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.