Page 123 - Principles of Catalyst Development
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CATALYST  PREPARATION                                            III








                                           AI 2 0 3
            Figure 6.13.  Phases of nickel compounds present after calcination of nickel alumina catalysts.

            difficulties. (168)  Variations  of this  procedure  are  practiced  in  which  alkali
            solution is slowly injected through a hyperdermic syringe into the turbulence
            of the  mixer.
                After washing,  the treated support is  dried to  remov'e  excess moisture
            from  the pores. This operation is  not as  critical  as  in  support preparation,
            since  the  active  component  is  firmly  anchored  to  the  surface.  However,
            precautions  should  be  taken  to  avoid  rapid  heating  that  generates  large
            internal steam pressures. Calcination decomposes the deposited hydroxides
            and carbonates into stable oxides or metals, depending on the atmosphere.
            Temperatures  are  determined by the  conditions  necessary  for  decomposi-
            tion.  For example, nickel  hydroxide deposited on alumina decomposes to
            the oxide at  300°C,  whereas  nickel  hydrosilicate or silicate  is  stable up to
            600°C.  (168)  Side  reactions also occur,  either parallel  or series,  between the
            original deposit, the oxide, and the support. Unlike coprecipitated systems,
            interaction  is  restricted  to  surface  layers  and  may  be  only  a  few  atomic
            dimensions thick. For example, the surface of NiOj A1 20 3 ,  prepared through
            precipitation  and calculation, is  shown in  Fig.  6.13.
                Whether NiAl 20 4-type substrates are undesirable because they rob the
            system  of  NiO  or  whether  they  contribute  some  benefit.,  for  example  by
            stabilization  during  reduction,  is  a  debatable  point.  The  relative  amounts
            of NiO and Ni[AI2]04 depend on the calcination temperature, which should
            exceed any anticipated  process temperatures.
                Precipitation is the preferred deposition route for loadings higher than
            10% -20%.  Below this  value,  other techniques are  usually  practiced.


            6.4.2.  Adsorption

                Support materials  exposed to  metal  salt solutions adsorb equilibrium
            quantities of salt ions and obey adsorption isotherms as shown in Fig. 6.14.
            Adsorption is  an excellent method for depositing small amounts.  Powders
            or particles  are  dehydrated  and  "soaked"  in  the  appropriate  solution  for
            suitable periods.  Deposition is  uniform, providing all  POTl!S  are penetrated
            during the soaking time.
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