Plasma Chemistry and Plasma Processing (v.34, #5)

Non-Thermal Plasmas for VOCs Abatement by Gang Xiao; Weiping Xu; Rongbing Wu; Mingjiang Ni; Changming Du; Xiang Gao; Zhongyang Luo; Kefa Cen (1033-1065).
Volatile organic compounds (VOCs) are some of the most common air pollutants emitted from commercial and industrial processes; VOCs may also reduce indoor air quality. Increased environmental awareness, however, has resulted in stringent regulations controlling VOCs emission and has motivated researchers to develop various kinds of treatment. Non-thermal plasma (NTP) processes are regarded as promising methods for VOCs abatement. This paper reviews the state of the art and achievements of NTP for VOCs abatement and includes a description of several reactor configurations based on different discharge principles. Of particular interest are NTP-catalytic systems, characterized by higher energy efficiencies and lower byproduct production than the NTP-alone systems. Physical–chemical effects of NTP-catalytic systems occurring during plasma catalytic processes are discussed. The NTP decomposition mechanisms for toluene, naphthalene and trichloroethylene are discussed in detail. Influences of various processing parameters are summarized, and comments are given based on removal efficiencies and operational costs.
Keywords: Volatile organic compounds; Non-thermal plasma; Plasma catalysis; Decomposition

This paper is devoted to the analysis of optical and electrical characteristics of positive streamers (namely the emitted light and current signals, electric field, velocity and electrical charge) propagating in mineral oil, tetra-ester and toluene, in a point-plane electrode arrangement, under alternating current voltage. Correlations are established between the different characteristics of streamers especially between the velocity and the current derivative indicating that the streamers present also a certain inductive nature, contrarily to the generally used assumptions where streamers propagating are considered of only resistive and capacitive nature. The frequency spectra of the current pulses are also investigated and enlighten all similarities and differences between the studied oils. Each type of discharge can be well identified in relation with the nature of the liquid by the analysis of the frequency behaviour of the corresponding currents or emitted lights. The behaviour of each current streamer is found to be highly influenced by the liquid temperature of evaporation and its electronic affinity.
Keywords: Liquid dielectrics; Positive streamers; Currents; Light emissions; Correlations; Frequency analysis

Hydrogen Peroxide Production in an Atmospheric Pressure RF Glow Discharge: Comparison of Models and Experiments by C. A. Vasko; D. X. Liu; E. M. van Veldhuizen; F. Iza; P. J. Bruggeman (1081-1099).
The production of $$hbox {H}_2hbox {O}_2$$ H 2 O 2 in an atmospheric pressure RF glow discharge in helium-water vapor mixtures has been investigated as a function of plasma dissipated power, water concentration, gas flow (residence time) and power modulation of the plasma. $$hbox {H}_2hbox {O}_2$$ H 2 O 2 concentrations up to 8 ppm in the gas phase and a maximum energy efficiency of 0.12 g/kWh are found. The experimental results are compared with a previously reported global chemical kinetics model and a one dimensional (1D) fluid model to investigate the chemical processes involved in $$hbox {H}_2hbox {O}_2$$ H 2 O 2 production. An analytical balance of the main production and destruction mechanisms of $$hbox {H}_2hbox {O}_2$$ H 2 O 2 is made which is refined by a comparison of the experimental data with a previously published global kinetic model and a 1D fluid model. In addition, the experiments are used to validate and refine the computational models. Accuracies of both model and experiment are discussed.
Keywords: Atmospheric pressure plasma; Hydrogen peroxide; Chemical kinetics modelling; Water vapor chemistry; RF glow discharge

Comparison of Aluminium Nanostructures Created by Discharges in Various Dielectric Liquids by Ahmad Hamdan; Cédric Noël; Jaafar Ghanbaja; Thierry Belmonte (1101-1114).
Synthesis of aluminium-containing nanoparticles (NPs) by electrical discharges was performed in three dielectric liquids (heptane, liquid nitrogen and water) with aluminium electrodes. The nature of the liquid plays an essential role in the synthesis yield and in the structural properties of NPs. Time-resolved optical emission spectroscopy of selected emission lines emitted during the discharge and its time afterglow was used to observe the chemical changes occurring in the gas phase. It turns out that in heptane and liquid nitrogen, crystalline metallic NPs (from 5 to 10 nm in diameter) are synthesized and oxidized next into amorphous alumina when they are in contact with air, once the liquid is evaporated. In heptane, the transformation of the liquid itself into hydrogenated amorphous carbon creates a kind a matrix in which the aluminium NPs are embedded. Sometimes, a protective graphite shell grows around the NPs and protects them from any further oxidation. In water, these crystalline metallic NPs are synthesized during the first 800 ns of the discharge process, when oxidation is limited by the outward flux of the metallic vapour. They are oxidized next in water. A second type of alumina NPs (several 10 s of nm in diameter) are produced from 800 ns on. They are likely formed from AlO molecules and no longer from aluminium atoms. In every liquid, sub-micrometric particles are also found due to droplet emission from the liquid well created during impacts of spark discharges on electrodes.
Keywords: Discharge in liquids; Aluminium nanoparticles; Growth mechanism

A multi-needle-to-plate pulsed discharge plasma reactor was designed to investigate its potential for polyvinyl alcohol-containing wastewater (PVA) treatment. The effects of some operational parameters such as PVA initial concentration, pulse peak discharge voltage, air flow rate, solution pH value, and iron additives on PVA degradation were examined. The results indicated that PVA could be effectively degraded from aqueous solutions. PVA degradation efficiency was 76.0 % within 60 min’s discharge plasma treatment with 1.5 mmol L−1 Fe2+ addition. Decreasing PVA initial concentration and increasing pulse peak discharge voltage were both beneficial for PVA degradation. There existed appropriate air flow rate for obtaining great PVA degradation efficiency in the present study. A little acid environment was conducive to PVA degradation. The presence of Fe2+ and Cu2+ could both benefit PVA degradation, and the increment of Fe2+ and Cu2+ concentrations to a certain extent could enhance its degradation efficiency, as well as energy yield. PVA possible degradation mechanisms were discussed, and the degradation processes were mainly triggered by the reactions of PVA with $$^{ cdot } { ext{OH}}$$ · OH radicals.
Keywords: Pulse discharge plasma; PVA; Degradation

Plasma-Induced Synthesis of CuO Nanofibers and ZnO Nanoflowers in Water by Xiulan Hu; Xin Zhang; Xiaodong Shen; Hongtao Li; Osamu Takai; Nagahiro Saito (1129-1139).
Fiber-shaped cupric oxide (CuO) nanoparticles and flower-shaped ZnO nanoparticles were facilely synthesized by plasma-induced technique directly from copper and zinc electrode pair in water, respectively. The phase composition, morphologies and optical property of nanoparticles have been investigated by energy dispersive X-ray analysis, X-ray powder diffraction, transmission electron microscopy and UV–vis. The in situ analysis by an optical emission spectroscopy clarified the formation mechanism. Plasma was generated from the discharge between a metal electrode pair in water by a pulse direct current power. CuO and ZnO nanoparticles were synthesized via almost the same formation mechanism, which were prepared via the rapid energetic radicals’ bombardment to electrodes’ surface, atom vapour diffusion, plasma expansion, solution medium condensation, and in situ oxygen reaction and further growth. This novel plasma-induced technique will become a potential application in nanomaterials synthesis.
Keywords: Plasma-induced; Nanomaterials; Oxidation; Growth; Optical properties

Plasma-Catalytic Oxidation of Toluene on MnxOy at Atmospheric Pressure and Room Temperature by Meijuan Lu; Rong Huang; Peitao Wang; Limin Chen; Junliang Wu; Mingli Fu; William Wen; Bichun Huang; Daiqi Ye (1141-1156).
MnxOy/SBA-15 catalysts were prepared via the impregnation method and utilized for toluene removal in dielectric barrier discharge plasma at atmospheric pressure and room temperature. The catalysts were characterized by X-ray diffraction, N2 adsorption–desorption, Raman spectroscopy, X-ray photoelectron spectroscopy, H2 temperature-programmed reduction, and O2 temperature-programmed desorption methods. The characterization results indicated that manganese loading did not influence the 2D-hexagonal mesoporous structure of SBA-15. The catalyst had various oxidation states of manganese (Mn2+, Mn3+, and Mn4+), with Mn3+ being the dominant oxidation state. Toluene removal was investigated in the environment of pure N2 and 80 % N2 + 20 % O2 plasma, showing that the toluene removal efficiency and CO2 selectivity were noticeably increased by MnxOy/SBA-15, especially in the presence of 5 % Mn/SBA-15. This activity was closely related to the high dispersion of 5 % Mn on SBA-15 and the lowest reduction temperature exhibited by this catalyst. Mn loading increased the yield of CO2 in the N2 plasma and promoted the deep oxidation of toluene. During toluene oxidation, oxygen exchange might follow a pathway, wherein bulk oxygen was released from the MnxOy/SBA-15 surface; gas-phase O2 subsequently filled up the vacancies created on the oxide. Each of the manganese oxidation states played an important role; Mn2O3 was considered as a bridge for oxygen exchange between the gas phase and the catalyst, and Mn3O4 mediated transfer of oxygen between the catalyst and toluene.
Keywords: Dielectric barrier discharge (DBD); MnxOy/SBA-15; Toluene removal; Oxygen exchange

Fourier transform infrared spectroscopy of $$hbox {CH}_{4}/hbox {N}_{2}$$ CH 4 / N 2 and $$hbox {C}_{2}hbox {H}_{m}/hbox {N}_2$$ C 2 H m / N 2 ( $$m = 2, 4, 6$$ m = 2 , 4 , 6 ) gas mixtures in a medium pressure (300 mbar) dielectric barrier discharge was performed. Consumption of the initial gas and formation of other hydrocarbon and of nitrogen-containing HCN and $$hbox {NH}_{3}$$ NH 3 molecules was observed. $$hbox {NH}_{3}$$ NH 3 formation was further confirmed by laser absorption measurements. The experimental result for $$hbox {NH}_{3}$$ NH 3 is at variance with simulation results.
Keywords: Dielectric barrier discharge; Hydrocarbon/nitrogen gas mixture; Infrared spectroscopy; HCN and NH3 formation

Plasma spray-physical vapor deposition (PS-PVD) is a novel coating process based on plasma spraying. In contrast to conventional methods, deposition takes place not only from liquid splats but also from nano-sized clusters and from the vapor phase. This offers new opportunities to obtain advanced microstructures and thus to comply with growing demands on modern functional coatings. In this study, different process conditions were investigated with regard to the application of the PS-PVD process for ceramic thermal barrier coatings. Plasma characteristics were calculated under chemical equilibrium conditions by minimizing the Gibbs energy. The plasma-feedstock interaction was modeled taking into account the particular conditions at very low pressure. Since the plasma is highly rarefied, the small feedstock particles are in the free molecular flow regime. Hence, continuum methods commonly used in fluid mechanics and heat transfer approaches with continuous boundary conditions are not appropriate; alternative methods based on the kinetic theory of gases are required. The experimental results confirm the predictions about the degree of vaporization made by such calculations. In particular, they show that the feedstock treatment mainly takes place within the very first trajectory segment between injector and jet expansion.
Keywords: Plasma spraying; PS-PVD; Free molecular flow; Drag force; Heat transfer

Thin film deposition processes emit large amounts of NF3 and by-product particles, which are of great concerns in the semiconductor industry, from the environmental and economic points of view. With an objective to overcome these concerns, plasmas are generated from a cylindrical reactor placed before a vacuum pump. The discharge stability is evaluated by monitoring the changes in the plasma images with the pressure. By using a particle sampler and a particle trap, the size and quantity of the by-product particles are compared during plasma-on and plasma-off. The effects of adding O2 and H2O to the by-products of the NF3 abatement process are investigated by analyzing the spectra obtained with a Fourier transform infrared spectroscopy. Further, the H2O flow rate is optimized for the highest destruction and removal efficiency of NF3. Finally, the applicability of our device to the after-treatment equipment is discussed.
Keywords: Low-pressure plasma reactor; Cylindrical electrode; NF3 ; By-product particles; Eco-friendly processing

Response of Linear, Branched or Crosslinked Polyethylene Structures on the Attack of Oxygen Plasma by R. Mix; J. F. Friedrich; D. Neubert; N. Inagaki (1199-1218).
Linear, branched and crosslinked polyethylenes (PE) were exposed to the low-pressure oxygen plasma for 2–120 s. In the following the samples were washed with solvents to remove low-molecular weight oxidized material and to excavate the subjacent polymer structure for microscopic characterization. X-ray photoelectron spectroscopy (XPS) measurements provided information about changes in elemental composition and chemical structure of PE after plasma exposure and washing. The calculation of the concentration of tertiary C atoms using XPS data was a measure of branches and crosslinking in the polymer before and after exposure to oxygen plasma. Linear PE was most sensitive towards oxygen plasma and showed the highest concentration in tertiary C atoms after plasma exposure. On the other hand branched PE types, which possess originally more tertiary carbon atoms, have lost two-third of them after 2 s oxygen plasma exposure. Branched PE show also topological changes at their surface as detected by atomic force microscopy. Differential scanning calorimetry measurements confirmed strong changes in crystallinity and molecular orientation of linear PE already after 120 s exposure to the oxygen plasma interpreted as amorphization. These effects should be interpreted as result of crosslinking caused by the recombination of dangling bond sites.
Keywords: Linear and branched polyethylene; Oxygen plasma; Functionalization; Crosslinking

Chemical Investigation on Various Aromatic Compounds Polymerization in Low Pressure Helium Plasma by Mihai Asandulesa; Ionut Topala; Yves-Marie Legrand; Stephanie Roualdes; Vincent Rouessac; Valeria Harabagiu (1219-1232).
Remarkable properties of plasma polymer films are greatly dependent not only on the chemical structure of precursor but also on the reactor design and the deposition conditions. In many industrial applications it is a challenge to control the plasma polymer structure. In this paper we investigate the chemical transformation of various aromatic compounds, such as activation and fragmentation of substituent-part, aromatic ring opening, during plasma polymerization process. Polymerized films are deposited in a low-frequency capacitively coupled plasma-enhanced chemical vapour deposition reactor, working at low pressure. The chemical composition of plasma-polymerized films is elucidated by Fourier-transform infrared spectroscopy, solid-state carbon-13 nuclear magnetic resonance spectroscopy, and X-ray photoelectron spectroscopy. Based on spectroscopic measurements, the intermediary reactions during film growth may be presumed.
Keywords: Aromatic compounds; Intermediary reactions; NMR; Plasma polymerization; XPS

Decomposition of Acetaldehyde Using an Electron Beam by Youn-Suk Son; Junghwan Kim; Jo-Chun Kim (1233-1245).
This study investigated the decomposition characteristics of acetaldehyde using an electron beam. The removal efficiency (RE) in air, O2, N2 and He atmospheres at 10 kGy were 88, 89, 94 and 35 %, respectively. By varying the initial concentration (C0), G-values at 240 ppm (C0) were maintained from 6.4 to 7.0 molecules/100 eV, while the G-values at 34 and 60 ppm (C0) decreased from 4.5 to 1.1 and from 6.6 to 2.0 molecules/100 eV when the absorbed dose increased from 2.5 to 10 kGy. The RE of acetaldehyde at 96 % relative humidity was approximately 10–15 % higher than that at dry air when the absorbed doses were 5–10 kGy. Increasing the water supply did not provide additional improvement of the RE at 2.5 kGy. CO, CO2, O3 and trace VOC compounds such as C2H4O2, C7H6O, C6H6, C7H8 and C8H10 were detected as by-products.
Keywords: Electron beam; Acetaldehyde; Benzene; G-value; Radical