Page 66 - Principles of Catalyst Development
P. 66

CATALYTIC  MATERIALS                                              53
           weaker  bond.  Since  this  mode  is  not  so  strongly  held  and  presumably  is
           more active,  planes with  0.35-nm  ensembles, such as  [100]  and [ItO], are
           expected to show higher activity than those without, e.g., [111]. Also, atomic
           separation  is  a  function  of metal-metal  distances  in  the  unit cell,  leading
           to  the "volcano" curve, shown  in  Fig.  4.3.
                Such a simple treatment illustrates the essential features of the geometric
           or  multiplet  theory.(96)  This  model  was  used  to  explain  many  types  of
           reactions,  now  called  demanding  or  structure  sensitive, (97)  that  require
           unique groupings.  Unfortunately, it  was  possible to explain any trend, and
           useful predictions did not result.  Also, as more sophisticated surface struc-
           ture determination became possible, it was clear that surface symmetry and
           dimensions do not always  follow  those  of the bulk. The geometric theory,
           in  its  original  form,  is  no  longer  fashionable.  However,  elements  of this
           model  survive  today  in  the  concept  of reaction  ensembles,  believed to  be
           decisive in explaining a  large number of hydrocarbon reactions over metal
           films  and high-area supported catalysts.(95)  In addition to geometric group-
           ings, this approach considers the symmetry of emerging orbitals that interact
           with  adsorbing  molecules.  These  features  will  be  discussed  after  we  first
           consider the  historical  developments in  the  competing electronic theory.
                Based on the rigid band theory,(98) the electronic theory considered the
           metal  as  a  collective  source of electrons  and  electron  holes,  characterized


                            -  ENTHALPY  OF  ETHYLENE  ADSORPTION

                                                   Rh


                        10- 1
                    ~
                    I
                    ~
                    E   10- 2
                    u
                    I-
                    Z
                    <1:
                    l-  10- 3
                    (/)
                    Z
                    0
                    ()
                    W   10- 4
                    l-
                    e(
                    ex:
                       10- 5
                           0.30         0.35         0.40         0.45
                                       M - M  DISTANCE,  nm
                   Figure 4.3.  Ethylene hydrogenation  as  a  function  of atomic distance. 124 )
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