Ammonia Synthesis and Decomposition

Numerous studies of the kinetics of ammonia synthesis and decomposition have been made. With a few exceptions, work has tended to show that the slow step in the synthesis of ammonia is the chemisorption of nitrogen and the slow step for the decomposition is the desorption of nitrogen. Furthermore, it turns out that the decomposition and synthesis of ammonia usually involve in the rate expression a term where y/x is close to 1.5. In 1940, Temkin and Pyzhev derived an equation consistent with both of these observations [M. I. Temkin and V. Pyzhev, Acta Physiochim. U.R.S.S. 12, 327 (1940)]. It has formed the basis for most of the kinetic treatments of ammonia synthesis and decomposition in recent years. 

HansenJB (1995), Kinetics of ammonia synthesis and decomposition , p. 149 in Ammonia Catalysis and Manufacture (Ed.) Nielsen, A., Springer-Verlag, Heidelburg, Germany. 

Fig. 1.24 Rates of ammonia synthesis and decomposition over various metal catalysts Decomposition at 0.2kPa 0.8kPa, synthesis on 5% M—K/AC at 80 kPa and on M powder at 5 MPa (M-transition metals)...

Figure 9.5. One-dimensional potential energy diagrams for ammonia synthesis and decomposition on platinum and iron (a) Polycrystalline platinum (b) iron/ ...

Kinetics of Ammonia Synthesis and Decomposition on Heterogeneous Catalysts... 

The kinetics of ammonia synthesis is treated in detail in chapter a. The experimental results on the kinetics of ammonia synthesis and decomposition will be included in the present chapter to the extent that these results illustrate aspects of the mechanism. 

The rate-determining step of ammonia synthesis and decomposition on Ni powder was studied using tracer isotopes (determination of the stoichiometric number) at 743 K. Dissociative adsorption and desorption were the slowest steps [225]. Dissociative adsorption was studied in few cases. The slight extent of chemisorption of N2 at 523 K to 573 K was speculated to be atomic species on Ni powder [226]. A spectroscopic study of surface nitride on polycrystalline Ni was carried out under a N2 pressure of 200 Torr [227]. 

This review has illustrated, that the large amount of work done on the kinetics of ammonia synthesis and decomposition has stimulated new concepts of reaction kinetics catalyzed by heterogeneous catalysts. The concept of a non-uniform surface vis-a-vis a homogeneous surface is just one example, which is still... 

The fourth chapter is written by Dr. John Bogild Hansen and is entitled, Kinetics of Ammonia Synthesis and Decomposition on Heterogeneous Catalysts"". The scope of the chapter is limited to promoted and non-promoted iron catalysts. The chapter includes discussions of the Temkin-Pyzhev rate equation, it deals with rate equations derived from the Langmuir isotherm and reports on the kinetics based on surface science techniques. In the last section of the chapter a discussion of transfer phenomena - as these are found in experimental reactors and in industrial converters - is included. 

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