Investigation of nitrogen fixation in low-pressure microwave plasma via rotational–vibrational NO and N2 kinetics

Abstract

A pulsed microwave surfaguide-type discharge used for nitrogen fixation in N[Formula: see text]–O[Formula: see text] gas mixtures is characterized by optical emission spectroscopy. Results show that both rotational and vibrational temperatures are elevated in the active zone near the waveguide, decaying along the discharge tube in both upstream and downstream. The characteristic length of optical emission from NO([Formula: see text]-[Formula: see text]) transition gets contracted when pressure increases, specifically at [Formula: see text] Torr. The degree of vibrational non-equilibrium (defined as the ratio between vibrational and rotational temperatures) is decreased by a factor of two when pressure changes from 0.6 to 10 Torr. Non-equilibrium likely disappears as the discharge pressure rises, resulting in a gas temperature elevation. A correlation between gas residence time, pulse duration, and characteristic times for different energy transfer channels is discussed. The rotational–vibrational dynamics differs for NO and N[Formula: see text] during the pulse. Both species lose vibrational excitation at the beginning of the pulse, whereas N[Formula: see text] gets re-excited again during the second half of the pulse, which may occur as a result of an efficient pumping-up effect through the vibrational–vibrational energy transfer. At the same time, vibrational relaxation of NO takes place primarily due to a strong vibrational–translational exchange via NO–O[Formula: see text] and NO–O collisions.

Spatio-temporal analysis of power deposition and vibrational excitation in pulsed N2 microwave discharges from 1D fluid modelling and experiments

“…Both emission bands overlap with the intensifier operating range. The wavelength dependence of the intensity of the peaks in each band is related to the population of vibrational levels of N 2 (A,B,C), which is not trivial to predict [12,49]; it is therefore assumed that the shape of the bands does not vary relevantly within the conditions analysed in this work. This allows us to consider the quantum efficiency as constant, meaning the only factor determining the emission profile would be the emitters' density.…”

“…Another way to exploit the different timescales of chemical reactions, vibrational excitation, and gas heating is through the modulation of the input power in time by using a pulsed power input [6,8,11,12]. The end of the non-equilibrium time window measured in a 25 mbar pure nitrogen discharge by van de Steeg et al [11] was attributed to the raise in N atoms number density causing fast quenching of vibrations.…”

“…The end of the non-equilibrium time window measured in a 25 mbar pure nitrogen discharge by van de Steeg et al [11] was attributed to the raise in N atoms number density causing fast quenching of vibrations. In the works by Bahnamiri et al [6,12] the matching between residence time and pulse duration was proposed as a way to achieve optimal NO x yield through vibrational non-equilibrium; this was maintained during the whole pulse due to the vibrationaltranslational (V-T) collision rates being ∼100 times lower than other processes favouring vibrational energy loading into N 2 .…”

Insight in NO synthesis in a gliding arc plasma via gas temperature and density mapping by laser-induced fluorescence

“…The statistical factors were simulated with LIFBASE [26], using the T rot measured by LIF. Considering the pressure and T gas in the GAP, the vibrational-translational relaxation time is estimated to have the order of magnitude of a few µs [27,28]. As, both the time required by the gas to reach the probed region and the typical duration of the arc is of the order of a few ms, T vib = T rot is assumed for the evaluation of Φ v=0 .…”