REACTION OF MOLYBDENUM CLUSTERS WITH MOLECULAR NITROGEN

Reactions of molybdenum clusters with molecular nitrogen have been investigated by using a fast-flow reactor equipped with a laser vaporization cluster source and laser ionization time-of-flight mass spectrometer for detection of clusters. Absolute rate coefficients are reported for reaction of clusters in the range 3-35 metal atoms, at temperatures 279, 300, and 372 K in He buffer gas at 1 and 2 Torr pressure. Rate coefficients generally decrease with increasing temperature, as expected for a mechanism involving formation of a weakly bound precursor complex. By comparison with surface chemistry studies of nitrogen adsorption on molybdenum and tungsten, the reactivity trend with cluster size is interpreted as reflecting the geometric configurations of metal atoms on the surfaces of the clusters. It is suggested that structural transitions occur in molybdenum clusters in the size range 13-20 metal atoms. Evidence is described that suggests that cluster reaction products have nitrogen bound in both atomic and molecular forms. Bulk surface like reactivity of the clusters appears to be established in the range 25-30 metal atoms.