Iron-ammonia catalysts surface heterogeneity

Kummer and Emmett 311) studied the chemisorption of carbon monoxide on an iron ammonia catalyst by the same method and found a partial mixing of the desorbed gas, as if the surface consisted of a heterogeneous complex of homogeneous parts. Similar results were obtained by Eischens 312). 

Three more experimental proofs for surface heterogeneity of iron ammonia catalysts will be mentioned. 

The first use of chemisorption in the study of heterogeneous catalysis was introduced by Emmett during the study of iron-based catalyst for ammonia S3mthesis which used the chemical adsorption of CO and CO2 to measure the surface area of active Fe iron and promoters of K2O and AI2O3. He obtained the following instructive revelation Although content of promoters is very little, they cover most of the surface of the catalyst, which shows that the promoters tend to occupy the surface phase. Since then, many researchers have used chemisorption to study the effects of various components in the traditional iron catalyst, as well as the relationship between the mutative trends of various component and changes in activity... 

A heterogeneous catalyst is in a different phase or state of matter than the reactants. Most commonly, the catalyst is a solid and the reactants are liquids or gases. These catalysts provide a surface for the reaction. The reactant on the surface is more reactive than the free molecule. Many times these homogeneous catalysts are finely divided metals. Chemists use an iron catalyst in the Haber process, which converts nitrogen and hydrogen gases into ammonia. The automobile catalytic converter is another example. 

A heterogeneity was demonstrated for the surfaces of iron, tungsten, and also for oxide catalysts consisting of various phases, in accordance with poisoning experiments on these catalysts. One of the important functions of promoters is to develop active sites on the surface of certain catalysts, as, e.g., on the promoted iron catalysts used in the ammonia synthesis. 

As shown in Fig. 7.18, the ammonia synthesis reaction is highly structure sensitive. The most reactive face of iron is the Fe(lll) surface planel l The different phases tested in the reactivity study are shown in Fig. 7.18. The preference for this surface in the hydrogenation of nitrogen implies that the barrier for the activation of N2 on this surface is the lowestl l. Interestingly, on this surface, the site responsible for N2 dissociation consists of seven Fe atoms, as in the enzyme FeyMoSg cofactor. It illustrates that heterogeneous catalyst can also contain unique sites that are responsible for maximal substrate activation. 

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. 

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. 

These books in the series will deal with particular techniques used in the study of catalysts and catalysis these will cover the scientific basis of the technique, details of its practical applications, and examples of its usefulness. The volumes concerned with an industrial process or a class of catalysts will provide information on the fundamental science of the topic, the use of the process or catalysts, and engineering aspects. For example, the inaugural volume. Principles of Catalyst Development, looks at the science behind the manufacture of heterogeneous catalysts and provides practical information on their characterization and their industrial uses. Similarly, an upcoming volume on ammonia synthesis will extend from the surface science of single iron crystals to the design of reactors for the special duty of ammonia manufacture. It is hoped that this approach will give a series of books that will be of value to both academic and industrial workers. 

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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