Enhancing methane dissociation with nickel nanoclusters

Abstract

Size dependency of methane adsorption and dissociation on nickel nanoclusters containing 6, 13 and 19 atoms are studied using density functional theory (DFT) calculations. Methane physisorption was identified only on top sites on all the nanoclusters. Elongation of the dissociating C–H bond was found on Ni6 and Ni13 nanoclusters while on Ni19 no such elongation was found. Transition state calculations revealed that barrier for methane dissociation on Ni6 and Ni13 clusters are nearly half the barrier for dissociation on Ni19 cluster. Comparison of activation energies with this and the previous studies, suggest no geometry dependence i.e., only electronic effect prevails for nanoclusters containing less than ∼13 nickel atoms. For smaller nanoclusters (Ni6 and Ni13) the elongation of the dissociating C–H bond in the adsorbed state acts as the precursor state for dissociation, thereby reducing the energy barrier for methane dissociation. However, no sign of precursor formation is observed in the case of Ni19 nanocluster and hence a large barrier. Semi-classical dissociation probability plots show high dissociation probabilities for Ni6 and Ni13 nanoclusters compared to Ni19 clusters and low-indexed nickel surfaces.