We investigated the efficiency and formation mechanism of ammonia generation in recombining plasmas generated from mixtures of N 2 and H 2 under various plasma conditions. In contrast to the Haber-Bosch process, in which the molecules are dissociated on a catalytic surface, under these plasma conditions the precursor molecules, N 2 and H 2 , are already dissociated in the gas phase. Surfaces are thus exposed to large fluxes of atomic N and H radicals. The ammonia production turns out to be strongly dependent on the fluxes of atomic N and H radicals to the surface. By optimizing the atomic N and H fluxes to the surface using an atomic nitrogen and hydrogen source ammonia can be formed efficiently, i.e., more than 10% of the total background pressure is measured to be ammonia. The results obtained show a strong similarity with results reported in literature, which were explained by the production of ammonia at the surface by stepwise addition reactions between adsorbed nitrogen and hydrogen containing radicals at the surface and incoming N and H containing radicals. Furthermore, our results indicate that the ammonia production is independent of wall material. The high fluxes of N and H radicals in our experiments result in a passivated surface, and the actual chemistry, leading to the formation of ammonia, takes place in an additional layer on top of this passivated surface.
Abstract. A low pressure recombining Ar plasma to which mixtures of N 2 and O 2 were added has been studied to explore the relevance of surface related processes for the total chemistry. The abundances of the stable molecules N 2 , O 2 , NO, N 2 O and NO 2 have been measured by means of a combination of infrared tunable diode laser absorption spectroscopy and mass spectrometry.A gas phase chemical kinetics model was developed in chemkin to investigate the contribution of homogeneous interactions to the conversion of the feedstock gases N 2 and O 2 . At a partial pressure of N 2 plus O 2 less than 8 Pa, significant discrepancies between measured and calculated concentrations of N 2 O and NO 2 were observed, indicating that heterogeneous processes are dominating the chemistry in this pressure range. At a partial pressure of N 2 plus O 2 higher than 40 Pa and a relatively high fraction of admixed O 2 we observed a fair agreement between measured and calculated concentrations of NO molecules, indicating that homogeneous processes (notably Natom induced) are more dominant than heterogeneous processes.
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