Exploring the Reaction Mechanisms and Kinetics of NH2 Radical with Singlet and Triplet O2 Molecules: Implications for Modeling Ammonia Oxidation.

Abstract

The detailed mechanism of how the electronically excited species of air plasma promote the combustion of ammonia remains unclear. Herein, the reactions of NH₂ radical with both singlet and triplet oxygen molecules are investigated by high-level theoretical calculations. A minimal-energy crossing point is found on the ²A″ and ²A' potential energy surfaces of H₂NOO radical, which plays an important role on the product distributions of NH₂ + O₂(a¹Δ_g) reaction. The temperature- and pressure-dependent rate constants of the reactions are predicted by Rice-Ramsperger-Kassel-Marcus theory and master equation simulations. Subsequently, the influence of O₂(a¹Δ_g) on the ignition delay time of ammonia is assessed by updating the existing ammonia combustion model with the computed reaction parameters. The results indicate that 5% O₂(a¹Δ_g) in the total oxygen dramatically accelerate the ignition of ammonia by more than one order of magnitude at ≈1000 K and 1 atm, mainly via the NH₂ + O₂(a¹Δ_g) = H₂NO + ³O reaction. Further, the catalytic effects of ammonia, water, and formic acid on the isomerization of the adduct, H₂NOO radical, of NH₂ + O₂ are studied, which will have implications for modeling the oxidation kinetics of ammonia in both atmospheric and combustion conditions.