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

Abstract

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.

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

“…For a detailed model see Refs. 3,30,38 . A quantitative description of NH 3 production is not attempted in the present work since this surface reaction model would require the input of several unknown parameters.…”

“…The formation of NH 3 is quantitatively not yet fully understood, but it is generally assumed that NH 3 is produced mainly by surface reactions at the plasma surrounding wall 3,30,38 . Atomic nitrogen and hydrogen originating from electron-impact dissociation in the plasma are expected to synthesize on the surface resulting in NH x (x = 1, 2) and finally in NH 3 .…”

“…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.…”

Reaction mechanisms of atomic layer deposition of TaNx from Ta(NMe2)5 precursor and H2-based plasmas

“…Our results relate well to a similar study by Van Helden et al on expanding thermal plasma reactors (with different geometries and volumes), the maximum in NH 3 production was found to occur in the same range, i.e., 60% -70% H 2 , and had the same magnitude, i.e., 2% -10% NH 3 produced. 27 Instead of a H 2 -N 2 plasma also an NH 3 plasma can be employed during ALD. When employing such plasma, a large part of the NH 3 is dissociated into H 2 and N 2 which leads to an increase in the total number of particles.…”

Kinetic and Diagnostic Studies of Molecular Plasmas Using Laser Absorption Techniques

“…TDLAS can also be used to measure neutral gas temperatures [2] and to investigate dissociation processes of molecular low temperature plasmas [3][4][5][6]. The main applications of TDLAS until now have been for investigating molecules and radicals in fluorocarbon etching plasmas [2,5,7], in plasmas containing hydrocarbons [6, 8-14, 60,63] and nitrogen, hydrogen and oxygen [58,59,61,62]. A wide variety of low molecular weight free radicals and molecular ions has been detected by TDLAS in purely spectroscopic studies e.g.…”