Page 30 - Principles of Catalyst Development
P. 30

16                                                       CHAPTER  I
            energy Ea ,  so the stronger the bond formed the faster the rate and the longer
            the adsorbed species remains on the surface. Current models for chemisorp-
            tion include geometric (or ensemble) and electronic (ligand) effects in which
            a  given  molecule  finds  the  right  symmetry  or  orbitals  at  the  surface.(23)
            Understanding these  features  is  important  in  design  of new  or improved
            catalysts and will  be treated in Chapter 4.
                Also  part  of  the  adsorption  step  is  surface  migration  or  diffusion.
            Although  we  visualize  the adsorptive  process as  chemical  bonding with  a
            surface  site,  the  adsorbed  molecule  does  not  stay  at that site  but  "hops"
            around  from  one  site  to  another.  Activation  energy  for  this  process  is
            approximately 0.3-0.5  of ilHa.  For example,  with  ilHa  =  -30 kcal mole-  1
            (a typical  value)  the  molecule  makes  5 x  10 14   "hops"  per second!(25)  The
            surface  is  truly  a  dynamic  system  with  molecules  arriving,  leaving,  and
            migrating constantly.  It  is  within this framework that we  consider the next
            step of the catalytic mechanism.



            1.4.4.  Surface Reaction
                In this sea of constant movement, two adsorbed molecules destined to
            react must come together.  For instance, the  Ns species of mechanism (1.2)
            finds  itself in  surface contact with  Hs.  If surface geometry and energetics
            are appropriate then a  surface reaction  occurs. Thus

                                              H
                                               I
                                      N  H    N
                                      I   I    I   I
                                     S+S  -+  S+S                         (1.23 )

                If the  Ns species  is  held  too strongly  it  will  either be too immobile to
            find  Hs  or  disinclined  to  break  its  bonds  with  S  and  react.  If  held  too
            weakly, it  will  desorb before reacting and the surface concentration of Ns
            is  low.  Thus  we  find  the  situation  shown  in  Fig.  1.7,  in  which  the  rate  of
            N H ~ synthesis over different metals passes through a maximum when plotted
            against the  enthalpy of nitrogen  adsorption.(26)
                Curves of this  shape  are  often  found  when  correlating catalytic  rates
           against  some  measure  of  adsorption  strength  and  are  called  "volcano"
           curves.
                Formulating rate equations from  rate-determining steps such as (1.23)
           is  indeed a  formidable task. The reader is  referred to standard texts on this
           topic.(27.2S)  For our purpose in  appreciating the role of catalytic steps, it  is
           sufficient to note that most success is  achieved with very simple models.  By
   25   26   27   28   29   30   31   32   33   34   35