Guided ion beam studies of the reactions of Co n+ (n=1-18) with N2: Cobalt cluster mononitride and dinitride bond energies.

The reactions of Co n+ (n=1-18) with N2 are measured as a function of kinetic energy over a range of 0-15 eV in a guided ion beam tandem mass spectrometer. A variety of Co m +, Co m N+, and Co m N2+ (m<or=n) product ions are observed, all in endothermic processes, with collision-induced dissociation dominating the products for all clusters. Bond dissociation energies for both cobalt cluster nitrides and dinitrides are derived from threshold analysis of the energy dependence of the endothermic reactions using several different approaches. These values show only a mild dependence on cluster size over the range studied, although the Co13+-N bond energy is relatively weak. The bond energies of Co n+-N for larger clusters suggest that a reasonable value for the desorption energy of atomic nitrogen from bulk phase cobalt is 6.3+/-0.2 eV, which is somewhat lower than the only available value in the literature, an estimate based on the enthalpy of formation of bulk cobalt nitride. The trends in the cobalt nitride thermochemistry are also compared to previously determined metal-metal bond energies, D 0(Co n+-Co), and to D 0(Fe n+ -N). Implications for catalytic ammonia production using cobalt versus iron are discussed.