Abstract: above atmospheric) pressure regimes (1–3.5 bar). It was demonstrated that these operational conditions significantly influence both the discharge dynamics and the process efficiencies of O2 and CO2 discharges. For the case of the O2 DBD, the pressure rise results in the amplification of the discharge current, the appearance of emission lines of the metal electrode material (Fe, Cr, Ni) in the optical emission spectrum and the formation of a granular film of the erosion products (10–300 nm iron oxide nanoparticles) on the reactor walls. Somewhat similar behavior was observed also for the CO2 DBD. The discharge current, the relative intensity of the CO Angstrom band measured by Optical Emission Spectroscopy (OES) and the CO2 conversion rates could be stimulated to some extent by the rise in pressure. The optimal conditions for the O2 DBD (P = 2 bar) and the CO2 DBD (P = 1.5 bar) are demonstrated. It can be argued that the dynamics of the microdischarges (MD) define the underlying process of this behavior. It could be
demonstrated that the pressure increase stimulates the formation of more intensive but fewer MDs. In this way, the operating pressure can represent an additional tool to manipulate the properties of the MDs in a DBD, and as a result also the discharge performance.

Abstract: Understanding the nature and effect of the multitude of plasma–surface interactions in plasma catalysis is a crucial requirement for further process development and improvement. A particularly intriguing and rather unique property of a plasma-catalytic setup is the ability of the plasma to modify the electronic structure, and hence chemical properties, of the catalyst through charging, i.e. the absorption of excess electrons. In this work, we develop a quantum chemical model based on density functional theory to study excess negative surface charges in a heterogeneous catalyst exposed to a plasma. This method is specifically applied to investigate plasma-catalytic CO2 activation on supported M/Al2O3 (M=Ti, Ni, Cu) single atom catalysts. We find that (1) the presence of a negative surface charge dramatically improves the reductive power of the catalyst, strongly promoting the splitting of CO2 to CO and oxygen, and (2) the relative activity of the investigated transition metals is also changed upon charging, suggesting that controlled surface charging is a powerful additional parameter to tune catalyst activity and selectivity. These results strongly point to plasma-induced surface charging of the catalyst as an important factor contributing to the plasma-catalyst synergistic effects frequently reported for plasma catalysis.

Abstract: Although plasma catalysis is gaining increasing interest for various environmental applications, the underlying mechanisms are still far from understood. For instance, it is not yet clear whether and how plasma streamers can propagate in catalyst pores, and what is the minimum pore size to make this happen. As this is crucial information to ensure good plasma-catalyst interaction, we study here the mechanism of plasma streamer propagation in a catalyst pore, by means of a twodimensional particle-in-cell/Monte Carlo collision model, for various pore diameters in the nm range to μm-range. The so-called Debye length is an important criterion for plasma penetration into catalyst pores, i.e. a plasma streamer can penetrate into pores when their diameter is larger than the Debye length. The Debye length is typically in the order of a few 100 nm up to 1 μm at the conditions under study, depending on electron density and temperature in the plasma streamer. For pores in the range of ∼50 nm, plasma can thus only penetrate to some extent and at
very short times, i.e. at the beginning of a micro-discharge, before the actual plasma streamer reaches the catalyst surface and a sheath is formed in front of the surface. We can make plasma streamers penetrate into smaller pores (down to ca. 500 nm at the conditions under study) by increasing the applied voltage, which yields a higher plasma density, and thus reduces the Debye length. Our simulations also reveal that the plasma streamers induce surface charging of the catalyst pore sidewalls, causing discharge enhancement inside the pore, depending on pore diameter and depth.

Abstract: Electrospinning is a versatile technique for the fabrication of polymer-based nano/microfibers. Both physical and chemical characteristics of pre-electrospinning polymer solutions affect the morphology and chemistry of electrospun nanofibers. An atmospheric-pressure plasma jet has previously been shown to induce physical modifications in polylactic acid (PLA) solutions. This work aims at investigating the plasma-induced chemistry in organic solutions of PLA, and their effects on the resultant PLA nanofibers. Therefore, very broad range of gas, liquid, and solid (nanofiber) analyzing techniques has been applied. Plasma alters the acidity of the solutions. SEM studies illustrated that complete fiber morphology enhancement only occurred when both PLA and solvent molecules were exposed to preelectrospinning plasma treatment.
Additionally, the surface
chemistry of the PLA nanofibers
was mostly preserved.

Abstract: Plasma generation inside catalyst pores is of utmost importance for plasma catalysis, as the existence of plasma species inside the pores affects the active surface area of the catalyst available to the plasma species for catalytic reactions. In this paper, the electric field enhancement, and thus the plasma production inside catalyst pores with different pore shapes is studied with a two-dimensional fluid model. The results indicate that the electric field will be significantly enhanced near tip-like structures. In a conical pore with small opening, the strongest electric field appears at the opening and bottom corners of the pore, giving rise to a prominent ionization rate throughout the pore. For a cylindrical pore, the electric field is only enhanced at the bottom corners of the pore, with lower absolute value, and thus the ionization rate inside the pore is only slightly enhanced. Finally, in a conical pore with large opening, the electric field is characterized by a maximum at the bottom of the pore, yielding a similar behavior for the ionization rate. These results demonstrate that the shape of the pore has a significantly influence on the electric field enhancement, and thus modifies the plasma properties.

Abstract: Climate change, environmental pollution control, and resource utilization efficiency, as well as food security, sustainable agriculture, and water supply are among the main challenges facing society today. Expertise across different academic fields, technologies,anddisciplinesisneededtogeneratenewideastomeetthesechallenges. This “white paper” aims to provide a written summary by describing the main aspects and possibilities of the technology. It shows that plasma science and technology can make significant contributions to address the mentioned issues. The paper also addresses to people in the scientific community (inside and outside plasma science) to give inspiration for further work in these fields.

Abstract: In this work we study energy transport in a gliding arc discharge with two diverging flat
electrodes in argon gas at atmospheric pressure. The discharge is ignited at the shortest electrode
gap and it is pushed downstream by a forced gas flow. The current values considered are
relatively low and therefore a non-equilibrium plasma is produced. We consider two cases, i.e.
with high and low discharge current (28 mA and 2.8mA), and a constant gas flow of 10 lmin −1 ,
with a significant turbulent component to the velocity. The study presents an analysis of the
various energy transport mechanisms responsible for the redistribution of Joule heating to the
plasma species and the moving background gas. The objective of this work is to provide a
general understanding of the role of the different energy transport mechanisms in arc formation
and sustainment, which can be used to improve existing or new discharge designs. The work is
based on a three-dimensional numerical model, combining a fluid plasma model, the shear stress
transport Reynolds averaged Navier–Stokes turbulent gas flow model, and a model for gas
thermal balance. The obtained results show that at higher current the discharge is constricted
within a thin plasma column several hundred kelvin above room temperature, while in the low-
current discharge the combination of intense convective cooling and low Joule heating prevents
discharge contraction and the plasma column evolves to a static non-moving diffusive plasma,
continuously cooled by the flowing gas. As a result, the energy transport in the two cases is
determined by different mechanisms. At higher current and a constricted plasma column, the
plasma column is cooled mainly by turbulent transport, while at low current and an unconstricted
plasma, the major cooling mechanism is energy transport due to non-turbulent gas convection. In
general, the study also demonstrates the importance of turbulent energy transport in
redistributing the Joule heating in the arc and its significant role in arc cooling and the formation
of the gas temperature profile. In general, the turbulent energy transport lowers the average gas
temperature in the arc, thus allowing additional control of thermal non-equilibrium in the
discharge.

Abstract: Plasma catalysis is gaining increasing interest for various environmental applications. Catalytic
material can be inserted in different shapes in the plasma, e.g., as pellets, (coated) beads, but also
as honeycomb monolith and 3DFD structures, also called ‘structured catalysts’, which have high
mass and heat transfer properties. In this work, we examine the streamer discharge propagation
and the interaction between plasma and catalysts, inside the channels of such structured catalysts,
by means of a two-dimensional particle-in-cell/Monte Carlo collision model. Our results reveal
that plasma streamers behave differently in various structured catalysts. In case of a honeycomb
structure, the streamers are limited to only one channel, with low or high plasma density when
the channels are parallel or perpendicular to the electrodes, respectively. In contrast, in case of a
3DFD structure, the streamers can distribute to different channels, causing discharge
enhancement due to surface charging on the dielectric walls of the structured catalyst, and
especially giving rise to a broader plasma distribution. The latter should be beneficial for plasma
catalysis applications, as it allows a larger catalyst surface area to be exposed to the plasma.

Abstract: A gliding arc (GA) plasma has great potential for CO2 conversion into value-added chemicals, because of its high energy efficiency. To improve the application, a 2D/3D fluid model is needed to investigate the CO2 conversion mechanisms in the actual discharge geometry. Therefore, the complex CO2 chemical kinetics description must be reduced due to the huge computational cost associated with 2D/3D models. This paper presents a chemistry reduction method for CO2 plasmas, based on the so-called directed relation graph method. Depending on the defined threshold values, some marginal species are identified. By means of a sensitivity analysis, we can further reduce the chemistry set by removing one by one the marginal species. Based on the socalled flux-sensitivity coupling, we obtain a reduced CO2 kinetics model, consisting of 36 or 15 species (depending on whether the 21 asymmetric mode vibrational states of CO2 are explicitly included or lumped into one group), which is applied to a GA discharge. The results are compared with those predicted with the full chemistry set, and very good agreement is reached. Moreover, the range of validity of the reduced CO2 chemistry set is checked, telling us that this reduced set is suitable for low power GA discharges. Finally, the time and spatial evolution of the CO2 plasma characteristics are presented, based on a 2D model with the reduced kinetics.

Abstract: This study focuses on a magnetically stabilized gliding arc (MGA) plasma. Two fully coupled flow-plasma models (in 3D and 2D) are presented. The 3D model is applied to compare the arc dynamics of the MGA with a traditional gas-driven gliding arc. The 2D model is used for a detailed parametric study on the effect of the external magnetic field. The results show that the relative velocity between the plasma and feed gas is generated due to the Lorentz force, which can increase the plasma-treated gas fraction. The magnetic field also helps to decrease the gas temperature by enhancing heat transfer and to increase the electron number density. This work shows the potential of an external magnetic field to control the gliding arc behavior, for enhanced gas conversion at low gas flow rates.

Abstract: We studied hydrogen sulfide (H2S) decomposition into hydrogen (H2) and sulfur (S2) in a gliding arc plasmatron (GAP) and microwave (MW) plasma by a combination of 0D and 2D models. The conversion, energy efficiency, and plasma distribution are examined for different discharge conditions, and validated with available experiments from literature. Furthermore, a comparison is made between GAP and MW plasma. The GAP operates at atmospheric pressure, while the MW plasma experiments to which comparison is made were performed at reduced pressure. Indeed, the MW discharge region becomes very much contracted near atmospheric pressure, at the conditions under study, as revealed by our 2D model. The models predict that thermal reactions play the most important role in H2S decomposition in both plasma types. The GAP has a higher energy efficiency but lower conversion than the MW plasma at their typical conditions. When compared at the same conversion, the GAP exhibits a higher energy efficiency and lower energy cost than the MW plasma.

Abstract: The spatiotemporal dynamics of volume and surface positive and negative streamers in a pintoplate volume dielectric barrier discharge is investigated in this study. The discharge characteristics are found to be completely different for positive and negative streamers. First, the spatial propagation of a positive streamer is found to rely on electron avalanches caused by photo-electrons in front of the streamer head, whereas this is not the case for negative streamers. Second, our simulations reveal an interesting phenomenon of floating positive surface discharges, which develop when a positive streamer reaches a dielectric wall and which explain the experimentally observed branching characteristics. Third, we report for the first time, the interactions between a positive streamer and dielectric pores, in which both the pore diameter and depth affect the evolution of a positive streamer.

Abstract: A pulsed microwave surfaguide discharge operating at 2.45 GHz was used for the conversion of molecular nitrogen into valuable compounds in several gas mixtures: N2 :O2 , N2 :O2 :CO2 and N2 :CO2 . The ro-vibrational absorption bands of the molecular species were monitored by a Fourier transform infrared apparatus in the post-discharge region in order to evaluate the relative number density of species, specifically NO production. The effects of specific energy input, pulse frequency, gas flow fraction, gas admixture and gas flow rate were studied for better understanding and optimization of the NO production yield and the corresponding energy cost (EC). By both the experiment and modelling, a highest NO yield is obtained at N2 :O2 (1:1) gas ratio in N2 :O2 mixture. The NO yield reveals a small growth followed by saturation when pulse repetition frequency increases. The energy efficiency start decreasing after the energy input reaches about 5 eV/molec, whereas the NO yield rises steadily at the same time. The lowest EC of about 8 MJ mol−1 corresponding to the yield and the energy efficiency of about 7% and 1% are found, respectively, in an optimum discharge condition in our case.

Abstract: Nanosecond pulsed discharge plasma shows a high degree of non-equilibrium, and exhibits relatively high conversions in the dry reforming of methane. To further improve the application, a good insight of the underlying mechanisms is desired. We developed a chemical kinetics model to explore the underlying plasma chemistry in nanosecond pulsed discharge. We compared the calculated conversions and product selectivities with experimental results, and found reasonable agreement in a wide range of specific energy input. Hence, the chemical kinetics model is able to provide insight in the underlying plasma chemistry. The modeling results predict that the most important dissociation reaction of CO₂and CH₄is electron impact dissociation. C₂H₂is the most abundant hydrocarbon product, and it is mainly formed upon reaction of two CH₂radicals. Furthermore, the vibrational excitation levels of CO₂contribute for 85% to the total dissociation of CO₂.

Abstract: The use of microfluidic devices in the field of plasma-liquid interaction can unlock unique possibilities to investigate the effects of plasma-generated reactive species for environmental and biomedical applications. So far, very little simulation work has been performed on microfluidic devices in contact with a plasma source. We report on the modelling and computational simulation of physical and chemical processes taking place in a novel plasma-microfluidic platform. The main production and transport pathways of reactive species both in plasma and liquid are modelled by a novel modelling approach that combines 0D chemical kinetics and 2D transport mechanisms. This combined approach, applicable to systems where the transport of chemical species occurs in unidirectional flows at high Péclet numbers, decreases calculation times considerably compared to regular 2D simulations. It takes advantage of the low computational time of the 0D reaction models while providing spatial information through multiple plug-flow simulations to yield a quasi-2D model. The gas and liquid flow profiles are simulated entirely in 2D, together with the chemical reactions and transport of key chemical species. The model correctly predicts increased transport of hydrogen peroxide into the liquid when the microfluidic opening is placed inside the plasma effluent region, as opposed to inside the plasma region itself. Furthermore, the modelled hydrogen peroxide production and transport in the microfluidic liquid differs by less than 50% compared with experimental results. To explain this discrepancy, the limits of the 0D–2D combined approach are discussed.