Kinetics and Mechanism of Ammonia Synthesis

Unsurprisingly, in view of the early technological significance of NH3 synthesis, this synthesis was probably more intensively studied than any other catalytic reaction. A variety of literature addresses elementary steps as well as the reaction rate under industrial conditions, as, for example, reviewed by Appl (1999), Parrauto and Bartholomew (1997), Nielsen (1971), and Aparicio and Dumesic (1994). In the following, only main aspects are discussed. 

According to Gerhard Ertl (born 1936, Nobel Prize in Chemistry in 2007), the reaction scheme of ammonia synthesis may be formulated by the following sequence of elementary steps  

One of the first rate equation for engineering purposes was published by Temkin and Pyzhev (1940)  

Based on this rate equation, several modifications were subsequently developed that describe even more accurate experimental results (Appl, 1999 Farrauto and Bartholomew, 1997). A problem with Eq. (6.1.9) - at least for laboratory studies where the syngas may be free of NH3 - is the fact that the reaction rate becomes, formally, infinitely high for zero NH3 concentration (reactor entrance). To avoid this, the following equation is often used  

External heat transfer effects (temperature difference between bulk gas phase and catalyst particles 1K) and concentration gradients in the boundary gas layer are negligible (Appl, 1999). 

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Studies of the Kinetics and Mechanisms of Ammonia Synthesis and Hydrodesulfurization on Metal Single-Crystal Surfaces... 

A. Ozaki, K. Aika Catalytic Activadon of Dinitrogen The third chapter is a comprehensive and critical review of studies on the catalytic activation of dinitrogen, including chemisorption and coordination of dinitrogen, kinetics and mechanism of ammonia synthesis, chemical and instrumental characterization of active catalysts, and homogeneous activation of dinitrogen including metal complexes (353 references). 

Calculations using a numerical model for the kinetics and mechanism of ammonia synthesis show that the reaction orders and activation energy in this model are not constant but show some dependence on the reaction conditions for the catalyst [396],... 

Many works were devoted to the mechanism and kinetics of ammonia synthesis. In accordance with the outline of this article, only the main results of studies of ammonia synthesis by the author with co-workers will be presented here other works will be cited, but in connection with our studies. The discussions of kinetics and the mechanism of ammonia synthesis in the reviews published in this series (90) and in the contribution by Nielsen et al. (91) can serve as supplementary sources of information. The contribution by Malina (92) contains interesting historical data and an extensive list of references. 

The kinetics of the synthesis of ammonia in the presence of an industrial catalyst have been investigated.25 The study was performed in a flow system at various working temperatures. The kinetics and mechanisms of several reactions of NH3 have been studied,26 including ... 

A. Ozaki, H. Taylor, and M. Boudart. Kinetics and Mechanism of the Ammonia Synthesis. Proc. R. Soc. London A258 47 (1960). 

Urbas, A. D. (1978a), Kinetics and Mechanisms of Chemical Processes in Glow Discharges in Nitrogen, Hydrogen, Ammonia and Their Mixtures, Ph. D. Dissertation, Institute of Petrochemical Synthesis of USSR Academy of Sciences, Moscow. 

In 1993, almost 20 years later, another symposium was held in the honor of H. Topsoe and A. Nielsen, two of the leaders in industrial ammonia synthesis, and there, a general agreement was reached that the essential aspects of the kinetics and mechanism of this process are now more or less understood (20). This progress has essentially to be attributed to the large number of experimental studies applying surface physical techniques to well-defined, single-crystal surfaces (the surface science approach) that in combination with quantum-chemical calculations and theoretical treatment of the kinetics of the various elementary reactions (microkinetics) provided a rather closed picture. 

The prebiotic synthesis of adenine was confirmed by C. U. Lowe, M. W. Rees and R. Markham, and indirectly by Ponnamperuma and co-workers. The former authors also found the fourth common biological purine, hypoxanthine, in the reaction product from HCN and ammonia mixtures. It may be noted that the above sequence of reactions follows the same direction (first imidazole, then pyrimidine eyclization) as the biochemical synthesis of purines. The kinetics and mechanisms of HCN oligomerization as well as the potential role of the HCN trimer and tetramer for purine synthesis were studied by several investigators, particularly by J. P. Ferris and L. Orgel and coworkers. The yields of purines from HCN are quite low but after isolating the appropriate intermediates, e.g. aminomalonodinitrile (AMN), diaminomaleodinitrile (DAMDN), AICN, AICAI, AICA, and condensing them with formamidine or other one-carbon compounds, substantial yields of adenine and the other purines can be obtained. 

Mechanism and Kinetics of Ammonia Synthesis If all the experimental evidence presented in the preceding sections is put together, the reaction scheme for the catalytic synthesis of ammonia on iron-based catalysts can unequivocally be formulated in terms of the following steps ... 

It can be seen from the above-mentioned discussion that the Temkin s theory of catalytic reaction kinetics on heterogeneous surfaces is confirmed not only by overall reaction kinetic data of ammonia synthesis, isotherms and adsorptive rates of nitrogen on iron catalysts, more importantly, perhaps, but also some very useful generalization results derived from this theory. For instance, Temkin s equation is obtained based on two steps or simplified two steps mechanism. So, it can apply to any kind of catalytic reactions. The problem is Are some simplifications required by reasonable formation of two step mechanism, and can the assumption of rate determining step be made for non-uniform surface The answer of Temkin s theory is positive, and especially Temkin s theory of catalytic reaction kinetics on non-uniform plays an important role in solving the selection and improvement of catalysts. 

Dm-ing the early stages in the development of ammonia synthesis catalysts, German chemists investigated over 20,000 catalyst formula proportions and spent amazing efforts into it. Over the century, the compositions, properties, reaction mechanism and kinetics of the catalysts have been extensively studied in the world and we have accmnulated large munber of data and information. Now, it is possible to design the ammonia synthesis catalysts on those bases. 

This book comprises 10 chapters which can be classified into four parts. The first part deals with the catalyst itself, including the development (Chapter 1), chemical components and physical structure (Chapter 3) preparation and reduction (Chapters 4 5) and the performance evaluation of the catalysts (Chapter 7). Those of ruthenium catalysts are solely put in Chapter 6. The second part is about the reaction mechanism and kinetics of ammonia synthesis (Chapter 2). The third part is a combination of the above two, namely, is centered on the relationship between the performance of catalysts and reaction, which includes reaction condition, reactor, process and application condition and its impact on the economic benefit of... 

The synthesis of ammonia from its elements ranks as one of the most important discoveries in the history of the science of catalysis, not only because of its industrial application in which synthetic fertilizers have contributed enormously to the survival of mankind, but also from the viewpoint of fundamental science. Even today, some eighty years after the first demonstration of ammonia synthesis, many original scientific papers on the mechanism of the catalytic synthesis of ammonia are still published. Every time a new method, technique, or concept has appeared in the field of heterogeneous catalysis, it has been applied to this reaction. Specific examples of these applications over the years include the concepts of gas equilibrium, activated adsorption, structure sensitivitystoichiometric number and kinetic studies, " nonuniform surfaces, the measurements of surface area, surface composition and promoter distributions, and the use of isotopic and spectroscopic techniques. In particular, various surface science techniques have been applied successfully to this reaction system over well-defined single crystal surfaces in recent years. In this way the effect of promoters on the iron catalyst has been elucidated. Accordingly, the history of ammonia synthesis parallels not only that of industrial catalysis, but also the development of the science of catalysis. 

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 validity of this model was assessed by comparison with the experimental data of Nielsen [60]. The maximum deviation between observed and calculated rate constants for the ammonia synthesis is 1.4, despite the wide span of pressures from 1 to 300 atm suggests good representation of the kinetics and mechanism by the model. 

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