Page 109 - Principles of Catalyst Development
P. 109

CATALYST  PREPARATION                                            97
            6.2.1.  Metal Salt Solution

                The  first  step is  to  prepare a  solution  (water is  the  preferred  solvent)
            of a  metal salt, MnXm' destined to become the oxide MxOy. The solubility
            of the  salt  must  be  sufficient  to  give  convenient  volumes  at  prescribed
            temperatures. If necessary, organic solvents are used. The amount of solvent
            is  determined by the  quantity  of oxide desired,  size  of laboratory vessels,
            and  requirements of other steps in  the preparation. Choice of the  anion is
            based on many factors, such as solubility, impurities, availability, cost, and
            potential  problems.
                The  last  item  refers  to  the  difficulty  often  encountered  in  removing
            adsorbed  anions  from  the  precipitated  oxide.  As  we  shall  see,  the  nature
            of X  (e.g., Cl-, NO;-, or SO~-) influences the stability of the precipitate. A
            certain amount adsorbs on the particles. These must be removed, either by
            washing  or volatilization  during drying  and  calcination.  Chlorides  left  on
            the catalyst increase acidity, sulfates form either S02 or H 2S, depending on
            conditions, and deactivate other components.  Nitrates  produce obnoxious
            fumes during calcination. Certain compromises are necessary, for example,
            oxalates  are  the  best  but  are  not  always  readily  available  and  sometimes
            evolve toxic compounds upon calcination.  Sulfates are the  least expensive
            but are  difficult  to  remove.

           6.2.2.  Controlled Precipitation
                The  objective  of this  step  is  to  precIpitate  a  sol,  a  colloidal  particle
            10_,10' nm  in  diameter. Sol  particles do not settle, are difficult to filter,  and
           are  not visible  except with  an  ultra  microscope. They are the  beginning of
           the  process  leading to  the  formation  of porous structure in  the  catalyst.  If
           precipitation  is  too  vigorous,  then  massive  particles  are  formed  and  lack
           the  necessary  properties for  high  surface catalysts.
                Precipitation  occurs in  three  phases:  supersaturation, nucleation, and
           growth.  Pertinent  parameters producing supersaturation are shown in  Fig.
           6.2. Solubility curves are a function of temperature and pH. In the supersat-
           urated region the system is  unstable and precipitation occurs with any small
           disturbance.  Precipitation  can  be  rapid  and  agglomeration  severe.  Slow
           growth  is  possible  in  the  metastable  solutions,  but  only  if  unfavorable
           conditions  are  avoided.  This  metastable  supersaturation  region  is
           approached either by increasing the concentration through evaporation (A
           to  C),  lowering  the  temperature  (A  to  B),  or  increasing  the  pH  (which
           effectively  moves  the  solubility curve  to  0  and A  into  the  supersaturation
           region).  This last  approach is  the  most convenient method. The  reaction
                                                                          (6.1)
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