Page 126 - Principles of Catalyst Development
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114                                                      CHAPTER  6











                          Figure 6.15.  Shell  adsorption  of chloroplatinic acid.

                This technique may be extended with strongly adsorbing acids such as
            oxalic, tartaric, and citric. These adsorb preferentially on the outside, forcing
            platinum  ions deeper into the  particles, as  shown in  Fig.  6.16.
                Profile  (a),  for  example, is  useful  in  cases such as  automobile exhaust
            oxidation.  Platinum-poisoning  lead  contamination  adsorbs  in  the  outer
            platinum-free shell of alumina, which acts as a guard.(169)  Applications for
            (b) and (c)  include reactions  with  negative order and diffusionally limited,
           consecutive reactions.


           6.4.3.  Ion  Exchange
                Ion exchange in catalyst preparation is  very similar to ionic adsorption
           but involves exchange of ions other than protons. Lower valency ions, such
           as Na t, exchange with ions having higher charge, for example Ni '  ,accord-
                                                                     2
           ing to the equilibrium

                                                                          (6.8)

           Ion exchange is  useful  in  removing harmful agents  and adding promoters.
           During washing with  ammonium  solutions,  NH~ ions are  exchanged with
           impurities such as  Na' to remove potential poisons. For controlled loadings
           of active components or promoters, the catalyst is soaked in excess solutions
           containing the exchange ions.  Loading or extent of exchange is  controlled
           with  soaking time.  Even  multiple exchange  is  possible.  Figure 6.17  shows







                         ---INCREASING  ACID  STRENGTH          •

                          (a)         (b)         (c)         (d)
                      Figure 6.16.  Pellet  concentration  rrotiles  with  adsorbing acids.
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