The reactions of FeO with O3, H2 , H2O, O2 and CO2

Abstract

The FeO(X ⁵Δ) radical plays a central role in the atmospheric chemistry of meteor-ablated iron and in the iron-catalysed inhibition of flames. This paper reports a kinetic study of a series of FeO reactions, using the pulsed two-photon dissociation at 248 or 193 nm of ferrocene vapour in the presence of NO₂ or O₃, respectively, followed by time-resolved laser-induced fluorescence spectroscopy of FeO at 582.0 nm [FeO(D′  ⁵Δ₄–X ⁵Δ₄)]. For the recombination reactions of FeO with O₂, CO₂ and H₂O, ab initio quantum calculations on FeO₃, FeCO₃ and Fe(OH)₂ were coupled with RRKM theory to show that the first two reactions were close to their low pressure limits under the experimental conditions, yielding k(FeO+O₂+N₂, 196–519 K)=(3.86±0.07)×10⁻³⁰ (T/300 K)^+0.50±0.06 and k(FeO+CO₂+N₂, 233–475 K)=(3.09±0.22)×10⁻³¹ (T/300 K)^{−1.19±0.24} cm⁶ molecule⁻² s⁻¹, at the 95% confidence level. By contrast, the much faster FeO+H₂O (+N₂) reaction was found to be in the fall-off region over the experimental pressure range; an RRKM fit to the results yields: log₁₀(k_rec,0)=−31.05+4.438log₁₀T−1.218log₁₀²T, k_rec,∞=5.35×10⁻¹⁰ exp(−611/T), F_c=0.28, with an uncertainty of 15% over the experimental range of 298–527 K and 3.1–8.3 Torr. The corresponding reactions with He as the third body were slower by factors of 2–4. The reaction between FeO and O₃ is fast with a small T-dependence: k(FeO+O₃→FeO₂+O₂ , 194–341 K)=(2.94±0.43)×10⁻¹⁰ exp[(−1.45±0.29) kJ mol⁻¹/RT] cm³ molecule⁻¹ s⁻¹. No reaction was observed between FeO and H₂, indicating an upper limit of k(FeO+H₂→Fe+H₂O, 320 K)⩽7×10⁻¹⁴ cm³ molecule⁻¹ s⁻¹. Finally, the atmospheric implications of these results are discussed.