Ammonia synthesis mechanism

Shift Conversion. Carbon oxides deactivate the ammonia synthesis catalyst and must be removed prior to the synthesis loop. The exothermic water-gas shift reaction (eq. 23) provides a convenient mechanism to maximize hydrogen production while converting CO to the more easily removable CO2. A two-stage adiabatic reactor sequence is normally employed to maximize this conversion. The bulk of the CO is shifted to CO2 in a high... 

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. 

Studies of the Kinetics and Mechanisms of Ammonia Synthesis and Hydrodesulfurization on Metal Single-Crystal Surfaces... 

Fig. 4.2 Mechanism and energy diagram for ammonia synthesis on iron. (Energies in l

As an example, the Temkin-Pyzhev rate expression for ammonia synthesis reproduces the experimentally observed kinetics quite well. However, this rate expression was originally derived from a proposed mechanism which had both the wrong key intermediates and the wrong rate-limiting step. 

As an example we will consider the mechanism for ammonia synthesis. We include Ar in the gas phase to illustrate the effect of inerts. This mechanism is rich enough to illustrate most of the features discussed below. 

Table 4.9 Dissociative and associative mechanisms in ammonia synthesis on solid...

The methanation reaction is a highly exothermic process (AH = —49.2 kcal/ mol). The high reaction heat does not cause problems in the purification of hydrogen for ammonia synthesis since only low amounts of residual CO is involved. In methanation of synthesis gas, however, specially designed reactors, cooling systems and highly diluted reactants must be applied. In adiabatic operation less than 3% of CO is allowed in the feed.214 Temperature control is also important to prevent carbon deposition and catalyst sintering. The mechanism of methanation is believed to follow the same pathway as that of Fischer-Tropsch synthesis. 

Many works were devoted to the mechanism and kinetics of ammonia synthesis. In accordance with the outline of this article, only the main results of studies of ammonia synthesis by the author with co-workers will be presented here other works will be cited, but in connection with our studies. The discussions of kinetics and the mechanism of ammonia synthesis in the reviews published in this series (90) and in the contribution by Nielsen et al. (91) can serve as supplementary sources of information. The contribution by Malina (92) contains interesting historical data and an extensive list of references. 

We note that the very possibility of observing the equilibrium (307) supports the notion that the reaction rate is determined by stage 1 of mechanism (295). At temperatures somewhat lower than that of ammonia synthesis, the reaction rate of this stage becomes negligible, but the equilibrium of stage 2 still is established sufficiently rapidly. 

Let us now consider an example of a non-linear mechanism, including a reaction that involves two molecules of some intermediates. The probable reaction mechanism for ammonia synthesis on an iron catalyst can be represented as... 

Thus, here we have two independent routes. For a linear mechanism of ammonia synthesis on an iron catalyst we will have... 

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. 

The conversion of dinitrogen to ammonia is one of the important processes of chemistry. Whereas the technical ammonia synthesis requires high temperature and pressure (1), this reaction proceeds at room temperature and ambient pressure in nature, mediated by the enzyme nitrogenase (2). There is evidence that N2 is bound and reduced at the iron-molybdenum cofactor (FeMoco), a unique Fe/Mo/S cluster present in the MoFe protein of nitrogenase. Although detailed structural information on nitrogenase has been available for some time (3), the mechanism of N2 reduction by this enzyme is still unclear at the molecular level. Nevertheless, it is possible to bind and reduce dinitrogen at simple mono- and binuclear transition-metal systems which allow to obtain mechanistic information on elemental steps involved... 

A. Ozaki, K. Aika Catalytic Activadon of Dinitrogen The third chapter is a comprehensive and critical review of studies on the catalytic activation of dinitrogen, including chemisorption and coordination of dinitrogen, kinetics and mechanism of ammonia synthesis, chemical and instrumental characterization of active catalysts, and homogeneous activation of dinitrogen including metal complexes (353 references). 

The mechanism of the catalytic ammonia synthesis. The measurement of adsorption on solids. Discontinuities in adsorption isotherms. ... 

The concept of mechanical fixation of metal on carbon makes catalytic applications at high temperatures possible. These applications require medium-sized active particles because particles below 2nm in size are not sufficiently stabilised by mechanical fixation and do not survive the high temperature treatment required by the selective etching. Typical reactions which have been studied in detail are ammonia synthesis [195, 201-203] and CO hydrogenation [204-207]. The idea that the inert carbon support could remove all problems associated with the reactivity of products with acid sites on oxides was tested, with the hope that a thermally wellconducting catalyst lacking strong-metal support interactions, as on oxide supports, would result. 

In the majority of cases, the last step in the preparation of catalytically active metals is a reduction. The precursor is very frequently an oxide. An oxychloride is the real precursor of active platinum and some noble metals if chlorometal complexes (e.g. chloroplatinic acid) are used. It may be advantageous to use still other precursors and to reduce them directly without any intermediary transformation to oxide. On the other hand, nearly all catalytic metals are used as supported catalysts. The only notable exception is iron for ammonia synthesis, which is a very special case and then the huge body of industrial experience renders scientific analysis of little relevance. The other important metals are Raney nickel, platinum sponge or platinum black, and similar catalysts, but they are obtained by processes other than reduction. This shows the importance of understanding the mechanisms involved in activation by reduction. 

In the current volume a variety of subjects is treated by competent authors. These subjects deal with new techniques of surface investigations with the microbalance, with the elucidation of reaction mechanisms by the concept of intermediates, and with specialized studies of the ammonia synthesis, hydrogenations, carbon monoxide oxidation and hydrocarbon syntheses. In addition, Volume V contains an extensive critical review of Russian literature in catalysis. 

Poisoning of iron catalysts during ammonia synthesis by sulfur compounds has received relatively little attention (154, 240-244). Nevertheless, the previous work provides information on the poisoning mechanism and interesting examples of how oxide promoters may influence the sulfur poisoning behavior of a catalytic metal. 

Industrial catalysts for ammonia synthesis must satisfy the following requirements (1) high catalyst activity at the lowest possible reaction temperatures, (2) the highest possible insensitivity to oxygen- and chlorine-containing catalyst poisons, (3) long life, and (4) mechanical strength. 

---