REACTION OF TUNGSTEN CLUSTERS WITH MOLECULAR NITROGEN

Reactions of tungsten clusters with molecular nitrogen have been inves tigated by using a fast-flow reactor equipped with a laser vaporizatio n cluster source and time-of-flight mass spectrometer detector. Absolu te rate coefficients are reported for reaction of W-n clusters in the range n=4-26, at temperatures 277, 300, and 370 K in He buffer gas at 1 and 2 Torr pressure, For smaller clusters with n<15, complexes with N-2 are formed with binding energies near 16 kcal mol(-1), and act as precursors to dissociation of N-2 on the clusters. A sharp jump in the binding energy occurs at a cluster size of 15 metal atoms, and may si gnal the onset of atomic as opposed to molecular binding of Nz on the cluster. It is suggested that the change in reactivity at n=15 is corr elated with a structural transition of the clusters, from relatively c lose-packed to more open structures. The reactivity of tungsten cluste rs with N-2 is compared with that of molybdenum clusters and tungsten metal surfaces. (C) 1996 American Institute of Physics.