Quasi-atmospheric Pressure Research Articles

Isomer-sensitive characterization of low temperature oxidation reaction products by coupling a jet-stirred reactor to an electron/ion coincidence spectrometer: case of n-pentane.

Through the use of tunable vacuum ultraviolet light generated by the DESIRS VUV synchrotron beamline, a jet-stirred reactor was coupled for the first time to an advanced photoionization mass spectrometer based upon a double imaging PhotoElectron PhotoIon COincidence (i2PEPICO) scheme. This new coupling was used to investigate the low-temperature oxidation of n-pentane, a prototype molecule for gasoline or diesel fuels. Experiments were performed under quasi-atmospheric pressure (1.1 bar) with a residence time of 3 s for two equivalence ratios (1/3 and 0.5) with a fuel initial mole fraction of 0.01. The measured time-of-flight mass spectra are in good agreement with those previously obtained with other photoionization mass spectrometers and, like those previous ones, display several m/z peaks for which the related species assignation is ambiguous. This paper shows how the analysis of the coincident mass-tagged Threshold PhotoElectron Spectra (TPES) together with first principle computations, consisting of the determination of the adiabatic ionization energies and the spectra of some products, may assist products' identification. The results mostly confirm those previously obtained by photoionization mass spectrometry and gas chromatography, but also allow a more accurate estimation of the 1-pentene/2-pentene mole fraction ratio. Our data also indicate a higher formation of acetone and methyl ethyl ketone than what is predicted by current models, as well as the presence of products that were not previously taken into account, such as methoxyacetylene, methyl vinyl ketone or furanone. The formation of three, four and five membered ring cyclic ethers is confirmed along with linear ketones: 2- and 3-pentanone. A significant general trend in indicating higher amounts of ketones than are indicated by gas chromatography is noted. Finally, TPES of alkenylhydroperoxides are also provided for the first time and constrains on the isomers identification are provided.

First Study of the Pyrolysis of a Halogenated Ester: Methyl Chloroacetate

The pyrolysis of a halogenated ester, methyl chloroacetate (MC), under dilute atmosphere and quasi-atmospheric pressure was studied at temperatures from 473 to 1048 K using an alumina tubular reactor. MC was chosen as a surrogate to model the thermal decomposition of ethyl bromoacetate, a chemical warfare agent. A maximum MC conversion of 99.8% was observed at a residence time of 2 s, a temperature of 1048 K, and an inlet mole fraction of 0.01. The following products were quantified: CO, CO2, HCl, methane, ethylene, ethane, propene, chloromethane, dichloromethane, vinyl chloride, chloroethane, and dichloroethane. For the first time, a detailed kinetic model of MC pyrolysis was developed and gave a good prediction of the global reactivity and the formation of most of the major products. Flow rate and sensitivity analyses were made to highlight the different pathways of decomposition during the MC pyrolysis. In a first attempt to extrapolate the results obtained with methyl chloroacetate to ethyl bromoacetate, simulations were run with a modified version of the model developed in this study taking into account the differences in bond dissociation energies induced by the change of the chlorine atom by a bromine one.

Effects of N2 and NH3 plasma exposure on the surface topography of p-GaN under quasi-atmospheric pressure

We used atomic force microscopy to characterize the surface topography of p-GaN exposed to N2 and NH3 plasma under quasi-atmospheric pressure using a microwave-excited plasma source with a microstrip line structure. The exposure time was varied from 2 to 20 min at a substrate temperature of 700 °C. Under both N2 and NH3 plasma exposure for 2 min, the ridge-shaped features on the surface of as-grown p-GaN dulled immediately and the surface roughness decreased remarkably, whereas the atomic step structure of the surface was maintained. The step crossing and bunching of the surface disappeared with increasing exposure time to both types of plasma. However, increasing the NH3 plasma exposure time to 20 min led to the formation of pits and appearance of particles along the step edges, resulting in drastic roughening of the surface. Thus, GaN surfaces can be smoothed without the destruction of their step structures via moderate plasma exposure under quasi-atmospheric pressure, and these plasma sources could prospectively be used in metal-organic chemical vapor deposition systems for nitride semiconductor growth.

A study of thermal decomposition of bromoethane

The pyrolysis of bromoethane under dilute atmosphere and quasi-atmospheric pressure was studied at temperatures from 600 to 975 K using a fused silica jet stirred reactor (JSR) and an alumina tubular reactor (TR). In the JSR, a complete conversion was observed at a residence time of 2 s, a temperature of 975 K and an inlet mole fraction of bromoethane of 0.01. In the TR, the full conversion was observed from 925 K. In both reactors, the following species were quantified: methane, ethylene, acetylene, ethane, 1,3-butadiene, vinylacetylene and vinyl bromide. A new detailed kinetic model was developed and gave a good prediction for both reactors of the global reactivity and the major products. Flow rate and sensitivity analyses have been performed to better understand the important reaction pathways during bromoethane pyrolysis. They showed, in contrast to previous studies, that the radical mechanism is far from being negligible and that the sole analysis of reaction products can be misleading as the radical mechanism mainly lead to ethylene and HBr which are also the two products from the molecular decomposition reaction of bromoethane.

A study of chlorobenzene pyrolysis

The pyrolysis of chlorobenzene under dilute atmosphere and quasi-atmospheric pressure was studied at temperatures from 800 to 1150 K using a fused silica jet stirred reactor (JSR) and from 800 to 1250 K in an alumina tubular reactor. Chlorobenzene was chosen as a surrogate to model the thermal decomposition of polychlorinated biphenyls (PCBs). In the jet stirred reactor, a maximum chlorobenzene conversion of 48.5 % was observed at a residence time of 2 s, a temperature of 1150 K and an inlet mole fraction of chlorobenzene of 0.005. The following species were quantified: benzene (the major product), HCl, methane, ethylene, acetylene, biphenyl, 1-, 2- and 3-chlorobiphenyl, biphenylene and six isomers of dichlorobiphenyls. In the tubular reactor, a maximum chlorobenzene conversion of 95% was observed at a temperature of 1250 K under the same conditions as in the jet stirred reactor. The same reaction products were detected but with a larger formation of acetylene and methane and a smaller production of chlorinated and bicyclic compounds. The effect of two H-atom donors, methane and hydrogen, has been investigated in the JSR. Hydrogen addition has a strong inhibiting effect on the formation of chlorinated and bicyclic products. A new detailed kinetic model was developed and gave a good prediction of the global reactivity, the formation of most major products, and the effect of the addition of hydrogen and methane. Flow rate and sensitivity analyses have been made to explain the effect of H-atom donors.

Surface Processing of Polyester Canvas using Atmospheric Pressure Air Glow Discharge Plasma

A staggered contact electrode structure was proposed in this study. Through pressing the electrode closely to the surface of polyester canvas, the electric field distribution with intensive electric field in short gap while weak electric field in long gap was formed, which was conducive to inhibit the emergence of filamentary discharge. Specifically, such electric field distribution could restrain the development degree of electron avalanche so that the quasi atmospheric pressure glow discharge plasma with nearly uniform luminescence could be generated in air gaps between the electrode and the surfaces of polyester canvas. Moreover, the device based on multi groups of staggered contact electrode structures was employed to achieve the large-area and continuous processing of polyester canvas. Through scanning electron microscope observations and peel strength tests, it could be observed that the surface roughness of polyester fabrics was obviously enhanced and the adhesion property of the canvas was greatly improved after treatment. The peel strength value was increased from 3.5 to 7.3 N/mm.

Products from the Oxidation of Linear Isomers of Hexene

The experimental study of the oxidation of the three linear isomers of hexene was performed in a quartz isothermal jet-stirred reactor (JSR) at temperatures ranging from 500 to 1100 K including the negative temperature coefficient (NTC) zone, at quasi-atmospheric pressure (1.07 bar), at a residence time of 2 s and with dilute stoichiometric mixtures. The fuel and reaction product mole fractions were measured using online gas chromatography. In the case of 1-hexene, the JSR has also been coupled through a molecular-beam sampling system to a reflectron time-of-flight mass spectrometer combined with tunable synchrotron vacuum ultraviolet photoionization. A difference of reactivity between the three fuels, which varies with the temperature range has been observed and is discussed according to the changes in the possible reaction pathways when the double bond is displaced. An enhanced importance of the reactions via the Waddington mechanism and of those of allylic radicals with HO2 radicals can be noted for 2- and 3-hexenes compared to 1-hexene.

An experimental and modeling study of the low- and high-temperature oxidation of cyclohexane

The experimental study of the oxidation of cyclohexane has been performed in a jet-stirred reactor at temperatures ranging from 500 to 1100K (low- and intermediate temperature zones including the negative temperature-coefficient area), at a residence time of 2s and for dilute mixtures with equivalence ratios of 0.5, 1, and 2. Experiments were carried out at quasi-atmospheric pressure (1.07bar). The fuel and reaction product mole fractions were measured using online gas chromatography. A total of 34 reaction products have been detected and quantified in this study. Typical reaction products formed in the low-temperature oxidation of cyclohexane include cyclic ethers (1,2-epoxycyclohexane and 1,4-epoxycyclohexane), 5-hexenal (formed from the rapid decomposition of 1,3-epoxycyclohexane), cyclohexanone, and cyclohexene, as well as benzene and phenol. Cyclohexane displays high low-temperature reactivity with well-marked negative temperature-coefficient (NTC) behavior at equivalence ratios 0.5 and 1. The fuel-rich system (ϕ=2) is much less reactive in the same region and exhibits no NTC. To the best of our knowledge, this is the first jet-stirred reactor study to report NTC in cyclohexane oxidation. Laminar burning velocities were also measured by the heated burner method at initial gas temperatures of 298, 358, and 398K and at 1atm. The laminar burning velocity values peak at ϕ=1.1 and are measured as 40 and 63.1cm/s for Ti=298 and 398K, respectively. An updated detailed chemical kinetic model including low-temperature pathways was used to simulate the present (jet-stirred reactor and laminar burning velocity) and literature experimental (laminar burning velocity, rapid compression machine, and shock tube ignition delay times) data. Reasonable agreement is observed with most of the products observed in our reactor, as well as the literature experimental data considered in this paper.

New experimental evidence and modeling study of the ethylbenzene oxidation

A study of the oxidation of ethylbenzene has been performed in a jet-stirred reactor (JSR) at quasi-atmospheric pressure (800Torr), at temperatures ranging 750–1100K, at a mean residence time of 2s and at three equivalence ratios ϕ (0.25, 1, and 2). Reactants and 25 reaction products were analyzed online by gas chromatography after sampling in the outlet gas. A new mechanism for the oxidation of ethylbenzene was proposed whose predictions were in satisfactory agreement with the measured species profiles obtained in JSR and with flow reactor data from the literature. A flow rate analysis has been performed at 900K showing the important role of the combinations with HO2 radicals of resonance stabilized radicals obtained from ethylbenzene by H-atom abstractions. Other important reactions of ethylbenzene are the ipso-additions of H- and O-atoms and of methyl radicals to the aromatic ring.

Detailed Product Analysis during Low- and Intermediate-Temperature Oxidation of Ethylcyclohexane

An experimental study of the oxidation of ethylcyclohexane has been performed in a jet-stirred reactor with online gas chromatography, under quasi-atmospheric pressure (800 Torr), at temperatures ranging from 500 to 1100 K (low- and intermediate-temperature zone including the negative temperature coefficient area), at a residence time of 2 s, and for three equivalence ratios (0.25, 1, and 2). Ethylcyclohexane displays important low-temperature reactivity with a well-marked negative temperature coefficient behavior. In addition to 47 products with a mass lower than ethylcyclohexane which have been quantified, many species with a C(8)H(14)O formula (molecular weight of 126) were detected by GC-MS and 7 of them were quantified. These molecules are cyclic ethers, ketones, and aldehydes with the same carbon skeleton as the reactant. Experiments were also carried on under the same conditions for two other C(8) hydrocarbons, n-octane and 1-octene, showing that the reactivity of ethylcyclohexane is close to that of the alkene and lower than that of the alkane. Simulations using a detailed kinetic model of the literature allow a good prediction of the global reactivity and of the main hydrocarbon products for temperatures above 800 K. The main reaction channels leading to the observed reaction products at both low (below 800 K) and intermediate temperature (above 800 K) are discussed.

Detailed product analysis during the low temperature oxidation of n-butane.

The products obtained from the low-temperature oxidation of n-butane in a jet-stirred reactor (JSR) have been analysed using two methods: gas chromatography analysis of the outlet gas and reflectron time-of-flight mass spectrometry. The mass spectrometer was combined with tunable synchrotron vacuum ultraviolet photoionization and coupled with a JSR via a molecular-beam sampling system. Experiments were performed under quasi-atmospheric pressure, for temperatures between 550 and 800 K, at a mean residence time of 6 s and with a stoichiometric n-butane/oxygen/argon mixture (composition = 4/26/70 in mol%). 36 reaction products have been quantified, including in addition to the usual oxidation products, acetic acid, hydrogen peroxide, C(1), C(2) and C(4) alkylhydroperoxides and C(4) ketohydroperoxides. Evidence of the possible formation of products (dihydrofuranes, furanones) derived from cyclic ethers has also been found. The performance of a detailed kinetic model of the literature has been assessed with the simulation of the formation of this extended range of species. These simulations have also allowed the analysis of possible pathways for the formation of some obtained products.

ChemInform Abstract: Hydrogenation of Olefins on Platinum.

Hydrogenation of buta-1,3-diene was studied on the three low-index planes of platinum under quasi-atmospheric pressure and hydrogenation of 2-methyl-buta-1,3-diene was investigated on Pt(111) in the same conditions. Kinetic results and especially selectivity were analyzed and compared to each other. Special attention has been given to the adsorbed species and their relative abundance on the surface. A mechanism has been proposed which explains the variations observed in the studied systems. In no case is the reaction governed by the dissociation of hydrogen and the selectivity depends on the stability of the hydrocarbon species on the surface.

Experimental study of the oxidation of methyl oleate in a jet-stirred reactor.

The experimental study of the oxidation of a blend containing n-decane and a large unsaturated ester, methyl oleate, was performed in a jet-stirred reactor over a wide range of temperature covering both low and high temperature regions (550-1100 K), at a residence time of 1.5 s, at quasi atmospheric pressure with high dilution in helium (n-decane and methyl oleate inlet mole fractions of 1.48 × 10-3 and 5.2 × 10-4) and under stoichiometric conditions. The formation of numerous reaction products was observed. At low and intermediate temperatures, the oxidation of the blend led to the formation of species containing oxygen atoms like cyclic ethers, aldehydes and ketones deriving from n-decane and methyl oleate. At higher temperature, these species were not formed anymore and the presence of unsaturated species was observed. Because of the presence of the double bond in the middle of the alkyl chain of methyl oleate, the formation of some specific products was observed. These species are dienes and esters with two double bonds produced from the decomposition paths of methyl oleate and some species obtained from the addition of H-atoms, OH and HO2 radicals to the double bond. Experimental results were compared with former results of the oxidation of a blend of n-decane and methyl palmitate performed under similar conditions. This comparison allowed highlighting the similarities and the differences in the reactivity and in the distribution of the reaction products for the oxidation of large saturated and unsaturated esters.

Experimental study of the oxidation of large surrogates for diesel and biodiesel fuels

The experimental study of the oxidation of two blend surrogates for diesel and biodiesel fuels, n-decane/ n-hexadecane and n-decane/methyl palmitate (74/26 mol/mol), has been performed in a jet-stirred reactor over a wide range of temperatures covering both low, and high-temperature regions (550–1100 K), at a residence time of 1.5 s, at quasi atmospheric pressure with high dilution in helium (hydrocarbon inlet mole fraction of 0.002) and at stoichiometric conditions. Numerous reaction products have been identified and quantified. At low and intermediate temperatures (less than 1000 K), the formation of oxygenated species such as cyclic ethers, aldehydes and ketones has been observed for n-decane, n-hexadecane, and methyl palmitate. At higher temperature, the formation of these species was not observed any more, and small amounts of unsaturated species (olefins and unsaturated methyl esters) have been detected. Results obtained with methyl palmitate and n-hexadecane have been compared in order to highlight similarities and differences between large n-alkanes and methyl esters.

Hydrogenation of olefins on platinum

Hydrogenation of buta-1,3-diene was studied on the three low-index planes of platinum under quasi-atmospheric pressure and hydrogenation of 2-methyl-buta-1,3-diene was investigated on Pt(111) in the same conditions. Kinetic results and especially selectivity were analyzed and compared to each other. Special attention has been given to the adsorbed species and their relative abundance on the surface. A mechanism has been proposed which explains the variations observed in the studied systems. In no case is the reaction governed by the dissociation of hydrogen and the selectivity depends on the stability of the hydrocarbon species on the surface.

Hydrogenation of isoprene on Pt(111)

The hydrogenation of isoprene (2-methyl-1,3-butadiene) was investigated on Pt(111) under quasi-atmospheric pressure and over a temperature range of 200 K. The surface was characterized by LEED and AES. Special attention has been given to the rate-determining adsorbed species; their relative abundances on the surface, governed by temperature and pressure parameters, showed evidence for their competitive influence on kinetics. However, only temperature affected product selectivity. As for sulfur poisoning, adsorption site blocking and hence mobility inhibition for adsorbed molecules can explain the shape of the deactivation curves.

Hydrogenation of 1,3-butadiene on Pt(111): Comparison with results on Pt(110) and Pt(100)

The adsorption of butadiene was realized under low pressure, p=10 −7 Torr and studied by Auger Electron Spectroscopy (A.E.S.). The reaction of butadiene hydrogenation was then investigated on Pt(111) under quasiatmospheric pressure. The surface was characterized by low energy electron diffraction (LEED) and Auger electron spectroscopy (AES) before and after each catalytic test. It was shown that the surface quickly reached a first saturation state equivalent to one butadiene molecule per 2 to 3 platinum atoms which did not reduce its catalytic activity. A mechanism was proposed involving butadiene as the most abundant surface species and the hydrogenation of C 4H 7 as the rate determining step. The influence of sulphur adsorbed in controlled quantities, θ s<1 was also investigated. Sulphur did not change the mechanism of the reaction but slightly improved the selectivity in butenes. Total poisoning occurred for θ s=0.5. Finally, a short review of the results concerning the hydrogenation of butadiene on the three low index faces of platinum is given. The most important result was an increase of the activity with increasing surface roughness.

Butadiene hydrogenation on platinum(100)

Butadiene hydrogenation was investigated on the (100) plane of platinum under quasiatmospheric pressure. The surface was characterized by LEED and A.E.S. before and after each catalytic test. The activity and the selectivity of the reaction were measured as a function of temperature and reactive pressures. In steady state conditions, the surface is covered by a well reproducible amount of carbon species (equivalent to 1C per platinum atom). A model is proposed according to which hydrogen dissociation and hydrocarbon adsorption are not competitive and the rate determining step is the hydrogenation of the half hydrogenated product C 4H 7 (a); moreover, there is good evidence that butane is mainly formed by selective hydrogenation of 1-butene. The influence of adsorbed sulphur in well controlled quantity was investigated: results showed the hydrogen dissociation and hydrocarbon adsorption become competitive, the decrease of activity as a function of sulphur coverage is not linear, and the selectivity in butenes is improved. These results are compared with results already obtained on Pt (110).