Studies of reactions of atoms in a discharge flow stirred reactor. Part 2.—O+H2+CO system

Abstract

A discharge–flow stirred reaction system has been used to measure the rate constant for the reaction O(³P)+H₂→ OH+H over the temperature range 315–490 K. Below 350 K, carbon monoxide (⩽ 5%) was added to generate the bluish O+CO chemiluminescence, the intensity of which served to measure oxygen atom decay rates. An expression k₁=(3.1±0.5)× 10¹⁰ exp (–4950±300 K/T) dm³ mol^–1 s^–1 summarised the measurements above 350 K, but sharp positive deviation was found in the Arrhenius plot of k₁ below this temperature, for which quantum mechanical tunnelling in the transition state provides a likely interpretation.

At 425 K in O/H₂/N₂ or Ar systems with CO mole fractions up to 60%, HCO radicals are syn-synthesised by the combination reaction H+CO+M = HCO+M (19) and destroyed by the reactions H+HCO → H₂+CO (20) O+HCO → CO+OH (21a) O+HCO → CO₂+H.(21b)

In systems where large mole fractions of CO were present, the [H]/[O] ratios were established from the linear increase in the oxygen atom decay rate from entry to exit with increasing additions of nitric oxide, using the O+CO chemiluminescence as indicator. In 27 experiments in N₂ carriers and 36 experiments in Ar carriers with 90 ⩽[CO]/[O]⩽ 950 and 0.2 ⩽[H]/[O]⩽ 3, the measured oxygen atom decay parameters were analysed iteratively by computer. Combination of these two types of experimental results yielded k₁₉(M = N₂)=(1.44±0.12)× 10⁸ dm⁶ mol^–2 s^–1 and k₁₉(M = Ar)=(0.97±0.09)× 10⁸ dm⁶ mol^–2 s^–1 for 425 K, the latter in agreement with temperature extrapolated literature values. A well-defined ratio A=k₂₀/k₂₁= 2.1±0.3 was also obtained, together with an estimate of B=k_21a/k₂₁= 0.4±0.2. A simplified collisional model is used to interpret the values of A and B.