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

Abstract

High Temperature Kinetics / Gas-phase / Molecularity / Metal Atoms / Metal Oxidation / Rate Coefficients / Activation Energies / Sb / As / O 2 / SbO Bond EnergyExamples 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 1165Ϫ1495 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 ϩ O2 → SbO ϩ O, is pressure-independent with k(T) ϭ 6.3 ϫ10 Ϫ11 exp (Ϫ 9107 K/T) cm 3 molecule Ϫ1 s Ϫ1 . The 2σ precision limits are Ϯ 8% and the corresponding estimated accuracy limits are Ϯ 24%. The results suggest that D 0 (Sb-O) ϭ 444 Ϯ 28 kJ mol Ϫ1 . The results are compared to those of other endothermic metal-atom O 2 abstraction reactions, all of which have pre-exponentials on the order of 10 Ϫ10 cm 3 molecule Ϫ1 s Ϫ1 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 ϩ O2.

Metal ion-molecule kinetics at combustion temperatures: The reaction of Ca+ with O2

“…Very small intercepts may be present at the lower temperatures as well; these are statistically insignificant, but increase with temperature. It is tempting to attribute this to an abstraction channel competing effectively with reaction (1) with increasing temperature, as has been observed for several neutral metal -atom oxidation reactions [4]. However, this process…”