Environmental Engineering Reference
In-Depth Information
1.4.1.1
Co-precipitation by Titration
In this method, a metal salt solution is titrated against a basic solution dur-
ing which not only solids of LDHs structure occur, but also other solid
phases are formed, due to the precipitation of less soluble metal hydroxides
prior to the precipitation of the LDHs.
1.4.1.2
Co-precipitation at Low Supersaturation
Co-precipitation at constant pH and at temperature between 60-80
C
by the slow addition of mixed solution of M(II) and M(III) salts in the
chosen ratio and the base solution. h e rate of addition has to be con-
trolled in order to obtain a more homogeneous phase. h ese conditions
are best to obtain higher crystallinity of the material due to the fact that
at constant pH the rate of crystal growth is generally lower than the rate
of nucleation [3].
h is method is most ot en used, as it is the most ef ective one. For
example, Marchi
et al.
obtained an amorphous precipitate of Cu/Co/
Al hydrotalcites catalysts by co-precipitation at high supersaturation
while under the conditions of low supersaturation a crystalline HT was
precipitated [48].
°
1.4.1.3
Co-precipitation at High Supersaturation
In this method, a solution containing a mixture of the metal ion is quickly
added to the basic solution and mixed rapidly. h is results in the formation
of LDHs which are generally not very crystalline, but sometimes amor-
phous product is obtained due to the high number of nuclei formation.
h e precipitates obtained by this method are washed with distilled water
in order to eliminate adsorbed ions and are dried at temperatures up to
120
°
C [1, 49].
1.4.2 Hydrothermal Synthesis
Hydrothermal preparation methods have been employed in order to con-
trol the particle size and distribution. h ere are two routes to hydrother-
mal treatment. In the i rst case, materials are treated at temperatures more
than 373K in a pressured autoclave. Here, the LDHs may be synthesized
from precursor such as MgO and Al
2
O
3
or from mixtures obtained from
the decomposition of the nitrate forms of these precursors [1, 50, 51]. More
specii cally, the precursors are placed in an autoclave set at temperature
above 596K and pressure between 10-130 MPa for a i xed time period.
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