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

The temperature- and pressure-dependent rate constants of NH₂ + ¹O₂ reaction are investigated theoretically for the first time. The importance of the calculated reactions for the ignition delay time of ammonia and the consumption pathways of NH₂ radical in ammonia oxidation is investigated by kinetic simulations.

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(a ¹Δ _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(a ¹Δ _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(a ¹Δ _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(a ¹Δ _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.

Zum Volltext