Page 42 - Principles of Catalyst Development
P. 42

28                                                       CHAPTER  2
            invoked to explain and predict differences in, for example, metals or crystal
            planes.
                Semiconducting oxides and sulfides constitute a large class of catalytic
            materials. '45 )  Electron donor and acceptor levels provide redox-type activa-
           tion,  but  surface  configurations  are  more  complicated  than  with  metals.
            Greater geometric complexity leads to more selective redox reactions, such
           as partial oxidation, hydrodesulfurization, and denitrogenation. Insulators
           do  not  readily  promote  charge  transfer,  but  surface  sites  with  localized
            protons are favored. (46) Acid-like  in  nature, these sites promote carbonium
            ion  mechanisms,  resulting  in  typically  acid-catalyzed  reactions  such  as
           isomerization or cracking.
               With these brief introductory remarks, the reader is  referred to Chapter
           4  for  more  explicit descriptions of active components.

           2.3.2.  Support
                Supports, or carriers,  perform many functions,  but  most  important is
           maintenance  of high  surface  area  for  the  active  component.  This  is  best
           illustrated  with  platinum,  an  important  catalytic  metal  widely  used  for
           catalytic  reforming(1 I)  and  automobile  exhaust  clean-up. (47)  For  high
           activity,  platinum  crystallites  must  have  the  highest surface area  possible.
           Figure 2.3 shows the relationship between dispersion, defined as the fraction
           of platinum atoms on the surface of the spherical crystallite, and diameter
           of the  sphere.  Dispersion  decreases  very  rapidly  between  1  and  10 nm.
           Ideally,  platinum  crystallites should be  as  small  as  possible,  but certainly

                 1.0  ,..----=:----r--------r--------,.----...,



               ...
               z
               ......
              z  (/)
                 .5
              z
              Q
              (f)
              Il:
              W
              11.
              (f)
              0
                  0
                  0.1             1.0           10             100
                                CRYSTALLITE  DIAMETER,  nm
                      Figure 2.3.  Dispersion versus  crystallite diameter for platinum.
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