Nitrogen, dissociation

An important question for the reaction mechanism for NH3 synthesis is whether any H is added to N2 before the fission of the N-N bond. Mechanisms involving the additions of H before the fission of the N-N bond are referred to as associative while mechanisms involving N are called dissociative. Even if the kinetic expression by Temkin and Pyzhev was derived from considerations for an associative mechanism, the evidence for the associative mechanism is rather meager. 

Attempts have been made to observe the intermediates under conditions where the formation of NH3 could be hoped for. By field ion mass spectroscopy of a Fe tip at room temperature the observation of the ion N2 [510, 511] and the observation of N2 by laser Raman spectroscopy of the catalyst [647] has been interpreted as evidence for the associative mechanism [647]. Interestingly, other studies using field ion mass spectroscopy have shown data incompatible with the associative mechanism [648] or no reaction [649]. Presumably the reaction conditions in these experiments are too far removed from conditions where NH3 may be formed in appreciable amounts. 

More direct experimental evidence, such as N-isotopic exchange over the catalyst, the study of chemisorption of N2, and the synthesis of NH3 over single crystals, leaves no doubt that the mechanism is dissociative. 

---

Figure 7.19. (Left-hand side) Comparison between experimental sticking coefficients of N2 on Fe(l 11) and the prediction on the basis of Eq. (57) with an activation energy of 0.03 eV. (Right-hand side) Potential energy diagram for molecular nitrogen dissociating on Fe(l 11).

The new crystalline solid, metastable in liquid nitrogen, dissociates at ambient... 

It has been proposed that this reaction intermediate could decompose to produce HCN and CH3 [55], Chemiluminescence from alkanes can be greatly enhanced by addition of HC1. The proposed explanation is that energy transfer from active nitrogen dissociates HC1 to produce chlorine atoms, which have rapid hydrogen-atom abstraction reactions with alkanes,... 

Figure 3.36. Nitrogen dissociation on W(100). (a) Experimental measurements of the dissociation probability S as a function of En and Ts. (b) Experimental measurements of only the direct component of dissociation probability S as a function of Et and 6f. (a) and (b) from Ref. [339]. (c) Dissociation probability S from first principles classical dynamics, separated into a dynamic trapping fraction and a direct dissociation fraction, (d) Approximate reaction path for dynamic trapping mediated dissociation from the first principles dynamics. The numbers indicate the temporal sequence, (c) and (d) from Ref. [343].

In earlier work no reaction was observed to occur when metallic samples at floating potential were exposed to nitrogen discharge. It is difficult to elucidate the details of the plasma conditions used in work180 on plasma nitriding reported in the literature but two effects appear to be the probable cause of the contradictory results that have been obtained 1) impurities and 2) a low degree of nitrogen dissociation. 

The two phase model describes all the principle features of the desorption kinetics, suggesting that recombinative desorption under conditions where the coverage is less than saturation occurs by the recombination of N atoms from a dilute phase on the Cu(l 11) surface. This behaviour is the same as that observed for H recombinative desorption on many surfaces [63]. Desorption from the dilute phase is preferred over direct decomposition of the nitride islands because this leaves the copper surface in its equilibrium (111) orientation, rather than as an unstable Cu(l 00) overlayer [99]. As a result we expect that detailed balance can be used to relate measurements of recombination from the N covered Cu(l 1 1) surface with nitrogen dissociation on bare Cu(l 1 1) terraces. In contrast, if desorption occurred via decomposition of reconstructed copper nitride islands then detailed balance arguments would not reveal anything about the energetics or dynamics of N2 dissociation on a Cu(l 1 1) surface. 

Metallurgy. In this application ammonia is cracked to produce a dissociated ammonia that consists of 75 percent hydrogen and 25 percent nitrogen. Dissociated ammonia is used in a number of metal treatment proceses.57... 

Results obtained from the dissociation pressures of Slade and Higson (3) are obviously not reliable. Two other more extensive sets of dissociation pressure data (6, 7) have been reported for the mononitride phase. Brauer and Schnell (6) measured the nitrogen dissociation pressures for VN between 1573 and 1873 K, while Kozheurov et al. (7) determined equilibrium pressures for VN (x = 0.55-0.9) at temperatures in the range 1573-1923 K. 

The acid dissociation constant of the free ligand, when the proton attached to the nitrogen dissociates, is approximately 10 14 B, and the effect of metal chelation is to increase this dissociation constant by a factor of 10B to 108. The values of Ka, and Ka the stepwise dissociation constants, were determined spectrophotometrically and potentiometrically, and the acid-strengthening effect on the NH group was found to depend markedly on the chelated metal ion (95). The order of increasing acidity is Mn(II) < Cd(II) < Zn(II) < Ni(II) < Fe(II). The values of pKa, and pKat for any one metal were found to be within 1.3. It therefore appears that these... 

The nucleic bases are all weak acids with pk values from 10 to 12. NH protons of imidazole N9 or of the lactam nitrogens dissociate by 50% at pH 10-12. Mercury or silver salts react with these nitrogen atoms to form salts (Scheme 8.5.3). Nucleosides are formed if the mercury or silver salts are combined with halosides of deoxyribose. 

The measurements of the nitrogen dissociation pressure of Th3N4(cr) have been discussed in Section X. 1.1.2, where they are used to calculate the enthalpy of formation of ThN(cr). The selected value for the Gibbs energy of formation is ... 

Illustrative examples of intramolecular oxidations of remote groups in nitroaromatic ions are the redox reactions occurring in ionized benzotriazoles and triazolopyridines bearing o-nitroaryl substituents on nitrogen. Dissociative ionization apparently causes... 

A completely different strategy for the surface functionalisation of CNTs with nitrogen-containing groups is the treatment of CNTs under atomic nitrogen flow obtained by molecular nitrogen dissociation in an Ar -t N2 microwave plasma. X-ray photoelectron spectroscopy of the nanotube surface demonstrated the presence of amides, oximes and mainly amine and nitrile groups. ... 

The three types of solids, metals, covalent semiconductors or insulators, and ionic compounds (including oxides) have characteristic surface reactions. In organic catalysis only metals and ionics are considered (Table 6.5), while in CVD all three types of solid surfaces are of interest. Metals absorb hydrogen and nitrogen dissociatively while ionic substrates have redox reactions or acid/base reactions with molecules. Oxidation of gases is often catalyzed by the surface of metal oxides. So is deposition of oxides by oxidation and hydrolysis of metal-containing precursors. When mixed oxides (e.g., perovskites) are deposited care must be taken to ensure a sufficient availability of the separate components. 

Candler, G., Olejniczak, J. Harrold, B. (1997). Detailed simulation of nitrogen dissociation in stagnation regions, Phys. Fluids 9(7) 2108-2117. 

---