Page 51 - Principles of Catalyst Development
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STRUCTURE  OF  CATALYSTS                                          37
                Promotion  of  the  active  component  may  be  either  structural  or
           electronic. (67)  For  metals,  there  are  many  well  documented  examples.
           Ammonia synthesis catalysts consist of iron doubly promoted with alumina
            and potassium. (2) It was long thought that the role of alumina was to prevent
            iron sintering upon reduction and that potassium poisoned acid sites inad-
            vertently  introduced  by  the  alumina. (68)  Now,  there  are  indications  the
            alumina  promotes  the  iron  by  stabilizing  [111]  surface  planes  which  are
            500 times more active than other planes. (67)  The potassium also doubles as
            an electronic modifier by donating electrons from the ionized state. (14) These
            improve  7T-bonding  of nitrogen,  leading to  faster  dissociation and  higher
            rates.  Similar  effects  have  been  reported  for  hydrogenation  of  carbon
            monoxide on nickel. (67)
                Another successful promotion is in catalytic reforming, where addition
           of  rhenium  to  platinum  leads  to  vastly  improved  performance,  due  to
           decreased  hydrogenolysis  of the  hydrocarbons. (J 1)  The  rhenium  functions
            perhaps as  an electronic promoter, but its  exact role is  still  being debated.
           Chemically, the  rhenium  preferentially bonds  with  low  coordination sites
           on crystal  plane corners,  edges  and steps.  Since these  atoms appear to  be
            involved with the hydrogenolysis, coke-forming reactions are suppressed.  (67)
                With  oxidic  active  components,  promoter  interactions  can  also  be
           structural or electronic. A good example is CoMo/ A1 20 3 , discussed in detail
           in  the  next section.
                Table 2.5 lists common promoters and their mode of action. In principle,
           any additive  which  enhances or inhibits catalytic functions  can be  classed
           as  a  promoter.



           4.  EXAMPLE-HDS  CATALYSTS


                Here  we  demonstrate the  unique  role  of all  three  components  in  the
           case  of  CoMo/ Al 20 3  catalysts,  used  in  hydrodesulfurization  processes.
           Many  different  types  of organic  sulfur  compounds  are  involved  and  the
           reaction  follows  a  series  (15)


                                                   +H,
                                 R-S          R~  -- R                    (2.5)


           Hydrogenolysis  of C-S  bonds  is  followed  by  hydrogenation.  Petroleum
           fractions are desulfurized for many reasons; to protect catalysts, to improve
           product quality, and to prevent environmental pollution. (69) Feedstocks vary
           from  light  naphthas  to  heavy  residua  and  require  reactors  of increasing
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