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, D_o(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 ⁵D_i) 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.