The Nature of Rate Limiting Step

One important question about the mechanism of ammonia synthesis is the nature of the rate limiting step. The question about the nature of the rate 

For the associative mechanism with N2, adsorption as the rate limiting steps one finds a kinetic expression of the same form as for the dissociative mechanism with N2 dissociation being rate limiting. However, this has not prevented the use of experimental reaction orders for NH3 synthesis [607, 662] and for NH3 decomposition [663-666] as arguments for N2 adsorption being the rate limiting step. 

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