Sintering of Ammonia Synthesis Catalysts

Whether operated in pure or impure synthesis gas, the catalyst may lose activity irreversibly due to a mass transfer process in which the catalytically active material loses surface area due to crystal growth. Such a process is usually known as sintering. The sintering process is very temperature-dependent, but may, as suggested above, also be promoted by impurities in the gas phase. 

In Chapter 2 the reduction of the synthesis catalyst and the development of the active phase are described. The reduced catalyst contains more than 90% iron in the metallic state. The iron is present as small (20-30 nm) crystals imbedded in a promoter matrix with some potassium at the surface. A number of crystalline compounds has been identified in the promoter phases. Their main function is to keep the iron crystals separated from each other. 

The active catalyst is obtained by the reduction of promoted magnetite. On reduction, a weight loss of about 25%, the crystal density of the iron phase increases from 5.1 to 7.9 gcm , but the overall particle dimensions are unchanged. The result is a porosity of approximately 50% and a catalyst consisting of iron crystallites, which have an internal surface area typically above 10 m g when measured with the BET method. 

Metallic iron has a fairly high surface tension. Miedema suggests a value of 2.42 J m at 700 K for the (100) surface of iron that corresponds to 119 kJ mol of iron surface atoms. 

There is, therefore, a substantial chemical potential for reducing the iron metal surface by crystal growth. However, since the iron crystallites are physically separated, the growth mechanism has to take place across the boundaries of the structural promoter phases. 

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