Reactivities and collision-induced dissociation of vanadium oxide cluster cations are investigated using a triple quadrupole mass spectrometer coupled with a laser vaporization source. The dominant peaks in the mass distribution correspond to cluster ions with stoichiometries of (VO 2 ) n (V 2 O 5 ) m (O 2 ) q + . Collision-induced dissociation studies of the vanadium oxide species V 2 O 4-6 + , V 3 O 6-9 + , V 4 O 8-10 + , V 5 O 11-13 + , V 6 O 13-15 + , and V 7 O 16-18 + show that VO 2 , VO 3 , and V 2 O 5 units are the main building blocks for most of these clusters. The reaction pathways observed for these vanadium oxide clusters include molecular association, cracking, dehydration, and oxygenation of the neutral hydrocarbons with the reactivities of specific clusters differing from species to species. For example, V 3 O 7 + is very efficient in the dehydrogenation of 1,3-butadiene and in the cracking of 1-butene. On the other hand, V 3 O 6 + produces only molecular association products with these same reactants. To help explain these differences in reactivity, calculations on the molecular structure of some of these cluster ions were also undertaken, and the findings are presented.

Transition metal oxides are widely used as both catalysts and catalytic supports in industrial processes. However, the mechanisms by which these materials function as catalysts and the structure-reactivity relationships are not well understood. In particular, there is a paucity of information on the specific sites responsible for the catalytic activity of bulk surfaces. A valuable approach to identifying the active sites of transition metal oxides is to study the chemistry of gas phase metal oxide clusters. A comprehensive program is underway in our laboratory in which reactivity studies of transition metal oxide clusters are carried out using a tandem mass spectrometer system coupled to a laser vaporization source. The desired transition metal oxide cluster cations are mass-selected and then injected into a reaction cell, whereby reactions with various organic molecules are investigated. The advantage of this technique is that the reactivity of specific clusters can be probed independently of other clusters, thereby providing insight into the intermediates and mechanisms at the active sites present on transition metal oxide catalysts of which the gas-phase clusters are representative models. Hence by employing gas-phase techniques, the effect of varying the composition, stoichiometry, oxidation state, charge state, degree of coordinative saturation, and size of the metal oxide clusters on the reactivity is determined. The findings provide valuable information about reaction intermediates, reaction mechanisms, and structure-reactivity relationships. Therefore, these gas-phase studies provide an understanding of the function of transition metal oxide catalysts at the molecular level that is expected to provide knowledge that will find use in the design of more efficient catalysts. This article provides an overview of findings derived in our laboratory for reactions of group V transition metal oxide clusters, with particular emphasis on the mechanism of oxygen transfer to small organic molecules.

Erratum: "Ligand and metal binding energies in platinum carbonyl cluster anions: Collision-induced dissociation of Pt m − and Pt m (CO) n " [J. Gas-phase thermochemical stabilities of cluster ions [( N 2 ) m ( Ar ) n ] + with (m+n)=1-5 Unimolecular dissociation of trivalent metal cluster ions: The size evolution of metallic bonding Ligand and metal binding energies in platinum carbonyl cluster anions: Collision-induced dissociation of Pt m − and Pt m ( CO ) n −The formation and structure of gas-phase vanadium oxide cluster anions are examined using a guided ion beam mass spectrometer coupled with a laser vaporization source. The dominant peaks in the anion total mass distribution correspond to clusters having stoichiometries of the formϪ , and V 7 O 16-18Ϫ indicate that VO 2 , VO 3 , and V 2 O 5 units are the main building blocks of these clusters. There are many similarities between the anion mass distribution and that of the cation distribution studied previously. The principal difference is a shift to higher oxygen content by one additional oxygen atom for the stoichiometric anions (V x O y Ϫ ) as compared to the cations with the same number of vanadium atoms, which is attributed to the extra pair of electrons of the anionic species. The oxygen-rich clusters, V x O y ͑O 2 ͒ Ϫ , are shown to more tightly adsorb molecular oxygen than those of the corresponding cationic clusters. In addition, the bond dissociation thresholds for the vanadium oxide clusters ⌬E(V ϩ -O)ϭ6.09 Ϯ0.28 eV, ⌬E(OV ϩ -O)ϭ3.51Ϯ0.36 eV, and ⌬E(O 2 V Ϫ -O)ϭ5.43Ϯ0.31 eV are determined from the energy-dependent collision-induced dissociation cross sections with Xe as the collision partner. To the best of our knowledge, this is the first bond dissociation energy reported for the breaking of the V-O bond of a vanadium oxide anion.

Reactions of mass-selected group V transition metal oxide cluster ions (V x O y ( , Nb x O y ( , and Ta x O y ( ) with ethane (C 2 H 6 ) and ethylene (C 2 H 4 ) were investigated. The major reaction channels observed during the reactions of C 2 hydrocarbons with M x O y + were association and oxygen transfer. The association channel, M x O y C 2 H n + , where n ) 4 or 6, was common to most of the group V transition metal oxide cluster cations examined. However, a reaction channel corresponding to the loss of an oxygen atom from the mass-selected metal oxide cluster, producing M x O y-1 + , was only observed during the reactions of (V 2 O 5 ) n + , where n ) 1, 2, or 3, with ethane and ethylene and during the reactions of ethylene with Nb 2 O 5 + . It is proposed that this reaction pathway is oxygen transfer from the mass-selected metal oxide cluster cation to the neutral hydrocarbon. This oxygen transfer channel is the major pathway observed during the course of reactions of V 2 O 5 + , V 4 O 10 + , and V 6 O 15 + with ethane and ethylene, but this reaction pathway is minor or nonexistent in the case of reactions of C 2 H 6 and C 2 H 4 with stoichiometrically equivalent niobium and tantalum oxide cluster cations. Additionally, the reactions of M x O ywith C 2 hydrocarbons were also examined. In contrast to the cation results, no reaction products were observed in studies of the interaction of group V transition metal oxide cluster anions with ethane and ethylene. The studies reveal that the identity of the metal, charge state, cluster stoichiometry, and geometric structure strongly influence the ability of the metal oxide cluster to transfer an oxygen atom to the neutral C 2 hydrocarbon.

The reactions of selected vanadium oxide cluster cations with carbon tetrachloride were studied using a triple quadrupole mass spectrometer coupled with a laser vaporization source. The vanadium oxide species VO2 +, V2O4 - 6 +, V3O6 - 8 +, V4O8 - 11 +, V5O11 - 13 +, V6O13 - 15 +, and V7O16 - 18 + demonstrated several pathways for reaction with CCl4. The chloride ion transfer reaction is the dominant reaction pathway for the smaller clusters that contain three or fewer vanadium atoms. For the larger clusters, the abstraction of two chlorine atoms with the transfer of oxygen to the neutral reactant molecule dominates their reactions with CCl4, leading to the production of phosgene. This is consistent with the reactions that occur over the condensed-phase vanadium oxide catalyst for the degradation of carbon tetrachloride to phosgene and carbon dioxide. In addition, oxidation reactions are observed to occur more readily for clusters that contain vanadium atoms with oxidation states lower then +5.