Ammonia molecular precursor

Studies of coadsorption at Cu(110) and Zn(0001) where a coadsorbate, ammonia, acted as a probe of a reactive oxygen transient let to the development of the model where the kinetically hot Os transient [in the case of Cu(110)] and the molecular transient [in the case of Zn(0001)] participated in oxidation catalysis16 (see Chapters 2 and 5). At Zn(0001) dissociation of oxygen is slow and the molecular precursor forms an ammonia-dioxygen complex, the concentration of which increases with decreasing temperature and at a reaction rate which is inversely dependent on temperature. Which transient, atomic or molecular, is significant in chemical reactivity is metal dependent. 

The conversion of the molecular precursors into boron nitride requires a ceramisation under an ammonia flow. Ammonia is used as a reductive atmosphere, and its ability to replace halo or amino groups on the borazine framework is well known [12,13]. Ammonia is also a curing reagent in the borazine polymerisation [25]. So, for the preparation of BN from molecular precursors, one usually needs to perform a low temperature ceramisation, up to 600°C at least, under an ammonia flow. The conversion of each precursor into BN has been studied using TGA up to 1000°C in order to optimise the ceramisation conditions and the properties of the obtained ceramic. The measurements were realised in a pure ammonia flow up to 650°C and then under nitrogen. The three tested compounds presented original behaviours related to their formulation and their reactivity towards ammonia. 

In previous papers by the authors [9,10], the temperature-programmed desorption of N2 from an iron-based catalyst has been studied experimentally. The microkinetic analysis of these results is based on the kinetic simulation of ammonia synthesis by Stoltze and Nprskov [22-24] using the approach by Dumesic and Trevino [2]. On Fe single crystal surfaces it was possible to detect a di-molecular precursor labelled a-N2 — forN2 dissociation... 

The preexponential term for the dissociation of nitrogen held in the molecular precursor state (A5 2) is below the theoretical value of 102 This is in accord with the measured, extremely low sticking probability of atomic nitrogen/28.35) initial sticking probability of ammonia on an Fe(lll) surface has been measured by Ertl and co-workers " to be 0.1, and so the A-factor for the adsorption of ammonia (A s ) has been given a value of 10 . 

Figure 1.10 shows surface micrographs of CoMo intermetallic alloys after carburization in an attempt to synthesize a bimetallic (ternary) carbide. Such phases are of increasing interest as catalysts for a variety of industrial processes, including hydro-treating to remove S, N, and O impurities from fossil fuels, ammonia synthesis, water gas shift reaction to produce hydrogen, and fuel cell catalysts [44 7]. They are typically formed by gas reactions of complex oxide precursors or by molecular precursor or chemical synthesis [48-50] routes. The use of intermetallic precursors offers the potential to leverage a different set of precursor stoichiometries and structures to form new complex... 

Caprolactam [105-60-2] (2-oxohexamethyleiiiiriiQe, liexaliydro-2J -a2epin-2-one) is one of the most widely used chemical intermediates. However, almost all of the aimual production of 3.0 x 10 t is consumed as the monomer for nylon-6 fibers and plastics (see Fibers survey Polyamides, plastics). Cyclohexanone, which is the most common organic precursor of caprolactam, is made from benzene by either phenol hydrogenation or cyclohexane oxidation (see Cyclohexanoland cyclohexanone). Reaction with ammonia-derived hydroxjlamine forms cyclohexanone oxime, which undergoes molecular rearrangement to the seven-membered ring S-caprolactam. 

Relevant to the synthesis of ammonia over iron catalysts is the observation of Ertl et al. (54) that potassium preadsorbed by an iron catalyst ((7 = 0.1) increased the rate of synthesis at 430 K by a factor of about 300. This effect the authors attributed to an enhancement of the heat of adsorption of molecular nitrogen due to transfer of electronic charge from potassium to the surface of the iron catalyst. This would be entirely in keeping with the precursor model proposed for nitrogen chemisorption (55). 

Constitution XV for sucrose has up to the present satisfied all demands made upon it. Like its precursors, I and II (page 6), it was not incompatible with physical properties of sucrose such as the magnetic rotation, or the parachor, although the latter claim has been denied. Von Lippmann lists a great many early determinations of the physical properties of the sugar more recent measurements include the heat of combustion, the molecular weight in liquid ammonia, and various optical and electrical constants. ... 

Submicrometer-size droplets of methyldichlorosilane were produced at cryo-scopic temperatures and reacted with ammonia to give high-molecular-weight solid silazane derivatives in a rapid reaction. On calcination these precursors ended as spherical mixed silicon nitrides of a- and p-Si3N4 (70). 

A large number of a, 3-didehydro-a-amino acids have been identified as constituents of relatively low molecular weight cyclic compounds from microbial sources. However, the presence of a,p-didehydroalanine in bacterial as well as in mammalian histidine ammonia lyase and in phenylalanine ammonia lyase shows that the occurrence of a,p-didehydro-a-amino acids is not limited to small molecules alone 8 These residues are incorporated in natural sequences by posttranslation modification. a,p-Didehydro-a-amino acids have also been postulated to be precursors in the biosynthesis of several heterocyclic metabolites including penicillin and cephalosporin 9 Other well-known compounds containing ,( -di-dehydro-a-amino acids are nisin 10,11 (a food preservative112 ), subtilin (a broad spectrum antibiotic) 13 and some of the metabolites isolated from Streptomyces strains such as gri-seoviridin 14 ... 

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