NH₃and NH_x<3radicals synthesis downstream a microwave discharge sustained in an Ar-N₂-H₂gas mixture. Study of surface reactive processes and determination of rate constants

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 or H2 flowing downstream the discharge. NHs is adsorbed on the stainless 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) and ksg 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 producing NH2s and NH3.

Measurement and modeling of neutral, radical, and ion densities in H2-N2-Ar plasmas

“…The latter is also of interest in nuclear fusion research where N 2 is puffed into the plasma edge of the hydrogen plasma to reduce the local heat load of the so-called divertor region [14][15][16][17][18][19] . Argon addition to H 2 -N 2 plasmas is applied in fusion plasmas 16,17 as well as for industrial nitriding treatments of ferrous and nonferrous materials for improving the mechanical properties or corrosion resistance 20,21 . In many of these plasmas the dominant plasma-surface-interaction processes are driven by radical species or ions or a synergistic interaction, respectively, between them.…”

“…Jang and Lee 29 previously examined a few aspects in an inductively coupled H 2 -N 2 plasma by measuring n e , T e and ion signal intensities. In addition, there exists a variety of studies about H 2 -N 2 plasmas in which the densities of background gas species 13,20,[30][31][32][33] , radicals 7,8,20,32,34 , electrons 34,35 and ion signals 7,36 as well as the surface loss probability of H and N (see Ref. 7 ) were reported.…”

“…Argon admixtures to hydrogen and nitrogen containing plasmas were examined in the studies of Refs. 20,26,31,35,36 .…”

Detailed study of the plasma-activated catalytic generation of ammonia in N2-H2 plasmas

“…The formation of ammonia is generally ascribed to stepwise addition reactions between adsorbed nitrogen and hydrogen containing radicals at the surface and incoming N and H containing radicals. [15][16][17][18] To gain a better understanding of plasma-surface intera͒ Author to whom correspondence should be addressed; Electronic mail: r.engeln@tue.nl actions that take place during the generation of stable molecules, we have chosen to investigate the formation of ammonia in plasmas of mixtures of nitrogen and hydrogen in more detail. Plasmas containing nitrogen and hydrogen are also extensively studied because of their widespread applications in research and industrial environments.…”

Final Project Report for Grant DE-FG03-00ER54581 Selective Control of Chemical Reactions With Plasmas

“…The combination of H 2 , N 2 , and Ar is used commonly in industrial applications and has been explored by several researchers. [26][27][28][29] In this work the combination of H 2 and N 2 shows that the T e is dominated by the presence of N 2 ( Figure 10). The T e in an Ar/3% N 2 plasma was relatively stable with increasing H 2 concentration.…”