Page 67 - Principles of Catalyst Development
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54                                                       CHAPTER  4
            by the  band structure  shown  in  Fig.  4.4.  Moving  from  right  to  left  across
            the  periodic table  means less  d-electrons  are available to  fill  the bands of
            energies corresponding to collectivized d-orbitals.  Levels are filled success-
            ively until the  Fermi  level  is  reached.  A certain number of vacant levels or
            d-holes are available for bonding with adsorbates; the lower the Fermi level
            the  larger the  number and  stronger the adsorption.  Thus,  another version
            of the  ethylene  hydrogenation  volcano  curve,  shown  in  Fig.  4.5,  results.
            Ethylene hydrogenation rate is presumably related to the strength of adsorp-
            tion through the number of d-holes.(7!1 This theory began to lose credibility
            when  experiments  on  well-characterized  Ni-Cu  alloys  become  possible.
            The rigid band model predicts that alloying with Cu should result in some
            "intermediate" atom  as  nickel  accepts electrons from  copper. The  d-band
            fills  and  the  Fermi  level  increases  until,  at  55%  Cu,  no  more  holes  are
            available.  Catalytic  activity,  such  as  ethylene  hydrogenation  in  Fig.  4.5,
            should be zero at this point. However, measurements on Ni-Cu alloys with
            well-characterized  surface  compositions/  IOO )  shown  in  Fig.  4.6,  do  not
            support this.
                Much  of the earlier work  on  alloy  films(24)  was  confused  by  the  fact
            that  binary  systems  are  very  susceptible  to  surface  enrichment  by  one  of
            the components.I!D!)  Techniques for  measuring alloy surface compositions
            have led  to a  much greater understanding. (102,103.104)  Current views are that












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           (;)
           a:
           w
           z
           w

                                                        Fe   Co  Ni        Cu







                           1/d                            ENERGY
                           Figure 4.4.  Electronic band  structure of d·metals.
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