Reactions in AB5-NH3 systems

Article abstract of DOI:10.1007/s11176-005-0077-5

Treatment of intermetallic compounds LaNi₅ and SmCo₅ with ammonia (initial ammonia pressure 0.6-0.7 MPa) in the presence of 10 wt% NH₄Cl as an activator results in hydrogenation of these compounds or, in the case of SmCo

Full text of DOI:10.1007/s11176-005-0077-5

Russian Journal of General Chemistry, Vol. 74, No. 11, 2004, pp. 1641 1645. Translated from Zhurnal Obshchei Khimii, Vol. 74, No. 11, 2004,
pp. 1765 1769.
Original Russian Text Copyright
2004 by Fokin, Shul’ga, Tarasov, Fokina, Korobov, Burlakova, Shilkin.
Reactions in AB NH Systems
5
3
V. N. Fokin, Yu. M. Shul’ga, B. P. Tarasov, E. E. Fokina,
I. I. Korobov, A. G. Burlakova, and S. P. Shilkin
Institute of Chemical Physics Problems, Russian Academy of Sciences, Chernogolovka, Moscow oblast, Russia
Received January 10, 2003
Abstract Treatment of intermetallic compounds LaNi and SmCo with ammonia (initial ammonia pres-
5
5
sure 0.6 0.7 MPa) in the presence of 10 wt% NH Cl as an activator results in hydrogenation of these com-
4
pounds or, in the case of SmCo , in disproportionation to form Sm Co H . In some cases, the intermetallic
5
2
7 x
matrix decomposes under this treatment.
Intermetallic compounds AB (A = La, Sm; B =
Ni, Co) are used as metal hydride accumulators of
hydrogen and for production of permanent magnets
of ammonium chloride (10 wt% with respect to the
amount of the intermetallic compound).
5
As determined by X-ray diffraction, LaNi is a
5
[
1].
single-phase compound with the lattice constants, a
5
.013 and c 3.986
(CaCu5 structure); SmCo5
Intermetallic compounds LaNi and SmCo (struc-
5
5
(CaCu structure, a 5.001 and c 3.971 ) contains a
5
tural type CaCu ) have a hexagonal lattice with the
5
minor amount of Sm Co located in the intergranular
space and promoting cracking and crushing of the
SmCo sample in the course of its hydrogenation.
2
7
parameters a 5.017 and c 3.982, and a 5.003 and c
3
.978
, respectively. These compounds readily
5
absorb hydrogen to form LaNi H6.8 and SmCo H [2,
5
5
4
3
]. After hydrogenation completion, LaNi H6.8 lib-
5
The conditions of reactions of NaNi and SmCo
5
5
erates hydrogen to form -solid solution [2], and
SmCo H loses hydrogen to form SmCo H . The
latter compound is converted into SmCo H at 70
8
2
products is 0.2 0.3 m g [4]. The particle size of
powders prepared by the hydride dispersion procedure
is several microns.
with ammonia and composition, lattice constants and
specific surface area of the reaction products are
summarized in the table. As seen from the table, the
reaction of LaNi and SmCo with ammonia at 150 C
5
4
5
2
5
1
0 C and is completely dehydrogenated at 190
00 C. The specific surface area of the hydrogenation
5
5
(
sample no. 1 and 8) for 25 h yields a solid solution
2
1
of hydrogen in the AB matrix. As a result, the lattice
5
constants of the intermetallic compounds slightly
increase: a 5.015 and c 3.990 for La Ni H and a
5
5
0.5
5
.001 and c 4.034
for SmCo H . The specific
5 0.3
Our previous study of reactions in the systems Ti
NH and ScFe(Ni) NH [5, 6] suggest that introduc-
tion of hydrogen and nitrogen into the matrix of the
2
1
surface area of La Ni H (1.3 m g ) is substantially
larger than that of hydrogen-treated LaNi (0.2
5
5
0.5
3
2
3
5
2
1
0
.3 m g ). The specific surface area of SmCo H
5 0.3
intermetallic compounds AB would allow preparation
5
2
1
(
0.3 m g ) is similar to that of the sample prepared
of nitrogen-containing hydrides and, by disproportiona-
tion of the intermetallic compounds under the action
of ammonia, of finely divided products: transition
metal (Co or Ni) and hydrides of nitrides of metal
A, in the form of homogeneous mixtures.
2
1
by the hydride dispersion procedure (0.27 m g ) [4].
As determined by chemical analysis, the chlorine
content in the hydronitriding product decreased from
2
.3 to 1.2% after washing with two portions of
ethanol and then to 0.9% after boiling for 10 min in
water. The nitrogen content in the sample decreased
to 1.6% (0.5% nitrogen loss) after this treatment.
In this work we studied experimentally intercala-
tion of nitrogen and hydrogen into the lattice of
intermetallic compounds AB5 during treatment of
The reaction of LaNi with ammonia at 200 C
sample no. 2) yields metallic nickel and unknown
these compounds with NH at different temperatures
5
3
(
in the presence of NH Cl promoter. Reaction of LaNi5
4
X-phase along with the -LaNi H . phase (a 5.015
and SmCo with ammonia (so-called hydronitriding
2
0 5
5
process) was studied at the initial ammonia pressure
of 0.56 0.70 MPa and 150 500 C and in the presence
and c 3.987 ). As determined by thermal analysis,
the X-phase endothemally decomposes at 160 and
1
070-3632/04/7411-1641 2004 MAIK Nauka/Interperiodica

1642
FOKIN et al.
Conditions of reactions between AB intermetallic compounds and ammonia and parameters of the reaction products
5
Reaction conditions
Reaction products
Sample
no.
AB5
lattice
parameters,
surface area,
T,
C
, h
p(NH ), MPa
composition
3
2
1
m g
1
2
LaNi5
LaNi5
150
25
25
0.64
0.64
LaNi H
a 5.015, c 3.990
a 5.015, c 3.987
1.3
6.8
5
0.5
200
250
300
350
LaNi H
+
5
0.5
X-phase (traces) +
Ni (traces)
3
4
5
LaNi5
LaNi5
LaNi5
28
25
23
0.7
LaNi H
+
+
a 4.981, c 4.024
2.5
1.7
1.8
5
0.5
X-phase +
Ni
a 3.526
a 5.023, c 3.983
0.68
0.68
LaNi H
5
0.5
X-phase +
Ni
LaNi Hx (traces) +
5
X-phase +
Ni
a 3.523
a 3.526
6
7
8
9
LaNi5
LaNi5
400
450
150
200
250
26
25
25
23
26
0.68
0.56
0.64
0.64
0.62
X-phase + Ni +
LaN (traces)
X-phase + Ni +
LaN
a 3.524
a 5.290
2.1
0.3
1.4
6.5
SmCo5
SmCo5
SmCo5
SmCo H
+
a 5.001, c 4.034
5
0.3
Sm Co H (traces)
2
7
x
SmCo H
+
a 5.090, c 4.069
5
0.3
Sm Co H (traces)
2
7
x
1
1
1
1
0
1
2
3
SmCo H
+
+
a 5.021, c 4.003
a 5.020, c 23.182
5
0.3
Sm Co H
+
2
7
x
-Co
SmCo5
SmCo5
SmCo5
300
350
400
23
25
26
0.56
0.64
0.64
SmCo H
a 5.004, c 4.064
a 4.943, c 23.558
a 2.504, c 4.073
0.7
3.9
0.5
5
0.3
Sm Co H
+
2
7
x
-Co
SmCo Hx (traces) +
5
Sm Co H
+
a 4.929, c 23.323
a 2.510, c 4.055
a 5.034, c 24.564
a 5.073
2
7
x
-Co
Sm Co H
+
2
7
x
SmN +
-
Co
Co +
a 2.515, c 4.022
a 3.542
1
4
5
SmCo5
SmCo5
450
500
25
26
0.64
0.70
SmN +
Co
SmN +
Co
a 5.046
a 3.541
a 5.045
a 3.543
1.0
0.8
1
2
40 C to form fine powders with developed surface
When SmCo is treated with ammonia at 250 C
5
2
1
area (6.8 m g ).
for 26 h (sample no. 10), the hydride phase decom-
poses into metallic cobalt and the compound with a
lower cobalt content Sm Co H . The lattice param-
SmCo , unlike LaNi , is more stable with respect
5
5
2
7
x
to ammonia. At 200 C, SmCo reacts with ammonia
eters of SmCoO H
X-ray diffraction, are a 5.021, c 4.003 and a 5.020,
c 23.182 , respectively (cf.: Sm Co crystallizes in
and Sm Co H , determined by
5
5
0.3 2 7 x
(
(
sample no. 9) to form the solid solution SmCo H
5
0.3
a 5.090 and c 4.069 ). The specific surface area of
2
7
2
1
the reaction products is 1.4 m g .
the hexagonal system with a 5.043, c 24.312
[7]).
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 74 No. 11 2004

REACTIONS IN AB NH3 SYSTEMS
1643
5
The specific surface are of the reaction products of
2
1
SmCo with NH at 250 C reaches 6.5 m g and is
2
5
3
higher by a factor of tens then that of the hydride
dispersion products, which indicates high dispersity
of the material. The average particle size estimated
assuming their spherical shape is 110 nm. The
chlorine content in the reaction products decreases
from 4.7 to 0.9% after washing with two portions of
ethanol.
4
0
0
0
2
2
0
The yield of the X-phase and metallic Ni after
treatment of LaNi with ammonia at 250 300 C in-
creases (sample nos. 3 and 4). At 350 C (sample no.
1
5
4
0
5
), only traces of LaNi H are formed. The specific
5 x
1
.0
0.5
0
0.5
1.0
H, T
surface area of the products of the reaction at 250
2
1
3
50 C is 1.7 2.5 m g .
Magnetization diagram of (1) SmCo and (2) reaction
The reaction of SmCo with ammonia (sample
5
5
products of SmCo with ammonia at 250 C.
nos. 11 and 12) at 300 350 C is accompanied by
further degradation of SmCo H . At 300 C an equi-
5
5
x
the hydronitriding products obtained at 250 C (the
temperature providing maximal dispersion, specific
molar mixture of SmCo H
and Sm Co H is
5
0.3
2 7 x
formed, whereas at 350 C Sm Co H prevails. The
2
7
x
2
1
surface area 6.5 m g ) do not contain SmCo . Indeed,
lattice parameters of SmCo H
and Sm Co H
5
5
0.3
2
7
x
although the X-ray patterns of the reaction products
prepared at 300 C are a 5.004 and c 24.064, and a
.943 and c 23.558 , respectively. The specific
surface area of the products of reaction of SmCo with
contain only strong reflections of SmCo H and
4
5
0.3
Sm Co H , small X-ray amorphous particles of
2
7
x
5
2
1
SmCo may be present in the sample. X-ray amor-
ammonia at 250 350 C is 0.7 3.9 m g .
5
phous SmCo powders prepared by grinding under an
5
LaNi undergoes more dramatic transformations
5
inert atmosphere have high coercivity ( 0.4 T) [8].
during treatment with NH at 400 C (sample no. 6)
3
The coercivity H of the reaction products of SmCo
c
5
to form traces of lanthanum nitride LaN. At 450 C,
the yield of this product noticeably increases (sample
no. 7). LaN crystallizes in the cubic system with a
with ammonia at 250 C is 0.006 0.007 T irrespective
of washing of the sample. The coercivity only slightly
increases after washing.
5
.290 . The specific surface area of the hydronitrid-
2
1
The reaction of SmCo with ammonia in the pres-
ing product formed at 450 C is 2.1 m g .
5
ence of NH Cl can be described by Eqs. (1) (4).
4
As in the case of LaNi , the products of the reac-
5
1
50 200 C, NH Cl
4
tion of SmCo with ammonia at 400 450 C substan-
5
SmCo5 + NH3
SmCo5H0.3 + N2, (1)
SmCo H
tially differ in composition from those obtained at
lower temperatures. At 400 C (sample no. 13), the
hydride phase completely decomposes, the content of
Sm Co H (a 5.034, c 24.564 ) decreases, and
2
50 350 C, NH Cl
4
5
SmCo + NH3
5
0.3
(
2)
+ Sm2Co7Hx + -Co + N2,
400 C, NH4Cl
2
7
x
SmCo + NH3
SmCo Hx
5
7
samarium nitride (a 5.073 ) and a mixture of -Co
a 2.515, c 4.022 ) and cubic cobalt (a 3.542 ) are
formed. At 450 C (sample no. 14), Sm Co H com-
+
SmN + -Co + Cocubic
,
(3)
(
4
50 500 C, NH Cl
4
2
7
x
SmCo5 + NH3
SmN+ Cocubic + H2. (4)
pletely disappears, and a homogeneous mixture of
samarium nitride SmN (a 5.046 ) and cubic Co (a
Similar reactions occur in the LaNi NH system.
5 3
2
1
3
.541 ) with the specific surface area of 1 m g is
Reactions (1) (4) show how the initial intermetallic
formed. The composition of the hydronitriding prod-
ucts formed at 500 C (sample no. 15) is the same.
compound SmCo transforms into the final reaction
5
products (SmN and Cocubic) with increasing tempera-
ture. In the process, hydrogen loosens the SmCo5
matrix. This scheme incompletely describes decom-
position of the intermetallic compound. Analysis of
the hydronitriding products washed with water or/and
It is known that the coercivity of micron SmCo5
powders increases with decreasing particle size. The
dependence of the specific magnetization M of SmCo5
(curve 1) and the products of its reaction with ammonia
at 250 C (curve 2) on the external field H is shown in
the figure. As seen from the magnetization diagrams,
ethanol to remove large amounts of NH Cl shows
intercalation of not only hydrogen but also nitrogen
4
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 74 No. 11 2004

1644
FOKIN et al.
in the metallic matrix. Hydridonitrides SmCo H N or
for 3 h, cooled to 20 C, and heated again. After a
required number of the heating-cooling cycles,
ammonia was transferred into a buffer vessel and
ammonium chloride was removed, if necessary, by
sublimation in a vacuum at 180 200 C for 2 h or by
washing of the sample with water and/or ethanol. The
reactor was discharged under an inert atmosphere.
5
x
y
Sm Co H or, which is more possible, their mixtures
2
7
x
are stable for a time and can strongly affect the
magnetic properties of the initial intermetallic com-
pound.
Formation of mixed hydridonitrides is confirmed
by the results of chemical analysis: washed products
of SmCo nitriding at 150 C contain 1.6% nitrogen
NH Cl and other chlorine-containing impurities
were removed from the hydronitriding products of
5
4
dissolved in SmCo H . The content of dissolved
5
x
nitrogen in the products of hydronitriding of SmCo5
at 300 C increases to 2.55%. However, no reflections
of SmN are observed on the X-ray pattern of the
samples.
SmCo by the following procedure. Absolute ethanol
5
(10 ml) was added to a weighed portion of the sample
( 0.5 g), and the mixture was stirred for 3 h. Usually
this procedure was repeated two times. If the chlorine
content in the sample was 1 2%, the sample was
additionally washed with distilled water. In some
cases the mixture was boiled. To remove water traces,
the sample was stirred in absolute diethyl ether for
It should be noted that that the specific surface area
of the hydronitriding products of SmCo increases
5
2
1
from 0.3 to 6.5 m g with increasing reaction
temperature from 150 to 250 C. This indicates the
increase in the dispersity of the powder. Nitrogen ac-
cumulation in the intermetallic compound lattice
promotes dispersion of the sample.
1
0 min. The product was analyzed for hydrogen,
nitrogen, and chlorine.
X-ray diffraction studies were performed on a
ADP-1 diffractometer (CrK radiation) controlled by
a computer. The determination error of lattice con-
As determined by DTA, mixed hydronitrides of
SmCo prepared by hydronitriding at 150 300 C are
5
stants was no higher than 0.005
.
almost completely dehydrogenated at 150 200 C. The
nitrogen loss is completed at 560 C.
The specific surface area of the samples was cal-
culated by the Brunauer Emmett Teller procedure
EXPERIMENTAL
[9] from the low-temperature adsorption of krypton.
Prior to the measurements, volatile products were
removed from the solid phase by heating of the
sample at 300 C for 15 h in a vacuum (1.3 10 Pa).
Intermetallic compounds AB were prepared by
melting of the corresponding mixtures of pure metals
La (99.8%), Sm (99.8%), Ni (99.9%), and Co
5
3
(
99.99%) in an electric arc furnace with noncon-
The magnetization curves were measured on an
RARC M-4500 vibration magnetometer. SmCo5
powder was prepared by the hydride dispersion proce-
dure. After 10 hydrogenation dehydrogenation cycles,
hydrogen was removed and the sample was treated
with CO, for 5 h under a pressure of 0.5 MPa. The
specifics surface area of the resulting powder was
sumable tungsten electrode in an atmosphere of high
purity nitrogen. The alloys were homogenized by
annealing at 600 C for 250 h in evacuated quartz
ampules and quenched in cold water. Just before use,
the oxide film was mechanically removed from the
alloy surface and then the samples were washed with
ethanol and diethyl ether and crushed under argon to
2
1
0.48 m g .
1
5 mm particle size.
Thermal analysis was performed in an argon flow
on a C-3434 derivatograph with microprocessor
control.
Ammonium chloride was dried at 150 C in a
vacuum. The purity of ammonia dried with metallic
sodium was 99.99%.
The hydrogen content in the reaction products was
determined by combustion of samples in an oxygen
flow, and the nitrogen and chlorine content, by the
Kjeldahl procedure and turbidimetric analysis, re-
spectively.
The reactions of intermetallic compounds AB with
5
ammonia were performed in a 60-ml high-pressure
lab unit. A mixture of AB powder with 10 wt%
NH Cl was ground in a mortar under argon. A
5
4
weighed portion of this mixture (1 2 g) was placed in
a stainless steel container which was loaded in the
autoclave. The autoclave was evacuated to 1 Pa for
ACKNOWLEDGMENTS
3
0 min at room temperature. Ammonia was fed, and
the reactor was kept for 30 min at room temperature.
Then the reactor was heated at required temperatures
This work was financially supported by the Rus-
sian Foundation for Basic Research (project no. 01-
03-33241).
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 74 No. 11 2004

REACTIONS IN AB NH3 SYSTEMS
1645
5
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