Gas-phase Reaction Mechanism Research Articles

Properties of c-C4F8 inductively coupled plasmas. II. Plasma chemistry and reaction mechanism for modeling of Ar/c-C4F8/O2 discharges

Gas mixtures containing Ar, c-C4F8, O2, and CO are often used for the plasma etching of silicon dioxide. Gas phase reaction mechanisms are required for first principles modeling of these systems to both provide insights to the plasma chemistry and to help optimize the process. In this article, results from computational and experimental investigations of the plasma chemistry of inductively coupled plasmas (ICPs) sustained in Ar, O2, Ar/c-C4F8 and O2/c-C4F8 gas mixtures with and without magnetic confinement are discussed. These results were used to develop a reaction mechanism for low-pressure and low-temperature plasmas sustained in mixtures initially consisting of any combination of Ar/c-C4F8/O2/CO. Predictions for ion saturation current and ion mass fractions were compared to experiments for validation. The consequences of charge exchange of fluorocarbon species with Ar+ and CO+ on the ratio of light to heavy fluorocarbon ion densities in Ar/c-C4F8/O2/CO plasmas are discussed. We found that the electron density and ion saturation current significantly increase with the addition of Ar to c-C4F8 but weakly depend on the addition of O2. The ratio of light to heavy fluorocarbon ion densities increases with power, especially for ICPs with magnetic confinement.

Trends and perspectives: Architectures for integrating legacy information systems with modern bar code technology

The homogeneous ignition of CH4/air, CH4/O2/H2O/N2, and CH4/O2/CO2/N2 mixtures over platinum was investigated experimentally and numerically at pressures 4 bar ⩽ p ⩽ 16 bar, temperatures 1120 K ⩽ T ⩽ 1420 K, and fuel-to-oxygen equivalence ratios 0.30 ⩽ φ ⩽ 0.40. Experiments have been performed in an optically accessible catalytic channel-flow reactor and included planar laser induced fluorescence (LIF) of the OH radical for the determination of homogeneous (gas-phase) ignition and one-dimensional Raman measurements of major species concentrations across the reactor boundary layer for the assessment of the heterogeneous (catalytic) processes preceding homogeneous ignition. Numerical predictions were carried out with a 2D elliptic CFD code that included elementary heterogeneous and homogeneous chemical reaction schemes and detailed transport. The employed heterogeneous reaction scheme accurately captured the catalytic methane conversion upstream of the gaseous combustion zone. Two well-known gas-phase reaction mechanisms were tested for their capacity to reproduce measured homogeneous ignition characteristics. There were substantial differences in the performance of the two schemes, which were ascribed to their ability to correctly capture the p–T–φ parameter range of the self-inhibited ignition behavior of methane. Comparisons between measured and predicted homogeneous ignition distances have led to the validation of a gaseous reaction scheme at 6 bar ⩽ p ⩽ 16 bar, a pressure range of particular interest to gas-turbine catalytically stabilized combustion (CST) applications. The presence of heterogeneously produced water chemically promoted the onset of homogeneous ignition. Experiments and predictions with CH4/O2/H2O/N2 mixtures containing 57% per volume H2O have shown that the validated gaseous scheme was able to capture the chemical impact of water in the induction zone. Experiments with CO2 addition (30% per volume) were in good agreement with the numerical simulations and have indicated that CO2 had only a minor chemical impact on homogeneous ignition.

Catalytic combustion of gasified biomass in a platinum monolith honeycomb reactor

The characteristics of catalytic ignition and reaction of single and multiple fuels including simulated gasified biomass are investigated both experimentally and numerically. A two-dimensional channel flow code with the multi-step gas phase and surface reaction mechanisms is used to clarify the detailed interaction mechanism of the main gasified biomass species on the surface. The light-off characteristics for single and multiple fuels with varying concentrations and global residence times are identified. For gasified biomass, the preheat temperature needed for light-off is found to be determined by carbon monoxide, which has the largest adsorption probability on platinum surface. The observed two-step rising of the reactor outlet temperature also provides evidence for the promotion of methane light-off by hydrogen and carbon monoxide. The operational domain for catalytic combustion of gasified biomass in a platinum monolith honeycomb reactor is bounded by curves of carbon monoxide concentration for ignition, residence time and methane concentration for conversion ratio, and the sinter temperature of the catalyst.

Kinetic and mechanistic study on the chemical vapor deposition of titanium dioxide thin films by in situ FT-IR using TTIP

The effect of the gas-phase reaction on the precursor decomposition, deposition rate and the properties of TiO2 films from MOCVD with titanium tetraisopropoxide (TTIP) has been investigated. In situ Fourier transform infrared spectroscopy measurement was performed to study the gas-phase reaction mechanism at 300–600 °C. We concluded that the formation of propene, H2O and isopropanol was the key species to understand the real CVD processes. The TiO2 film was deposited in low pressure chemical vapor deposition (LPCVD) and remote plasma enhanced chemical vapor deposition (remote-PECVD) reactor at the deposition temperature from 160 to 420 °C, which was characterized by SEM, XRD, Ellipsometer and atomic force microscopy. The deposition kinetics was dependent on the decomposition temperature of TTIP in different ambient (N2, N2/O2 and N2/O2 plasma). Anatase TiO2 films were deposited with PECVD even at low temperature below 280 °C with the three times higher deposition rate than that of LPCVD.

Metalorganic precursor decomposition and oxidation mechanisms in plasma-enhanced ZrO2 deposition

We investigated the gas phase reaction mechanisms in the ZrO2-deposition plasma using zirconium tert-butoxide (ZTB) as a metalorganic precursor, Ar as a carrier of the ZTB vapor, and O2 as an oxidant using quadrupole mass spectrometry (QMS). Zirconium containing ions including Zr+, ZrO+, ZrO2H+, ZrO3H3+, and ZrO4H5+ were clearly observed in the plasma, and ions of higher zirconium oxidation states become progressively favored at higher O2-to-ZTB carrying Ar flow rate ratio (O2/Ar), increased chamber pressure, and decreased microwave power. The average oxidation state calculated from the partition of ZrOxHy+ varied from 0.5 to 2.1 in the process range covering O2/Ar of 0 to 4, pressure of 5 to 40 mTorr, and power of 150 to 700 W. Based on the QMS analyses, we proposed two main opposing reaction paths responsible for the complex gas phase reactions, i.e., serial dissociations and serial oxidations. The increase in the electron temperature and density resulted in the shift of ZrOxHy+ to lower oxidation states by enhancing the dissociation of Zr–O bond and by depleting oxygen through gas phase reactions. The repartitioning of the ZrOxHy+ species was also contributed by their different Zr–O dissociation energies. To ascertain the effect of various process variables, we monitored the time evolutions of O2+ and ZrOxHy+ intensities as we abruptly change a process variable: the chemical effect (O2/Ar) led to gradual changes in their intensities, whereas physical effects (pressure and power) caused an abrupt step change in accordance with an instant response of the plasma electrons.

Mass Spectrometric Studies of Physical, Thermochemical and Reactive Properties of Xenon Fluorides, Xenon Oxides and Xenon Oxyfluorides

Using a reactor with a flowing diffusion cloud coupled to a high-resolution, low-energy electron-impact ionization mass spectrometer, mechanistic, kinetic and thermochemical characteristics of gas-phase reactions with the participation of charged and neutral xenon oxides, xenon fluorides and xenon oxyfluorides have been investigated. Ionization energies for XeF, XeF2, XeF4, XeO3, XeO4, XeOF4 molecules and appearance energies for the ions formed from these molecules were obtained. Based on experimental and reference data, the enthalpies of XeO3 and XeOF4 formation were refined and a number of binding energies in the parent and fragment ions were calculated. For electron-impact ionization, the ionization cross-sections for Xe, XeF2, XeF4 and XeOF4 proved to correlate with a semi-empirical principle of full ionization. Based on the temperature dependencies of saturated vapor pressures for XeO4, XeOF4 and XeO2F2, their enthalpies of evaporation, sublimation and melting were determined. The mechanisms of gas-phase reactions between H atoms and neutral XeF2, XeF4, XeF6, XeO4 and XeOF4 were studied.

The flame preheating effect on numerical modelling of soot formation in a two-dimensional laminar ethylene–air diffusion flame

Numerical modelling of soot formation is conducted for an axisymmetric, laminar, coflow diffusion ethylene–air flame by two different methods to investigate the effect of flame preheating. The first method cannot account for the preheating effect, while the second one can. A detailed gas-phase reaction mechanism and complex thermal and transport properties are used. The fully coupled elliptic equations are solved. A simple two-equation soot model is used to model the soot process coupled with detailed gas-phase chemistry. The numerical results are compared with experimental data and indicate that the flame preheating effect has a significant influence on the prediction of soot yields. Both methods give reasonable flame temperature and soot volume fraction distributions. However, quantitatively the second method results in improved flame temperatures and soot volume fractions, especially in the region near the fuel inlet, although the maximum flame temperatures from both methods are slightly lower than that from the experiment.

Role of Hydrogen-Bonded Intermediates in the Bimolecular Reactions of the Hydroxyl Radical

Because of their importance in atmospheric and combustion chemistry, the rate coefficients and mechanisms of gas-phase reactions of the OH radical have been studied extensively, and the kinetic dat...

High-quality SiO 2 film deposition using active reaction by oxygen radical

A novel film deposition method called radical shower chemical vapor deposition (RS-CVD) has been developed for high-quality gate-oxide (SiO 2) film formation on low-temperature poly-Si TFT-LCD. RS-CVD is characterised by a plasma damage-free deposition on a large area substrate at a low temperature, although discharge is used to generate the activated species of oxygen radicals. In film deposition, by using a reaction of SiH 4 with oxygen radicals, it is possible to grow SiO 2 films under low substrate temperature at low deposition pressure. Dependencies of film properties on deposition parameters are investigated to optimize the RS-CVD process. The parameter dependencies are interpreted with consideration of the gas phase reaction mechanisms, which account for the observable consequences of Si–OH bonds quantity incorporated in the films. Throughout this study, it is concluded that it is necessary for the film quality to reduce the contamination of intermediate species containing Si–OH bonds in the films and to control the excess reaction in the gas phase. Besides, the increase in the oxygen radical quantity is effective for the improvement of film properties.

Effects of gas and soot radiation on soot formation in a coflow laminar ethylene diffusion flame

A computational study of soot formation in an undilute axisymmetric laminar ethylene-air coflow jet diffusion flame at atmospheric pressure was conducted using a detailed gas-phase reaction mechanism and complex thermal and transport properties. A simple two-equation soot model was employed to predict soot formation, growth, and oxidation with interactions between the soot chemistry and the gas-phase chemistry taken into account. Both the optically thin model and the discrete-ordinates method coupled with a statistical narrow-band correlated-K based wide band model for radiative properties of CO, CO 2, H 2O, and soot were employed in the calculation of radiation heat transfer to evaluate the adequacy of using the optically thin model. Several calculations were performed with and without radiative transfer of radiating gases and/or soot to investigate their respective effects on the computed soot field and flame structure. Radiative heat transfer by both radiating gases and soot were found to be important in this relatively heavily sooting flame studied. Results of the optically thin radiation model are in good agreement with those obtained using the wide band model except for the flame temperature near the flame tip.

Introductory lecture. Advanced laser spectroscopy in combustion chemistry: from elementary steps to practical devices.

In recent years a large number of linear and nonlinear laser-based diagnostic techniques for nonintrusive measurements of species concentrations, temperatures, and gas velocities in a wide pressure and temperature range with high temporal and spatial resolution have been developed and have become extremely valuable tools to study many aspects of combustion. Beside the nonintrusive diagnostics of technical combustion devices the kinetics and microscopic dynamics of elementary chemical combustion reactions can be investigated in great detail by laser spectroscopy. These investigations show, that a small number of relatively simple elementary steps like H + O2-->OH + O, H2O2-->2OH, O + N2-->NO + N, NH2 + NO-->H2O + N2, OH + N2H control a large variety of combustion phenomena and pollutant formation processes. Laminar flames are ideal objects to develop the application of laser spectroscopic methods for practical combustion systems and to test and improve the gas-phase reaction mechanism in combustion models. Nonintrusive laser point and field measurements are of basic importance in the validation and further development of turbulent combustion models. Nonlinear laser spectroscopic techniques using infrared-visible sum-frequency generation can now bridge the pressure and materials gap to provide kinetic data for catalytic combustion. Finally, the potential of laser techniques for active combustion control in municipal waste incinerators is illustrated.

Elimination of the concentration dependence in mass isotopomer abundance mass spectrometry of methyl palmitate using metastable atom bombardment

An important problem in mass isotopomer abundance mass spectrometry (MIAMS) is the dependence of measured mass isotopomer abundances on sample concentration. We have evaluated the role of ionization energy on mass isotopomer abundance ratios of methyl palmitate as a function of sample concentration. Ionization energy was varied using electron impact ionization (EI) and metastable atom bombardment (MAB). The latter generates a beam of metastable species capable of ionizing analyte molecules by Penning ionization. We observed that ionization of methyl palmitate by EI (70 eV) showed the greatest molecular ion fragmentation and also showed the greatest dependence of relative isotopomer abundance ratios on sample concentration. Ionization using the 3 P 2 and 3 P 0 states of metastable krypton (9.92 and 10.56 eV, respectively) resulted in almost no molecular ion fragmentation, and the isotopomer abundances quantified were essentially independent of sample concentration. Ionization using the 3 P 2 and 3 P 0 states of metastable argon (11.55 and 11.72 eV, respectively) showed molecular ion fragmentation intermediate between that of EI and MAB(Kr) and showed an isotopomer concentration dependence which was less severe than that observed with EI but more severe than that observed with MAB(Kr). The observed decrease in the dependence of isotopomer abundance on sample concentration with a decrease in molecular ion fragmentation is consistent with the hypothesis that proton transfer from a fragment cation to a neutral molecule is the gas phase reaction mechanism responsible for the concentration dependence. Alternative explanations, e.g., hydrogen abstraction from a neutral molecule to a molecular cation, is not supported by these results. Moreover, the MAB ionization technique shows potential for eliminating one source of error in MIAMS measurements of methyl palmitate, in particular, and of fatty acids methyl esters, in general.

A theoretical study of the addition reactions of HF, H 2O, H 2S, NH 3 and HCN to carbodiimide and related heterocumulenes

Using ab initio methods at different levels of theory (RHF/6-31G ∗, B3LYP/6-311G ∗∗ and MP2/6-311++G ∗∗), we have investigated the reaction mechanism in gas phase of the addition of one molecule of HF, H 2O, H 2S, NH 3, and HCN to carbodiimide, carbon dioxide and ketene. In this last case both modes of addition were studied, i.e. across the carbon–carbon and the carbon–oxygen double bond. The potential energy surfaces were explored and the stationary points representing the reactants, transition states and products were localized. In all cases, the bond order and electron densities at the bond critical points were evaluated. Although our results correspond in all cases to large energy barriers for the 1:1 addition processes, these barriers are related to the results obtained using several molecules of water or ammonia.

Tropospheric formation of hydroxymethyl hydroperoxide, formic acid, H2O2, and OH from carbonyl oxide in the presence of water vapor: a theoretical study of the reaction mechanism.

We have carried out a theoretical investigation of the gas-phase reaction mechanism of the H2COO+ H2O reaction, which is interesting for atmospheric purposes. The B3LYP method with the 6-31G(d,p) and 6-311 + G(2d,2p) basis sets was employed for the geometry optimization of the stationary points. Additionally, single-point CCSD(T)/6-311 + G(2d,2p) energy calculations have been done for the B3LYP/6-311 + G(2d,2p) optimized structures. The reaction begins with the formation of a hydrogen-bond complex that we have calculated to be 6 kcalmol(-1) more stable than the reactants. Then, the reaction follows two different channels. The first one leads to the formation of hydroxymethyl hydroperoxide (HMHP), for which we have calculated an activation barrier of deltaGa(298) = 11.3 kcalmol(-1), while the second one gives HCO + OH + H2O, with a calculated activation barrier of deltaGa(298) = 20.9 kcalmol(-1). This process corresponds to the water-catalyzed decomposition of H2COO, and its unimolecular decomposition has been previously reported in the literature. Additionally, we have also investigated the HMHP decomposition. We have found two reaction modes that yield HCOOH+H2O; one reaction mode leads to H2CO + H2O2 and a homolytic cleavage, which produces H2COOH + OH radicals. Furthermore, we have also investigated the water-assisted HMHP decomposition, which produces a catalytic effect of about 14 kcalmol(-1) in the process that leads to H2CO + H2O2.

Spatial Distribution of H Atom Density in High-Density, Helicon-Wave H2 Plasmas Measured by Laser-Induced Fluorescence

Spatial distributions of H atom density in high-density, helicon-wave H2 plasmas were measured by (2+1)-photon laser-induced fluorescence spectroscopy. In low-power, high-pressure discharges, hill-type distributions, i.e., the H atom density in the plasma column was higher than that in the outside region, were observed. The hill-type density distribution can be explained by known mechanisms of gas-phase reactions, diffusion, and surface loss of H atoms. On the other hand, in high-power, low-pressure discharges, deep dips were observed in the spatial distribution of the H atom density. The dip region corresponded to the location of the plasma column, suggesting significant heating of H atoms.

Mass spectrometric studies of organic ion/molecule reactions.

In the past 25 years, a tremendous amount of work has been published on the ion/molecule reactions of organic species. This review provides an overview of the areas where gas phase ion chemistry has made a contribution to our understanding of fundamental organic reaction processes. It is clear that the gas phase work can provide insights into subtle features of reaction mechanisms that could not be addressed by conventional condensed phase methods. The study of ion/molecule reactions has already had a major impact on the way that organic chemists think about reaction mechanisms and interpret substituent effects. Moreover, it has heightened our awareness of the importance of solvation effects and how they can alter not only absolute rates but also relative rates, leading in some cases to complete reversals in reactivity patterns. A large body of work could not be included in this review due to space limitations. For example, the study of thermochemistry in the gas phase (i.e., acidities, basicities, bond strengths, binding energies, etc.) has provided a wealth of data that has been exceptionally useful in interpreting organic reaction mechanisms. This has spilled over into the study of organometallic systems, and several groups are making major headway in using mass spectrometry to probe the stability and reactivity of transition metal species. Finally, work involving chemical ionization has provided abundant information on gas phase reaction mechanisms. The future appears to be very promising for the study of gas phase organic reaction mechanisms. In particular, the emergence of new ionization techniques and more powerful mass analyzers will allow chemists to explore a wider range of species. Although still at an early stage, the gas phase study of biochemical transformations offers great promise and has been facilitated by electrospray and matrix assisted laser desorption ionization methods. In addition, these techniques provide a means for introducing important, metal-centered catalytic species into the gas phase and exploring the details of their reactivity. Finally, mass spectrometry continues to play a major role in the study of atmospheric ion chemistry and is providing important kinetic as well as mechanistic data.

Integrating factors and the compression of gas-phase reaction mechanisms

Integrating factors have been used to temporarily compress the differential equations describing the rates of change of the species of a gas-phase reaction mechanism, resulting in a faster simulation of gas-phase chemistry. The processing time requirements have been reduced by up to a factor of 3.7, by combining the use of Bernoulli's integrating factors with the assumption of linear time dependence within each substep of a multistep integration. No formal loss in accuracy relative to the original system was observed, with a suite of 2511 test cases having average errors of the same magnitude as the iteration error allowed within the numerical solver. The method thus presents a significant means for reducing the simulation time required by atmospheric reaction-transport models.

On the cis -> trans isomerization of the square-planar [Pt(Cl)(SnCl3)(PH3)2] compound: ab initio gas phase reaction mechanism and solvent effects using continuum models

A isomerizacao cis ® trans do composto quadratico plano d8 [Pt(Cl)(SnCl3)(PH3)2 ] foi investigada utilizando-se o nivel ab initio de calculo MP4(SDQ)//MP2. As estruturas otimizadas, localizadas na superficie de energia potencial em fase gasosa, indicam que esta reacao se processa atraves de um estado de transicao quase-tetraedrico. A influencia dos efeitos eletronicos dos ligantes no mecanismo da reacao foi investigada utilizando-se o metodo de Analise de Decomposicao de Carga (CDA), o qual forneceu suporte para a compreensao do forte efeito trans do ligante SnCl3. O efeito devido ao solvente na energia de reacao em fase gasosa foi avaliado utilizando-se os modelos continuos SCRF e IPCM. Em ambos os casos um aumento na barreira de energia para o processo foi observado, sendo que a estabilidade termodinâmica dos isomeros cis e trans foi alterada pela solvatacao.

125 酸水素燃焼による高エクセルギーを利用した往復動蒸気機関の開発(OS-1 熱回収によるエネルギーの有効利用(2))

Reaction of hydrogen and oxygen mixture in compressed water had been studied based on the principle of corresponding states and the gas phase reaction mechanism. Calculations have been carried out for the cases of heating the mixture of water, hydrogen, and oxygen under the constant volume. The results show that reaction starts at the temperatures higher than 760 K. It is also pointed out that hydrothermal reaction may enable the control of the ignition processes in diesel combustion of the hydrogen and oxygen mixture.

Intrinsic low-dimensional maniforlds of heterogeneous combustion processes

The method of intrinsic low-dimensional manifolds (ILDMs) has been shown to be a useful tool to reduce detailed reaction mechanisms. Previous studies have been concentrated on the gas-phase kinetics. In this paper we show that this method can be applied to heterogeneous combustion processes, too. Calculations have been performed using a well-known surface reaction system (methane-air mixure on platinum) in a laminar stagnation configuration, including detailed surface and gas-phase reaction mechanisms and a detailed transport model. Using a local eigenvalue analysis of the surface chemistry, which follows the same ideas as for the gas phase, the relaxed modes of the surface kinetics and the local dimesion of the ILDMs for surface species can be determined. A mathematical model is presented which allows a combined use of gas-phase and surface ILDMs in heterogeneous reacting flow calculations.