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

Abstract

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.