A new model of the surface of a fused iron catalyst for ammonia synthesis is proposed. It is assumed that the iron surface is covered with a double layer due to wetting by promoter oxides. The first sublayer in the direct vicinity of the iron atoms is filled with oxygen atoms and the free adsorption sites are present in this sublayer. The second sublayer located over the first one is filled with promoter atoms. The number of oxygen atoms and free adsorption sites is determined by the nature of the promoter. Every atom of alkali metal needs one oxygen atom to be stable on the iron surface under ammonia synthesis conditions; one atom of alkali earth element needs two oxygen atoms; one atom of aluminum needs three. The number of the free adsorption sites is directly related to the number of oxygen atoms on the surface. The more oxygen atoms on the iron surface, the less free adsorption sites are available for dinitrogen adsorption. The surface area of the iron catalyst is also dependent on the number of oxygen atoms on the surface. Therefore, elements such as Ca and Al, which need a high concentration of oxygen on the surface, lead to the formation of the catalyst exhibiting the highest surface area. Between the surface, the iron bulk, and the spacers located between the iron crystallites the thermodynamic equilibrium is maintained. On the basis of this model an interpretation of some properties of the iron catalyst is presented.

The model of nucleation where adsorption of reactant is a rate-limiting step has been considered. Assuming the adsorption range model, a numerical simulation has been made. The dependency of bulk concentration and surface coverage versus time and thermogravimetric curves are presented. The crystallite size is suggested to be the key factor of the nucleation rate. Theoretical considerations have been compared with the experimental results of the iron nitriding reaction.

The catalytic ammonia decomposition over iron and iron nitride, Fe 4 N, under the atmosphere of ammonia-hydrogen mixtures of different amounts of ammonia in the temperature range of 400-550°C by means of thermogravimetry has been studied. A differential tubular reactor with mixing has been used. The ammonia concentration in the gas phase during all the process was analysed. The balance between the inlet and outlet ammonia quantity has been used to determine a degree of ammonia conversion and the values of decomposition reaction rate. The activation energy of ammonia decomposition reaction over Fe and Fe 4 N was found to be 68 and 143 kJ/mol, respectively.

The reaction of ammonia decomposition and nitriding reaction as an example of the parallel reactions were studied. A surface reaction was assumed as the rate limiting step. The experiments were carried out in the range of temperatures from 623 to 723 K. Mixtures of different iron nitrides (gamma'-Fe(4)N, epsilon-Fe(3-2)N) were obtained. Differential tubular reactor with thermogravimetric (TG) measurement and analysis of the gas phase composition in the reaction volume was used. Reacting gases flowing through the reactor were mixed. Effective reactor volume was determined. The rate constants for ammonia decomposition and ammonia adsorption process at critical point between alpha-Fe and gamma'-Fe(4)N phases were estimated. The number of collisions and the sticking coefficient of ammonia over alpha-Fe phase were also assessed.

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