Overlooked hydroperoxyl radical reactions in ammonia oxidation under combustion conditions†

The present work focuses on highlighting bimolecular reactions in the NH₃ system involving HO₂, an important radical at intermediate combustion temperatures. The reaction mechanism generator (RMG) tool was used to identify potentially significant reactions, and the automated rate calculator (ARC) tool was used to automatically compute rate coefficients at the ΛCCSD(T)/aug-cc-pVTZ-F12//B2PLYP-D3/aug-cc-pVTZ level of theory. Several reactions explored in this work, such as N + HO₂ ⇌ NH + O₂, NH + HO₂ ⇌ NH₂ + O₂, NNH + HO₂ ⇌ N₂H₂ + O₂, and HNO₂ + HO₂ ⇌ NO₂ + H₂O₂, have not been thoroughly investigated in the existing literature. In particular, the reaction HNO + HO₂ ⇌ HNOH + O₂, though known, lacks a precise rate coefficient in recent chemical kinetic models for ammonia. This study provides computed rate coefficients for 10 hydrogen abstraction and disproportionation reactions involving HO₂ in the NH₃ system. The reaction rate coefficients computed here may improve future low- and intermediate-temperature oxidation models of NH₃.