Plasma Chemistry and Plasma Processing (v.29, #6)

The Effect of Temperature on the Plasma-Catalytic Destruction of Propane and Propene: A Comparison with Thermal Catalysis by Tarryn Blackbeard; Vladimir Demidyuk; Sarah L. Hill; J. Christopher Whitehead (411-419).
A comparison has been made of plasma-catalysis with thermal-catalysis and plasma alone for the removal of low concentrations of propane and propene from synthetic air using a one-stage, catalyst-in discharge configuration. In all cases, plasma-catalysis produces better hydrocarbon destructions (~40%) than thermal catalysis at low temperatures. At higher temperatures, little difference is observed between plasma-catalytic and thermal-catalytic operation. Plasma operation by itself had a similar effectiveness to plasma-catalysis at low temperatures but was significantly lower (up to 50%) as the temperature was raised. By examining the form of the temperature dependence for the plasma-catalytic destruction processes, it is possible to phenomenologically distinguish two contributions to the destruction; one that is specifically plasma-induced and another (at higher temperatures) in which both plasma and thermal activation have similar mechanisms.
Keywords: Plasma catalysis; Non-thermal plasma; Atmospheric pressure; Plasma; Propane; Propene

The effect of O2 and H2O vapor on the Nitric oxide (NO) removal rate, the NO2 generation rate and the discharge characteristics were investigated using the dielectric barrier discharge (DBD) reactor at 1 atm pressure and at room temperature (20°). The results showed that the O2 present in the flue gas always hampered the removal of NO and the generation of N2O, but that the O2 could enhance the generation of NO2 in the NO/N2/O2 mixtures. Furthermore, with the increase of oxygen, the average discharge current gradually decreases in the reactor. The H2O present in N2/NO hindered the removal of NO and the generation of NO2 but had no impact on the average discharge current in the reactor in the NO/N2/H2O mixtures in which the HNO2 and HNO3 was detected. The energy efficiency of the DBD used to remove the NO from the flue gas was also estimated.
Keywords: Dielectric barrier discharge; NO conversion; Average discharge current; Oxygen; Water vapor

Combined Reforming and Partial Oxidation of CO2-Containing Natural Gas Using an AC Multistage Gliding Arc Discharge System: Effect of Stage Number of Plasma Reactors by Nongnuch Rueangjitt; Wariya Jittiang; Krittiya Pornmai; Jintana Chamnanmanoontham; Thammanoon Sreethawong; Sumaeth Chavadej (433-453).
The effect of stage number of multistage AC gliding arc discharge reactors on the process performance of the combined reforming and partial oxidation of simulated CO2-containing natural gas having a CH4:C2H6:C3H8:CO2 molar ratio of 70:5:5:20 was investigated. For the experiments with partial oxidation, either pure oxygen or air was used as the oxygen source with a fixed hydrocarbon-to-oxygen molar ratio of 2/1. Without partial oxidation at a constant feed flow rate, all conversions of hydrocarbons, except CO2, greatly increased with increasing number of stages from 1 to 3; but beyond 3 stages, the reactant conversions remained almost unchanged. However, for a constant residence time, only C3H8 conversion gradually increased, whereas the conversions of the other reactants remained almost unchanged. The addition of oxygen was found to significantly enhance the process performance of natural gas reforming. The utilization of air as an oxygen source showed a superior process performance to pure oxygen in terms of reactant conversion and desired product selectivity. The optimum energy consumption of 12.05 × 1024 eV per mole of reactants converted and 9.65 × 1024 eV per mole of hydrogen produced was obtained using air as an oxygen source and 3 stages of plasma reactors at a constant residence time of 4.38 s.
Keywords: Natural gas; Reforming; Partial oxidation; Gliding arc discharge; Plasma

The physical processes and chemical reactions that take place inside different temperature plasma zones in water are only partially understood. The present study uses the emission spectroscopy and hydrogen peroxide measurements as indicators of the processes that take place on the gas–liquid boundary and inside plasma. Based on the hydrogen peroxide measurements with negative and positive high-voltage polarities as a function of solution conductivity, it was concluded that the main difference between positive polarity plasma and negative polarity plasma lies in the active radical concentration inside plasma. Data suggested that in the positive polarity electrical discharge the hydrogen peroxide concentration depends on the solution pH, whereas in the negative polarity discharge, it depends on the solution conductivity. Also, only in the negative polarity discharge do some of the electrons that are emitted from the high voltage electrode diffuse into the bulk where they react with the solutes.
Keywords: Positive and negative electrical discharge in water; Hydrogen peroxide; Aqueous electrons

Evidence of Substrate Melting of NiCr Particles on Stainless Steel Substrate by Experimental Observation and Simulations by A. T. T. Tran; S. Brossard; M. M. Hyland; B. J. James; P. Munroe (475-495).
Single NiCr splats were plasma-sprayed onto a polished stainless steel substrate held at room temperature. The splat-substrate interface was characterized by focused ion beam and transmission electron microscopy. The frequent observation of NiO particles, particularly in pores within the splat, and at the periphery of splat, suggests that the principal oxidation process occurs at the substrate surface, where the splats are exposed to a water vapor-rich environment. It was also observed that the splat adhered well in some locations where elemental-diffusion and jetting of the substrate occurred, suggestive of substrate melting. A three-dimensional numerical model was developed to simulate the impact of a splat onto a substrate. The simulation shows that the observation of the central pore in the splat and the phenomenon of substrate melting may occur. Based on these results, the effect of water release on oxide formation and splat morphology can be explained.
Keywords: Numerical simulation; Substrate melting; FIB; TEM

Nano-Particle Sizing in a Thermal Plasma Synthesis Reactor by Lu Jia; François Gitzhofer (497-513).
The radio frequency inductively coupled thermal plasma synthesis process, based on the use of solution precursors as the process feedstock, has been employed for the production of ceria (CeO2) nano-powders. A sampling probe has been developed to continuously withdraw synthesized nano-powders from all desired positions within the plasma chamber for subsequent analysis. Using this probe, it was possible to study the 3D mapping of the plasma synthesis process. A flow of helium was introduced into the sampling probe to quench sampled particles and to prevent further particle growth within the sampling probe. Numerical simulations of the plasma flow were performed to study the influence of the probe tip geometry on the plasma flow. The reactor wall product collection method was also applied for sampling probe performance verification. The effects of selected plasma power and reactor pressure on the synthesized nano-powders size were investigated with this sampling probe. The results indicated that size distribution of the synthesized nano-powders is locally monomodal, with particles sizes as small as 4 nm being synthesized.
Keywords: Nano-powders; Sampling probe; Induction plasma

The enhancement of hydrophilicity of DC air and oxygen plasma treated cotton fabric and its effect on the antimicrobial efficacy when treated with neem leaf extract is reported in this paper. The axial and radial ion density distribution between the electrodes was studied using Langmuir probe to place the fabric between the electrodes for effective plasma treatment. The effect of system parameters viz., process gas pressure, DC power density and plasma exposure time on the hydrophilicity and hence the antimicrobial efficacy after the neem leaf extract treatment was analysed and optimised. The functional group present in the cellulose units of the cotton fabric before and after plasma treatment was identified and estimated using standard tests and analysed using ATR–FTIR spectra. The surface morphology of untreated and plasma treated cotton fabric was analyzed with the help of SEM micrograph. The mean pore diameter of the fabric matrix was calculated and air permeability was measured before and after plasma treatment to account for the improved capillarity due to plasma treatment. The formation of functional groups with increased polarity and improved capillary action of the plasma treated fabric enhances its hydrophilicity which in turn improves the sorption of neem leaf extract and hence the antimicrobial activity.
Keywords: Hydrophilicity; Antimicrobial activity; DC plasma; Langmuir probe; Ion density; Neem leaf extract; ATR–FTIR spectrum and SEM micrograph

Thin films from acrylic acid and Ar mixtures were deposited on polypropylene films, aluminum foils and silicon 100 wafers by radiofrequency (RF) plasma polymerization. Different deposition conditions were investigated, varying the gas-mixture feed composition and the reactor geometry. For every tested condition, stable coatings (as assessed by long time immersion in phosphate buffer saline) were obtained by varying the RF power input. The influence of the gas carrier on the layer stability was discussed. The films were further characterized by water contact angle measurements, attenuated total reflectance infrared spectroscopy, X-ray photoelectron spectroscopy and atomic force microscope. Moreover, retention of carboxylic acid groups in the stable layers were investigated by means of ion-exchange reaction with thionin acetate. Results show a strong influence of the gas feed composition and the reactor geometry on the chemical structure of the deposited coatings, especially in their carboxylic groups concentration.
Keywords: Acrylic acid; ATR FTIR; Plasma polymerization; Structure retention

Oxidation of Dimethyl Sulfide in Air Using Electron-Beam Irradiation, and Enhancement of its Oxidation Via an MnO2 Catalyst by Teruyuki Hakoda; Muhammed Alamgir Zaman Chowdhury; Akihiko Shimada; Koichi Hirota (549-557).
The decomposition of dimethyl sulfide (DMS) at initial concentrations of 4.5–18.0 ppmv in air was studied under electron-beam (EB) irradiation. Doses to decompose 90% of input DMS were 2.5 kGy for 4.5 ppmv, 3.4 kGy for 10.6 ppmv, and 3.9 kGy for 18.0 ppmv. HCOOH, (CH3)2SO, and trace CH3OH and (CH3)2SO2 were produced as irradiation products in addition to CO2 and CO. Application of an O3 decomposition catalyst to an irradiated sample gas led to an enhancement in the oxidation of DMS and its products into CO2 and the decomposition of O3. For 10.6 ppmv DMS/air, the mineralization ratio increased from 41% via only EB irradiation to 100% via the combination treatment at 6.3 kGy. The yield of CO2 to COx increased from 5.3 to 87.6% by combination with catalytic oxidation. This combination treatment enables the irradiation energy used to deodorize gas streams containing DMS to be reduced.
Keywords: Electron beam; Dimethyl sulfide; Odor; Decomposition; Catalyst

Depth Profiled XPS Analysis of a Polymerized Silicon-Carbon Thin Film by Paul R. Scott; David M. Wieliczka; Michael B. Kruger (559-566).
An in situ study of the chemical properties of a polymerized silicon-carbon thin film has been completed. The deposited film was created by plasma enhanced chemical vapor deposition of trimethylsilane onto an aluminum substrate. Depth profiled X-ray photoelectron spectra were obtained. An analysis of the data shows varying chemistry throughout the film as well as an interaction between silicon and aluminum/aluminum oxide at the bi-layer interface.
Keywords: XPS; PVD; PECVD; Trimethylsilane; Plasma polymerization; Corrosion protection; Aluminum; Silicon; Carbon