Ammonia oxidation at high pressure and intermediate temperatures

Ammonia oxidation experiments were conducted at high pressure (30 bar and 100 bar) under oxidizing and stoichiometric conditions, respectively, and temperatures ranging from 450 to 925 K. The oxidation of ammonia was slow under stoichiometric conditions in the temperature range investigated. Under oxidizing conditions the onset temperature for reaction was 850–875 K at 30 bar, while at 100 bar it was about 800 K, with complete consumption of NH₃ at 875 K. The products of reaction were N₂ and N₂O, while NO and NO₂ concentrations were below the detection limit even under oxidizing conditions. The data were interpreted in terms of a detailed chemical kinetic model. The rate constant for the reaction of the important intermediate H₂NO with O₂ was determined from ab initio calculations to be 2.3 × 10² T^2.994 exp (−9510 K/T) cm³ mol⁻¹ s⁻¹. The agreement between experimental results and model work was satisfactory. The main oxidation path for NH₃ at high pressure under oxidizing conditions is NH₃⟶^+OH NH₂⟶^+HO₂,NO₂ H₂NO⟶^+O₂ HNO⟶^+O2 NO ⟶^+NH₂ N₂. The modeling predictions are most sensitive to the reactions NH₂ + NO = NNH + OH and NH₂ + HO₂ = H₂NO + OH, which promote the ammonia consumption by forming OH radicals, and to NH₂ + NO = N₂ + H₂O and NH₂ + NO₂ = N₂O + H₂O, which are the main chain-terminating steps.