Probing High-Temperature Amine Chemistry: Is the Reaction NH3 + NH2 ⇄ N2H3 + H2 Important?

Abstract

The reaction NH3 + NH2 ⇄ N2H3 + H2 (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 k1 of 1.5 × 1013 exp(-58.9 kcal mol-1/RT) cm3 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 NH3 pyrolysis data supported a very low value of k1. However, this finding opens up a novel unresolved issue. Current kinetic models cannot capture the NH3 oxidation behavior in a number of laminar flow reactor and jet-stirred reactor experiments without adopting an improbably high value for k1. Important oxidation steps might be underestimated or missing from mechanisms.