Methane activation by cobalt cluster cations, Con+ (n=2-16): Reaction mechanisms and thermochemistry of cluster-CHx (x=0-3) complexes

The kinetic energy dependences of the reactions of Co-n(+) (n=2-16) with CD4 are studied in a guided ion beam tandem mass spectrometer over the energy range of 0-10 eV. The main products are hydride formation, ConD+, dehydrogenation to form ConCD2+, and double dehydrogenation yielding ConC+. These primary products decompose to form secondary and higher order products, ConCD+, Con-1D+, Con-1C+, Con-1CD+, and Con-1CD2+ at higher energies. Adduct formation of ConCD4+ is also observed for the largest cluster cations, n >= 10. In general, the efficiencies of the single and double dehydrogenation processes increase with cluster size, although the hexamer cation shows a reduced reactivity compared to its neighbors. All reactions exhibit thresholds, and cross sections for the various primary and secondary reactions are analyzed to yield reaction thresholds from which bond energies for cobalt cluster cations to D, C, CD, CD2, and CD3 are determined. The relative magnitudes of these bond energies are consistent with simple bond order considerations. Bond energies for larger clusters rapidly reach relatively constant values, which are used to estimate the chemisorption energies of the C, CD, CD2, and CD3 molecular fragments to cobalt surfaces.