Sequential attachment of hydrogen molecules to niobium cluster cations

The reactivity of niobium cluster cations Nb-n(+) (n = 6-12) towards molecu lar hydrogen was studied using an FT-ICR mass spectrometer combined with a laser-ablation/supersonic-jet external ion source. The reaction rate patter ns for the sequential adsorption of hydrogen on niobium cluster cations as well as the H-2 saturation coverage pattern were measured. It was found tha t for the Nb-8(+) and Nb-11(+) clusters, the adsorption of an H-2 molecule can increase the reactivity of the cluster toward another H-2 molecule. Thi s suggests that the adsorption of a hydrogen molecule can substantially cha nge the electronic configuration and/or geometric structure of a niobium cl uster and therefore supports the chemisorption mechanism for H-2 adsorption on Nb-n(+) clusters. (C) 1999 Elsevier Science B.V. All rights reserved.