Iron-ammonia catalysts reduction

In the pre-World War II days there was little work done at Princeton on synthesis of catalysts. Copper catalysts were made by reduction of Kahlbaum copper oxide, the iron ammonia catalysts were obtained from the Fixed Nitrogen Laboratory through the courtesy of Dr. Paul Emmett, the nickel on kleselguhr catalyst was obtained from DuPont. Platinum on asbestos was made in the laboratory by soaking asbestos with chloroplatinic acid and then igniting it, mixed chromite catalysts were precipitated and calcined and a study was made. 

Most commercial liquid ammonia contains up to several ppm of colloidal iron compounds, possibly the iron oxide catalyst commonly used in manufacturing ammonia. Reduction converts these compounds to colloidal iron which strongly catalyzes the reaction between alcohols and sodium and potassium. The reaction of lithium with alcohols is also catalyzed by iron but to a markedly lesser degree. The data in Table 1-4 illustrate the magnitude of these catalytic effects. The data of Table 1-5 emphasize how less than 1 ppm... 

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 reasoning which led the author to make this first shot in the dark regarding the usefulness of combinations of solid compounds as ammonia catalysts was as follows If we assume that a labile iron nitride is an interminate in the catalytic ammonia synthesis, every addition to the iron which favors the formation of the iron nitride ought to be of advantage. In other words, the hypothesis was used that surface catalysis acts via the formation of intermediate compounds between the catalyst and one or more of the reactants. An experimental support for this theory was the fact that a stepwise synthesis via the formation and successive hydrogen reduction of nitrides had been carried out with calcium nitrides (Haber), and cerium nitrides (Lipski). Later, the author found molybdenum nitride as being the best intermediate for such a stepwise synthesis. 

From these results, it is concluded that, in a fully reduced catalyst, FeAl204 is not present furthermore, the aluminum inside the iron particle is present as a phase that does not contain iron (e.g., A1203), and this phase must be clustered as inclusions 3 nm in size. These inclusions may well account for the strain observed by Hosemann et al. From the Mossbauer effect investigation then, the process schematically shown in Fig. 17 was suggested for the reduction of a singly promoted iron synthetic ammonia catalyst. Finally, these inclusions and their associated strain fields provide another mechanism for textural promoting (131). 

Table 16 gives a composition survey of commercial ammonia catalysts in the years 1964-1966. The principal component of oxidic catalysts is more or less stoichiometric magnetite, Fe304, which transforms after reduction into the catalytically active form of a-iron. 

With regard to the sulfur bound on the catalyst surface, differences exist between the various types of ammonia catalysts, especially between those that contain alkali and alkaline earth metals and those that are free of them. Nonpromoted iron and catalysts activated only with alumina chemisorb S2N2 and thiophene. When treated with concentrations that lie below the equilibrium for the FeS bond, a maximum of 0.5 mg of sulfur per m2 of inner surface or free iron surface is found this corresponds to monomolecular coverage [382], [383], The monolayer is also preserved on reduction with hydrogen at 620 °C, whereas FeS formed by treatment above 300 °C with high H2S concentrations is reducible as far as the monolayer. For total poisoning, 0.16-0.25 mg S/m2 is sufficient. Like oxygen, sulfur promotes recrystallization of the primary iron particle. 

Iron-zeolite catalysts present an important type of materials with broad application for selective oxidations (i.e. benzene hydroxylation) and environmentally important processes, like SCR reduction of NOx or N2O decomposition. In the case of SCR reaction they could provide a convenient substitution of the vanadia-based system using environmentally problematic ammonia, by more convenient paraffin as a reducing agent. Unfortunately, the efficiency in utilization of paraffin is inferior in comparison to ammonia, namely due to paraffin nonselective oxidation by oxygen catalyzed by unspecified iron-oxide type species typically present in the iron-zeolite catalysts. The mostly used preparation processes include impregnation from water solutions, ion exchange procedures, both in water solution or solid state, as well as gas phase CVD. 

Preparation of the H8gg carbide was started with a commercial synthetic ammonia catalyst containing 92.6% FesOi, 4.2% MgO, 0.7% SiOj, and 0.4% Cr203. Reduction and carburization were carried out in apparatus similar to that previously described (Hofer and Peebles, 49). A stream of purified hydrogen was passed over the sample held at 450° for 82 hours. The reduced sample was then treated at 240° for 539 hours with purified carbon monoxide, until the carbon/iron ratio became nearly constant, and closely corresponded to FeaC. The reaction vessel was always opened in a carbon dioxide atmosphere to prevent oxidation. 

Julie Doherty, MSc thesis, UCD, Selective Catalytic Reductions of Nitric Oxide with Ammonia and Urea over Copper and Iron-based catalysts, 2004. 

Apostolescu N, Geiger B, HizbuUah K, Jan MT, Kureti S, Reicher D, Schott F, Weisweiler W (2006) Selective catalytic reduction of nitrogen oxides by ammonia on iron oxide catalysts. Appl Catal B 62 (1—2) 104—114... 

The above discussion was only for the reduction of the iron oxide catalyst in pure hydrogen. In industrial devices, usually the mixture gas of H2 + N2 is used as reducing reagent. Therefore, the actual reduction process is also accompanied by the formation of ammonia, and the process will become more complex. 

Since the realization of ammonia synthesis on the industrial scale in 1916 there have been no fundamental changes in the composition of the iron synthesis catalyst. Essentially, potassium carbonate, aluminum oxide, and small amounts of other promoters are fused with magnetite, followed by reduction in situ as described fully in Chapter 2,... 

Preparation of finely-divided iron/carbon catalysts for ammonia synthesis by the controlled reduction of graphite/ferric chloride intercalation compounds. K. Kalucki, W. Morawski, and W. Arabczyk (Politechnika Szczecinska). PL 141907 (1988). 

Iron(III) bromide [10031-26-2], FeBr, is obtained by reaction of iron or inon(II) bromide with bromine at 170—200°C. The material is purified by sublimation ia a bromine atmosphere. The stmcture of inoa(III) bromide is analogous to that of inon(III) chloride. FeBr is less stable thermally than FeCl, as would be expected from the observation that Br is a stronger reductant than CF. Dissociation to inon(II) bromide and bromine is complete at ca 200°C. The hygroscopic, dark red, rhombic crystals of inon(III) bromide are readily soluble ia water, alcohol, ether, and acetic acid and are slightly soluble ia Hquid ammonia. Several hydrated species and a large number of adducts are known. Solutions of inon(III) bromide decompose to inon(II) bromide and bromine on boiling. Iron(III) bromide is used as a catalyst for the bromination of aromatic compounds. 

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