Plasma in contact with liquid is widely studied in research involving water purification, nanoparticle synthesis, biomedical applications, and so on. However, plasma–liquid interactions, which are physical and chemical phenomena occurring at a plasma–liquid interface, have not yet been fully understood. To deepen our understanding of the interactions, in this study, we have investigated where specifically short-lived active species generated by plasma react with solutes at the plasma–liquid interface. We adopted chloride (Cl ⁻ ), bromide (Br ⁻ ) and iodide (I ⁻ ) ions as indicators of the reaction region of the short-lived active species, because it is known that these halide ions are located closer to the gas–liquid interface in an order of I ⁻ , Br ⁻ , and Cl ⁻ . A DC argon plasma was irradiated to 2.1 mol l ⁻¹ sodium chloride (NaCl), bromide (NaBr), and iodide (NaI) solutions, and we found that chlorine was not detected after 600 s plasma irradiation while bromine and iodine were observed after 30 s plasma irradiation. To explain the different characteristics of halogen (chlorine, bromine, and iodine) generation, we postulated that, because of the surface activity and low kinetic energy of the short-lived active species generated by plasma, they could react with the solutes only at the topmost layer of the water surface, which is an atomic monolayer in liquid in contact with the gas phase. Based on our suggestion, we could say that the Cl ⁻ is unable to react with the active species into the chlorine because the ions were excluded from the topmost layer, and that, by contrast, the Br ⁻ and I ⁻ were able to react into the bromine and iodine, respectively, because some of these ions were located in the topmost layer. For understanding the plasma–liquid interactions more, a difference between chemical reactions induced by plasma and conventional electrolysis has been also discussed.

A unique nitriding technique with the use of an atmospheric-pressure pulsed-arc plasma jet has been developed to offer a non-vacuum, easy-to-operate process of nitrogen doping to metal surfaces. This technique, however, suffered from a problem of excess nitrogen supply due to the high pressure results in undesirable formation of voids and iron nitrides in the treated metal surface. To overcome this problem, we have first established a method to control the nitrogen dose amount supplied to the steel surface in the relevant nitriding technique. When the hydrogen fraction in the operating gas of nitrogen/hydrogen gas mixture increased from 1% up to 5%, the nitrogen density of the treated steel surface drastically decreased. As a result, the formation of voids were suppressed successfully. The controllability of the nitrogen dose amount is likely attributable to the density of NH radicals existing in the plume of the pulsed-arc plasma jet.

Water purification by non-equilibrium atmospheric pressure plasma has attracted much attention and is expected to be a next-generation method. However, general approaches to improve the energy efficiency of the water purification have not been revealed. Therefore, to investigate important factors for increasing its energy efficiency, we developed coaxial cylindrical plasma reactors where pulsed streamers were generated between a high-voltage electrode and running water film. To evaluate the performance of the plasma reactors, we measured hydroxyl (OH) radicals in solution based on a chemical probe method using disodium terephthalic acid (NaTA) and decolorized indigo carmine solution. Our experimental results showed that the production rate of the OH radicals was approximately 20 nmol/s and that the energy efficiency of the decolorization was on the order of 10 g/kWh. In addition, we found that controlling liquid flow based on the Coandă effect and introducing the intermittent operation of the streamer discharges to use post-discharge reactions increased the energy efficiency by a factor of approximately 3.5, which indicated that these approaches are effective to improve the performance of the water purification by plasma.

Although sulfonated carbon catalysts are considered promising solid acid catalysts for cellulose conversion, most carbon sulfonation processes require concentrated sulfuric acid (18 M) at elevated temperatures. This work investigates a novel sulfonation mechanism for carbon acid catalysts via a gas–liquid interfacial plasma sulfonation system under atmospheric pressure conditions with a dilute sulfuric acid solution (1 M) at 38 °C, and the by-products of the plasma sulfonation process were investigated by in-situ and ex-situ diagnosis. The results show that a high gas temperature (>1050 K) around the plasma allows H₂SO₄ droplet decomposition, and active species (•OH, SO₃, and HOSO₂•) generated at the gas–liquid interface were grafted on the defects of the carbon materials and subsequently formed sulfonic acid groups (0.36 mmol g⁻¹) and total acid groups (4.16 mmol g⁻¹) on the carbon network. This study aimed to provide significant insight into the understanding of the sulfonation mechanisms of an emerging plasma-based process for carbon acid catalysts, which is important for the further development of an environmentally friendly sulfonation process for acid catalysts for biomass conversion.