Unexpected Odd–Even Oscillation in the Enhanced Chemical Activities of the Ru n (n = 2–14) Nanoclusters for H2O Splitting

Nanoclusters usually display exotic physical and chemical properties due to their intriguing geometric structures in contrast to their bulk counterparts. In general, the more energetically stable the nanocluster, the weaker the reagent reacts with it; however, to date, it is still open whether all reactions are subject to such a fundamental constraint. Here, using first-principles calculations within density functional theory in consideration of van der Waals corrections and Gaussian 09 program, we investigate the energetic and kinetic properties of water molecules adsorption on small Ru n (n = 2–14) clusters. It is found that almost all of the studied Ru n clusters possess superior activities toward H2O adsorption and dissociation, due to the enlarged desorption energies and reduced dissociation barriers when compared with the bulk Ru(0001) counterpart. More interestingly, though the stabilities of Ru n clusters exhibit significant odd–even oscillation behavior, i.e., the even-numbered nanoclusters are distinctly more stable than their neighboring odd-numbered cases, the H2O molecule adsorption on the even-numbered Ru n clusters (such as n = 4, 6, 8, 10) leads to larger adsorption energies. We reveal that such intriguing activity can be explicated by a geometric effect, namely, the lowly coordinated atoms contribute the lowest-unoccupied molecular orbital protruding out of the cluster to capture the lone-pair electrons from H2O molecule and determine the size-dependent chemical activities. These findings shed new insight into highly efficient and economic nanocatalysts design for the field of water splitting.