Plasma-liquid System Research Articles

The formation mechanism of aqueous hydrogen peroxide in a plasma-liquid system with liquid as the anode

We investigate the formation of aqueous hydrogen peroxide (H2O2aq) in a DC discharge plasma-liquid system with liquid as the anode. The theoretical analysis and experimental results show that the H2O2aq formation process is mostly controlled by the aqueous electron-induced reactions in the liquid zone directly affected by the plasma. It is shown that the low H2O2aq yield in this system is caused by quenching the dissolved OH radicals through aqueous electrons and alkalization in the plasma-directly-affected liquid zone.

Plasma Activated Synthesis of Macromolecular Compound

The paper is devoted to study of the plasma-activated synthesis of organic compounds with optically activity. Synthesis was carried out in plasma-liquid system with a rotating gliding discharge submerged in a liquid. The initial reagents of synthesis were ethanol, ammonia, and CO2. The possibility of the influence of the electric field direction on the optical activity of the products of plasma-activated synthesis is shown.

Ethanol-controlled peroxidation in liquid-anode discharges

Discharge plasma–liquid systems provide an interesting research field for many physical and chemical processes. These fascinating processes can be studied by using systems with liquid as anode and cathode, and the yield of aqueous hydrogen peroxide (H2O2aq) in the plasma–liquid system with the liquid anode is significantly lower than that with the liquid cathode. We improve the H2O2aq yield in the plasma–liquid system with the liquid anode by adding appropriate amount of ethanol in the liquid. A seven-time increase in the production rate is reached when the ethanol volume concentration is 5%, while further increase in the ethanol concentration will decrease the production rate. The H2O2aq production improvement is ascribed to the atomic oxygen-induced hydrogen abstraction reactions from ethanol which in situ improve the concentration of the H2O2 building brick, the OH radicals, in the liquid. However, excessive ethanol can scavenge the in situ formed OH radicals through the hydrogen abstraction reaction, by which the H2O2aq formation is inhibited.

The pH value control in air plasma–liquid system by sodium bicarbonate

The pH value of an aqueous solution usually decreases when treated by air discharge plasma due to the formation of HNO2 and HNO3 in solution. Herein, we found that the existence of a pH-buffer material, sodium bicarbonate (NaHCO3), can maintain or even increase the solution pH value in an air plasma–liquid system depending on the discharge conditions such as the temperature and the voltage polarity of the liquid. When acidic compounds (for instance, HNO2 and HNO3) are produced by the plasma–liquid interactions, the bicarbonate ions (HCO3) existing in the solution can combine with the produced hydrogen ions (H+) to form carbonic acid molecules (H2CO3−), resulting in the maintaining of the solution pH value. However, the formed H2CO3 might be decomposed to produce water (H2O) and carbon dioxide (CO2) by the discharge plasma-induced heat, leading to a continuous consumption of H+ ions in the solution. As the quantity of H+ ions consumed by the decomposition of H2CO3 is greater than the quantity of H+ ions produced by air discharge plasma, the solution pH value will increase.

The formation pathways of aqueous hydrogen peroxide in a plasma-liquid system with liquid as the cathode

Using an aqueous solution of NaCl as the cathode, we explored the atmospheric pressure Ar discharge plasma generated above the solution surface. The formation pathways of aqueous hydrogen peroxide (H2O2aq) in this system were investigated. Dimethyl sulfoxide was used as a scavenger of hydroxyl (OH) radicals to investigate the contribution of dissolved OH radicals to the H2O2aq. The results indicate that the H2O2aq is mainly formed by the combination of the dissolved OH radicals at the plasma-affected thin liquid layer, while the H2O2 formed in the gas phase and the H2O2aq formation inside water by the plasma-induced ultraviolet radiation have no contribution.

On the quantification of the dissolved hydroxyl radicals in the plasma-liquid system using the molecular probe method

Hydroxyl (OH) radical is one of the most important reactive species produced by plasma-liquid interactions, and the OH in liquid phase (dissolved OH radical, OHdis) takes effect in many plasma-based applications due to its high reactivity. Therefore, the quantification of the OHdis in a plasma-liquid system is of great importance, and a molecular probe method usually used for the OHdis detection might be applied. Herein, we investigate the validity of using the molecular probe method to estimate the [OHdis] in the plasma-liquid system. Dimethyl sulfoxide is used as the molecular probe to estimate the [OHdis] in an air plasma-liquid system, and usually the estimation of [OHdis] is deduced by quantifying the OHdis-induced derivative, the formaldehyde (HCHO). The analysis indicates that the true concentration of the OHdis should be estimated from the sum of three terms: the formed HCHO, the existing OH scavengers, and the H2O2 formed from the OHdis. The results show that the measured [HCHO] needs to be corrected since the HCHO consumption is not negligible in the plasma-liquid system. We conclude from the results and the analysis that the molecular probe method generally underestimates the [OHdis] in the plasma-liquid system. If one wants to obtain the true concentration of the OHdis in the plasma-liquid system, one needs to know the consumption behavior of the OHdis-induced derivatives, the information of the OH scavengers (such as hydrated electron, atomic hydrogen besides the molecular probe), and also the knowledge of the H2O2 formed from the OHdis.

What Are the Effective Reactants in the Plasma-Induced Wastewater Treatment?

We perform a series of experiments in a plasma-liquid system, aiming to explore in real time the change of a synthetic wastewater (methyl orange, MO) under an atmospheric pressure DC plasma exposure. The results indicate that the short-lived hydroxyl radicals generated from the plasma-liquid interactions are the predominant reactants in the MO molecules decomposition, while the long-lived species such as the plasma-induced hydrogen peroxide (H2O2) are ineffective. Nevertheless, the plasma-generated H2O2 can contribute to the MO decomposition by producing hydroxyl radicals from reacting with Fe2+ (Fenton's reaction) or by producing peroxynitrite from reacting with NO2−. Moreover, the MO decomposition rate in the case of liquid cathode is found to be significantly faster than that in the case of liquid anode.

Plazmovo-ridynna systema zi zvorotnym vykhrovym potokom dlya plazmovo-katalitychnoho reformuvannya

A plasma-liquid system with the reverse vortex flow and a liquid electrode, which was designed for the plasma-catalytic reforming of hydrocarbons, has been studied. Discharge operation modes with the solid and liquid cathodes are compared, including the discharge voltage dependences on the distance between the upper flange and the liquid surface. The influence of the water content in a plasma-forming gas on the average energy of plasma electrons is analyzed.

Synthesis of Nanosilver in Plasma-Liquid Microdischarge System

This paper presents the results of investigation of the plasma-liquid system with the secondary discharge supported by atmospheric pressure microdischarge in the vortex Ar flow. The plasma treated fluids were aqueous solutions of AgNO<sub>3</sub>. The microplasma discharge was powered by a DC supply. The plasma channel behaviour was characterized by photo/video recording, also plasma was studied using emission spectroscopy technique. The working liquid and firm products created after the treatment were studied also.

An Atmospheric Pressure Plasma Setup to Investigate the Reactive Species Formation

Non-thermal atmospheric pressure ('cold') plasmas have received increased attention in recent years due to their significant biomedical potential. The reactions of cold plasma with the surrounding atmosphere yield a variety of reactive species, which can define its effectiveness. While efficient development of cold plasma therapy requires kinetic models, model benchmarking needs empirical data. Experimental studies of the source of reactive species detected in aqueous solutions exposed to plasma are still scarce. Biomedical plasma is often operated with He or Ar feed gas, and a specific interest lies in investigation of the reactive species generated by plasma with various gas admixtures (O2, N2, air, H2O vapor, etc.) Such investigations are very complex due to difficulties in controlling the ambient atmosphere in contact with the plasma effluent. In this work, we addressed common issues of 'high' voltage kHz frequency driven plasma jet experimental studies. A reactor was developed allowing the exclusion of ambient atmosphere from the plasma-liquid system. The system thus comprised the feed gas with admixtures and the components of the liquid sample. This controlled atmosphere allowed the investigation of the source of the reactive oxygen species induced in aqueous solutions by He-water vapor plasma. The use of isotopically labelled water allowed distinguishing between the species originating in the gas phase and those formed in the liquid. The plasma equipment was contained inside a Faraday cage to eliminate possible influence of any external field. The setup is versatile and can aid in further understanding the cold plasma-liquid interactions chemistry.

An Atmospheric Pressure Plasma Setup to Investigate the Reactive Species Formation.

Non-thermal atmospheric pressure ('cold') plasmas have received increased attention in recent years due to their significant biomedical potential. The reactions of cold plasma with the surrounding atmosphere yield a variety of reactive species, which can define its effectiveness. While efficient development of cold plasma therapy requires kinetic models, model benchmarking needs empirical data. Experimental studies of the source of reactive species detected in aqueous solutions exposed to plasma are still scarce. Biomedical plasma is often operated with He or Ar feed gas, and a specific interest lies in investigation of the reactive species generated by plasma with various gas admixtures (O2, N2, air, H2O vapor, etc.) Such investigations are very complex due to difficulties in controlling the ambient atmosphere in contact with the plasma effluent. In this work, we addressed common issues of 'high' voltage kHz frequency driven plasma jet experimental studies. A reactor was developed allowing the exclusion of ambient atmosphere from the plasma-liquid system. The system thus comprised the feed gas with admixtures and the components of the liquid sample. This controlled atmosphere allowed the investigation of the source of the reactive oxygen species induced in aqueous solutions by He-water vapor plasma. The use of isotopically labelled water allowed distinguishing between the species originating in the gas phase and those formed in the liquid. The plasma equipment was contained inside a Faraday cage to eliminate possible influence of any external field. The setup is versatile and can aid in further understanding the cold plasma-liquid interactions chemistry.

Plasma-liquid system with reverse vortex airflow of „tornado” type with liquid electrode

Plasma-liquid system with «tornado»-type reverses vortex airflow and liquid water electrode was investigated. Discharge behavior and water electrolysis were studied using Nikon L100 video camera. The current-voltage characteristics of discharge in different operating modes were obtained. Plasma was analyzed by emission spectroscopy. The electron temperature and its distribution along the plasma torch were determined based on the relative intensities of two hydrogen atomic lines (Нa, Нb). Vibrational and rotational temperatures were calculated.

Plasma-liquid system with secondary discharge based on rotating gliding discharge

Plasma-liquid system with secondary discharge based on rotating gliding discharge

An Ambient-Pressure Plasma-Based Method for the Synthesis of Metal Nanoclusters in Aqueous Electrolytes

Small metal clusters are finding exciting new applications in a variety of fundamental and technological areas including the petroleum industry.1,2 As has been amply emphasized in the literature, a reduction in size brings about increases in the surface to volume ratio, a factor of key economic importance to catalysis, including electrocatalysis, where active materials are often rare and expensive. In addition, the small dimensions elicit modifications in the electronic structure and often in the surface morphology of the clusters and thus in their chemical and electrochemical properties, including catalytic and electrocatalytic activities and redox potentials.3 Whereas classical thermodynamics predicts monotonic changes in the redox potential of metals as a function of their size, the properties of very small metal clusters of the type to be investigated in this proposed project will be governed by quantum mechanical effects. This contribution described a new continuous plasma-liquid reactor system based on a direct current (dc) atmospheric-pressure microplasma formed at the surface of an open air, laminar flow liquid water jet has been designed and constructed. The water jet is formed by pressurizing a capillary tube ending in a rectangular nozzle in a horizontal geometry yielding for suitable pressures a highly stable solution jet over distances on the order of several cm (see Fig. 1). This allows the dc microplasma to be stably formed at the surface of an aqueous solution by placing a counter electrode downstream to pass current through the plasma-liquid interface (see Fig. 2). As has been shown by Sankaran and coworkers using a similar system,8 this tactic allows for electrons to enter the solution, leading, upon thermalization, to the reduction of metal ions leading, upon agglomeration, to the formation of metal clusters. A unique aspect of this strategy involves placement of the plasma source downstream a jet of the liquid containing the metal ions emerging from the nozzle of a capillary. This arrangement allows metal clusters of increasing size to be formed along the length of the jet, via diffusion and agglomeration (see Fig. 3). This time-space correlation will make it possible to study the properties of arrays of clusters by suitable spectroscopic probes, including optical absorption and fluorescence, placed at various positions along the jet, while the liquid jet-plasma system is operating under a constant set of conditions. It is also envisioned that introduction of a reactant into the solution prior to its ejection through the nozzle will allow the catalytic properties of clusters of various sizes to be probed by monitoring the presence of the product along the jet length by spectroscopic and/or spectrometric means.9 In addition, strategies will be devised to explore the redox properties of these unsupported clusters by injecting the jet containing the clusters into solutions containing judiciously selected redox couples while monitoring spectroscopically the composition of the media.

Ignition process investigation of the gas discharge in the gas-liquid system

The results of experimental investigations of DC discharge ignition under reduced pressure between the liquid anode and metal cathode are shown. The discharge ignition process for distilled water and prepared solutions is considered. The obtained data will be used in the future to obtain an adequate mathematical description of the processes in the plasma-liquid system. During investigations it is fixed the occurrence of cathode streamer with a deficiency of the charge carriers. It is noted that by using a liquid solution, in which impurities are present, that affect the structural changes in the water, the instantaneous breakdown with a bright flash of discharge ignition occurs at the streamer formation at the anode. The presence of bubbles helps to formation and migration of the charged particles that helps to maintaining a discharge. The obtained data allow a better understanding of the regularities of the plasma-chemical treatment process of liquid environment.

Bioglycerol Conversion in the Reverse Vortex Gas Flow Plasma-Liquid System (Tornado-Type) with Liquid Electrode and Addition of CO2

Bioglycerol Conversion in the Reverse Vortex Gas Flow Plasma-Liquid System (Tornado-Type) with Liquid Electrode and Addition of CO2

Assessment of the roles of various inactivation agents in an argon-based direct current atmospheric pressure cold plasma jet

Three types of gases, pure argon (99.999%), argon with 2% oxygen, and argon with 2% oxygen and 10% nitrogen were used as operating gases of a direct current atmospheric pressure cold plasma jet to inactivate Staphylococcus aureus (S. aureus) suspended in a liquid. The inactivation efficacies for the plasma jets operating in the three gases decrease from Ar/O2(2%) to Ar/O2(2%)/N2(10%) to pure Ar. Optical emission spectroscopy, electron spin resonance spectroscopy, high performance liquid chromatography, and atomic absorption spectrophotometry were employed to identify and monitor the reactive species in the plasma-liquid system for the three operating gases and revealed the presence of O, 1O2, OH, NO, H2O2, O3, and NO3−/NO2− as well as Cu+/Cu2+. The S. aureus inactivation results indicate that atomic oxygen (O) is the key inactivation agent, while other species play a lesser role in the inactivation progress studied here.

Plasma-Assisted Reforming of Ethanol in Dynamic Plasma-Liquid System: Experiments and Modeling

This paper presents the results of experimental and theoretical investigations of the process of nonthermal plasma-assisted reforming of aqueous ethanol solutions in the dynamic plasma-liquid system based on the electric discharge in a gas channel with liquid wall. The experiments show possibilities and efficiency of low-temperature plasma-chemical conversion of liquid ethanol into synthesis gas. The numerical modeling clarifies the nature and explains the kinetic mechanism of nonequilibrium plasma-chemical transformations in the system.