Iron nitride catalysts fused

Studies of the Fischer-Tropsch synthesis on nitrided catalysts at the Bureau of Mines have been described (4,5,23). These experiments were made in laboratory-scale, fixed-bed testing units (24). In reference 5, the catalyst activity was expressed as cubic centimeters of synthesis gas converted per gram of iron per hour at 240°C. and at a constant conversion of 65%. Actually, the experiments were not conducted at 240°C., but the activity was corrected to this temperature by the use of an empirical rate equation (25). Conditions of catalyst pretreatment for one precipitated and two fused catalysts are given in Table IV. 

Catalyst Fused iron Iron nitride Thoria with... 

A number of substances show considerable activity as ammonia catalysts. Fe, Os, and Re and nitrides of Mo, W, and U are the best known. Iron in the form of promoted iron catalysts is by far the most important, maybe the only type in industrial use, and except for a few comparisons, iron catalysts will be the only type dealt with in this paper. Furthermore, the discussion will be limited to the type of catalysts made by fusing iron oxides together with the promoter components and subsequently reducing the catalysts. This limitation is not too important, since this type of catalyst is the one most widely used and also the type on which most fundamental work has been done. 

Thus, ammonia does not reduce magnetite at an appreciable rate at temperatures below 450°C., and it appeal s that at 450°C. and above, the reduction may be accomplished by decomposition products of ammonia rather than by ammonia itself. This contention is based on the fact that the reduction of fused catalysts with ammonia at 450°C. and 550°C. appeared to be an autocatalytic process that is, the rate of reduction increased with time in the initial part of the experiment. Reduction with hydrogen does not appear to be autocatalytic. It may be postulated that a-iron and nitride formed in the reduction are better catalysts for the ammonia decomposition than iron oxide. 

Hall, Dieter, Hofer, and Anderson (19) studied reactions of nitrides and carbonitrides in a reduced, fused iron catalyst (Bureau of Mines number D3001). The results of these experiments were in general similar to those of Jack, and most of the differences may be explained by the differences in the type of iron employed. The major discrepancy was that in the catalyst of large surface area and small crystallite size, the <-carbonitride phase was found under conditions under which massive iron is converted to the f-phase. Since the transformation of the - to the f-phase involves only slight changes in the lattice positions of iron atoms and small changes in the x-ray pattern, it is possible either that this transformation did not occur in the catalyst or that the pattern of the f-phase could not be distinguished from that of the e-phase in the diffuse diffraction patterns. 

The reduction of nitrides of iron in pure hydrogen was very rapid, and at 200°C. nitrogen was virtually completely removed from fused iron catalysts in three hours. The hydrogenation of carbonitrides was considerably slower than that of nitrides, and the rate varied inversely with the carbon content of the carbonitride (Table III and reference 19). In... 

Fig. 7. Product from Fischer-Tropsch synthesis with nitrided fused iron catalyst. Reprinted by permission of the copyright holder, the American Chemical Society.

Nitrided iron catalysts are remarkably stable toward oxidation and deposition of elemental carbon in the synthesis at both 7.8 and 21.4 atm. Shultz, Seligman, Lecky, and Anderson (23) described composition changes in nitrided fused, sintered, and precipitated catalysts in the synthesis. In a test (X218) with 1H2 + ICO synthesis gas at 7.8 atm., the catalyst was sampled frequently for analysis. X-ray diffraction patterns showed only the e-nitride or carbonitride phase, except for weak... 

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. 

Comparative tests at the Piltsbur i Energy Technology Center with a Fe-Cu - K catalyst revealed a better stability for the nitrided iron catalyst [63]. The precipitated nitrided iron catalyst gave a higher percentage of oxygenates than did nitrided fused iron. 

Anderson et al. (1964) studied fused iron catalysts which had either been reduced or reduced and nitrided prior to use in fixed-bed reactors, determining reaction kinetics and the effects of the extent of reduction and particle size on catalyst activity. Particle sizes ranged from 42—60 mesh to 4—6 mesh. The catalyst activity increased with smaller particle size until the diameter reached about 0.3 mm for the most active catalysts tested. Catalyst particles were modeled as an active layer of catalyst surrounding an inert core, with the depth of the active layer governed by the reduction temperature. Their calculations allowed them to estimate the effective reactant dif-fusivity, and they were also able to quantify the depth of the active layer of catalyst. Variations in catalyst activity were attributed to the diffusion of reactant through a wax-filled pore and the depth of the active layer. 

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