Electronic factors determining the methane bond breaking process on small aluminum clusters

In order to understand the catalytic activity of small metal clusters as a function of their size, we have studied the interaction of CH₄ with Al₄ and Al₅ neutral and charged clusters, as well as neutral thermally expanded clusters in the two lowest lying spin states, using density functional theory. These calculations, via extended search, are used to determine the stable positions of H and CH₃ near the cluster, and the transition state to break the H─CH₃ bond. In order to understand the factors underlying the reactivity of the clusters, we have analyzed the electronic structure at the transition state. By an analysis of the change of the electronic density of states close to the transition state, we identify the orbitals involved in the bond breaking process. In conjunction with our previous studies of Al₂ and Al₃ clusters, we find that the small Al clusters, except for Al₅, lower the CH₃─H dissociation barrier with respect to the gas-phase value, although Al lacks occupied d-orbitals. Still, Al₅ does not catalyze methane bond breaking, which is attributed to the required interaction with low-lying Al sp-states. Furthermore, in all cases where stable methyl-aluminum-hydrides are possible, the recombinative desorption of methane is studied by vibrational analysis and application of transition state theory.