Page 121 - Principles of Catalyst Development
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CATALYST PREPARATION 109
on the internal surface of porous particles require'> special attention to avoid
pore diffusional limitations that give uneven distributions. This must be
weighed against the advantages of better control of pore size distribution
and easier handling operations. Active components deposited in large
concentrations onto powders change mechanical and surface properties, so
that subsequent pelleting or extrusion may be more difficult. In laboratory
preparations, deposition onto powders is usually practiced. Commercial
manufacturers find preformulation more convenient, efficient, and
economical.
6.4.1. Precipitation
I n precipitation, the objective is to achieve a reaction of the type
----
B<lse
Metal salt solution + Support Metal hydroxide
oxalate powder '<"OH or carbonate
KOIl
nitrate particle NH,01l on support
sulfate NaHCO,
NJ:C0 1
chloride ( 6.5)
The choice of salt or alkali depends on factors similar to those considered
for single oxide precipitations, the most important being the possible harm-
ful effects on the final catalyst. Powders or particles are slurred with an
amount of salt solution sufficient to give the required loading. It is helpful
to carry out preliminary heating or evacuation to ensure that the pores are
properly filled with solution. The sequence of events is shown in Fig. 6.12.
Enough alkali solution is added to cause precipitation, the powder is
then filtered or otherwise separated and washed to remove alkali ions,
reagent anions, and excess deposit on the outside of particles. Two processes
are involved in the deposition: (1) precipitation of sols in bulk and pore
fluid, and (2) interaction with the support surface. Best results occur when
the OH groups of the support surface enter into reaction (6.5) so that the
pH of the surface region is higher than in the bulk solution. Precipitation
then occurs preferentially and uniformly on the surface. For example, silanol
groups of silica result in surface deposits of hydrosilicates rather than
hydroxides. (16R) The effect is not as pronounced with alumina, where the
surface acts more like nucleating centers for sol formation. Often the process
is merely one of sol adsorption or impregnation.
Rapid nucleation and growth in the bulk solution must be avoided,
since sols are then too large to enter the pores easily and associate only
with the outside of the particle. This is most likely to occur in the vicinity
of the alkali droplets entering the solution. Rapid mixing is essential.