Arbeiten zu instabilen komplexen Hydriden des Aluminiums

155 Pages·2010·2.74 MB·German

by  Pawelke, Roland Hermann

Arbeiten zu instabilen komplexen Hydriden
des Aluminiums

Versuche zur Entwicklung neuartiger Verbindungen für die
reversible chemische Speicherung von Wasserstoff

Inaugural-Dissertation zur Erlangung der Doktorwürde
der Fakultät für Chemie und Biochemie
der Ruhr-Universität Bochum

vorgelegt von
Roland Hermann Pawelke
geboren in Wuppertal

Bochum, Mai 2010

Dekan: Prof. Dr. Wolfgang Schuhmann

Gutachter: Prof. Dr. Ferdi Schüth
Prof. Dr. Roland A. Fischer

Tag der mündlichen Prüfung: 13.07.2010

Die der Dissertationsschrift zugrunde liegenden Arbeiten wurde von August 2006
bis August 2009 am Max-Planck Institut für Kohlenforschung in Mülheim an der
Ruhr unter Leitung von Prof. Dr. F. Schüth durchgeführt.

Danksagung

Ich danke meinem akademischen Lehrer Prof. Dr. Ferdi Schüth für das mir
erwiesene Vertrauen, die anspruchsvolle Themenstellung und seine stete
Diskussionsbereitschaft. Bei der Bearbeitung dieses Themas wurden mir
großzügige Unterstützung und viel Freiheit gewährt.

Weiterhin möchte ich Prof. Dr. Roland A. Fischer für die freundliche Übernahme
des Koreferates danken.

Den Dres. Claudia Weidenthaler und Bodo Zibrowius danke ich für die
unschätzbare Hilfe bei Aufnahme und Auswertung von Röntgen- bzw. MAS-
NMR-Experimenten.

Meinen Kollegen aus der Wasserstoffgruppe, Prof. em. Dr. Borislav Bogdanović
Dr. Michael Felderhoff, Klaus Hauschild und André “Pommerin“ Pommerin danke
ich für die gute und lehrreiche Zeit.

Ferner möchte ich den Mitgliedern der Arbeitsgruppe Schüth für das kollegiale
Arbeitsumfeld und die ungezwungene Atmosphäre danken. Allen weiteren nicht
namentlich genannten Mitarbeitern des Max-Planck-Institutes möchte ich an
dieser Stelle meinen Dank und meine Anerkennung dafür aussprechen, dass sie
tagtäglich dazu beitragen, diesen Ort zu etwas Besonderem in der deutschen
Forschungslandschaft zu machen.

Ars requirit totum hominem

Thu nicht alles / was Du kannst,
Sag nicht alles / was Du weißt,
Glaub nicht alles / was Du hörst!

Johann Rudolph Glauber

The Virgin crowding all sail, made after her four young keels, and
thus they all disappeared far to leeward, still in bold, hopeful chase.

Herman Melville, Moby Dick

Meiner Familie
Abstract

The efficient storage of hydrogen is a problem yet to be mastered on the way to-
wards common fuel cell implementation. Most, if not all, efforts in contemporary
metal hydride hydrogen storage research aim by concept at the destabilization of
stable systems. But after more than a decade of extensive research, the potential
of this approach seems to be exploited while no turnkey solution is in sight. In
this work, the usual paradigm is inverted for the first time and the issue ap-
proached upside down via the stabilization of instable systems.

Complex aluminium hydrides (alanates) are a logical compromise of storage ca-
pacity and reversibility while the hydrogen evolution is not troubled by formation
of volatile byproducts toxic to fuel cell components. The s-block alanates (e.g. Ti-
doped NaAlH ) have been exhaustively investigated as hydrogen storage mate-
4
rials; but alanates of 3d-elements have actually received very limited attention
since the discovery of the alanate ion at all. A common feature of 3d-alanates is
their pronounced thermal instability (usually T < -78 °C), making their investiga-
tion at best tricky. Apart from their mere, more or less safeguarded existence,
almost nothing is known about these compounds.

Although the characteristics of these compounds are not appealing for an indi-
vidual hydrogen storage material, it seems worthwhile to consider these materi-
als as building blocks for more advanced hydrogen storage systems. Such a sys-
tem may be approached by the combination of an instable 3d-alanate with a sta-
ble alkali alanate; quaternary alanate systems of 3d-elements are entirely un-
known. Chemical reason advocates KMII(AlH ) with M = 3d-metal as target
4 3
compounds. These systems may be approached via a mechanochemically acti-
vated metathesis between a ternary chloride perovskite KMIICl starting material
3
and a QAlH alkali alanate with Q = Li, Na, K at a ratio of 1:3. The preparation of
4
chloride perovskites KMIICl (M = Ti, Cr-Zn) was easily achieved from KCl and
3
MIICl by a ball milling reaction.
2
The subsequent mechanochemical alanate metathesis was studied on the ex-
amples of manganese and zinc under 270 bar H . The reaction proceeded read-
2
ily in all cases as indicated by formation of the respective alkali halide without
substantial decomposition of the alanate. All transition metal metathesis products
were amorphous to x-rays. The phenomenon of non-decomposition was obvi-
ously irrespective to the nature of the 3d-element and alkali alanate reactant, but
the former aspect showed an influence on the product pattern of the metathesis
reaction.

The KZn(AlH ) system was found to be not coherent as indicated by formation of
4 3
KAlH . Further investigations by XRD-diffraction, 27Al-MAS-NMR spectroscopy,
4
DSC- and thermolysis measurements allowed the conclusion, that Zn(AlH ) is
4 2
formed as an amorphous intermediate that decomposes subsequently to AlH
3
and ZnH . The completeness of this latter process seems to be influenced by the
2
cation of the respective QCl alkali halide matrix (Q = Li, Na, K). The decomposi-
tion enthalpy of ZnH was estimated -64 kJ mol-1 H . The investigation of the
2 2
KMn(AlH ) samples by XRD-diffraction, thermolysis and DSC-measurements
4 3
gave hint towards the existence of a KMn(AlH ) system.
4 3

The application of hydrogen pressure (~ 270 bar) during milling was found to
account only for a minor proportion of the stabilizing effect and the reaction
yields basically the same products when run under 1 bar argon (shown for the
KZnCl /3 LiAlH system). However, the latter conditions favoured an increased
3 4
tendency towards alanate decomposition; this observation could be explained
with the occurence of metatstable intermediates that are stabilized by
hydrogen pressure. Since neither the 3d-metal, nor the alkali alanate showed
a decisive influence on alanate stability in this reaction either, the reason for
the global phenomenon of non-decomposition is most likely related to the
structure of the KMIICl perovskite reactant. This conclusion was reinforced by
3
a cross-experiment with ZnCl and LiAlH at a ratio of 1:2 and utter
2 4
decomposition of the alanate anion was observed.
A reaction mechanism hypothesis capable of explaining this difference has
been developed. It is astonishing that the decomposition of notoriously
instable 3d-metal alanate systems may be at least substantially delayed just
by starting from a ternary chloride perovskite KMIICl instead of a binary MIICl
3 2
reactant. The potential of this unparalled reaction has to be further exploited;
these investigations may contribute substantially to a better overall understan-
ding of instable metal hydride systems.

A second concern of this work is the investigation of the unanswered questions
within the Ca/Al/H-system. CaAlH is a potentially reversible metal hydride, but its
5
preparation in good crystallinity and characterization were a then unsolved chal-
lenge. It was found that Ca(AlH ) reacts with LiH in a mechanochemical reaction
4 2
to CaAlH and LiAlH via an unknown phase; CaAlH of excellent crystallinity can
5 4 5
be isolated in high yield and purity.

It could be shown that the unknown phase has supposedly the composition
LiCa(AlH ) . The decomposition enthalpy of CaAlH was found to be 25 kJ mol-1
4 3 5
H , the activation energy of the decomposition has been estimated 124 kJ mol-1
2
CaAlH . Doping of CaAlH with three mol % TiCl was found to lower the decom-
5 5 3
position enthalpy from 25 to 19 kJ mol-1 H , but the activation energy of the de-
2
composition was increased from 124 to 183 kJ mol-1 CaAlH by this measure. Al-
5
though the doping with TiCl yielded a thermodynamic destabilization of CaAlH ,
3 5
system kinetics changed for the worse. This observation could be explained by a
decomposition mechanism hypothesis accounting for the semi-polymeric struc-
ture of CaAlH .
5

The doped and pristine CaAlH samples were decomposed at 300 °C and mech-
5
anochemically rehydrogenated at 280 bar H for three hours. Whereas no hydro-
2
gen uptake was observed for the pristine CaH /Al-system, the Ti-doped sample
2