Reaction mechanism Ammonia synthesis

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... 

The reactant is adsorbed on the catalyst s surface. As a reactant molecule attaches to the surface of the catalyst, its bonds are weakened and the reaction can proceed more quickly because the bonds are more easily broken (Fig. 13.36). One important step in the reaction mechanism of the Haber process for the synthesis of ammonia is the adsorption of N2 molecules on the iron catalyst and the weakening of the strong N=N triple bond. 

The reaction with ammonia or amines, which undoubtedly proceeds by the SnAt mechanism, is catalyzed by copper and nickel salts, though these are normally used only with rather unreactive halides. This reaction, with phase-transfer catalysis, has been used to synthesize triarylamines. Copper ion catalysts (especially cuprous oxide or iodide) also permit the Gabriel synthesis (10-61) to be... 

MECHANISM FIGURE 18-11 Nitrogen-acquiring reactions in the synthesis of urea. The urea nitrogens are acquired in two reactions, each requiring ATP. (a) in the reaction catalyzed by carbamoyl phosphate synthetase 1, the first nitrogen enters from ammonia. The terminal phosphate groups of two molecules of ATP are used to form one molecule of carbamoyl phosphate. In other words, this reaction has two activa-... 

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. 

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. 

Careful inspection of the reported photocatalytic reactions may demonstrate that reaction products can not be classified, in many cases, into the two above categories, oxidation and reduction of starting materials. For example, photoirradiation onto an aqueous suspension of platinum-loaded Ti02 converts primary alkylamines into secondary amines and ammonia, both of which are not redox products.34) ln.a similar manner, cyclic secondary amines, e.g., piperidine, are produced from a,co-diamines.34) Along this line, trials of synthesis of cyclic imino acids such as proline or pipecolinic acid (PCA) from a-amino acids, ornithine or lysine (Lys), have beer. successfuL35) Since optically pure L-isomer of a-amino acids are available in low cost, their conversion into optically active products is one of the most important and practical chemical routes for the synthesis of chiral compounds. It should be noted that l- and racemic PCA s are obtained from L-Lys by Ti02 and CdS photocatalyst, respectively. This will be discussed later in relation to the reaction mechanism. 

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... 

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... 

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 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. 

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. 

Nielsen et al. investigated ammonia synthesis on a commercial Topsoe catalyst, KM IR, over a wide temperature range. They found evidence that a different reaction mechanism predominates below and above 330 °C [170]. Also, at low temperatures, chemisorbed hydrogen blocks the catalyst surface [171]. The latter finding is in agreement with the observations of Ertl s group [123],... 

Reaction Mechanism on Non-Iron Catalysts. Non-iron systems which exhibit some potential to catalyze ammonia synthesis can be divided into the following groups [172] ... 

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