Chemical and phase transformations in the systems hydrogen-sorbing intermetallic compound-diborane

Article abstract of DOI:10.1023/A:1026384214275

The reactions of the intermetallic compounds CeFe₂, CeCo ₂, and λ₃-ScFe₂ with B₂H ₆ at 4.8 × 10³ Pa, 293-573 K, and various contact times were studied. CeFe₂ decompose

Full text of DOI:10.1023/A:1026384214275

Russian Journal of General Chemistry, Vol. 73, No. 6, 2003, pp. 865 867. Translated from Zhurnal Obshchei Khimii, Vol. 73, No. 6, 2003,
pp. 914 917.
Original Russian Text Copyright
2003 by Kravchenko, Kalinnikov, Shilkin.
Chemical and Phase Transformations in the Systems
Hydrogen-Sorbing Intermetallic Compound Diborane
S. E. Kravchenko, G. V. Kalinnikov, and S. P. Shilkin
Institute of Problems of Chemical Physics, Russian Academy of Sciences, Chernogolovka, Moscow Oblast, Russia
Received June 4, 2001
3
Abstract The reactions of the intermetallic compounds CeFe , CeCo , and -ScFe with B H at 4.8 10
2
2
3
2
2 6
Pa, 293 573 K, and various contact times were studied. CeFe decomposes in the presence of diborane to give
2
CeH1.9 2.2, Ce Fe H8 10, -Fe, CeCo decomposes into the cerium trihydride CeH and -Co, whereas
2
17
2
2.7 2.8
3-
ScFe forms an ScFe H
hydride phase. Therewith, with CeFe and CeCo , diborane decomposes into
2
2
0.4 0.5 2 2
solid nonvolatile X-ray amorphous various-composition compounds (BH ) , as well as H and B H , while
m n
2
5 9
with
-ScFe , into H , B H , B H , B H , and (BH ) .
2 2 4 10 5 9 5 11 m n
3
For certain intermetallic compounds formed by rare-
earth metals, scandium, yttrium, etc. (A) and 3d-transi-
tion metals (B), hydrogen absorption and dissociation
occur under mild conditions and fairly fast, while with
different rates, according to reaction (1) [1, 2].
hydrogenating agent, a carrier of bound and active
hydrogen, diborane. The intermetallide ScFe melts
2
congruently and has two polymorphous modifications:
phase (structural type MgZn ) formed at tempera-
1
2
3
tures higher than 1473 K and
phase (MgNi struc-
2
tural type) formed at 1873 K. In the melt with a scan-
dium content of 36.5 at%, a Laves cubic phase 2-
P1, T1
AB + xH
AB H ,
(1)
n
2
n x
P2, T
2
ScFe1.74 (MgCu structural type) was found, formed
2
by a peritectic reaction at 1798 K [5]. The compounds
CeFe and CeCo are formed by peritectic reactions at
P1
P2, T2
T1; n = 1 5.
2
2
1
213 and 1296 K, respectively, and crystallize in the
However, the formation of AB H hydride phases
n
x
MgCu structural type [5].
2
by reaction (1) is not a single possible way of reaction
of an intermetallic compound with hydrogen. As
shown previously, decomposition of intermetallides
The aim of the present work was to study reactions
of the intermetallic compounds -ScFe , CeCo , and
CeFe with diborane as a carrier of chemically bound
3
2
2
under the action of H is more reasonable thermo-
2
2
dynamically. Therewith, depending on the chemical
nature of intermetallide and conditions (pressure and
temperature), reaction (2) gives one or more X-ray
amorphous products [3, 4], or results in the decomposi-
tion of the starting intermetallide into binary hydride
and active hydrogen.
As follows from the data in the table, at room
temperature in the presence of CeFe , diborane de-
2
composes evolving hydrogen, and the intermetallide
undergoes profound phase transformations involving
its decomposition and formation of several crystal
phases: -Fe, CeH , and Ce Fe H . In all the cases
studied, the crystal lattice period of CeH varies
AH and 3d-transition metal B [2] by reaction (3), or
X
the decomposition of the intermetallide into the binary
hydride and a new, B-enriched AB H hydride phase
x
2
17
y
m
y
by reaction (4) [2, 4].
x
within 0.5578 0.5564 nm, implying formation of the
hydride CeH1.9 2.2. The Ce Fe H hydride phase
P, T
2
17
y
ABn + xH2
X-ray amorphous products, (2)
crystallized in the Th Ni structural type (a 0.8651
2
17
0
and c 0.8385 nm), with the unit cell volume in-
P, T
0
ABn + xH2
AHx + nB,
(3)
(4)
creased by 6% compared with Ce Fe . By analogy
2
17
with the Ce Co H hydride phase whose unit cell
P, T
2
17 4.7
ABn + xH2
AHx + AB H , m > n.
volume is increased by 3.2% compared with Ce Co
m
y
2 17
[
6] and assuming that the changes in the unit cell
volume, related to one absorbed hydrogen atom
( V/H), for the Ce Fe and Ce Co
phases should be close to each other, the y value in
In this connection we chose as model objects ScFe2,
CeCo , and CeFe , whose reactions with molecular
oxygen are described by reactions (1 4), and for the
2
2
2
17
2
17
1
070-3632/03/7306-0865$25.00 2003 MAIK Nauka/Interperiodica

866
KRAVCHENKO et al.
3
Conditions and results of the reactions of CeFe , CeCo , and ScFe with B H at the initial pressure P 4.8 10 Pa in
2
2
2
2 6
the static mode
Starting
intermetallic
compound
Reaction conditions
Reaction products
T, K
time, h
gas phase
solid phase
CeFe2
CeFe2
CeFe2
CeFe2
CeFe2
CeFe2
CeFe2
CeFe2
CeC°2
CeC°2
CeC°2
CeC°2
293
293
293
293
323
373
423
473
293
323
373
473
0.5
3.5
24
96
4
4
1
1
B H , H
2 6
CeFe2
CeFe2, -Fe
-Fe, CeH , CeFe ,
-Fe, CeH , Ce Fe H
-Fe, CeH , Ce Fe H
x 2 17
-Fe, CeH , Ce Fe H , (BH )
-Fe, CeH , Ce Fe H , (BH )
x 2 17 y m n
-Fe, CeH , Ce Fe H , (BH )
CeCo , X-ray amorphous products
2
2
2
2
2
2
B H , H
2
6
B H , H
2
6
x
2
B H , H
2
6
x 2 17
y
y
B H , H
2
6
B H , H , B H
2
6
2
5
9
9
x 2 17 y m n
B H , H , B H
2
6
2
5
x 2 17 y m n
a
144
1
B H , H
2
6
2
a
B H , H
CeCo , X-ray amorphous products
2
6
2
2
a
1
0.5
B H , H , B H
X-ray amorphous products
2
6
2
5
9
9
B H , H , B H
CeH
, X-ray amorphous
2
6
2
5
2.7 2.8
a
products
CeH2.7 2.8
ScFe2
CeC°2
ScFe2
ScFe2
ScFe2
ScFe2
ScFe2
573
293
293
388
423
473
1.5
24
96
7
2
0.5
,
-Co, (BHm)n
B H , H
2
6
2
B H , H
ScFe2
2
6
2
B H , H , B H , B H , B H
ScFe Hz, ScFe , (BH )
2
6
2
5
9
4
10
10
5
11
2 2 m n
B H , H , B H , B H
ScFe H , ScFe , (BH )
2 z 2 m n
2
6
2
5
9
9
4
B H , H , B H
ScFe H , (BH )
2
6
2
5
2 z m n
a
Solid nonvolatile X-ray amorphous products containing hydrogen and no boron (according to chemical and IR spectral analysis).
Ce Fe H can be estimated at 8 10. The crystal
periods a 0.2505 and c 0.4074 nm. The latter values
0 0
2
17
y
lattice period of -Fe in all experiments was the same
a0 0.2865), which is nicely consistent with published
are in close agreement with published data (a 0.2507
0
(
and c 0.4070 nm [8]).
0
data (a 0.2866 nm [7]). At reaction temperatures
0
increased to 373 423 K, along with H and B H , in
Unlike CeFe and CeCo , which decompose or pass
2
2
6
2
2
the gas phase we found
solid products, up to 10% of nonvolatile variable-
composition boranes (BHm)n.
5% of B H , whereas in
into the X-ray amorphous state at moderate tempera-
tures under the action of both molecular hydrogen and
diborane (carrier of bound H ), ScFe reacts with
5
9
2
2
B H at 388 473 K to form a hydride phase with the
crystal lattice periods a0 0.5030 0.5040 and c0
2
6
The intermetallide CeCo reacts with B H at an
2
2
6
appreciably lower rate than with CeFe . At 293 323 K,
among solid reaction products, some unreacted
1.640 1.653 nm, implying the formula ScFe N0.4 0.5.
2
2
In this temperature range in the gas phase, along with
diborane and H , the higher boranes B H , B H , and
CeCo is present along with X-ray amorphous hydro-
2
2
4
10
5
9
gen-containing compounds, while diborane decom-
B H are present, whose total concetration decreases
5 11
poses with H evolution. At 373 473 K, in the gas
with increasing temperature, while certain boranes,
such as B H and B H , do not form at 473 K.
2
phase we found, along with hydrogen and diborane,
4
10
5
11
2
% of B H , and among solid product, along with
5 9
X-ray amorphous nonvolatile variable-composition
boron- and hydrogen-containing products, a cerium
hydride with the crystal lattice period a varying
For a more detailed study of thermolysis of B H
2 6
on ScFe N hydride phases we prepared the hydride
2
z
phase ScFe H with the crystal lattice periods a0
0
2 2.7
within 0.5546 0.5542 nm, which suggests the formula
CeH2.7 2.8. At 573 K, diborane decomposes com-
pletely, and among solid products we found, along
with cerium trihydride and boron- and hydrogen-con-
taining X-ray amorphous compounds, -Co which
crystallized in the hexagonal syngony with the lattice
0.5257 and c 1.7039 nm, which agrees well with
0
published data (a 0.5255 and c 1.7040 nm [9]).
0
0
Since on contact of B H with -ScFe at 293 473 K
2
6
3
2
in static conditions the gas phase always contains
hydrogen, then thermolysis of diborane in the pre-
sence of ScFe N
we performed with equimolar-
2
2.7
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 73 No. 6 2003

CHEMICAL AND PHASE TRANSFORMATIONS IN THE SYSTEMS
867
amounts of B H and H in dynamic conditions at a
with an accuracy of no more than 0.0002 nm for
starting alloys and 0.0004 nm for reaction products.
2
6
2
contact time of 30 s, an initial pressure of the gas
mixture of 0.15 MPa, and a temperature of 398 K. At
a 20% conversion of B H , in the gas phase we found
The gas mixtures containing hydrogen and boranes
were analyzed on an LKhM-8MD chromatograph with
a thermal conductivity detector on a column (length
2
6
6
0% of B H , 5% of B H , and 25% of B H and in
4 10 5 11 5 9
the solid phase, along with the hydride phase on the
basis of -ScFe , about 10% of nonvolatile variable-
1
m, diameter 0.003 m) packed with tricresyl phos-
3
2
phate (30 wt%) on Celite 545, rate of carrier gas
composition boranes (BHm)n.
(He, Ar) 30 ml/min.
The IR spectra of solid X-ray amorphous reaction
products of diborane with CeFe , CeCo , and ScFe
The IR spectra of the gas and solid phases were
obtained on a UR-20 spectrometer at 400 3600 cm
in cells with KBr windows.
1
2
2
2
lack absorption bands characteristic of bridging B H
1
vibrations (1915 and 1602 cm ) and contains absorp-
Hydrogen and boron were determined by standard
procedures. Hydrogen was determined by burning a
sample in oxygen in a device for semimicroanalysis
and boron, by potentiometric titration with alkali of
mannitolboric acid after binding metals with Trilon B.
tion bands due to skeletal stretching vibrations (980
1
and 1080 cm ), as well as absorption bands at 2700
750 cm 1 due to terminal B H vibrations. These
2
findings led us to conclude that the solid nonvolatile
boron- and hydrogen-containing compounds comprise
polymeric chains including B H fragments.
Diborane was prepared by the Schlezinger proce-
n
m
dure from boron trifluoride etherate and NaBH in
4
Thus, the considered reactions of intermetallic
compounds with diborane follow the same regularities
reactions of the same compounds with molecular
hydrogen and can be represented by reactions (1) (4).
diglyme (B H was prepared by L.A. Gavrilova). Pure
2
6
grade boron trifluoride etherate was purified by di-
stillation at 333 K directly before use. Sodium boro-
hydride (purity > 99.0%) was prepared by crystalliza-
tion of technical product from 1 N NaOH and dried
in a vacuum of 1.33 10 1 Pa of 393 K.
EXPERIMENTAL
The reactions of intermetallic compounds with di-
borane were studied in a laboratory device equipped
with gas samplers and capable of operating both at
reduced and increased pressures. All preliminary
works, including sampling of solid products, were
performed under argon.
The intermetallic compounds CeFe , CeCo , and
2
2
ScFe were prepared by alloying metal charge (purity,
2
%
: cerium 99.71, scandium 99.84%, cobalt 99.99%,
and iron 99.96) on a copper water-cooled floor of an
arc furnace with a nonconsumable tungsten electrode
under a high-purity nitrogen pressure of 0.2 MPa.
The deviation of resulting from preset compositions
was no more than 0.05 wt%. For homogeneous alloys
the samples were realloyed three times. Homogeniz-
ing annealing was performed at 973 K for 400 h in
evacuated quartz ampules with subsequent quenching
in ice water. To prevent reaction of alloys with quartz
they were wrapped in a molybdenum foil. As a re-
sidual oxygen getter we placed in the ampules ti-
tanium turnings. By neutron activation data, the
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2
2
.8 10 %.
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2
2
2
structure (a 0.7288 and 0.7155 nm, respectively).
0
6
7
ScFe crystallized in the MgNi structural type with
2
2
the lattice periods a 0.4970 and c 1.630 nm, nicely
0
0
1
983, vol. 53, no. 7, p. 1443.
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RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 73 No. 6 2003