DEHYDROGENATION AND PHYSISORPTION OF SATURATED-HYDROCARBONS (N-BUTANEAND ISOBUTANE) ON NB-X(+)

Niobium cluster cations, Nb-x(+), are produced from a laser vaporizati on/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 increa sing cluster size (x=1-9). Dehydrogenation products from the butane mo lecules are observed within the small cluster size range (x=1-6). The degree of dehydrogenation increases with cluster size (x=1-6); but rig ht after x=7, no dehydrogenation is observed and only molecular adsorp tion takes place. The product distribution for isobutane is found to b e more scattered to channels of different extent of dehydrogenation th an that for n-butane. The results are explained on the basis of the pr ompt insertion and the lowering of the activation barrier by the charg e-induced polarization. (C) 1998 American Institute of Physics. [S0021 -9606(98)02344-7].