NH3 and NHx

Abstract

NH3 and NHx<3 radicals are produced downstream a microwave discharge containing Ar-N2-H2 gas mixture. The chemical mechanism under investigation consists of heterogenous reactions between adsorbed species NH or NH2 (denoted NHs and NH2s) and H orH2 flowing downstream the discharge. NHs is adsorbed on thestainless steel reactor wall and reacts with H or H2 producing NH2s or . Then, part of NH2s produced reacts with H atoms producing ; another part is desorbed from the tube wall: . We assume that NH3 is spontaneously and totally desorbed. From the balance equations, we determine analytical relations for NH2s, NH2 and NH3 concentrations. We then measure values of reaction rate constants and compare the numerical results to measurements performed in the afterglow by means of mass spectrometer versus the %H2 injected in the discharge. We measure values in two different initial gas mixtures, 98.7% Ar-1.3% N2 and 66.6% Ar-33.3% N2. In the first gas mixture, k1, k2(NHs), k3(NHs) andksg range between 1×10-17 and 2×10-17 m3 s-1, 0.035 and 0.045 m s-1, 9 and 11 m s-1, and 0.30 and 0.35 m-1 s-1, respectively.In the second gas mixture, as expected, similar values are found for k1 and ksg but the other two values increase by a factor of 5. Such an increase for k2(NHs) and k3(NHs) is probably due to the increase of the (NHs) concentration on the reactor wall. The recombination coefficient γ is deduced from the previous rate constant values. We find γ1 = 4.12×10-4, γ2 = 4.91×10-6 and γ3 = 7.93×10-4, using the mean values of reaction rate constants determined for k1, k2 and k3, respectively, in the first gas mixture. To our knowledge, these results have never been published before. They are in good agreement with values given in the literature for other similar mechanisms. Finally, we conclude that the loss of H atoms on the reactor wall mainly results in producingNH2s and NH3.