Studies of Reactions of Atoms in a Discharge Flow Stirred Reactor: Part 2.—O + H2 4- 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 A'i = (3.1 ±0.5) x IO10 exp (-4950 + 300 K/rj dm3 mol-1 s"1 summarised the measurements above 350 K, but sharp positive deviation was found in the Arrhenius plot of ki 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 O + HCO -> CO + OH O + HCO -> CO2 + H. (20) (21a) (216) 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 £19(M = N2) = (1.44±0.12)x 10s dm6 mob2 s-‘ and Aj9(M = Ar) = (0.97 + 0.09) x 108 dm6 mol'2 s-1 for 425 K, the latter in agreement with temperature extrapolated literature values. A well-defined ratio A = k20fk21 = 2.1+0.3 was also obtained, together with an estimate of B = k2ia/k2i = 0.4±0.2. A simplified collisional model is used to interpret the values of A and B.