Both agglomeration and grain growth are undesirable for a number of applications of ultradisperse oxide powders obtained from co-precipitated precursors.
These problems can be solved by freezing of as-obtained residues followed by
freeze-drying. Removal of ice at low temperatures and reduced pressures hinders
the interparticle bridging process almost completely. Taking into account that
precipitation/co-precipitation results in the formation of nanosized residue particles
able to retain their size during further processing, cryogel synthesis is a useful
preparation method for multicomponent nanomaterials [26–63].
In the case of oxide solid solutions [28–31] or complex oxide formation [26, 32–
39], a high chemical homogeneity of the precursor promotes fast and uniform
formation of the target product during its thermal processing. However, even in
the case of non-reacting components, it was shown that cryogel synthesis is useful
for the synthesis of nanocomposites [27, 40–54]. It is also possible to synthesize
core–shell oxide particles by this method when precipitation of the components is
performed not simultaneously, but sequentially [55].
All these features are also attractive for the synthesis of solid-state catalysts for
various reactions in the gas and liquid state. The low thermal stability of silver and
noble metal oxides allows one to obtain the metal catalyst with oxide promoter
and/or support in a single synthesis. In this case, the starting gel contains hydroxides
of both components. Thermal processing of freeze-dried multicomponent cryogels
at 600–1,300
C causes the decomposition of Al hydroxides to oxides, whereas the
corresponding compounds of Ag [40–42], Pt, and Pd [43–45] are decomposed to
metals. The introduction of a carbon support into the starting multicomponent
solution, followed by liquid phase reduction by NaBH 4 or ethylene glycol and,
finally, freezing and freeze-drying produces PtNi and PtRh catalysts without additional heat treatment (Fig. 2) [27, 46, 47]. Similar methods can be used for the
preparation of free-standing PtAu nanoparticle catalyst [48] and well-dispersed
water-soluble CdTe quantum dots in montmorillonite clay host media [54].
Variation of the thermal processing conditions allows modification of the micromorphology of the particles thus formed, and ensures the maximum sinterability of
oxide powders in order to obtain dense functional ceramics at the lowest possible
temperatures [56, 57]. The absence of hard agglomerates in the starting powder is
crucial for the production of optically transparent, fully dense ceramics [33] that
can be used as the active body of solid state lasers instead of single crystals [58–
60]. The products of cryogel synthesis can also be used as templates in
topochemical reactions with easy melting components. When the reaction of
complex oxide formation is limited by diffusion of the mobile component into
the precursor particles, the size and the shape of the complex oxide particles can be
controlled by directed modification of the particles of cryogel precursor [61–63].
Similar approaches and methods have been applied to single component
cryogels. As the chemical homogeneity of the cryogel is not critical in this case,
the main reason to use cryogel precursors is to protect their unique micromorphology during drying. In most cases, the cryogel precursors are used to obtain various
oxide nanoparticles, e.g., Co 3 O 4 [23], CeO 2 [64], V 2 O 5 [65], and ZrO 2
[66]. Cryogels of silica are widely used as sorbents or cryogenic thermal insulators
228
O.A. Shlyakhtin
These problems can be solved by freezing of as-obtained residues followed by
freeze-drying. Removal of ice at low temperatures and reduced pressures hinders
the interparticle bridging process almost completely. Taking into account that
precipitation/co-precipitation results in the formation of nanosized residue particles
able to retain their size during further processing, cryogel synthesis is a useful
preparation method for multicomponent nanomaterials [26–63].
In the case of oxide solid solutions [28–31] or complex oxide formation [26, 32–
39], a high chemical homogeneity of the precursor promotes fast and uniform
formation of the target product during its thermal processing. However, even in
the case of non-reacting components, it was shown that cryogel synthesis is useful
for the synthesis of nanocomposites [27, 40–54]. It is also possible to synthesize
core–shell oxide particles by this method when precipitation of the components is
performed not simultaneously, but sequentially [55].
All these features are also attractive for the synthesis of solid-state catalysts for
various reactions in the gas and liquid state. The low thermal stability of silver and
noble metal oxides allows one to obtain the metal catalyst with oxide promoter
and/or support in a single synthesis. In this case, the starting gel contains hydroxides
of both components. Thermal processing of freeze-dried multicomponent cryogels
at 600–1,300
C causes the decomposition of Al hydroxides to oxides, whereas the
corresponding compounds of Ag [40–42], Pt, and Pd [43–45] are decomposed to
metals. The introduction of a carbon support into the starting multicomponent
solution, followed by liquid phase reduction by NaBH 4 or ethylene glycol and,
finally, freezing and freeze-drying produces PtNi and PtRh catalysts without additional heat treatment (Fig. 2) [27, 46, 47]. Similar methods can be used for the
preparation of free-standing PtAu nanoparticle catalyst [48] and well-dispersed
water-soluble CdTe quantum dots in montmorillonite clay host media [54].
Variation of the thermal processing conditions allows modification of the micromorphology of the particles thus formed, and ensures the maximum sinterability of
oxide powders in order to obtain dense functional ceramics at the lowest possible
temperatures [56, 57]. The absence of hard agglomerates in the starting powder is
crucial for the production of optically transparent, fully dense ceramics [33] that
can be used as the active body of solid state lasers instead of single crystals [58–
60]. The products of cryogel synthesis can also be used as templates in
topochemical reactions with easy melting components. When the reaction of
complex oxide formation is limited by diffusion of the mobile component into
the precursor particles, the size and the shape of the complex oxide particles can be
controlled by directed modification of the particles of cryogel precursor [61–63].
Similar approaches and methods have been applied to single component
cryogels. As the chemical homogeneity of the cryogel is not critical in this case,
the main reason to use cryogel precursors is to protect their unique micromorphology during drying. In most cases, the cryogel precursors are used to obtain various
oxide nanoparticles, e.g., Co 3 O 4 [23], CeO 2 [64], V 2 O 5 [65], and ZrO 2
[66]. Cryogels of silica are widely used as sorbents or cryogenic thermal insulators
228
O.A. Shlyakhtin