Observation and the Origin of Magic Compositions of Co_<i>n</i>O_<i>m</i>^– Formed in Oxidation of Cobalt Cluster Anions

Abstract

To obtain atomistic insights into the early stage of the oxidation process of free cobalt cluster anions Co_<i>n</i>^–, the reaction of Co_<i>n</i>^– (<i>n</i> ≤ 10) with varied pressure of O₂ was studied experimentally and theoretically. Population analysis of the oxidation products Co_<i>n</i>O_<i>m</i>^– as a function of <i>m</i> revealed two types of magic compositions: the population decreases abruptly upon addition of a single O atom to and removal of a single O atom from the magic compositions. Magic compositions of the former type were further divided into oxygen-rich (<i>n</i>:<i>m</i> ∼ 3:4) and oxygen-poor (<i>n</i>:<i>m</i> ∼ 1:1) series. The oxygen-rich compositions most likely correspond to fully oxidized states, since the compositions are comparable to those of Co₃O₄ in the bulk. Their appearance is ascribed to the significant reduction of binding energies of O atoms to fully oxidized clusters. In contrast, oxygen-poor compositions correspond to the intermediates of the full oxidation states in which only the surface is oxidized on the basis of theoretical prediction that oxidation proceeds by bonding O atoms sequentially on the surface of Co_<i>n</i>^– while retaining its morphology. Their appearance is ascribed to the kinetic bottleneck against internal oxidation owing to significant structural change of the Co_<i>n</i> moiety. In contrast, magic compositions of the latter type are associated with the abrupt increase of survival probability as anionic states during the relaxation of internally hot Co oxide clusters based on the <i>m</i>-dependent behaviors of adiabatic electron affinities determined by photoelectron spectroscopy