Experimental and theoretical study of the reaction Fe + O2+ N2→ FeO2+ N2
Abstract
The recombination reaction between Fe and O2 has been studied by the pulsed photodissociation at 193.3 nm of ferrocene vapour to produce Fe atoms in an excess of O2 and N2 bath gas, followed by time-resolved laser-induced fluorescence spectroscopy of atomic Fe at 248.3 nm [Fe(x5F°5–a 5D4)]. This yields k(288 < T/K < 592)=(4.34 ± 0.96)× 10–30 exp[–(16.94 ± 0.67) kJ mol–1/RT] cm6 molecule–2 s–1, where the quoted uncertainty is 2σ. Ab initio quantum calculations on FeO2 indicate that the ground electronic state is a superoxide with an isosceles triangular geometry, FeO2(7A1). The bond energy, Do(Fe—O2), is estimated to be 180 ± 50 kJ mol–1. These calculations, combined with RRKM theory, demonstrate that the positive temperature dependence of this recombination reaction is caused by barriers in the entrance channels of the manifold of potential-energy surfaces that arise from the interaction of Fe(a 5Di) and O2(3Σ–g). The recommended rate coefficient between 150 and 2000 K is given by log(k/cm6 molecule–2 s–1)=– 161.87 + 119.32x– 36.057x2+ 3.6303x3, where x= log(T), and the uncertainty in k is estimated to be ±25%. The implications of this result for the chemistry of meteor-ablated Fe in the upper atmosphere are then considered.