The Catalytic Oxidative Coupling of Methane

One of the great challenges in the field of heterogeneous catalysis is the conversion of methane to more useful chemicals and fuels. A chemical of particular importance is ethene, which can be obtained by the oxidative coupling of methane. In this reaction CH₄ is first oxidatively converted into C₂H₆, and then into C₂H₄. The fundamental aspects of the problem involve both a heterogeneous component, which includes the activation of CH₄ on a metal oxide surface, and a homogeneous gas-phase component, which includes free-radical chemistry. Ethane is produced mainly by the coupling of the surface-generated CH radicals in the gas phase. The yield of C₂H₄ and C₂H₆ is limited by secondary reactions of CH radicals with the surface and by the further oxidation of C₂H₄, both on the catalyst surface and in the gas phase. Currently, the best catalysts provide 20% CH₄ conversion with 80% combined C₂H₄ and C₂H₆ selectivity in a single pass through the reactor. Less is known about the nature of the active centers than about the reaction mechanism; however, reactive oxygen ions are apparently required for the activation of CH₄ on certain catalysts. There is spectroscopic evidence for surface O^? or O ions. In addition to the oxidative coupling of CH₄, cross-coupling reactions, such as between methane and toluene to produce styrene, have been investigated. Many of the same catalysts are effective, and the cross-coupling reaction also appears to involve surface-generated radicals. Although a technological process has not been developed, extensive research has resulted in a reasonable understanding of the elementary reactions that occur during the oxidative coupling of methane.     

Radiolysis of 2-propanol in the presence of 1-(2-furanyl)-1-hexanone; The yields of hydrogen and methane

Hydrogen and methane produced during the -radiolysis of 2-propanol have been measured in the absence and presence of 0.1 and 0.01 M ketone (1-(2-furanyl)-1-hexanone) at different doses. In the absence of ketone at 100 kGy, the hydrogen and methane yields were found to be G(H₂)=5.1 and G(CH₄)=2.02. At lower ketone concentrations the hydrogen and methane yields were G(H₂)=4.35 and G(CH₄)=1.83, while higher concentrations decreased the above values to G(H₂)=2.95 and G(CH₄)=1.33, respectively. The radiolytic mechanism is discussed in detail.     

Determinations of Intermediate Neutral Species in Hydrocarbon Discharge Plasma

This work reports laser ionization combined with Time-Of-Flight (TOF) mass spectrometry investigation on intermediate species in the hydrocarbon plasma of atmospheric-pressure fast-flow pulsed dc-discharge. All neutral intermediate species including transient radicals from benzene/Ar discharge have been characterized by a molecular beam sampling combined with TOF mass spectrometry. This shows that with a hydrocarbon gas mixture of 0.3% C₆H₆ in Ar discharge the intermediate species consist of simple radicals (such as C₂, C₅H₅, C₇H₇) and polycyclic organic molecules (C₁₀H₈, C₁₃H₁₀, C₁₄H₁₀). Theoretical studies on total energies and ionization potentials of the intermediate species have been carried out using the hybrid density functional theory. Effect of the ionization potential on mass spectral intensity has been discussed. Based on the observed data, the possible major neutral reaction channels of the plasma chemistry have been discussed. The developed experimental method has implications in volatile organic compounds removing and impurities diagnosis in Tokamak edge-plasma.     

The influence of hydrogen on the kinetics of plasmapyrolytic methane conversion

The plasma pyrolysis of methane in the presence of excess hydrogen was kinetically modeled by means of a single pulse shock tube (SPST) technique. Pyrolytic decomposition of methane and the product formation including quenching was investigated as a function of temperature and dwell time in order to elucidate the kinetic role of hydrogen. Methane pyrolysis with an excess of hydrogen leads to lower conversion degrees and lower product yields as compared to pure methane. The decay curves may be described kinetically by first- and second-order ate equations, depending on reaction conditions. An excess of hydrogen causes a decrease in the overall rate constants for the methane decomposition and a straightening of the Arrhenius plots. According to the reaction mechanisms the recombination of primary radicals, especially CH₃, with H₂ molecules and H atoms is significant.     

Spectroscopic and voltammetric studies of the cobalt (II) complex of Morin bound to calf thymus DNA

Absorption and emission spectra, viscometric and electrochemical studies have been carried out on the interaction of Morin (2, 3, 4, 5, 7-pentahydroxyflavone) and its Co complex, CoL₂·3H₂O[L = Morin (2-OH group deprotonated)], with calf thymus DNA. In the presence of DNA, the complex exhibits a hypochromism in the u.v.–vis. spectra and a large enhancement in emission spectra suggests that the complex binds to DNA via a weak partial intercalation, revealed by competitive experiments, viscosity and by electrochemical studies. The binding constant is ca. 2 × 10³ M^–1 at 20 °C. Both ZnL₂·3H₂O and CoL₂·3H₂O complexes have the same molecular structure, ZnL₂·3H₂O shows the spectral characteristics and electrochemical behaviour which agrees with observations for other intercalators in the presence and absence of DNA, whereas the CoL₂·3H₂O complex shows different spectral characteristics and electrochemical behaviour to that of ZnL₂·3H₂O, which suggests that the mode and affinity of the complex CoL₂·3H₂O binding to DNA are different from that of ZnL₂·3H₂O. Both ZnL₂·3H₂O and CoL₂·3H₂O complexes exhibited different antitumour activity. So the binding mode and affinity of complexes to DNA may play an important role in determining the antitumour activity.     

Microwave Plasma Jet in Water: Characterization and Feasibility to Wastewater Treatment

Plasma–liquid interactions have gained escalated interests over the last decade due to their potentials in many applications. The simultaneous generation of physicochemical phenomena of interest promotes itself to the top of the promising technologies for liquid processing. Here, we study the physics of a microwave plasma jet (MWPJ) submerged into water and its feasibility to wastewater treatment. We investigate the plasma and bubble dynamics using high-speed imaging. The effects of the argon flow rate, additive gas, and microwave power on the dynamics are examined highlighting the retreating behaviors of plasma channels due to the losses of electrons and power caused by nearby water surface. The addition of N₂ (<?5%) to Ar flow results in an oscillatory motion of the foremost edge of the plasma channel. We characterize the submerged MWPJ using a time- and space-averaged optical emission spectroscopy. We found the dominant OH (A–X) molecular band and atomic Ar lines with pure Ar flow indicating the effective dissociation of water. Meanwhile, the addition of N₂ leads to an intense emission of NH (A–X) molecular band. Finally, we assess the submerged MWPJ as a viable method for water purification based on the degradation of methylene blue (popular model compound). We find a significant improvement in the efficiency by adding 1–3% of N₂ to the Ar, which should be attributed to a combined effects of NH radicals, having high redox potential, and the backward reactions of H₂O₂ to form OH radicals with NO and NO₂.     

Molecular Beam Mass Spectrometry System for Characterization of Thermal Plasma Chemical Vapor Deposition

A molecular beam mass spectrometry system for in situ measurement of the concentration of gas phase species including radicals impinging on a substrate during thermal plasma chemical vapor deposition (TPCVD) has been designed and constructed. Dynamically controlled substrate temperature was achieved using a variable thermal contact resistance method via a backside flow of an argon/helium mixture. A high quality molecular beam with beamtobackground signal greater than 20 was obtained under film growth conditions by sampling through a small nozzle (75 m) in the center of the substrate. Mass discrimination effects were accounted for in order to quantify the species measurements. We demonstrate that this system has a minimum detection limit of under 100 ppb. Quantitative measurements of hydrocarbon species (H, H₂, C, CH₃, CH₄, C₂H₂, C₂H₄) using Ar/H₂/CH₄ mixtures and silicon species (Si, SiH, SiH₂, SiCl, SiCl₂, Cl, HCl) using Ar/H₂/SiCl₄ mixtures were obtained under thermal plasma chemical vapor deposition conditions.     

Chemical composition and properties of films produced from hexamethyldisilazane by plasma-enhanced chemical vapor deposition

Relationships between the chemical composition of the gas phase and the properties of SiC_xN_yH_z films produced from hexamethyldisilazane by plasma-enhanced chemical vapor deposition have been studied. The plasma composition has been examined by optical emission spectroscopy. Thermal analysis of the films with simultaneous mass spectrometric detection of released gases has been performed. On the basis of the results and published data, mechanisms for the formation of films by plasma polymerization have been proposed and the film growth at a low plasma power and high reactor temperatures has been found to follow the heterogeneous mechanism.     

FTIR study of the reaction of ethyl radicals with O2 and Cl2 in air at 295 K

Using Fourier transform infrared spectroscopy, the ethene yield from the reaction of C₂H₅ radicals with O₂ has been determined to be 1.50 ± 0.09%, 0.85 ± 0.11%, and <0.1% at total pressures of 25, 50, and 700 torr, respectively. Additionally, the rate constant of the reaction of C₂H₅ radicals with molecular chlorine was measured relative to that with molecular oxygen. (1) A ratio k₆/k₇ = 1.99 ± 0.14 was measured at 700 torr total pressure which, together with the literature value of k₇ = 4.4 × 10^?12 cm³ molecule^?1s^?1, yields k₆ = (8.8 ± 0.6) × 10^?12 cm³ molecule^?1s^?1. Quoted errors represent 2σ. These results are discussed with respect to previous kinetic and mechanistic studies of C₂H₅ radicals.     

Spectroscopic Characterization of a Steam Arc Cutting Torch

Optical emission spectroscopy has been used to investigate the characteristics of a plasma jet produced by a steam arc cutting torch operated in air at atmospheric pressure. A procedure has been developed for simultaneous determination of temperature and pressure in the plasma jet as well as an effective nonequilibrium factor. It is based on comparison of a few experimental and simulated spectral quantities. The experimental data were obtained from the spectrum of H_β and OII lines centred at 480 nm. The existence of the shock wave structure characteristic of an underexpanded jet can clearly be deduced from the measured properties. In the first expansion region, the centreline pressure drops from about 1.4 atm at the nozzle exit to about 0.7 atm a few tenths of millimeter downstream. On the contrary, the centreline temperature remains almost unchanged within this region and reaches the value of about 23,000 K.     

Sound velocity in different reacting thermal plasma systems

In most cases the energy dissipated in plasma jets used either,for heating or spraying is varied by changing the are current, the total gas floc+rate, and composition. However, when doing so, conditions are reached where the plasma jet may become supersonic. To predict such conditions or to characterize supersonic plasma jets the knowledge of the sound velocitya is mandatory The goal of this paper is to calculatea versus plasma forming gas composition, temperature, and pressure. Rigorous calculation would imply the knowledge of the chemical reaction kinetics, sound velocity depending strongly on this. Unfortunately such kinetics are generally lolknown for plasma jet floras and the only possibility is to determine the equilibrium sound velocitya calculated through the isentropic coefficient T. This coefficient has been calculated taking into account the dissociation and ionization reactions at equilibrium for temperatures ranging from 300 to 25,000 K and pressures between 0.1 and 1 Mpa for Ar, H₂, He, Ar-He, Ar-H₂, O₂, N₂, air, .steam, and methane.a often called the frozen sound velocity, was also calculated using (ratio of specific heats) instead of .     

On the Interaction of Cold Atmospheric Pressure Plasma with Surfaces of Bio-molecules and Model Polymers

We review studies of surface-interaction mechanisms for a surface microdischarge (SMD) and an atmospheric pressure plasma jet (APPJ) with model polymers and biomolecules in our laboratory. We discuss the influence of plasma source type, operating parameters, and gaseous environments on surface modifications and biological deactivation. We focus on mild, remote conditions where the visible plasma plume does not contact the surface. For an APPJ fed with Ar, the interaction of the plasma plume with O₂ and/or N₂ gaseous environments leads to oxidation and surface-bound NO_x even on materials containing neither oxygen nor nitrogen. The APPJ also modifies photo-sensitive polymers. Using optical filters, these modifications were shown to result in part from irradiation with vacuum ultraviolet (VUV) photons in a spectral range corresponding to Ar excimer emission. No VUV-induced effects were seen for the SMD source operated with O₂/N₂. SMD treatments using O₂/N₂ mixtures result in surface oxidation and nitridation. A new surface-bound species, NO₃, has been measured on the polymers and biomolecules. Depending on the gas chemistry and film molecular structure, the NO₃ surface concentration can reach 10 %. Both surface NO₃ on plasma-treated films of lipopolysaccharide (LPS), an immune stimulating biomolecule found in bacteria such as E. c oli, and overall surface oxidation correlate with LPS biological deactivation as evaluated using an enzyme-linked immunosorbent assay. Ambient humidity was studied using the SMD and was found to decrease overall surface modifications including NO₃ and biodeactivation for O₂-rich conditions. Lastly, we discuss possible mechanisms and compare our results with published simulation studies.     

Pyrolysis and Oxidation of Methane in a RF Plasma Reactor

The chemical kinetic effects of RF plasma on the pyrolysis and oxidation of methane were studied experimentally and computationally in a laminar flow reactor at 100 Torr and 373 K with and without oxygen addition into He/CH₄ mixtures. The formation of excited species as well as intermediate species and products in the RF plasma reactor was measured with optical emission spectrometer and Gas Chromatography and the data were used to validate the kinetic model. The kinetic analysis was performed to understand the key reaction pathways. The experimental results showed that H₂, C₂ and C₃ hydrocarbon formation was the major pathways for plasma assisted pyrolysis of methane. In contrast, with oxygen addition, C₂ and C₃ formation dramatically decreased, and syngas (H₂ and CO) became the major products. The above results revealed oxygen addition significantly modified the chemistry of plasma assisted fuel pyrolysis in a RF discharge. Moreover, an increase of E/n was found to be more beneficial for the formation of higher hydrocarbons while a small amount of oxygen was presented in a He/CH₄ mixture. A reaction path flux analysis showed that in a RF plasma, the formation of active species such as CH₃, CH₂, CH, H, O and O (¹D) via the electron impact dissociation reactions played a critical role in the subsequent processes of radical chain propagating and products formation. The results showed that the electronically excitation, ionization, and dissociation processes as well as the products formation were selective and strongly dependent on the reduced electric field.     

X-ray emission spectroscopy and the electronic structure of heterocyclic compounds

The electronic structure of the pyridine molecule has been investigated by x-ray emission spectroscopy. The NK _y and CK _y emission spectra have been measured. Ab initio and MNDO calculations have been carried out and individual bands in the spectra have been identified subsequently. The calculations produce spectral contours which approximate those of the experimental spectra.For Communication 3 see [1].Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 11, pp. 1484–1487, November, 1993.     

Isobutane Made Practical as a Reagent Gas for Chemical Ionization Mass Spectrometry

As a reagent gas for positive- and negative-mode chemical ionization mass spectrometry (CI-MS), isobutane (i-C₄H₁₀) produces superior analyte signal abundance to methane. Isobutane has never been widely adopted for CI-MS because it fouls the ion source more rapidly and produces positive CI spectra that are more strongly dependent on reagent gas pressure compared with methane. Isobutane was diluted to various concentrations in argon for use as a reagent gas with an unmodified commercial gas chromatograph-mass spectrometer. Analyte spectra were directly compared using methane, isobutane, and isobutane/argon mixtures. A mixture of 10% i-C₄H₁₀ in argon produced twice the positive-mode analyte signal of methane, equal to pure isobutane, and reduced spectral dependence on reagent gas pressure. Electron capture negative chemical ionization using 1% i-C₄H₁₀ in argon tripled analyte signal compared with methane and was reproducible, unlike pure isobutane. The operative lifetime of the ion source using isobutane/argon mixtures was extended exponentially compared with pure isobutane, producing stable and reproducible CI signal throughout. By diluting the reagent gas in an inert buffer gas, isobutane CI-MS experiments were made as practical to use as methane CI-MS experiments but with superior analytical performance.

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Graphical Abstract ?

     

A shock-tube study of the kinetics of reaction of hydroxyl radicals with H2, CO,CH4, CF3H,C2H4, and C2H6

Concentration-time profiles have been measured for hydroxyl radicals generated by the shock-tube decomposition of hydrogen peroxide in the presence of a variety of additives. At temperatures close to 1300°K the rate constants for the reaction are found to be in the ratio 0.18:0.19:0.59:1.00:2.33:2.88 for the additives CO:CF₃H:H₂:CH₄:C₂H₄:C₂H₆, respectively.     

Direct determination of the equilibrium constant and thermodynamic parameters in the reaction of pentadienyl radicals with O2

Reaction of pentadienyl radicals (C₅H₇) with O₂ has been studied by a combination of pulsed laser photolysis and photoionization mass spectrometry. These radicals could be generated either by the photolysis of 1,3-pentadiene or by the two-step reaction of carbon tetrachloride photolysis followed by the H-atom abstraction reaction of Cl atom with 1,4-pentadiene. The equilibrium between pentadienyl radicals, O₂ and pentadienylperoxy radicals could be observed over the range 268–308 K. An analysis of the temporal signal of pentadienyl radicals was used to evaluate the equilibrium constant. Third-law analysis was used to evaluate the enthalpy change for the reaction C₅H₇ + O₂ ⇌ C₅H₇O₂. The observed CO bond energy in the C₅H₇O₂ adduct was found to be 56.0 ± 2.2 kJ·mol^–1, which is lower than the values of peroxy radicals formed with allyl and cyclohexenyl radicals which have an allylic resonance structure.     

Low-temperature luminescence of aqueous systems

The emission and emission excitation spectra of various hydroxides, acids and halide salts, separately and in combination, have been measured in aqueous, methanolic and ethanolic solutions. The shifts of spectra as a function of solvent, the disappearance of emission in ethanol and the enhancement of the emissivity of the acids by the addition of halide salts suggest that the emitting entity is an exciplex of the hydrogen atom with water. The candidacy of(H₃O^.) or, better [(H₂O)_nH^.] as the emitter accords with all experimental information; however, GAUSSIAN 80 calculations for the lowest-energy quartet state suggest that this state is dissociative in the gas phase.     

A Study of the Influence of the Surrounding Gas on the Plasma Jet and Coating Quality During Plasma Spraying

Coating quality is affected by arc and plume instabilities during plasma spraying. In closed chamber plasma spraying, gradual drift is one of the intermediate instabilities, which is mainly due to the electrode erosion. This work focuses on the source of the gradual drift of the plasma jet and the influence on coating quality. The ambient state inside the chamber was controlled by a ventilation system and a vacuum system. The variation in the plasma jet was observed by a particle flux image device based on a CCD camera. The optical spectrum of the plasma plume was measured and analyzed through an optical spectrometer. The results indicated that the addition of hydrogen to plasma gas induced the change in the plasma jet length and width with changing rates depending on the chamber state and the ventilation power. With poor ventilation, the intensity of H_α emission was found to become gradually stronger while H_β and H_γ were found to become weaker. On closing the chamber and retaining enough ventilation power, it was observed that the ambient gas slowly turned red. Simultaneously, the coating weight and thickness were slightly decreased meanwhile the porosity ratio was obviously increased. The red ambient gas has been proved to be able to acidify the city water with pH value decreased from 7 to 1–3. Without hydrogen, the plasma jet was found to be stable without reddening and variation, but the plasma enthalpy was unfortunately low.     

Hydrogen abstraction by hydrocarbon radicals. I. Interactions between 1-phenyl ethyl radicals and 1-phenyl ethanol as well as benzyl alcohol

Hydrogen abstraction by 1-phenylethyl radicals (?H) from phenylmethyl-carbinol (HROH) and benzyl alcohol (H₂R′OH) has been studied in the liquid phase at 120°C. 1-Phenylethyl radicals have been generated by thermal decomposition of azo-bis-1-phenyl ethane and the formation of ethylbenzene (RH₂), acetophenone (RO), and 2,3-di-phenyl butane (R₂H₂) has been monitored during the reaction. In order to optimize the experimental conditions, a mechanism has been assumed for the various pathways of the disappearance of ?H and by using estimated rate parameters a presimulation was performed. The relative rate constants obtained are: and where k_H refers to the hydrogen abstraction while k_t is the combination rate coefficient of the radicals ?H.