Molecular nitrogen dissociative chemisorption

The similarity of the reactivity patterns for niobium and cobalt and the non-reacti vi ty of iron with nitrogen suggests that dissociative chemisorption is taking place. Dissociation of molecularly chemisorbed nitrogen is an activated process on all metals(35) and is most exothermic for the early metals in the periodic tab e(36). The limited observations on clusters seems to be consistent with these trends. 

If the degree of dissociation is high enough the metal surface is readily saturated due to direct adsorption of atoms which can also occupy single isolated sites (the dissociative chemisorption of molecular nitrogen requires two adjacent empty surface sites). 

Haber process, has been shown to involve the dissociative chemisorption of molecular nitrogen to give surface adsorbed nitrogen atoms (4). These subsequently form N—H bonds followed by desorption of the ammonia. 

Much has been written about the high reactivity of steps at metal surfaces and there are several examples in chemisorption. For example, the 110 plane of W has a low sticking probability ( 3 x 10 3) for nitrogen chemisorption at 300 K, but on the stepped 320 plane, with 110 terraces, s0 = 0.70. On the other hand, a singular, step-free W 100 surface is also very reactive (s0 = 0.59) and it has been suggested that the reactivity of stepped planes such as 320 is related to the structural similarity of the steps to the 100 plane itself [47]. A further dramatic illustration of the dependence of s0 on the presence of steps is demonstrated by the work of Salmeron et al. [353] on the stepped Pt surface. Using a molecular beam source, they demonstrated that the reactivity for dissociative chemisorption of H2 (as indicated by the formation of HD from a mixture of H2 and D2) was 7 times higher when the beam was directed at the steps than when it was directed over the steps, as shown in Fig. 20. 

The promoter potassium facilitates the dissociation of N2 and thus increases the rate of formation of NH3. In investigations of the chemisorption of N2 on the less active (100) and (110) iron surfaces, it was shown that low concentrations of potassium increase the heat of chemisorption of molecular nitrogen by 16 kJ/mol and increase the rate of N2 dissociation 300-fold. 

The mechanism of the synthesis reaction remains unclear. Both a molecular mechanism and an atomic mechanism have been proposed. Strong support has been gathered for the atomic mechanism through measurements of adsorbed nitrogen atom concentrations on the surface of model working catalysts where dissociative N2 chemisorption is the rate-determining step (17). The likely mechanism, where (ad) indicates surface-adsorbed species, is as follows ... 

Clearly the molecular events with iron were complex even at 80 K and low NO pressure, and in order to unravel details we chose to study NO adsorption on copper (42), a metal known to be considerably less reactive in chemisorption than iron. It was anticipated, by analogy with carbon monoxide, that nitric oxide would be molecularly adsorbed on copper at 80 K. This, however, was shown to be incorrect (43), and by contrast it was established that the molecule not only dissociated at 80 K, but NjO was generated catalytically within the adlayer. On warming the adlayer formed at 80 K to 295 K, the surface consisted entirely of chemisorbed oxygen with no evidence for nitrogen adatoms. It was the absence of nitrogen adatoms [with their characteristic N(ls) value] at both 80 and 295 K that misled us (43) initially to suggest that adsorption was entirely molecular at 80 K. 

It was proposed by Brill that the rate-determining step in ammonia synthesis was molecular rather than dissociative nitrogen chemisorption. On this basis, the following equation was derived for the kinetics of ammonia synthesis ... 

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