Ignition of hydrogen–methane–air mixtures over Pd foil at atmospheric pressure

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The chain character of the third self-ignition limit of hydrogen-oxygen mixtures and flame propagation at atmospheric pressure

It is shown that the characteristic time of the hydrogen-oxygen reaction without the participation of reaction chains is thousands of times longer than the characteristic time of heat removal from the reactor under third self-ignition limit conditions. As a result, reaction mixture self-heating does not exceed several degrees and, contrary to commonly accepted views, cannot cause thermal ignition. It is also shown that the reason for self-ignition under these conditions is the excess rate of chain branching compared with chain termination responsible for the formation of chain avalanches. For the same reason, layer-by-layer ignition during laminar flame propagation is also a chain process. Self-heating arises as a result of the development of chain combustion and strengthens chain avalanches. Explosion and detonation inhibition shows that chain avalanches play an important role in these processes.     

Features of the propagation of laminar spherical flames initiated by a spark discharge in mixtures of methane,pentane, and hydrogen with air at atmospheric pressure

Using high-speed digital color cinematography, we studied the propagation of a laminar spherical flame in stoichiometric mixtures of hydrogen, methane, and pentane with air in the presence of additives at atmospheric pressure in constant-volume reactors, and derived quantitative data on the time of formation of a stable flame front. Cellular flames caused by gas-dynamic instability attributable to convective flows arising during the afterburning of gas were observed in hydrocarbon-air stoichiometric mixtures diluted with inert additives. It was found that the effect of additives of carbon dioxide and argon (>10%) and minor additives of CCl₄ on the combustion of hydrocarbons, and of propylene on the combustion of hydrogen-rich mixtures, lead to periods of delay in the development of a laminar spherical flame; in addition, additives of propylene promote the combustion of hydrogen poor mixtures.     

Radio-frequency induction plasmas at atmospheric pressure: Mixtures of hydrogen,nitrogen, and oxygen with argon

Numerical calculations are reported which simulate atmospheric-pressure radiofrequency induction plasmas consisting of either pure argon or mixtures of argon with hydrogen, nitrogen, or oxygen. These calculations are compared to observations of laboratory plasmas generated with the same geometry and run conditions. The major features of the laboratory plasmas are predicted well by the calculations: the pure argon plasma is the largest, with the argon-oxygen plasma slightly smaller. The argon-nitrogen plasma is considerably smaller and the argon-hydrogen plasma is the shortest, although somewhat fatter than the argon-nitrogen case. The calculations are not entirely successful in predicting the exact location of the plasmas relative to the coils. A likely explanation is that there is significant uncertainty regarding the actual power coupled to the laboratory plasmas.     

Reactions of chloroethenes with atomic chlorine in air at atmospheric pressure

Relative rate method at the temperature of 298 K and pressure of 1013 hPa and GC-MS detection were used for the study of kinetics of the reactions of Cl atoms with H₂C=CCl₂, cis-ClHC=CHCl, trans-ClHC=CHCl, ClHC=CCl₂, and Cl₂C=CCl₂. The reaction products were identified by FTIR spectroscopy. A mechanism for the atmospheric degradation of chloro-ethenes has been suggested.     

Transport of uranium free particles in a d.c. arc plasma burning in air at atmospheric pressure

The transport parameter ψ was calculated for the free particles of uranium in a d.c. arc plasma burning in air. Using the “wire method” the volatilization rate (Q_w), the total concentration of free particles (_nt), the axial velocity (_vj) of uranium particles and the plasma cross section (S) were measured. The transport parameter was calculated for cylindrical symmetry of the arc. The total particle concentration calculated by the wire method was compared to values obtained by absolute intensity measurements of ion and atom spectral lines of uranium. This led to an estimate of the molecular concentration of uranium in the d.c. arc plasma.     

Reactions of primary and secondary butoxy radicals in oxygen at atmospheric pressure

The reaction pathways of n-butoxy and s-butoxy radicals have been investigated by TLC and HPLC analysis of end products, particularly peroxides and carbonyl compounds. The butoxy radicals were produced by the pyrolysis of very low concentrations of the corresponding dibutylperoxide in an atmosphere of oxygen and nitrogen, at atmospheric pressure. The decomposition reaction (3) s-BuO → C₂H₅ + CH₃CHO and the reaction (2) s-BuO + O₂ → HO₂ + CH₃COC₂H₅ have been studied, and the ratio k₃/k₂ has been determined in the temperature range 363–503 K by kinetic modeling of the formation of the observed acetaldehyde and methylethylketone. The rate constant k₃ obtained was: A good agreement was observed between experimental data and RRKM theory. The implications of the results for atmospheric chemistry and combustion are discussed. At room temperature, the reaction with O₂, yielding HO₂ radicals and methylethylketone is, by far, the main channel for s-BuO radicals. In the field of low temperature combustion, the decomposition of s-BuO radicals producing C₂H₅ and CH₃CHO is the main pathway; the route s-BuO + O₂ decreases tremendously in importance as the temperature is raised above 393 K.     

An experimental study of the electrospraying of water in air at atmospheric pressure

Water solutions with electrical conductivities ranging from that of the deionized water up to 2 S/m have been electrosprayed in air through narrow silica tubes. Results show unambiguously that steady cone jets of water in air without the assistance of glow discharge can be formed for the range of electrical conductivities we have explored. The absence of corona discharge has been proven not only for the good agreement between the experimental results and the scaling laws given in the cone-jet literature but also for the independence of the spray current on the atmosphere (air or CO(2)) in which water was being electrosprayed. Other regimes such as the electric dripping and the assisted glow discharge cone-jet mode that appear in the electrospraying of water in air at room temperature have also been investigated.     

Ignition of hydrogen–oxygen and stoichiometric hydrogen–methane–oxygen mixtures on hot wires at low pressures

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Solubilities of cefodizime disodium in aqueous alcohol mixtures at atmospheric pressure and different temperatures

Using dynamic method and the laser monitoring observation technique, the solubility of cefodizime disodium in water + (ethanol, 1-propanol, and 2-propanol) was measured as a function of temperature from 278.15 K to 318.15 K under atmospheric pressure. The experimental data were correlated with a simple model of molecular thermodynamics for solubility of solid in liquid. The model parameters were fitted, and the solution enthalpies Δ_solH and solution entropies Δ_solS were estimated. Δ_solH and Δ_solS are all positive. The endothermic effect of solution process may be due to the fact that the newly bond energy between cefodizime disodium and solvent molecules is not powerful enough to compensate the energy needed to break the original association bond in various solvents, and the system needs to absorb heat from surroundings and manifests as Δ_solH > 0. The reason for the entropy increase during the dissolution process is that the solutes disrupt the alignment of solvent molecules and therefore reduced the degree of order of the system while they were dissolved in various solvents. The positive Δ_solH and Δ_solS revealed that the dissolution process of cefodizime disodium was an entropy-driven process.     

Positive and negative gas-phase ion chemistry of chlorofluorocarbons in air at atmospheric pressure

This paper presents a report on the ionization/dissociation of some representative chlorofluorocarbons (CFCs) induced by corona discharges in air at atmospheric pressure. Both positive and negative ions formed from Freons 1,1,1-trichlorotrifluoroethane (CFC 113a), 1,1,2-trichlorotrifluoroethane (CFC 113), and 1,1,1,2-tetrachlorodifluoroethane (CFC 112a) were analyzed using an atmospheric pressure chemical ionization mass spectrometry (APCI-MS) instrument. Energy-resolved mass spectra were obtained by modulating the kinetic energy of the ions via adjustment of the sampling cone potential (V(cone)). Positive ion spectra of the CFCs (M) at low V(cone) show no signals due to either M(+)* or MH(+) but only those due to species [M - Cl](+) and CX(3)(+) (X = Cl, F), likely formed via C-Cl and C-C bond cleavages following ionization via charge exchange. Charge localization in the products of C-C bond cleavage in M(+)* is driven by the stability of the neutral fragment. At low V(cone) the hydrates [M - Cl](+)(H(2)O) are also observed. In the case of 1,1,2,-trichlorotrifluoroethane, [M - F](+) species also form as a result of ion-molecule reactions. As V(cone) is increased collision-induced dissociation of [M - Cl](+) and [M - F](+), i.e., the perhalogenated cations C(2)X(5)(+) (X = Cl, F), takes place via carbene elimination. In some cases such elimination is preceded or accompanied by rearrangements involving transfer of halogen from one carbon to the other. Evidence is also presented for the occurrence of a condensation reaction of C(2)Cl(3)F(2)(+) with water to form a C(2)Cl(2)F(2)HO(+) species via elimination of HCl. Negative ion spectra are dominated by Cl(-) and its ion-neutral complexes with M and with water. Additional components of the plasma include ion-neutral complexes O(3)(-)(M), the molecular anion M(-) (observed only with 1,1,2-trichlorotrifluoroethane), and an interesting species corresponding to [M - Cl + O](-). The origin and structure of these [M - Cl + O](-) species are discussed in terms of available thermochemical and reactivity data and current mechanistic views concerning reaction of O(2)(-) with halogenated compounds. The observation of both positive and negative ions containing oxygen is of special relevance to development of new processes for the treatment of volatile organic compounds (VOCs) based on oxidative decomposition induced by corona discharges in air at room temperature and pressure.     

Modified Markó’s aerobic oxidation of alcohols under atmospheric pressure with air or molecular oxygen at room temperature

A modified version of Markó’s aerobic oxidation procedure, using highly pure (99.995+%) CuCl with 4,7-diphenyl-1,10-phenanthroline (dpPhen), DBAD, and Cs₂CO₃ (98% purity) successfully oxidized primary and secondary alcohols to the corresponding aldehydes and ketones in excellent yield at room temperature with either air or molecular oxygen under atmospheric pressure.     

Saturation effects in the laser ablation of stainless steel in air at atmospheric pressure

A pulsed Nd : YAG laser was used to generate a plasma from stainless steel targets in air at atmospheric pressure. Laser focusing was found to be an important factor in the ablation process. The influence of focal conditions on spatial profiles of plasma, emission intensity and averaged ablation rate (AAR, μm pulse^–1) of stainless steel samples as a function of laser energy are discussed. At high energies and depending on laser beam focusing, ablation efficiency tends to decrease compared to that at lower energies. This effect can be due to plasma shielding and air breakdown. The averaged ablation rate was found to be dependent on the thickness of the sample. This effect results in shielding of the incoming laser beam and redeposition of removed material in the crater. By focusing the beam inside the material free expansion of plasma is allowed, resulting in more efficient erosion of the sample at larger energies. For comparative purposes, data on ablated mass per pulse are presented. Received: 25 January 1999 / Revised: 7 April 1999 / Accepted: 30 April 1999     

A mass spectrometry study of alkanes in air plasma at atmospheric pressure

The positive APCI-mass spectra in air of linear (n-pentane, n-hexane, n-heptane, n-octane), branched [2,4-dimethylpentane, 2,2-dimethylpentane and 2,2,4-trimethylpentane (i-octane)], and cyclic (cyclohexane) alkanes were analyzed at different mixing ratios and temperatures. The effect of air humidity was also investigated. Complex ion chemistry is observed as a result of the interplay of several different reagent ions, including atmospheric ions O₂^+•, NO⁺, H₃O⁺, and their hydrates, but also alkyl fragment ions derived from the alkanes. Some of these reactions are known from previous selected ion/molecule reaction studies; others are so far unreported. The major ion formed from most alkanes (M) is the species [M − H]⁺, which is accompanied by M^+• only in the case of n-octane. Ionic fragments of C _n H_2n ^+1/+ composition are also observed, particularly with branched alkanes: the relative abundance of such fragments with respect to that of [M − H]⁺ decreases with increasing concentration of M, thus suggesting that they react with M via hydride abstraction. The branched C₇ and C₈ alkanes react with NO⁺ to form a C₄H₁₀NO⁺ ion product, which upon collisional activation dissociates via HNO elimination. The structure of t-Bu⁺(HNO) is proposed for such species, which is reasonably formed from the original NO⁺(M) ion/molecule complex via hydride transfer and olefin elimination. Finally, linear alkanes C₅–C₈ give a product ion corresponding to C₄H₇⁺(M), which we suggest is attributed to addition of [M − H]⁺ to C₄H₈ olefin formed in the charge-transfer-induced fragmentation of M. The results are relevant to applications of nonthermal plasma processes in the fields of air depuration and combustion enhancement.     

Saturation effects in the laser ablation of stainless steel in air at atmospheric pressure

A pulsed Nd?:?YAG laser was used to generate a plasma from stainless steel targets in air at atmospheric pressure. Laser focusing was found to be an important factor in the ablation process. The influence of focal conditions on spatial profiles of plasma, emission intensity and averaged ablation rate (AAR, μm pulse^–1) of stainless steel samples as a function of laser energy are discussed. At high energies and depending on laser beam focusing, ablation efficiency tends to decrease compared to that at lower energies. This effect can be due to plasma shielding and air breakdown. The averaged ablation rate was found to be dependent on the thickness of the sample. This effect results in shielding of the incoming laser beam and redeposition of removed material in the crater. By focusing the beam inside the material free expansion of plasma is allowed, resulting in more efficient erosion of the sample at larger energies. For comparative purposes, data on ablated mass per pulse are presented.     

Colloid and nanodimensional catalysts in organic synthesis: II. The hydrogenation of alkenes with hydrogen at atmospheric pressure

Colloid particles of metals of varying valence are found to be relatively cheap and easily prepared catalyst permitting the the hydrogenation under mild conditions. Procedure of reduction of unsaturated compounds with gaseous hydrogen at atmospheric pressure and low temperaturehas been developed. Colloid solutions of metals from the series iron, nickel, cobalt, chromium, manganese or their mixtures were prepared in situ directly in the reaction mixture. Selective the hydrogenation of multiple carbon-carbon bonds without the reduction of a series of functional groups was observed.     

The role played by oxygen plasma on Teflon: relevance to the concept of "cryptoelectrons"

Dr Williams (AIP Adv., 2012, 2, 010701) suggested that cleaning Teflon by high pressure oxygen plasma may have affected our result that Cu(2+) and Pd(2+) ions can be absorbed but not chemically reduced by a Teflon surface rubbed by PMMA (Phys. Chem. Chem. Phys., 2012, 14, 5551). In response, we show that this treatment does not affect the adsorption of Cu(2+) and Pd(2+). We reaffirm our statement that Cu(2+) and Pd(2+) ions can be adsorbed by a Teflon surface only after rubbing with the organic polymers, not before rubbing.