Thermochemistry of the activation of N ₂ on iron cluster cations: Guided ion beam studies of the reactions of Fe _n ⁺ (n=1-19) with N ₂

The kinetic energy dependences of the reactions of Fen+ (n=1-19) with N2 are studied in a guided ion beam tandem mass spectrometer over the energy range of 0-15 eV. In addition to collision-induced dissociation forming Fem+ ions, which dominate the product spectra, a variety of Fem N2+ and Fem N+ product ions, where m≤n, is observed. All processes are observed to exhibit thresholds. Fem+ -N and Fem+ -2N bond energies as a function of cluster size are derived from the threshold analysis of the kinetic energy dependences of the endothermic reactions. The trends in this thermochemistry are compared to the isoelectronic D0 (Fen+ -CH), and to bulk phase values. A fairly uniform barrier of 0.48±0.03 eV at 0 K is observed for formation of the Fen N2+ product ions (n=12, 15-19) and can be related to the rate-limiting step in the Haber process for catalytic ammonia production.