Ammonia synthesis kinetics

These pioneers understood the interplay between chemical equiUbrium and reaction kinetics indeed, Haber s research, motivated by the development of a commercial process, helped to spur the development of the principles of physical chemistry that account for the effects of temperature and pressure on chemical equiUbrium and kinetics. The ammonia synthesis reaction is strongly equiUbrium limited. The equiUbrium conversion to ammonia is favored by high pressure and low temperature. Haber therefore recognized that the key to a successful process for making ammonia from hydrogen and nitrogen was a catalyst with a high activity to allow operation at low temperatures where the equiUbrium is relatively favorable. 

The elementary steps in the kinetic model for ammonia synthesis are ... 

The kinetics of the ammonia synthesis have been discussed as an example of micro-kinetic modeling in Chapter 7. Here we present a brief description of the process, concentrating on how process variables are related to the microscopic details and the optimization of the synthesis. 

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

Ammonia Synthesis Kinetics over Fe Crystal Surfaces. 

Kinetics of Ammonia Synthesis on Re Crystal Surfaces. The kinetics of the ammonia synthesis reaction on Re were studied on the Re(ll50) surface which is composed of atoms having seven-fold coordination. The rate of production at 70 K and 20 atm. ... 

Annable, D., Application of the Temkin kinetic equation to ammonia synthesis in large-scale reactors, Chem. Eng. Sci. 1(4), 145-153 (1952). 

As an example, the Temkin-Pyzhev rate expression for ammonia synthesis reproduces the experimentally observed kinetics quite well. However, this rate expression was originally derived from a proposed mechanism which had both the wrong key intermediates and the wrong rate-limiting step. 

One or more steps may form a dead end in the form of an intermediate formed through an elementary reaction and consumed exclusively by the reverse of this step. Although the dead-end will not contribute to the overall reaction rate, the step may affect the kinetics if the intermediate is strongly adsorbed on the surface. The poisonous effect of H2O in ammonia synthesis is an example. 

Antonelli and co-workers have recently demonstrated that room temperature stoichiometric ammonia synthesis is possible with their mesoporous titanium and niobium oxide catalysts. In this study, they proposed that the ammonia species are formed via the reaction activated nitrogen with the underlying moisture of the support. Reversible, inter-conversion of and NH2 species via exposure to moist air for aluminophosphate oxynitride catalysts has been observed by FTIR and XPS by Marquez and co-workers. There has been a lot of interest in the literature in the development of novel routes for the low temperature stoichiometric conversion of nitrogen to ammonia, e.g.. However, in principle this could be realised by the nitridation of Li, followed by hydrolysis, although the kinetics would be very slow. 

From the viewpoint of the experimenter, the ammonia synthesis reaction is an advantageous subject for kinetic studies since it proceeds only in one direction without any by-products the activity of catalysts is usually sufficiently stable, it being an important condition for the success of kinetic investigations. 

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. 

Although (305) is sufficient for the description of ammonia synthesis kinetics under the conditions of commercial production, some essential... 

An experimental study of kinetics of ammonia synthesis on iron (101), cobalt, and nickel (96) catalysts, at ammonia concentrations much lower than that at equilibrium, showed that at pressures of the order of 1 atm the second of these possibilities is realized.7 When far from equilibrium, the... 

The character of the chemisorption of nitrogen can be also judged from the results of studies of ammonia synthesis kinetics at the reversible poisoning of the catalyst with water vapor (102,103). If a gas mixture contains water vapor, an adsorption-chemical equilibrium of adsorbed oxygen, hydrogen gas, and water vapor sets in on the iron catalyst. 

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