Page 134 - Principles of Catalyst Development
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122                                                      CHAPTER  6
            caustic soda at  50°C  is  added slowly to give the  reaction

                                                                         ( 6.11)

            with evolution of hydrogen. The catalyst is washed free of caustic and stored
            under distilled water. These catalysts are used for a wide range of selective
            hydrogenation  reactions  using  liquid  phase slurry  reactors.

           6.5.3.  Reduced  Fused  Oxides

                The  ammonia  synthesis  catalyst,  Fe-AlzOr K 20, is  the  best  example
           of reduced fused oxides. (2)  I ron oxide, Fe304, and appropriate amounts of
           AI 20) and K 20  are fused together at about 1 500°C. When thoroughly mixed,
           the melt is  cooled, crushed and sieved for use.  Sizes  range from  1 to 2 cm.
            Loaded directly as  the oxides, the catalyst is  reduced with feed  gas,  which
           removes the oxygen component of the Fe)04Iattice, leaving behind a matrix
           of Fe  crystallites  with  high  porosity.  Collapse of the structure  at  reaction
           temperatures (400-500°C) is  prevented by the alumina, serving as a spacer.
            Promotion  is  achieved  by the  K~O, which  is  believed to  poison  acid sites
           preventing the release of NH"  to induce exposure of [Ill] planes of higher
           activity, and to donate electrons to the iron so that nitrogen chemisorption
           is  aided. (14)


           6.6.  SPECIAL  TYPES

               Here we  consider common types of catalysts not included in  previous
           descriptions.

           6.6.1.  Mixed Oxides

               In  cases  where  intimate  interaction  is  not  necessary  and  where  no
           synergistic  effects  exist,  mechanical  mixing  of two  oxides  (or other com-
           ponents)  may be all  that is  necessary.(181) This is accomplished by mulling
           with suitable  mechanical devices.  Early  methanol  catalysts, such  as  ZnO-
           ZnCuZ04, were prepared in this manner.

           6.6.2.  Cemented Oxides

               When  mechanical strength and thermal stability are  paramount, as  in
           steam  reforming,  this  is  accomplished by  cementing components together
           with  some  type  of ceramic  cement,  such  as  calcium  aluminate. (182)  Early
           steam reforming catalysts  were of this type.
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