Iron-ammonia catalysts reduction temperature

An even more effective homogeneous hydrogenation catalyst is the complex [RhClfPPhsfs] which permits rapid reduction of alkenes, alkynes and other unsaturated compounds in benzene solution at 25°C and 1 atm pressure (p. 1134). The Haber process, which uses iron metal catalysts for the direct synthesis of ammonia from nitrogen and hydrogen at high temperatures and pressures, is a further example (p. 421). 

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 in situ XRD results of Fe304-based ammonia synthesis catalyst are shown in Fig. 7.36. ° It summarizes the evolution of diffracted intensities of three main iron reflections with increasing temperature. First of all, from Fig. 7.36 it can be seen clearly that the growth of the reflections of a-iron is asymmetric, at the end of the reduction the intensity ratio of the first three iron reflections (110), (200) and... 

A commercially available ammonia synthesis catalyst is usually supplied with the iron phase in the form of magnetite, which first must be reduced to metallic iron before the catalyst is used. The reduction time is typically from three to five days, although the actual time required is dependent on the plant design and on limitations of equipment, such as the start-up heater. The general principles of reduction are outlined below. More detailed information to suit a specific plant can be obtained from catalyst suppliers. The principal factors governing a plant reduction are the water content of the circulating gas, the gas flowrate, the reduction pressure, and the reduction temperature. 

Aniline may be made (I) hy Ihe reduction, with iron or tin in HOI, of nitrobenzene, and (2) by the amination of chlorobenzene by healing with ammonia to a high temperature corresponding to a pressure of over 200 atmospheres in the presence of a catalyst (a mixture of cuprous chloride and oxide). Aniline is the end-point of reduction of most mono-nitrogen substituted benzene nuclei, as nitrosobenzene, beta-phenylhydroxylamine. azoxybenzene, azobenzene, hydrazobenzene. Aniline is detected by the violet coloration produced by a small amount of sodium hypochlorite. 

Thus, ammonia does not reduce magnetite at an appreciable rate at temperatures below 450°C., and it appeal s that at 450°C. and above, the reduction may be accomplished by decomposition products of ammonia rather than by ammonia itself. This contention is based on the fact that the reduction of fused catalysts with ammonia at 450°C. and 550°C. appeared to be an autocatalytic process that is, the rate of reduction increased with time in the initial part of the experiment. Reduction with hydrogen does not appear to be autocatalytic. It may be postulated that a-iron and nitride formed in the reduction are better catalysts for the ammonia decomposition than iron oxide. 

The subsequent reduction of the magnetite is of crucial importance to the quality of the catalyst. It is normally carried out with synthesis gas in the pressure reactor of the ammonia plant at not too high pressures (70 to 300 bar, depending on the plant type) and at temperatures between 350 and 400°C, whereupon highly porous a-iron is formed ... 

Catalytic reductions have been carried out under an extremely wide range of reaction conditions. Temperatures of 20 C to over 300 C have been described. Pressures from atmospheric to several thousand pounds have been used. Catal3rsts have included nickel, copper, cobalt, chromium, iron, tin, silver, platinum, palladium, rhodium, molybdenum, tungsten, titanium and many others. They have been used as free metals, in finely divided form for enhanced activity, or as compounds (such as oxides or sulfides). Catalysts have been used singly and in combination, also on carriers, such as alumina, magnesia, carbon, silica, pumice, clays, earths, barium sulfate, etc., or in unsupported form. Reactions have been carried out with organic solvents, without solvents, and in water dispersion. Finally, various additives, such as sodium acetate, sodium hydroxide, sulfuric acid, ammonia, carbon monoxide, and others, have been used for special purposes. It is obvious that conditions must be varied from case to case to obtain optimum economics, yield, and quality. 

The ammonia synthesis reaction is an equilibrium reaction between hydrogen, nitrogen and ammonia, in the presence of magnetic iron oxide. The conversion is a function of pressure, temperature and the reactants ratio. Raising the reaction pressure increases the equilibrium conversion therefore increasing the heat of reaction produced inside the catalyst bed. This causes an increase in temperature resulting in a rise in spacial velocity. These results in a reduction in the reactor volume required compared to a reactor operating at lower pressure. The equilibrium reaction is defined by ... 

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. 

At present, the two major types of ammonia synthesis catalyst are the fused iron catalyst and the ruthenium catalyst, but the fused iron catalyst is still the primary catalyst in use. A fused iron catalyst may be classified in several ways according to the operating temperature as medium-temperature and low-temperature according to the state before use as pre-reduction and oxidized state or according to shape as irregular shape and regular shape. The ruthenium catalyst is a low temperature catalyst, but is not widely used because of its high price. 

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