Theoretical investigation of the interaction of CH4 with Al2 and Al3 neutral and charged clusters

Abstract

We have studied the interaction of CH4 with Al2 and Al3 neutral and charged 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 all cases, stable methyl-aluminum-hydrides are possible. The H desorption is studied by means of vibration analysis and application of transition state theory. A common observed trend is that, in breaking the H–CH3 bond, the interacting H atom is attached to the “surface” of the clusters attracting some negative charge of ≈0.2e. The charge transfer is illustrated using the corresponding orbitals near the transition state in conjunction with the computed Mulliken population analysis. Thermal vibrations, generally, do not enhance the reaction. In all exothermic cases, the binding energy toward CH3+HAlncharge increases with increasing charge of the original Aln(q=−1,0,1) cluster. Although Al lacks occupied d-orbitals, the small Al clusters reduce the (free methane) CH3–H dissociation barrier except for Al3(q=−1,0). The relevant reactions in desorption require ∼400–700 °C.