The Reaction between Sodium Hydroxide and Atomic Hydrogen in Atmospheric and Flame Chemistry

We report the first direct kinetic study of the gas-phase reaction NaOH + H → Na + H₂O, which is central to the chemistry of sodium in the upper atmosphere and in flames. The reaction was studied in a fast flow tube, where NaOH was observed by multiphoton ionization and time-of-flight mass spectrometry, yielding k(NaOH + H, 230-298 K) = (3.8 ± 0.8) × 10^-11 cm³ molecule ^-1 s^-1 (at 2σ confidence level), showing no significant temperature dependence over the indicated temperature range and essentially in agreement with previous estimates of the rate constant in hydrogen-rich flames. We show, using theoretical trajectory calculations, that the unexpectedly slow, yet T-independent, rate coefficient for NaOH + H is explained by severe constraints in the angle of attack that H can make on NaOH to produce H₂O. This reaction is also central to explaining Na-catalyzed flame inhibition, which has been proposed to occur via the sequence Na + OH (+ M) → NaOH followed by NaOH + H → Na + H₂O, thereby effectively recombinating H and OH to H₂O. RRKM calculations for the recombination of Na and OH yield k(Na + OH + N₂, 300-2400 K) = 2.7 × 10^-29 (300/T)^1.2 cm⁶ molecule^-2 s^-1, in agreement with a previous flash photolysis measurement at 653 K and Na-seeded flame studies in the 1800-2200 K range. These results therefore provide strong evidence to support the mechanism of flame inhibition by Na.