Carburization during

It has been suggested [21,22] that the presence of Cu and K increases the rates and extent of Fe304 carburization during reaction and the FTS rates, by providing multiple nucleation sites that lead to the ultimate formation of smaller carbide crystallites with higher active surface area. In the present investigation, Cu- and K-promoted iron catalysts performed better than the unpromoted catalysts in terms of (1) a lower CH4 selectivity, (2) higher C5+ and alkene product selectivi-ties, and (3) an enhanced isomerization rate of 1-alkene. 

Iron catalysts present special problems for the determination of the adsorbed species. The iron may be carburized during the reaction and this leads to a change in the bulk composition of iron catalyst with time on stream. Bianchi et al. determined that iron was partly converted into a mixture of e -Fe2 2C and x Fe2 5C during the reaction using Mossbauer spectroscopy. The adsorbed carbon surface species were determined to be present in three forms small amounts of reactive CH species which produced the bulk of the hydrocarbon products during reaction, a carbidic species with some associated hydrogen, and inactive graphitic carbon species. 

Ohno, K Hino, M Kinetic analysis of iron carburization during smelting reduction, Science and technology of innovative ironmaking for aiming at energy half consumption, Japan, November, 2003,17-20. 

Also, manganese added to cobalt on activated carbon catalysts resulted in a decrease in bulk carbide formation during reduction and a decrease in the subsequent deactivation rate.84 Magnesium and yttrium added to the support in alumina-supported cobalt catalysts showed a lower extent of carburization. This was explained by a decrease in Lewis acidity of the alumina surface in the presence of these ions.87... 

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. 

DICTRA (Diffusion simulation software) (Borgenstam et al. 2000). DICTRA (coupled with Thermo-Calc) is a software for the simulation of diffusion in multi-component alloy systems (processes which can be simulated are for example the homogenization of alloys, micro-segregation during solidification, carburizing, and decarburizing, of steels, etc.). 

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... 

Podgurski, Kummer, DeWitt, and Emmett (14) carburized reduced fused catalysts with propane, butane, and pentane at 325°C. Although x-ray patterns of Hagg carbide were found, the carbon content of these preparations approached 7.5 weight-% as a limit rather than 9.1% which was obtained upon carburization with carbon monoxide. Hall (15) found that during carburization of a reduced fused catalyst (Bureau of Mines number D3001) in n-butane at 300°C., C increased only to 0.22, and the carbide phase was cementite rather than Hagg carbide. Methane is too stable thermodynamically to carburize iron rapidly at low temperatures however, at 500°C. relatively pure cementite may be prepared with methane (15). 

Okoli, S., Haubner, R. and Lux, B. (1991), Carburization of tungsten and tantalum filaments during low-pressure diamond deposition. Surf. Coat. Technol., 47(1-3) 585-599. 

HP microalloys were developed during the 1990s. The microalloys enhanced carburization resistance and improved high-temperature creep-rupture resistance.88... 

Upon heating, only the carbon atoms in contact or in the near-neighborhood react with tungsten. All others must to be transported to the tungsten surface, either by surface diffusion (slow) or by chemical vapor transport (CVT), i.e., formation and subsequent dissociation of methane (fast). Thus, during the initial stage of the carburization, the availability of carbon atoms is quite high and becomes less later. 

Figure 3.21 shows the change of the product composition with carburization time in hydrogen for a tungsten-carbon black mixture at 1119°C. After only 10 minutes at this temperature most of the metal was transformed to W2C, which was ffien only slowly converted to WC. This result is characteristic for the temperature range of 1050 to 1850 °C and W particle sizes of 1.3 to 20 pm [3.71]. It is remarkable that W2C forms during carburization even at 900 °C, which is well below its eutectoid decomposition teniperature of 1250°C. 

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