Iron nitride catalysts selectivity

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. 

Kam FS, Shultz JP, Anderson RB Kinetics of the Fischer-Tropsch synthesis on iron catalysts. Pressure dependence and selectivity of nitrided catalysts, J Phys Chem 64(4) 446—451, 1960. 

Bureau of Mines studies by M. D. Schlesinger, now in progress, indicate that nitrided fused iron catalysts operate successfully in the slurry process with about the same selectivity as observed in the fixed-bed tests. 

Alkali promotion is vital for Fe catalysts. The basicity of the surface determines its activity and, in particular, the selectivity toward longer chain hydrocarbons (see the section Control of Selectivity ). Thus, the effect of K2O is much more pronounced than that of the weaker base Li20. The presence of silica, which reacts with the alkali to form less basic silicates, lowers the basicity of alkali-promoted catalysts (20). Thus, the presence of silica in alkali-promoted Fe catalysts lowers the heavy hydrocarbon selectivity. CO2 chemisorption can be used as a measure of the surface basicity. It has been found that chemisorbed nitrogen lowers the amount of CO2 that can subsequently be chemisorbed, which indicates that surface nitrides lower the basicity. This is in agreement with the observation that nitriding iron catalysts results in a lower heavy hydrocarbon selectivity (14). CO2 chemisorption data nevertheless need to be interpreted with care. For instance, promotion with CaO increases the CO2 chemisorption but it has little effect on the FT selectivity. 

Ba-Ru/BN and Ba-Ru/AC catalysts have the same reaction kinetics. When compared with the melting iron catalysts, the influence of ammonia concentration on Ba-Ru/BN catalysts is relatively small. In given reaction conditions (temperature, pressure, H2/N2 rate and concentration of ammonia etc.), the optimum activity of Ru/BN can be obtained by selecting the appropriate surface area of boron nitride, the content of Ru and promoter, size and density of grain. Moreover, the useful... 

---