Effects of pressure and temperature on the reactions of niobium cluster cations with hydrogen and deuterium

The effects of pressure and temperature on the reactivities of niobium clus ter cations Nb-n(+)(n=4-16) toward molecular hydrogen and H-2/D-2 mixture w ere studied using a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer combined with a laser-ablation/supersonic-jet external ion so urce. It was shown that the rate constants of the reactions of most Nb-n(+) clusters with the first H-2 molecule exhibit pronounced fall-off behavior in the total pressure range of 10(-6)-10(-4) Torr. For the Nb-n(+)+H-2/D-2 reactions, the H/D scrambling in the reaction products and the apparent kin etic isotope effects were also found to be strongly pressure-dependent. The observed phenomena are consistent with a reaction mechanism that involves a sequence of reversible stages of dissociative chemisorption of H-2 (or D- 2) molecules on Nb-n(+) clusters. The fall-off effects can explain the appa rent negative temperature dependence for the Nb-n(+)+H-2 reaction rate cons tants, the difference between the kinetic data obtained in FT-ICR and fast- flow reactor studies, and the observed enhanced kinetic isotope effects. In direct evidence was obtained showing that the low reactivities of certain N b-n(+) clusters (n=8, 10, and 16) toward hydrogen are due to activation bar riers rather than low chemisorption energies. (C) 2001 American Institute o f Physics.