Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
V.P. Balema et al. / Journal of Alloys and Compounds 329 (2001) 108–114
109
ence of Ti- and/or Fe-based catalysts. The experimental
data presented below demonstrate that the high catalytic
activity of TiCl4 reported in Ref. [8] is associated with in
situ formation of a microcrystalline intermetallic Al3Ti
phase from TiCl4 and LiAlH4 during ball-milling. Once
formed, Al3Ti acts as a heterogeneous dehydrogenation
catalyst.
atmosphere. The 27Al NMR spectra were acquired in a
Chemagnetics MAS probe using single-pulse excitation
and a spinning rate of 20 kHz. A two-pulse phase
modulation (TPPM) decoupling scheme was used to
eliminate the line broadening due to the heteronuclear
1
dipolar interaction with H nuclei [14]. The 27Al spectra
reported in this work use the d scale, with positive values
downfield, and are referenced with an aqueous solution of
aluminum nitrate.
2. Experimental
Differential thermal analysis (DTA) of the powder
samples was carried out on a Perkin-Elmer DTA 7 unit
between 20 and 3008C with a heating rate of 108C/min in
an argon atmosphere. Aluminum oxide crucibles were used
as holders and aluminum oxide was used as a reference
material. Chemical analysis of selected samples was
performed using an ICP-AES technique on a Thermo
Jerrell Ash IRIS spectrometer according to a standard
ICP-AES procedure [15].
LiAlH4 ($98 wt.% pure) and TiCl4 (99.995 wt.% pure)
were purchased from Sigma–Aldrich. TiH2 (99 wt.% pure)
was purchased from Alfa. The intermetallic compounds
were prepared from titanium, iron and aluminum (all were
99.999 wt.% pure), purchased from various commercial
vendors. All operations on lithium aluminohydride,
titanium hydride and titanium tetrachloride were carried
out in a helium atmosphere in a glove box. Ball-milling of
various quantities of materials, usually 0.5–1.0 g, was
performed using 21 g of steel balls in a Spex mill in a
hardened-steel vial sealed under helium. Forced air-cooling
of the vial was employed to prevent its heating during
ball-milling experiments. The majority of hydrogen formed
during solid-state transformations promoted by mech-
anochemical processing of LiAlH4 in the presence of
different catalysts remained inside the vial and was de-
tected during its opening. However, it is feasible that some
hydrogen gas escaped the vial during the processing and/
or was absorbed by the vial walls, and therefore no
measurements of the quantities of hydrogen built-up inside
the vial during the processing were carried out.
3. Results and discussion
Unlike complex boron- [16] and magnesium-based
hydrides [17], little is known about mechanically induced
solid-state transformations of complex derivatives of
aluminum hydride. Except for a few reports on mechani-
cally induced solid-state reduction of several transition
metal halogenides by alkali metal aluminohydrides [18,19]
and on mechanochemical preparation of complex
aluminohydrides of Li, Na, Mg and Sr [5,9,20–23], no
systematic data about mechanochemical behavior of this
class of materials is available in the literature.
Intermetallic alloys with Al3Ti, Al22Fe3Ti8 and Al3Fe
stoichiometries, approximately 10 g each, were prepared
by arc-melting mixtures of pure metals in an argon
atmosphere on a water-cooled copper hearth. To ensure
alloy homogeneity, samples were turned over eight times
during arc-melting. Since weight losses during the prepara-
tion of alloys did not exceed 0.8 wt.%, the compositions of
the alloys were assumed unchanged.
As we reported in our earlier communication [8], ball-
milling of material consisting of 97 mol% of LiAlH4 and 3
mol% of TiCl4 for 5 min at room temperature causes
complete transformation of LiAlH4 into Li3AlH6, Al and
H2. Only Bragg peaks corresponding to the microcrystal-
line Li3AlH6, Al and LiCl can be seen in the X-ray powder
diffraction pattern of the ball-milled hydride (Fig. 1). The
differential thermal and gas-volumetric analyses of the
mixture (i.e. only one thermal event associated with the
decomposition of Li3AlH6 was observed, and the amount
of hydrogen released during heating to 6508C corresponds
to the amount of produced Li3AlH6) confirmed the results
of X-ray powder diffraction indicating that the transforma-
tion proceeds according to Eq. (1), and showed that no
other nano-crystalline or amorphous hydride phases unde-
tectable by X-ray powder diffraction formed during the
mechanochemical processing (see Ref. [8] for more de-
tails):
The X-ray powder diffraction (XRD) characterization of
the obtained materials was carried out on a Scintag powder
diffractometer using Cu Ka radiation. A full profile
Rietveld analysis of the powder diffraction data, which
were collected at room temperature with a 0.028 2Q step,
was employed for crystal structure refinement. To protect
air-sensitive hydride samples from the atmospheric oxygen
and moisture during X-ray powder diffraction experiments,
the sample holder containing a hydride powder was
covered with an X-ray-transparent polymer film in a glove
box under helium.
27Al nuclear magnetic resonance (NMR) experiments
were performed on a Chemagnetics Infinity spectrometer
operated at 9.4 T (104.2 MHz). Following the preparation,
the samples were transferred to the 3.2-mm magic angle
spinning (MAS) rotors within a glove box in a helium
2.91LiAlH4 1 0.09TiCl4 5 0.85Li3AlH6 1 0.36LiCl
1 2.06Al 1 0.09[Ti] 1 3.27H2
(1)
We also found that LiAlH4 is stable during the mech-
Balema
