Plasma Chemistry and Plasma Processing (v.33, #4)

Conversion of Methane and Carbon Dioxide in a DBD Reactor: Influence of Oxygen by Torsten Kolb; Jan H. Voigt; Karl-Heinz Gericke (631-646).
A continuous plug flow reactor supported by a dielectric barrier discharge (DBD) is used to study the conversion of methane, carbon dioxide, and oxygen at different compositions. The three studied gases were diluted with helium to 3 % with an overall flow rate of 200 sccm. The 13.56 MHz plasma was ignited at atmospheric pressure. The product stream and the inlet flow were analyzed by a FTIR spectrometer equipped with a White-cell and by a quadrupole mass spectrometer. The DBD reactor generates hydrogen, carbon monoxide, ethane, ethene, acetylene, formaldehyde, and methanol. Additional oxygen in the feed has positive effects on the yield of methanol, formaldehyde and carbon monoxide and reduces the total consumed energy. The hydrogen yield reaches its maximum at medium amounts of oxygen in the inlet flow. The conversion of methane increases to a limiting value of about 35 %. Methane rich feeds increase the yield of hydrogen, ethane and methanol. On the other hand, additional oxygen has a negative influence on the produced amount of C2 hydrocarbons. The conversion of methane and carbon dioxide as well as the yield of synthesis gas components and C2 hydrocarbons increases by changing the plasma power to higher values.
Keywords: Cold plasma; Dielectric barrier discharge; Methane; Oxygen; Online monitoring

Comparative Study of Methane Activation Process by Different Plasma Sources by Dae Hoon Lee; Young-Hoon Song; Kwan-Tae Kim; Jae-Ok Lee (647-661).
In this paper, we compare the characteristics of methane activation by diverse plasma sources. The test conditions of reactant flow rate and composition are fixed for each plasma source to eliminate any possible misleading effects from varying test conditions. Among the diverse characteristics of each plasma source, we focus on the electron energy and degree of thermal activation in evaluating the cost-effectiveness of methane decomposition. The reaction is evaluated based on the selectivity of specific products, including H2, C2H6, and C2H2. Among the tested plasma sources, those that provide a somewhat thermal environment have a rather high degree of warmness, resulting in higher methane conversion and lower operational costs. As the non-thermal characteristics of the plasma sources become stronger, the selectivity of C2H6 increases. This reflects C2H6 formation from the direct collision of CH4 with high-energy electrons. On the other hand, as the degree of warmness increases, the selectivity of H2 and C2H2 increase. The results give an insight into possible tools for process control or selectivity control by varying the degree of warmness in the plasma source. The process optimization and cost reduction of methane activation should be based on this concept of selectivity control.
Keywords: Plasma source; Methane activation; Optimization; Product selectivity

Plasma Assisted Synthesis and Physicochemical Characterizations of Ni–Co/Al2O3 Nanocatalyst Used in Dry Reforming of Methane by Nader Rahemi; Mohammad Haghighi; Ali Akbar Babaluo; Mahdi Fallah Jafari; Pooya Estifaee (663-680).
To assess the effects of plasma treatment a Ni–Co/Al2O3 nanocatalyst (10 % Ni and 3 % Co) was prepared via impregnation method followed by treatment with a non-thermal plasma to be investigated in a catalytic dry reforming of methane. The impregnated and plasma-treated nanocatalysts were characterized using XRD, FESEM, EDX, TEM, BET, FTIR, and XPS techniques. The XRD patterns confirmed the presence of nickel as NiO and NiAl2O4 and cobalt as Co3O4 on alumina support. Small NiO, NiAl2O4, and Co3O4 crystals observed in plasma-treated nanocatalyst, exhibited a good dispersion of active phase in this catalyst. The average particles size in plasma-treated sample obtain by FESEM micrograph were shown to be smaller than that of impregnated sample and the morphology was more homogenous and relatively agglomeration-free in plasma-treated Ni–Co/Al2O3 nanocatalyst. According to BET analysis, specific surface area of plasma-treated sample was 58 % higher than the non-treated catalyst. TEM analysis showed that particles of active phase were fairly small and well-dispersed on Al2O3 as a result of the plasma treatment. Better dispersion of active metal on the surface of plasma-treated sample was confirmed by XPS analysis. The plasma-treated sample showed higher yield and conversion at all temperature ranges investigated and was more resistant to coke formation compared to the non-treated sample. The results from the characterization and reaction studies suggests that plasma treatment may be a promising method for obtaining more active and stable nanocatalysts for dry reforming of methane.
Keywords: Dry reforming; Synthetic gas; Ni–Co/Al2O3 ; Non-thermal plasma

Effect of Water Vapor on NO Removal in a DBD Reactor at Different Temperatures by Tao Wang; Bao-Min Sun; Hai-Ping Xiao; Dong Wang; Xiang-yu Zhu; Ya-feng Zhong (681-690).
The present work investigates experimentally the effect of H2O vapor on the removal of NO at elevated temperatures. Breakdown voltage, discharge characteristics and NO removal efficiency were studied under various conditions of water vapor content. The experimental results indicate H2O can greatly enhance the NO removal efficiency from a NO/O2/N2/C2H4/H2O system, but the breakdown voltage increases as the relative humidity of the gas increases. Moreover, the effect of temperature on NO removal at a relative gas humidity of 30 % was analyzed. With an increase in temperature, E/N increased, producing more active species and energetic electrons; electron detachment also became significant at high temperature and the rates of major reactions were promoted, intensifying the conversion of NO.
Keywords: Dielectric barrier discharge; NO removal; Water vapor; Temperature

Improvement of Atmospheric Water Surface Discharge with Water Resistive Barrier by Xiaoping Wang; Zhongjian Li; Jinhui Zeng; Xingwang Zhang; Lecheng Lei (691-705).
Atmospheric water surface discharge is a promising method for water treatment. The selection of discharge gap distance must take a pair of conflicting aspects into account: the chemical efficiency grows as the discharge gap distance decreases, while the spark breakdown voltage decreases as the gap distance decreases. To raise the spark breakdown voltage and the chemical efficiency of atmospheric pressure water surface discharge, resistive barrier discharge is introduced in this paper. Both the high voltage electrode and the ground electrode are suspended above water surface to form an electrode-water-electrode discharge system. The water layer plays the role of a resistive barrier which inhibits the growth rate of discharge current as voltage increases. Experiments conducted at different discharge gap distances and water conductivities indicate that both the spark breakdown voltage and the chemical efficiency are remarkably raised in comparison with traditional water surface discharge. After parameter optimization, the discharge reactor is scaled up with activated carbon fiber electrodes and advantages of water resistive barrier discharge are kept.
Keywords: Water resistive barrier discharge; Water surface discharge; Spark breakdown; Chemical efficiency

Surface Modification of Smectite Clay Induced by Non-thermal Gliding Arc Plasma at Atmospheric Pressure by Antoine Tiya Djowe; Samuel Laminsi; Daniel Njopwouo; Elie Acayanka; Eric M. Gaigneaux (707-723).
Smectite clay from Sabga (west-Cameroon) was treated in aqueous suspension by gliding arc plasma to modify its surface properties. The evolution of the modifications was followed with the exposure time and post-discharge duration using Fourier transformed infra red spectroscopy and scanning electron microscopy. X-ray diffraction and nitrogen physisorption analyses were also performed to evaluate if both crystalline and textural properties of the material are affected by the treatment. The results obtained show that the plasma treatment causes the breakdown of structural bounds at the clay surface and induces the formation of new hydroxyl groups (Si–OH and Al–OH) on the clay edges. Crystallinity, sheet structure and textural properties are not significantly affected by the plasma treatment. However, it should be noted that an intensive treatment of the clay lowers the pH of the suspension, which subsequently induces an acid attack of the clay. In such case, the specific surface area of the clay increases. This study demonstrates that gliding arc plasma treatments can be used to activate clay minerals for environmental application.
Keywords: Smectite clay; Gliding arc plasma; Humid air; Hydroxyl groups

Plasma-Assisted Synthesis of TiO2 Nanorods by Gliding Arc Discharge Processing at Atmospheric Pressure for Photocatalytic Applications by E. Acayanka; A. Tiya Djowe; S. Laminsi; C. C. Tchoumkwé; S. Nzali; A. Poupi Mbouopda; P. T. Ndifon; E. M. Gaigneaux (725-735).
The present study explores a new method of synthesis of TiO2 nano-particles in an aqueous medium from TiCl3 precursor by non-thermal plasma in humid air as feeding gas obtained at atmospheric pressure. The precursor solution, TiCl3 is oxidized by strongly reactive species generated by gliding arc plasma (HO· = 2.85 V/SHE) to produce titanium oxide powders. The synthesized powder was characterised by X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy, FTIR spectroscopy, nitrogen physisorption, and UV–Vis spectroscopy. The results obtained showed that the material consists of rod-shaped nanoparticles of rutile and anatase phases. The presence of TiO2 phases was confirmed by FTIR spectrum and textural analyses showed that the material is mesoporous with specific surface area of 158 m2 g−1. UV–Visible spectrum of the plasma-synthesized TiO2 sample showed that it absorbs in the UV–A region leading to effective use as a photocatalyst under visible light.
Keywords: Non-thermal plasma; Gliding arc discharge; Nanorods; Titanium dioxide; Photocatalyst

Effect of Inorganic Ions on the Oxidation of Methyl Violet with Gliding Arc Plasma Discharge by Ya-na Liu; Shu-fa Zhu; Hui Tian; Ming Zhou; Juan Miao (737-749).
Gliding arc discharge process was used for the treatment of methyl violet wastewater. First, the intermediate products were studied by gas chromatography coupled with mass spectrometry and Fourier transform infrared spectrometry, and ultraviolet–visible spectrometer. Second, the effects of inorganic anions including chloride (Cl), carbonate (CO3 2−), sulfate (SO4 2−), phosphate (PO4 3−), nitrate (NO3 ) on the degradation efficiency of methyl violet were examined. The research results indicated that hydroxyl radicals attacked carbon atom that situated in the center of dye molecule, and the conjugating structure of methyl violet was destroyed, and dye was degraded and decolored, so a possible degradation pathway was proposed by the analysis of intermediate products detected. The methyl violet degradation rate decreased with increasing anions concentrations, and their order of sequence according to the inhibition reaction was CO3 2− > Cl > SO4 2− > NO3 .
Keywords: Inorganic ions; Gliding arc; Degradation efficiency

·OH Treatment for Killing of Harmful Organisms in Ship’s Ballast Water with Medium Salinity Based on Strong Ionization Discharge by Yubo Zhang; Mindong Bai; Cao Chen; Xiangying Meng; Yiping Tian; Nahui Zhang; Zhe Yu (751-763).
A novel discharge mode consisting of alternate discharge of a micro-streamer and micro-glow was developed to induce the formation of ·OH radicals in ballast water. A series of ·OH killing experiments were then conducted using medium salinity ballast water. Five species of algae from three different phyla and three kinds of bacteria were killed by ·OH radicals in compliance with the D-2 ballast water standard of International Maritime Organization. Moreover, the chlorophyll-a was fully discolored when the total reactive oxidants was 2.5 mg/L, indicating that the algae had died. Overall, the quality of medium salinity ballast water with heavy pollution was greatly improved. These results indicate that the use of ·OH radicals is an effective method for the treatment of ship’s ballast water.
Keywords: Micro-streamer discharge; Micro-glow discharge; ·OH radical; Medium salinity ballast water; D-2 ballast water standard

Decomposition of Chlorobenzene by Thermal Plasma Processing by P. Fazekas; E. Bódis; A. M. Keszler; Zs. Czégény; Sz. Klébert; Z. Károly; J. Szépvölgyi (765-778).
Decomposition of chlorobenzene as a model molecule of aromatic chlorinated compounds was studied in radiofrequency thermal plasma both in neutral and oxidative conditions. Optical emission spectroscopy was applied for the evaluation of the plasma excitation and molecular rotational-vibrational temperature. Atomic (C, H, O) and molecular (CH, OH, C2) radicals were identified, while the morphology of the formed soot was characterized by electron microscopy. Organic compounds adsorbed on the surface of the soot after plasma processing were comprised of various polycyclic aromatic hydrocarbons (PAH) and chlorinated PAH molecules. Their amount was greatly affected by experimental conditions, especially the oxygen content and plate power. The higher input power reduced the ring number of the PAH molecules. Addition of oxygen significantly reduced the amount of both PAHs chlorinated PAH molecules but enhanced the formation of polychlorinated benzene compounds.
Keywords: Chlorobenzene; Decomposition; RF thermal plasma; Optical emission spectroscopy; Gas chromatography mass spectrometry

High Speed Video Camera and Electrical Signal Analyses of Arcs Behavior in a 3-Phase AC Arc Plasma Torch by Christophe Rehmet; Frederic Fabry; Vandad Rohani; François Cauneau; Laurent Fulcheri (779-796).
A 3-phase AC plasma torch has been developed and aims at overcoming some limits of the classical DC torches in terms of efficiency, cost and reliability. However, the arc behavior in 3-phase plasma torch remains poorly explored. This paper is dedicated to the high speed video camera at 100,000 frames per second and electrical signal analyses of arcs behavior in a 3-phase AC arc plasma torch. First, a reference case at 150 A, in nitrogen as working gas, has been deeply analyzed. Afterwards, a parametric study based on current and inter-electrode gap has been carried out. Results show that only one arc can exist at a given time and arcs rotate by switching from a pair of electrodes to another one, following the maximal electrical gap potential. However, a particular “abnormal” arc behavior was sometimes observed. Indeed, the arc motion within the inter-electrode gap increases the heat exchange and stabilizes the 3-phase discharge whereas the system is unbalanced when the arc is in the periphery. The analysis highlights that the arc motion is strongly influenced by the electrode jet velocity and repulsive Lorentz forces. The parametric study shows that the current increases both jet velocity and arc discharge stability. Elsewhere, the increase of the inter-electrode gap can also stabilizes the electrical 3-phase arc discharge. Furthermore, the correlation between arc motion and current waveform is highlighted. This work is likely to open the way toward a better understanding of 3-phase discharges in the perspective of their further optimization.
Keywords: High speed video camera; Arc motion; 3-Phase AC plasma torch; Electric arc

Effect of H2 Flow Rate on High-Rate Etching of Si by Narrow-Gap Microwave Hydrogen Plasma by Takahiro Yamada; Hiromasa Ohmi; Hiroaki Kakiuchi; Kiyoshi Yasutake (797-806).
For the purpose of realizing a low-cost production process of silane (SiH4) gas, we have proposed the high-rate etching of metallurgical-grade Si by narrow-gap microwave hydrogen plasma. In this paper, effect of hydrogen gas flow rate (0–10 L/min) on the etch rate has been investigated and correlated with the relative variation of hydrogen-atom density estimated by actinometry. By decreasing hydrogen gas flow rate, the etch rate gradually increases up to the maximum value of 11 μm/min at 2 L/min. This increase is well correlated with the increase of hydrogen-atom density due to the longer residence time of hydrogen molecules in the plasma. On the other hand, when the gas flow rate is lower than 2 L/min, the etch rate abruptly decreases with decreasing gas flow rate in spite of the increase of hydrogen-atom density. From the surface observations and Raman measurements, it is found that the decrease in etch rate in the lower flow rate range is attributed to the formation of microcrystalline Si particles due to the decomposition of generated-SiH4 molecules in the plasma.
Keywords: Silane; Solar cell; Metallurgical-grade Si; Etching; Hydrogen plasma; Actinometry

Poly-silicon based superhydrophobic surface (water contact angle >150°) is being fabricated and its electrowetting properties have been studied. The polysilicon thin film has been deposited over patterned gold electrodes. The polysilicon film is structured to form nanoscale features using Reactive Ion Etching. A thin film of HfO2 high k-dielectric is deposited over the structured polysilicon surface. The surface was chemically modified with Trichloro (1H, 1H, 2H, 2H-perfluorooctyl) silane (PFOS). Such a surface showed Superhydrophobic behavior with water contact angle of 172° and roll off angle <3°. The electrowetting properties of the fabricated device was studied by applying a DC voltage between the gold electrode and the droplet. The electrowetting commences when the applied voltage was 18 V and the contact angle is reduced to 152°. As the applied voltage was increased there was decrease in contact angles.
Keywords: Superhydrophobic surface; Contact angle; Electrowetting; Polysilicon and reactive ion etching

Polymer light-emitting devices were fabricated utilizing plasma polymerized thin films as emissive layers. These conjugated polymer films were prepared by RF plasma enhanced chemical vapor deposition using naphthalene as monomer. The effect of different applied powers on the chemical structure and optical properties of the conjugated polymers was investigated. Fourier transform infrared (FTIR) and Raman spectroscopies confirmed that a conjugated polymer film with a 3-D cross-linked network was developed. By increasing the power, products tended to form as highly cross-linked polymer films. The fabricated devices showed broadband Electroluminescence (EL) emission peaks with center at 535–550 nm. Photoluminescence (PL) spectra of plasma polymers showed different excimeric emissions, resulted from crosslinked architecture. As the plasma power increased, the optical properties showed two different domains; up to 200 W, EL, PL and UV–Vis spectra red-shifted and broadened significantly. At higher powers, a reverse behavior was observed. Also, the relation between the film structure and plasma species was investigated using optical emission spectroscopy.
Keywords: Polymer light emitting diode; Plasma polymerization; Electroluminescence; Photoluminescence; Conjugated polymer