Page 110 - Principles of Catalyst Development
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98                                                       CHAPTER  6



                               PRECIPITATION          /
                                                    /
                                                   /
                         z                       /
                         o
                         I-                    /
                         <   SUPERSATURATED  /                .D
                         a:      REGION   /                  .,11
                         I-
                         z           \   ,/  B~-r--... A  ••••••
                         UJ
                         ()         /
                         Z        ,,/ ,,'<..        .. ,.,/""
                         o
                             ,..              ., ...... .
                         (J    ,,/                 ,.
                                            ..
                                           .'   '   SOLUTION
                             SOLUBILITy ••• ••••
                             CURVE   ••••
                                ......
                                        TEMPERATURE
                             Figure 6.2.  Metastable, supersaturated states.
            is  controlled  by  increasing  the  pH  through  addition  of a  basic  solution.
            Commonly  used  reagents  are  sodium,  potassium,  and  ammonium
           hydroxides, carbonates, and bicarbonates. The choice of which one to  use
           is  determined  by  subsequent  action  of the  associated  cations.  Table  6.2
           gives  the  precipitation conditions for  common transition  hydroxides.
                Particles within the supersaturated region develop in a two-step process;
           nucleation and growth. Nucleation may proceed spontaneously through the
           formation  of M(OH)"  entities or be initiated with  "seed" materials. These
           are  solid  impurities  such  as  dust,  particle  fragments,  or  rough  edges  on
                                  Table 6.2.  Precipitation  pH
                                  for Hydrous Oxides (25°(')"

                                     Hydroxide     pH

                                      Mg(OH),     10.5
                                      Mn(OH),      8.6
                                     Co(OH),       6.8
                                      Ni(OH),      6.7
                                      Fe(OH),      5.5
                                     Cu(OH),       5.3
                                     Cr(OH),       5.3
                                     Zn(OH),       5.2
                                     AI(OH),       4.1
                                      Fe(OH),      2.0
                                 ------
                                 u  Reference  1,\7.
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