A Kinetic Study About the Reactions of NH₂(X̄²B₁) Radicals with Saturated Hydrocarbons in the Gas Phase

Abstract

The reactions of NH2(X̄2B1) with the hydrocarbons methane (1), ethane (2) and propane (3) were investigated behind incident shock waves at temperatures between 1500 K and 2100 K. The decay of NH2 was measured by laser absorption. NH2 was formed by the thermal decomposition of hydrazine. The following rate constants were obtained for the temperature range mentioned: k1 = (1.2±0.6)·1013 exp(‐(63.4±3.4) kJ/mol/RT) cm3 mol−1 s−1 k2 = (9.7±0.3)·1012 exp(‐(48.0±1.8) kJ/mol/RT) cm3 mol−1 s−1 k3 = (1.7±0.4)·1013 exp(‐(44.6±2.0) kJ/mol/RT) cm3 mol−1 s−1. These measurements extend the results obtained at low temperatures by Demissy and Lesclaux (330 K–250 K) and by Hack et al. (400 K–1080 K) into the range of interest for combustion processes. The apparent energies of activation fit rather well into an Evans Polanyi plot. They correlate with the energies of activation of the corresponding reactions of OH(X2II)and those of CH3(X2A2) radicals.

A shock tube study of the reaction NH2 + CH4 → NH3 + CH3 and comparison with transition state theory

“…Mebel and Lin [5] have computed potential energy surfaces (PES) for this and several other abstractions of H-atoms from alkanes by NH 2 radicals. The data reported in [4] are one order of magnitude lower than the calculated k 1 -values from [5]. Thus, there is considerable uncertainty in the value of the rate coefficient for reaction (R1) at higher temperatures.…”

“…As seen in Fig. 2, the influence of other secondary reactions are almost negligible because most of the NH 2 radicals produced by (R1) react with excess CH 4 . For our experimental conditions, the interpretation of experimental data is straightforward as the reaction is essentially pseudo-first-order, as evidenced by the fact that variation of the initial CH 4 concentration does not have a significant impact on the results as shown in Fig.…”

“…The measured values of k 1 are tabulated in Table I together with experimental conditions, and they are plotted in Fig. 3 together with the data from [2] and [4]. In Fig.…”

“…[3] studied this reaction in an isothermal discharge flow reactor using laser-induced fluorescence detection of NH 2 in the temperature range 743-1023 K. Hennig and Wagner [4] measured k 1 -values in the temperature range 1740-2140 K by monitoring NH 2 radicals behind incident shock waves, and reported k 1 -values that are a factor of 2 lower than in [2] and with a smaller temperature dependence. Mebel and Lin [5] have computed potential energy surfaces (PES) for this and several other abstractions of H-atoms from alkanes by NH 2 radicals.…”

“…Thus, there is considerable uncertainty in the value of the rate coefficient for reaction (R1) at higher temperatures. The objectives of this study were to measure the rate coefficient in the temperature range similar to that in [4] and to compare all the data with transition state theory by calculating the PES, and applying transition state theory, making an appropriate tunneling correction, in order to compute rate coefficients.…”