Page 29 - Principles of Catalyst Development
P. 29

CATALYTIC  FUNCTIONS                                              IS
           giving


                                                                         ( 1.20)


            Rates of pore diffusion-controlled reactions can be increased by decreasing
           the particle radius or increasing the diffusivity. The latter can only be done
           by increasing the  pore radius  (without decreasing  ()  or Sg).
                Another  important  conclusion  results  from  equation  (1.20).  For  the
           simple-order rate equation


                                                                         (1.21)

           substitution from  equation  (1.20)  gives

                                                                         (1.22 )

                Extreme pore diffusion limitation leads to one half the normal activation
           energy and moves the order closer to unity. These facts will be useful when
            we  discuss  diagnostics.

            1.4.3.  Adsorption

                Having  made  its  way  to  the  interior  surface  of the  porous  particle,
           molecule  A  is  now  ready  for  the  first  chemical  step,  adsorption  on  the
           surface. In catalysis, adsorption is almost always chemisorption. Chemisorp-
           tion results from chemical bonds between the molecule (adsorbate) and the
           solid surface (adsorbent). It is therefore very specific,(23) and receptive sites
           for chemisorption must exist.  Physical adsorption comes from general van
           der Waals forces, which are physical in origin, weaker than chemisorption,
           and  not  specific.  Chemisorption  stops  when  a  monolayer  of  adsorbed
           molecules is  formed.  It is  activated with energies around 10 kcal mole-I, is
           exothermic  with  enthalpy  changes  of  -15  to  -40 kcal mole -\ is  slowly
           reversible or even irreversible, and is  the key step in  activation of reaction
           intermediates.
               The  rate  of chemisorption  is  governed  by the  frequency  of collisions
           with  the  surface  and  the  probability  of  "sticking"  with  chemical  bond
           formation. The former is a physical phenomenon, dependent on temperature
           and pressure.  For example, at one atmosphere pressure and 25°C,  3 x  10 23
           molecules strike each square centimeter of surface each second. If all "stick,"
           the surface is covered in 3 x  10- sec.(26) The probability of chemical bonding
                                       9
           is  exponentially proportional to  the  enthalpy change, flHa ,  and activation
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