Magnetite catalyst catalytic activity

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]. 

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. 

The above-mentioned experimental results indicated that, in the ranges of Fe +/Fe + < 1 i.e., the first peak, the relation between catalytic activity and the Fe +/Fe + ratio is consistent with those results of traditional catalysts, in which the precursor is magnetite phases (Fig. 3.27). The facts of the decreasing activity with increasing of Fe +/Fe + ratio from 0.5 to 1, also coincides well with the results... 

The contents of the mixed structural promoters in iron oxides can determine the surface features, while the iron oxide precursors can influence the catalyst surface area, in order to perform the catalytic activity. Prom this point of view, the most optimum compositions of the structural promoters for wiistite catalysts are not identical to that of those magnetite catalysts. 

In Chapter 3, the authors proposed the single-phase principle of the preparation of fused iron catalyst. It is clearly pointed out that high catalytic activity can be achieved when wiistite or magnetite phase exists separately in the catalyst. When wiistite and magnetite coexist in the catalyst, the catal3dic activity is always low. In the FeO phase region, the activity of the catalyst decreases due to the formation of a new phase a-Fe in the case of f > 11. 

According to this model, the proportion of the sample accessible by photoemission should be reduced to metal within a short period of time, i.e., UPS should detect metallic iron shortly after the surface has reached the reaction temperature of ca 700 K. This was not observed. Instead, the gas-solid interface only becomes metallic after reduction in the bulk of the sample has terminated. The core level data, on the other hand, showed the expected behavior, with an early formation of iron metal nuclei and subsequent growth to form particles large enough to exhibit the spectral parameters of bulk iron. It is therefore concluded that the reduction process produces two interfaces the one, an outer gas-solid interface which remains in the magnetite form until the reaction on the other, an inner metal-metal oxide interface, is completed. For catalytic activity, only the gas-solid interface is relevant. The catalyst therefore only becomes active if the whole of the bulk is converted from oxide to metal, although the majority of the material is still not involved in catalysis. 

The catalytic activity of an ammonia catalyst may be reduced in the presence of certain chemical compounds, referred to as poisons. These may be gaseous, occurring as minor components of the synthesis gas, or as solids introduced to the catalyst during the manufacturing process, as impurities in the natural magnetite from which the catalyst is made. The latter will not be dealt with here, since they are already covered in Chapter 2. 

It can be calculated that a H2S/H2 ratio of 4.7 x 10 is required to establish the Fe/FeS equilibrium at TOOK, using data from Barin et but a H2S/H2 ratio of only 10 to affect the surface properties as extrapolated from Grabke s data. Similarly, a H2O/H2 ratio of 0.15 is required to establish the equilibrium between magnetite and iron at 700 K, but only ppm levels of water are required to affect the catalytic activity of an ammonia synthesis catalyst. 

Alumina is incorporated as a sohd solution of the iron aluminate spinel, hercynite, in the crystal lattice. The alumina concentration should be less than the solubility of alumina in magnetite. This corresponds to a maximiun content of about 3% alumina. Any excess of alumina does not go into solid solution, and leads to a reduction in catalytic activity, particitlarly when using catalysts promoted with alumina. The presence of alumina as a structural promoter also leads to the formation of wustite and stabihzes the reduced catalyst. Small amounts of magnesia can also dissolve into magnetite and act as a promoter. The calcium component exists in the form of ferrites or alirminates by neutrahzing acidic components—such as silica—and protects the potash that activates the catalyst. 

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