Laser-initiated NO reduction by NH₃: Total rate constant and product branching ratio measurements for the NH₂ + NO reaction

The total rate constant for the NH₂ + NO reaction has been measured in the temperature range 305-1037 K using a laser photolysis mass spectrometric technique by probing the rates of H₂O formation and NO decay. A weighted least-squares analysis of our data gives the following total rate constant expression: k_t = 8.29 × 10¹³T^-0.57 exp(300/T) cm³/(mol s). The branching ratios for the two product channels of the NH₂ + NO reaction, N₂H + OH (α) and N₂ + H₂O (β), have also been measured in the temperature range 300-1200 K by detecting CO₂ (formed by the reaction of OH as the added CO) and NO for α and H₂O for β. The value of α was found to increase gradually from 0.1 at 300 K to 0.28 at 1000 K, with a concomitant decrease in β from 0.9 at 300 K to 0.72 at 1000 K. At temperatures between 1000 and 1200 K, α rapidly increases to 0.47 according to the result of our modeling of observed NO decay rates. The drastic upturn in the value of α above 1000 K confirms the results of our recent study of the NH₃ + NO reaction by FTIR spectrometry as well as the conclusion reached by modeling of NH₃-NO flame speeds that α ≥ 0.5 above 1500 K. The absolute rate constants of the two branching reactions are recommended for future applications over the temperature range 300-2000 K.