Gas-phase niobium cluster reactivity with isobutane

Abstract

A laser ablation fast-flow reactor apparatus, equipped with a laser ionization time-of-flight mass spectrometer detector, has been used to investigate the reactivity of gas-phase Nbn with isobutane. Absolute second-order rate coefficients k(2) for the removal of Nbn by isobutane were measured at 279, 300, and 371 K in a continuous flow of He carrier gas held at 1 or 2 Torr total flow tube pressure. The rate coefficients for a given cluster species were found to be pressure independent, and became larger with increasing temperature. A gradual increase in k(2) with increasing cluster size has been observed. This increase is simply explained by a direct correlation of k(2) with the hard-sphere collision cross section. Pseudo-first-order kinetic plots for all cluster reactions with isobutane were linear with no bimodal character evident, indicating an insensitivity to structural forms of Nbn. Activation energies have been estimated, and their values were found to be close to zero for all Nbn cluster reactions with isobutane. The small magnitudes of k(2) relative to khs have been attributed to a reaction mechanism involving a ‘‘tight’’ transition-state complex. Product masses, after the reaction of a bare Nb cluster with one or more isobutane molecules, clearly show dehydrogenation to be an active process. A pseudo-first-order kinetic model for a consecutive bimolecular reaction scheme has been solved for the Nb2/isobutane system, and is in good agreement with the experimental data.