Nitride iron, promotion

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. 

The main emphasis was laid, in this initial work, on Haber s catalysts, e.g., osmium and uranium compounds, as well as on a series of iron catalysts. Some other metals and their compounds which we tested are, as we know today, less accessibble to an activation by added substances than iron. Therefore, they showed no improvement or only small positive effects if used in the form of multicomponent catalysts. Finally, the substances which we added to the metal catalysts in this early stage of our work were mostly of the same type as those which had proved to favor the nitride formation, e.g., the flux promoting chlorides, sulfates, and fluorides of the alkali and alkaline earth metals. Again, we know today that just these compounds do not promote, but rather impair the activity of ammonia catalysts. 

Molybdenum In its pure form, without additions, it is the most efficient catalyst of all the easily obtainable and reducible substances, and it is less easily poisoned than iron. It catalyzes in another way than iron, insofar as it forms analytically easily detectable amounts of metal nitrides (about 9% nitrogen content) during its catalytic action, whereas iron does not form, under synthesis conditions, analytically detectable quantities of a nitride. In this respect, molybdenum resembles tungsten, manganese and uranium which all form nitrides during their operation, as ammonia catalysts. Molybdenum is clearly promoted by nickel, cobalt and iron, but not by oxides such as alumina. Alkali metals can act favorably on molybdenum, but oxides of the alkali metals are harmful. Efficiency, as pure molybdenum, 1.5%, promoted up to 4% ammonia. 

It should be noted that the results for the formic acid decomposition donor reaction have no bearing for ammonia synthesis. On the contrary, if that synthesis is indeed governed by nitrogen chemisorption forming a nitride anion, it should behave like an acceptor reaction. Consistent with this view, the apparent activation energy is increased from 10 kcal/mole for the simply promoted catalyst (iron on alumina) to 13-15 kcal/mole by addition of K20. Despite the fact that it retards the reaction, potassium is added to stabilize industrial synthesis catalysts. It has been shown that potassium addition stabilizes the disorder equilibrium of alumina and thus retards its self-diffusion. This, in turn, increases the resistance of the iron/alumina catalyst system to sintering and loss of active surface during use. 

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. 

The group of metals forming low-stability or unstable nitrides includes Mn, Fe, Co, Ni, Tc and Re. As in the case of iron a clear structural sensitivity was found for rhenium but the role of promoters remains the subject of discussion. There are also indications of structure sensitivity for cobalt and nickel. It was attempted to improve the activity of the classical magnetite catalyst by alloying with nickel or cobalt. The only commercial catalyst is a cobalt containing magnetite [392],... 

There are good NH3 decomposition catalysts such as ICI-47-1 (10 weight % nickel on alirmina) Haldor Topsoe DNK-2R (triply promoted iron-cobalt) SUD-Chemie 27-2 (nickel oxide on alirmina) various supported nitrided catalysts (e.g. molybdenum nitride and nickel molybdenum nitride on - a alumina) and ruthenium modified nickel oxide on alumina. 

The corrosion properties of nitrided (570 °C, 4 h) and untreated iron were examined in a borate solution of pH 8.4 with and without chlorides or ammonia [72]. Enhanced pitting resistance and enhanced anodic currents were observed for nitrided Fe and also for untreated Fe in the presence of ammonia. The observed increase of the magnetite in the passive layer is due to the ammonia-promoted conversion of ... 

Also, recent modehng has shown that the presence of high concentrations of nitrogen species in the combustion products of propellants can reduce gun barrel erosion by promoting the formation of iron nitride rather than iron carbide on the interior surface of the barrel [11]. 

German chemist in catalysis. He obtained a doctorate in chemical philosophy at Leipzig University nnder the supervision of Bodenstein after twists and turns. In 1904, he joined BASF and studied on ammonia synthesis as an assistant of Bosch. He noticed in study of iron nitrides that trace components changed catalytic performance dramatically and conducted studies of multi-component catalysts. He is a genius who found the doubly promoted iron catalysts only in more year. Haber and Bosch were awarded Nobel Prize for their contribntion to the technology of ammonia synthesis. It should be noted that the contribntion of Mittasch was no less than those of them. 

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... 

These metals can form nitrides, but it is unlikely that bulk nitrides are present under ammonia synthesis conditions. Alkali promoters are generally required although their role is uncertain, perhaps influencing both nitrogen dissociation and ammonia inhibition. This group also includes the alloys of iron. 

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