Page 112 - Principles of Catalyst Development
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100 CHAPTER 6
The surface may be positively or negatively charged depending on the
pH. With low pH acid solutions, the equilibrium is driven toward positive
surfaces. The surface becomes less positive and finally negative as the pH
increases. However, the effective charge on the surface is partially neutral-
ized by counterions in the solution. In Fig. 6.4, a positively charged particle
attracts anions in the solution. These originate from the bases used during
precipitation or may be electrolytes added during aging. Counterions form
a space charge, part of which is held sufficiently strongly to be carried along
as the particle moves with Brownian motion. The result is an effective
charge, called the zeta potential. Both the original charge and the neutraliz-
ing counterions respond to pH changes, resulting in the zeta potential curves
shown in Fig. 6.5.
The zeta potential determines the rate of gelation. If the charge is high,
particles effectively repel one another and avoid contact. If it is low, then
thermal motion leads to collision and coalescence. These rates are highest
at the isoelectronic point, where the zeta potential is zero.
These effects are demonstrated in Fig. 6.6 showing surface areas and
gelation times for silica sols. (ISS) From a practical point of view, it is
necessary to compromise between large surface areas and reasonable gela-
tion times.
Silica gels, such as those in Fig. 6.6, are prepared by mixing solutions
of water glass (sodium silicate with Si02/Na20 = 3.22) and HC!. This is
an example of condensation and produces (HO)3Si-O-Si(OHh sols of
about 1.5 nm at a pH of 6. Gelation times of 10 min result in gels so stiff
they may be cut into cubes. The hydrogel has a pore volume of 2.0 cm 3 g-l
and contains about 60% - 70% H 20.
COUNTERIONS
- /
/-
Figure 6.4. Double layer structure of a charged particle.