Thermal reduction of NO by NH₃: Kinetic modeling of the NH₂+NO product branching ratio

The product-branching ratio for the reaction NH₂+NO→HN₂+OH (1) and N₂+H₂O (2) has been determined in the temperature range of 950-1200 K by pyrolysis of Ar-diluted NH₃+NO mixtures and at 1173 and 1200 K by pyrolysis of Ar-diluted NH₃-NO-CO mixtures. Analysis of the pyrolyzed and unpyrolyzed mixtures was carried out by the FTIR spectrometric method. Kinetic modeling of the decay rates of NH₃ and NO and the production rate of CO₂ by varying the branching ratio, α₁=k₁/(k₁+k₂), and keeping the known total rate constant (k_t=k₁+k₂=9.6×10¹⁴T^-0.85 cm³ mol^-1 s^-1) unchanged at each temperature allows determination of the value of α₁ with good reproducibility. Kinetic modeling of α₁, using previously measured H₂O formation data [28] from the NH₃+NO reaction in the temperature range of 1123-1273 K was also carried out. The branching ratio was found to increase rapidly from 0.27 at 950 K to 0.58 at 1273 K, which is fully consistent with the sharply rising trend recently reported from NH₃-NO flame studies, in which α₁∼0.5 at 1500 K and rose to 0.9 at 2000 K. These new findings cast doubt on the much lower values, α₁≤0.17 at T≤1173 K, obtained by laser kinetic measurements.