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摘要
Cross sections for chemisorption of N(2) onto Al(44)(+/-) cluster ions have been measured as a function of relative kinetic energy and the temperature of the metal cluster. There is a kinetic energy threshold for chemisorption, indicating that it is an activated process. The threshold energies are around 3.5 eV when the clusters are in their solid phase and drop to around 2.5 eV when the clusters melt, indicating that the liquid clusters are much more reactive than the solid. Below the melting temperature the threshold for Al(44)(-) is smaller than for Al(44)(+), but for the liquid clusters the anion and cation have similar thresholds. At high cluster temperatures and high collision energies the Al(44)N(2)(+/-) chemisorption product dissociates through several channels, including loss of Al, N(2), and Al(3)N. Density functional calculations are employed to understand the thermodynamics and the dynamics of the reaction. The theoretical results suggest that the lowest energy pathway for activation of dinitrogen is not dynamically accessible under the experimental conditions, so that an explicit account of dynamical effects, via molecular dynamics simulations, is necessary in order to interpret the experimental measurements. The calculations reproduce all of the main features of the experimental results, including the kinetic energy thresholds of the anion and cation and the dissociation energies of the liquid Al(44)N(2)(+/-) product. The strong increase in reactivity on melting appears to be due to the volume change of melting and to atomic disorder.
- 出版日期2010-9-22
