Page 73 - Principles of Catalyst Development
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60                                                       CHAPTER  4

                        .8
                            :  CORNER
                            I          ~(111] FACE
                            I
                     C/)   •7 ·  I
                     w      I  I
                     I-     I  I
                     en  .6   I
                     w
                     ()
                     ~  .5
                     a:
                     :::>
                     C/)  .4
                     u.
                     o
                     z
                     o
                     ~
                     ()
                     «                                   (100]  FACE
                     a:
                     u.
                                             (111]-(100]  EDGE

                         o        2   3   4   5    6   7   8    9  10
                                          RADIUS,  nm

                      Figure 4.11.  Distribution of surface sites  with  crystallite size. II08 )

            (Fig.  4.12). (I 101  Real  crystallites  are  not  ideal  cubo-octahedra.  Imperfect
           planes develop other types of sites, as shown in Fig. 4.13.(83)  Low coordina-
           tion  sites  such  as  corners,  edges,  steps,  and  kinks  are  high  energy  sites.
           There is evidence that reactions such as hydrogenolysis occur preferentially
           on these sites,19S 1  so that selectivity patterns may depend on crystallite size
           or surface  morphology.  Also,  blocking  promoters,  such  as  Cu  in  Ni,  and
           poisons, such as S on Pt, bond with these sites and account for the pronoun-
           ced effect of these agents in decreasing hydrogenolysis.(831 Experiments such
           as  these  have  led  to  much  interest  in  the  theoretical  properties  of these
           small crystallites.( 111,IIZ)
               The  question  of strong  metal  support  interactions  (SMSI)  has  been
           mentioned  in  Chapter 2.  There  is  little  question  that  the effect  exists,  but
           the  mechanism  is  still  far  from  clear.(63)  For  metals  supported  on  easily
           reducible  oxides  such  as  Ti0 2 ,  reduction  at  high  temperatures  results  in
           loss of hydrogen adsorption and reactivity. Mild oxidation reverses the loss,
           verifying speculation that Ti3+ sites are generated and, in some way, poison
           orbital participation. Similar but less  pronounced effects are observed with
           common supports such as Alz0 3  and Si0 2 .  Many examples of SMSI effects
           and  possible  explanations  are  given  in  reviews.(59,63)  Although  interesting
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