Page 17 - Principles of Catalyst Development
P. 17

CATALYTIC  FUNCTIONS                                               3

                 100~--~----~----~----~----~----~--~
                                                         3H 2 +N 2 "2NH 3
               z                                           H 2 /N 2 =3
               o   80
               (f)
               a:
               w
               >  60
               z
               o
               ()
               ~  40
               :)
               a:
               III
               ....J   20
               :)
               o
               w

                           100     200    300     400    500     600    700
                                      TEMPERATURE,OC
                           Figure  1.2.  Conversions for ammonia synthesis(l)


                Our definition contains within it four implications that are the founda-
           tion  for  understanding  the  functions  of the  catalyst.  First,  as  emphasized
           in  Fig.  1.1,  equilibrium  is  unchanged,  together  with  all  thermodynamic
           properties  such  as  !l 0" !l H"  and  K r •  The  catalyst  promotes  only  those
           reactions  for  which  the  Gibbs  free  energy  change  !lOr  is  less  than  zero.
            Feasibility is  better defined  by  !lO~, which  is  not the  same  as  !lOr  but  is
           more easily  calculated.  Table  1.1  gives  general  criteria for  whether or not
           a reaction  is  feasible.


                        TABLE 1.1.  Criteria for Thermodynamic  Feasibility

                                                  Feasibility

           1.  Very  negative
              <  - 10 kcal  mole~'   Very  high  equilibrium conversions  possible
           2.  Moderately  negative
             o to  -10 kcal mole-I   Fairly  high  equilibrium  conversions  possible
           3.  Moderately  positive   Low  equilibrium  conversions depending upon
             o to  10 kcal mole  I    process conditions
           4.  Very  positive       Ver)  low  equilibrium  conversions, generally
              > 10 kcal mole-- '      not  signiiicant  reactions
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