A. Reizer

Kinetics of activation of the industrial and model fused iron catalysts for ammonia synthesis

Abstract The paper summarizes our results published for many years and also adds new information. Kinetic data concerning the reduction of the oxidized forms of model as well as industrial catalysts used in ammonia synthesis were obtained in dry and in wet atmosphere containing 1% water vapour. The data for the industrial catalyst have been reassessed using three kinetic models. Modifications applied to the classical Seth-Ross model of the shrinking-core type resulted in the best fitting of this equation to the experimental data. The failure of the crackling core model to describe the kinetic data in a quantitative way is tentatively explained. The reduction of model catalysts containing enhanced amounts of wustite and/or potassium proceeds initially linearly with time. The effect of promoters, and especially of potassium, is discussed in more detail. The magnetite-alumina subsystem is responsible for the retardation effect of the water vapour on the reduction rate. A hypothesis is formulated that — in the presence of wustite or potassium — the inhibitive, Al-rich, hydrated surface layer is not effective in hindering the progress of the reduction process.

Characterization of unreduced fused iron catalyst for ammonia synthesis

Abstract A procedure for the characterization of iron catalysts is proposed. For this purpose, some model iron catalysts (obtained by fusion) for ammonia synthesis, in unreduced form, were studied. In order to characterize the catalysts, the following properties have to be established: (i) the content of major and minor cations, including Fe2+, and the total amount of iron; (ii) phase composition, insofar as the major phases are considered; (iii) homogeneity on the macro- and micro-scale, insofar, at least, as the major cations and major phases are considered; (iv) non-stoichiometry of the major iron oxides. In order to check the proposed procedure, five samples of the model catalysts (unpromoted, potassium oxide-promoted, alumina-promoted and potassium oxide/alumina-promoted) were prepared under industrial conditions and analysed according to the programme outlined above. It was found that the Fe2+ content varies significantly on either the macro- or micro-scale, this property being reflected in the wustite abundance. Wustite is present in two separate phases, one being rich in iron. This feature is due to slow cooling of the fused material. On the other hand, the magnetite phase is almost stoichiometric and well crystallized. The magnitude of the effective fields acting on iron nuclei at room temperature is a measure of the amount of alumina built into the magnetite phase. Mossbauer spectroscopy was found to be a very useful tool for the determination of the composition of the major phases and of the stoichiometry of iron-containing phases. The contents of impurities can be well approximated by the content of unreducible oxides at their highest degree of oxidation.

The simultaneous effect of promoters and water on the reduction of an iron catalyst for ammonia synthesis

Abstract The rate of reduction of magnetite by gaseous hydrogen is slightly affected by water vapour (1%). However, this effect of water vapour is significant in iron catalysts for ammonia synthesis of the KM I type. Promoted magnetite is the main component of the iron catalyst and it is concluded therefore that the influence of water is applicable only when promoters are present. The validity of the core-and-shell reduction model, assuming a Langmuir-Hinshelwood kinetic equation which describes the reaction at the oxide/iron interface, is discussed on the basis of the kinetic data for unpromoted and promoted iron catalysts. It is found that the model is generally valid, except for the case of advanced reduction of promoted catalyst in a moist atmosphere.

Kinetics of reduction of an iron catalyst for ammonia synthesis

The kinetics of reduction of an iron catalyst have been studied at 450–550 °C. The overall kinetic equation was of the “mixed-control” type. The equation of the surface reaction was of the Langmuir-Hinshelwood type with the adsorption of only water vapor taken into account.

The effect of water on the reduction of an iron catalyst for ammonia synthesis

Abstract The reduction of an iron catalyst of KM I type by a hydrogen:nitrogen (3:1) gas mixture containing 2,600 – 10,000 ppm of water vapour was studied thermogravimetrically at 500°C. The kinetic core-and-shell model, proposed previously [1], based on the Langmuir-Hinshelwood equation is valid at the beginning of the process. The effect of water causes the model, for unknown reason, to become invalid during a further period of reduction.

Simultaneous effect of phase composition and water vapour on the reduction of iron catalyst for ammonia synthesis

Abstract Water vapour causes a dramatic decrease in the reduction rate of an iron catalyst for ammonia synthesis and also affects the topochemistry of the reduction. Core-and-shell behaviour and reduction of an internal type have been observed in dry and wet atmospheres, respectively. Magnetite is the main component of the catalyst. Some industrial catalysts, e.g. Topsoe KM I, also contain a small amount (ca. 10%) of wustite. A sample of singly aluminium-promoted catalyst was divided into two parts; one part was oxidized to magnetite and the other was reduced to wustite. It has been shown that the reduction of the magnetite sample was completely retarded by 10 000 ppm of water vapour, whereas that of the wustite was only slightly affected. The results indicate that the retardation effect of water vapour is due to the magnetite phase promoted with aluminium. This result, supported by already published porosimetric and Mossbauer data and microscopic observations, explain the topochemistry of reduction of the iron catalyst in dry and wet atmospheres.

The topochemistry of the reduction of an iron catalyst for ammonia synthesis

Abstract Reduction of KM I iron catalyst at 500°C by dry and moist H2-N2 (3:1) atmosphere was followed using scanning electron microscope. Dry atmosphere reduction follows a ‘core and shell’ pattern, whereas use of the moist atmosphere leads to reduction of an internal type, qualitatively explained by the crackling model of Park and Levenspiel. The decrease of specific reduction rate caused by a small amount of water is emphasized.

The Activation of Iron Catalyst for Ammonia Synthesis

The activation of ammonia catalysts is essentially a reduction of non-porous magnetite particles with hydrogen to yield the internal porous structure of the iron catalyst. The reduction is studied by means of a “core-and-shell” model where the gas-solid reaction proceeds on the interface between the core and shell. Effective diffusion coefficients are calculated from experimental pore size distribution measurements. Different reaction rate expressions were tested on experimental reduction curves for cases with and without water addition, and it is shown that it is necessary to include the adsorption of water in the rate expression. Finally, some limitations in the model are described.

The Kinetics of Activation of Industrial and Model Iron Catalysts for Ammonia Synthesis in Dried and Wet Atmosphere

The oxide precursor of the catalyst is activated by a hydrogen reduction in situ in industrial reactors or in especially built installations [1]. The kinetic data are of primary importance for the technological design of the reduction process. Kinetic equations for the reduction of industrial catalyst in dried gas phase were previously proposed by us [2, 3]. Their significance was emphasized in the review articles [4, 5]. Water evolved during the reduction significantly retards the reduction rate. The retarding effect is due to the alumina [6]. It implies a necessity of a study of industrial and model catalysts in dried and wet atmosphere.