Page 119 - Principles of Catalyst Development
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CATALYST  PREPARATION                                            107

                Both  nickel  and  aluminum  preCIpitate  hydrous  oxide  sols.  For  best
           combinations, care must be taken to ensure that precipitation occurs simul-
           taneously. Variations of pH within the vessel or during addition of the alkali
           result  in  preferential  precipitation  of  one  component,  yielding  product
           nonuniformity. One laboratory approach is  to use the apparatus shown in
           Fig.  6.11.(165)
               The inner flask contains a solution of6N NaOH, the outer appropriate
           mixtures of nickel and aluminum nitrates. Tests are made to find the correct
           ratio  between the  two  solutions  such that,  after  mixing  and  precipitation,
           the  final  pH  is  7.  Inverting  and  shaking  the  jar  gives  instantaneous
           coprecipitation.
               When sodium bicarbonate is  used as the precipitating agent, a distinct
           nickel,  aluminum  hydroxy carbonate compound  is  formed.  1163 ) This  helps
           ensure  the  homogeneity  desired  in  this  type  of material.  Washing  is  best
           achieved with decantation. Usually 5-6 washes are sufficient. After filtering,
           the  light green hydrous  oxide is  dried,  crushed,  and calcined. Calcination
           of coprecipitated  oxides  initiates solid state  reactions  in  addition to  those
           given  for single oxides.  In  this  case


                                                                          (6.3)

               Nickel  aluminate  is  a  well-defined  spinel  that  is  difficult  to  reduce.
           Formation  of  pseudospinels  are  not  limited,  however,  to  stoichiometric


                                            GLASS  ROD


                                                      GLASS  JAR





                                                      MIXED
                                                      SOLUTION

                                                     ALKALI
                                                     SOLUTION







                           Figure 6.11.  Apparatus for  rapid coprecipitation.
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