The Mechanism of NH3 Synthesis

The key step in the synthesis of NH3 form N2 + 3H2 is to dissociate the N-N triple bond in the N2 molecule. The direct gas phase reaction would involve extremely endothermic and exothermic reactions. The resulting activation energies would be prohibitively high according to the principle of Sabatier. 

All imaginable reaction mechanisms can be separated into two main cases. In the associative mechanism H is added to the N2 molecule before dissociation of the N-N bond, e.g. 

Very endothermic or exothermic reaction steps are avoided if the N-N bond is broken synchronously with the addition of H to the N-atoms. 

In the dissociative mechanism the N-N bond is dissociated before any N-H bond is formed, e.g. 

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A number of compounds including H2O, H2S and the halogens are strong poisons for NH3 synthesis. This is reviewed in detail in Sect. . In the present section, the literature on poisoning will be reviewed to the extent it illustrates the mechanism of NH3 synthesis. A common feature of the poisons is that they are adsorbed at least as strongly on the surface of the catalyst as some of the intermediates of the reaction. 

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