Page 124 - Principles of Catalyst Development
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112                                                      CHAPTER  6












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                                       CONCENTRA TlON
                          Figure 6.14.  Isotherm  for  adsorption of catalytic ions.

                Adsorption from solutions may be either cationic or anionic depending
            on  the  properties  of  the  surface.  In  general,  zeolites  are  strong  cation
            exchangers, silica is a weak cationic adsorber, alumina adsorbs both cations
            and anions weakly, magnesia is  a strong anion adsorber and carbon prefers
            to form  charge-transfer complexes with electron donation but also weakly
            adsorbs cations.  (157)  With  the  exception of zeolites,  that follow  a  different
            mechanism,  the  governing  processes  for  ionic  adsorption  involve  equili-
            brium  reactions of the type


                                                                          (6.6)


           and

                                                                          (6.7)

           Equilibrium is determined by the concentrations ofC C and A- in the solution
           and by the pH. For cations, relative adsorption strength is largely a function
           of the  polarizing power of the  ions.  A cation adsorber C,  has  affinities  in
           the order C + > C3+ > C + > C+  - H+.  Adsorption of anions  depends  on
                                  2
                      4
           anionic  polarizability and  ionic charge,  so that  SO~- > r  >  Br- >  Cl- >
           F-,  etc.  Ionic adsorption  characteristics  of catalytically  important  groups
           are given  in Table 6.3.
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