Page 111 - Principles of Catalyst Development
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CATALYST PREPARATION 99
vessel surfaces. The rate of nucleation may be accelerated by deliberate
addition of seed nuclei. Growth then proceeds through adsorption of ions
on the surface of the seeded particle. This growth process proceeds at a
rate that is a function of concentration, temperature, and pH.
Thus the relative rates of nucleation and growth may be independently
controlled to some extent. If nucleation is faster than growth, the system
produces a narrow distribution of small particles. Fast growth results in
narrow distributions of larger particles. When the rates are similar, wide
size distributions result.
Precipitation of sols may also be initiated by condensation, by hydroly-
sis of organic soluble complexes, and by oxidation or reduction.(156)
6.2.3. Agglomeration and Gelation
Sols have large surface energies which the system strives to minimize
through particle growth or agglomeration. (156) There are three types of
agglomerates: hydrogels, flocks, and massive precipitates. Hydrogels are
the most important in catalysis.
Hydrogels have a three-dimensional, loosely bound structure, as shown
in Fig. 6.3. Small particles set up long-range order through hydrogen bonding
via the interstitial water molecules. Better long-range homogeneity is
achieved with gels derived from sols with a narrow size distribution.
However, these particles are mobile and grow through a process of collision
and coalescence. In order to appreciate factors controlling this growth, it
is necessary to examine the nature of interaction between these hydrous
oxides and water molecules,
OH OH 0-
I I I
H 20 + -M-O-M- ~ -M- + H;O ( 6.2)
OH OH OH;
I I I
H 20 + -M-O-M- ~ -M- + OH-
PARTICLE DIAMETER 1.5 nm
o 0 d~O'~
0.9 nm
o 0 0 0 O-r
00000
Figure 6.3. Structure of a hydrogel.