Plasma-induced Reactions Research Articles

Ultrafast Dual Activation of C(sp3)-H and C(sp2)-H Bonds in an Arc Plasma-Initiated Microdroplet.

This study demonstrates a method that utilizes arc plasma-induced microdroplet reactions to synthesize dual-activated products with C(sp3)-N and C(sp2)-O bonds starting from C-H bonds. This innovative process utilizes arc- and microdroplet-generated hydroxyl radicals and water dimer radical cations, opening new possibilities for the multisite derivatization of small molecules.

A review on plasma-based CO2 utilization: process considerations in the development of sustainable chemical production

Plasma-based processes, particularly in carbon capture and utilization, hold great potential for addressing environmental challenges and advancing a circular carbon economy. While significant progress has been made in understanding plasma-induced reactions, plasma-catalyst interactions, and reactor development to enhance energy efficiency and conversion, there remains a notable gap in research concerning overall process development. This review emphasizes the critical need for considerations at the process level, including integration and intensification, to facilitate the industrialization of plasma technology for chemical production. Discussions centered on the development of plasma-based processes are made with a primary focus on CO2 conversion, offering insights to guide future work for the transition of the technology from laboratory scale to industrial applications. Identification of current research gaps, especially in upscaling and integrating plasma reactors with other process units, is the key to addressing critical issues. The review further delves into relevant research in process evaluation and assessment, providing methodological insights and highlighting key factors for comprehensive economic and sustainability analyses. Additionally, recent advancements in novel plasma systems are reviewed, presenting unique advantages and innovative concepts that could reshape the future of process development. This review provides essential information for navigating the path forward, ensuring a comprehensive understanding of challenges and opportunities in the development of plasma-based CCU process.

Film Deposition by Controlling Plasma-induced Surface Reactions

Film Deposition by Controlling Plasma-induced Surface Reactions

Green Chemistry Applied to Ground Coffee Volatile Compounds Modification Aiming Coffee Aroma Improvement

Aroma has a major role in coffee acceptance by consumers. This work has assessed the application of cold plasma technology to induce modifications in ground coffee beans without resorting to solvents or additives, thus applying a green chemistry approach. Cold plasma improved and modulated coffee aroma, increasing desirable and decreasing undesirable aroma notes. Ground coffee beans were subjected to dielectric-barrier discharge (DBD) plasma at different excitation frequencies. Its volatile chemical profile was determined by GC-MS. The odor activity value technique was used to evaluate the aroma profile. Extensive chemical changes occurred with aldehydes, furans, and pyrazine compounds. This work showed that setting proper excitation frequency enables modulating changes toward oxidation and C-C bond scission. Chemical reaction pathways were postulated and discussed. DBD plasma technology enhanced the importance of the desirable cocoa note from 74.4 to 88.6% and decreased the significance of the green and fruity notes from 8.5 to 3.2% and 2.7 to 1.9%, respectively. Plasma-induced reactions can help mitigating aroma defects.

Modification of Nitrogen Doped Carbon Quantum Dots Emissive States Using Plasma-Induced Non-Equilibrium Electrochemistry

Highly luminescent nanomaterials with ambient stability, environmental friendliness, low-cost and abundance are in demand for various applications in biological sensing, imaging and optoelectronics [1-6]. Many of the photoluminescent nanomaterials are semiconductors and usually contain toxic elements, heavy and expensive metals which have limited their applications. Carbon nanoparticles are a promising alternative to semiconductor nanocrystals as next generation green nanomaterials due to excellent biocompatibility, low cytotoxicity and solution processability which results in ease of production and incorporation in devices [7]. In the current work, nitrogen doped carbon quantum dots (N-CQDs) are synthesized by plasma-induced non-equilibrium electrochemistry utilizing a direct-current atmospheric pressure microplasma. The direct current microplasmas used in this work serve as a reliable and highly reproducible synthesis method for tuning the optical properties of carbon-based quantum dots. The outcome of precursors and discharge current affecting the surface chemistry and optical properties are studied using various characterization techniques, which have contributed to determine the mechanisms leading to QD formation from the plasma-induced reactions at the interface. The synthesized N-CQDs are crystalline with graphitic core doped with nitrogen and functionalized surface. The luminescence which originates from the combined effect of core and surface states can be finely controlled by changing synthesis conditions. These N-CQDs have potential to be used as an active material in next-generation solar cells or even as down-converters for high energy photons to be absorbed by a lower bandgap transporter [8]. The particularly high photoluminescence quantum yield (33% to 68%) can be exploited for optoelectronic applications.

Plasma-enhanced catalytic reduction of SO2: Decoupling plasma-induced surface reaction from plasma-phase reaction

Complex processes take place when coupling nonthermal plasma with heterogenous catalysis, obscuring mechanistic understanding of the plasma–catalysis synergy. Simultaneous homogenous plasma-phase reactions and heterogenous surface catalytic reactions occur in the system, which makes distinguishing the plasma’s contributions very challenging. In this work, we perform a careful and rigorous evaluation of surface reactions under plasma conditions. We use SO2 hydrogenation as a probe reaction to examine plasma-induced surface reactions, desorption, and dissociation of SO2 adsorbate. A temperature-programmed plasma-induced surface reaction approach is used to decouple plasma-induced surface reactions from plasma-phase reactions. The qualitative and quantitative analyses reveal a new Eley-Rideal reaction between plasma-generated atomic hydrogen in the gas-phase and strongly adsorbed SO2 over alumina, a reaction that is not thermally feasible. Furthermore, plasma causes partial desorption of weakly adsorbed SO2 species and enhances the strong chemisorption of SO2 over supported iron sulfide catalyst. Moreover, plasma facilitates H2 chemisorption and reaction with iron sulfide, producing sulfur vacancies. This work sheds light on the possible origin of plasma–catalysis synergy and offers fundamental insight about the underlying mechanisms of plasma-assisted SO2 hydrogenation reaction.

Mixed H2O/H2 plasma-induced redox reactions of thin uranium oxide films under UHV conditions.

X-ray photoelectron spectroscopy (XPS) has been used to study the effect of mixed H2O/H2 gas plasma on the surfaces of UO2, U2O5 and UO3 thin films at 400 °C. The experiments were performed in situ under ultra-high vacuum conditions. Deconvolution of the U4f7/2 peaks into U(IV), U(V) and U(VI) components revealed the surface composition of the films after 10 min plasma exposure as a function of H2 concentration in the feed gas of the plasma. Some selected films (unexposed and exposed) were also analysed using grazing-incidence X-ray diffraction (GIXRD). The XPS results show that U(V) is formed as a major product upon 10 minutes exposure of UO3 by a mixed H2O/H2 plasma in a fairly wide H2 concentration range. When starting with U(V) (U2O5), rather high H2 concentrations are needed to reduce U(V) to U(IV) in 10 minutes. In the plasma induced oxidation of UO2, U(V) is never observed as a major product after 10 minutes and it would seem that once U(V) is formed in the oxidation of UO2 it is rapidly oxidized further to U(VI). The grazing incidence X-ray diffraction analysis shows that there is a considerable impact of the plasma and heating conditions on the crystal structure of the films in line with the change of the oxidation state. This structural difference is proposed to be the main kinetic barrier for plasma induced transfer between U(IV) and U(V) in both directions.

Characterization of Cosmic Grain Analogs Formed at Low Temperature from Small Hydrocarbon Precursors in the NASA Ames COSmIC Facility

Abstract Here, we present the results of the first solid-phase ex situ analysis of cosmic grain analogs produced at low temperature (<200 K) in the NASA Ames COsmic SImulation Chamber (COSmIC) from small hydrocarbon precursors, methane (CH4) and acetylene (C2H2), seeded in an argon supersonic jet expansion and submitted to a plasma discharge. The plasma-induced chemical reactions, initiated between the precursor molecules and their atomic and molecular fragments, radicals and ions, produce larger molecules and eventually solid particles that are collected in situ under controlled conditions. Scanning electron microscopy (SEM) imaging was used to provide insight on the morphology and growth structure of the grains produced in COSmIC, and to investigate how the precursors used to produce the grains affect these parameters. This SEM study has shown that under identical experimental conditions with fixed physical and chemical parameters (precursor density, temperature, energy, and reaction time), heavier precursors in the initial mixture produce larger grains and in larger quantity, most likely as a result of a more complex chemistry: most of the grains produced in the Ar/CH4 (95:5) gas mixture ranged from 15 to 385 nm in diameter with an average density of 2.1 grains μm−2, while the grains produced in the Ar/C2H2 (95:5) gas mixture ranged from 40 to 650 nm with a density of 3.5 grains μm−2. Changes in the morphology were also observed, with grains produced from acetylene (C2H2) precursors tending to be more spherical than grains produced from methane (CH4) precursors. This change in morphology could be associated with different stages of growth formation at low temperature from a more “planar” growth at first, followed by coagulation into more spherical particles. This study demonstrates that the COSmIC experimental setup can be used to investigate carbon grain formation from small gas-phase molecular precursors at low temperature (<200 K), i.e., under a temperature regime that is representative of the dust condensation zone and outer region of circumstellar envelopes.

Impact of plasma processed air (PPA) on phenolic model systems: Suggested mechanisms and relevance for food applications

Cold plasma is considered to be a novel, non-thermal, chemical-free and eco-friendly disinfection and surface modification technology. Plasma treatment of air to generate the so called plasma processed air (PPA) induces the formation of reactive oxygen (ROS) and nitrogen species (RNS). As a result, PPA has a different chemical composition compared to untreated air and suits therefore as an alternative method for microbial disinfection. However, depending on the product properties of the food matrix and its composition, a number of plasma-induced reactions also need to be taken into consideration.This necessitates also the elucidation and understanding of the basic interactions of plasma species with bioactive compounds. The intention here is to avoid the degradation of these valuable substances and to prevent other undesirable effects in future food related applications.In the present study, the effects of PPA treatment on selected antioxidants such as pyrocatechol and derivatives of hydroxycinnimic acid were investigated in model systems to specify possible reactions induced. Antioxidant capacity, pH value, UV–Vis spectroscopy, RP-HPLC and LC-MS analysis were applied to identify reaction products providing information on possible changes induced in food matrices by PPA treatment.Exposure to PPA caused a perceptible color change towards yellow-brown accompanied by a strong reduction of the pH and the formation of insoluble sediments in the model solutions. The accumulation of nitrate, nitrite, but not of hydrogen peroxide was shown. LC-MS analysis demonstrated the formation of plasma-modified derivatives in all tested systems. The main reactions in liquid model solutions exposed to PPA were attributed to oxidation, nitration and polymerization of the phenolic compounds.

Plasma Induced Non-Equilibrium Electrochemistry for Synthesis of Nitrogen Doped Carbon Quantum Dots Applied to Third-Generation Solar Cells

Highly luminescent nanomaterials with ambient stability, environmental friendliness, low-cost and abundance are in demand for various applications in biological sensing, imaging and optoelectronics [1-6]. Many of the photoluminescent nanomaterials are semiconductors and usually contain toxic elements, heavy and expensive metals which have limited their applications. Carbon nanoparticles are a promising alternative to semiconductor nanocrystals as next generation green nanomaterials due to excellent biocompatibility, low cytotoxicity and solution processability which results in ease of production and incorporation in devices [7]. In the current work, nitrogen doped carbon quantum dots (N-CQDs) are synthesized by one-step atmospheric pressure microplasma process. The direct current microplasmas used in this work serve as a reliable and highly reproducible synthesis method for tuning the optical properties of carbon-based quantum dots in colloid form which result as being highly stable and environmentally-friendly. The outcome of precursors and discharge current affecting the particle morphology and optical properties are studied using various characterization techniques, which have contributed to determine the mechanisms leading to QD formation from the plasma-induced reactions at the interface. The synthesized N-CQDs are crystalline with graphitic core doped with nitrogen and functionalized surface. The particle size and luminescence can be finely controlled to give either excitation dependent or fixed wavelength emission in the visible region. The synthesis conditions can be easily controlled by changing the precursors or the discharge current of the plasma to give rise to required particle size and absorbance in the ultraviolet-visible region. These N-CQDs have potential to be used as an active material in next-generation solar cells or even as down-converters for high energy photons to be absorbed by a lower bandgap transporter [8]. The possible synthesis mechanisms have been analysed and potential chemical pathways leading to the formation of the QDs are described. The particularly high photoluminescence quantum yield (33% to 68%) can be exploited for applications.

Plasma-induced redox reactions synthesis of nanosized α-, γ- and δ-MnO2 catalysts for dye degradation

Mesoporous α- and δ-MnO2 with specific surface areas of 98 and 186 m2/g, were obtained via plasmachemical-reduction of KMnO4 solutions induced by flashing them with a gliding arc plasma operated at respectively high and low voltages. Correspondingly, γ- and δ-MnO2 with 48 and 289 m2/g, were obtained via plasmachemical-oxidation of Mn(CH3COO)3 solutions. At lower humid airflow, the plasma also led during the reduction of KMnO4 to changes of structure and morphology, and to increased specific surface areas. These changes are hypothesized to be due to a variation of the amount of radicals generated by the plasma along its creation parameters. Performances of the prepared catalysts were evaluated in the oxidation of Tartrazin Yellow (TY) chosen as a model dye polluting wastewaters. TY bleaching was significantly different from one plasma-synthesized material to another. α-MnO2, being the best of the plasma synthesized material appears to be recyclable and resistant to dissolution. This contribution confirms the importance of mastering the structure and texture of MnO2 catalysts during their synthesis. Gliding arc plasma is revealed as an easy and efficient method to achieve so.

Hydrogen production in liquid water by femtosecond laser-induced plasma

Development of alternative methods for hydrogen production from liquid water requires novel research approaches. In the present study, we investigated the yet unexplored method of water splitting by femtosecond laser pulses (100 fs, λ = 800 nm) focused into a quartz cell containing ultra-purified water. As a result of plasma-induced radical reactions, the formation of stable products including molecular H2 and H2O2 was identified and quantified as a function of irradiation time, repetition rate and pulse energies up to 1 mJ. The observed dependencies provided new insight into the basic chemical processes taking place in the plasma generated in water by focused laser pulses. It was found that the energy efficiency of hydrogen production increased with the decrease of pulse energy and repetition rate. Under appropriate irradiation conditions, an advantageous stoichiometry of water decomposition only into pure H2 and H2O2 (without O2) was achieved. An original way of process intensification (a series of separate laser beams of adequately low energy generating plasma at different places, a sufficiently fast water flow through the irradiation zones) was proposed in order to improve the energy yield of hydrogen generation and to ensure the evolution of pure hydrogen as the only gaseous product of the water splitting. Taking into account the suggested solution, the predicted energy efficiency of hydrogen production of 0.4 g H2/kWh was found to be comparable to other plasma methods for H2 generation. Our results show that direct production of pure H2 using femtosecond laser pulses is a fully reasonable process that carries considerable potential for both research and application.

Investigation of Nonthermal Plasma Assisted Charcoal Gasification for Production of Hydrogen-Rich Syngas

The motivation of this work is to investigate experimentally the influence of nonthermal plasma (NTP) application on the reaction kinetics of atmospheric pressure steam gasification of charcoal using a thermostatically controlled drop tube reactor. A gliding-arc generator provides about 1 kW electrical power NTP. For comparison thermal gasification is investigated under comparable flow and specific energy input conditions providing additional heat to the steam. Optical temperature measurement 20 cm flow down of the NTP zone is utilized to characterize the specific enthalpy of the reactive flow. The composition of produced syngas is measured by a gas analyzer and used for the calculation of gas flow rates. The results show a NTP-enhancement on the production of individual syngas components (H2, CO, CH4), especially on hydrogen production by around 39%. The syngas-based carbon conversion and hydrogen release are calculated from the carbon and hydrogen balance between the correspondent content in syngas and in the feedstock. The NTP promoted the carbon conversion and hydrogen release by 25% and 31%, respectively. The first-order reaction kinetics are determined by data-fitting in an Arrhenius diagram. The plasma enhanced the reaction rate coefficients by 27%. Based on experimental results and other literature, possible plasma-induced reactions are proposed.

Temperature-programmed plasma surface reaction: An approach to determine plasma-catalytic performance

Plasma-enhanced heterogeneous catalysis offers a promising alternative to thermal catalysis due to the synergy between the plasma and the solid catalyst. However, there is only a limited mechanistic insight about the interactions of highly energetic electrons and excited molecules with heterogeneous catalysts in plasmas. Accurate performance comparison in a plasma-catalytic setting is complicated because of the intricate nature of the plasma-catalyst system: simultaneous reactions occurring in the gas-phase and at the catalytic surface; the dependence of the discharge on dielectric properties of the packed catalyst bed; and the dependence of permittivity and polarization of the catalyst on plasma parameters. Here, we present a method of temperature-programmed plasma surface reaction (TPPSR) that allows decoupling gas-phase processes from the surface plasma-induced reactions. Using this method we reveal the main reasons of apparent synergy between plasma and heterogeneous catalyst for the case of carbon dioxide hydrogenation. Experiments with isotopically labelled CO2 and temperature-programmed plasma reaction experiments in flow of CO2/H2 prove a substantial role of gas-phase dissociation/hydrogenation for the observed catalyst activity and selectivity. The product distribution and reaction pathways do not significantly depend on the discharge parameters. Taking into account overheating of the catalytic bed for comparison of catalytic activity with and without plasma, it was concluded that energy dissipation also plays an important role. The observed plasma enhancement is in part due to the acceleration of electron-induced surface reactions.

Temperature dependence on plasma-induced damage and chemical reactions in GaN etching processes using chlorine plasma

Plasma-induced damage (PID) on GaN was optimally reduced by high-temperature chlorine plasma etching. Energetic ion bombardments primarily induced PID involving stoichiometry, surface roughness, and photoluminescence (PL) degradation. Chemical reactions under ultraviolet (UV) irradiation and chlorine radical exposure at temperatures higher than 400 °C can be controlled by taking into account the synergism of simultaneous photon and radical irradiations to effectively reduce PID.

Plasma-catalytic decomposition of nitrous oxide over γ-alumina-supported metal oxides

This work investigated the decomposition of dilute N2O from gas streams with various oxygen contents by using a plasma-catalytic process over metal oxide catalysts supported on γ-Al2O3. Among the metals explored (Ru, Co, Cu, V, etc.), Ru was found to be the best catalyst for the decomposition of N2O in a plasma-catalytic reactor, and most of the experiments were conducted with alumina-supported Ru. The effects of applied voltage, reaction temperature, O2 content, gas flow rate and initial N2O content on the decomposition efficiency and byproducts formation were examined. Compared to the catalyst-alone case, the presence of plasma enhanced the decomposition efficiency by 30–50%, depending on the operating condition. The increase in the oxygen content from 0 to 20% largely decreased the catalytic decomposition efficiency, whereas in the presence of plasma N2O could be successfully decomposed even at 20% O2 content. The decomposition efficiency was not a strong function of the initial N2O concentration in the range of 225–1800ppm, exhibiting pseudo first-order reaction kinetics. Without O2, there were negligible byproducts, but in the presence of O2, NO and NO2 were formed mainly due to the plasma-induced reactions between background molecules such as N2 and O2. The results obtained in this work showed the feasibility of plasma-catalytic process for the abatement of N2O.

Diagnostics of plasma-biological surface interactions in low pressure and atmospheric pressure plasmas

Mechanisms of plasma-surface interaction are required to understand in order to control the reactions precisely. Recent progress in atmospheric pressure plasma provides to apply as a tool of sterilization of contaminated foodstuffs. To use the plasma with safety and optimization, the real time in situ detection of free radicals - in particular dangling bonds by using the electron-spin-resonance (ESR) technique has been developed because the free radical plays important roles for dominantly biological reactions. First, the kinetic analysis of free radicals on biological specimens such as fungal spores of Penicillium digitatum interacted with atomic oxygen generated plasma electric discharge. We have obtained information that the in situ real time ESR signal from the spores was observed and assignable to semiquinone radical with a g-value of around 2.004 and a line width of approximately 5G. The decay of the signal was correlated with a link to the inactivation of the fungal spore. Second, we have studied to detect chemical modification of edible meat after the irradiation. Using matrix-assisted laser desorption/ionization time-of-flight mass spectroscopy (MALDI-TOF-MS) and ESR, signals give qualification results for chemical changes on edible liver meat. The in situ real-time measurements have proven to be a useful method to elucidate plasma-induced surface reactions on biological specimens.

Development of an In situ Infrared Spectroscopic Measurement of Plasma-Induced Reactions in Ethanol

An in situ monitoring system for plasma-induced reactions in ethanol was developed by infrared absorption spectroscopy in multiple internal reflection geometry. We monitored in situ the preferential decrease in the IR peak intensity of CH3 and OH groups in liquid ethanol. In addition, we monitored in situ the preferential inclusion of OCH3 and free OH groups deposited on a multiple internal reflection prism. Unlike optical emission spectroscopy, the IR-based technique detects non-light-emitting compounds. It presents as a powerful alternative tool for monitoring liquid-phase plasma reactions.

Effects of water vapor on flue gas conditioning in the electric fields with corona discharge

Sulfur dioxide (SO2) removal via pulsed discharge nonthermal plasma in the absence of ammonia was investigated to determine how electrostatic precipitators (ESPs) can effectively collect particulate matter less than 2.5μm in diameter from flue gas. SO2 removal increased as water vapor concentration increased. In a wet-type plasma reactor, directing a gas-phase discharge plasma toward the water film surface significantly enhanced the liquid-phase oxidation of HSO3− to SO42−. Comparisons of various absorbents revealed that the hydroxyl radical is a key factor in plasma-induced liquid-phase reactions. The resistivity, size distribution, and cohesive force of fly ash at different water vapor contents were measured using a Bahco centrifuge, which is a dust electrical resistivity test instrument, as well as a cohesive force test apparatus developed by the researchers. When water vapor content increased by 5%, fly ash resistivity in flue gas decreased by approximately two orders of magnitude, adhesive force and size increased, and specific surface area decreased. Therefore, ESP efficiency increased.

Real-time in situ electron spin resonance measurements on fungal spores of Penicillium digitatum during exposure of oxygen plasmas

We report the kinetic analysis of free radicals on fungal spores of Penicillium digitatum interacted with atomic oxygen generated plasma electric discharge using real time in situ electron spin resonance (ESR) measurements. We have obtained information that the ESR signal from the spores was observed and preliminarily assignable to semiquinone radical with a g-value of around 2.004 and a line width of approximately 5G. The decay of the signal is possibly linked to the inactivation of the fungal spore. The real-time in situ ESR has proven to be a useful method to elucidate plasma-induced surface reactions on biological specimens.