Gas-phase Chemical Models Research Articles

Extinction characteristics of catalyst-assisted combustion in a stagnation-point flow reactor

As a fundamental study to understand physical and chemical characteristics in catalyst-assisted combustion, numerical simulations of a stagnation-point flow combustor with a catalytic surface are performed. The combustible mixture of methane and air is blown on top of a platinum surface, forming a classical stagnation-point flow configuration. This geometry not only represents an on-chip microcombustor considered in recent studies, but it also serves as a canonical problem of combined heterogeneous/homogeneous combustion subjected to flow straining, which is a key parameter that governs the quenching and flammability limit. One-dimensional similarity formulation is derived with full consideration of detailed surface and gas-phase chemical kinetic models. Parametric studies are conducted to investigate the effects of strain rate, equivalence ratio, heat loss on the combustion, and extinction modes. The steady results showed that catalysis can largely extend the extinction limit, while suppressing the gas-phase reaction at lower strain rates. It was also found that the extension of the catalytic reaction quenching limit is sensitive to the mixture composition, suggesting the dominance of chemical aspects in catalytic combustion. The temperature versus strain rate response curves exhibit multiple branches of stable solutions, implying a possibility of hysteresis behavior in a coupled homogeneous–heterogeneous reactor. Extensive parametric studies in terms of the mixture equivalence ratio and the conductive heat loss parameter have revealed three distinct steady response regimes: a surface-dominant monotonic response, continuous two-branch response, and separated response showing an isolated surface-reaction island. The results are expected to provide insight into improving the overall combustion stability and efficiency of catalyst-assisted combustors.

CH$_\mathsf{3}$OH abundance in low mass protostars

We present observations of methanol lines in a sample of Class 0 low mass protostars. Using a 1-D radiative transfer model, we derive the abundances in the envelopes. In two sources of the sample, the observations can only be reproduced by the model if the methanol abundance is enhanced by about two order of magnitude in the inner hot region of the envelope. Two other sources show similar jumps, although at a lower confidence level. The observations for the other three sources are well reproduced with a constant abundance, but the presence of a jump cannot be ruled out. The observed methanol abundances in the warm gas around low mass protostars are orders of magnitude higher than gas phase chemistry models predict. Hence, in agreement with other evidences, this suggest that the high methanol abundance reflects recent evaporation of ices due to the heating by the newly formed star. The observed abundance ratios of CH3 OH, H2 CO, and CO are in good agreement with grain surface chemistry models. However, the absolute abundances are more difficult to reproduce and may point towards the presence of multiple ice components in these regions.

Detection of steam in the circumstellar disk around a massive Young Stellar Object

We report on the observation of hot water vapor (steam) in the inner AU of a young massive star located in the star- forming region IRAS 08576-4334. The water lines are detected in a medium resolution (R ∼ 10 000) K-band spectrum taken by the infrared spectrometer ISAAC mounted on the VLT-ANTU. The water vapor is at a mean temperature of 1565 ± 510 K, cooler than the hot CO found in the same object, which is at � 1660 K; and the column density is N(H2O) = (2.5 ± 0.4) × 10 18 cm −2 . The profile of both H2O and CO lines is best reproduced by the emission from a Keplerian disk. To interpret the data, we also investigate the formation of molecules and especially CO and water vapor in the inner hot and dense part of disks around young high mass stars using a pseudo time-dependent gas-phase chemical model. Molecules are rapidly photodissociated but this destruction is compensated for by an efficient formation due to fast neutral-neutral reactions. The ability of CO molecules to self-shield significantly enhances its abundance. Water molecules are sufficiently abundant to be detectable. The observed H2O/CO ratio is reproduced by gas at 1600 K and by an enhanced UV field over gas density ratio IUV/nH = 10 −4 −10 −6 .T he simulations support the presence of CO and H2O molecules in the inner disks around young massive stars despite the strong UV radiation and show that the OH radical plays an essential role in hot gas chemistry.

Development and initial evaluation of a dynamic species‐resolved model for gas phase chemistry and size‐resolved gas/particle partitioning associated with secondary organic aerosol formation

A module for predicting the dynamic evolution of the gas phase species and the aerosol size and composition distribution during formation of secondary organic aerosol (SOA) is presented. The module is based on the inorganic gas‐aerosol equilibrium model Simulating the Composition of Atmospheric Particles at Equilibrium 2 (SCAPE2) and updated versions of the lumped Caltech Atmospheric Chemistry Mechanism (CACM) and the Model to Predict the Multiphase Partitioning of Organics (MPMPO). The aerosol phase generally consists of an organic phase and an aqueous phase containing dissolved inorganic and organic components. Simulations are presented in which a single salt (either dry or aqueous), a volatile organic compound, and oxides of nitrogen undergo photo‐oxidation to form SOA. Predicted SOA mass yields for classes of aromatic and biogenic hydrocarbons exhibit the proper qualitative behavior when compared to laboratory chamber data. Inasmuch as it is currently not possible to represent explicitly aerosol phase chemistry involving condensed products of gas phase oxidation, the present model can be viewed as the most detailed SOA formation model available yet will undergo continued improvement in the future.

Radiation effects on combustion and pollutant emissions of high-pressure opposed flow methane/air diffusion flames

Numerical simulations of one-dimensional opposed-flow laminar methane/air diffusion flames with detailed gas chemistry and global soot kinetics were conducted using the Sandia OPPDIF code to understand the interactions between radiation and chemistry. The gas-phase chemistry model was based on GRI-Mech 2.11, and Lindstedt's global soot kinetics model was used for soot formation calculations. The OPPDIF code was modified to include the effects of the radiation source term and the equations for soot mass fraction and soot particle number density based on soot kinetics. The radiation properties of the gas molecules and the soot particles, including a treatment for the self-absorption of radiation, were considered. Interactions between soot and gas-phase chemistry were considered. The effects of radiation heat loss, pressure, and injection velocity were studied for methane/air diffusion flames. The effects of radiation heat loss, thermophoresis, soot oxidation by O 2 and OH radicals, and depletion of acetylene by soot formation were all found to be important in determining soot concentrations, especially for high-pressure flames. Soot oxidation by OH radical dominates soot oxidation by O 2 at atmospheric pressure. However, the OH oxidation rate decreases with increasing pressure, and becomes comparable to the O 2 oxidation rate, which is very low compared with the overall soot formation rate at all pressures. The neglect of radiation heat loss leads to significantly higher estimates of soot and NO emissions compared with the estimates including radiation heat loss, particularly at high pressures. The effects of radiation, including those of self-absorption are found to be significant, particularly at lower stretch rates.

Physical-chemical modeling of the low-mass protostar IRAS 16293-2422

We present detailed gas-phase chemical models for the envelope of the low-mass star-forming region IRAS 16293-2422. By considering both time- and space-dependent chemistry, these models are used to study both the physical structure proposed by Schoier et al. (2002), as well as the chemical evolution of this region. A new feature of our study is the use of a detailed, self-consistent radiative transfer model to translate the model abundances into line strengths and compare them directly with observations of a total of 76 transitions for 18 chemical species, and their isotopes. The model can reproduce many of the line strengths observed within 50%. The best fit is for times in the range of 3 x 10 3 -3 x 10 4 yrs and requires only minor modifications to our model for the high-mass star-forming region AFGL 2591. The ionization rate for the source may be higher than previously expected - either due to an enhanced cosmic-ray ionization rate, or, more probably, to the presence of X-ray induced ionization from the center. A significant fraction of the CO is found to desorb in the temperature range of 15-40 K; below this temperature ∼90% or more of the CO is frozen out. The inability of the model to explain the HCS + , C 2 H, and OCS abundances suggests the importance of further laboratory studies of basic reaction rates. Finally, predictions of the abundances and spatial distributions of other species which could be observed by future facilities (e.g. Herschel-HIFI, SOFIA, millimeter arrays) are provided.

Arc process parameters for single-walled carbon nanotube growth and production: experiments and modeling.

Collarets rich in single-walled carbon nanotubes (SWCNTs) have been grown using a direct current arc method. Arc process parameters such as current, pressure, and anode to cathode distance were varied experimentally and by modeling to provide an optimal working window. The best collaret yields were obtained when helium was used as a buffer gas. Mixing helium with argon in the buffer permits controlling nanotube diameters. In addition to an experimental study, a modeling approach was developed assuming local thermal equilibrium and homogenous and heterogeneous neutral chemistry. The gas-phase chemical model involves 81 neutral carbon species (C1, C2, . . ., C79, C60F, C70F) and 554 reactions with rates taken from data of Krestinin and Moravsky. Axial profiles of temperature, C atom, C2 radical, and fullerene distributions in the reactor are predicted as a function of process parameters. Carbon nanotube growth is considered by a set of surface reactions simulating open nanotube growth. Because nanotube surface chemistry is controlled by the local terminated bond and not by the bulk nanotube bond, a mechanistic approach based on the formal resemblance between the bonding and the structure of open nanotube and other carbon surfaces is proposed to explain nanotube growth. Predicted growth rates are in the range of 100 to 1000 microm/min.

First detection of triply-deuterated methanol

We report the first detection of triply-deuterated methanol, with 12 observed transitions, towards the low-mass protostar IRAS 16293-2422, as well as multifrequency observations of 13CH3OH, used to derive the column density of the main isotopomer CH3OH. The derived fractionation ratio [CD3OH]/[CH3OH] averaged on a 10'' beam is 1.4%. Together with previous CH2DOH and CHD2OH observations, the present CD3OH observations are consistent with a formation of methanol on grain surfaces, if the atomic D/H ratio is 0.1 to 0.3 in the accreting gas. Such a high atomic ratio can be reached in the frame of gas-phase chemical models including all deuterated isotopomers of H3+.

First Detection of Doubly Deuterated Hydrogen Sulfide

This work was carried out using the Caltech Submillimeter Observatory and presents the observational study of HDS and D2S toward a sample of Class 0 sources and dense cores. We report the first detection of doubly deuterated hydrogen sulfide (D2S) in two dense cores and analyze the chemistry of these molecules, aiming to help understand the deuteration processes in the interstellar medium. The observed values of the D2S/HDS ratio and upper limits require an atomic D/H ratio in the accreting gas of 0.1-1. The study presented in this Letter supports the hypothesis that formaldehyde, methanol, and hydrogen sulfide are formed on the grain surfaces, during the cold prestellar core phase, where the CO-depleted gas has large atomic D/H ratios. The high values for the D/H ratios are consistent with the predictions of a recent gas-phase chemical model that includes H and its deuterated isotopomers, H2D+, D2H+, and D (Roberts, Herbst, & Millar).

2D InP etching simulation under high density plasma of chlorine

ABSTRACTA gas phase kinetic model of chlorine in an ICP reactor (Inductive Coupled Plasma) combined with a surface model have been developed to study the etching profile evolution of InP material. A gas phase chemistry model is used to predict the main neutral and charged specie fluxes impinging upon etched InP surface. These particle fluxes are then injected as input parameters into both a Monte-Carlo sheath model and a 2D surface model to predict the etch profile topography. The coupling between the gas kinetic model, sheath and surface models allows a direct prediction of the InP etch profile evolution versus reactor parameters (pressure, source power, Cl2 flow rate, DC bias on the substrate‥). A parametric study is carried out to show the role of some plasma parameters on etch rate, anisotropy and adsorbed InP surface state by chlorine.

The chemistry of compact planetary nebulae

We report high-sensitivity millimetre observations of several molecular species (13CO, HCN, HNC, CN, HCO+ and N2H+) in a sample of compact planetary nebulae. Some species such as HCO+ and CN are particularly abundant compared to envelopes around AGB stars or even interstellar clouds. We have estimated the following average values for the column densities ratios: , , and . Thus, the chemical composition of the molecular envelopes in these compact PNe appears somewhat intermediate between the composition of proto-PNe (such as CRL 2688 or CRL 618) and well evolved PNe (such as the Ring, M4–9, or the Helix). From observations of the CO isotopomers, we have estimated that the 12C/13C ratio is in the range . These values are below those expected from standard asymptotic giant branch models and suggest non-standard mixing processes. The observed molecular abundances are compared to very recent modelling work, and we conclude that the observations are well explained, in general terms, by time-dependent gas-phase chemical models in which the ionization rate is enhanced by several orders of magnitude with respect to the average interstellar value. Thus, our observations confirm that the chemistry in the neutral shells of PNe is essentially governed by the high energy radiation from the hot central stars. The complexity of the chemical processes is increased by numerous factors linked to the properties of the central star and the geometry and degree of clumpiness of the envelope. Several aspects of the PN chemistry that remains to be understood are discussed within the frame of the available chemical models.

Optimization of the chemical vapor deposition process for carbon nanotubes fabrication

A coupled boundary-layer laminar-flow hydrodynamic, heat-transfer, gas-phase chemistry and surface chemistry model is developed to analyze, at the reactor length-scale, chemical vapor deposition (CVD) of carbon nanotubes from a gas mixture consisting of methane (carbon precursor) and hydrogen (carrier gas) in the presence of cobalt catalytic particles in a cylindrical reactor. The model allows determination of the gas-phase fields for temperature, velocity, and species concentration as well as the surface species coverages, the carbon nanotubes growth rate and the deposition rate of amorphous carbon. Experimentally determined carbon deposition rates and carbon nanotubes growth rates at different processing conditions are used to validate the model. The model is also coupled with the Genetic Algorithm to determine the process parameters (the gas temperature and velocity at the reactor inlet, the reactor-wall temperature, the pressure, and the mole fraction of methane in the gas mixture) which maximize the carbon nanotubes yield while minimizing the amount of deposited amorphous carbon.

H 3++HD↔H 2D ++H 2: low-temperature laboratory measurements and interstellar implications

The system of reactions H 3 ++HD↔H 2D ++H 2 has been studied in a low-temperature multipole ion trap at a nominal temperature of 10 K . The rate coefficient k 1 for the forward reaction has been found to be 3.5×10 −10 cm 3 s −1 at 10 K , a value significantly smaller than the currently accepted value of 1.5×10 −9 cm 3 s −1 . The rate coefficient k −1 for the backward reaction has been found to be much higher than the value derived from the equilibrium coefficient of ∼10 −18 cm 3 s 1 . For normal-hydrogen, a value of k −1=4.9×10 −11 cm 3 s −1 has been deduced while for almost pure para-hydrogen (purity 99±1%) the value drops to k −1=7.3×10 −13 cm 3 s −1 . The results are discussed on the basis of the well-known energy levels of the involved molecules and the potential energy surface of the H 4D + intermediate. Zero-point energy plays a key role; however, there are additional complications due to the formation of rotationally excited H 2D +, if even traces of ortho-H 2 ( o-H 2) are present. The consequences of the results for the chemistry of cold clouds are illustrated using an evolutive gas-phase chemical model. There is strong evidence, that the new results significantly reduce the efficiency of isotope fractionation via gas-phase reactions. The experimental results also indicate the need for state-to-state rate coefficients in order to correctly simulate the o-H 2 induced non-equilibrium conditions prevailing both in the low temperature ion trap and in interstellar clouds.

Trouble in Polar Paradise

G aping ozone holes, collapsing ice sheets, rising temperatures, aboriginal villages swept away: These and other disturbing headlines from the high latitudes show that rapid climate change is under way, and that even the far corners of Earth are not sheltered from its effects. In this special issue, we examine how the polar regions have fared in recent decades and how they may be transformed in a future, warmer world. At the center of global-change research is how the dynamic behavior of the atmosphere and the oceans contributes to the weather patterns that have affected much of the world since the late 20th century. Moritz et al . (p. [1497][1]) discuss how the Arctic climate has changed over the past several decades. By comparing observed Arctic climate variations to modeled patterns, they illustrate how large-scale climate dynamics may have varied in response to human-induced warming. In a News story, Kerr (p. 1490) discusses the implications of an ice-free Northwest Passage in the Arctic Ocean, which many readers may see in their lifetimes. Goldman (p. 1493) describes the hazards of melting permafrost, and Stone (p. [1493][2]) provides a snapshot of an American Association for the Advancement of Science project on pollution and Arctic ecology on Russia's Kola peninsula. One of the greatest concerns related to global warming is how far sea levels will rise if the vast continental ice sheets of Greenland and Antarctica begin to melt. Rignot and Thomas (p. 1502) review what is known about the mass balance of polar ice sheets, based on data collected over the past decade. They show that those ice sheets are much more dynamic than previously thought. In a News story, Kaiser (p. 1494) describes how the Antarctic Peninsula's disintegrating ice shelves may serve as a natural laboratory for studying the melting of mainland ice sheets. Recent changes in Antarctic seabird populations may reflect complex responses, connected with sea ice change, to regional warming. Croxall et al . (p. [1510][3]) explain that more regional environmental data, better ecosystem modeling, and improved knowledge of physical and biological forcing factors are required to understand and predict the responses of seabirds to climate change. The combination of recent harvest-driven changes and those caused by global warming may produce rapid shifts rather than gradual changes. Because of its large surface area, snow is highly reactive. Domine and Shepson (p. 1506) present an overview of how the composition of near-surface air is perturbed by snow and how snowpack chemistry affects snow composition, snowpack interstitial air, and the overlying atmosphere. As a result, the composition of the atmosphere over snowpacks is very different from that predicted by gas-phase chemistry models. Processes taking place in surface snow must be taken into account when ice cores are used to reconstruct past climates. For those interested in knowing more about careers in the frigid high latitudes, a Science 's Next Wave Web feature ([nextwave.sciencemag.org/cgi/content/full/2002/02/26/1][4]) examines how nearly two dozen scientists have made work at the poles a part of their lives. [1]: /lookup/doi/10.1126/science.1076522 [2]: /lookup/doi/10.1126/science.297.5586.1493b [3]: /lookup/doi/10.1126/science.1071987 [4]: http://nextwave.sciencemag.org/cgi/content/full/2002/02/26/1

Chemistry as a probe of the structures and evolution of massive star-forming regions

We present detailed thermal and gas-phase chemical models for the envelope of the massive star-forming region AFGL 2591. By considering both time- and space-dependent chemistry, these models are used to study both the physical structure proposed by van der Tak et al. ([CITE], [CITE]), as well as the chemical evolution of this region. The model predictions are compared with observed abundances and column densities for 29 species. The observational data cover a wide range of physical conditions within the source, but significantly probe the inner regions where interesting high-temperature chemistry may be occurring. Taking appropriate care when comparing models with both emission and absorption measurements, we find that the majority of the chemical structure can be well-explained. In particular, we find that the nitrogen and hydrocarbon chemistry can be significantly affected by temperature, with the possibility of high-temperature pathways to HCN. While we cannot determine the sulphur reservoir, the observations can be explained by models with the majority of the sulphur in CS in the cold gas, SO2 in the warm gas, and atomic sulphur in the warmest gas. Because the model overpredicts CO2 by a factor of 40, various high-temperature destruction mechanisms are explored, including impulsive heating events. The observed abundances of ions such as HCO+ and N2H+ and the cold gas-phase production of HCN constrain the cosmic-ray ionization rate to ~ s-1, to within a factor of three. Finally, we find that the model and observations can simultaneously agree at a reasonable level and often to within a factor of three for , with a strong preference for yrs since the collapse and formation of the central luminosity source.

Surface hydrogenation in diffuse and translucent interstellar clouds

We investigate the eects of grain-surface hydrogenation reactions on gas phase chemical models of diuse and translucent clouds in the interstellar medium. Models in which gas phase species freeze out on to dust without release do not match observed column densities well. Expanding on previous work we extend our models to include the release of C, N, O, S and CO following hydrogenation on the grain surface. The results show that such mechanisms do improve the ability of chemical models to reproduce observed abundances, not only through the release of otherwise frozen-out species but also because of the additional hydrogenating reactions. For example, the predicted column densities of NH in diuse clouds and H2 Sa nd NH 3 in translucent clouds match observations better in models with grain-surface hydrogenation.

Detection of Triply Deuterated Ammonia in the Barnard 1 Cloud

We report the detection of the ground-state rotational transition JK = 10 → 00 (0a → 0s) of triply deuterated ammonia at 309.91 GHz in the Barnard 1 cloud, obtained with the Caltech Submillimeter Observatory. The observed, integrated, line intensity of 0.307 ± 0.019 K km s-1 implies an ND3 column density of (2 ± 0.9) × 1012 cm-2, for excitation temperatures in the range 5-10 K. Using previously published H2 and NH3 column density estimates in this source, we derive an ND3 fractional abundance with respect to H2 of (1.5 ± 1) × 10-11 and an ND3-to-NH3 abundance ratio of ~8 × 10-4. The observed abundance ratios can be explained in the framework of gas-phase chemical models, in which the dissociative recombination of partially deuterated ions results in a somewhat higher probability for the ejection of hydrogen atoms than deuterium.

Optimization of the chemical vapor deposition process for carbon nanotubes fabrication

A coupled boundary-layer laminar-flow hydrodynamic, heat-transfer, gas-phase chemistry and surface chemistry model is developed to analyze, at the reactor length scale, chemical vapor deposition (CVD) of carbon nanotubes from a gas mixture consisting of methane (carbon precursor) and hydrogen (carrier gas) in the presence of cobalt catalytic particles in a cylindrical reactor. The model allows determination of the gas-phase fields for temperature, velocity, and species concentration as well as the surface-species coverages, the carbon nanotubes growth rate and the deposition rate of amorphous carbon. Experimentally determined carbon deposition rates and carbon nanotubes growth rates at different processing conditions are used to validate the model. The model is also coupled with the genetic algorithm to determine the process parameters (the gas temperature and velocity at the reactor inlet, the reactor-wall temperature, the pressure, and the mole fraction of methane in the gas mixture) which maximize the carbon nanotubes yield while minimizing the amount of deposited amorphous carbon.

New models of interstellar gas-grain chemistry - I. Surface diffusion rates

In recent years it has become evident that large differences can exist between model results of grain-surface chemistry obtained from a rate equation approach and from a Monte Carlo technique. This dichotomy has led to the development of a modified rate equation method, in which a key element is the artificial slowing down of the diffusion rate of surface hydrogen atoms. Recent laboratory research into the surface diffusion rate of atomic hydrogen suggests that atomic hydrogen moves more slowly on grains than heretofore assumed. This research appears to lessen the need for modifications to the rate equation method. Based on the new laboratory work, we have developed appropriate models of gas-phase and grain-surface chemistry in quiescent dense cloud cores to examine the chemical effects of slowing down the rate at which atomic H can scan over dust surfaces. Furthermore, we have investigated the effect of slowing down the rate at which all species can move over grain surfaces.

The Role of Outflows and C Shocks in the Strong Deuteration of L1689N

Recent detections of doubly deuterated ammonia and formaldehyde in L134N and L1689N have renewed interest in deuterium fractionation in cold, dense molecular gas. Chemical models show that the level of fractionation depends critically on the physical conditions, namely, the temperature and density of the molecular gas. Therefore, detailed studies of cold, dense molecular cores are required to constrain the existing chemical networks. We have mapped the core of the remarkable star-forming molecular cloud L1689N in a number of molecular tracers, including singly deuterated species, in order to obtain detailed information on the morphology and kinematics of the molecular gas, as well as the physical conditions in the dense core and the shocked regions associated with the molecular outflows emanating from the embedded far-infrared source IRAS 16293-2422. Our data suggest the presence of two regions of interaction between the molecular outflows and the dense ambient cloud, one of them associated with the SiO peak E1, the other one near the peak of deuterated and doubly deuterated molecular species ~90'' east of the IRAS source. The observed intensities of the CO (3-2), (4-3), and (6-5) emission at these two locations are consistent with those predicted by C shock models with velocities of 15-25 and 8-10 km s-1, respectively. The relatively low shock velocity derived for the deuterium peak explains why no SiO emission has been detected at this location. Deuterated molecular species, such as DCO+ and DCN, in the core of L1689N are kinematically and morphologically distinct from other molecular tracers of the dense gas, such as HCO+, H13CO+, and HCN. The peak of deuterated molecules is also displaced from the extended millimeter dust continuum source located in this region. We use formaldehyde observations to derive the excitation conditions in the dense gas and, subsequently, the D/H ratios in HCO+ and HCN at several locations via large velocity gradient modeling. These ratios are typically ~1% toward the locations of the IRAS source and the SiO peak E1, with much higher values, ~10%, derived toward the deuterium peak. These values fall within the range of predictions of gas-phase chemical models for dense and cold gas, provided that significant accretion of H2O, CO, and other molecular species on the dust grains has occurred. However, some discrepancies between the observations and model predictions remain. For example, contrary to gas-phase model predictions, we derive a higher deuterium fractionation in HCN compared to HCO+.