HYDROCARBON REACTIVITY WITH EARLY TRANSITION-METAL ATOMS AND NEUTRAL DIATOMIC METAL BRIDES IN THE GAS-PHASE

Absolute rate coefficients for the reactions of several hydrocarbons with early d-block metal atoms and their corresponding neutral diatomic oxides have been measured in the gas phase at 296 K, using a fast-flow reactor apparatus. Atomic Zr, Nb, and Ta vapors were generated by laser vaporization and entrained in a stream of He buffer gas. Metal atom reactivity with CH4, C3H8, i-C4H10, or C2H4 was monitored after introduction of the reactant gas downstream of the metal vapor source. Diatomic metal oxide reactivity was also investigated, using the same metals and Mo with the previously mentioned gases, by addition of an oxidant gas close to the source region. We report second-order rate coefficients for the reactions of Zr, Nb, Ta, ZrO, NbO, MoO, and TaO with CH4, C3H8, i-C4H10, and C2H4 Trends in the rate coefficient data show that the activation energy of a given reaction correlates with the bond dissociation energy of the weakest C-H bond of the hydrocarbon involved. A C-H bond insertion step followed by the elimination of H-2 is considered to be the operative mechanism for the reaction of both metal atoms and metal oxides with saturated hydrocarbons.