Interaction of hydrogen with the intermetallic compounds Sc2Al and Sc2Ni

Article abstract of DOI:10.1134/S0020168506050086

The reactions of the intermetallic compounds Sc₂Al and Sc ₂Ni with hydrogen have been investigated. The results demonstrate that Sc₂Ni reacts with hydrogen even at room temperature, to form amorphous Sc₂NiH₅. The reaction is irreversible at room temperature. Vacuum extraction of hydrogen between room temperature and 900°C leads to partial decomposition of Sc₂NiH₅ and the formation of crystalline ScH₂ and ScNi₂. Sc ₂Al is nonreactive with hydrogen at room temperature. Heating at a hydrogen pressure of 5 MPa leads to Sc₂Al hydrogenolysis starting at 225°C and the formation of ScH₂ and metallic Al. Pleiades Publishing, Inc., 2006.

Full text of DOI:10.1134/S0020168506050086

ISSN 0020-1685, Inorganic Materials, 2006, Vol. 42, No. 5, pp. 491–495. © Pleiades Publishing, Inc., 2006.
Original Russian Text © G.S. Burkhanov, N.B. Kol’chugina, O.D. Chistyakov, V.N. Verbetsky, A.A. Salamova, E.Yu. Andreeva, E.S. Volkova, 2006, published in Neorganicheskie
Materialy, 2006, Vol. 42, No. 5, pp. 551–555.
Interaction of Hydrogen
with the Intermetallic Compounds Sc2Al and Sc2Ni
a
a
a
b
G. S. Burkhanov , N. B. Kol’chugina , O. D. Chistyakov , V. N. Verbetsky ,
b
b
b
A. A. Salamova , E. Yu. Andreeva , and E. S. Volkova
Baikov Institute of Metallurgy and Materials Science, Russian Academy of Sciences,
Leninskii pr. 49, Moscow, 119991 Russia
a
b
Moscow State University, Vorob’evy gory 1, Moscow, 119899 Russia
e-mail: salamova@hydride.chem.msu.ru
Received October 31, 2005
Abstract—The reactions of the intermetallic compounds Sc2Al and Sc2Ni with hydrogen have been investi-
gated. The results demonstrate that Sc2Ni reacts with hydrogen even at room temperature, to form amorphous
Sc2NiH5. The reaction is irreversible at room temperature. Vacuum extraction of hydrogen between room tem-
perature and 900°C leads to partial decomposition of Sc2NiH5 and the formation of crystalline ScH2 and ScNi2.
Sc2Al is nonreactive with hydrogen at room temperature. Heating at a hydrogen pressure of 5 MPa leads to
Sc2Al hydrogenolysis starting at 225°C and the formation of ScH2 and metallic Al.
DOI: 10.1134/S0020168506050086
INTRODUCTION
The starting mixture was arc-melted in a water
cooled copper hearth using a nonconsumable tungsten
electrode under a high-purity helium atmosphere at a
pressure of ꢀ105 Pa. The samples were remelted three
or four times to improve the chemical homogeneity of
the alloys. Interaction of Sc2Al and Sc2Ni with hydro-
gen was studied using a purpose-designed hydrogena-
tion apparatus at pressures from 0.001 to 10 MPa and
also by high-temperature vacuum extraction in the
range 25–900°C. The hydrogen content of hydrides
was evaluated using a modified van der Waals equation.
The intermetallics and hydrides were characterized
by x-ray diffraction (XRD) on a DRON-2 powder dif-
fractometer (CuKα radiation, silicon standard). In data
processing, we used RIETAN-200 software. The sam-
ples were ground in an agate mortar, and the resultant
powders were placed in a quartz holder and secured
with x-ray amorphous film. All manipulations were
performed in a nitrogen-filled box.
Among the fundamental issues in hydrogen energy
development, the creation of novel hydrogen storage
and transport materials with tailored sorption character-
istics and dissociation pressures of their hydrides is of
particular importance. At the same time, there are many
intermetallic compounds with unique properties which
have not yet found practical application on a proper
scale. Among such compounds are rare-earth-rich
intermetallics, such as R3M and R2M (R = rare-earth
element, M = transition metal), which may be of prac-
tical interest and are potentially attractive as model sys-
tems for gaining insight into the fundamental aspects of
the chemistry of hydrides. Convincing evidence of
equilibrium interaction between such intermetallics
and hydrogen is, however, still lacking. This issue was
addressed earlier in several reports [1–8]. The interest-
ing, though somewhat contradictory, results of previous
studies led us to focus on this family of rare-earth-rich
compounds. The scandium compounds of this family,
in particular Sc2Al and Sc2Ni, have been studied very
little. In this paper, we report our findings on the inter-
action of Sc2Al and Sc2Ni with hydrogen.
RESULTS AND DISCUSSION
Structural characterization of Sc2Al and Sc2Ni.
As shown by XRD examination, Sc2Al has the Ni2In
(hP6) structure with lattice parameters a = 4.88 Å and
c = 6.166 Å, and Sc2Ni has the Ti2Ni structure (sp. gr.
Fm3d) with a = 11.318 Å. The indexing results agree
well with earlier data [10] and demonstrate that both
compounds were essentially phase-pure.
EXPERIMENTAL
Sc2Al and Sc2Ni were synthesized from high-purity
metals: 99.956% Sc (bisublimed), 99.99% Al, and
99.99% Ni. Commercial scandium was purified by vac-
uum sublimation in a graphite-heater furnace at a resid-
ual pressure of 1.33 × (10–3–10–4) Pa [9].
Interaction of Sc2Al with hydrogen. Attempts to
hydride Sc2Al at room temperature were unsuccessful:
491

492
BURKHANOV et al.
400
300
200
100
0
–100
–200
–300
10
20
30
40
50
60
70
2θ, deg
Fig. 1. XRD pattern of the decomposition products of hydrided Sc Al; the upper and lower tick marks show the positions of reflec-
2
tions from ScH and Al, respectively.
2
in the pressure range 5–7 MPa, no reaction with hydro-
gen was detected. During slow heating at a hydrogen
pressure of 5 MPa, reaction began at 225°C. The
amount of absorbed hydrogen was 4.0 hydrogen atoms
per formula unit (H : Sc2Al = 4). The XRD pattern of
the resultant compound is displayed in Fig. 1.
The heat of formation of scandium dihydride is
rather high, 50 kcal/mol [11], whereas that of Sc2Al is
about 7 kcal/mol [12]. It seems, therefore, likely that,
under the conditions of this study, the hydriding of
Sc2Al occurs according to the scheme
Sc2Al + 2H2
2ScH2 + Al,
(1)
As is well known [1, 2], the reaction between hydro-
gen and an intermetallic compound may follow differ-
ent paths, leading to drastically different reaction prod-
ucts. The most likely reaction mechanism is hydro-
genolysis, resulting in complete decomposition of the
compound and the formation of a stable AHx hydride
and an AmBn intermetallic compound (enriched in the
metal nonreactive with hydrogen) or B metal. Being the
most thermodynamically favorable process, hydro-
genolysis is the least interesting for practical applica-
tion. To ascertain which process—the formation of a
single-phase intermetallic hydride or hydrogenolysis—
is more likely, one should compare the enthalpies of
formation of the parent intermetallic compound and
expected hydriding products. In particular, a relatively
low heat of formation of a rare-earth intermetallic com-
pound in combination with a relatively high heat of
i.e., follows the most thermodynamically favorable
mechanism—hydrogenolysis.
This is supported indirectly by the fact that an
attempt to isothermally decompose the synthesized
hydride was unsuccessful: reducing the pressure from 5
to 0.01 MPa caused no hydrogen release. This may be
interpreted as evidence for the formation of the very
stable compound ScH2, whose decomposition temper-
ature exceeds 1200°C and was not attained in our
experiments.
Analysis of phase transformations in R–Al–H sys-
tems [4, 5] shows that the R3Al, R2Al, R3Al2, and RAl
(R =Y, Sc, La, Ce, Pr, Ho, Er) intermetallics are to some
formation of its hydride favors the hydrogenolysis extent subject to hydrogenolysis or do not react with
process.
hydrogen at all. La3Al reacts with hydrogen most
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INTERACTION OF HYDROGEN WITH THE INTERMETALLIC COMPOUNDS
493
readily, experiencing hydrogenolysis according to the
scheme
The hydriding of such compounds under mild con-
ditions leads to the formation of x-ray amorphous
hydrides which contain more hydrogen than do the
products of hydrogenolysis under severe conditions.
Calculations indicate that the amount of hydrogen
absorbed by Sc2Ni during hydriding exceeds that
needed for the reaction under more severe conditions,
which would be accompanied by hydrogenolysis and
the formation of a rare-earth hydride and, possibly, Ni
or an Sc–Ni alloy. This gives grounds to assume that the
x-ray amorphous phase is a hydride of the parent inter-
metallic compound:
2.4La3Al + 9.3H2
6.2LaH3 + LaAl2.4.
(2)
The mechanism underlying the interaction of these
compounds with hydrogen was analyzed by Seme-
nenko et al. [5], who studied the reactions of RxAly (R =
Ce, Pr, Ho, Er, Sc, Y) intermetallics with hydrogen in
broad temperature and pressure ranges by XRD and
differential thermal analysis. Their results demonstrate
that, at 573 K, hydrogenolysis leads to the formation of
rare-earth hydrides and aluminum-enriched intermetal-
lics or rare-earth hydrides and Al metal. The phases
forming at 300 K and lower temperatures are x-ray
amorphous superstoichiometric hydrides and crystal-
line hydrogenolysis products.
Sc2Ni + H2
Sc2NiH5 (am).
(3)
The properties of the x-ray amorphous hydride were
studied by two-step vacuum extraction of hydrogen.
The purpose of the first step was to determine the onset
temperature of hydrogen release. The sample was
heated from room temperature to 310°C, where hydro-
gen release began. XRD examination showed that, after
heating to 310°C, the sample remained amorphous. In
the second step of vacuum extraction, we examined the
possibility of hydrogen desorption during heating from
310 to 900°C. Hydrogen release was only observed in
the range 310–350°C, and the amount of desorbed
hydrogen corresponded to H : Sc2Ni = 2. Heating the
sample from 350 to 900°C produced no changes in this
ratio. The relationship between the amount of hydrogen
released as a result of vacuum extraction and that
absorbed during hydriding by reaction (3) provides
indirect evidence that, during heating, amorphous
Sc2NiH5 decomposes to form crystalline hydrogenoly-
sis products, one of which is, as usually, a dihydride:
Sc2Ni. The hydrogen content of scandium dihydride
corresponds to the amount of hydrogen that was not
desorbed during heating from 350 to 900°C, since, as
mentioned above, the decomposition temperature of
scandium hydrides exceeds 1200°C:
In light of this, it is reasonable to assume that the
hydriding of Sc2Al occurred according to scheme (1),
i.e., by the hydrogenolysis mechanism, leading to the
formation of the stable hydride ScH2. This is also sup-
ported by the amount of absorbed hydrogen,
H : Sc2Al ꢀ 4, which corresponds to the stoichiometry
of reaction (1). Direct evidence of this reaction path is
provided by the XRD results for the hydriding products
(Fig. 1): all of the observed diffraction peaks corre-
spond to scandium dihydride (sp. gr. Fm3m, a =
4.775 Å) and aluminum (sp. gr. Fm3m, a = 4.038 Å). In
addition, the diffraction line profiles point to partial
amorphization of the hydriding products.
The present results, as well as some of the earlier
data, are in conflict with the report by Antonova and
Chernogorenko [3] that, after repeated hydriding–
dehydriding cycles, the sorption capacity of Sc2Al for
hydrogen is H : Sc2Al = 4. They described a reversible
hydriding process and thermally stable hydrides which
might be used as hydrogen storage materials. Their
conclusions, however, appear highly questionable
because neither the starting alloys nor the hydriding
products in that study were characterized by XRD.
2Sc2NiH3(am) = 3ScH2(cr) + ScNi2(cr).
(4)
The above assumptions based on high-temperature
vacuum extraction results were confirmed by XRD
examination of the sample after vacuum extraction of
hydrogen in the temperature range 300–900°C (Fig. 2).
As expected, high-temperature heat treatment was
shown to result in the decomposition of amorphous
Sc2NiH5 and subsequent crystallization of the hydro-
genolysis products: all of the observed diffraction
peaks corresponded to scandium dihydride (sp. gr.
Fm3m, a = 4.775 Å) and ScNi2 (sp. gr. Fd3m, a =
6.923 Å).
Note that Semenova et al. [6] investigated the reac-
tions of the Sc2Ni, ScNi, ScNi2, Sc2Ni7, and ScNi5
intermetallics with hydrogen in the temperature range
20–900°C at a hydrogen pressure of 0.3 MPa. The max-
imum absorption capacity was determined to be 3.42,
3.04, 1.84, 3.2, and 1.71 wt %, respectively, which cor-
Interaction of Sc2Ni with hydrogen. Sc2Ni was
hydrided at room temperature at 5 MPa using the same
hydrogenation apparatus. A characteristic feature of the
hydriding process in this system is a long induction
period (several hours). The amount of absorbed hydro-
gen was H : Sc2Ni = 5.0. An attempt to isothermally
decompose the synthesized hydride was unsuccessful,
which attests to a very low equilibrium pressure in the
system Sc2Ni hydriding products–hydrogen, i.e., to sta-
bility of the hydride.
According to XRD data, the hydriding of Sc2Ni led
to the formation of an x-ray amorphous compound or a
mixture whose composition can be evaluated from ear-
lier data on the hydriding of rare-earth-rich compounds
with Al [2, 4, 5] and R3Ni compounds [7, 8].
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494
BURKHANOV et al.
300
250
200
150
100
50
0
–50
–100
–150
–200
10
20
30
40
50
60
70
80
2θ, deg
Fig. 2. XRD pattern of the decomposition products of amorphous Sc NiH after vacuum extraction of hydrogen in the temperature
2
5
range 300–900°C; the upper and lower tick marks show the positions of reflections from ScH and ScNi , respectively.
2
2
responds to the stoichiometries Sc2NiH4.6, ScNiH3.65
,
ScH2 rules out the use of Sc2Ni as a hydrogen storage
ScNi2H3.02, Sc2Ni7H1.92, and ScNi5H8.7. All of these material.
hydrides were reported to be thermally stable, with the
lowest decomposition temperature of 200°C
(ScNiH3.65), but the decomposition of this hydride did
not reach completion even at 400°C. Since Semenova
ACKNOWLEDGMENTS
This work was supported by the Russian Foundation
for Basic Research, project nos. 04-03-32194a and
03-03-33023.
et al. [6] presented no XRD data, comparison with our
results is impossible.
CONCLUSIONS
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hydrogen desorption from scandium dihydride (above
1200°C) prevents Sc2Al from being used as a hydrogen
storage material.
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