On the role of the termolecular reactions 2O2 + H2 → 2HO2 and 2O2 + H2 → H + HO2 + O2 in formation of the first radicals in hydrogen combustion: ab initio predictions of energy barriers

Abstract

We have investigated the role of termolecular reactions in the early chemistry of hydrogen combustion. We performed molecular chemical dynamics simulations using ReaxFF in LAMMPS to identify potential initial reactions for a 1 : 4 mixture of H₂ : O₂ in the NVT ensemble at density 276.3 kg m⁻³ and ∼3000 K (∼4000 atm) and ∼4000 K (∼5000 atm), and then characterized the saddle points for those reactions using ab initio methods: CCSD(T) = FC/cc-pVTZ//MP2/6-31G, CCSD(T) = FULL/aug-cc-pVTZ//CCSD = FC/cc-pVTZ and CASSCF MP2/6-31G//MP2/6-31G. The main initial reaction is H₂ + O₂ → H + HO₂, frequently occurring in the presence of a second O₂ as a third body; that is, 2O₂ + H₂ → H + HO₂ + O₂. The second most frequent reaction is 2O₂ + H₂ → 2HO₂. We found three saddle points on the triplet PES of these termolecular reactions: one for 2O₂ + H₂ → H + HO₂ + O₂ and two for 2O₂ + H₂ → 2HO₂. In the latter case, one has a symmetric structure consistent with simultaneous formation of two HO₂ and the other corresponds to a bimolecular reaction between O₂ and H₂ that is “interrupted” by a second O₂ before going to completion. The classical barrier height of the symmetric saddle point for 2O₂ + H₂ → 2HO₂ is 49.8 kcal mol⁻¹. The barrier to H₂ + O₂ → H + HO₂ is 58.9 kcal mol⁻¹. The termolecular reaction will be competitive with H₂ + O₂ → H + HO₂ only at sufficiently high pressures.