Page 72 - Principles of Catalyst Development
P. 72

CATALYTIC  MATERIALS                                              59
               The  model  is  consistent  with  observed  patterns  in  adsorption  and
           reactivity across the periodic table. Both spacing and orbital occupancy are
           involved, so that combinations of ensemble and ligand effects are necessary
           to  explain  dependencies.
                Finally, alloying patterns are justified.  Figure 4.6  shows cyclopropane
           hydrogenation  decreasing  uniformly  with  surface eu concentration.  Each
           nickel  atom  retains  its  orbital integrity,  and the  effect  of copper is  simply
           to  dilute.  For instance, if n  surface  nickel sites are required to generate an
           absorbed  species,  then  a  surface  copper  fraction feu  results  in  an  activity
           decrease,  AI A o, given  by


                                       A.         )n
                                      Ao = (1  - feu                      (4.1 )

                An  additional  feature  of the  localized  bond  model  emerges.  Small
           crystallites of metals exist as  pseudospherical cubo octahedra(108) as shown
           in  Fig.  4.10.
                In the ideal form of this structure, only [100]  and [111]  planes occur,
           but atoms  are also  situated  at sites  corresponding to corners and at edges
           between  these planes. These have  lower coordination than plane sites and
           exhibit different orbital symmetries. The distribution of these sites is shown
           in Fig. 4.11, suggesting that adsorption and reaction involving low coordina-
           tion sites increases as crystallite size decreases, whereas those occurring on
           face  sites  increase  with  sizeyo9)  Thus  a  crystallite  size  effect  is  seen, as
           verified  by  experiments  on  selectivity  factors  in  cyclohexane  reactions
















                                 1   CORNER  SITES
                                2   EDGE  SITES,  [111], [111]
                                3   EDGE  SITES,  [111], [100]
                                4   F ACE  SIT E S ,  [1 00]
                                5   FACE  SITES,  [111]
                            Figure 4.10.  Surface sites  on  cubo-octahedra.
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