J.D. Skalny

Oxidative decoloration of dyes by pulsed discharge plasma in water

Abstract Degradation of organic dyes by the pulsed discharge plasma between needle-to-plane electrodes in contaminated water has been investigated in three discharge modes: (i) streamer, (ii) spark, (iii) spark–streamer mixed mode. The process of the decoloration has been found to be most effective if the discharge operates in the spark–streamer mixed mode in dye solutions. The decoloration rate during the pulsed discharge plasma treatment was dependent on the initial pH values. The decoloration rate was increased when more acidic condition was used, especially in the case of streamer discharge mode. The decoloration rate at the pH value of 3.5 was found to be approximately three times higher than that at a pH value of 10.3. A small effect of initial pH during the decoloration process by spark and spark–streamer discharge mode means that the physical effects, such as shock-wave and ultraviolet radiation, may play an important role in the oxidation process. It was found that the decoloration rates in the case of spark and spark–streamer mixed discharge modes, which are characterized by high intensity ultraviolet radiation, were found to be much higher than that in the case of streamer discharge that is characterized by low intensity ultraviolet radiation. In addition, the considerable increase in the decoloration efficiency of H 2 O 2 containing solutions can be attributed to the increase in hydroxyl radicals’ concentration. These are produced by ultraviolet light photo-dissociation of H 2 O 2 molecules in water surrounding the plasma channel.

Electron impact ionization of CH4: ionization energies and temperature effects

The threshold energy and the threshold behaviour for electron impact ionization of CH4 were investigated at two temperatures, 293 and 693 K. The study was performed with a crossed electron–molecule beam apparatus with an electron energy resolution of 120 meV full width at half maximum. The values of the ionization energies (IEs) and the threshold behaviour were determined using a fitting procedure involving a convolution of the cross section and the electron energy distribution function. At 293 K the following IEs were obtained: IE(CH4+/CH 4) = 12.65 ± 0.4 eV, IE1(CH3+/CH 4) = 13.58 ± 0.1 eV (ion pair) and IE2(CH3+/CH 4) = 14.34 ± 0.1 eV. At 693 K a red shift in these IEs of about 0.14 eV was observed for both CH4+/CH 4 and CH3+/CH 4 processes, which reflects the change in the internal energy of CH4 with increasing temperature. In addition, at 293 K IEs were determined also for the small fragment ions, i.e. IE(CH2+/CH 4) = 15.1 ± 0.1 eV, IE(CH+/CH 4) = 19.8 ± 0.1 eV and IE(C+/CH 4) = 20.5 ± 0.2 eV.

Experimental study of negative corona discharge in pure carbon dioxide and its mixtures with oxygen

The products of a negative corona discharge in both pure CO2 and mixtures of CO2 + O2 have been studied using a coaxial cylindrical electrode geometry with particular emphasis on the production of ozone. The discharge current in pure CO2 was found to be highly sensitive to the presence of trace concentrations of molecular oxygen and to changes in the flow speed through the discharge. The effect of dissociative electron attachment to ozone on the discharge current was studied by measurements of ozone and CO production. The ozone concentration increases monotonically with increasing content of oxygen in the mixture with carbon dioxide, whereas the CO concentration exhibits a flat maximum for oxygen concentrations of around 4%. A simple kinetic model of the dominant chemical processes is described and compared with the experimental results.

Low energy electron attachment to SF5CF3

Low energy electron attachment to the potent greenhouse gas SF5CF3 is studied at high energy resolution by means of mass spectrometric detection of the product anions. A large dissociative electron attachment (DA) cross-section forming SF5−+CF3 is observed within a very narrow energy range close to zero eV. In addition, comparatively weak resonances are observed near 1 eV yielding the fragment ions CF3− and F−. Some implications for the atmospheric lifetime of SF5CF3 and hence its global warming potential (GWP) are considered.

Dissociative electron attachment to using a high-resolution crossed-beams technique

Using a crossed electron - molecule beam ion source in combination with a quadrupole mass spectrometer we have studied the electron energy dependence of the dissociative attachment process (at electron energies from about 0 - 2 eV and with an energy resolution of about 60 meV) in a target gas range of about 300 - 420 K. Utilizing the improved energy resolution of the present experimental set-up it was possible to determine the accurate shape and magnitude of the cross section function in this low-energy range. This leads to the conclusion that close to 0 eV energy (below about 0.15 eV) the reaction proceeds via s-wave capture, whereas at higher energy (above 1 eV) autodetachment significantly determines the shape of the cross section. Moreover, the present measurements allow us to clarify previously reported differences in the absolute cross section, in the number of peaks and in the energy positions of these peaks. Finally, by analysing the measured strong temperature dependence of the cross section peak at about zero electron energy the activation barrier for this dissociative attachment was determined to be in good agreement with thermochemical data deduced from swarm experiments.

A Study of Ozone Generation by Negative Corona Discharge Through Different Plasma Chemistry Models

Abstract The Steady radial distribution of chemical species in a wire‐to‐cylinder ozone generator filled with pure oxygen has been computed by applying four different plasma chemistry models of increasing complexity. The most complete model considers ten species (e, O2 +, O2 −, O3 −, O−, O2, O2(1Δg), O2(1∑g +), O and O3) and 79 reactions, including ionization by electron impact, electron attachment and detachment, electron-ion recombination, charge transfer, etc. The chemical model is coupled with the electrical model through Poissons equation. The spatially averaged ozone density has been computed as a function of the current intensity and compared with the experimental values obtained by UV spectroscopy.

Dissociative electron attachment to ozone using a high-resolution crossed beams technique

Abstract The production of O− and O2− by dissociative attachment to ozone has been studied as a function of electron energy (from about 0 up to about 9 eV) in a recently constructed high-resolution crossed beams apparatus combined with a quadrupole mass spectrometer. After absolute calibration of the ozone gas density and taking into account the different detection efficiency absolute attachment cross sections for both fragment anions could be deduced. The corresponding peak values are 2.8∗x10 −17 cm 2 at an electron energy of 1.33 eV and 1.83∗x10 −17 cm 2 at an electron energy of 1.05 eV for the O− and O2− production, respectively.

Dissociative electron attachment to CF2Cl2

Using a recently constructed high resolution crossed electron/molecular beam apparatus consisting of a hemispherical electron monochromator and a quadrupole mass spectrometer we have measured the relative production cross sections for CI– and F– via electron attachment to CF2Cl2. The relative Cl– cross section is placed on an absolute scale by reference to an absolute rate coefficient using a calibration method involving integration of the measured anion signal. The most efficient Cl– production process is at about zero energy and its magnitude is resolution limited. The present high resolution value of 6 × 10−16 cm2 compares well with an earlier value reported by Chen and Chantry. A second peak is detected at around 0.8 eV in accordance with some of the earlier beam and swarm measurements. The observed production of F– has an appearance energy of 1.9 eV and the energy of maximum cross section is 3.36 eV, the latter value comparing well with several previous studies.

Low-energy electron attachment to mixed ozone/oxygen clusters

Abstract Electron attachment to a cluster beam formed by adiabatic expansion of a mixture of O 3 (1%) and O 2 (99%) is studied in the energy range 0–4 eV. Despite the initial large excess of oxygen molecules, the dominant attachment products are undissociated cluster ions (O 3 ) m − including the monomer O 3 − , while oxygen cluster ions (O 2 ) n − appear with comparatively low intensity. This behaviour is explained by an enrichment of ozone in the cluster formation process and the preferential formation of O 3 − from mixed clusters. The structured energy dependence of the cross section of O 3 − formation is interpreted in terms of three different mechanisms, in the low-energy region by s-wave capture, around 1 eV via Feshbach resonances, and above 1.5 eV by self-scavenging, i.e. inelastic scattering of the primary electron involving low-lying electronic states of neutral ozone and subsequent attachment of the slowed-down electron to another ozone molecule in the same cluster.

Ozone generation in a negative corona discharge fed with N2O and O2

Ozone production in a negative corona discharge has been studied experimentally at atmospheric pressure in mixtures of N2O+O2 at ambient temperature. Ozone formation was found to be dramatically reduced with increase in the content of N2O in the mixture. The reaction of O(1D) with nitrous oxide is the most likely process reducing the rate of ozone generation. A considerable decrease in the mean discharge current at a constant voltage was also observed with increasing content of nitrous oxide in the mixture and is attributed to the formation of anions in the discharge. The low values of the calculated mobility of negative charge carriers in the drift region of the discharge is caused by the formation of O− · (N2O)n and NO−·(N2O)n cluster anions in the drift region of the negative corona discharge.