Iron-ammonia catalysts properties

The present paper focuses on the interactions between iron and titania for samples prepared via the thermal decomposition of iron pentacarbonyl. (The results of ammonia synthesis studies over these samples have been reported elsewhere (4).) Since it has been reported that standard impregnation techniques cannot be used to prepare highly dispersed iron on titania (4), the use of iron carbonyl decomposition provides a potentially important catalyst preparation route. Studies of the decomposition process as a function of temperature are pertinent to the genesis of such Fe/Ti02 catalysts. For example, these studies are necessary to determine the state and dispersion of iron after the various activation or pretreatment steps. Moreover, such studies are required to understand the catalytic and adsorptive properties of these materials after partial decomposition, complete decarbonylation or hydrogen reduction. In short, Mossbauer spectroscopy was used in this study to monitor the state of iron in catalysts prepared by the decomposition of iron carbonyl. Complementary information about the amount of carbon monoxide associated with iron was provided by volumetric measurements. 

Other substances decrease or annihilate, even in traces, the catalytic properties of iron. Such catalyst poisons had already been known as a nuisance in the catalytic oxidation of sulfur dioxide. With the ammonia catalysis several elements, particularly sulfur proved to be harmful, even in amounts of 4oo of one per cent. Chlorine, phosphorus and arsenic showed a similar behavior (30), just as certain metals, such as lead, tin and zinc. 

Lunin V.V., Sichov N.N., Kruglova M.A., Grigoryan E.H., Nayborodenko Y.S., Effect of Method of Starting Alloies Preparation on Physicochemical Properties of Iron-Containing Catalysts for Ammonia Synthesis, Zhum. Phiz. Khim.,69, No. 6 (1995) 987 (Rus). 

Lin J. Studies on the Mechanical Properties of Fused Iron Ammonia Synthesis Catalysts. Degree Thesis. Hangzhou Zhejiang University of Technology, 2005. 

Based on the research results of monometallic catalysts, scientists also studied on bimetallic catalysts for N2 activation. They realized that the adsorption energy of N2 determines the catalysts properties. Under specific reaction conditions, it can estimate adsorption energy of N2 on catalyst. The catalytic efficiency of the elements for the synthesis and decomposition of ammonia was correlated with the chemisorption energy of nitrogen. An inverted parabolic function (volcano curve) was obtained by Ozaki et in which iron, ruthenium, and osmium mark the top of the volcano. 

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... 

Hardness on the Mohs scale is often above 8 and sometimes approaches 10 (diamond). These properties commend nitrides for use as crucibles, high-temperature reaction vessels, thermocouple sheaths and related applications. Several metal nitrides are also used as heterogeneous catalysts, notably the iron nitrides in the Fischer-Tropsch hydriding of carbonyls. Few chemical reactions of metal nitrides have been studied the most characteristic (often extremely slow but occasionally rapid) is hydrolysis to give ammonia or nitrogen ... 

It is well established that ultrasmall metal clusters on supports have catalytic properties distinct from those properties of large bulk-like particles, as illustrated by the selective oxidation of propylene to propylene oxide by gold, alkene and arene hydrogenation catalysis,and CO oxidation. In these examples, the catalytic properties improve as the clusters become smaller. On the other hand, a reduction in size of the metal cluster can lead to less desirable catalytic properties as seen for ammonia synthesis on iron. Various explanations have been offered to account for the unique properties of nanoscaled metal catalysts, however, much remains to be understood. Clearly, experimental and theoretical studies will be required to develop an in-depth under-... 

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. 

The same type of fused iron catalyst may exhibit different structures and activities after reduction under different conditions (e.g., temperature, pressure, space velocity and gas composition etc.). Reduction condition is the external factor which affects the physical-chemical properties of catalysts. Thus, different reduction conditions are required for catalysts with different t3rpes, particle sizes or different types and content of promoters. The selection of the optimized reduction condition is very important to obtain a high performance for ammonia synthesis catalysts. It is the main reason to study the reductive performance and related kinetics of catalysts. 

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... 

There are various-type catalysts for ammonia synthesis in the world. They may be divided into three series of Al, A2 and A3 according to the precursor of iron oxide and the chemical composition as shown in Table 8.39. The activity of catalysts follows the order A3 > A2 > Al. Typical physical parameters and catalytic properties of catalyst are shown in Table 8.40. 

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. 

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. 

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. 

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