Reactions of simple hydrocarbons with Nb_n⁺: Chemisorption and physisorption on ionized niobium clusters

In a Fourier transform-ion cyclotron resonance mass spectrometer the gas phase reactivities of niobium clusters Nb_n⁺ (n =1-28) with molecular hydrogen, water, methane, ethane, n-propane, n-heptane, cyclohexane, acetylene, ethylene, allene, benzene, propene, toluene, xylene, and acetonitrile were investigated under single collision conditions as well as the reactivities of oxidized niobium cluster cations with ethylene and benzene. The reactions of larger clusters with a variety of unsaturated hydrocarbons are believed to proceed via long lived "physisorbed" addition intermediate complexes, which subsequently rearrange to form "chemisorbed," extensively dehydrogenated final products. The overall reaction seems to proceed with near collision rates, almost independent of cluster size. In some cases also the physisorbed primary products are stabilized and detected. Their yields depend sensitively on the specific nature of the reactant, and on the niobium cluster size n. Fully saturated hydrocarbons unable to form the long lived complexes are unreactive with respect to the larger (n>7) Nb_n⁺ clusters. Smaller clusters with n≤7 seem to react by a different, prompt reaction mechanism. The rate of this reaction steeply decreases, and the degree of product dehydrogenation increases with n.