Ammonia synthesis reduction process

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. 

The most noteworthy multistage element cycles in which bacteria play important roles are the nitrogen and sulfur redox cycles. The fixation of nitrogen is a reductive process that provides organisms with nitrogen in a form usable for the synthesis of amino acids, nucleic acids, and other cell constituents. In essence, the overall conversion to the key intermediate, ammonia, can be represented as ... 

This has obvious advantages over the process seen for glutamate synthesis via the reductive amination of 2-oxoglutarate, in that it no longer requires the intervention of free ammonia. We thus have the situation that some organisms are able to carry out the fixation of ammonia via reductive amination, whereas others manipulate via transamination the amino acid structures obtained from protein in the diet. 

By analyzing energy barriers for product desorption under ammonia synthesis, CO hydrogenation, and NO reduction by CO, we can refine the models further. For these three processes, the reaction conditions are very different. The ammonia synthesis process is weakly exothermic, whereas the CO hydrogenation reaction has... 

Reduction in price of raw material (nitric acid) by the commercialization of the Haber-Bosch process for ammonia synthesis. 

Figure 1. The Nitrogenase Reaction. The electron transfer proteins ferredoxin (Fd) and flavodoxin (Fid) serve to couple the nitrogenase reaction to metabolically generated reducing equivalents. Ammonia synthesis requires 8 electrons 6 for the reduction of dinitrogen and 2 for the coupled, obligatory synthesis of H2. These reactions are catalyzed by the terminal component in the complex, the MoFe-protein. The electrons are transferred to the MoFe-protein from the Fe-protein in a process coupled to the hydrolysis of 2ATP/electron (Howard and Rees, 1994,1996).

The conversion of dinitrogen to ammonia is one of the important processes of chemistry. Whereas the technical ammonia synthesis requires high temperature and pressure (1), this reaction proceeds at room temperature and ambient pressure in nature, mediated by the enzyme nitrogenase (2). There is evidence that N2 is bound and reduced at the iron-molybdenum cofactor (FeMoco), a unique Fe/Mo/S cluster present in the MoFe protein of nitrogenase. Although detailed structural information on nitrogenase has been available for some time (3), the mechanism of N2 reduction by this enzyme is still unclear at the molecular level. Nevertheless, it is possible to bind and reduce dinitrogen at simple mono- and binuclear transition-metal systems which allow to obtain mechanistic information on elemental steps involved... 

In the majority of cases, the last step in the preparation of catalytically active metals is a reduction. The precursor is very frequently an oxide. An oxychloride is the real precursor of active platinum and some noble metals if chlorometal complexes (e.g. chloroplatinic acid) are used. It may be advantageous to use still other precursors and to reduce them directly without any intermediary transformation to oxide. On the other hand, nearly all catalytic metals are used as supported catalysts. The only notable exception is iron for ammonia synthesis, which is a very special case and then the huge body of industrial experience renders scientific analysis of little relevance. The other important metals are Raney nickel, platinum sponge or platinum black, and similar catalysts, but they are obtained by processes other than reduction. This shows the importance of understanding the mechanisms involved in activation by reduction. 

Present practice in the Haber-Bosch process for utilizing water-gas in ammonia synthesis results in the production of a gas containing 1 to 2 per cent carbon monoxide, a concentration too high to be removed practically in a catalytic preferential oxidation process because of reduction... 

The nitrate reduction is an important process, since it facilitates the entrance of NO3 into the plant metabolism. The resorbed ammonia nitrogen, urea nitrogen and amino acids can be easily utilized by the plants. Nitrates accepted are used for the synthesis of nitrogen-containing organic substances only after their reduction to ammonia. This reduction is sensitive to the environmental conditions. In the case of a lack of saccharides as energy sources, or if the activity of the reductases is lessened, nitrate nitrogen can be accumulated in plant tissues. 

Figure 7.21. Scheme of the restructuring process of iron induced by water vapor and the presence of aluminum oxide. The oxidation of iron permits the migration of the metal on top of the aluminum oxide. The formation of FeAl204 may facilitate this process. Upon reduction in nitrogen and hydrogen, iron is left in active and stable (111) orientation for ammonia synthesis on top of FeAL04. 

The synthesis of a water-soluhle metallodendrimer containing six cationic cyclopentadienyliron moieties was also reported (249). This dendrimer was examined as a redox catalyst for the cathodic reduction of nitrates and nitrites to ammonia. Star-shaped polyaromatic ether complexes of cyclopentadienyliron were recently reported hy Ahd-El-Aziz and co-workers (250). These complexes contained up to 15 cationic cyclopentadienyliron moieties pendent to aromatic rings in the star branches (111). Electrochemical analysis of these star polymers showed that the iron centers underwent reversible reduction processes. 

The catalysts for ammonia synthesis are porous particles with weenie and interlaced micro-pores. The active sites playing the role of surface catalysis are distributed on the internal surfaces formed by these micro-pores. The internal surface area of ammonia synthesis after reduction is about 10m -g -15m -g , and the external surface area is only 0.1 m g F So, the surface area playing the role of surface catalysis mainly is internal surface. The equivalent diameter of catalyst particles used in industrial ammonia reactor is between 1.5 mm and 13 mm, and the inhibition effect of diflfusion should be considered in real ammonia synthesis rates. When designing industrial reactor, the resistance of external diffusion can be neglected by increasing contact between gas flow and external sm-face of catalysts. The catalytic reaction processes for ammonia synthesis pertain to considerable internal diffusion process in most cases. 

Fig. 5.1 Schematic diagram of reduction process in (N2 + 3H2) for ammonia synthesis cataiyst...

The formation of reduction product ce-Fe of ammonia synthesis catalyst is a crystal growth process, which follows the principle and law of crystallography. The basic requirement for this process is that it should form the small a-Fe crystal without growth or conglutination. This requirement is related with the reaction rate, particularly temperature and the concentration of another product H2O, in particular reoxidation of a-Fe should be avoided. Thus, reduction conditions with low-temperatures, high-space velocities and low-water vapor concentrations are required in actual operation. 

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