SCHEDULED MAINTENANCE
Incorporation of lanthanides in alumina matrices by a sol–gel process employing heterometallic alkoxides, M[Al(OPri)4]3 , as precursors
Abstract
Spectroscopic studies of the hydrolysis of heterometallic alkoxides,
M[Al(OPr
i
)
4
]
3
, M=La, Ce, suggest that
they do not dissociate during the early stages of hydrolysis. Gels from
these heterometallic alkoxides can be prepared by two methods (i) direct
hydrolysis in Pr
i
OH, and (ii) peptization with acetic acid.
Changes that occur in the structure and surface properties of gels during
heat treatment are monitored by BET surface area measurements and X-ray
powder diffraction (XRD) studies. No crystalline phase is observed below
500 °C in xerogels derived from La[Al(OPr
i
)
4
]
3
and Ce[Al(OPr
i
)
4
]
3
or
their mixtures. The XRD of the gel derived from
La[Al(OPr
i
)
4
]
3
shows diffraction peaks
at 900 °C due to LaAlO
3
. Cerium oxide starts to separate
out at 600 °C in xerogels prepared from
Ce[Al(OPr
i
)
4
]
3
, and at 700 °C in
xerogels prepared from mixtures of La[Al(OPr
i
)
4
]
3
and Ce[Al(OPr
i
)
4
]
3
. Raman
spectra of the gels prepared from the mixtures of
La[Al(OPr
i
)
4
]
3
and
Ce[Al(OPr
i
)
4
]
3
show that the
CeO
2
crystallite sizes are >100 Å, and CeO
2
, in fact, contains lanthanum in a mixed fluorite structure.
High-resolution electron microscopy confirms that the lanthanum is part of
the CeO
2
fluorite structure even in lanthanum rich areas. X-Ray
photoelectron spectroscopic (XPS) analysis of the lanthanum containing
gels, heated to 500 and 1000 °C, shows a peak at 836.5 eV for La
3d
5/2
, which could be due to the presence of
M–O–Al type bridges in the samples. The contribution of
µ‴ in the Ce 3d peaks suggests that both oxides,
CeO
2
and Ce
2
O
3
, are present in cerium
containing gels. The presence of lanthanum does not affect the
distribution of Ce
III
and Ce
IV
in our
samples.
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