CHEMISTRY AND KINETICS OF SIZE-SELECTED COBALT CLUSTER CATIONS AT THERMAL ENERGIES .2. REACTIONS WITH O2

The size-dependent chemistry and kinetics of gas-phase reactions of Co(n)+ (n = 2-9) with O2 are examined by using a SIDT-LV (selected ion drift tube with laser vaporization source) operated at thermal energies. Co(n)+ are observed to display a very high reactivity toward O2, and the clusters tend to undergo successive oxidation reactions. The bimolecular reaction rate constants measured for the primary reactions display a strong correlation between the size of the clusters and their reactivity. As in the case of reactions with other reactant molecules such as CO, clusters containing four and five cobalt atoms exhibit aa higher reactivity toward oxygen than clusters of neighboring size. The primary reactions result mainly in a replacement of a Co atom by one O2, which suggests that the oxygen and cobalt atoms in the formed cobalt oxide clusters are bound together by strong chemical bonds. The formed oxide clusters are also very reactive toward oxygen. Except for a few cases, most of the oxide cluster reactions with oxygen proceed via either switching or attachment pathways. The successive oxidation reactions of Co(n)+ virtually terminate when oxide clusters with stoichiometric structures of (CoO)4(CoO2)n+ (n = 0-3), Co2O4,5+, or Co3O4,5+ are formed. Compared with the results obtained by other methods, the present work provides another important example demonstrating that SIDT-LV is a very effective technique to examine the reactions of size-selected metal cluster cations at thermal energies.