Nitrogen storage properties based on nitrogenation and hydrogenation of rare earth-iron intermetallic compounds R2Fe17 (R=Y, Ce, Sm)

Article abstract of DOI:10.1016/S0925-8388(99)00105-X

The nitrogen storage properties for the rare earth-iron intermetallic compounds, R₂Fe₁₇ (R=Y, Ce, and Sm), were investigated. These intermetallic compounds formed the corresponding metal nitrides by heating in a mixed gas of NH₃-H₂ at 350-450°C and the nitrogen was incorporated into interstitial sites of the crystal lattices. The nitrogen stored as the metal nitrides was reversibly released as NH₃ by the following heating in H₂ at 450°C. An amount of the nitrogen released per unit volume of these intermetallic compounds is larger than that of a conventional nitrogen container charged at 15 MPa. The nitrogen storage capacity of Sm₂Fe₁₇N_x was increased by repeating the nitrogenation-hydrogenation cycle owing to the formation of FeN_x/RN_y composites with large surface areas derived from the starting intermetallic compound through the cycle. Furthermore, the nitrogen storage characteristics of Sm₂Fe₁₇ powders were effectively improved by surface loading with Ru metal that is active for ammonia generation.

Full text of DOI:10.1016/S0925-8388(99)00105-X

Journal of Alloys and Compounds 288 (1999) 141–146
L
Nitrogen storage properties based on nitrogenation and hydrogenation of
rare earth–iron intermetallic compounds R2Fe17 (R5Y, Ce, Sm)
*
Masahiro Itoh, Ken-ichi Machida, Hiroharu Nakajima, Kazuhiro Hirose, Gin-ya Adachi
Department of Applied Chemistry, Faculty of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan
Received 22 February 1999; received in revised form 27 February 1999
Abstract
The nitrogen storage properties for the rare earth–iron intermetallic compounds, R2Fe17 (R5Y, Ce, and Sm), were investigated. These
intermetallic compounds formed the corresponding metal nitrides by heating in a mixed gas of NH3 –H2 at 350–4508C and the nitrogen
was incorporated into interstitial sites of the crystal lattices. The nitrogen stored as the metal nitrides was reversibly released as NH3 by
the following heating in H2 at 4508C. An amount of the nitrogen released per unit volume of these intermetallic compounds is larger than
that of a conventional nitrogen container charged at 15 MPa. The nitrogen storage capacity of Sm2Fe17Nx was increased by repeating the
nitrogenation–hydrogenation cycle owing to the formation of FeNx /RNy composites with large surface areas derived from the starting
intermetallic compound through the cycle. Furthermore, the nitrogen storage characteristics of Sm2Fe17 powders were effectively
improved by surface loading with Ru metal that is active for ammonia generation.
1999 Elsevier Science S.A. All rights reserved.
Keywords: Nitrogen storage; Rare earth intermetallic compounds; Nitrogen absorption–desorption; Atomic nitrogen; Interstitial type metal nitrides
1. Introduction
of Sm2Fe17 is classified as a Th2Zn17-type one with a
space group of R3m and is formed when rare earth metals
¯
Rare earth–iron intermetallic compounds, R2Fe17, pro-
vide the corresponding metal nitrides (R2Fe17Nx) by
heating in N2 or a mixed gas of NH3 –H2 [1] and, among
them, Sm2Fe17N3 has been particularly noted as one of the
promising materials for high-performance permanent mag-
nets such as bonded ones [2–6]. Usually, the nitrogenation
for Sm2Fe17 is made in N2, but it needs the much more
hard conditions of reaction pressure (|10 MPa), tempera-
ture (|5508C) and time (|72 h) than the nitrogenation in
the NH3 –H2 mixture [1]. In the latter case, a significant
amount of nitrogen can be interstitially introduced in the
R2Fe17 crystal lattice (x5|7) under the more moderate
conditions than in N2 (at 0.1 MPa and |4508C for |3 h)
[3–6] and then, in a similar manner as metal hydride
systems [7], the excess nitrogen in R2Fe17Nx (x.3) is
reversibly desorbed by heating at a temperature of around
5008C in Ar to form the R2Fe17N3 compounds. This
behavior of nitrogen absorption and desorption has a close
relation to the sites for nitrogen in the crystal lattices.
Crystal structures of Sm2Fe17Nx and Y2Fe17Nx are shown
in Fig. 1(a) and (b), respectively [1]. The crystal structure
(R) are Ce, Pr, Nd, Sm, Gd and Tb. Meanwhile, the R2Fe17
compounds of R5Y, Dy, Ho, Er, Tm, and Lu crystallize in
another Th2Ni17-type structure (space group5P63 /mmc)
[1]. Buschow et al. have concluded from neutron diffrac-
tion measurements of Th2Fe17N3 with the same as the
Th2Zn17-type structure that the nitrogen occupies the 9e
¯
sites represented by the space group R3m symmetry [8].
However, the 9e sites can only accommodate three nitro-
gen atoms per R2Fe17 formula unit and it has also deduced
from the neutron diffraction measurement of Th2Fe17Dx
(x55) that the excess nitrogen in Sm2Fe17Nx (3,x,7)
occupies other sites such as 18g sites [8]. In the case of
Y2Fe17Nx, the nitrogen shares 6h sites of the Th2Ni17-type
crystal cell up to x53, so that the excess one occupies 12i
sites [9]. Therefore, the nitrogen absorbed more than x57
as R2Fe17Nx is released down to x53 since the nitrogen on
the 9e or 6h sites is stable but not on the 18g or 12i sites.
This fact suggests that the R2Fe17 compounds store large
amounts of nitrogen in the crystal lattices in a similar
manner as hydrogen storage materials. In addition, since
the nitrogen in R2Fe17Nx exists as a monoatomic state [9],
it is expected that the nitrogen in R2Fe17Nx is considerably
active and the nitrogen atom stored above x53 in the
crystal lattice are regenerated as ammonia by replacing the
*Corresponding author. Tel.: 181-6-68797352; fax: 181-6-6879-7354.
E-mail address: adachi@ap.chem.eng.osaka-u.ac.jp (G. Adachi)
0925-8388/99/$ – see front matter
PII: S0925-8388(99)00105-X
1999 Elsevier Science S.A. All rights reserved.

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M. Itoh et al. / Journal of Alloys and Compounds 288 (1999) 141–146
Fig. 1. Crystal structures of (a) Sm2Fe17Nx and (b) Y2Fe17Nx.
atmosphere from Ar to H2 in the above-mentioned treat-
ment process for homogenizing the metal nitrides.
In this paper, the nitrogenation and hydrogenation of
R2Fe17 were studied in streams of NH3 –H2 mixture and
H2 to characterize the nitrogen absorption and desorption
property based on the formation of R2Fe17Nx, together
with composite materials, Ru/Sm2Fe17, in which the Ru
metal was loaded on the surface in order to promote the
reactions for them. A novel nitrogen utilization system via
ammonia was proposed from the discussions on the
nitrogen storage characteristics using the nitrogen absorp-
tion–desorption cycle for R2Fe17.
solution containing 0.06 g of Ru3(CO)12, the resulting
composite materials were dried in N2 at room temperature
overnight and heated at 3008C in H2 for 3 h.
The R2Fe17 or Ru/R2Fe17 powder (1.0 g) charged in a
conventional fixed-bed quartz tube reactor (12 mm o.d.)
was nitrogenated in a stream of mixed NH3 –H2 gas (molar
ratio51:1) with a space velocity of 1200–4000 h21 at
350–4508C for 3 h (nitrogen absorption step), followed by
the subsequent hydrogenation process performed in a H2
stream with the same space velocity at 4508C for 3 h as the
absorption step. The nitrogen storage capacity of R2Fe17
powders was evaluated from the difference between the
nitrogen contents directly measured on a nitrogen/oxygen
analyzer (Horiba, EMGA 550) for the metal nitrides before
and after the nitrogen desorption (R2Fe17Nx and
R2Fe17Nx2d ). The structural change was checked every
step for the nitrogenation and hydrogenation treatment by
measurements of powder X-ray diffraction (XRD) by a
MAC Science M18XHF-SHA diffractometer using Cu Ka
radiation equipped with a curved graphite monochlometer.
The nitrogen species (NH3) stripped from the metal
nitrides by H2 in the desorption step was accumulated in a
cold trap (21968C) and then it was analyzed on a mass
spectrometer. The amount of regenerated nitrogen as
ammonia was quantified by the difference between the pH
values of an H2SO4 solution trap before and after passing
2. Experimental
The intermetallic compounds R2Fe17 (R5Y, Ce and Sm)
were prepared from appropriate amounts of rare earth
metal ingots or shots (.99.9% in purity) and Fe rods
(.99.95% in purity) by melting on an induction electric
furnace, and then annealed in Ar at 11008C for 48 h. The
ingots obtained were ground into powders with a particle
size below 50 mm (BET surface area50.15–0.30 m2 g21).
Further, some of them were activated by the Ru metal
cluster particles derived from Ru3(CO)12: After 1.0 g of
the Sm2Fe17 powder was impregnated with a hexane

M. Itoh et al. / Journal of Alloys and Compounds 288 (1999) 141–146
143
gas eluted from the metal nitride in the nitrogen desorption
step. Nitrogen absorption and desorption properties of the
Sm and Fe metals used as the starting materials were also
characterized on their powders (BET surface area5|0.16
m2 g21) by the same methods as used for the R2Fe17
compounds.
The catalytic activity of ammonia decomposition over
the Sm2Fe17, Ru/Sm2Fe17 and Fe was measured by a gas
chromatograph (Shimadzu GC-8A) in a temperature range
from 200 to 7008C. The inlet gas composition was 5%
NH3 and He as a balance with a flow rate of 20 ml min21
.
The decomposition yield from NH3 to N2 over the
Sm2Fe17 powders was calculated by integrating their peak
area ratio.
3. Results and discussion
Figs. 2 and 3 show the typical XRD patterns measured
on powdered Sm2Fe17 and Y2Fe17 samples nitrogenated in
NH3 –H2 mixed gas at 4508C for 3 h and hydrogenated in
H2 gas at 4508C for 3 h, together with the as-obtained one.
As shown in Figs. 2(b) and 3(b), the diffraction peaks
shifted to the lower angle side than those of the obtained
after the nitrogenation treatment (nitrogen absorption). This
indicates that nitrogen is interstitially incorporated into
Fig. 3. XRD patterns of the Y2Fe17 powders : (a) as obtained, (b)
nitrogenation in NH3 –H2 mixed gas at 4508C for 3 h, and (c) hydro-
genation in H2 gas at 4508C for 3 h.
their crystal lattices as accompanied with lattice expansion.
After the subsequent hydrogenation treatment (nitrogen
desorption), their XRD patterns inversely shifted to the
higher angle side, suggesting lattice shrinkage for releasing
nitrogen from their crystal lattices (see Figs. 2(c) and 3(c)).
A part of the R2Fe17Nx compounds decomposed after the
hydrogenation to a-Fe as shown in Figs. 2(c) and 3(c)
because the resulting metal nitrides were metastable. The
large amounts of the nitrogen introduced into the crystal
lattices lowered the crystallinity. The intensities of diffrac-
tion peaks for these intermetallic compounds were
weakened and broaden with the nitrogenation treatment.
Mass spectra for the nitrogen species regenerated from
the Sm2Fe17Nx and Y2Fe17Nx powders heated in H2 at
4508C for 3 h are shown in Fig. 4. The mass signals
appeared at the m/z values of 2, 15, 16 and 17, which were
assigned to H12 , NH1, NH21 and NH13 ion species,
respectively. Since NH3 is responsible for the fraction ion
species of NH1, NH21 and NH31, and H12 is mainly
originated from the H2 stripping gas used here, it is
concluded that the nitrogen absorbed in the R2Fe17 crystal
lattices is regenerated as NH3 by stripping with H2. In
addition, the amounts of ammonia product evaluated from
the difference between the pH values of H2SO4 solution
trap observed before and after the heat treatment in H2
were in good accordance with the converted values as NH3
from the desorbed nitrogen amounts determined directly by
Fig. 2. XRD patterns of the Sm2Fe17 powders : (a) as obtained, (b)
nitrogenation in NH3 –H2 mixed gas at 4508C for 3 h, and (c) hydro-
genation in H2 gas at 4508C for 3 h.

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M. Itoh et al. / Journal of Alloys and Compounds 288 (1999) 141–146
the nitrogen analysis for the metal nitride powders. It can
be seen from these results that the nitrogen released from
the R2Fe17Nx powders were converted to ammonia at
almost 100% yield.
The nitrogen absorption and desorption characteristics of
R2Fe17, Ru/Sm2Fe17, Sm and Fe powders observed on the
nitrogenation in the NH3 –H2 mixed gas at 350 or 4508C
for 3 h and on the hydrogenation in H2 at 4508C for 3 h
are summarized in Table 1. Even for the nitrogenation at
3508C, Ru/Sm2Fe17 absorbed a significant amount of
nitrogen to form Ru/Sm2Fe17N4.9 while the nitrogen
content was decreased to x52.6 in the subsequent desorp-
tion step, due to a high catalytic activity of Ru metal for
the NH3 dissociation and recombination [10], although the
no loaded Sm2Fe17 powder was nitrogenated only to x5
3.1 under the same conditions. From a calculation based on
the difference of nitrogen content (Dx52.3) between Ru/
Sm2Fe17N4.9 and Ru/Sm2Fe17N2.6, the quantity of the
nitrogen desorbed from 1 cm3 of the metal nitride (r5ca.
7.6 g cm23) was equivalent to 13.13 mmol of NH3,
because the atomic nitrogen stored in the crystal lattice
were regenerated as NH3 at about 100% yield. It is noted
that this amount of the nitrogen regenerated from 1 cm3 of
the metal nitride is comparable to the storage capacity of
conventional high-pressure nitrogen containers charged at
15 MPa (6.12 mmol of N2 gas per 1 cm3 of their
containers’ volume). However, the diffusion rate of nitro-
gen at 3508C in the R2Fe17 crystal lattice is not enough to
form the metal nitrides uniformly, and the core region of
R2Fe17 particles has not yet completely nitrogenated only
Fig. 4. Typical mass spectra for the nitrogen species eluted from (a)
Sm2Fe17Nx and (b) Y2Fe17Nx powders with H2.
Table 1
Nitrogen absorption–desorption characteristics of R2Fe17Nx, Ru/R2Fe17Nx, SmNx and FeNx
Starting
material
Cycle
Nitrogen content (x)a
Regenerated
ammonia
mmol cm23
(metal nitride)b
Number
Temp./8C
After
absorption
After
desorption
Sm2Fe17
Ru/Sm2Fe17
Y2Fe17
Ce2Fe17
Sm2Fe17
1
1
1
1
1
11
1
11
1
1
1
350
350
450
450
450
450
450
450
450
700
350
450
3.1
4.9
6.1
6.6
6.8
8.4
7.5
8.6
0
2.0
2.6
3.0
3.7
3.7
2.9
3.6
2.8
0
6.25
13.13
17.05
16.96
17.59
31.61
22.41
36.43
0
Ru/Sm2Fe17
Sm
Fe
1.0
0.01
0.14
0.98
0
0.01
1.07
1.43
18.39
1
a Nitrogen contents calculated from the nitrogen analysis data measured on the nitrogenated and hydrogenated samples.
b Amounts of the ammonia regenerated from 1 cm3 of the metal nitrides were evaluated on the basis of differences between the nitrogen contents of the
individual samples in the 1st or 11th absorption/desorption cycle. The corresponding figure for the conventional high-pressure N2 containers (15 MPa) is
6.12 mmol of N2 gas per 1 cm3 of volume.

M. Itoh et al. / Journal of Alloys and Compounds 288 (1999) 141–146
145
by heating for 3 h. Therefore, the complete absorption of
nitrogen may need much more time for the nitrogenation
than 3 h.
On the other hand, the nitrogenation at 4508C absorbs a
higher amount of nitrogen in the R2Fe17 crystal lattice than
at 3508C, due to the enhancement of the diffusion rate of
nitrogen. Amounts of the ammonia regenerated from the
R2Fe17Nx and Ru/Sm2Fe17Nx powders were evaluated to
be 16.96–17.59 and 22.41 mmol per 1 cm3 of the
intermetallic compounds, respectively, and these values
were 1.4 to 1.8 times larger than the nitrogen atom content
of the N2 gas stored per unit volume of the conventional
high-pressure nitrogen containers (12.24 mmol). What was
not observed was the obvious differences in the amount of
ammonia regeneration among the individual intermetallic
compounds. It might be understood that the difference
between the crystal Th2Zn17-type structure for Ce and Sm
and the Th2Ni17-type structure for Y has little influence on
the nitrogen storage capacities of them for the similarity in
their crystal structures. In our results for the Laves phase-
type compounds (MFe2; M5Ti, Mo, and Nb), the nitrogen
storage capacities for these compounds was depressed with
decreasing the standard enthalpy value for the nitride
formation of M elements [11]. The standard enthalpy
values of nitride formation are almost the same as one
another for the rare earth metals in this study. Therefore,
the nitrogen storage capacity of rare earth intermetallic
compounds is not affected by such nitride formation
enthalpy.
On the other hand, since the Sm metal used as one of the
starting materials was inactive for the dissociation of NH3
even at 4508C, only a trace of nitrogen was introduced in
the crystal lattice while SmN was formed at 7008C. Fe
metal also hardly reacted with NH3-H2 mixture at 3508C,
although in the case of the R2Fe17-type intermetallic
compounds, a large amount of nitrogen is incorporated into
their lattices. At 4508C, however, a large amount of
nitrogen was introduced in the Fe metal bulk as FeNx, and
a significant amount of NH3 (18.39 mmol per 1 cm3 of
FeNx) was subsequently regenerated. The XRD pattern of
nitrogenated Fe metal powder was assigned according to
that reported on an interstitial Fe4N compound [12]. This
indicates that the nitrogen storage capacity of Fe metal is
due to the formation of interstitial metal nitride Fe4N in a
similar manner as R2Fe17Nx.
Fig. 5. Decomposition properties of ammonia over Sm2Fe17
Sm2Fe17 and Fe powders.
, Ru/
because of the high catalytic activity of Ru metal for the
NH3 dissociation and recombination [10]. The ammonia
decomposition characteristics over Sm2Fe17, Ru/Sm2Fe17
and Fe powders fairly agrees with their nitrogen storage
properties summarized in Table 1. Since the catalytic
activity of Fe metal and Fe4N is lower than that of R2Fe17
and R2Fe17Nx, the Fe metal cannot absorb the large
amount of nitrogen compared with them even by the
nitrogenation at 4508C.
The nitrogen storage capacity of Sm2Fe17Nx was in-
creased with repeating the nitrogen absorption–desorption
cycle. An amount of the ammonia regenerated from the
Sm2Fe17Nx and Ru/Sm2Fe17Nx powders in the 11th cycle
(31.61 and 36.43 mmol) was considerably higher than that
in the 1st cycle (17.59 and 22.41 mmol) (see Table 1).
However, the XRD pattern of Sm2Fe17Nx was mixed with
those of a-Fe and amorphous phases after several nitrogen
absorption–desorption cycles. This finding means that a
kind of composite material, FeNx /SmNy (y5|1), which is
derived from Sm2Fe17 by repeating the nitrogen absorp-
tion–desorption cycle, gives an excellent nitrogen absorp-
tion and desorption property, although the XRD patterns of
Fe4N and SmN have not yet been observed on the
composite sample. It is concluded that the FeNx /RNy
composite material is also highly active for the dissociation
of NH3. In the study of ammonia synthesis using rare earth
intermetallics, rare earth–iron intermetallics decomposed
to a-Fe and the corresponding rare earth nitrides after the
reaction of N2 –H2 mixed gas under a pressure of 7 MPa
and high catalytic activity was observed over the a-Fe/rare
earth nitrides composite materials [13]. The increase of the
amount of ammonia obtained in the 11th cycle is ascribed
to the increase of the surface area due to the crack growth
on the surface and decomposition of the Sm2Fe17 by
repeating nitrogen absorption–desorption cycle and the
high activity of ammonia dissociation over the FeNx /RNy
composite materials.
Fig. 5 shows the decomposition properties of ammonia
over Sm2Fe17, Ru/Sm2Fe17, and Fe powders. The surface
areas of these powders are 0.15–0.30 m2 g21 measured by
the conventional BET method and nearly the same. The
initial temperature of ammonia decomposition over the
Sm2Fe17 powder is lower than that over the Fe powder.
This result indicates that the activity for ammonia de-
composition over the Sm2Fe17 powder is higher than that
of the Fe powder. Furthermore, ammonia was decomposed
on the Ru/Sm2Fe17 composite powder at a lowered
temperature compared with no Ru metal loaded Sm2Fe17
Schematic models for the nitrogen absorption and

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M. Itoh et al. / Journal of Alloys and Compounds 288 (1999) 141–146
the nitrogenation by heating in the mixed gas of NH3 –H2
and denitrogenation by heating in H2 gas. The nitrogen
storage capacities per unit volume through absorption–
desorption for these samples is superior to those of
conventional high pressure nitrogen containers charged at
15 MPa. More improvement for the nitrogen storage
capacity of R2Fe17 than the above one is attained by
loading the Ru metal on their powder surface because of
the acceleration for the nitrogenation step. The atomic
nitrogen incorporated in the compounds is regenerated as
ammonia in the nitrogen desorption step at almost 100% of
yield. In addition, the nitrogen storage capacity of
Sm2Fe17Nx is increased with repeating the nitrogen ab-
sorption–desorption cycle due to the formation of FeNx /
SmNy fine particle composites.
Acknowledgements
This work was partly supported by Grant-in-Aid for
Scientific Research Nos. 07242245, 08232249, 09218234,
and 09874143 from the Ministry of Education, Science,
Sports, and Culture of Japan.
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