The behavior of molecules in microwave-induced plasmas studied by optical emission spectroscopy. 1. Plasmas at atmospheric pressure

The behavior of molecules in different atmospheric microwave-induced plasmas (MIPs) has been studied by means of optical emission spectroscopy. This is in order to obtain more insight into molecular processes in plasmas and to investigate the feasibility of emission spectroscopy for the analysis of molecular compounds in gases, e.g. flue gases. Various molecular species (i.e. N₂, CO₂, H₂O, SF₆ and SO₂) have been introduced into discharges in argon or in molecular gases such as carbon dioxide or nitrogen. The plasmas were created and sustained by a guide-surfatron or a torch in the power range of 150 W to 2 kW. Only nitrogen sometimes yielded observable emission from the non-dissociated molecule (first and second positive system). Using other molecular gases, only dissociation and association products were observed (i.e. atomic species and diatomic molecules such as CN, C₂, CO, OH, NH and N₂⁺). The intensities of these products have been studied as a function of the concentration of introduced molecules, the position in the plasma and the composition of the plasma environment. Since in most cases the same diatomic association products are seen, observed associated molecules can only to some extend be related to the molecules originally present in the plasma gas. Therefore, it will be difficult to use atmospheric microwave discharges for the analysis of gas mixtures under the experimental conditions studied.     

The chemical reduction of small inorganic gases in an electrothermal plasma reactor

Chemical reduction of small inorganic gases is accomplished using an electro-thermal plasma reactor. This benchscale reactor maintains a highly reactive plasma zone in a fluidized bed of carbon particles through which an electrical current is discharged. The carbon particles function as current-controlling media, heat sinks and reaction sites. Chemicals introduced into this medium are subjected to a variety of energy sources and chemically reactive species, which result in chemical reduction. It is shown that the inorganic gases, H₂O, CO₂, NO, NO₂ and SO₂, are chemically reduced in the plasma zone of the reactor. The end products consist of hydrogen, carbon monoxide and nitrogen gases. Additionally, elemental sulfur is deposited onto the carbon particles.     

Micro- and nanosecond laser TiN coating/steel modification: Morphology studies

Morphology effects induced during interaction of μs- (Transversely Excited Atmospheric (TEA) CO₂ laser) or ns- (HF laser) pulses with titanium nitride (TiN) coating, deposited on austenitic stainless steel AISI 316, were studied. Experiments were carried out in regime of focused laser beam in air at atmospheric pressure. The used laser fluences were found to be sufficient for inducing intensive surface modifications of the target. The energy absorbed from the CO₂ as well as HF laser beam is mainly converted into thermal energy, causing different effects like ablation, appearance of hydrodynamic features, etc. Morphology characteristics obtained during ns-pulses irradiation (HF laser) were different to those initiated by μs-pulses (TEA CO₂ laser). The changes on the target surface in form of massive resolidifed droplets and crown-like structures were observed only for ns- (HF laser) pulses. It was found that these effects are a consequence of higher temperature and better coupling of the HF laser radiation with the target. Recent investigations of ps-Nd:YAG laser interaction with the same TiN coating showed that morphology picture is quite different including the reduction of thermal effect. The article is published in the original.     

Direct analysis of powder samples using transversely excited atmospheric CO<Subscript>2</Subscript> laser-induced gas plasma at 1 atm

A novel method for the direct and sensitive analysis of powder samples has been developed by utilizing the characteristics of a transversely excited atmospheric (TEA) CO₂ laser. In this study, a powder sample was placed in a container and covered by a metal mesh; the metal mesh functions to control the blowing-off of the powder. The container was then perpendicularly attached on a metal surface. When a TEA CO₂ laser (1.5 J, 200 ns) was focused on the metal surface, a large hemispherical gas plasma (radius of around 8 mm) with long emission lifetime (several tens of microseconds) was produced without ablating the metal surface. The high-speed expansion force of the gas plasma samples the powder covered by the metal mesh and fine powder particles are sent into the gas plasma region to be dissociated and excited. Sensitive semi-quantitative analysis was made on organic powder samples such as powdered rice, starch, seaweed (agar), and supplements. The detection limit of heavy metals of Cr in powdered mineral supplement was approximately 0.55 mg/kg.     

Effect of Electrical Pulse Forms on the CO2 Reforming of Methane Using Atmospheric Dielectric Barrier Discharge

Methane conversion using an electric discharge has been studied for many years. Recently, many research groups have developed high-frequency pulsed plasma reaction for methane conversion to higher hydrocarbons and synthesis gas. CO₂ reforming of methane to synthesis gas has also attracted considerable interest as a method of utilization of the greenhouse gases, CO₂ and CH₄, which occupy most of man-made greenhouse gases. In this study, the influence of pulse form of applied voltage on methane and carbon dioxide conversions and product selectivity has been investigated using a cylindrical type DBD reactor. For this purpose, two kinds of power supply were compared, that is, AC power supply which has a high-frequency sinusoidal wave form, and AC pulse power supply which has modified AC pulse wave form. The conversions of methane and carbon dioxide were enhanced using pulsed plasma. The lower pulse width was more profitable economically.     

A quantitative study of carbon monoxide and carbon dioxide evolution during thermal degradation of flame retarded epoxy resins

Thermogravimetric analysis (TGA) combined with infrared analysis of the evolved gases analysis (EGA) has been used to study the thermal degradation behaviour of epoxy resin both in air and nitrogen. The mass loss as a function of temperature has been correlated with the evolution of carbon monoxide (CO) and carbon dioxide (CO₂), and oxygen consumption as measured using an oxygen analyser. An analytical technique has been developed to quantitatively measure the carbon monoxide and dioxide gases evolved. The effect of a range of flame retardants containing phosphorus, nitrogen and halogen elements on CO and CO₂ evolution during thermal degradation of flame retarded epoxy resins has also been observed.     

Adsorption of gases on layered silicates at high pressures

Equilibrium adsorption of nitrogen, carbon dioxide, and argon was examined on the sodium and pyridinium forms of montmorillonite and on the hydrogen form of bentonite. The measurements were carried out at 303, 343, 373, and 400 K over pressure ranges of 0.1–90 MPa (Ar and N₂) and 0.1–6 MPa (CO₂). The amount of nitrogen vapor adsorbed was determined at 77 K and pressures from 0 to 0.1 MPa. The porous structure parameters of the studied samples were determined using adsorption isotherms of nitrogen, argon, and carbon dioxide vapors. At elevated temperatures and pressures >10 MPa, Ar and N₂ adsorption processes on the Na-form of montmorillonite and Ar adsorption on bentonite are activated, since the amounts of the gases adsorbed and adsorption volumes increase with temperature. No activated adsorption is observed for carbon dioxide adsorption on these adsorbents. A comparison of the excess adsorption isotherms of gases on the Py-form of montmorillonite and H-form of bentonite shows that adsorption in micropores predominates for the Py-form of montmorillonite, whereas for the Na-form of bentonite and H-form of bentonite adsorption occurs mainly in meso- and macropores.     

Carbon dioxide separation from hydrogen and nitrogen by fixed facilitated transport in swollen chitosan membranes

The separation of carbon dioxide from hydrogen and nitrogen at high temperatures would be valuable to fuel cell, flue gas purification, and ammonia processes. A feed gas mixture of carbon dioxide, hydrogen and nitrogen (10% CO₂, 10% H₂, and 80% N₂) was used to evaluate water-swollen chitosan as a facilitated transport membrane for these applications. The amino group of the chitosan repeating unit could be the fixed carrier that facilitates carbon dioxide transport in the presence of water.     

Surface modifications of TiN coatings by a pulsed TEA CO<Subscript>2</Subscript> laser: Coating thickness effects

Interactions of a transversely excited atmospheric (TEA) CO₂ laser, pulse duration ∼2 μs (initial spike FWHM ∼120 ns), with polycrystalline titanium nitride (TiN) coatings deposited on high-quality steel (AISI 316 or M2) were studied. The experiments were carried out in a regime of high laser energy densities: 25, 48, and 50 J/cm². The energy absorbed from the laser beam was partially converted to thermal energy and the effects of the TiN coating thickness on the morphological changes were considered. The morphological features and processes that accompany the interaction can be summarized as follows: (i) exfoliation of the TiN coating in the central zone of the irradiated area (for coating thickness of 1 μm) or appearance of grainy structure (for coating thicknesses 3 and 10 μm); (ii) appearance of hydrodynamic changes in the surrounding peripheral zone; and (iii) appearance of plasma in front of the target during sample irradiation. The text was submitted by the authors in English.     

Silicon Dioxide Coating of Titanium Dioxide Nanoparticles from Dielectric Barrier Discharge in a Gaseous Mixture of Silane and Nitrogen

The coating of titanium dioxide nanoparticles with silicon dioxide has been carried out by dielectric barrier discharge (DBD) plasma treatments to enhance the thermostability of Titania for applications at high temperature processes. During the first coating processing step, a closed film of silicon nitride was produced via plasma treatment in a gaseous mixture of silane and nitrogen, while atmospheric surface contaminations got mainly removed. In the second processing step, the DBD plasma treatment in oxygen or air was used to convert the silicon nitride mainly into silicon dioxide. Remaining carbon impurities at the interfaces between titanium dioxide and silicon nitride after the nitrogen/silane plasma treatment were subsequently removed simultaneously. Atomic force microscopy and X-ray photoelectron spectroscopy were employed to study the DBD plasma treatments of the TiO₂ nanoparticles.     

Solubility of some carrier gases in stationary liquid phases used in gas-liquid chromatography

A simple gas chromatographic technique for the determination of the solubility of gases in low-volatile liquids was proposed. The procedure is based on the introduction of a certain volume of the liquid saturated with the gas at atmospheric pressure into a gas chromatograph. The solubility of carrier gases (helium, hydrogen, nitrogen, methane, and carbon dioxide) in various stationary liquid phases (SLP), such as pentadecane, polydimethylsiloxane PMS-100, and polyethylene glycol PEG-600, was studied. The carrier gases studied can be arranged in the following series by solubility in SLP: He<H₂<N₂<CH₄<CO₂. This order coincides with the series reflecting change in the retention values in GLC for different carrier gases. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 923–925, May, 1999.     

Modelling of carbon dioxide: methane separation using titanium dioxide nanotubes

The separation of carbon dioxide (CO₂) and methane (CH₄) mixture is of considerable interest in order to purify natural gas, and one suggestion is that titanium dioxide (TiO₂) nanotubes might be exploited to separate a gaseous mixture of methane and carbon dioxide. In this study, we employ both Coulomb’s law and the Lennard–Jones potential to determine the total energy of adsorption CO₂ and CH₄ into a TiO₂ nanotube. The CH₄ is a nonpolar molecule, and therefore the Coulombic interaction may be neglected. The total energy of the systems is evaluated utilizing the continuous approximation, which assumes that the two gas molecules are spheres of certain radii, while the tube is modelled as a cylinder. Further, both electrostatic and van der Waals potentials are determined and expressed in the exact analytical formulae. The numerical results predict that a single molecule of CO₂ or CH₄ can be encapsulated into the tube. On assuming both gases may form clusters with the same proportion of atom species, a cluster of CO₂ will not be adsorbed into the tube when its radius exceeds 3.32?. On the other hand, a cluster of CH₄ can be encapsulated into an appropriate radius of TiO₂ nanotube. These results indicate that TiO₂ nanotubes may be useful in the purification of CH₄.     

Automatic Carbon,Hydrogen, Nitrogen,Sulfur Analyzer Ciiemistry of Sulfur Reactions

Abstract

An automatic analyzer for the simultaneous microdetermination of carbon, hydrogen, nitrogen, and sulfur is described. The method is based on the uncatalyzed, dynamic, flash-combustion of the sample in an oxygen/helium atmosphere in a quartz tube. Separation of the combustion gases, N₂, CO₂, SO₂, and H₂O is accomplished by using gas chromatography and a thermal conductivity detector. Reactions of SO₂ formation are given in detail.     

Acid and basic functionalities of nitrogen and carbon dioxide plasma‐treated polystyrene

The choice of plasma gas can determine the interaction between material and plasma and therefore the applications of the treated materials. Nitrogen plasma can integrate functional groups such as primary amines and carbon dioxide plasma can incorporate carboxylic groups on the surface of polymers. For specific adhesion such as bio‐adhesion, polar groups must be attached to the surface to enhance bio‐film formation but the acidic or basic character also controls the adhesion mechanism. Nitrogen and carbon dioxide plasmas are chosen to treat the surface of polystyrene and to show the effects of different functionalizations, i.e. attachment of acid or basic groups and degradation are compared in the present work. Nitrogen‐containing plasma induces mainly weak degradation at a rate of ～0.13 µg cm^?2s^?1. The roughness of the treated surface remains mostly unchanged. Functionalization leads to amino group attachment at a concentration of 1.2 sites nm^?2. We found that carbon dioxide plasma treatment shows more drastic degradation with a rate three times higher than that of nitrogen plasma and can create more functional groups (4.5 sites nm^?2) at mild plasma treatment. However, the roughness of the surface is altered. In both cases the aromatic groups are degraded through the plasma treatment (again this is more evident with the CO₂ plasma) and the induced functionalization was shown to be quick (the upper monolayer of polystyrene film can be functionalized rapidly). Copyright © 2005 John Wiley & Sons, Ltd.     

Solid-state potentiometric gas sensors—current status and future trends

The development of potentiometric sensors for monitoring environmental gases has become a well-established direction in sensor technology. Various types of potentiometric sensors employing solid electrolytes for in situ measurements of such gases as oxygen, hydrogen, carbon dioxide, sulfur oxides, carbon monoxide, nitrogen oxides, and hydrocarbons are reviewed. Particular concern was given to the CO₂ potentiometric sensor which is an example of successful commercial application. The construction details, working mechanism, and performance of different types of potentiometric gas sensors are given. Special emphasis is given for the mixed-potential electrodes, which seems to be the principal direction for the future research and development of the sensor science and technology. Additionally, the future use of potentiometric sensors for the detection of other environmental gases is discussed.     

Graphene Oxide Membranes with Heterogeneous Nanodomains for Efficient CO2 Separations

Achieving high membrane performance in terms of gas permeance and carbon dioxide selectivity is an important target in carbon capture. Aiming to manipulate the channel affinity towards CO₂ to implement efficient separations, gas separation membranes containing CO₂‐philic and non‐CO₂‐philic nanodomains in the interlayer channels of graphene oxide (GO) were formed by intercalating poly(ethylene glycol) diamines (PEGDA). PEGDA reacts with epoxy groups on the GO surface, constructing CO₂‐philic nanodomains and rendering a high sorption capacity, whereas unreacted GO surfaces give non‐CO₂‐philic nanodomains, rendering low‐friction diffusion. Owing to the orderly stacking of nanochannels through cross‐linking and the heterogeneous nanodomains with moderate CO₂ affinity, a GO‐PEGDA500 membrane exhibits a high CO₂ permeance of 175.5 GPU and a CO₂/CH₄ selectivity of 69.5, which is the highest performance reported for dry‐state GO‐stacking membranes.     

Influence of polar groups on the wetting properties of vertically aligned multiwalled carbon nanotube surfaces

In this work, we have studied superhydrophilic and superhydrophobic transitions on the vertically aligned multiwalled carbon nanotube (VACNT) surfaces. As-grown, the VACNT surfaces were superhydrophobic. Pure oxygen plasma etching modified the VACNT surfaces to generate superhydrophilic behavior. Irradiating the superhydrophilic VACNT surfaces with a CO₂ laser (up to 50?kW?cm^?2) restored the superhydrophobicity to a level that depended on the laser intensity. Contact angle and surface energy measurements by the sessile drop method were used to examine the VACNT surface wetting. X-ray photoelectron spectroscopy (XPS) showed heavy grafting of the oxygen groups onto the VACNT surfaces after oxygen plasma etching and their gradual removal, which also depended on the CO₂ laser intensity. These results show the great influence of polar groups on the wetting behavior, with a strong correlation between the polar part of the surface energy and the oxygen content on the VACNT surfaces. In addition, the CO₂ laser treatment created an interesting cage-like structure that may be responsible for the permanent superhydrophobic behavior observed on these samples.     

Application of copper sulfate pentahydrate as an ammonia removal reagent for the determination of trace impurities in ammonia by gas chromatography

Rapid analysis of trace permanent gas impurities in high purity ammonia gas for the microelectronics industry is described, using a gas chromatograph equipped with a phtoionization detector. Our system incorporates a reactive precolumn in combination with the analytical column to remove the ammonia matrix peak that otherwise would complicate the measurements due to baseline fluctuations and loss of analytes. The performance of 21 precolumn candidate materials was evaluated. Copper sulfate pentahydrate (CuSO₄·5H₂O) was shown to selectively react with ammonia at room temperature and atmospheric column pressures, without affecting the hydrogen, oxygen, nitrogen, methane or carbon monoxide peak areas. To prevent loss of trace carbon dioxide, an additional boron trioxide reactant layer was inserted above the copper sulfate pentahydrate bed in the reactive precolumn. Using the combined materials, calibration curves for carbon dioxide proved to be equivalent in both ammonia and helium matrix gases. These curves were equivalent in both matrix gases. The quantitative performance of the system was also evaluated. Peak repeatabilities, based on eight injections, were in the range of 4.1–8.2% relative standard deviation; and detection limits were 6.9 ppb for H₂, 1.8 ppb for O₂, 1.6 ppb for N₂, 6.4 ppb for CH₄, 13 ppb for CO, and 5.4 ppb for CO₂.     

Poly(Ionic Liquid)-Derived Carbon with Site-Specific N-Doping and Biphasic Heterojunction for Enhanced CO2 Capture and Sensing

CO₂ capture is a pressing global environmental issue that drives scientists to develop creative strategies for tackling this challenge. The concept in this contribution is to produce site-specific nitrogen doping in microporous carbon fibers. Following this approach a carbon/carbon heterojunction is created by using a poly(ionic liquid) (PIL) as a “soft” activation agent that deposits nitrogen species exclusively on the surface of commercial microporous carbon fibers. This type of carbon-based biphasic heterojunction amplifies the interaction between carbon fiber and CO₂ molecule for unusually high CO₂ uptake and resistive sensing.     

CN emission spectroscopy study of carbon plasma in nitrogen environment

Spectroscopic emission diagnostics of a carbon plasma created by an excimer KrF laser pulse at three laser fluences (12, 25 and 32 J/cm²) is performed under nitrogen ambient at pressures of 0.5 and 1 mbar. By following the time evolution of the radical CN spectral emission profiles, we notice, at a certain distance from the target surface, the existence of twin peaks for the time of flight distribution. This double structure depends on laser fluence and gas pressure parameters. The first peak moves forward in relation with the plasma expansion whereas the second peak moves backward and it is attributed to CN species undergoing oscillations or reflected shocks.