Abstract An electronic-specific kinetic model for nitrogen-argon plasmas is developed by the reduction of a state-of-the-art vibronic-specific model. The model is used to study the nonequilibrium recombination of high-temperature nitrogen-argon plasmas, initially in local thermodynamic equilibrium at high temperature and atmospheric pressure, and forced to cool rapidly. Simulations using the electronic-specific model are performed along the axis of the tube. The results are compared with measurements of electron densities, ground state species (N), and several excited electronic states of N, N 2 (B, C), and N 2 + (B), obtained in two sets of experiments. The simulations and measurements generally agree within a factor of 3, while the nonequilibrium degree is typically several orders of magnitude. A detailed analysis of the main processes governing the recombination of the plasma indicates that the transfer of electronic energy from N 2 (A) to N( 2 P) and the three-body recombination of N 2 by N-impact play key roles in the recombination kinetics. We show that accurate predictions of species densities depend primarily on accurate predictions of N 2 (A) and N( 2 P) densities, which must be the focus of future research efforts.

Abstract Hydrogen peroxide (H 2 O 2 ) plays a key role in plasma-induced chemistry for various applications. The mechanisms governing the production of aqueous hydrogen peroxide, H 2 O 2aq , in non-thermal plasma-liquid interactions (PLIs), particularly the significant production rate dependence on liquid electrode polarity, have resulted in persistent controversy in the literature. We conduct spatiotemporal measurements of the gas phase H 2 O 2 density at the interface of a pulsed helium plasma jet impinging on a liquid electrode, using photo-fragmentation laser-induced fluorescence (PF-LIF). We show that gas phase H 2 O 2 densities are unexpectedly similar for both discharge polarities. In contrast, complementary measurements of the liquid phase H₂O₂ aq concentration show that the H₂O₂ aq production is more than tenfold higher for a liquid cathode discharge compared to the liquid anode discharge. While this disparity cannot be reconciled by the solvation of gas phase H 2 O 2 , it can be explained by the enhanced H 2 O 2aq yield in the liquid cathode configuration being predominantly driven by the production of aqueous hydroxyl radicals, OH aq , that are formed by incident positive ions. This hypothesis is further supported by experiments using various OH aq scavengers as well as modelling results. The reported findings highlight that ion-driven liquid-phase chemistry is the dominant mechanism responsible for the polarity dependence in H 2 O 2 synthesis by plasmas in contact with an aqueous electrode.

Abstract Plasma-catalysis holds great potential for efficient CO 2 recycling but the effectiveness of this approach depends on how plasma sources and catalytic materials are coupled. Fluidized bed reactors are interesting because they exhibit increased surface contact area between material particles and gas phase, and improved heat transfer. A low-pressure DC glow discharge in fluidized bed configuration (FB-GD), ignited with or without fluidized Al 2 O 3 particles, is investigated with optical emission spectroscopy. A decrease in oxygen atom density through the fluidization of the material and an increase in the intensity of CO systems, attributed to increased CO density and to a lesser extend to electric field changes, is observed in comparison to the plasma alone. This indicates that fluidized particles indeed cause a reduction in the O presence leading to an increase in CO density. The rotational temperature does not significantly change, despite the more efficient heat transfer to the wall expected in FB-GDs. This is attributed to the higher current density induced by the confinement of the plasma in the center of the tube by the charged particles.
The plasma-assisted catalytic behavior is further investigated by infrared absorption spectroscopy downstream of the FB-GD, showing superior conversion performance compared to the glow discharge alone. The development of this innovative route is crucial to understanding the enhancement of plasma-surface interaction for CO 2 recycling.

Abstract Direct non-oxidative methane (CH 4 ) conversion to value-added hydrogen (H 2 ) and C 2 products remains hindered by fundamental catalytic scaling constraints and rapid surface deactivation at elevated temperatures. Plasma-enabled catalysis offers a promising route to overcome the thermodynamic and kinetic barriers of direct non-oxidative methane upgrading at mild conditions, yet control over C–C product selectivity and catalyst stability remains elusive. Here, we establish a unified mechanistic framework including Langmuir–Hinshelwood (L–H) and Langmuir–Rideal (L–R) mechanisms that disentangles the roles of plasma excitation (including vibrationally activated species and radicals), surface temperature ( T sur ), and catalyst binding energy in steering CH 4 conversion to H 2 and C 2 hydrocarbons. Through a combination of density functional theory (DFT) informed microkinetic modeling, in situ and ex situ surface characterization, and product quantification under dielectric barrier discharge conditions, we show that vibrationally excited CH 4 lowers activation barriers selectively for dissociative chemisorption, enabling surface activation across a wide range of transition metal catalysts at low thermal energy input. We find that once CH 4 is dissociatively chemisorbed, the branching between C 2 H 2 , C 2 H 4 , and C 2 H 6 is governed by surface properties (carbon binding energy, T sur , etc), regardless of plasma excitation. The DFT informed microkinetic model decouples the effects of molecular activation from surface properties and indentifies operating windows that maximize target yields while suppressing carbon accumulation and subsequent catalytic inactivation. Experiments on polycrystalline Cu/Al 2 O 3 , Ni/Al 2 O 3 , and Pt/Al 2 O 3 validate these predictions, revealing catalyst-dependent branching toward ethane or ethylene and distinct deactivation profiles. We unify these trends into a generalized three-dimensional plasma-thermal-catalytic design space, from which reduced descriptors such as T vib / T sur in the limit of vibrationally excited L–H pathways emerge as predictive metrics. These results enable rational tuning of methane conversion pathways and unlock selective C 2 formation using earth-abundant metals under mild plasma conditions.

Abstract By controlling the seed electron concentration through pre-ionization, the transition between glow discharge and arc discharge modes in an argon-mercury gas discharge tube can be effectively regulated. This enables stable operation at extremely low power levels and broadens its application range. This study proposes a control method using dielectric barrier discharge (DBD) as a pre-ionization source, investigating the process from the perspective of discharge mode evolution. By adjusting parameters such as the pre-ionization voltage, frequency, and spatial location, the evolution of discharge modes and plasma characteristics under different conditions were systematically analyzed. The results indicate that this method effectively modulates the mode transition process, expanding the sustainable discharge regime from a single arc mode (negative impedance) to a coexisting state of arc and glow modes (positive impedance). The corresponding minimum maintainable electron density decreased from 1.68 × 10 11 cm -3 to 6.43 × 10 9 cm -3 , and the current lower limit was reduced from 32 mA to 0.45 mA, with all discharges exhibiting hysteresis behavior. Each influencing factor affects the mode transition by altering the power available for seed electron generation. Further investigation into the origin of the mode transition revealed that the strong localized electric field generated by DBD pre-ionization bombards the filament cathode, driving an ionization process that dominates seed electron emission under low electron density conditions. These findings confirm that DBD pre-ionization is the decisive factor in inducing controllable switching between thermionic and field emission modes, providing a feasible strategy for achieving low-density operation and multi-mode discharge control in argon-mercury discharge tubes.

Abstract A detailed characterization of electron parameters, particularly temperature and number density, is essential to understand the physics driving non-equilibrium plasma jets sustained by microwave electromagnetic fields. These parameters are typically extracted from optical emission spectroscopy, a widely used and non-intrusive diagnostic method. Single point measurements or along a line-of-sight to extract a profile are the most applied techniques, while performing 2D spatially resolved measurement continues to be a challenge. Although filter-based imaging techniques offer spatial resolution, spectral information is limited to only a few emission lines. Conversely, acquiring full spectra across a 2D region requires sequential point-by-point scanning, making real-time 2D mapping complex. In this study, we address these limitations by coupling hyperspectral imaging with collisional-radiative modeling to simultaneously extract and map, with an exceptionally high spatial resolution, the electron temperature, electron number density, and argon molecular ion number density. The results are obtained in both contracted (100% Ar) and expanded (Ar + 0.35% Xe) microwave plasma jets expanding into ambient air. The electron temperature, electron density, and molecular ion density exhibit distinct and structured spatial distributions, with electron temperature peaking and molecular ions accumulating at the edges.

Abstract The temporal evolution of the electron energy distribution function (EEDF) in a 2 Hz pulsed positive secondary streamer in atmospheric-pressure air was measured using the laser Thomson scattering (LTS) method. The rotational Raman scattering signal, which was superimposed on the LTS signal, changed because of the fast gas heating in the streamer. The photo-detachment of electrons from negative ions resulted in stronger LTS signal intensities and slower decay rates than expected based on the actual electron density. However, this did not affect the EEDF measurements, because the photo-detached electrons rapidly relaxed thermally. The non-Maxwellian EEDF corresponding to a high reduced electric field E / N ≈ 130 Td was observed in the initial phase of the secondary streamer ( t = 20 ns ), which subsequently decreased to E / N ≈ 80 − 100 Td and remained nearly constant throughout the later phase ( t = 40 − 100 ns ). These findings indicate that the production rates of several reactive species are higher in the initial phase of the secondary streamer, rather than in the later phase. This paper also presents important considerations for the application of the LTS method to atmospheric-pressure plasmas, particularly under conditions in which the air density is significantly higher than the electron density.

Abstract We present a combined 2D-0D model to investigate reactive oxygen and nitrogen species (RONS) production by the kINPen plasma jet, as well as transport of these species throughout the gas and liquid during treatment of a well plate. This treatment setup is typically used in biomedical experiments for plasma treatment of (cancer) cells. Our 2D-axisymmetric CFD model describes various transport phenomena, both in the gas and liquid phase, and provides input for a 0D chemical kinetics simulation of the Ar plasma propagating in humid air. After validation of our model through comparison with experimental data from literature, we simulate the plasma chemistry to compare operation of a free jet (without a treated substrate) with the treatment of well plates of different sizes, to gain a deeper insight into how these well plates can affect RONS production by the plasma. Indeed, while most diagnostic investigations of RONS production by the kINPen are performed with it operating as a free jet, it is mostly used in biomedical experiments by treating a well-plate containing, e.g. a liquid and (cancer) cells. We find that the resulting differences between the conditions in the plasma plume significantly affect the produced RONS mixture. This will affect the deposition of RONS into the liquid and, by extension, the outcome of the cell treatment. While it is important to keep these effects in mind, we thus advise that modelling and diagnostics of plasma jets should be performed for the same conditions and geometry as used for biomedical treatments.