Transitions in Order and Molecularity with Temperature in Gaseous Metal Oxidation Reactions. The Sb-O₂ System

Examples of reactions which are termolecular near room temperature and gradually switch over to complex-forming bimolecular reactions with increasing temperature are discussed. The Sb/O₂ system, which is known from D. Husain´s work to be termolecular at 300 K, has been studied here from 1165-1495 K at 12 to 52 mbar in a high-temperature fast-flow reactor (HTFFR). Sb was vaporized from a crucible and atomic resonance absorption spectrometry was used to measure the rates of Sb consumption. Under these conditions the reaction, identified as Sb + O₂ → SbO + O, is pressure-independent with k(T) = 6.3×10^-11 exp (-9107 K/T) cm³ molecule^-1 s^-1. The 2σ precision limits are ±8% and the corresponding estimated accuracy limits are ±24%. The results suggest that D₀(Sb-O) = 444±28 kJ mol^-1. The results are compared to those of other endothermic metal-atom O₂ abstraction reactions, all of which have pre-exponentials on the order of 10-10 cm³ molecule^-1 s^-1 and activation energies for abstraction close to the endothermicity. This allows assessments of the relative importance of bi-, and ter- molecular kinetics, as illustrated for As + O².