Page 43 - Principles of Catalyst Development
P. 43

STRUCTURE  OF  CATALYSTS                                         29
            in the 0.5-5 nm range. Platinum spheres of this size can be made as colloidal
            platinum  black. (48)  Attempts  to  use  the  colloid  as  a  catalyst  at  reaction
            temperatures rapidly leads to sintering or agglomeration of the crystallites
            through the mechanism shown in  Fig.  2.4.
                Crystallites  contact  each  other  through  thermal  motion,  becoming
            especially agitated at higher temperatures. Above the "Tamm" temperature
            O.5Tm,  where  Tm  is  the  melting  point,  bulk  metal  atoms  acquire  enough
            thermal energy to  migrate within the crystallite.  Even at 0.3 Tm  (the Huttig
            temperature) surface atoms have enough energy to overcome weaker surface
            crystal  forces,  diffuse, and form  necks as shown in  Fig.  2.4.  If surface and
            bulk atoms are mobile enough, two smaller crystallites coalesce to a larger
            one, thereby decreasing the surface energy.  Typical  growth  is  both a ther-
            modynamic and kinetic effect.  Patterns showing a  dependence on melting
            point are demonstrated  in  Figure 2.5.(49)
                Supports function  as stable surfaces over which the active component
            is  dispersed  in  such  a  way  that sintering is  reduced. (43)  The support itself


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                          Figure 2.4.  Sintering of colloidal  metal  crystallites.
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