Synthesis of ammonia on iron

The synthesis of ammonia from its elements, N2 + 3H2 2NH3, is one of the largest industrial processes based on heterogeneous catalysis [1-4]. Almost 90% of the present world capacity of about 200 million tons per year is used for the production of fertilizers. Almost all plants employ promoted catalysts based on iron, quite similar to the one developed by A. Mittaschin 1910 attheBadische Anilin and Sodafabrik (BASF) [5]. The above reaction could be successfully realized for the first time in the laboratory in 1909 by Fritz Haber, and was then transferred into a technical 

Reactions at Solid Surfaces. By Gerhard Ertl Copyright 2009 John Wiley Sons, Inc. 

Remarkably, despite this enormous technical importance and numerous laboratory studies, the actual mechanism of the Haber-Bosch process remained unclear for many decades. There was agreement that adsorption of nitrogen is the rate-limiting step, but the question whether the nitrogen species involved in the reaction is molecular or atomic in nature was not conclusively resolved [6]. 

The actual problem in studpng the surface chemistry of the real catalyst becomes evident from the inspection of an electron micrograph from the Mittasch catalyst reproduced in Fig. 6.1 [7]. The doubly promoted catalyst is formed from a precursor consisting essentially of Fe304 with small concentrations of potassium, aluminum, and calcium oxides as listed in the first row of the table of Fig. 6.1. The surface composition differs considerably from that of the bulk and changes further upon reduction. The working catalyst consists of particles with about 30 nm size and a specific surface area of around 20 m /g. Under reaction conditions, these 

Bulk composition Surface Unreduced Reduced Cat. active spot 

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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 ... 

Fig. 13. Schematic potential energy diagram for the catalytic synthesis and decomposition of ammonia on iron. The energies are given in kj/mol to convert...

Since the formulation of his concepts in 1939 Kobozev has carried out many investigations in the attempt to verify their general validity. In the decomposition of H202 and the oxidation of Na2SCh, the specific effect (173) of small amounts of iron (0.0005 to 40%) added to copper on carbon and the converse in which small amounts of copper were added to iron on carbon were studied. The activity of these catalysts was very effectively promoted by these additives, the extent of promotion being proportional to the concentration of the additives. The catalytic synthesis of ammonia by iron supported on carbon or asbestos was also studied. The results of this study and similar studies of catalytic... 

Effects of Potassium on the Adsorption of Ammonia on Iron Under Ammonia Synthesis Conditions... 

Effects of Potassium on the Adsorption of Ammonia on Iron Under Ammonia Synthesis Conditions The changes in the apparent reaction order dependence in ammonia partial pressure suggest that to elucidate the effects of potassium on both iron single crystals and the industrial catalyst, it is necessary to understand the readsorption of gas-phase ammonia on the catalyst surface during ammonia synthesis The fact that the rate of ammonia synthesis is negative order in ammonia synthesis. Once adsorbed, the ammonia has a certain residence time (t) on the catalyst which is determined by its adsorption energy on iron [t cx tq exp (AH /RT)]... 

M. Temkin and V. Pyzhev. Kinetics of the Synthesis of Ammonia on Promoted Iron Catalysts. J. Phys. Chem. (USSR) 13 851 (1939). 

The reactant is adsorbed on the catalyst s surface. As a reactant molecule attaches to the surface of the catalyst, its bonds are weakened and the reaction can proceed more quickly because the bonds are more easily broken (Fig. 13.36). One important step in the reaction mechanism of the Haber process for the synthesis of ammonia is the adsorption of N2 molecules on the iron catalyst and the weakening of the strong N=N triple bond. 

In general, TPR measurements are interpreted on a qualitative basis as in the example discussed above. Attempts to calculate activation energies of reduction by means of Expression (2-7) can only be undertaken if the TPR pattern represents a single, well-defined process. This requires, for example, that all catalyst particles are equivalent. In a supported catalyst, all particles should have the same morphology and all atoms of the supported phase should be affected by the support in the same way, otherwise the TPR pattern would represent a combination of different reduction reactions. Such strict conditions are seldom obeyed in supported catalysts but are more easily met in unsupported particles. As an example we discuss the TPR work by Wimmers et al. [8] on the reduction of unsupported Fe203 particles (diameter approximately 300 nm). Such research is of interest with regard to the synthesis of ammonia and the Fischer-Tropsch process, both of which are carried out over unsupported iron catalysts. 

Data on the rate of synthesis or decomposition of ammonia on a number of metals give activation energies of ammonia decomposition, E, close to 40 kcal/mol, as in the case of iron catalysts, and m = 0.5 (107). 

Nielsen, A., An Investigation on Promoted Iron Catalysts for the Synthesis of Ammonia," 3rd ed. Giellerups, Copenhagen, 1968. 

Scholten, J. J. F., Chemosorption of Nitrogen on Iron Catalists in Connection with Ammonia Synthesis. Croniger, Amsterdam, 1959. 

Table 5.1 shows an application of XPS to the study of the promoted iron catalyst used in the Haber synthesis of ammonia. The sizes of the various electron intensity peaks allows a modest level of quantitative analysis. This catalyst is prepared by sintering an iron oxide, such as magnetite (Fe304) with small amounts of potassium nitrate, calcium carbonate, aluminium oxide and other trace elements at about 1900 K. The unreduced solid produced on cooling is a mixture of oxides. On exposure to the nitrogen-hydrogen reactant gas mixture in the Haber process, the catalyst is converted to its operative, reduced form containing metallic iron. As shown in Table 5.1, the elemental components of the catalyst exhibit surface enrichment or depletion, and the extent of this differs between unreduced and reduced forms. 

Fastrup, B. (1994) Temperature programmed adsorption and desorption of nitrogen on iron ammonia synthesis catalysts, and consequences for the microkinetic analysis of NH3 synthesis. Top. Catal., 1, 273. 

M. W. Kellogg has developed a new technology in the synthesis of ammonia. They employ a ruthenium on graphite as the catalyst on Kellogg Advanced Ammonia Process (KAAP). The process is the first to employ a non-iron based catalyst and was co-developed with British Petroleum Ventures. The KAAP has been commercialized since 1994, and has been used in an increasing number of projects. 

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