Iron catalysts microkinetic analysis

Fastrup, B. (1994) Temperature programmed adsorption and desorption of nitrogen on iron ammonia synthesis catalysts, and consequences for the microkinetic analysis of NH3 synthesis. Top. Catal., 1, 273. 

The ammonia synthesis reaction is one of the most widely studied reactions and it has been discussed previously in this text. (See Example 1.1.1, Table 5.2.1, and Section 5.3.) In this section, results from the microkinetic analysis of ammonia synthesis over transition metal catalysts containing either iron or mthenium will be presented. 

A microkinetic analysis of ammonia synthesis over transition metals is presented in Section 7.3. Use the results of that analysis to explain how adsorbed nitrogen atoms (N ) can be the most abundant reaction intermediate on iron catalysts even though dissociative chemisorption of N2 is considered the rate-determining step. 

In previous papers by the authors [9,10], the temperature-programmed desorption of N2 from an iron-based catalyst has been studied experimentally. The microkinetic analysis of these results is based on the kinetic simulation of ammonia synthesis by Stoltze and Nprskov [22-24] using the approach by Dumesic and Trevino [2]. On Fe single crystal surfaces it was possible to detect a di-molecular precursor labelled a-N2 — forN2 dissociation... 

The rate constants used in all the three models reflect a feature that the sm-face of catalyst is almost covered by nitrogen atoms. The crucial point is that there is also this case in programmed-temperatm-e desorption experiments over the surface of single crystal iron. Therefore, by the microkinetics analysis, the reason that the relation between experimental results obtained at ultrahigh vacuum conditions and at high pressm-e reactor could be established is because the important kinetic steps... 

B. Fastrup, Microkinetic analysis of ammonia synthesis based on siarface reaction studies of iron catalysts as compared to single-crystal studies, J. Catal. 168 (1997) 235. 

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