Page 16 - Principles of Catalyst Development
P. 16

2                                                         CHAPTER  I



               50  -                                  rate  catalytic
                                                                    = 3  x  10 13
                                                     rate  homogeneous
               40


                30           o
                             E
                            ~
            ..   20         W                     c
             w
            ...J   r-                             o
            0
             ~  10
            ...J
            <t
            U
            :.::   0
            )--
            l!)
            a::  -10
            w
            z
            w
               -20
              -30 l
                  I
                                       REACTION  -
                        Figure  1.1.  Catalytic reaction  path for  ammonia synthesis.



                                         H2  -+  2Ha                     ( 1.2a)

                                         N2  -+  2Na                     (1.2b)

                                    N" + Ha  -+  NHa                      (I .2c)

                                  NHa + Ha  -+  (NH 2)a                  ( 1.2d)

                                (NH 2 )a  + Ha  -+  (NH 3 )a             (l.2e)


                                     (NH]L  -+  NH J                      ( 1.2f)
                The rate-determining step is (1.2b), nitrogen adsorption, which requires
            an  activation  energy  of only  12 kcal mole-I.  Rates  accelerate enormously,
            by  a  factor  of  IOiJ  at  50()°C.  Notice  that  initial  and  final  enthalpies  are
            unchanged,  so  that  equilibrium  conversion,  X"  is  the  same.  Conversions
            follow  the  relationships shown  in  Fig.  1.2.
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