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.