Page 208 - Principles of Catalyst Development

CATALYST DEACTIVATION 197

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RADIUS, nm
Figure 8.8. Crystallite size distribution changes for crystallite migration of nickel on silica at
500 DC."OOI

For catalyst designers, these mechanisms point the way to possible
corrective modification. First, promoters may be used to prevent collisions
between migrating crystallites. Figure 8.10 shows an example of this in a
Cu-ZnO-AI20) catalyst used in low-temperature water-gas shift processes.
Second, intercrystallite transport can be restricted by promoters acting as
preferential adsorbents. I n Fig. 8.10, copper crystallites are protected from
migration by zinc oxide, but sinter in the presence of chlorine. However,
high surface area zinc oxide preferentially adsorbs chlorine, further protect-
ing the copper crystallites.
Third, both crystallite and atomic migration may be retarded by
modification of the substrate. Alumina surfaces have cationic vacancies
which encourage coordination with transition ions. As sites for nickel
adsorption, these vacancies provide a mechanism whereby metal atoms
(and parts of crystallites) "hop" over the surface. By adding magnesia to
form surface MgAI:004, these vacancies are filled and migration inhibited.
In fact, there is some support for the model that nichl oxide-alumina
preparations result in nickel oxide dispersed on nickel aluminate patches
(see Fig. 6.13), upon which nickel crystallites resist sintering. Data on wetting