Grain-growth kinetics and microstructure evolution of nanocrystalline iron

2 Grain-growth kinetics and microstructure evolution of nanocrystalline iron 

Active phase is the source of the catalytic activity which value has a direct relationship with the microstructure of active phase. Ammonia synthesis reaction is the most sensitive to structure. The results from single crystal measurements by field-ion microscopy shows that the opened iron (ill) and (211) surface is 

The result above is obtained under strictly controlled conditions in laboratory, but the single surface of a-iron and fully exposed a-iron (111) surface are hard to get during the preparation and reduction of catalysts. Even more important, because of the existence of promoter, 50% or more surface of a-iron is covered by K2O or KOH and the structure of active phase changes because of the addition of the promoter, and even the active site distribution or active order on the crystal plane of a-Fe are greatly changed. As a result, the impact of microstructure changes on the mechanism of activity is worth exploring. 

As can be seen from Fig. 7.43, the average value of micro-stress decrease when temperature increasing after 10 min rapid decrease of stress, it basically unchanged later. The rate of increase of particle size distribution at low temperatures is small from wide to narrow, representing the self-spread movement on the edge of nanocrystalline Fe particle is relatively slow, and the crystal growth activation energy is about 100 kJ mol F 

At high-temperature, initial growth of particle is rapid, the particle size distribution has become very wide, the crystal growth activation energy increases to 175kJ mol which mean nano-iron crystal strengthens its self-spread ability at the edge of coarse particles. 

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