Electronic and Geometric Effects on Chemical Reactivity of 3d-Transition-Metal-Doped Silver Cluster Cations toward Oxygen Molecules

We report electronic and geometric structures of 3d-transition-metal-doped silver cluster cations, Ag_N-1M⁺ (M = Sc-Ni), studied by chemical reaction with oxygen molecules. The evaluated reaction rate coefficients for small sizes, N, are 2-6 orders of magnitude higher than those of undoped Ag_N ⁺, whereas those for large N are comparable with those of Ag_N ⁺. The low reactivity at large sizes is attributed to a geometric effect, that is, encapsulation of the dopant atom, which provides an active site located on the surface of the cluster in small sizes. In addition, a reactivity minimum is observed for Ag_N-1M⁺ with M = Sc, Ti, V, Fe, Co, and Ni at a specific size, where the cluster possesses 18 valence electrons including 3d electrons. With the aid of density functional theory calculations, the reactivity minimum is suggested to be due to an electronic effect, that is, formation of a closed electronic shell by the 18 valence electrons, implying delocalized 3d electrons. Ag₁₃Cr⁺ and Ag₁₂Mn⁺, possessing 18 valence electrons as well, are noted to be exceptions, where d electrons are supposed to be localized on the dopant atom because of the half-filled nature of Cr and Mn 3d orbital.