Fischer-Tropsch carburized

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. 

Hofer, L. J. E., Cohn, E. M., and Peebles, W. C. 1949. Isothermal decomposition of the carbide in a carburized cobalt Fischer-Tropsch catalyst. J. Phys. Coll. Chem. 53 661-69. 

Temperature-programmed reduction combined with x-ray absorption fine-structure (XAFS) spectroscopy provided clear evidence that the doping of Fischer-Tropsch synthesis catalysts with Cu and alkali (e.g., K) promotes the carburization rate relative to the undoped catalyst. Since XAFS provides information about the local atomic environment, it can be a powerful tool to aid in catalyst characterization. While XAFS should probably not be used exclusively to characterize the types of iron carbide present in catalysts, it may be, as this example shows, a useful complement to verify results from Mossbauer spectroscopy and other temperature-programmed methods. The EXAFS results suggest that either the Hagg or s-carbides were formed during the reduction process over the cementite form. There appears to be a correlation between the a-value of the product distribution and the carburization rate. 

The TPR-XAFS technique confirmed that doping Fischer-Tropsch synthesis catalysts with Cu and alkali (e.g., K) remarkably promotes the carburization rate relative to the undoped catalyst. The EXAFS results suggest that either the Hagg or e-carbides were formed during the reduction process over the cementite form. A correlation is observed between the a-value of the product distribution and the carburization rate. 

Li, S., Li, A., Krishnamoorthy, S., and Iglesia, E. 2001. Effects of Zn, Cu, and K promoters on the structure and on the reduction, carburization, and catalytic behavior of iron-based Fischer-Tropsch synthesis catalysts. Catal. Lett. 77 197-205. 

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. 

Iron catalysts used in Fischer-Tropsch synthesis are very sensitive to conditions of their preparation and pretreatment. Metallic iron exhibits very low activity. Under Fischer-Tropsch reaction conditions, however, it is slowly transformed into an active catalyst. For example, iron used in medium-pressure synthesis required an activation process of several weeks at atmospheric pressure to obtain optimum activity and stability.188 During this activation period, called carburization, phase... 

Carburization Studies of Iron Fischer-Tropsch Catalysts... 

The experiments and results reported by Hofer, Cohn, and Peebles are as follows A 3-g. sample of raw cobalt-thoria-kieselguhr Fischer-Tropsch catalyst was reduced in flowing purified hydrogen for 40 hours at 400°. The sample was then carburized with purified carbon monoxide... 

Ribeiro MC, Jacobs G, Davis BH, Cronauer DC, Kropf AJ, Marshall CL (2010) Fischer — Tropsch synthesis an in-situ TPR-EXAFS/XANES investigation of the influence of Group I alkali promoters on the local atomic and electronic structure of carburized iron/ silica catalysts. J Phys Chem C 114 7895-7903... 

Iron is the industrial Fischer-Tropsch catalyst and is applied in practice. Reduced iron interacts strongly with carbon. Because the activation energy for carbon diffusion into the metallic iron lattice is low (40-65 kJ/mol), the metal converts to iron carbides during reaction. According to Niemantsverdriet et al, the initial rate of the Fischer-Tropsch reaction is low because the carburization process consumes most of the carbon. When the iron particles become saturated, carbon stays at the surface where it is available for the actual Fischer-Tropsch reaction. Molybdenum is also converted to a carbide or an oxide when exposed to synthesis gas in this state it is an active catalyst. However, the early transition metals form stable but unreactive compounds in synthesis gas and are inactive as Fischer-Tropsch catalysts. 

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