Page 127 - Principles of Catalyst Development
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CATALYST  PREPARATION                                            115

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                           Z
                           0
                           (J)
                           a:
                           w   50
                           >
                           z
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                           z
                           w
                           ::i:
                           ::>   0
                           <..>   0      10       20      30
                                      PERCENT  H  POSITIONS
                                      EXCHANGED  WITH  LA
                       Figure 6.17.  Ion  exchange of NaY  with  H+  and  La)'  ions." 70 )

            the  results  of double exchange  of NaY faujasite  with  H+(NH1) and  La3+
            ions. 1170 )
                Because of the large number of ion exchange possibilities, this method
            promises  to  be  important  for  the  modification  of catalytic  materials.  For
            example,  hydrous  oxide  ion  exchangers  of the  type  Na xTi0 2x(OH)  are
            prepared  with  a  wide  concentration  range  of  sodium,  which  is  then
            exchanged with  transition and  noble metal  ions. (171)

            6.4.4.  Impregnation

                Also  known  as  "incipient wetness,"  impregnation is  the  simplest and
            most direct method of deposition.(l72) The object is  to fill  the pores with a
            solution of metal salt of sufficient concentration to give the correct loading.
            Figure  6.18  shows  the  sequence  of steps.  The  support,  usually  in  particle
            form, is  heated or evacuated to  remove pore moisture. Thiis  is  not essential
            but speeds  diffusion  of the  solute  into  the  pores.  Solution,  in  an  amount
            just  sufficient  to  fill  the  pores  and  wet  the  outside  of  the  particles,  is
            introduced. Although this may be calculated from measured pore volumes,
            it is  sometimes more reliably determined  with  preliminary tests on aliquot
            samples.
                Drying  is  necessary  to  crystallize  the  salt on  the  pore  surface.  If not
            performed properly, this step can result in irregular and uneven concentra-
            tion  distributions.  For  example,  Fig.  6.19  demonstrates  how  the  rate  of
            drying affects pore and particle profilesY731  If the rate is  too slow, evapor-
            ation  occurs  at  the  miniscus,  which  retreats  down  the  pore.  Some  salt
            deposition occurs but most of the solute merely concentrates deeper in the
            pore.  When finally  crystallized, the salt is  located at the bottom  of a  pore
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