Fischer-Tropsch synthesis surface carbon

Bertole, C. J., Kiss, G., and Mims, C. A. 2004. The effect of surface-active carbon on hydrocarbon selectivity in the cobalt-catalyzed Fischer-Tropsch synthesis. J. Catal. 223 309-18. 

In Fischer-Tropsch synthesis the readsorption and incorporation of 1-alkenes, alcohols, and aldehydes and their subsequent chain growth play an important role on product distribution. Therefore, it is very useful to study these reactions in the presence of co-fed 13C- or 14 C-labeled compounds in an effort to obtain data helpful to elucidate the reaction mechanism. It has been shown that co-feeding of CF12N2, which dissociates toward CF12 and N2 on the catalyst surface, has led to the sound interpretation that the bimodal carbon number distribution is caused by superposition of two incompatible mechanisms. The distribution characterized by the lower growth probability is assigned to the CH2 insertion mechanism. 

Fischer-Tropsch synthesis can be regarded as a surface polymerization reaction since monomer units are produced from the reagents hydrogen and carbon monoxide in situ on the surface of the catalyst. Hence, a variety of hydrocarbons (mainly n-paraffines) are formed from hydrogen and carbon monoxide by successive addition of C, units to hydrocarbon chains on the catalyst surface (Equation 12.1). Additionally, carbon dioxide (Equation 12.3) and steam (Equations 12.1 and 12.2) are produced C02 affects the reaction just a little, whereas H20 shows a strong inhibiting effect on the reaction rate when iron catalysts are used. 

As catalysis proceeds at the surface, a catalyst should preferably consist of small particles with a high fraction of surface atoms. This is often achieved by dispersing particles on porous supports such as silica, alumina, titania or carbon (see Fig. 1.2). Unsupported catalysts are also in use. The iron catalysts for ammonia synthesis and CO hydrogenation (the Fischer-Tropsch synthesis) or the mixed metal oxide catalysts for production of acrylonitrile from propylene and ammonia form examples. 

Iron has a rich surface coordination chemistry that forms the basis of its important catalytic properties. There are many catalytic applications in which metallic iron or its oxides play a vital part, and the best known are associated with the synthesis of ammonia from hydrogen and nitrogen at high pressure (Haber-Bosch Process), and in hydrocarbon synthesis from CO/C02/hydrogen mixtures (Fischer-Tropsch synthesis). The surface species present in the former includes hydrides and nitrides as well as NH, NH2, and coordinated NH3 itself. Many intermediates have been proposed for hydrogenation of carbon oxides during Fischer-Tropsch synthesis that include growing hydrocarbon chains. 

The rale of the Boudouard reaction is mcreased by struciuial promoters proportional to the increase of the iron surface area (85). Fleetronic promoters not only enhance the catalyst activity but atscarbon deposition (57). This effect can be controlled by the addition of SiO . Thus, in order to minimize carbon depcKirion during Fischer Tropsch synthesis, it is necessary to control the catalyst basicity (851. 

Chain growth during the Fischer-Tropsch synthesis is controlled by surface polymerization kinetics that place severe restrictions on our ability to alter the resulting carbon number distribution. Intrinsic chain growth kinetics are not influenced strongly by the identity of the support or by the size of the metal crystallites in supported Co and Ru catalysts. Transport-limited reactant arival and product removal, however, depend on support and metal site density and affect the relative rates of primary and secondary reactions and the FT synthesis selectivity. 

Carbon forms play important roles as intermediates, catalyst additives and deactivating species in Fischer-Tropsch synthesis on iron catalysts. Deactivation may be due to poisoning or fouling of the surface by atomic carbidic carbon, graphitic carbon, inactive carbides or vermicular forms of carbon, all of which derive from carbidic carbon atoms formed during CO dissociation (ref. 5). While this part of the study did not focus on the carbon species responsible for deactivation, some important observations can be made to this end. 

R.T.K. Baker, D.J.C. Yates, and J.A. Dumesic, Filamentous Carbon Formation over Iron Surfaces, in Coke Formation on Metal Surfaces, in Coke Formation on Metal Surfaces, eds. L.G. Albright and R.T.K. Baker, American Chemical Society, Washington D.C., 1982, p. 1. D.J. Dwyer, Iron Fischer-Tropsch Catalysts Surface Synthesis at High Pressure, Prep. ACS Div. Pet. Chem. 29 (1984) 715. 

The reactivity of surface carbon relevant to Fischer-Tropsch synthesis begins to be understood from the recent works on iron clusters containing coordinatively unsaturated "carbidic" carbon or 2-CH ligands ... 

Here Z is a Ni surface site. The equation they derive is complex but can be simplified (see Table 4) for full-scale application. These workers point out that the same equation can be derived from a mechanism involving surface carbon as an intermediate similar to carbide theories for Fischer-Tropsch synthesis. In that case steps (b) and (c) in the above equation would be replaced by (b ) and (c ). 

Liquid-phase Fischer-Tropsch synthesis has been investigated using a slurry-bed reactor. The catalytic activity of ultrafine particles (UFP) composed of Fe was shown to be greater than that of a precipitated Fe catalyst. The difference was interpreted as caused by the different nature of surface structure between these catalysts, whether porous or not. The obtained carbon number distributions over alkali-promoted Fe UFP catalysts were simulated by a superposition of two Flory type distributions. It is ascertained that the surface of alkali-promoted UFP catalysts possesses promoted and unpromoted sites exhibiting different chain growth probabilities. 

Aligned multiwall CNT arrays were synthesized as a basis for a microstructured catalyst, which was then tested in the Fischer-Tropsch reaction in a microchannel reactor [269]. Fabrication of such a structured catalyst first involved MOCVD of a thin but dense A1203 film on a FeCrAlY foam to enhance the adhesion between the catalyst and the metal substrate. Then, multiwall CNTs were deposited uniformly on the substrate by controlled catalytic decomposition of ethene. Coating the outer surfaces of the nanotube bundles with an active catalyst layer results in a unique hierarchical structure with small interstitial spaces between the carbon bundles. The microstructured catalyst was characterized by the excellent thermal conductivity inherent to CNTs, and heat could be efficiently removed from the catalytically active sites during the exothermic Fischer-Tropsch synthesis. 

A controversial issue related to cobalt catalysts in Fisher-Tropsch synthesis is the structure-sensitive character of this reaction. Iglesia and co-workers [126,127] reported a large increase in activity when the cobalt particle size was decreased from 200 nm to 9 nm, whereas the specific activity [turnover frequency (TOF)] was not influenced by the cobalt particle size. However, other authors have reported that the TOF suddenly decreased for catalysts with cobalt particle sizes smaller than 10 nm [122,128]. Bezemer et al. [125] were the first to investigate the influence of cobalt particle size in the range 2.6 to 27 nm on performance in Fischer-Tropsch synthesis on well-defined catalysts supported on carbon nanofibers. It was found that the TOF for CO hydrogenation was independent of cobalt particle size for catalysts with particles larger then 6 nm (at atmospheric pressure) or 8 nm (at 35 bar). But both the TOF and the C5+ selectivity decreased for catalysts with smaller particles. It was proposed that the cobalt particle size effects could be attributed to a strucmre-sensitivity characteristic of the reaction, together with a CO-indnced reconstmction of the cobalt surface. 

P. Biloen, J.L. Helle, and W.M.H. Sachtler. Incorporation of Surface Carbon into Hydrocarbons during Fischer-Tropsch Synthesis Mechanistic Implications. J. Catal. 58 95 (1979). 

The hydrocarbon synthesis and the elemental carbon formation are competing for the available supply of carbide. Higher partial pressures of hydrogen are known to increase the rate of hydrocarbon synthesis and therefore the rate of elemental carbon formation would be expected to decrease with increase of HjrCO ratio. This explanation assumes that bulk carbide (as distinguished from surface carbide) is a necessary intermediate in the Fischer-Tropsch synthesis. As shown by the recent work of Emmett and his associates (75) using radioactive carbon as a tracer, this assumption is very probably incorrect. 

The variation in promoter ability of (i) potassium and rubidium carbonates and (ii) the other carbonates may be attributable to subtle effects the active carbonates have on the surfaces and active sites of the catalyst. These effects may be caused by differences in basicity of the carbonates. According to Dry et al. (23), the promoter which is the strongest base is the most effective. The influence of base promoters on Fischer-Tropsch synthesis depends on their effect on the heat of adsorption of carbon monoxide and hydrogen on the catalyst, e.g., K2O increases the heat of carbon monoxide adsorption at low coverage and decreases the initial heat of hydrogen adsorption. 

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