen concentration
€ ssbauer char(about 1 ppm). Samples for XRD, FTIR and Mo
acterizations were loaded and sealed in the glove box and
then transferred outside to avoid contamination by moisture
and air.
Li3N þ Fe þ 1/2N2 ¼ Li3FeN2
14173
(6)
There are two kinds of ternary nitrides made of Li and Fe,
one is Li3FeN2 and the other is Li3xFexN (0 < x < 1). We, at the
present stage, cannot exclude the presence of amorphous
€ ssbauer spectrum is
Li3xFexN in the sample, because its Mo
very similar to that of Li3FeN2 [30]. After ball milling using a
SPEX-8000 mill under Ar atmosphere, Fe can be detected both
€ ssbauer spectrum
from the XRD pattern (Fig. 1b) and Mo
(Fig. 2b), which may be originated from the decomposition of
Li3xFexN. Furthermore, the content of Li3FeN2 after ball
milling treatment is about 83.9% from the fitting result of
€ ssbauer spectrum, which is consistent with the yield of
Mo
Li3FeN2 estimated from the sample weight gain.
The thermal decomposition behavior of Li3FeN2 was characterized by TPD method. The Ar-TPD profile shows that N2 is
Results and discussion
Synthesis of Li3FeN2
Li3FeN2 was prepared following a solid-state route [27].
Equivalent Li3N and metallic Fe were heated gradually under
150 psi N2. As shown in Fig. S1, the pressure increased with
temperature until a turning point at about 590 C evidencing
the beginning of the reaction 6. After held at 700 C for 1 h, the
pressure did not change anymore, which means the reaction
has ended. The X-ray powder diffraction (XRD) pattern of the
as-prepared sample was shown in Fig. 1a. All of the diffraction
€ ssbauer
peaks can be assigned to the Li3FeN2 phase. The Mo
spectrum of the sample (Fig. 2a) also confirms that the major
Fe species is Li3FeN2. Calculated from the sample weight gain,
the yield of Li3FeN2 is ca. 83%.
Fig. 1 e XRD patterns of as-synthesized Li3FeN2 (a), ball
milled Li3FeN2 (b), and Li3FeN2 samples containing
0.86 wt.% (c), 2.35 wt.% (d) and 4.56 wt.% (e) hydrogen.
€ ssbauer spectra of as-synthesized Li3FeN2 (a),
Fig. 2 e Mo
ball milled Li3FeN2 (b), and Li3FeN2 samples containing
0.86 wt.% (c), 2.35 wt.% (d) hydrogen. The dotted lines
represent the measured data, and solid lines represent the
fitted data. The green line corresponds to Li3FeN2, the blue
line is Fe, and red line for the sum of the fitted lines. (For
interpretation of the references to color in this figure
legend, the reader is referred to the web version of this
article.)
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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 e n e r g y 4 1 ( 2 0 1 6 ) 1 4 1 7 1 e1 4 1 7 7
released from the sample at temperatures above 500 C
(Fig. S2), and meanwhile, Fe and Li3N peaks can be observed
from the XRD pattern of the product after the Ar-TPD (Fig. S3),
which indicates the reaction 6 is reversible at high temperatures, and higher N2 pressure is needed to prepare Li3FeN2 of
high yield.
Thermodynamic analysis of reaction between Li3FeN2 and
H2
The ideal scenario of Li3FeN2 as a candidate material for
hydrogen storage is that the hydrogenation and dehydrogenation can be fully reversible via the following reaction:
Li3FeN2 þ 2H2 ¼ LiNH2 þ 2LiH þ FeN
(7)
During this process, Fe should be bonded with N as AlN to
allow a hydrogen storage capacity of 3.8 wt%. From thermodynamic viewpoint, however, FeN is much less stable
than AlN, which can be inferred from their standard molar
enthalpies of formation (DfH298K(FeN) ¼ 36 kJ/mol [31],
DfH298K(AlN) ¼ 311 kJ/mol). Under H2 atmosphere, FeN will
be reduced to metallic Fe (the reaction 8), which is thermodynamically allowed (DH ¼ 10 kJ/mol).
FeN þ 1.5H2 ¼ Fe þ NH3
(8)
NH3 generated from the reaction 8 can react easily
with LiH to form LiNH2, so in a closed reacting system, the
hydrogenation of Li3FeN2 should proceed in the following
path:
Li3FeN2 þ 2.5H2 ¼ 2LiNH2 þ LiH þ Fe
(9)
Through the reaction 9, 4.8 wt% H2 can be absorbed by
Li3FeN2. It should be highlighted that different from the reaction 7, the amount of Hdþ (bonded with N) is not equal to Hd
(bonded with Li) in the reaction 9. The increase in hydrogen
absorption is due to the redox of Fe.
Experimental studies of the hydrogenation of Li3FeN2
To improve the kinetics of hydrogenation of Li3FeN2, the asprepared Li3FeN2 sample was pretreated by ball milling to
reduce the particle size, which is evidenced by the increase
in peak width of Li3FeN2 phase (Fig. 1b). The hydrogenation
process was then recorded by heating the sample under a
hydrogen pressure of 10 bar and a ramping rate of 0.5 C/min.
As can be seen from Fig. 3, the ball milled Li3FeN2 can absorb
H2 at a temperature as low as 75 C, and the total amount of
absorbed H2 is ca. 4.56 wt %, which is close to the theoretical
value derived from the reaction 9 and is significantly greater
than the previously reported data [22]. To obtain more information about the phase change during the hydrogenation,
the products at different hydrogenation stages (the amount
of absorbed H2 is ca. 0.86, 2.35 and 4.56 wt %, respectively)
were collected and characterized by using XRD, FTIR and
€ ssbauer techniques. XRD measurements (Fig. 1c, d and e)
Mo
Fig. 3 e Temperature-programmed volumetric H2
absorption curve of the ball milled Li3FeN2.
disclose that LiNH2 and Fe were formed upon hydrogenation
and the intensities of their diffraction peaks increase gradually with the degree of hydrogenation. The Li3FeN2 phase,
on the contrary, gradually weakens. Langmi et al. also
observed the formation of Fe and LiNH2 in the hydrogenated
sample [22]. No other Fe-containing phase can be observed.
€ ssbauer spectra (Fig. 2c, d and e) also show that there are
Mo
only two detectable Fe species, i.e., Li3FeN2 and Fe, at
different hydrogenation stages. The contents of Li3FeN2 and
Fe at different stages of hydrogenation are listed in Table 1.
Furthermore, the formation of LiNH2 rather than Li2NH at the
beginning of hydrogenation was confirmed by FTIR (Fig. 4b, c
and d) characterization.
€ ssbauer analyses of
According to the quantitative Mo
the contents of Fe and Li3FeN2, we can calculate the
amount of hydrogen absorbed by Li3FeN2 at different hydrogenation stages, which are shown in Table 1. These
values are very close to the experimental data. Combining
the abovementioned experimental results and our knowledge about the hydrogenation of Li3N, we can conclude
that the reaction 9 take place during the hydrogenation of
Li3FeN2.
The reverse reaction 9 was investigated by means of ArTPD. As shown in Fig. 5, H2-release begins at a very low
temperature (ca. 50 C), and two peaks can be observed at 226
and 328 C, respectively. The first peak should be ascribed
mainly to the reaction of equivalent LiNH2 and LiH forming
Li2NH and H2, which has been intensively studied before. The
second higheretemperature H2 desorption event is associated with the co-productions of NH3 and N2 at temperatures
above 250 C where the decomposition of excess LiNH2
remained in the sample to Li2NH and NH3 should take place.
It is very likely that the formations of N2 and H2 are via the
decomposition of NH3, which will be discussed later. The
XRD pattern of the post-TPD sample shows the co-existences
of Li3FeN2 and Fe metal, as shown in Fig. 6, so the reaction (9)
is partially reversible under the condition applied in the
present study.
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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 e n e r g y 4 1 ( 2 0 1 6 ) 1 4 1 7 1 e1 4 1 7 7
Table 1 e Quantitative analyses of the contents of Li3FeN2 and Fe in the samples collected at different stages of
hydrogenation.
Content of Li3FeN2
Content of Fe
Theoretical H2 contenta
Experimental H2 content
a
Raw material
After milling
Stage 1
Stage 2
Stage 3
~100%
~0%
e
e
83.9%
16.1%
0
0
79.1%
20.9%
0.86 wt.%
0.86 wt.%
46.8%
53.2%
2.44 wt.%
2.35 wt.%
e
e
4.7 wt.%
4.56 wt.%
The amount of absorbed hydrogen calculated from the reaction 7 by referring to the contents of Fe and Li3FeN2.
Fig. 4 e FTIR spectra of as-synthesized Li3FeN2 (a), and
Li3FeN2 samples containing 0.86 wt.% (b), 2.35 wt.% (c) and
4.56 wt.% (d) hydrogen.
Fig. 6 e XRD pattern of the post-TPD sample.
results in the formation of LiNH2 and an amorphous FeeN
containing species [29]. In the present study, we employed
€ ssbauer technique to determine the chemical state and
Mo
coordination environment of Fe species. As shown in Fig. 7
Fig. 5 e Ar-TPD-MS profile of the hydrogenated Li3FeN2
sample.
Interaction of Li3FeN2 and NH3
The TPD (Fig. 5) and XRD (Fig. 6) measurements show that
LiNH2 decomposes to H2 and N2 in the presence of Fe and
Li2NH and gives rise to Li3FeN2. It is interesting to figure out
whether Li3FeN2 can react with ammonia. Therefore, the
reaction of Li3FeN2 and NH3 in a closed reactor was studied.
Previous XRD and X-ray absorption fine structure spectroscopy (XAFS) results show that the ammoniation of Li3FeN2
€ ssbauer spectrum of ammoniated Li3FeN2 with
Fig. 7 e Mo
the dotted line representing the measured data and solid
lines representing the fitted data. The green line
corresponds to Li3FeN2, the blue line is Fe, the magenta line
corresponds to FeN, the cyan line corresponds to the
unknown Fe species and the red line is the sum of the
fitted lines. (For interpretation of the references to color in
this figure legend, the reader is referred to the web version
of this article.)
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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 e n e r g y 4 1 ( 2 0 1 6 ) 1 4 1 7 1 e1 4 1 7 7
and Table S1, besides unreacted Li3FeN2 and Fe metal, FeN
and another unknown Fe species can be observed. This unknown Fe species may be ascribed to a metastable FeeNHx or
LieFeeNHx (x ¼ 1, 2) species, which easily decomposes to
NH3 at relatively lower temperatures. The presence of FeN
can be also nicely correlated with the formation of N2, which
is due to the decomposition of FeN to form Fe2N, at temperatures above 80 C as shown in the TPD profile of the
ammoniated Li3FeN2 [29].
Our previous results indicate that Li3FeN2 was a key intermediate in the catalytic decomposition of NH3 by the
Fe2NeLi2NH composite catalyst. The proposed chemical cycle
can be written as follows,
3Li2NH þ Fe2N ¼ 2Li3FeN2 þ 3/2H2
equivalent H2 to form 2 equivalent LiNH2, one LiH and one Fe.
Li3FeN2 can be partially regenerated upon dehydrogenation.
Li3FeN2 reacts with ammonia easily and can serve as a
catalyst precursor to produce H2 from catalytic decomposition of NH3.
Acknowledgments
The authors acknowledge financial support from Project of
National Science Fund for Distinguished Young Scholars
(51225206), National Natural Science Foundation of China
(51472237, 21273229) and the CAS-Helmholtz JRG Project
(XMXX201200020861).
(10)
Appendix A. Supplementary data
2Li3FeN2 þ NH3 ¼ 3Li2NH þ Fe2N þ 1/2N2
(11)
We thus use a ball milled Li3FeN2 sample as the catalyst
precursor to test its role in ammonia decomposition. Fig. 8
presents the catalytic activity of ball milled Li3FeN2 as a
function of temperature. It can be seen that ammonia
decomposition occurs right above 300 C, ca. 50 C lower than
that of as-prepared Li3FeN2 sample [29] which may be due to
the reduced particle size of ball milled sample. And this onset
temperature is ca. 125 C lower than that of Fe2N. At 475 C, its
activity is ca. 15 times of Fe2N. It should be noted that catalyst
sample collected ex situ contains Fe2N and Li2NH showing the
transforming of Li3FeN2 to Li2NH and Fe2N under elevated
temperatures and NH3 pressure.
Conclusion
Li3FeN2 was synthesized by reacting equivalent Li3N and
Fe with a yield of about 83%. Li3xFexN (0 < x < 1) is likely
co-produced because no other Fe species can be detected
€ ssbauer spectroscopy. Li3FeN2 can react with 2.5
by Mo
Fig. 8 e NH3 conversion as a function of temperature for
30 mg of ball milled Li3FeN2 and Fe2N samples at an NH3
flow rate of 30 ml/min.
Supplementary data related to this article can be found at
http://dx.doi.org/10.1016/j.ijhydene.2016.05.108.
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