Session IR4: Plasma Liquid Interaction III

Generating enhanced chemical reactions inside highly charged microscale droplets for remote delivery of reactive radicals and high purity nanomaterials

The OH^· radical plays an important role in areas including atmospheric chemistry, water and pollution remediation, disinfection, tumour therapy and protein folding studies. Atmospheric pressure plasmas (APP) are very efficient generators of radicals including OH^·. Most configurations involve plasmas that are in contact or directly coupled to the substrate leading to complexity in species diagnostics and control. The RF plasma configuration can limit plasma and photon fluxes as well as electric field and current effects to the near electrode afterglow region, allowing remote delivery of mainly radical chemical species only and further restricted to predominantly OH^· and H₂O₂, which can be useful for validation of chemical models. However radical recombination in flight over extended plasma-free distances has not been characterised and may limit the technological benefits of remote delivery. We report measured OH^· and H₂O₂ fluxes over distances up to 200 mm from the RF plasma. We then present details of enhanced delivery using low-density aerosol streams. The fundamental mechanisms derive from a complex interplay between plasma and liquid microdroplets, involving charged and neutral species bombardment, interfacial electric field generation, solvated electron and chemical species surface reactions, evaporation and high-density vapour layer. While complex, the ultra-small volume, high surface to volume ratio and the well-defined spherical geometry offer opportunities for modelling within and beyond the plasma region. We present initial outcomes from one-dimensional spherical simulation of liquid reaction – diffusion chemistry with reference to direct measurements of downstream OH^· and H₂O₂ fluxes as well as measurements of droplet charge, from which estimates of electron and ion fluxes to droplet within the plasma can be obtained.     

    

Analyses of chemical reactions in plasma generated within humid oxygen bubbles with highly concentrated ozone

We have used plasma generated within oxygen bubbles for generation of liquid-phase hydrogen peroxide. In this study, effects of additions of ozone and water vapor to the bubbles on the hydrogen peroxide generation were investigated. Ozone addition slightly increased rate and energy efficiency of hydrogen peroxide generation when the bubbles were not humidified. With humidification, on the other hand, different tendencies were observed depending on the plasma input power. The rate and efficiency increased with increasing ozone concentration when the input power was high. At ozone concentration of 100 g/m^_3, twice the rate and efficiency were achieved as without ozone addition. However, the hydrogen peroxide generation was suppressed with such highly concentrated ozone with low input power. The reactions in the plasma were analyzed by numerical simulations with a global model which included electron-impact reactions and chemical reactions.

    

Measurement of Radicals Generated by Plasma in Contact with Dilute Sulfuric Acid by Using Electron Spin Resonance (ESR) Method

Catalyst for hydrolysis of cellulose to glucose is very important to highly efficient biofuel process. Our group has achieved to generate a high-performance hydrolysis catalyst, which was formed by sulfonating carbon material, by using plasmas in contact with dilute sulfuric acid. However, the mechanism of the sulfonation process is not fully understood. In this study, to better understand the sulfonation mechanism, we investigated radicals generated by plasma in contact with the dilute sulfuric acid by using an electron spin resonance (ESR) method and a spin trapping reagent of CYPMPO. When the plasma was generated by negative pulsed voltage and irradiated to the 500 mM dilute sulfuric acid for 300 s, we observed an ESR spectrum derived from CYPMPO-OH, but any radicals formed from the sulfuric acid, such as sulfate ion radicals (SO₄⁻), were not detected. On the other hand, in the case of the positive polarity, unknown peaks in addition to those derived from the CYPMPO-OH were observed in the 500 mM dilute sulfuric acid after 300 s plasma irradiation.   

Creation of reaction species by an atmospheric pressure plasma jet when treating liquids

Versatility of effects induced in liquids after plasma treatments has facilitated development of various applications in the fields of medicine, biology, water treatment etc. Plasma treated liquids (PTL) may contain different long-lived chemical species, but their occurrence and concentrations depend on the plasma system used. Some of the fundamental and significant reactions have been specified, but due to complexity of the chemistry in both plasma and liquid and their interaction, obtaining a wider prospect is still remote. To investigate interaction between atmospheric pressure plasma and a liquid target, in this study we associated results of spectrally resolved imaging and optical emission spectroscopy with quantification of reactive oxygen and nitrogen species (RONS) formed in the treated sample. For creation of the plasma above the sample we used a plasma jet operating in kHz regime with He/Ar as working gases. Relation between plasma and liquid properties has been established for wide range of different plasma parameters and several liquid targets. Obtained results provided relation between creation of RONS in the gas phase and in PTL.   

Numerical simulation of chemical reactions in PBS-like solution exposed to atmospheric-pressure plasmas

Atmospheric-pressure plasma (APP) has been employed in various medical and biological research applications, including wound healing, bacteria sterilization, virus inactivation, and cancer treatment. In this study, zero-dimensional (0D, i.e., global) and one-dimensional (1D) numerical simulations were used to investigate chemical reactions, especially the generation and loss of reactive oxygen and nitrogen species (RONS) in a phosphate-buffered saline (PBS)-like solution (a pH buffer solution with NaCl) exposed to APPs [1,2]. First, we focused on the effects of charge-neutral reactive species generated in the gas-phase plasmas on the liquid-phase chemical reactions. It was found that hypochlorite ClO- generated in the solution consumed much of hydrogen peroxide H₂O₂ dissolved into the solution from the gas phase, which was in good agreement with an earlier experimental study [3]. Second, we examined the effects of injection of charged species of pulsed He plasmas into the PBS-like solution. The time-dependent injected fluxes of ions and electrons were evaluated with particle-in-cell/Monte Carlo (PIC/MCC) simulations of a pulsed He plasma at atmospheric pressure with a frequency of 10 kHz, an applied voltage of 1.5 kV, and a pulse duration of 15 nsec. It was found that, even if only charged species (e.g., He⁺, He₂⁺, and e^-) penetrated the PBS-like solution, chemically reactive species such as H₂O₂ and HOCl are generated in the solution (mostly in the reaction boundary layer, i.e., a thin solution layer at the gas-solution interface). The dynamics of liquid-phase chemical species due to the pulsed fluxes of gas-phase species from the plasma are also discussed.   

Experimental study of the plasma chemistry in atmospheric pressure plasma contacts with dilute sulfuric acid

Our previous study developed a gas-liquid interfacial plasma sulfonation system under atmospheric pressure conditions. During the plasma sulfonation process, droplets emit when the plasma attacks the solution surface, and some active species (•OH, HOSO2•) and long-lived species (SO2, SO3, H2O2) are generated. In this work, the gaseous elements (long-live species) were measured by the Fourier-transform infrared spectrometer (FTIR) and mass spectrometer (MS) during liquid-cathode and liquid-anode discharge. •OH generation was confirmed by optical emission spectroscopy. It is found that SO2 and some other species are only produced with droplets transmitted in the liquid-cathode discharge.   

Polymerization of EDOT on H2O by DBD treatment

We have previously reported synthesis of a freestanding film on an aqueous solution irradiated with dielectric barrier discharge (DBD). In this work, we have explored possibility of fabricating freestanding Poly-(3,4-ethylenedioxythiophene) (PEDOT) films with the same method using 3,4-ethylenedioxythiophene (EDOT) as the source liquid material. However, no films were formed by simple DBD irradiation on EDOT, although the liquid color changed from transparent to deep brown. Alternatively, we examined DBD irradiation on the EDOT droplet on H₂O. The amounts of EDOT and H₂O was 10µL and 1000µL, respectively. Applied voltage was bipolar-pulse voltage with amplitude of 5kV, frequency of 40kHz, and pulse width of 4 µs. The discharge gas was helium (2.5 L/min). The structure of the samples was investigated using Fourier-transform infrared (FT-IR) absorption spectroscopy. EDOT and H₂O of the sample before DBD irradiation were separated like water and oil. After the He-DBD irradiation for 10 minutes, a freestanding film colored with black was successfully formed on the surface of the liquid. In addition, the result of the IR spectra suggested that the film formed by DBD irradiation was a polymer of EDOT. Details will be presented in the conference.