Catalysts metallic iron

Iron as a Catalyst.—Metallic iron can, in certain circumstances, act as a catalyst, in this respect resembling, albeit feebly, the more noble elements of Group VIII. of the Periodic Table. Thus, for example, iron assists the production of ammonia from a mixture of nitrogen and hydrogen at 800° to 1000° C. under a pressure of 100 atmospheres. No ammonia is obtained under these conditions in a porcelain vessel if iron or any other catalyst be excluded.8... 

The addition of allcenes to alkenes can also be accomplished by bases as well as by the use of catalyst systems consisting of nickel complexes and alkylaluminum compounds (known as Ziegler catalysts), rhodium catalysts, and other transition metal catalysts, including iron. These and similar catalysts also catalyze the 1,4 addition of alkenes to conjugated dienes, for example. 

Nitrogen adsorption experiments showed a typical t)q5e I isotherm for activated carbon catalysts. For iron impregnated catalysts the specific surface area decreased fix>m 1088 m /g (0.5 wt% Fe ) to 1020 m /g (5.0 wt% Fe). No agglomerization of metal tin or tin oxide was observed from the SEM image of 5Fe-0.5Sn/AC catalyst (Fig. 1). In Fig. 2 iron oxides on the catalyst surface can be seen from the X-Ray diffractions. The peaks of tin or tin oxide cannot be investigated because the quantity of loaded tin is very small and the dispersion of tin particle is high on the support surface. 

The quadrupole doublet has an isomer shift corresponding to iron in the ferric or Fe " state. After reduction in H2 at 675 K the catalyst consists mainly of metallic iron, as evidenced by the sextet, along with some unreduced iron, which gives rise to two doublet contributions of Fe " and Fe " in the centre. The overall degree of iron reduction, as reflected by the relative area under the bcc ion sextet, is high. Fischer-Tropsch synthesis at 575 K in CO and FI2 converts the metallic iron into the Flagg carbide, Fe5C2. The unreduced iron is mainly present as Fe ". Exposure of the carburized catalyst to the air at room temperature leaves most of the carbide phase unaltered but oxidizes the ferrous to ferric iron. 

In the previous sections we have dealt mainly with the catalytic activity of pure substances such as metallic iron, ruthenium, copper, platinum, etc. Real catalyst, however, are often much more complex materials that have been optimized by adding remote amounts of other elements that may have a profound impact on the overall reactivity or selectivity of the catalyst. Here we shall deal with a few prominent examples of such effects. 

An XPS Investigation of iron Fischer-Tropsch catalysts before and after exposure to realistic reaction conditions is reported. The iron catalyst used in the study was a moderate surface area (15M /g) iron powder with and without 0.6 wt.% K2CO3. Upon reduction, surface oxide on the fresh catalyst is converted to metallic iron and the K2CO3 promoter decomposes into a potassium-oxygen surface complex. Under reaction conditions, the iron catalyst is converted to iron carbide and surface carbon deposition occurs. The nature of this carbon deposit is highly dependent on reaction conditions and the presence of surface alkali. 

This XPS investigation of small iron Fischer-Tropsch catalysts before and after the pretreatment and exposure to synthesis gas has yielded the following information. Relatively mild reduction conditions (350 C, 2 atm, Hg) are sufficient to totally reduce surface oxide on iron to metallic iron. Upon exposure to synthesis gas, the metallic iron surface is converted to iron carbide. During this transformation, the catalytic response of the material increases and finally reaches steady state after the surface is fully carbided. The addition of a potassium promoter appears to accelerate the carbidation of the material and steady state reactivity is achieved somewhat earlier. In addition, the potassium promoter causes a build up on carbonaceous material on the surface of the catalysts which is best characterized as polymethylene. 

Supported iron catalysts are notoriously difficult to reduce [6-8] and thus a substantial fraction of the iron can be expected to remain inactive for the catalysis of hydrogenation. Particular attention has therefore been paid to the preparation of Fe/MgO catalysts by several different methods and examination of their effectiveness in producing metallic iron of adequate specific surface area after reduction in hydrogen. The activity and selectivity for primary amine formation have been determined for the hydrogenation of ethanenitrile (acetonitrile) and propanenitrile. 

Carbon monoxide chemisorption was used to estimate the surface area of metallic iron after reduction. The quantity of CO chemisorbed was determined [6J by taking the difference between the volumes adsorbed in two isotherms at 195 K where there had been an intervening evacuation for at least 30 min to remove the physical adsorption. Whilst aware of its arbitrariness, we have followed earlier workers [6,10,11] in assuming a stoichiometry of Fe CO = 2.1 to estimate and compare the surface areas of metallic iron in our catalysts. As a second index for this comparison we used reactive N2O adsorption, N20(g) N2(g) + O(ads), the method widely applied for supported copper [12]. However, in view of the greater reactivity of iron, measurements were made at ambient temperature and p = 20 Torr, using a static system. 

Iron was present as Fe " in the calcined precursors. For all the catalysts the reduction procedure described in Sec. 2.1 resulted in incomplete reduction of the Fe to metallic iron. This is in agreement with the findings of previous authors [6,11]. The individual percentage reductions of Fe to Fe°, as determined by the separate gravimetric and volumetric measurements (Sec. 2.2), are shown in Table 1. The values are calculated on the assumption that all the Fe is reduced to Fe prior to the onset of reduction to Fe°. There is good agreement between the two methods. Table 1 also records the actual Fe/(Fe + Mg) ratio in the catalysts as determined by atomic absorption spectroscopy (AAS) on the calcined precursors. 

For preparative reactions, Lewis acid catalysts are used. Zinc chloride or ferric chloride can be used in chlorination, and metallic iron, which generates ferric bromide, is often used in bromination. The Lewis acid facilitates cleavage of the halogen-halogen bond. 

Macdonald, J.E. and Veinot, J.G.C. (2008) Removal of residual metal catalysts with iron/iron oxide nanoparticles from coordinating environments. Langmuir, 24 (14), 7169-7177. 

For the development of a selectivity model it is helpful to have a picture of the surface of the catalyst to ht the explanation of how the product spectrum is formed. The fundamental question regarding the nature of the active phase for the FT and water-gas shift (WGS) reactions is still a controversial and complex topic that has not been resolved.8 Two very popular models to describe the correlations between carbide phase and activity are the carbide9 and competition models.10 There are also proposals that magnetite and metallic iron are both active for the FT reaction and carbides are not active11. These proposals will not be discussed in detail and are only mentioned to highlight the uncertainty that is still present on the exact phase or active site responsible for the FT and WGS reactions. 

After calcinations, the precipitated iron catalyst is composed of a mixture of iron oxide phases before activation. The exact nature of this phase is not critical for the discussion and will be referred to in general as an oxide phase. During activation the catalyst is subjected to a reducing environment that will lead to the formation of either metallic iron if pure hydrogen is used or some iron carbide if the reduction is done with either CO or syngas. During reduction with a gas... 

After reduction in FI2 at 675 K the catalyst consists mainly of metallic iron, as evidenced by the sextet (S= 0.00 mm/s, H = 331 kOe), along with some unreduced iron, which gives rise to two doublet contributions of Fe2+ and Fe3+ in the center. The... 

When reduced Fe/Ti02 is used as a catalyst for the reaction between CO and H2 to form hydrocarbons (the Fischer-Tropsch synthesis) the spectrum changes entirely. All metallic iron has been converted into a new phase. The spectrum is that of a crystallographically well-defined iron carbide, namely the Hagg carbide, or %-Fe5C2. Apparently the strongly reducing atmosphere has affected the unreduced iron as well all ions are now present as Fe2+. 

Figure 5.9 Left Mossbauer spectra of a metallic iron catalyst after different periods of Fischer-Tropsch synthesis in CO+H2 at 240°C, showing the conversion of metallic iron (visible by the outer two lines in the upper two spectra) into iron carbides all spectra were recorded at room temperature. Right reaction rate of the Fischer-Tropsch synthesis (upper curve) and the relative contributions of metallic iron and various carbides to the Mossbauer spectra (from [22]).

The conversion of iron catalysts into iron carbide under Fischer-Tropsch conditions is well known and has been the subject of several studies [20-23], A fundamentally intriguing question is why the active iron Fischer-Tropsch catalyst consists of iron carbide, while cobalt, nickel and ruthenium are active as a metal. Figure 5.9 (left) shows how metallic iron particles convert to carbides in a mixture of CO and H2 at 515 K. After 0.5 and 1.1 h of reaction, the sharp six-line pattern of metallic iron is still clearly visible in addition to the complicated carbide spectra, but after 2.5 h the metallic iron has disappeared. At short reaction times, a rather broad spectral component appears - better visible in carburization experiments at lower temperatures - indicated as FexC. The eventually remaining pattern can be understood as the combination of two different carbides -Fe2.2C and %-Fe5C2. 

In catalyst characterization, diffraction patterns are mainly used to identify the crystallographic phases that are present in the catalyst. Figure 6.2 gives an example where XRD readily reveals the phases in an Fe-MnO Fischer-Tropsch catalyst [7], The pattern at the top is that of an MnO reference sample. The diffraction pattern of the reduced Fe-MnO catalyst shows a peak at an angle 29 of 57°, corresponding to metallic iron, and two peaks which are slightly shifted and broadened in comparison with the ones obtained from the bulk MnO reference. The Mossbauer spectrum of the reduced catalyst contains evidence for the presence of Fe2+ ions in a mixed (Fe,Mn)0 oxide [7], and thus it appears justified to attribute the distortion of the XRD peaks to the incorporation of Fe into the MnO lattice. Small particle size is another possible reason why diffraction lines can be broad, as we discuss below. 

When the Fe-MnO catalyst is analyzed after use in the Fischer-Tropsch reaction (the synthesis of hydrocarbons from CO and H2), the XRD pattern in Fig. 6.2 reveals that all metallic iron has disappeared. Instead, a number of weak reflections are visible, which are consistent with the presence of iron carbides, as confirmed by Mossbauer spectroscopy [7]. The conversion of iron to carbides under Fischer-Tropsch conditions has been studied by many investigators and has been discussed in more detail in Chapter 5 on Mossbauer spectroscopy. 

Figure 6.6 In situ XRD of an alumina-supported iron catalyst during reduction in H2 at 675 K reveals the transition of a-Fe202 (hematite) via Fe(04 (magnetite) to metallic iron as a function of time. The graph shows the degree of reduction of supported and unsupported oc-Fe203 as determined from the XRD measurements (from Jung and Thomson f 14]).

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As a result of CNT synthesis, catalyst metal nanoparticles (iron, cobalt, nickel) together with amorphous carbon and fullerenes are unavoidably present in the CNT soot. 

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