Probing High-Temperature Amine Chemistry: Is the Reaction NH 3 + NH 2 ⇄ N 2 H 3 + H 2 Important?

The reaction NH 3 + NH 2 ⇄ N 2 H 3 + H 2 (R1) has been identified as a key step to explain experimental results for pyrolysis and oxidation of ammonia. However, no direct experimental or theoretical evidence for the reaction has been reported. In the present work, the reaction was studied by ab initio theory and by reinterpretation of experimental data. We could not locate a transition state for R1 occurring as a direct process, but alternative mechanisms yielded an upper bound to k 1 of 1.5 × 10 13 exp(-58.9 kcal mol -1 / RT ) cm 3 mol -1 s -1 over 1000-2500 K, several orders of magnitude lower than values applied in modeling. Consistent with the theoretical work, re-evaluation of NH 3 pyrolysis data supported a very low value of k 1 . However, this finding opens up a novel unresolved issue. Current kinetic models cannot capture the NH 3 oxidation behavior in a number of laminar flow reactor and jet-stirred reactor experiments without adopting an improbably high value for k 1 . Important oxidation steps might be underestimated or missing from mechanisms.