Catalytic activity magnetite

In the Haber-Bosch process, ammonia is formed from the reaction between N2 and H2, using a Fe304 (magnetite) catalyst promoted with A1203, CaO, K20 and a moderately small amount of iron and other elements [25], In this mixture, the catalytically active... 

An answer to the first question was suggested by Lancet and Anders (1970). The principal meteoritic phases stable above 350-400 K (olivine, pyroxene, Fe, FeS) are not effective catalysts for the Fischer-Tropsch reaction, whereas the phases forming below this temperature (hydrated silicates, magnetite) are. P hough metallic iron is often regarded as a catalyst for this synthesis, the catalytically active phase actually is a thin coating of FCjO formed on the surface of the metal (Anderson, 1956)]. Thus CO may have survived metastably until catalysts became available by reactions such as ... 

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. 

The reduction of oxidic catalyst is generally effected with synthesis gas. The magnetite is converted into a highly porous, high surface area, highly catalytically active form of a-iron. The promoters, with the exception of cobalt, are not reduced [33]. 

Besides, over-reduction with formation of FeO and metallic iron should be avoided since these phases are not catalytically active in WGS and they catalyze undesired side reactions (i.e., methanation and CO disproportionation). Therefore, the reduction is always conducted in the presence of steam and with relatively low concentrations of the reductants in the process gas. Steam acts as a mild oxidant, thereby stabilizing the magnetite phase. 

The solid state structure of magnetite, a spinel(2.), contains iron cations in two different oxidation states (Fe " and Fe ) and in two lattice sites of different coordination (octahedral and tetrahedral) therefore, the catalytic surface of this material may be expected to provide a variety of possible sites capable of acting as adsorption or reaction centers. Also, it has been demonstrated that substitution of other cations for iron can significantly alter the catalytic activity for WGS (4,5). 

The effects of solid state alterations of the magnetite structure on the catalytic activity for WGS provide additional insight into the nature of the active sites. While gravimetric and chemisorptive studies provided a chemical picture of the active sites, a geometric or crystallographic description was lacking. Solid state probes of the active sites have supplied information on this aspect of the mechanism. 

The early development of catalysts for ammonia synthesis was based on iron catalysts prepared by fusion of magnetite with small amounts of promoters. However, Ozaki et al. [52] showed several years ago that carbon-supported alkali metal-promoted ruthenium catalysts exhibited a 10-fold increase in catalytic activity over conventional iron catalysts under the same conditions. In this way, great effort has been devoted during recent years to the development of a commercially suitable ruthenium-based catalyst, for which carbon support seems to be most promising. The characteristics of the carbon surface, the type of carbon material, and the presence of promoters are the variables that have been studied most extensively. 

Dofour et al. [17] investigated the influence of synthesis method, precursor and effect of Cu addition on the WGS activity of Fe-Cr-Co catalysts. They prepared FeCr, FeCrCu, FeCrCo and FeCrCuCo formulations by oxidation precipitation method, using chloride (Cl) and sulphate (S) metal precursors. The catalytic activity results of FeCrCo and FeCrCuCo catalysts are presented in Figure 2.2. All the materials prepared from sulphate precursor showed higher carbon monoxide conversion than those synthesized with chloride. As expected Cu-promoted catalysts show better activity than Fe-Cr-Co catalysts. For the catalysts synthesized by chloride precursor, in the case of cobalt, incorporation of this metal into the magnetite lattice could improve the covalency of Fe and... 

Replacement of Cr with Al has been investigated extensively in recent times. Several groups concluded that Cr can be successfully replaced with Al. In 2000 Araujo and Rangel [33] first time ever reported Fe-Al-Cu catalysts for WGS reaction. They prepared Fe-Al, Fe-Cu, Fe-Al-Cu catalysts by coprecipitation. They maintained the iron to dopant molar ratio of 10. Addition of Al to the iron oxide increases the catalytic activity of iron oxide slightly. However, addition of both Al and Cu to the iron oxide increases the WGS activity tremendously (34 x lO" mol g h ). The catalyst with both dopants showed higher activity than a chromium- and copper-doped commercial catalyst (25 X lO" mol g h ). This sample produces the active phase more easily than the other catalysts and shows resistance to a further magnetite reduction. 

The Haber-Bosch process has been known and used for over a century, and considerably little has changed over such a long period of time. In early work, Mittasch developed a highly active heterogeneous iron catalyst prepared from magnetite (Fe304) very similar catalyst formulations are still used in modern ammonia synthesis. It was also demonstrated that catalysts prepared from magnetite had superior catalytic activity in comparison to catalysts prepared from other iron oxides. 

Nevertheless, the iron oxalate/NH4 Y system heat-treated at high temperatures might be considered for catalytic applications since the decomposition products (metallic iron, iron carbide and magnetite) may be highly dispersed and. hence, may exert catalytic activity. Studies are in progress to prove this suggestion. 

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