Ammonia reaction with iron clusters

Parks E K, Welller B H, Bechthold P S, Hoffman W F, NIeman G C, Pobo L G and Riley S J 1988 Chemical probes of metal cluster structure reactions of Iron clusters with hydrogen, ammonia and water J. Chem. Rhys. 88 1622... 

Lewis Bases. A variety of other ligands have been studied, but with only a few of the transition metals. There is still a lot of room for scoping work in this direction. Other reactant systems reported are ammoni a(2e), methanol (3h), and hydrogen sulfide(3b) with iron, and benzene with tungsten (Tf) and plati num(3a). In a qualitative sense all of these reactions appear to occur at, or near gas kinetic rates without distinct size selectivity. The ammonia chemisorbs on each collision with no size selective behavior. These complexes have lower ionization potential indicative of the donor type ligands. Saturation studies have indicated a variety of absorption sites on a single size cluster(51). 

Niobium and cobalt clusters exhibit size-sensitive reactions with nitrogen with a reactivity pattern similar to that observed for hydrogen. The reactivity of rhodium clusters (n = 1-12) toward N2 has also been studied. In this case the atoms through the tetramer appear to be inert, with reactivity turning on at Rhj. Maximum reactivity occurs at Rh7, and subsequently drops off by roughly a factor of 2 in going from Rh, to Rh,. Iron clusters appear to be nearly unreactive toward N2. Attempts to induce low-pressure ammonia synthesis on gas-phase iron clusters indicate that hydrogenated iron clusters Fe H are also unreactive toward N2. ... 

The binding of ammonia to the cluster induces a change in electronic structure relative to that of the bare cluster. This is probed by reacting ammoniated clusters with hydrogen and comparing the reaction rate constants as a function of cluster size with the naked iron clusters. The absolute reaction rate constants toward H 2 for the fully ammoniated clusters are about an order of magnitude smaller than those for the bare clusters. The minima in reactivity observed for bare iron clusters are shifted to smaller cluster size for the ammoniated species for example, Fe,3 is reactive with H2, but upon... 

The proteins involved in the reduction of nitrogen to ammonia and other accessible forms contain several such clusters coupled with molybdenum centres. The structure of the central iron-molybdenum cluster at the centre of nitrogenase is shown in Fig. 10-9. Even with the detailed knowledge of this reaction site, the mode of action of nitrogenase is not understood. 

V-substituted nitrogenases as well as active nitrogenases with no metal but iron (see Section 8.22.4). It is also difficult to explain the huge differences in reaction rate and product distribution upon mutation of residues near the iron belt but distant from the molybdenum atom. Neither of these problems is fatal to the molybdenum model, because a few octahedral vanadium and iron N2 complexes produce ammonia upon protonation. In such models, distant belt-area mutations presumably cause a change in the hydrogen bonding network around the Mo site. Thus it is tenable to hold that all nitrogenases react with N2 at the octahedral metal site in the M cluster. 

Thus, it appears that high temperature chemisorption of N2 counts correctly, not the iron surface atoms, but the sites required for chemisorption of N2, the rate of which is the rate determining step in armnonia synthesis reaction. It would be wrong to conclude that the reaction is structure-insensitive because the turnover rate is independent of particle size when the sites are counted with chemisorption of N2. On the contrary, all the combined results confirm the view that a site for ammonia synthesis comprises a certain number of atoms, including C7 atoms, forming a multiple, or ensemble, or cluster. Perhaps these consist of the six atoms as shown in Fig. 2.30. ... 

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