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

Abstract

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.

Combustion in a Sustainable World: From Molecules to Processes

“…A carbon-free, hydrogen-rich energy vector such as ammonia can be used for hydrogen transport, chemical storage, and as a combustion fuel. Although its pyrolysis and oxidation have been investigated in detail, certain reactions in the ammonia system have remained unexplored, including that of NH 3 + NH 2 , which is the subject of the contribution of Marshall and Glarborg .…”

“…Although its pyrolysis and oxidation have been investigated in detail, certain reactions in the ammonia system have remained unexplored, including that of NH 3 + NH 2 , which is the subject of the contribution of Marshall and Glarborg. 34 In-situ laser diagnostic methods for the quantitative detection of HCN, a toxic byproduct of NO x formation reactions during the combustion of nitrogenous fuels, were developed and refined by Stuhr and Friedrichs, who applied mid-infrared frequency modulation spectroscopy to enable time-resolved, sensitive measurements of this species in shock tube experiments. 35 For the technical application of ammonia as a combustion fuel, its low reactivity must be enhanced by the addition of promoters in the fuel mixture.…”