Page 111 - Principles of Catalyst Development
P. 111

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.
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