Page 129 - Principles of Catalyst Development
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CATALYST PREPARATION 117
or at the particle center. When the drying rate is too fast, a temperature
gradient occurs. Vaporization deep in the pore forces solution toward the
outside, where most of the deposition takes place. The ideal situation is
when crystallization is slow enough to form uniform d(~posits. However,
since the support exists with a distribution of pore sizes, it is impossible to
satisfy optimum conditions for each. Only experiment can establish the best
procedures, but some non uniformity must always be expected. When con-
centration profiles are desired for process reasons, these effects may be used
to good advantage.(174,17S)
Calcination is important in these circumstances. Crystallized salt redis-
solves when the dehydrated catalyst is exposed to moist tnvironments and
subsequent process drying may violate optimum conditions. Calcination
converts the salt to an oxide or metal and essentially "freezes" the distribu-
tion. Other calcination effects, such as solid state reaction, also take place.
6.4.5. Activation
Activation is the final step in producing the deposited active component.
None is necessary if the oxide itself is the active state. Conditioning by the
process may be necessary, and this is examined in Chapter 8. If metals or
sulfides are required, then reduction or sulfiding is necessary,
In reduction, the deposited oxide is converted to the metal by treatment
with hydrogen, although other reducing agents such as CO or hydrazine
are also used. The amount of metal produced depends on which oxidic
compounds are present. For example, calcination of nickel salts deposited
on alumina results in an increasing amount of Ni[AI2]04 at higher tem-
peratures. Since Ni[AI2]04 is difficult to reduce, high concentrations
influence the final metal content as shown in Fig. 6.20.
Partial reduction is common, leading to some uncertainty about the
composition of the catalyst in the active system. Figure 6.13 shows nickel
crystallites in contact with Ni[AI 2]04, which may play some role in the
catalytic step. If Ni[AI 2]04 is not a factor, then better results may be possible
by eliminating calcination and reducing the deposited hydroxide or salt
directly.
The temperature of reduction is also important. Metal crystallite forma-
tion follows a sequence in which divalent nickel ions in the surface are first
reduced:
( 6.9)
Protons are necessary and are produced through hydrogen dissociation by
NiO itself. However, this requires some minimum temperatures, around