Niobium clusters, reactions with

Small ey(Ib) that cobalt and niobium clusters react in a very selective fashion with deuterium was rapidly followed by similar studies on a variety of other metal systems. Three groups simultaneously reported similar dramatic behavior for iron c usters( 2b, 3b). Di hydrogen or di deuteri urn addition reactions have been reported for vanadium( 3e), i ron(lc,2a-d,3b), cobalt(lb),... 

The reactions of some transition metal cluster ions have been described in a review by Parent and Anderson (201). The review covered reactions reported up to 1992 and so the reactions reported here are generally later than 1992. A recent review by Knickelbein (202) discusses the reactions of cation clusters of iron, cobalt, nickel, copper, silver, niobium, and tungsten with small molecules such as H2 and D2. Some of the reactions in Knickelbein s review are included in the following tables of reactions (Tables IV and V). Table IV gives examples of the reactions of transition metal cluster ions and includes the vaporization source, experimental apparatus, the reactants, and the observed product ions. A few examples from these tables will be selected for further discussion. 

Niobium and rhodium cluster anions have been prepared by laser vaporization and the reactions with benzene studied by FT-ICR/MS (58). The reactions of the anions and similar cations have been compared. With few exceptions the predominant reaction of the niobium cluster anions and cations was the total dehydrogenation of benzene to form the metal carbide cluster, [Nb C6]-. The Nb19 species, both anion and cation, reacted with benzene to form the coordinated species Nb 9C6I I6p as the predominant product ion. The Nb22 ions also formed some of the addition complex but the Nb2o Nb2i, and all the other higher clusters, formed the carbide ions, Nb C6. ... 

In contrast to the addition of only one benzene molecule to even large niobium cluster anions, small niobium cluster anions react by the addition of up to 5 N2 molecules and up to 15 CO molecules (229). The reactions of the niobium cluster anions Nb (n = 2-7) were qualitatively similar to the reactions of the cluster cations and in both cases CO was more reactive than N2. The rates of reaction generally increased with increasing size of the cluster but in many cases extensive fragmentation of the clusters was observed. 

As a specific example of the correlations we have observed, our experimental results demonstrate that the reactions of vanadium and niobium oxide cluster cations with methanol lead to dehydrogenation of methanol under single collision conditions producing neutral formaldehyde [16]. Formaldehyde is, indeed, a major product formed from methanol over condensed-phase vanadia surfaces [17]. Another study conducted in our laboratory showed that,, Clj products were formed during the reaction between certain clusters and CCl, leaving phosgene (COClj) as the most likely neutral product [15, 18]. The decomposition of... 

Zemski KA, Justes DR, Bell RC, Castleman AW Jr (2001) Reactions of niobium and tantalum oxide cluster cations and anions with n-butane. J Phys Chem A 105 4410 Bell RC, Castleman AW Jr (2002) Reactions of vanadium oxide cluster ions with 1, 3-butadiene and isomers of butane. J Phys Chem A 106 9893... 

The initial observation of remarkable size-dependent reactions of deuterium with clusters of cobalt and niobium was rapidly followed by other reports of dihydrogen addition reactions with clusters of many metals, in some cases containing more than 100 atoms. Further work was reported for nio-bium " and cobalt as well as for iron, vanadium, " nickel, platinum, rhodium, tantalum, and aluminum. ... 

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. ... 

Finally, the overall trend is toward antipathetic structure sensitivity iron atoms (n = 1), which have the highest IP, are not active (17). Of course, for supported small particles the size distribution would not ordinarily permit the observation of the detailed oscillations of Fig. 23. Other results for H2 chemisorption on bare cobalt and niobium clusters have been reported by Geusic et al. (16), and the reaction with 02 and H2S has been studied by Whetten et al. (355a). For the latter, the oscillations as the reactivity increases with n are much smaller than those of Fig. 23. 

Although the behavior of niobium and tantalum clusters in ethanolic solutions is expected to be similar to that in methanol, experiments indicate significantly faster reaction with methanol. 

It was a great thrust for reduced rare earth metal chemistry when it was discovered that sealed metal ampoules (mostly produced from niobium and tantalum tubing by He arc welding) were reaction containers inert enough to the corrosive molten salts to allow for pure products. Still, all constituents of air need to be excluded, which is possible, thanks to modem dry box techniques. The unintended incorporation of ubiquitous hydrogen or carbon led to a whole new world of cluster complexes with endohedral atoms, mostly with extended stmctmes, as in H Lu Cl or in CLu2 Cl2 (see Rare Earth Metal Cluster Complexes). 

Thus, recent experimental evidence seems to support the idea that growing small particles have maximum chemical reactivities, and certain sized/shaped small particles may have the highest reactivities. What size and/or shape varies with the metal in question and the reaction in question This information strongly supports three ideas (1) structure sensitivity in chemical reactions on metal surfaces is very important, (2) more than one atom is necessary to carry out at least some bond breaking processes, and (3) defect sites on growing small particles are extremely reactive (see Fig. 9). It has also been possible by pulsed laser vaporization to produce many types of gas phase metal clusters. Particularly interesting have been reactivity studies of niobium clusters Nb where X = 5-20. A definite cluster size dependence on reactivity was observed. Exposure... 

Mild oxidant agents often permit oxidations with retention of the cluster structures. One example of this kind of simple chemical electron-transfer reactions are the oxidations of niobium and tantalum clusters [M6Xi2] to the species with charges 3+ and 4+. The reaction with iron(III) chloride and cerium (IV) salts leads directly to the cations [M6Xi2] . ... 

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