Iron carbide, hydrocarbon synthesis

HTS catalyst consists mainly of magnetite crystals stabilized using chromium oxide. Phosphoms, arsenic, and sulfur are poisons to the catalyst. Low reformer steam to carbon ratios give rise to conditions favoring the formation of iron carbides which catalyze the synthesis of hydrocarbons by the Fisher-Tropsch reaction. Modified iron and iron-free HTS catalysts have been developed to avoid these problems (49,50) and allow operation at steam to carbon ratios as low as 2.7. Kinetic and equiUbrium data for the water gas shift reaction are available in reference 51. 

After reduction and surface characterization, the iron sample was moved to the reactor and brought to the reaction conditions (7 atm, 3 1 H2 C0, 540 K). Once the reactor temperature, gas flow and pressure were stabilized ( 10 min.) the catalytic activity and selectivity were monitored by on-line gas chromatography. As previously reported, the iron powder exhibited an induction period in which the catalytic activity increased with time. The catalyst reached steady state activity after approximately 4 hours on line. This induction period is believed to be the result of a competition for surface carbon between bulk carbide formation and hydrocarbon synthesis.(6,9) Steady state synthesis is reached only after the surface region of the catalyst is fully carbided. 

The steady state rates of hydrocarbon synthesis over the carbided iron surface are given in Table I. The reaction rates have been normalized to the physical surface area of the starting iron powder [18 M /g] and are reported in molecules/cm sec. A turnover... 

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+. 

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. 

Good evidence has been obtained that heterogeneous iron, ruthenium, cobalt, and nickel catalysts which convert synthesis gas to methane or higher alkanes (Fischer-Tropsch process) effect the initial dissociation of CO to a catalyst-bound carbide (8-13). The carbide is subsequently reduced by H2to a catalyst-bound methylidene, which under reaction conditions is either polymerized or further hydrogenated 13). This is essentially identical to the hydrocarbon synthesis mechanism advanced by Fischer and Tropsch in 1926 14). For these reactions, formyl intermediates seem all but excluded. 

The CO that is consumed in the process, besides forming hydrocarbons and C02 also caused the transformation of hematite to magnetite and subsequently the magnetite to iron carbide. Therefore, a better measure of Fischer-Tropsch synthesis (FTS) activity is the rate of CH4 formation plotted in Figure 28.1(b). It is seen that activation at 543 K makes the... 

The hypothesis of formation of oxygenated compounds as intermediate products was rejected by Eidus on the basis of experiments on the conversion over cobalt of methyl and ethyl alcohols and formic acid which were found to form carbon monoxide and hydrogen in an intermediate step of the hydrocarbon synthesis (76). Methylene radicals are thought to be formed on nickel and cobalt catalysts (76) by hydrogenation of the unstable group CHOH formed by interaction of adsorbed carbon monoxide and hydrogen, while on iron catalysts methylene radicals are probably formed by hydrogenation of the carbide (78,81). Carbon dioxide was found to interact with the alkaline promoters on the surface of iron catalysts as little as 1 % potassium carbonate was found to occupy 30 to 40% of the active surface area. The alkali also promotes carbide formation and the synthesis reaction on iron (78). 

Carbided cobalt reacts with hydrogen at synthesis conditions. However, the product of the conversion is methane only (113). Iron carbides are very stable against hydrogen at synthesis conditions. The decomposition of iron carbide with acids yields higher hydrocarbons in addition to methane (114). 

Conversion of the bulk of the catalyst metal to carbide during hydrocarbon synthesis is only observed in the case of iron catalysts. The carbide formation, which occurs parallel with the activity of iron catalysts, may have an important influence on the conditions of the catalyst structure and catalyst surface. The carbide formation, however, seems to be insufficient for the catalyst activity. Treatment of carbided iron catalysts with sulfur does not change the carbide content but makes the catalyst inactive for hydrocarbon synthesis (134). 

The possible steps of Fischer-Tropsch (FT) reaction and its catalysts (Fe, Co, Ru, Ni) represent a very complicated systemThe catalysts usually need a formation or self-organisation , meaning that the full activity will only be reached after a certain period. This means that for Fe-based catalysts, a part of the initial Fe oxide is transformed into iron carbide. This was investigated as early as 1948 by the tracer method.A fused iron catalyst was carbided with The synthesis product from CO/H2 = 1 1 reactant contained 10-15% labelled molecules, almost independently of the reaction conditions, even in repeated runs, indicating the minor role of carbide incorporation into hydrocarbons. The formation of a Fe-Al-Cu catalyst at 523 K and various H2/CO ratios required 100 to 2000 minutes. The yield of retained carbon decreased gradually, while the FT yield increased more abruptly after this period. [ 1... 

The role of carbides in the synthesis of hydrocarbons has been widely considered ever since the carbide theory was first postulated by Fischer and Tropsch in 1926 (20). Although recent experimental studies indicate that the carbide theory is largely incorrect, that is, that bulk-phase carbides are not intermediates in the formation of higher hydrocarbons, iron catalysts converted to Hagg carbide or cementite are usually more active than similar raw or reduced catalysts (21). (For a review of the carbide theory up to 1950, see p. 571 of reference 22.) The selectivity of carbided iron catalysts is essentially the same as that of corresponding reduced catalysts. Nitrides of iron are usually more active than reduced or carbided catalysts, and the catalyst selectivity is significantly different. 

The Fischer-Tropsch Synthesis (FTS) converts synthesis gas (a mixture of CO and H,) to hydrocarbons. Iron-based catalysts lose activity with time on stream (TOS). The rate of deactivation is dependent on the presence/absence of promoters such as potassium and/or binders such as silica [1.2]. Several possible causes of catalyst deactivation have been postulated [3] (i) Sintering, (ii) Carbon deposition, and, (iii) Phase transformations. With respect to phase transformations, there is considerable disagreement whether the active phase for the FTS is iron oxide or carbide [4,5]. In addition, certain reactor conditions, such as a high partial pressure of water, are known to cause a decline in activity [6]. 

Experiments have recently been completed by Kummer, DeWitt, and Emmett (75), using C as a tracer in the synthesis on an iron catalyst. The results are inconclusive. If the total catalyst surface is uniformly active in the synthesis, the results show that only a small fraction of the reaction proceeds by way of the carbide. However, if only occasional active patches of the surface are participating in the synthesis, then it is possible to interpret the results as indicating that all of the reaction proceeds by way of the carbide. The carbide intermediate hypothesis for the mechanism of the synthesis on iron catalysts, however, is probably incorrect. Thus, the results of recycle operations at low temperatures on iron catalysts show that alcohols are formed earlier in the synthesis than olefinic hydrocarbons. 

The fact that only a relatively small fraction of singly-branched hydrocarbons is produced in the synthesis on iron and cobalt catalysts is very difficult to explain on the basis of the carbide-methylene polymerization hypothesis. Branching should occur whenever a carbidic carbon is included in a methylene polymerization chain, and as there is according to this hypothesis a considerable supply of carbidic carbon, branching should occur much more often than is the case. Furthermore, there is no explanation offered by this hypothesis as to why the branch is limited to a single methyl group. 

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