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.
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