Page 31 - Principles of Catalyst Development
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CATALYTIC  FUNCTIONS                                             17


                                         \               I
                                         \              1
                                          \            I
                                           \          1
                                            \        1
                                            \       1
                                             \     I
                                              ,   I
                               N2  ADSORPTION   \/ NH3  DESORPTION
                                 CONTROLS       I,     CONTROLS
                                                ~
                        W
                        l-
                       e:(
                        a:











                                ENTHALPY  OF  ADSORPTION
                                          N2  OR  NH3
                           Figure  1.7.  Volcano curve for ammonia synthesis.

           assuming  that  all  sites  are  homogeneous  (same  activation  and  adsorption
           energy),  immobile,  and  noninteracting,  surface  concentrations  from  the
           Langmuir isotherm may be used to yield rate equations that fit experimental
           data as  well as  any others. The fact  that we  know that surface sites  do not
           generally  follow  these criteria  need  not  deter us  if we  suspect that  only a
           narrow segment of the spectrum of sites  is  involved in  the reaction. (28)
                If reaction  (1.23)  is  the  rate-determining step (RDS),


                                                                         (1.24)
           where  ON  and  OH  are the  fraction  of surface sites  occupied by  Ns and  Hs.
           From  Langmuir isotherms(29)


                                                                         (1.25 )


           where  K H ,  KN  are  equilibrium  constants  (or  adsorption  coefficients)  for
           dissociative  adsorption  of H2  and  N 2 ;  PH,  PN  are  partial  pressures  of H2
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