Iron oxide catalysts magnetite

The source of this discrepancy is unknown to us. Equation (349) is, undoubtedly, adequate for the description of the reaction kinetics on an iron-chromium oxide catalyst. The fact that in one of the works (124) magnetite without the addition of chromium oxide served as a catalyst can hardly be of consequence since a study of adsorption-chemical equilibrium (344) on an iron-chromium oxide catalyst (7% Cr203) (52) led to the value of the average energy of liberation of a surface oxygen atom that practically coincides with that found earlier (50) for an iron oxide catalyst with no chromium oxide. It may be suspected that in the first work (124) the catalyst was poisoned with sulfur of H2S that possibly was contained in unpurified C02... 

After performing FT synthesis on an unreduced iron oxide catalyst, Kuivila et al.12 observed 22% carbide in the bulk by Mossbauer spectroscopy, but only —3% carbide on the surface by XPS, and therefore concluded that a sub-surface carbide phase had formed beneath a magnetite surface layer. Based in part on this result, they conclude that magnetite is the active phase for FT synthesis. Reymond et a/.10 also observed substantial amounts of carbide by XRD, but little or no carbide by XPS. The observation of a 2-4 nm thick carbon layer on the carbide phase, but not on the magnetite, allows a reinterpretation of the data in these two papers. Sputtering of the surface carbon layer permits the XPS to see the underlying carbide, and therefore it is not necessary that the carbide be present beneath an oxide layer. Thus, measurement of low carbide signals by XPS cannot be interpreted to mean that carbide is absent from the catalyst surface, and therefore not an important phase in FT... 

The following discussion concentrates mainly on the ammonia synthesis reaction over iron catalysts and refers only briefly to reactions with non-iron catalysts. Iron catalysts which are generally used until today in commercial production units are composed in unreduced form of iron oxides (mainly magnetite) and a few percent of Al, Ca, and K other elements such as Mg and Si may also be present in small amounts. Activation is usually accomplished in situ by reduction with synthesis gas. Prereduced catalysts are also commercially available. 

The synthesis of iron oxide catalysts leads to the hematite (Fe203) phase after calcination. Before the WGS reaction the hematite phase is reduced to magnetite phase (Fe304), since magnetite is the active phase for the WGS reaction. Usually, the reduction is taking place in the presence of process gas between 350 and 450 °C. Process gas is a mixture of CO, CO2, H2 and water vapour. The representative reactions are shown as follows ... 

In Table 9.1 mechanism II is also called as associative mechanism. Several authors indicated that the reaction mechanism over iron oxide catalysts is dependent on the temperature. Armstrong and HUditch suggested that the activity of magnetite at lower temperatures may be limited by the dissociation of stream and this indicates that the associative mechanism is dominant at those conditions. 

Rethwisch and Dumesic [5] studied the adsorption of CO/CO2 and CO/CO/ H2O gas mixtures over iron oxide catalysts and concluded regarding the regenerative mechanism in the magnetite catalysts. In 1982 Lund and Dumesic [6] concluded that catalysts that are not active for adsorption of CO and CO2 are not active for water gas shift (WGS). Tinkle and Dumesic [7] performed... 

An increasing intensity of the diffraction peaks of hematite is observed when comparing the dried and calcined catalyst as shown in Fig. 2(a), indicating that hematite forms at M er temperatures. No obvious diffraction peaks to lithium such as lithium iron oxide (LiFcsOg) could probably be ascribed to the small fraction of lithium or overlapped peaks betwem hematite and lithium iron oxide. The diffraction peak intensity of magnetite in tested catalysts increases significantly. 

For a precipitated iron catalyst, several authors propose that the WGS reaction occurs on an iron oxide (magnetite) surface,1213 and there are also some reports that the FT reaction occurs on a carbide surface.14 There seems to be a general consensus that the FT and WGS reactions occur on different active sites,13 and some strong evidence indicates that iron carbide is active for the FT reaction and that an iron oxide is active for the WGS reaction,15 and this is the process we propose in this report. The most widely accepted mechanism for the FT reaction is surface polymerization on a carbide surface by CH2 insertion.16 The most widely accepted mechanism for the WGS reaction is the direct oxidation of CO with surface 0 (from water dissociation).17 Analysis done on a precipitated iron catalyst using bulk characterization techniques always shows iron oxides and iron carbides, and the question of whether there can be a sensible correlation made between the bulk composition and activity or selectivity is still a contentious issue.18... 

At first this new assumption seemed confirmed by the enhanced activities of catalysts which were obtained by pressing loose powders of iron oxide into dense tablets. Soon, however, it was found that magnetites of another origin than the Swedish specimen yielded no ammonia, or only negligible amounts, in spite of the dense structure of their reduction products. 

The principal iron oxides used in catalysis of industrial reactions are magnetite and hematite. Both are semiconductors and can catalyse oxidation/reduction reactions. Owing to their amphoteric properties, they can also be used as acid/base catalysts. The catalysts are used as finely divided powders or as porous solids with a high ratio of surface area to volume. Such catalysts must be durable with a life expectancy of some years. To achieve these requirements, the iron oxide is most frequently dis-... 

It is implicit in reaction 9.4 that the equilibrium yield of ammonia is favored by high pressures and low temperatures (Table 9.1). However, compromises must be made, as the capital cost of high pressure equipment is high and the rate of reaction at low temperatures is slow, even when a catalyst is used. In practice, Haber plants are usually operated at 80 to 350 bars and at 400 to 540 °C, and several passes are made through the converter. The catalyst (Section 6.2) is typically finely divided iron (supplied as magnetite, Fe304 which is reduced by the H2) with a KOH promoter on a support of refractory metallic oxide. The upper temperature limit is set by the tendency of the catalyst to sinter above 540 °C. To increase the yield, the gases may be cooled as they approach equilibrium. 

Table 5.1 shows an application of XPS to the study of the promoted iron catalyst used in the Haber synthesis of ammonia. The sizes of the various electron intensity peaks allows a modest level of quantitative analysis. This catalyst is prepared by sintering an iron oxide, such as magnetite (Fe304) with small amounts of potassium nitrate, calcium carbonate, aluminium oxide and other trace elements at about 1900 K. The unreduced solid produced on cooling is a mixture of oxides. On exposure to the nitrogen-hydrogen reactant gas mixture in the Haber process, the catalyst is converted to its operative, reduced form containing metallic iron. As shown in Table 5.1, the elemental components of the catalyst exhibit surface enrichment or depletion, and the extent of this differs between unreduced and reduced forms. 

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. 

Diffraction patterns can be used to identify the various phases in a catalyst. An example is given in Fig. 10.3b, where XRD is used to follow the reduction of alumina-supported iron oxide at 675 K as a function of time. The initially present oc-Fe2C>3 (haematite) is partially reduced to metallic iron, with Fe3C>4 (magnetite) as the intermediate. The diffraction lines from platinum are due to the sample holder [10]. 

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. 

Mechanism of the Promoter Effect. The action of the so-called structural promoters (stabilizers), such as A1203, is closely associated with their solubilities in the iron oxide matrix of the unreduced catalyst or with the capability of the regular crystallizing magnetite to form solid solutions with iron - aluminum spinels [33], [289]-[291]. The solid solutions of Fe304 and the spinel FeAl204 have a miscibility gap below 850 °C... 

---