Two reaction regimes in the oxidation of larger cationic tantalum clusters (Tan(+), n = 13-40) under multi-collision conditions

The reaction of cationic tantalum clusters (Tan(+), n = 13-40) with molecular oxygen is studied under multi-collision conditions and at different temperatures. Consecutive reaction proceeds in several steps upon subsequent attachment of O2. All cluster sizes exhibit fast reaction with oxygen and form a characteristic final reaction product. The time-dependent product analysis enables the fitting to a kinetic model with the extraction of all the rate constants. Determined rate constants reveal the existence of two different regimes, which are interpreted as a change in the reaction mechanism. Based on the temperature-dependent reaction behavior, it is proposed that the reaction changes from a dissociative to a molecular adsorption of oxygen on the clusters. It is found that both regimes appear for all cluster sizes, but the transition takes place at different intermediate oxides TanOx(+). In general it is observed that transition occurs later for larger clusters, which is attributed to an increased cluster surface.