Page 32 - Principles of Catalyst Development
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18                                                       CHAPTER  I
            and  N 2 •  The 1/2 power comes from dissociation of hydrogen and nitrogen
            molecules.  We  often  make simplifying assumptions where justified, e.g.,

                                                                         (1.26 )

            which gives

                                                                         ( 1.27)
            or more generally, the  Power Rate  Law

                                                                         (1.28 )

                This  demonstration  is  given  as  an  example only.  The  fact  that  (1.27)
            does  not  fit  experimental  data  is  taken  as  evidence  that  reaction  (1.23)  is
            not the  rate-determining step.(301
                Rate  equations  such  as  these,  based  on  proposed  mechanisms,  can
            become very complex. Often many alternative mechanisms fit  equally well.
            If confirmed,  a  given  rate  equation  and  its  associated  mechanism serve  as
            a  guide only.  Other techniques are always required to establish the validity
            of the  surface  mechanism.

            1.4.5.  Product Steps

                Steps  (5),  (6),  and  (7)  in  Fig.  1.5  are  the  reverse  of (3),  (2),  and  (1).
            Although  diffusion  out  is  not  likely  to  be  different  from  diffusion  in,
           desorption  of a  product  could  be  rate  determining  and  must  be  handled
           accordingly.


            1.4.6.  Global  Rates
                I f  steps  (1 )-( 4)  are  all  first  order  and  are  combined  as  a  series  of
           sequential  events,  then  a  first-order  reaction  with  a  "global"  rate constant
                                                           3
            ko  describes  the  entire  process.  It can be shown thae !)
                                                                         (1.29)




                If a  given  step is  much slower than the others (i.e.,  rate determining),
           that appropriate term  predominates  in  the global  rate constant.  If there  is
           more  than  one slow  step,  or if the  surface  reaction  is  not  first  order, then
           a global rate expression is too complicated and cannot be solved analytically.
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