Shock Tube Studies of the Reactions of Hydrogen Atoms. I. The Reaction H + NH3 → H2 + NH2

Abstract

The partial equilibrium state following the branched-chain explosion of shock-heated rich H2/O2/diluent mixtures contains a high concentration of H atoms. The conditions under which this state can be used as a source of H atoms for the study of elementary reactions have been investigated. A small amount of NH3 was added to H2/O2/inert gas mixtures in order to measure the rate of the reaction H + NH3 → H2 + NH2. The pseudo-first order decay of NH3 in an approximately ten-fold excess of H atoms was followed by a time-of-flight mass spectrometer which sampled from the reflected shock region in a shock tube. The rate coefficient for this reaction, determined over the temperature range 1500–2150 °K, is 1013.44±0.10 exp −(17 400 ± 1 300 cal mol−1)/RT cm3 mol−1 s−1.It is pointed out that, under certain stated conditions, the method can also be extended to study the rates of elementary reactions involving O atoms and OH radicals. From our experiments, upper limits on the rate coefficients of the reactions OH + NH3 → H2O + NH2 and O + NH3 → OH + NH2 over the temperature range 1620–1920 °K are 8 × 109T0.08 exp (−1100/RT) and 1 × 1013 exp (−6600/RT), respectively.