Page 125 - Principles of Catalyst Development
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CATALYST  PREPARATION                                            113
                            TABLE 6.3.  Adsorption of Catalytic Ions"

                                            Adsorption of element as

            Group   Cation      cr-               NO)"               SO~-
             IB     Cu H   Cationic < 4 M      Cationic         Anionic
                           Anionic> 4 M
                    Ag+    Insoluble           Cationic (weak)   Anionic> 0.05  M
                    Au H   Anionic> 0.1  M     Anionic (strong)   Anionic (weak)
             VIII3   NiH   Cationic            Cationic         Cationic
                    Pd H   Anionic> 0.1  M     Anionic          Anionic
                    Pt4+   Anionic> 0.1  M     Anionic          Anionic
             VIIJ 2   CO H   Cationic < 6 M   Cationic          Cationic
                           Anionic> 6 M
                    RhH    Anionic            Anionic           Cationic/ anionic
                    Ir4 +   Anionic> 0.1  M   Anionic           Anionic
             VIII,   Fe H   Cationic <  1 M   Cationic          Anionic> 0.1  M
                           Anionic> 1 M
                    Ru 4 +   Anionic>  I  M    Anionic          Anionic
                    OS4+   Anionic> 0.1  M    Anionic           Anionic

            a  Reference  157.
                Unfortunately,  saturation  amounts  are  generally  small.  With  nickel
            solutions  and  alumina,  for  example,  only  loadings  up  to  2%-3%  are
            possible.  Multiple  adsorptions  with  intermediate  calcination  give  higher
            loadings, but this is time consuming. Other methods are generally preferred.
            With platinum and other expensive noble metals, however, amounts of less
           than 1 % are often needed. Low loadings with high dispersion give satisfac-
           tory results. Supports are soaked in solutions of chloroplatinic acid, H 2PtCI 6 ,
           to yield desired levels of adsorbed (PtCI4)2-. Washing is  not necessary, nor
            desirable,  since  it  induces desorption.  Drying and  calcination  are  carried
           out as  usual,  with  the  chloroplatinic ion  decomposing to  platinum  oxide
           or platinum.
                Transport effects are encountered when  using large particles. Adsorp-
           tion of chloropiatinic acid is  so  rapid that diffusion  of the  solute  into  the
            pores controls the rate.  Deposition takes place in  an outer shell, as  shown
           in  Fig.  6.15.
                In  some  cases,  such  profiles  are  desirable.  For  example,  with  fast
           reactions  and  external  diffusion  resistance,  the  reaction  occurs  only  on
           the  outside  of the  pellet.  Platinum  deposited  deep  inside  the  particle  is
           wasted.  Shell deposition is  not satisfactory, however,  in  r,eactions  that are
           not diffusion  controlled, such as  catalytic reforming.  It can be avoided by
           adding hydrochloric acid to the solution. The  HCl  competes  with  chloro-
           platinic acid for adsorption sites, driving platinum deeper into the particle.
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