Page 18 - Principles of Catalyst Development
P. 18

4                                                        CHAPTER  I
                Second,  since  the  equilibrium  constant,  Kp ,  is  unchanged,  it  follows
            from  the  equality

                                                                           (1.3 )
            that the catalyst must accelerate both the forward  rate constant, k,  and the
            reverse,  k.  Although  not  a  factor  in  irreversible  reactions,  this  feature  is
            important  in  appreciating  the  role  of the  catalyst  in  normally  reversible
            situations. For example, materials that are known to function as hydrogena-
            tion catalysts will  also be good for dehydrogenation, if compatible with the
            necessarily  different  process conditions.
                Another  more  subtle  point  emerges.  Sometimes  studying  a  forward
            reaction  is  difficult,  while  the  reverse  is  easy.  An  example  is  ammonia
            synthesis. This reaction is reversible over the range of temperatures normally
            encountered in ind ustrial operations, 200-1200°C.  Figure 1.2 shows that the
            exothermic  synthesis  reaction  decreases  in  equilibrium  conversion  as  the
            temperature  increases.  Higher  yields  are  obtained  by  decreasing  the  tem-
            perature.  But  kinetic  rates  are  lower,  so  precision  suffers.  Also,  the
            stoichiometry of reaction  (1.1)  indicates  that increasing pressure  will  raise
            the equilibrium conversion.  In  the  case  of NH3  synthesis, this  amounts  to
            hundreds  of  atmospheres.  High-pressure  equipment  is  extremely  incon-
            venient for  most  laboratory studies.
                Ammonia decomposition, on the other hand, may be carried out under
            more  favorable  conditions.  Stoichiometry  favors  low  pressure,  so  normal
            atmospheric-pressure  equipment  is  sufficient.  Equilibrium  yields  increase
            with temperature and kinetic rates are measured with precision. This is  why
            ammonia decomposition, which is  less interesting, has historically received
           so  much  attention  in the  search  for  improved synthesis  catalysts.(2i
                The third implication from the definition is that more than one reaction
            may be involved, leading to different thermodynamically feasible products.
            A catalyst, in principle, promotes only one of these, leading to improvements
            in  selectivity  as  well  as  activity.  Since  the  catalyst  is  a  chemical,  reacting
           with  reactants  and  products  through  chemisorption  or  complexing,  its
           reactivity  depends  upon  its  own  chemical  structure.  We  see  this  demon-
           strated  in  the  simple decomposition  of formic  acid:(JI
                       Dehydration:      HCOOH  --+ H 20  +  CO
                                                   AI,Ol
                                                                          (1.4 )
                       Dehydrogenation:  HCOOH  --+ H 2 +C0 2
                                                   Metals
                More  important  industrial  examples  exist.  For instance,  by  changing
           the catalyst (and  process conditions)  we  may convert  H2  and CO  mixtures
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