Page 121 - Principles of Catalyst Development
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CATALYST  PREPARATION                                            109
           on the internal surface of porous particles require'> special attention to avoid
           pore  diffusional  limitations  that  give  uneven  distributions.  This  must  be
           weighed  against  the  advantages  of better control  of pore  size  distribution
           and  easier  handling  operations.  Active  components  deposited  in  large
           concentrations onto powders change mechanical and surface properties, so
           that subsequent pelleting or extrusion  may be more difficult.  In  laboratory
            preparations,  deposition  onto  powders  is  usually  practiced.  Commercial
           manufacturers  find  preformulation  more  convenient,  efficient,  and
           economical.

           6.4.1.  Precipitation
                I n  precipitation, the objective  is  to  achieve  a  reaction  of the type
                                                 ----

                                                    B<lse
                 Metal  salt solution   +  Support          Metal  hydroxide
                      oxalate           powder      '<"OH     or carbonate
                                                    KOIl
                      nitrate           particle   NH,01l     on support
                      sulfate                      NaHCO,
                                                   NJ:C0 1
                     chloride                                             ( 6.5)
           The choice of salt or alkali depends on factors  similar to those considered
           for single oxide precipitations, the most important being the possible harm-
           ful  effects  on  the  final  catalyst.  Powders  or  particles  are  slurred  with  an
           amount of salt solution sufficient to  give  the  required loading.  It  is  helpful
           to  carry out preliminary heating or evacuation to  ensure that the pores are
           properly filled  with solution. The sequence of events is  shown in  Fig.  6.12.
                Enough  alkali  solution  is  added to  cause  precipitation, the  powder is
           then  filtered  or  otherwise  separated  and  washed  to  remove  alkali  ions,
           reagent anions, and excess deposit on the outside of particles. Two processes
           are  involved  in  the  deposition:  (1)  precipitation  of sols  in  bulk  and  pore
           fluid,  and (2)  interaction with  the support surface.  Best  results occur when
           the  OH  groups of the support surface enter into  reaction  (6.5)  so  that  the
           pH  of the  surface region  is  higher than in  the  bulk solution.  Precipitation
           then occurs preferentially and uniformly on the surface. For example, silanol
           groups  of  silica  result  in  surface  deposits  of  hydrosilicates  rather  than
           hydroxides. (16R)  The  effect  is  not  as  pronounced  with  alumina,  where  the
           surface acts more like nucleating centers for sol formation. Often the process
           is  merely  one of sol  adsorption or impregnation.
                Rapid  nucleation  and  growth  in  the  bulk  solution  must  be  avoided,
           since  sols  are  then  too  large  to  enter  the  pores  easily  and  associate  only
           with  the  outside of the  particle. This  is  most  likely  to occur in  the vicinity
           of  the  alkali  droplets  entering  the  solution.  Rapid  mixing  is  essential.
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