Exploration of hydrogenation conditions of intermetallic in situ N. Kunkel , C. Reichert

Exploration of hydrogenation conditions of intermetallic
compounds via in situ DSC
N. Kunkel1, C. Reichert1 , P. Wenderoth1, H. Kohlmann2
1 Saarland
University, Inorganic Solid State Chemistry, Saarbrücken, Germany
2 University Leipzig, Inorganic Chemistry, Germany
Introduction
Metal hydrides are in the focus of current energy related research due to their applications in hydrogen storage. The formation of metal hydrides by hydrogenation of intermetallic compounds - a
crucial step for the use as hydrogen storage media - is often a multi-step process and sometimes accompanied by the intermediate appearance of metastable compounds [1, 2]. We aim at
investigating such rarely explored reaction pathways in order to better understand the basics steps of the hydrogenation process. A combination of in situ studies (DSC, X-ray and neutron
diffraction) are performed on the solid-gas reactions of metals or intermetallic compounds with hydrogen (deuterium) in order to study reaction pathways, preferred positions of hydrogen, and the
kinetic stability of metastable hydrides.
The instrument
Hydrogenation of Zintl-Phases
● Some Zintl-Phases are known to form hydrides which can be classified by two different types: ZintlPhase hydrides and polyanionic hydrides [3].
● DSC measurements are used to confirm or eventually improve the synthesis conditions described in
the literature.
● Conditions for new reactions can be found by simple DSC measurements.
To explore the reaction conditions of hydride formation a DSC
equipped with a high pressure cell is a useful instrument. Our
gas pressure cell allows to use pressure up to 70 bar hydrogen
which is combined with up to 450°C reaction temperature.
Note: In all DSC-diagrams exothermic reactions are in positive
direction on the y-axis.
Fig. 1:
The TA Instruments
Q1000
with
high
pressure cel.
● Example 1: SrGa2 – SrGa2H2: Conditions in literature are 50 bar pressure and 180°C [4].
In situ investigation of the hydrogenation of LiAl
Zintl-Phase SrGa2
● Hydrogen induced decomposition of the
Zintl phase LiAl (NaTl Type) [9] into LiH and
LiAl with decreased unit cell (Fig. 8).
● The significantly higher Debye Waller
factors in Li and Al positions after
hydrogenation hint to an increase in defect
concentration [10] VLi and VAl (Fig. 10):
LiAl + ½ D2 → LiD + VAl
LiLi + ½ D2 → LiD + VLi
Polyanionic hydride
SrGa2H2
Fig. 2: Structures of SrGa2 and its hydride (left, Sr = blue, Ga = yellow, H = red) and DSC-diagram (right), starting
from 50 bar hydrogen pressure. The sample was heated with 5°C/min up to 220°C and then held for 24h at this
temperature. The reaction starts slowly at about 110°C and the main peak grows at about 180°C.
● Example 2: YbGa2 – ?: YbGa2 is not yet known to form a hydride. DSC-measurements have been
done at various starting pressures to get to know if a reaction takes place.
● Cell Volume decreases by 1.633 ų
Fig. 8: Plot and second order polynomial fitting of the Rietveld refined lattice
parameters of LiAl vs Temperature from an in situ neutron powder diffraction
experiment
YbH2,67
+
Ga-rich phases
Zintl-Phase YbGa2
Fig. 3: Structure of YbGa2 (right, Yb = blue, Ga = yellow) and DSC-diagrams starting from 54 bar (left) and 5 bar (middle) hydrogen pressure.
The samples were heated with 5°C/min up to 220°C and 180°C, respectively and then held for 2h at this temperature. A reaction takes place but
Fig. 10: Plot of the Debeye-Waller factor of the Aluminium position
only the pseudo-cubic YbH2,67 and Ga-rich phases are found as reaction products.
in LiAl vs temperature form an in situ neutron powder diffraction
experiment
Fig. 9: In situ DSC of Li1,1Al at 50 bar hydrogen pressure up to 450°C, used
Hydrogenation of palladium rich intermetallics
as sample preparation method. The broad signal at the first run at 300°C
indicates probably the formation of LiH. The more narrow signals at 400 °C
belong to the formation of the orthorhombic phase mentioned on the right.
● New phase prepared via in situ DSC (see
Fig. 9)
● Indexed as orthorhombic with lattice
parameters:
a = 6.292 Å, b = 5.230 Å c = 4.901 Å.
In situ investigation of the hydrogenation of Dy5Pd2
Fig. 11: In situ DSC of the hydrogenation
of Dy5Pd2 at different hydrogen
pressures. Heating and cooling rates 10
K/min.
1st:
first
cycle
showing
exothermic
hydrogen
uptake
and
formation of dysprosium hydride; 2nd,
3rd: second and third cycles of the
measurement on the same sample
showing hydrogen ab- and desorption in
dysprosium hydrde DyH2+x [8].
Fig. 4: In situ DSC of α-MgPd3 under 5 bar hydrogen pressure.
Heating and cooling rates: 10 K/min. (1): first cycle up to 520 K
showing exothermic hydrogen uptake and formation of α-MgPd3Hx;
(2): second cycle up to 750 K showing exothermic transformation
from α- to β-MgPd3Hx and possibly further hydrogen uptake; (3, 4):
third and fourth cycle up to 750 K [5].
Fig. 5: In situ DSC of the hydrogenation of tetragonal InPd3 (ZrAl3 type) at
1.3 MPa hydrogen pressure. Heating and cooling rates 10 K/min. 1st: first
cycle showing exothermic hydrogen uptake and formation of InPd3H≈0.8;
2nd, 3rd: second and third cycles of the measurement on the same sample
without further thermal effects [6].
● Dy5Pd2 does not form a ternary hydride upon
hydrogenation, but decomposes to binary hydrides of
dysprosium and palladium
● In situ DSC reveals irreversible hydrogen uptake
in palladium rich intermetallics MPd3 (M = Mg, In, Tl)
● detailed reaction pathway for MgPd3 by combining
● reaction products are: DyH2 (at 0.3 MPa); DyH2 + PdH0.7
(at 1.4 MPa); DyH2 + DyH2+x + DyH3 + PdH0.7 (at 2.5 MPa)
in situ DSC with in situ neutron diffraction [5]:
α-MgPd3 ↔ α-MgPd3D0,42 ↔ α-MgPd3D0,79 ↔ α-MgPd3D0,94
● The formation of DyH2+x at T = 495 K and p(H2) = 2.5 MPa
follows a parabolic rate law (from neutron diffraction) [11]
ZrAl3 type
↓ ΔT
β-MgPd3 ↔ β-MgPd3D0,67
Fig. 6: In situ DSC of the hydrogenation of tetragonal TlPd3
(ZrAl3 type) at 1.5 MPa hydrogen pressure. Heating and
cooling rates 10 K/min. 1st: first cycle showing exothermic
hydrogen uptake and formation of TlPd3H; 2nd, 3rd: second
and third cycles of the measurement on the same sample
without further thermal effects [7].
Fig. 7: Crystal
structure of αMgPd3D0.67
Summary
AuCu3 type
● In situ DSC reveals irreversible α-β transition in
● Reaction pathways of hydrogenation (deuteration) reactions of intermetallic phases were studied
MPd3Hx (M = Mg, In, Tl, x ≤ 1)) [5, 6, 7]
● Combination of in situ DSC, in situ diffraction and
by combined in situ methods: thermal analysis, X-ray and neutron diffraction
● In situ experiments revealed the existence of several metastable metal hydrides.
theoretical calculations reveal the existence of five
new metastable phases: β-MgPd3 (AuCu3 type),
● In situ thermal analysis allows the determination of reaction conditions for the reaction of
intermetallic compounds with hydrogen
Mg0.25Pd0.75 (Cu type), α-MPd3H0.42<x<1.0 (filled
ZrAl3 type), InPd3 (AuCu type), α-TlPd3Hx≈0.2(filled
ZrAl3 type) [5, 6, 7, 8]
Acknowledgements
We thank the Deutsche Forschungsgemeinschaft for support and the Saarland University for the financial support with LGFG scholarships (Kunkel, Wenderoth).
Sylvia Beetz and Hermann Recktenwald (Saarland University) are gratefully acknowledged for technical assistance.
References
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