Reaction between hydrogen atoms and nitrogen dioxide

Abstract

The stoichiometry and mechanism of the overall reaction between H atoms and NO₂ has been studied by e.s.r. spectrometry combined with computer simulation. The stoichiometry 2H + 3NO₂→3NO + O₂+ H₂O customarily assumed in titrating H by NO₂ cannot be relied upon when the titration is conducted in an acid-washed quartz flow-tube. The concentration of NO₂ required precisely to consume an H concentration, [H]₀, of about 10^–9 mol cm^–3 or less can vary from about 1.1 [H]₀ to 1.5 [H]₀ depending on the activity of the surface. The concentrations of OH observed during reaction are also variable. On the H-rich side, the results can be explained by including in the reaction mechanism the surface reaction suggested by previous mass spectrometric work, namely H + OH→H₂O. (5) The kinetics of reaction (5) are complex and, when H is in moderate excess ([H]₀/[NO₂]₀≈1.2), the reaction is effectively zero order in [H]. When excess [NO₂] over [H] is present originally, the concentration of OH in the system is not determined solely by known gas-phase reactions including OH + NO₂+ M and OH + NO + M. Again it appears that one or more surface reactions of OH occur.