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 (ηN2) were obtained for R1 and R2. Rate constant expressions and their associated temperature ranges as well as ηN2 values are summarized below: kNH2+Ar↔NH+H+Ar(T)=1014.6±0.4⁡exp⁡⁡(−(73±6)/(RT))cm3mol−1s−1(2900−4600K,ηN2=4.4±0.7) kNH+Ar↔N+H+Ar(T)=1014.0±0.2⁡exp⁡⁡(−(71±3)/(RT))cm3mol−1s−1(2500−4600K,ηN2≈4±2) kNH+H↔N+H2(T)=1015.1±0.3exp⁡⁡(−(17±4)/(RT))cm3mol−1s−1(1650−4600K)