Iron-ammonia catalysts surface properties

In order to reveal the intrinsic relation between the surface properties and textures with the hump-type activity curve, the specific surface area of both the ammonia synthesis fused iron catalyst with different iron oxides as precursors and their active components were measured by the means of low temperature physical adsorption of N2 and selective chemisorptions of CO, CO2 as shown in Table 3.17. 

These sets of parameters appear to be rather arbitrary and may have been established by empirical methods. It is the purpose of this chapter to describe some of the underlying solid state chemical principles which will allow us to systematize the complex phenomena of activation. A description of the resulting micromorphology will be followed by an analysis of the activated surfaces. The properties of the resulting gas-solid interface, as described by elemental and structural compositions and their changes with time, determine the usefulness of the activated catalyst. Finally, an empirical model of the active catalyst surface is presented which provides the basis for the discussion of the process of ammonia synthesis in terms of a comparison between the technical catalyst and model surfaces based on single crystals of iron. 

The bulk and surface properties of the industrial ammonia catalyst have been discussed in detail in the preceding chapters. For the present purpose it suffices to remember that under working conditions it consists essentially of metallic iron... 

It can be calculated that a H2S/H2 ratio of 4.7 x 10 is required to establish the Fe/FeS equilibrium at TOOK, using data from Barin et but a H2S/H2 ratio of only 10 to affect the surface properties as extrapolated from Grabke s data. Similarly, a H2O/H2 ratio of 0.15 is required to establish the equilibrium between magnetite and iron at 700 K, but only ppm levels of water are required to affect the catalytic activity of an ammonia synthesis catalyst. 

The concept of paracrystallinity introduced by Hosemann et al. in 1966 was used earlier to describe the phenomenon of ammonia iron. They developed the theory of paracrystallinity from XRD data which seemed to explain the special properties of the activated iron catalyst. A three-dimensional, endotactic incorporation of hercynite (FeAl204) motives into the a-iron lattice was thought to create substitutional point defects in the crystal lattice leading to a modified bulk and surface structure of the activated catalyst material. The interplanar spacings change... 

Calculations relating adsorption properties measured on clean single-crystal surfaces at (very) low pressure with actual rates of ammonia synthesis at high pressures described later in this chapter require some measurement of the free metallic iron surface area of the reduced synthesis catalysts. The free metallic iron surface area is generally calculated from the extent of chemisorption of carbon monoxide. In view of the importance of the free metallic iron surface area in work dealing with the mechanism of ammonia synthesis, it is important to review the adsorption of carbon monoxide on different iron surfaces reduced under different conditions. 

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