Dehydrogenation and physisorption of saturated hydrocarbons (n-butane and isobutane) on Nb-x(+)

Niobium cluster cations, Nb-x(+), are produced from a laser vaporization/supersonic expansion cluster ion source, and their reactivities are studied using a fast-flow reactor. The reaction patterns of n-butane and isobutane are found to be quite similar. In general, the relative reactivity of the cluster cations shows a decreasing trend with increasing cluster size (x=1-9). Dehydrogenation products from the butane molecules are observed within the small cluster size range (x=1-6). The degree of dehydrogenation increases with cluster size (x=1-6); but right after x=7, no dehydrogenation is observed and only molecular adsorption takes place. The product distribution for isobutane is found to be more scattered to channels of different extent of dehydrogenation than that for n-butane. The results are explained on the basis of the prompt insertion and the lowering of the activation barrier by the charge-induced polarization. (C) 1998 American Institute of Physics. [S0021-9606(98)02344-7].