Ammonia synthesis reduction mechanism

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... 

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. 

The fact that fused iron catalysts of the synthetic ammonia type were successively used in many investigations of hydrocarbon synthesis for both fluidized and fixed catalyst bed operations is of interest in different respects. Due to this fact it is possible to make use of the valuable experience obtained during development work of the ammonia synthesis (73). This applies to the reduction, the tendency to oxidize, and the effect of promoters and poisons, and to a certain extent also to questions regarding the reaction mechanism. 

In analogy to the Schrock cycle, Nishibayashi et aL postulated a Chatt-like reaction mechanism, where the bimetallic complex breaks into two monometallic fragments in solution, followed by catalytic reduction at a single metal center. However, the proposed intermediates could not be observed, and the reaction mechanism remained unclear. In order to address this problem, DFT calculations on the mechanism of the ammonia synthesis catalyzed by the Nishibayashi system were performed. Importantly, Batista and coworkers found that the bimetallic complex is the effective catalyst instead of the monometallic species that was originally postulated to play this role. Moreover, the dinitrogen-bridged dinuclear structure remains intact throughout... 

The azide synthesis is a better method for preparing primary amines than alkylation of ammonia because it avoids the formation of secondary and tertiary amines. This method involves treating an alkyl halide with sodium azide followed by reduction (Mechanism 23.1). 

This book comprises 10 chapters which can be classified into four parts. The first part deals with the catalyst itself, including the development (Chapter 1), chemical components and physical structure (Chapter 3) preparation and reduction (Chapters 4 5) and the performance evaluation of the catalysts (Chapter 7). Those of ruthenium catalysts are solely put in Chapter 6. The second part is about the reaction mechanism and kinetics of ammonia synthesis (Chapter 2). The third part is a combination of the above two, namely, is centered on the relationship between the performance of catalysts and reaction, which includes reaction condition, reactor, process and application condition and its impact on the economic benefit of... 

The ammonia synthesis catalyst and the ammonia synthesis process are well suited for the application of the radial flow principle. The process lay-out is such that pressure drop is critical, the effectiveness factor on a large catalyst particle is significantly below unity, the catalyst is mechanically strong and does not shrink or settle significantly during reduction and/ or operation, and the catalyst can be made available with a very small particle size. 

The experiment was performed inside a UHV chamber with a dynamic atmosphere of 8 X 10" mbar hydrogen. The result is shown in Fig. 2.17, in which the water evolution detected by a QMS is compared to the evolution of the metallic character of the surface, as expressed by the intensity at the Fermi edge monitored by He I UPS (for the shape of the whole spectra, see Section 2.7). The water evolution curve indicates two steps in the reduction process, but only the second step leads to the formation of metallic iron in the region near the surface. This evidence provides further strong support for the suggested two-step nucleation mechanism found for the reduction of magnetite in its modified form, which is now suggested as a model for the activation of the ammonia synthesis catalyst. 

While 20torr of water vapor was needed to restructure clean iron single crystals, only 0.4 torr of water vapor is needed to restructure an Al O /Fe surface. It therefore seems that Al O provides an alternate and apparently more facile mechanism for the migration of iron. Upon reduction, metallic iron is left in a highly active orientation [such as Fe(lll) and Fe(211)] for the ammonia synthesis reaction, and the Al O stabilizes the active iron, since if the Al O were not present the iron would move to positions coincident with the bulk periodicity (see Fig. 4.22 for a schematic representation of the restructuring). 

In Fig. 4.25, the rate of ammonia synthesis versus % free iron surface, as determined by carbon monoxide TPD (see experimental section), is shown graphically. The rate of ammonia synthesis decreases roughly in proportion to the amount of iron covered by the aluminum oxide and potassium. The only mechanism for this reduction in rate is site-blocking, which occurs during initial reaction conversions (Pnhj ranges from 0 torr to 3 torr during this measurement). 

The second chapter Structure and Surface Chemistry of Industrial Ammonia Synthesis Catalysts is written by Dr. Per Stoltze. This chapter deals with the structure and surface chemistry of iron-based ammonia synthesis catalysts of the type used by industry. Certain studies of single crystal surfaces are included to the extent that they serve to add information to the main topic. This chapter includes a presentation of the unreduced catalyst the reduction process and the bulk and surface structure of a reduced catalyst. A thorough discussion is given of the different states of sorption of nitrogen and of chemisorption of hydrogen, carbon oxides, ammonia and oxygen. The last part of the chapter gives a detailed account of the mechanism of ammonia synthesis on iron. 

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. 

A common illicit synthesis of methamphetamine involves an interesting variation of the Birch reduction. A solution of ephedrine in alcohol is added to liquid ammonia, followed by several pieces of lithium metal. The Birch reduction usually reduces the aromatic ring (Section 17-14C), but in this case it eliminates the hydroxyl group of ephedrine to give methamphetamine. Propose a mechanism, similar to that for the Birch reduction, to explain this unusual course of the reaction. 

Fig. 5. Application of the algorithm on the ammonia mechanism, after the elimination of a, with only five mechanisms (m7, mg, m, m,2, and m,j) remaining active. This reduction in the number of mechanisms as the first few intermediates are eliminated is quite common however, the number of mechanisms tends to increase again at the end. The intermediate HI (flj) will be eliminated next. [Reprinted with permission from Mavrovouniotis, M. L., and Stephanopoulos, G. Synthesis of reaction mechanisms consisting of reversible and irreversible steps I. A synthesis approach in the context of simple examples . Ind. Eng. Chem. Res. 31, 1625-1637, (1992). Copyright 1992 American Chemical Society.]...

---