Mesoporous crystalline metal oxides
Abstract
Mesoporous monocrystalline metal oxides (e.g. Co₃O₄, Cr₂O₃, NiO, CeO₂, In₂O₃
and WO₃) templated by SBA-15 or KIT-6 were synthesised successfully by using a
simple solvent-free approach, the so-called solid-liquid method, which was the
principal development of methodology in this project. A metal-containing precursor,
whose melting point is lower than its decomposition temperature, was directly ground
with a mesoporous silica and impregnated into the pores of the silica template after
melting when the temperature was increased above its melting point. The liquid
precursor then decomposed to form metal oxide inside the silica pores when the
temperature was further increased to its decomposition temperature and
crystallization temperature of the oxide. The structural characterisations of these
porous metal oxides were performed by using TEM, XRD and N₂
adsorption/desorption techniques. The solid-liquid method is convenient and
solvent-free. On the other hand, its limitation is that the precursor must have a
melting point lower than its decomposition temperature.
A novel porous single crystal of rutile TiO₂ as well as anatase nanocrystal-silica
composite was also synthesised successfully for the first time using SBA-15 and
KIT-6 as templates. These materials have interesting properties of proton conductivity,
Li insertion and photoactivity. Likewise, the characterisation of porous TiO₂ was
achieved by using XRD, TEM, SAED and N₂ adsorption/desorption. The residual
SiO₂ component in porous TiO₂ was detected by using the EDX technique.
Porous cubic metal oxides of Co₃O₄, NiO, CeO₂ and In₂O₃ were prepared using
novel mesoporous silicas FDU-12 and SBA-16, which contain spherical nanocavities
linked together by smaller windows. These porous materials have larger surface areas
than those templated by SBA-15 and KIT-6. Unlike the cubic metal oxides, syntheses
of porous crystals of non-cubic metal oxides such as rhombohedral Cr₂O₃, Fe₂O₃ and
hexagonal TiO₂, WO₃ were not successful when using cage-containing mesoporous
silicas as templates. The three-dimensional arrangements of nanospheres in porous crystals of cubic oxides mentioned above were observed by TEM and the
corresponding larger surface areas were confirmed by N₂ adsorption/desorption
technique.
Additionally, fabrication of porous crystals of other metal oxides such as MgO,
ZnO and ZrO₂ were unsuccessful by using either mesoporous silicas or mesoporous
carbons as templates. Possible drawbacks of using mesoporous silica and carbon as
templates were discussed.
Type
Thesis, PhD Doctor of Philosophy
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