Ammonia synthesis INDEX

Structure Sensitivity over Pe. Table II presents the rates of ammonia synthesis over each of the low Miller index planes of Pe. 

Dumesic et al. (31 la-c) deduced that large particles are more active than small ones because their surface has a higher fraction of 7-coordi-nated Fe atoms (C7) than does the surface of small particles. This explanation is consistent with later results of Spencer et al. (312), who measured rates of ammonia synthesis at 525°C and 20 atm on low-index planes of iron. They find that the relative rates are 418 25 1 for the (111), (100), and (110) faces of iron. For the body-centered cubic lattice of iron, only the (111) surface exposes a large concentration of C7 atoms. On this face the TOF is about 18 s l (312). 

It is hard to find out the most appropriate equipment/method and performance index to measure and assess the quality of catalysts, which is difficult in unified definition and standardization of methods. So far, some methods for evaluating the activities of a few catalysts such as those for catalytic ammonia synthesis and catalytic cracking have been standardized. The evaluation methods are different due to the varieties of catalysts and the experiences of researchers. Sometimes, the technical details of evaluation methods are a part of patent, such as formula and preparation process is confidential. Even then, some basic conceptions and methods for the evaluation and testing of catalysts are still worthy being introduced, which is the main focus of this chapter. 

Pig. 7.31 Phase analysis of a technical catalyst precursor for ammonia synthesis K = KHC03, C = CaFe306, M = magnetite (Magnetite reflections are indexed in the upper diagram)... 

Nitrogen adsorption has been especially studied on iron, which is the basis of the industrial catalyst for ammonia synthesis. This catalytic reaction is now relatively well understood [21]. The main function of iron is to activate the very stable N2 molecule by dissociative adsorption. Results in Table 3 show that the Fe-N binding energy is approximately the same on the three main low-index planes. However, the rate of N2 dissociation decreases by many orders of magnitude when going from Fe(lll), the most active for ammonia synthesis, to the less active Fe(llO). 

A further method for the synthesis of the title compounds with only hydrogen as byproduct is the base-catalyzed dehydrogenative coupling (index D) of ammonia and tris(hydridosilylethyl)boranes, B[C2H4Si(R)H2]3 (R = H, CH3). Initially, the strong base, e.g. n-butyl lithium, deprotonates ammonia. The highly nucleophilic amide replaces a silicon-bonded hydride to form a silylamine and lithium hydride, which then deprotonates ammonia, resuming the catalytic cycle. Under the conditions used, silylamines are not stable and by elimination of ammonia, polysilazane frameworks form. In addition, compounds B[C2l-L Si(R)H2]3 can be obtained from vinylsilanes, H2C=CHSi(R)H2 (R - H, CH3), and borane dimethylsulfide. 

Contents S.A. Topham The History of the Catalytic Synthesis of Ammonia. - J. V, Sanders The Electron Microscopy of Catalysts. - B.E.Koel, G.A.Somoijai Surface Structural Chemistry. - Subject Index. -Author Index Volumes 1-6. 

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