Subsequent Gas-phase Reactions Research Articles

Chemical Variation among Protostellar Cores: Dependence on Prestellar Core Conditions

Abstract Hot corino chemistry and warm carbon chain chemistry (WCCC) are driven by gas–grain interactions in star-forming cores: radical–radical recombination reactions to form complex organic molecules (COMs) in the ice mantle, sublimation of CH4 and COMs, and their subsequent gas-phase reactions. These chemical features are expected to depend on the composition of the ice mantle, which is set in the prestellar phase. We calculated the gas–grain chemical reaction network considering a layered ice mantle structure in star-forming cores to investigate how the hot corino chemistry and WCCC depend on the physical condition of the static phase before the onset of gravitational collapse. We found that WCCC becomes more active if the temperature is lower, or the visual extinction is lower in the static phase, or the static phase is longer. The dependence of hot corino chemistry on the static-phase condition is more complex. While CH3OH is less abundant in the models with a warmer static phase, some COMs are formed efficiently in those warm models because there are various formation paths of COMs. If the visual extinction is lower, photolysis makes COMs less abundant in the static phase. Once the collapse starts and visual extinction increases, however, COMs can be formed efficiently. The duration of the static phase does not largely affect COM abundances. The chemical diversity between prototypical hot corinos and hybrid sources, in which both COMs and carbon chains are reasonably abundant, can be explained by the variation of prestellar conditions. Deficiency of gaseous COMs in prototypical WCCC sources is, however, hard to reproduce within our models.

Effects of the Gas-/Liquid-Phase Interactions on the Evolution of Bio-oil during Its Thermal Treatment

To clarify the interactions among bio-oil components during bio-oil pyrolysis, the gas- and liquid-phase interactions were decoupled and investigated, respectively. In this study, the bio-oil was separated into the aromatic-rich fraction and the aromatic-poor fraction, and these two fractions were pyrolyzed in a quartz basket with a baffle at the bottom to eliminate the liquid-phase interactions. The results showed that the gas-phase interactions promoted the coke formation via the cross-polymerization between the light compounds and aromatic compounds during bio-oil pyrolysis at 300–400 °C and the tar yields and content of the aromatics in the tar decreased correspondingly. At 500–800 °C, more large molecules were generated as a result of the intensified gas-phase interactions; thus, both the tar and coke yields were increased as well as the aromatics with more than three rings in the tar. Additionally, the increasing percentage of the product yields was higher at higher temperatures. The liquid-phase interactions promoted the formation of the non-evaporable large molecules, which affected the subsequent gas-phase reactions, leading to the increase in the heavy tar yields and decrease in the light tar yields. Meanwhile, the liquid-phase interactions promoted the formation of the primary coke, inhibited the formation of the secondary coke, and promoted the formation of the O-containing functional groups in the coke.

Modeling the Pressure Generation in Aluminum‐Based Thermites

AbstractThe paper proposes a new theoretical model based on local thermodynamic equilibrium enabling the prediction of gas generation during the reaction of aluminum‐based thermites. We demonstrate that the model has the capability to predict the total pressure and the partial pressures of its components as a function of the reaction extent and compaction. Al/CuO, Al/Bi2O3, Al/Sb2O3, Al/MoO3, and Al/WO3 thermites are modeled and their capability to generate pressure is compared. Simulation results are also validated through dedicated experiments and show general agreement beyond the state of the art. Mechanisms underlying pressure generation are detailed. A two‐stage process for the pressure increase in Al/CuO reaction, also observed experimentally, is shown to be driven by oxygen generation as produced by CuO and Cu2O vaporization through different kinetics. Comparison with experimental data stresses the issue of understanding the complex chemical processes taking place during vaporization and subsequent gas phase reactions and the need to determine their thermodynamic constants.

Hot wire chemical vapor deposition chemistry in the gas phase and on the catalyst surface with organosilicon compounds.

CONSPECTUS: Hot wire chemical vapor deposition (HWCVD), also referred to as catalytic CVD (Cat-CVD), has been used to produce Si-containing thin films, nanomaterials, and functional polymer coatings that have found wide applications in microelectronic and photovoltaic devices, in automobiles, and in biotechnology. The success of HWCVD is largely due to its various advantages, including high deposition rate, low substrate temperatures, lack of plasma-induced damage, and large-area uniformity. Film growth in HWCVD is induced by reactive species generated from primary decomposition on the metal wire or from secondary reactions in the gas phase. In order to achieve a rational and efficient optimization of the process, it is essential to identify the reactive species and to understand the chemical kinetics that govern the production of these precursor species for film growth. In this Account, we report recent progress in unraveling the complex gas-phase reaction chemistry in the HWCVD growth of silicon carbide thin films using organosilicon compounds as single-source precursors. We have demonstrated that laser ionization mass spectrometry is a powerful diagnostic tool for studying the gas-phase reaction chemistry when combined with the methods of isotope labeling and chemical trapping. The four methyl-substituted silane molecules, belonging to open-chain alkylsilanes, dissociatively adsorb on W and Ta filaments to produce methyl radical and H2 molecule. Under the typical deposition pressures, with increasing number of methyl substitution, the dominant chemistry occurring in the gas phase switches from silylene/silene reactions to free-radical short chain reactions. This change in dominant reaction intermediates from silylene/silene to methyl radicals explains the observation from thin film deposition that silicon carbide films become more C-rich with a decreasing number of Si-H bonds in the four precursor molecules. In the case of cyclic monosilacyclobutanes, we have shown that ring-opening reactions play a vital role in characterizing the reaction chemistry. On the other hand, exocyclic Si-H(CH3) bond cleavages are more important in the less-puckered disilacyclobutane molecules. Metal filaments are essential in HWCVD since they serve as catalysts to decompose precursor gases to reactive species, which initiate gas-phase reaction chemistry and thin film growth. We discuss the structural changes in metal filaments when exposed to various precursor gases. Depending on the nature of the radical intermediates formed from the hot-wire decomposition and subsequent gas-phase reactions, metal silicides and carbides can be formed. Overall, study of the gas-phase reaction chemistry in HWCVD provides important knowledge of the chemical species produced prior to their deposition on a substrate surface. This helps in identifying the major contributor to alloy formation on the filament itself and the film growth, and consequently, in determining the properties of the deposited films. An integrated knowledge of the gas-phase reaction chemistry, filament alloy formation, and thin film deposition is required for an efficient deposition of high-quality thin films and nanomaterials.

Competition of Silene/Silylene Chemistry with Free Radical Chain Reactions Using 1-Methylsilacyclobutane in the Hot-Wire Chemical Vapor Deposition Process

The gas-phase reaction chemistry of using 1-methylsilacyclobutane (MSCB) in the hot-wire chemical vapor deposition (CVD) process has been investigated by studying the decomposition of MSCB on a heated tungsten filament and subsequent gas-phase reactions in a reactor. Three pathways exist to decompose MSCB on the filament to form ethene/methylsilene, propene/methylsilylene, and methyl radicals. The activation energies for forming propene and methyl radical, respectively, are determined to be 68.7 ± 1.3 and 46.7 ± 2.5 kJ·mol(-1), which demonstrates the catalytic nature of the decomposition. The secondary gas-phase reactions in the hot-wire CVD reactor are characterized by the competition between a free radical chain reaction and the cycloaddition of silene reactive species produced either from the primary decomposition of MSCB on the filament or the isomerization of silylene species. At lower filament temperatures of 1000-1100 °C and short reaction time (t ≤ 15 min), the free radical chain reaction is equally important as the silene chemistry. With increasing filament temperature and reaction time, silene chemistry predominates.

Warm cores around regions of low-mass star formation

Warm cores (or hot corinos) around low-mass protostellar objects show a rich chemistry with strong spatial variations. This chemistry is generally attributed to the sublimation of icy mantles on dust grains initiated by the warming effect of the stellar radiation. We have used a model of the chemistry in warm cores in which the sublimation process is based on extensive laboratory data; these data indicate that sublimation from mixed ices occurs in several well-defined temperature bands. We have determined the position of these bands for the slow warming by a solar-mass star. The resulting chemistry is dominated by the sublimation process and by subsequent gas-phase reactions; strong spatial and temporal variations in certain molecular species are found to occur, and our results are, in general, consistent with observational results for the well-studied source IRAS 16293-2422. The model used is similar to one that describes the chemistry of hot cores. We infer that the chemistry of both hot cores and warm cores may be described by the same model (suitably adjusted for different physical parameters).

The chemistry of silicon in hot molecular cores

The gas phase chemistry of hot molecular cores is thought to depend upon the composition of evaporated ice mantle material and its subsequent gas phase reactions. We have modelled the silicon chemistry in these conditions from an SiH4 precursor and find substantial fractional abundances of SiO, H2SiO and HNSi on a timescale of a few 104 yr.

The role of initiation in oxidative coupling of methane

A generalized kinetic model, which satisfactory describes the main kinetic features of the gas-phase oxidative coupling of methane over a wide range of pressures and temperatures, was used to investigate the influence of an additional generation of methyl radicals on the process. It was shown that the additional generation of methyl radicals sufficiently enhances the yield of C 2 hydrocarbons up to the values obtained with the use of catalysts, but even with a very high rate of additional generation of radicals by means of an effective reaction CH 4+O 2→CH 3+HO 2 , subsequent gas-phase reactions do not permit to overcome a certain kinetic limit in C 2 yield. This is very similar to the results obtained in the catalytic oxidative coupling of methane and it is in good agreement with the assumption that at atmospheric and elevated pressures the main role of most catalysts in this process is an additional generation of methyl radicals.

The chemistry of silicon in hot molecular cores

The gas phase chemistry of hot molecular cores is thought to depend upon the composition of evaporated ice mantle material and its subsequent gas phase reactions. We have modelled the silicon chemistry in these conditions from an SiH4 precursor and find substantial fractional abundances of SiO, H2SiO and HNSi on a timescale of a few 104 yr.

Fourier transform mass spectrometric studies of isovalent rare earth ions Sc +, Y + and Lu + with methanol. Formation of dimethoxide—metal species M(OCH 3) +2

The primary and subsequent gas-phase reactions between methanol and the isovalent rare earth metal ions Sc +, Y + and Lu + have been investigated using a Fourier transform ion cyclotron resonance mass spectrometer. The three metal ions react exothermically with methanol. Among the primary reaction products obtained, only two are common to the three metals, the hydroxide—metal ions MOH +, and the methoxide—metal ions MOCH + 3. For Sc + and Y +, as primary products the metal oxide ions MO +, and the metal—formaldehyde complexes M(OCH 2) +, are also observed, as well as the formation of the metal—dihydride ion MH + 2 (only for Y +). All the primary ions undergo subsequent reactions with methanol, and the final product is always the M(OCH 3) 2(CH 3OH) + n species ( n = 0–3). The results are analysed to yield the lower and/or upper limits for bond energies: D°(M +OH) and D°(HOM +OH) > 92 kcal mol −1, D°(HOM +OCH 3) and D°(M +OCH 3) > 105 kcal mol −1 for M  and Lu, D°(HOSc +OCH 3) and D°(Sc +OCH 3) > 119 kcal mol −1, D°(ScOH) < 168 kcal mol −1, D°(YOH) < 175 kcal mol −1, and D°(LuOH) < 194 kcal mol −1.

Dynamic modeling of a pilot-scale fluidized-bed coal gasification reactor

This paper describes a dynamic mathematical model of a pressurized fluidized-bed coal-gasification reactor, developed and used to correlate data from a pilot-scale reactor. The model accounts for pyrolysis, oxidation, char gasification, and subsequent gas-phase reactions, fines elutriation, and heat losses from the reactor, and simulates both transient and steady-state operation. If the values of three model parameters are adjusted to fit data obtained during steady-state operation, the model yields good predictions of 11 reactor state variables and dynamic response to an upset in operating conditions, and so provides an excellent basis for adaptive supervisory control of the reactor.

A case for the importance of gas phase initiation and the absence of free radical processes in the thermal decomposition of silane

A critical analysis of a recent paper by Robertson, Hils and Gallagher (RHG) on the mechanism of the thermal pyrolysis of silane is presented, with special attention to the nature of the initiation reaction and subsequent gas phase reactions. It is argued that exclusive initiation at the walls, as proposed by RHG is indeed the major mode of initiation at total reaction pressures below 1 Torr. However, homogeneous gas phase initiation via the pressure-dependent unimolecular reaction SiH 4 → SiH 2 + H 2 dominates at total reaction pressures above 100 Torr. Subsequent gas phase reactions are non-chain processes involving silylene and silene intermediates. Free radical chain reactions, as proposed by RHG, are not involved.

Nitrogen pollutant formation in a pulverized coal combustor: Effect of secondary stream swirl

AbstractConcentrations of NO, NH3, and HCN, together with coal particle burnout and gas composition, were measured during combustion of a bituminous coal. Control of incoming secondary gas swirl level and overall stoichiometric ratio led to significant reductions in nitrogen oxide pollutant concentration. In‐situ measurements showed that coal particles were confined near the reactor center during rapid particle reaction. This took place in a locally fuel‐rich environment, producing near‐quantitative conversion of fuel‐nitrogen to NH3 and HCN, with some NO. Subsequent gas phase reactions of these nitrogen species were identified as important in establishing the ultimate NO concentration.

Measurement of the burning surface temperatures of propellant compositions by infra-red emission

The temperatures of extremely thin surface layers of burning solid propellant compositions have been determined by measurement of their infra-red radiation. Specific wavelengths, at which the flame gas emission is low, have been selected for the measurements. The path length of the radiating gaseous flame through which the surface is viewed has been reduced using a specially designed burner so that adequate correction for gas emission can be made. The surface temperatures of ammonium perchlorate compositions have been examined in some detail and the effect of the magnitude of the absorption coefficient of the material on the observed temperature has been demonstrated with this system. For example, at wavelengths at which the absorption coefficient is low, the surface temperatures of very weak fuel ammonium perchlorate powder mixtures appear to fall somewhat as the burning rate increases but in spectral regions of suitably high absorption coefficient the surface temperatures remain virtually constant, irrespective of the burning rate. The surface temperature for such mixtures burning at atmospheric pressure is 495° ± 15°C. Practical propellant compositions, based on ammonium perchlorate and containing rather more than stoichiometric proportions of fuel, give the same temperature provided that the hot gas emission can be successfully eliminated, but observed temperatures are in some instances raised (to a maximum of about 560°C) by gas interference. It is concluded that the process taking place at the burning ammonium perchlorate surface is an equilibrium sublimation process and that the rate of consumption of the solid is controlled by subsequent gas phase reactions among the dissociation products. The average surface temperature of double-base propellants increases with increase of burning rate, although it is likely that the liquid nitric ester component vaporizes at constant temperature. The retention of carbonaceous products near the burning surface of some compositions makes measurements of surface temperature inaccurate and has so far prevented the determination of the precise temperature dependence of the nitrocellulose decomposition. Double-base compositions have surface temperatures ranging from 300° to 400°C at atmospheric pressure. Binder-fuels currently used in composite propellants have surface temperatures close to 500°C when burning in a diffusion flame with oxygen.