Distribution of promoters

It is diagnostic of electronic/chemical state, is sensitive to point defects, and can be used to probe the distribution of promoters in catalytic oxides (67). Examples include effects of the distribution of antimony in Sb-Sn02 catalysts (used for selective hydrocarbon oxidation) on the electronic structure of the catalyst and mapping of point defects in titania catalysts. 

The uniform distribution of promoters in catalyst is the key and essential requirement for preparation of catalyst with high activity and stability. Promoters are present in the catalyst by following four states. The first one is congregated on the grain boundaries (such as K2O). These kinds of promoters are extremely easy to be dissolved in the hydrochloric acid. The second one is that it can form a solid solution with the host iron oxides (such as FeO AI2O3). These kinds of promoters are dissolved along with the dissolution of iron oxides, which is somewhat slower compared to the first one. The third one is to be distributed uniformly into the catalysts (such as MgO toward FeO). The promoters with the solubility larger than that of iron oxides will be dissolved out prior to the latter. The fourth one is those promoters which are unable to be dissolved in the hydrochloric acid, such as certain metasilicates etc. 

Fig. 3.60 Influence of iron oxide precursors on distribution of promoters measured by ESCA ( Al K A Ca)...

Induction furnace has many advantages compared with the resistance furnace. Since the current can be very large, the temperatme can reach above 2,000°C and is easy to be controlled. As a result, the melting time can be shortened. It also means that the oxidation degree of Fe +/Fe + can be accmately adjusted. In addition, the magnetic field formed by the primary coffer can maintain a continuous rotation of the melt, and the powerful magnetic line can play the role of automatic mix round, which helps the diffusion and distribution of promoters. This leads to the... 

Pig. 4.26 Distribution of promoter in catalysts by the water cooled and gas flow cooled methods... 

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