Neutral and ion chemistry in low pressure dc plasmas of H2/N2 mixtures: routes for the efficient production of NH3 and NH4+

Abstract

The chemistry in low pressure (0.8-8 Pa) plasmas of H(2) + 10% N(2) mixtures has been experimentally investigated in a hollow cathode dc reactor using electrical probes for the estimation of electron temperatures and densities, and mass spectrometry to determine the concentration of ions and stable neutral species. The analysis of the measurements by means of a kinetic model has allowed the identification of the main physicochemical mechanisms responsible for the observed distributions of neutrals and ions and for their evolution with discharge pressure. The chemistry of neutral species is dominated by the formation of appreciable amounts of NH(3) at the metallic walls of the reactor through the successive hydrogenation of atomic nitrogen and nitrogen containing radicals. Both Eley-Rideal and Langmuir-Hinshelwood mechanisms are needed in the chain of hydrogenation steps in order to account satisfactorily for the observed ammonia concentrations, which, in the steady state, are found to reach values ~30-70% of those of N(2). The ionic composition of the plasma, which is entirely due to gas-phase processes, is the result of a competition between direct electron impact dissociation, more relevant for high electron temperatures (lower pressures), and ion-molecule chemistry that prevails for the lower electron temperatures (higher pressures). At the lowest pressure, products from the protonation of the precursor molecules (H(3)(+), N(2)H(+) and NH(4)(+)) and others from direct ionization (H(2)(+) and NH(3)(+)) are found in comparable amounts. At the higher pressures, the ionic distribution is largely dominated by ammonium. It is found that collisions of H(3)(+), NH(3)(+) and N(2)H(+) with the minor neutral component NH(3) are to a great extent responsible for the final prevalence of NH(4)(+).

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

“…Plasmas containing H 2 -N 2 are extensively applied for thin film growth and material processing on the nanometer scale [1][2][3] . For example, thin films of silicon nitride which are fabricated by plasma-enhanced chemical vapor deposition (PECVD) in the semiconductor industry are used as passivation layers 4 and in the photovoltaic industry 3,5 .…”

Chemistry in glow discharges of H2/O2mixtures: diagnostics and modelling

“…Similar models applied to H 2 +D 2 , H 2 +N 2 and H 2 +Ar discharges can be found in refs. [37][38][39][40]. The model accounts for the main physico-chemical processes: electron impact dissociation and ionization, ion-molecule and ion-ion reactions, neutralization at the wall and heterogeneous chemistry.…”

“…In the steady state, a small fraction of these hot atoms survives and can be detected either in the line profiles of Balmer emission lines of H atoms [81] or indirectly in the energy resolved mass spectra of H + and O + ions [81][82][83]. However, previous works carried out on this reactor for different gas mixtures at comparable pressures [33,34,[36][37][38][39] have shown that the assumption of a Maxwellian distribution is not a bad approximation for the global kinetics, and we have adopted it here for lack of better information on the actual eedf. The limitations of this approximation should be taken into account in the interpretation of the results.…”

“…The kinetic description for heterogeneous reactions follows the formulation employed in refs. [37,38]. A detailed account of the treatment of diffusion of neutral species to the wall in the model can be found in ref.…”

“…Dissociative adsorption of O 2 at the walls, which has been experimentally studied by molecular beams and modelled on clean Fe surfaces [87,88], has been tested with model simulations and has very small influence on the present H 2 / O 2 plasmas. In previous works [37][38][39], adsorptions of neutral molecules were considered negligible as compared with those of atoms and radicals. Literature calculations suggest that the adsorption of water on surface iron atoms can take place without a barrier [89,90].…”