Page 133 - Principles of Catalyst Development
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CATALYST  PREPARATION                                           121
           of the material. Examples are high-loading dispersed metals, porous metals,
            and reduced fused  oxides.


            6.5.1.  High  Loading  Dispersed  Metals
                Coprecipitated  oxide  systems  such  as  NiO-AI 20 3  may  be  prepared
            over the complete composition range. High concentration, highly dispersed
            metal catalysts are produced by reducing the oxide structure. Compositions
            as high as 70-80 wt %  Nil AI 20 3  with crystallite sizes of 2-3 nm are formed
            in commercial processes. Differences between low loading of nickel catalysts
            by  adsorption  or  impregnation  and  high  loading  by  coprecipitation  are
           shown in  Fig. 6.23. The intimate interaction between nickel  and aluminum
           oxides in  coprecipitation leads to the occlusion of small amounts of Al 20 3
           in the nickel crystallites upon reduction. Termed "'paracrystallinity," defects
           so created are believed to impart increased activity and stability. (164) Another
           important example of this class is Cu-ZnO-AI 20 3 ,  used for low-temperature
           shift and methanol  synthesis.


           6.5.2.  Porous Metals

                There  is  a  class  of  active  porous  metals  known  as  Raney  metals.
           Although  nickel  is  the  most  common,  preparations  are  possible  with  Fe,
           Co, and Cu. These  metals  form  alloys  with  AI,  which  is  easily  leached out
           with caustic, leaving behind a porous metal with high surface area. (180) Raney
           n'ickel  is  prepared by  melting  nickel  (mp 1230°C)  and adding the  required
           amounts of Al  (mp 660°C). The alloy melt is  thoroughly mixed and rapidly
           quenched in  cold water.  After crushing and sieving to 0.3-0.5 nm, 40 wt %



















                 (a)  LOW  LOADING  Ni/AI 2 0 3   (b)  HIGH  LOADING  Ni/AI 2 0 3
                   Figure 6.23.  (a)  Low  loading and  (b)  high  loading  in  nickel  catalysts.
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