Distribution of promoters in unreduced 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. 

In catalyst K3, there still are the compounds of alumina, ash and the residue calcium oxides. This phenomenon has been observed in the result of the selective etching approaches. Magnesia together with small amounts of alumina and silicon dioxides are dissolved in the wiistite. The content of alumina dissolved in wiistite obtained by point-by-point analysis is 1.0% (mass fractions). This is unlike the one obtained from selective etching approach, the value of which is 0.3% (mass fractions). The possible reason for this is, the promoters in glass phases undertake transformation during mechanical treatments in order to prepare the sample for EDS analysis and seem to be transformed into the wiistite phases. The selective 

59 Relationship of weight percent of aluminium oxide etched from the precursor (related to the meiss of the precursor before etching) to the etching degree of iron. Total contents of AI2O3 in the precursors are marked on the right side of the cheirt area. For the catalysts KO and K4 the total content of AI2O3 (see Table 3.26) exceeds the scale of y-axis 

Ammonia Synthesis Catalysts Innovation and Practice Table 3.25 Tendencies of metal ions to occupy A site and B site 

Item Weak tended to occupy B site — Strong — tended to occupy A site — strong weak  

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