Iron clusters ammonia

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... 

From these results, it is concluded that, in a fully reduced catalyst, FeAl204 is not present furthermore, the aluminum inside the iron particle is present as a phase that does not contain iron (e.g., A1203), and this phase must be clustered as inclusions 3 nm in size. These inclusions may well account for the strain observed by Hosemann et al. From the Mossbauer effect investigation then, the process schematically shown in Fig. 17 was suggested for the reduction of a singly promoted iron synthetic ammonia catalyst. Finally, these inclusions and their associated strain fields provide another mechanism for textural promoting (131). 

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 model can be extended to predict the direction of adsorbate-induced shifts in cluster IP upon the chemisorption of other reactant molecules as well. To illustrate this, consider the chemisorption of ammonia. In this case the net charge donation is from a filled nonbonding orbital of NH3 to the metal cluster (metal-acceptor interaction), resulting in associative chemisorption of NH3 onto the cluster. In contrast to the situation for H2, adsorption of NH3 can be viewed as a reductive addition process with respect to the metal cluster, thus resulting in an increase in the Fermi level and a decrease in IP. This prediction is in excellent agreement with recent data for NH3 chemisorbed on iron clusters, which indicate that the IPs of the fully ammoniated (saturated) clusters are as much as 2 eV lower than those of the corresponding naked clusters. 

In the context of this model we thus expect that the addition of ammonia to an iron cluster will lower its ionization threshold. The magnitude of the IP decrease will be dependent on the number of ammonia molecules chemisorbed and on the size of the cluster. We except that if the ionization thresholds and reactivities toward hydrogen were measured for Fe (NH3) or Fe H2 (n variable, m constant), an IP-ln (rate constant) anticorrelation would be found. Expieriments to date have shown that, upon ammoniation, the minimum in reactivity of iron clusters toward hydrogen shifts to smaller cluster sizes and that the rate constant for hydrogen chemisorption for these ammoniated clusters is about a factor of 10 lower than that for the bare clusters. However, the number of chemisorbed ammonias is different for each cluster. Experiments involving metal clusters with a fixed number of chemisorbed ammonias is a needed probe of the detailed interaction between NH3 and a cluster. 

Effect of particle size on turnover rate for ammonia sjmthesis. Small particles of metallic iron supported on magnesia were prepared by Boudart et The iron particle size could be changed between 1.5 run and 30 run and determined, in part, by electron microscopy. X-ray diffraction, magnetic susceptibility and Moss-bauer spectroscopy. Agreement was satisfactory with particle size values obtained by selective chemisorption of carbon monoxide (if 2 Fe for 1 CO). Two results are noteworthy, (i) The turnover rate for ammonia synthesis increases by a factor of 35 as the iron particle size increases (Table 2.12). (ii) A pretreatment of the iron catalyst with ammonia increases the turnover rate by only 10% for the larger particles, but quite appreciable for iron clusters (Table 2.13). 

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). 

While chlorine is a poison for the ammonia synthesis over iron, it serves as a promoter in the epoxidation of ethylene over silver catalysts, where it increases the selectivity to ethylene oxide at the cost of the undesired total combustion to C02. In this case an interesting correlation was observed between the AgCl27Cl ratio from SIMS, which reflects the extent to which silver is chlorinated, and the selectivity towards ethylene oxide [16]. In both examples, the molecular clusters reveal which elements are in contact in the surface region of the catalyst. 

The LA process may be carried out in the presence of gaseous molecules or near a stream of gas (60) (e.g., oxygen) producing cluster anions such as GdvO( or ammonia producing complex cations such as Cr(NH3)J, where x = 2-15 (61). Metal complexes may also be obtained by LA (e.g., LA of an iron rod produces Fe+), but when the rod is coated with C60 and coronene, ions such as [C60Fe(coronene)]+ are produced (62). 

Nitrite reductase and sulfite reductase are enzymes found in choroplasts and in prokaryotes that reduce nitrite to ammonia and sulfite to sulfide (Scott et al., 1978). Sulfite reductase also catalyzes reduction of nitrite at a lower rate. Both enzymes contain a siroheme prosthetic group linked to an iron-sulfur cluster. In siroheme, the porphyrinoid moiety is present in the more reduced chlorin form. Because NO lies between nitrite and ammonia in oxidation state, it is a potential intermediate. 

One of the enzymes given in Table 23 is nitrogenase, which is responsible for the fixation of dinitrogen to give ammonia. Molybdenum probably serves as the binding site for N2, and is present in the iron-molybdenum cofactor, which is a molybdenum-iron sulfide cluster. Nitrogenase will be considered in Section 63.1.14, which deals with the nitrogen cycle. 

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. 

---