Catalysts for ammonia synthesis

N2 and CO, respectively [31,32], Empirical knowledge about the promoting effect of many elements has been available since the development of the iron ammonia synthesis catalyst, for which some 8000 different catalyst formulations were tested. Recent research in surface science and theoretical chemistry has led to a fairly complete understanding of how a promoter works [33,34],... 

Since theoretical calculation of effectiveness is uncertain and is moreover sensitive to operating conditions, for industrially important cases it is determined by such reaction tests. Common types of curve fits may be used. For ammonia synthesis catalyst, for instance, an equation is provided by Dyson Simon (IEC Fundam 7 605, 1968) in terms of temperature and... 

Apparent Activation Energy of Ammonia Synthesis Catalysts for Formic Acid... 

Final Purification. Oxygen containing compounds (CO, CO2, H2O) poison the ammonia synthesis catalyst and must be effectively removed or converted to inert species before entering the synthesis loop. Additionally, the presence of carbon dioxide in the synthesis gas can lead to the formation of ammonium carbamate, which can cause fouHng and stress-corrosion cracking in the compressor. Most plants use methanation to convert carbon oxides to methane. Cryogenic processes that are suitable for purification of synthesis gas have also been developed. 

Promoters. Many industrial catalysts contain promoters, commonly chemical promoters. A chemical promoter is used in a small amount and influences the surface chemistry. Alkali metals are often used as chemical promoters, for example, in ammonia synthesis catalysts, ethylene oxide catalysts, and Fischer-Tropsch catalysts (55). They may be used in as Httie as parts per million quantities. The mechanisms of their action are usually not well understood. In contrast, seldom-used textural promoters, also called stmctural promoters, are used in massive amounts and affect the physical properties of the catalyst. These are used in ammonia synthesis catalysts. 

Ammonia production from natural gas includes the following processes desulfurization of the feedstock primary and secondary reforming carbon monoxide shift conversion and removal of carbon dioxide, which can be used for urea manufacture methanation and ammonia synthesis. Catalysts used in the process may include cobalt, molybdenum, nickel, iron oxide/chromium oxide, copper oxide/zinc oxide, and iron. 

In comparison to most other methods in surface science, STM offers two important advantages STM gives local information on the atomic scale and it can do so in situ [51]. As STM works best on flat surfaces, applications of the technique in catalysis concern models for catalysts, with the emphasis on metal single crystals. A review by Besenbacher gives an excellent overview of the possibilities [52], Nevertheless, a few investigations on real catalysts have been reported also, for example on the iron ammonia synthesis catalyst, on which... 

Bridger, G.W. Snowden, C.B. (1970) Ammonia synthesis catalysts. In Catalyst Handbook. Wolfe Scientific Books, 126-147 Brindley, G.W. Bish, D.L. (1976) Green rust a pyroaurite type structure. Nature 263 353 Bromfield, S.M. Williams, E.G. (1963) An examination of the biological reduction method for estimating active iron in soils. J. Soil Sd. 14 346-359... 

Equation (341) solves the task of quantitatively describing the effect of water vapor, and also oxygen gas (the last being rapidly converted to water vapor at the conditions of the reaction), on the activity of commercial ammonia synthesis catalysts. This result is of practical importance for ascertaining the necessary degree of purity of the inlet gas mixture with respect to poisons containing oxygen (122). 

Fastrup, B. (1994) Temperature programmed adsorption and desorption of nitrogen on iron ammonia synthesis catalysts, and consequences for the microkinetic analysis of NH3 synthesis. Top. Catal., 1, 273. 

Potentially of equal importance is the relationship between strain and catalyst stability. A calculation of the contribution to the total free energy of a catalyst crystal caused by the presence of strain-inducing microscopic precipitates50 showed that the extra free energy increases with the size of the crystal and inhibits it from sintering. This theory is an interesting one since it provides a mechanism which the catalyst scientist can exploit in his search for stable, high surface-area materials. The theory predicts the equilibrium crystallite size of the iron crystals of an ammonia synthesis catalyst with acceptable accuracy. 

If sulfur is present as H2S or COS, it is a poison for many catalysts and will partly or completely inhibit the catalyst activity46. Carbon monoxide (CO) and carbon dioxide (CO2) can poison the ammonia synthesis catalyst so both of these compounds must be removed53. 

A frequent reason for the dependence of catalyst structure on the chemical potential in the gas phase containing all the reactants is the incorporation of molecules or atoms from the reaction mixture into the catalyst phases. Formation of subphases, often only in the near-surface region of the solid, fails to create phases with individual reflections but modifies the reflections of the starting precatalyst phase notably (see previous sections). This complication presents a massive problem in the analysis of working catalysts when significant partial pressures of products are important to the phase formation and when the necessary conversions cannot be reached in the experimental cell. The investigation of ammonia synthesis catalysts when insufficient partial pressures of the product ammonia prevent the formation of the relevant nitride phases is a prominent example of this limitation (Herzog et al., 1996 Walker et al., 1989). 

The thermodynamic equilibrium is most favourable at high pressure and low temperature. The methanol synthesis process was developed at the same time as NH3 synthesis. In the development of a commercial process for NH3 synthesis it was observed that, depending on the catalyst and reaction conditions, oxygenated products were formed as well. Compared with ammonia synthesis, catalyst development for methanol synthesis was more difficult because selectivity is crucial besides activity. In the CO hydrogenation other products can be formed, such as higher alcohols and hydrocarbons that are thermodynamically favoured. Figure 2.19 illustrates this. 

After C02 removal, final purification includes methanation (8) gas diying (9) and ciyogenic purification (10). The resulting pure synthesis gas is compressed in a single-case compressor and mixed with a recycle stream (11). The gas mixture is passed to the ammonia converter (12), which is based on the Kellogg Brown Root Advanced Ammonia Process (KAAP). It uses a precious metal-based, high-activity ammonia synthesis catalyst to allow for high conversion at the relatively low pressure of 90 bar. 

---