Page 134 - Principles of Catalyst Development
P. 134
122 CHAPTER 6
caustic soda at 50°C is added slowly to give the reaction
( 6.11)
with evolution of hydrogen. The catalyst is washed free of caustic and stored
under distilled water. These catalysts are used for a wide range of selective
hydrogenation reactions using liquid phase slurry reactors.
6.5.3. Reduced Fused Oxides
The ammonia synthesis catalyst, Fe-AlzOr K 20, is the best example
of reduced fused oxides. (2) I ron oxide, Fe304, and appropriate amounts of
AI 20) and K 20 are fused together at about 1 500°C. When thoroughly mixed,
the melt is cooled, crushed and sieved for use. Sizes range from 1 to 2 cm.
Loaded directly as the oxides, the catalyst is reduced with feed gas, which
removes the oxygen component of the Fe)04Iattice, leaving behind a matrix
of Fe crystallites with high porosity. Collapse of the structure at reaction
temperatures (400-500°C) is prevented by the alumina, serving as a spacer.
Promotion is achieved by the K~O, which is believed to poison acid sites
preventing the release of NH" to induce exposure of [Ill] planes of higher
activity, and to donate electrons to the iron so that nitrogen chemisorption
is aided. (14)
6.6. SPECIAL TYPES
Here we consider common types of catalysts not included in previous
descriptions.
6.6.1. Mixed Oxides
In cases where intimate interaction is not necessary and where no
synergistic effects exist, mechanical mixing of two oxides (or other com-
ponents) may be all that is necessary.(181) This is accomplished by mulling
with suitable mechanical devices. Early methanol catalysts, such as ZnO-
ZnCuZ04, were prepared in this manner.
6.6.2. Cemented Oxides
When mechanical strength and thermal stability are paramount, as in
steam reforming, this is accomplished by cementing components together
with some type of ceramic cement, such as calcium aluminate. (182) Early
steam reforming catalysts were of this type.