Activation of methane by size-selected iron cluster cations, Fe-n(+) (n=2-15): Cluster-CHx (x=0-3) bond energies and reaction mechanisms

The kinetic energy dependences of the reactions of Fe-n(+) (n=2-15) with CD 4 are studied in a guided ion beam tandem mass spectrometer over the energy range of 0-10 eV. All reactions exhibit thresholds and two main products, FenD+ and FenCD2+, are formed. These primary products decompose at higher e nergies to form secondary and higher order products, FenCD+, FenC+, Fen-1D, Fen-1CD2+, Fen-1CD+, and Fen-1C+. The cross-section magnitudes for the de hydrogenation products, FenCD2+, are observed to vary considerably as a fun ction of cluster size; subsequent dehydrogenation to form FenC+ becomes mor e facile for larger clusters. Thresholds for the various primary and second ary reactions are analyzed and bond energies for iron cluster cation bonds to C, CD, CD2, and CD3 are determined. As a function of cluster size, these bond energies rapidly reach relatively constant values, which are argued t o lie close to bulk phase values. The relative magnitudes in these bond ene rgies are consistent with simple bond order considerations. On the basis of this thermochemistry, we find that there are barriers to the primary dehyd rogenation reactions for all the clusters, except n=3 and 4. Evidence that this barrier for n greater than or equal to5 corresponds to the chemisorpti on step is discussed. (C) 2001 American Institute of Physics.