Page 130 - Principles of Catalyst Development
P. 130

1t8                                                      CHAPTER  6

                          0   100r----.-----r-----r----.-----~~
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                          ....J                  50%  NIOI AI 20  3
                          UJ                 REDUCTION  AT  400·C
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                                     300   400               700
                                   CALCINATION  TEMPERATURE.  °C
               Figure 6.20.  EHeet  of calcination  on  the  reduction  of coprecipitated  NiO-AI,O). i 176)
            300°C.  Where lower reduction temperatures are desired, initial dissociation
            is  facilitated  by  adding small  amounts  of CuO or  Pt. (1771
                The  next step is  nucleation of the  nickel atoms into crystallites
                                                                         (6.10)

           The  rate  of nucleation  depends  on  the  mobility of Nio  atoms,  which  is  a
            function  of both  temperature and the  nature of the  substrate.  Oxides that
            reduce  with  difficulty,  such  as  Ni[AI 2]04,  result  in  lower  mobilities.  The
            relative rates of reactions (6.9) and (6.10) determine the subsequent crystal-
           lite  size  and  distribution.  Fast  reduction  and slow  nucleation give  narrow
           distributions of small  crystallites.  Similar rates lead to broad distributions,
           and rapid nucleation to large crystallites. For a given substrate, temperature
           is  the  determining  factor.  Figure  6.21  shows  the  resulting  crystallite  size
           distribution  for  reduction  of Ni/Si0 2  at  400°C  and  500°c.(1781  The  lower
           the temperature,  the better the dispersion.
               Another  factor  is  hydrogen  purity.  Reaction  (6.9)  is  reversible.  The
           presence  of steam,  even  in  small  amounts,  influences  the  rate  and  extent
           of reduction.  Water  is  difficult  to  avoid  even  with  high-purity  hydrogen,
           since it  is  a  product. When removal of reaction water is  essential, the effect
           of the  velocity  of the  hydrogen  flow  becomes significant, as shown in  Fig.
           6.22.117HI
               The sensitivity of these parameters introduces grave doubts concerning
           control  and  reproducibility of reduction  procedures.  Very  careful  instruc-
           tions  must be  followed  for satisfactory  results.  Is it  then advisable to carry
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