Ammonia Synthesis—Complicated Kinetics

Although (305) is sufficient for the description of ammonia synthesis kinetics under the conditions of commercial production, some essential 

An experimental study of kinetics of ammonia synthesis on iron (101), cobalt, and nickel (96) catalysts, at ammonia concentrations much lower than that at equilibrium, showed that at pressures of the order of 1 atm the second of these possibilities is realized.7 When far from equilibrium, the 

7 The first possibility can become realistic at high pressures since at high partial pressures of hydrogen the rate of hydrogenation is large. 

From this interpretation of (325), it follows that m = m where m = 1 — n and m is the exponent in (305) found for the same catalyst from the reaction rate in the region where (305) applies. 

This conclusion was verified by studies of kinetics on each of the above-mentioned metals of the iron group at very low and near to equilibrium concentrations of ammonia. It was completely confirmed that for an iron catalyst m = 0.5 and m - 0.5 (101), for cobalt m = 0.2 and rri = 0.2 (96), for nickel m = 0.3 and m - 0.3 (96). 

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The state of iron ammonia catalysts is dealt with in the following chapters, and x-ray, magnetic, and electric data will be discussed together with adsorption measurements. Information about the catalysts combined with kinetic experiments has led to a fairly good qualitative understanding of ammonia synthesis on iron catalysts, but owing to the extremely complicated nature of the catalyst surface during reaction, a quantitative treatment based on data of catalyst and reactants will not be attained in the near future. 

Ammonia synthesis is a relatively simple reaction without the complication of any secondary reaction product, and is especially suitable for a theoretical approach to its kinetics. In fact, the most used kinetic equation for ammonia synthesis was developed by Temkin on the basis of theoretical assumptions about 50 years ago and is still used successfully by chemists and engineers. 

In solving (134) Di is usually considered constant, although Dj depends on the gas composition. This approximation seems reasonable considering the uncertainty in t. For simple reaction kinetics, the solution to (134) is given in terms of the Thiele modulus [102]. The more complicated kinetics for ammonia synthesis does not give a simple analytical solution. Generally, a numerical integration has to be carried out. 

Sargeson and his coworkers have developed an area of cobalt(III) coordination chemistry which has enabled the synthesis of complicated multidentate ligands directly around the metal. The basis for all of this chemistry is the high stability of cobalt(III) ammine complexes towards dissociation. Consequently, a coordinated ammonia molecule can be deprotonated with base to produce a coordinated amine anion (or amide anion) which functions as a powerful nucleophile. Such a species can attack carbonyl groups, either in intramolecular or intermolecular processes. Similar reactions can be performed by coordinated primary or secondary amines after deprotonation. The resulting imines coordinated to cobalt(III) show unusually high stability towards hydrolysis, but are reactive towards carbon nucleophiles. While the cobalt(III) ion produces some iminium character, it occupies the normal site of protonation and is attached to the nitrogen atom by a kinetically inert bond, and thus resists hydrolysis. 

A reaction in which reactants convert to products and products convert to reactants in the same reaction vessel naturally leads to an equilibrium, regardless of how complicated the reaction is and regardless of the nature of the kinetic processes for the forward and reverse reactions. Consider the synthesis of ammonia from nitrogen and hydrogen ... 

As examined in Section 4.5.4, the effectiveness factor is only constant and independent of the degree of conversion if we have an irreversible first-order reaction (Tab. 4.5.5, Example 4.5.6). This is not the case for NH3 synthesis, which is with regard to the kinetics much more complicated as we have a reversible nth order reaction according to the Eqs. (6.1.9) and (6.1.10). Hence, the effectiveness factor depends on the reactant (H2 and N2) and product (NH3) concentrations and thus on the axial position in a fixed bed reactor. This leads to a decrease of the intrinsic rate along the bed (at almost constant effective diffusion coefficients), and in return to an increase of the effectiveness factor from the reactor inlet to the outlet as shown in Figure 6.1.7 for an isothermal and therefore hypothetical ammonia reactor apart from the assumption of isothermality, the parameters used for the calculations correspond to an industrial reactor. 

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