Nitriding, iron

The reverse reaction to ammonia synthesis, the decomposition to nitrogen and hydrogen, is used in the nitriding of iron and carried out industrially at temperatures around 800 K and atmospheric pressure to produce surface-hardening. This dissolution reaction must also play a part in the synthesis of ammonia by the industrial process. The attempt to nitride iron by reaction with nitrogen gas is very slow under atmospheric pressure, presumably due to the stability of the nitrogen molecule. 

The preliminary plastic deformation considerably effects on the phase formation, structure, microhardness and thickness of nitrided layers in -Fe and Fe-Ni alloys. The high microhardness of the diffusion layers results from the formation of the s- and y- nitrides. Iron doping with Ni leads to changing of the s-, f-phases composition. The existence of narrow intervals of deformations of 3-8 % and 20-30 %, in which the considerable (about 2 times) rise of microhardness of surface nitrided layers due to accelerated formation of s- and f-phases, was found. 

Changes of Composition of Nitrided Iron Catalysts during the Synthesis 373... 

Alcohols Separated from Products of Nitrided Iron Catalysts Fe304-Al20a-Kj0 catalyst at 21.4 atmospheres Weight-% of total condensed product exclusive of water ... 

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... 

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. 

Formation of oxygenated products over nitrided iron catalysts 6I ] ... 

Fisoher-Tropsch Nitrided iron 10-30 190-200 Olefins + paraffins 40%, dcobols 80%... 

Kokes et al. also utilized a nitrided iron catalyst which produced a higher fraction of alcohol than was formed with the normal iron catalyst. The radioactivities for... 

Three reviews have been made (22,25,26) the latest (26) summarized Bureau of Mines tests of nitrided iron in fluid-bed, slurry, and oil circulation reactors of small pilot-plant size. In these units the nitrides operated nicely, having the longest life of any catalyst used in these reactors. The yields of C and Cp hydrocarbons and oxygenates are often larger than some would like. 

Nitrided iron catalysts are like other iron catalysts in sensitivity to poisoning by sulfur compounds and in the relative usage of H and CO, For all iron catalysts water is the main primary product and COp is produced by the water-gas shift, Nitrided iron produces low of wax, and they can be used in fluid-bed/or entrained reactors at temperatures below 260 C, where reactions leading to deterioration of the catalyst are slow. At... 

Mit] Mittemeijer, E.J., Vogels, A.B.P., Vanderschaaf P.J., Morphology and Lattice-Distortions of Nitrided Iron and Iron-Chromium Alloys and Steels , J. Mater. Sci., 15(12), 3129-3140 (1980) (Experimental, Meehan. Prop., Morphology, 34)... 

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. 

Similarly, iron oxide NPs confined inside the CNTs channels revealed easier reduction of the oxide at a lower temperature than those supported on the outer wall [129], Even if both in and out NPs exhibit similar conversion, confined NPs displayed higher selectivity to heavier hydrocarbons and were more stable. Cubic iron nitride FeN NPs were synthesized inside or outside CNTs [81]. The FeN confined within the CNTs exhibits higher activity than the nitride iron particles located on the CNT exterior walls in CO hydrogenation. The authors correlated this effect to the smaller particle size and the stronger retention of nitrogen atoms in the cnbic nitride lattice, resnlting in formation of more carbides in than out. 

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