Direct Laser Absorption Measurements of NH₃-Relevant Pyrolysis Reactions

The pyrolysis kinetics of NH₃ and NH₃-relevant radicals are of importance for the implementation of NH₃ as a fuel. The pyrolysis of NH₂ radicals was therefore studied behind reflected shock waves using 7-243 ppm of N₂H₄ to generate NH₂ in bath gases of Ar and N₂. Post-shock conditions spanned 2900-4600 K and 0.41-0.94 atm. Sensitive laser absorption diagnostics allowed NH and NH₂ concentrations to be monitored simultaneously under highly dilute conditions. Sensitivities of measured time-histories to secondary reactions were therefore suppressed, enabling low-uncertainty measurements of reaction rates for NH₂ + M ↔ NH + H + M (R1), NH + M ↔ N + H + M (R2), and the first direct, high-temperature measurements of NH + H ↔ N + H₂ (R3) in the written direction. The first chaperon efficiency measurements for N₂ relative to Ar (η_N2) were obtained for R1 and R2. Rate constant expressions and their associated temperature ranges as well as η_N2 values are summarized below: $k_{{\mathrm{NH}}_2+\mathrm{Ar}\leftrightarrow \mathrm{NH}+H+\mathrm{Ar}\left(T\right)={10}^{14.6\pm 0.4}\exp (-(73\pm 6)/\left(RT\right)){\mathrm{cm}}^3{\mathrm{mol}}^{-1}{s}^{-1}(2900-4600K,{\eta }_{N_2}=4.4\pm 0.7)}$ $k_{\mathrm{NH}+\mathrm{Ar}\leftrightarrow N+H+\mathrm{Ar}\left(T\right)={10}^{14.0\pm 0.2}\exp (-(71\pm 3)/\left(RT\right)){\mathrm{cm}}^3{\mathrm{mol}}^{-1}{s}^{-1}(2500-4600K,{\eta }_{N_2}\approx 4\pm 2)}$ $k_{\mathrm{NH}+H\leftrightarrow N+H_2\left(T\right)={10}^{15.1\pm 0.3}\exp (-(17\pm 4)/\left(RT\right)){\mathrm{cm}}^3{\mathrm{mol}}^{-1}{s}^{-1}(1650-4600K)}$.