REACTION OF MOLYBDENUM CLUSTERS WITH MOLECULAR NITROGEN

Reactions of molybdenum clusters with molecular nitrogen have been inv estigated by using a fast-flow reactor equipped with a laser vaporizat ion cluster source and laser ionization time-of-flight mass spectromet er for detection of clusters. Absolute rate coefficients are reported for reaction of clusters in the range 3-35 metal atoms, at temperature s 279, 300, and 372 K in He buffer gas at 1 and 2 Torr pressure. Rate coefficients generally decrease with increasing temperature, as expect ed for a mechanism involving formation of a weakly bound precursor com plex. By comparison with surface chemistry studies of nitrogen adsorpt ion on molybdenum and tungsten, the reactivity trend with cluster size is interpreted as reflecting the geometric configurations of metal at oms on the surfaces of the clusters. It is suggested that structural t ransitions occur in molybdenum clusters in the size range 13-20 metal atoms. Evidence is described that suggests that cluster reaction produ cts have nitrogen bound in both atomic and molecular forms. Bulk surfa ce like reactivity of the clusters appears to be established in the ra nge 25-30 metal atoms.