Ammonia Synthesis from Nitrogen and Hydrogen

Unlike the S5mthesis of ozone, that of ammonia is an exothermic reaction (I/2H2 + 3/2H2NH3 + 46 kJ). However, since activation is needed, this reaction also requires an energy input which applies both to a thermal reaction and to a reaction in electric discharge. Much research has been devoted to the latter [116, 301]. A certain reaction limit depending on the discharge type and the reaction conditions has been observed. 

The yields of ammonia by synthesis in electric discharge are still very far from the industrial yield obtained by the common synthesis (a catalytic reaction under high pressure) and representing about one gram per kJ, which is by two orders higher than the best yields obtained with electric discharge. 

It follows from data on the ammonia synthesis, realized by bombardment of a mixture of nitrogen and hydrogen with electrons of a given energy, that the minimum electron energy required for the production of ammonia is 17 eV. Since this figure is close to the 

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Graphite compounds have been described as catalysts for ammonia synthesis from nitrogen and hydrogen (14, Pll), for Fischer-Tropsch chemistry M13, R14), for paraffin isomerization iR15), and for Friedel-Crafts chemistry (07). 

So now we have established that chemical reactions are dynamic and reversible, and, as a result, chemical reactions are an equilibrium mixture of reactants and products. We have also seen that sometimes the reaction strongly favors the products (as in reacting gunpowder) and sometimes the reaction strongly favors the reactants (as in Haber s ammonia synthesis from nitrogen and hydrogen). We haven t answered one question. 

The industrial-scale availability of nitrogen and hydrogen at the turn of the 19th to the 20th century enabled a host of new applications. The BASF company, for example, succeeded in developing an ammonia synthesis from nitrogen and hydrogen in 1913. This paved the way for mass production of fertilisers. 

Technical ammonia synthesis from nitrogen and hydrogen by... 

Chemists and engineers use these and other factors to manipulate the rate of a particular reaction to suit their needs. For example, consider the synthesis of ammonia, NH3, from nitrogen and hydrogen. 

An even more effective homogeneous hydrogenation catalyst is the complex [RhClfPPhsfs] which permits rapid reduction of alkenes, alkynes and other unsaturated compounds in benzene solution at 25°C and 1 atm pressure (p. 1134). The Haber process, which uses iron metal catalysts for the direct synthesis of ammonia from nitrogen and hydrogen at high temperatures and pressures, is a further example (p. 421). 

It is well established that commercially important supported noble metal catalysts contain small metal crystallites that are typically smaller than a few nanometers. The surface of these crystallites is populated by different types of metal atoms depending on their locations on the surface, such as comers, edges, or terraces. In structure sensitive reactions, different types of surface metal atoms possess quite different properties. For example, in the synthesis of ammonia from nitrogen and hydrogen, different surface crystallographic planes of Fe metal exhibit very different activities. Thus, one of the most challenging aspects in metal catalysis is to prepare samples containing metal particles of uniform shape and size. If the active phase is multicomponent, then it is also desirable to prepare particles of uniform composition. 

For example, classic thermodynamic methods predict that the maximum equUi-brium yield of ammonia from nitrogen and hydrogen is obtained at low temperatures. Yet, under these optimum thermodynamic conditions, the rate of reaction is so slow that the process is not practical for industrial use. Thus, a smaller equilibrium yield at high temperature must be accepted to obtain a suitable reaction rate. However, although the thermodynamic calculations provide no assurance that an equUibrium yield will be obtained in a finite time, it was as a result of such calculations for the synthesis of ammonia that an intensive search was made for a catalyst that would allow equilibrium to be reached. 

The direct gas-phase synthesis of ammonia from nitrogen and hydrogen (the Haber process, 1908) is presently the cornerstone of the fertilizer industry ... 

FIGURE 8.10 A representation of the enthalpy changes for steps in the synthesis of ammonia from nitrogen and hydrogen. If AH° values for step 2 and for the overall reaction are known, then A H° for step 1 can be calculated. That is, the enthalpy change for the overall reaction equals the sum of the enthalpy changes for the individual steps 1 and 2, a statement known as Hess s law. 

The KAAP (Kellogg Advanced Ammonia process) process is the first high-pressure ammonia synthesis process that makes ammonia from nitrogen and hydrogen without the aid of an iron-containing catalyst.1 It is described in References 22-25. 

For constant volume processes, such as catalytic synthesis of ammonia from nitrogen and hydrogen in pressurised (about 300 atmospheres) reactor, the change in Work Function would be a measure of the feasibility of reaction,... 

The synthesis of ammonia from nitrogen and hydrogen releases energy (is exothermic). We can represent this situation by treating energy as a product ... 

This principle is of great practical importance. For example, the synthesis of ammonia from nitrogen and hydrogen is exothermic (the heat evolved is 11.0 kcal per mole of ammonia formed) hence the yield of ammonia is made a maximum by keeping the temperature as low as possible. The commercial process of manufacturing ammonia from the elements became practicable when catalysts were found which caused the reaction to proceed sufficiently rapidly at low temperatures. 

Investigations on the synthesis of ammonia from nitrogen and hydrogen have been surveyed in previous reviews (102-105). In what follows only especially salient points in conjunction with measurements of the rate of individual steps are discussed. 

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