Reaction Mechanisms of Ammonia Synthesis

Catalysis is one of chemical phenomena in nature. However, the development of theory on catalysis shows that it is a field of multi-disciplines with concentrated knowledge and technology. The achievements in catalytic science and technology can serve many scientific branches and industrial branches. 

The subjects of catalytic science include catalysis (cataljAic phenomena and principle) catalyst (composition, structure, performance and manufacturing method and principle) catalytic reaction kinetics (chemical kinetics and mechanism) as well as cataljAic reaction engineering (apparent kinetics inclucing transport process and reaction process and reactor design) etc. The main tasks of catalytic science are to elucidate the nature of catalytic active sites, the function of catalyst and reaction mechanism to explore the main factors influencing activity, selectivity and stabihty of catalyst to accumulate acknowledge for the exploitation and development of chemical catalysis and to open up its relatively new disciplines — bionic catalysis, photo catalysis, electro catalysis and photoelectric catalysis — to indicate 

It is necessary to get much information from other disciplines, especially organic metallic chemistry, surface sciences, solid-state chemistry, material sciences, biochemistry and bionic chemistry, chemical reaction engineering, chemical kinetics and dynamicology s because of the complexity of catalyst and cataljdic phenomena. 

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Figure 7.26. The activation energy for ammonia synthesis on Fe(lll) and K/Fe(lll). Within experimental error there is no change, suggesting that potassium does not change the reaction mechanism of ammonia synthesis [59].

The study on the pattern of nitrogen chemisorption is very important for the understanding of reaction mechanism of ammonia synthesis. But the chemisorption of nitrogen on the surface of transient metals is very complex. Taking chemisorption on iron catalyst for ammonia synthesis as an example, it was found that the formation rate of ammonia depends on pre-adsorption temperature if the adsorption of nitrogen was preceded first on the surface of activated catalyst, and then hydrogen was passed on surface of pre-adsorbed nitrogen. It is considered that there exist two adsorption patterns The first one is of L-type which appeared at 200°C of pre-adsorption temperature, in which the formation rate of ammonia was proportional to second one is of H-type which appeared at 400°C 40°C of... 

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