The mechanisms of ammonia synthesis reaction

Based on the principle of equilibrium state approximation, only one elementary step is the determining step for the three kinetic models listed in Table 2.5. The overall rate of a reaction is determined by the rate of the rate determining step (RDS) with slowest rate, while other steps are approximately in chemical or adsorption equilibrium. There are three opinions or possibilities for the rate determining step of ammonia synthesis reaction on fused iron catalyst (i) RDS is the dissociation of adsorbed dinitrogen N2(ad) 2N (ad) (ii) the surface reaction of adsorbed species N (ad) + H (ad) NH (ad) (iii) the desorption of adsorbed ammonia NH3(ad) NHs(g). 

it is generally accepted that the dissociated adsorption of N2 is the rate determining step, because it has been confirmed by many results from surface science experiments. The contradictions about the pathway of nitrogen dissociation adsorption still remains, as different conclusions were obtained from different experimental measures. Meanwhile, there are also different results for different metals and crystal faces under various experimental conditions. Some experiments showed that N2 was dissociated directly to nitrogen atoms on Fe sm-face, and others found that N2 were dissociated to nitrogen atoms by intermediates, based 

Napolskikh also considered that the dissociation adsorption of nitrogen was RDS. He found that the following reactions proceeded very slowly  

It was observed by labeled atoms that NH3 and N- + 1.5 H2 was in equilibrium, and N2 and 2 N— were not equilibrated during aimnonia synthesis. The rate of ammonia formation is much faster for 2N— + 3H2 than for N2-I-3 H2. 

Zwietering et al. measured the amount of N— at the conditions of ammonia synthesis by weight analysis and it was 1% of equilibrium adsorption amount over iron catalyst containing 0.85% AI2O3 at 0.52-0.69 of surface N— coverage and 568 K of temperature. Aika and Ozaki further verified that H— was not a factor in the RDS, by the determination of label atom experiments. 

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