Page 129 - Principles of Catalyst Development
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CATALYST  PREPARATION                                           117
           or at  the  particle  center.  When  the  drying  rate  is  too  fast,  a  temperature
           gradient occurs.  Vaporization deep in the  pore  forces  solution toward  the
           outside,  where  most  of the  deposition  takes  place.  The  ideal  situation  is
            when  crystallization  is  slow  enough  to  form  uniform  d(~posits.  However,
            since the support exists with a distribution of pore sizes, it is  impossible to
            satisfy optimum conditions for each. Only experiment can establish the best
            procedures, but some non uniformity must always be expected. When con-
            centration profiles are desired for process reasons, these effects may be used
            to good advantage.(174,17S)
                Calcination is important in these circumstances. Crystallized salt redis-
            solves when the dehydrated catalyst is  exposed to moist tnvironments and
           subsequent  process  drying  may  violate  optimum  conditions.  Calcination
            converts the salt to an oxide or metal and essentially "freezes" the distribu-
           tion.  Other calcination effects, such as solid state reaction, also take  place.


           6.4.5.  Activation
                Activation is the final step in producing the deposited active component.
            None is  necessary if the oxide itself is  the active state. Conditioning by the
            process  may be necessary,  and this  is  examined in  Chapter 8.  If metals  or
           sulfides  are  required, then reduction or sulfiding  is  necessary,
                In reduction, the deposited oxide is converted to the metal by treatment
           with  hydrogen,  although  other  reducing  agents  such  as  CO  or  hydrazine
           are  also  used.  The  amount  of metal  produced  depends  on  which  oxidic
           compounds are present.  For example, calcination of nickel  salts  deposited
           on  alumina  results  in  an  increasing  amount  of  Ni[AI2]04  at  higher  tem-
           peratures.  Since  Ni[AI2]04  is  difficult  to  reduce,  high  concentrations
           influence the final  metal  content as  shown  in  Fig.  6.20.
                Partial  reduction  is  common,  leading  to  some  uncertainty  about  the
           composition  of the  catalyst  in  the  active  system.  Figure 6.13  shows  nickel
           crystallites  in  contact  with  Ni[AI 2]04,  which  may  play  some  role  in  the
           catalytic step. If Ni[AI 2]04 is not a factor, then better results may be possible
           by  eliminating  calcination  and  reducing  the  deposited  hydroxide  or  salt
           directly.
               The temperature of reduction is also important. Metal crystallite forma-
           tion follows a sequence in which divalent nickel ions in the surface are first
           reduced:
                                                                          ( 6.9)
           Protons are necessary and are produced through  hydrogen dissociation  by
           NiO  itself.  However,  this  requires  some  minimum  temperatures,  around
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