Gas-phase Reaction Rate Research Articles

A reaction class approach for modeling gas phase reaction rates

We present a series of new tunneling models based on a reaction class approach. Reaction class consists of all reactions that have the same reactive moiety. One can expect that reactions in the same class share similarities in the shape of the potential energy surfaces along the reaction path. By exploring such similarities, we propose to use reaction path information from the parent (smallest) reaction in calculations of tunneling contributions of larger reactions in the class. This significantly reduces the computational cost while maintaining the accuracy of the model.

A two‐dimensional model with input parameters from a general circulation model: Ozone sensitivity to different formulations for the longitudinal temperature variation

Net heating and temperature derived from the middle atmospheric version of the National Center for Atmospheric Research's Community Climate Model (MA CCM2) history tapes are used to evaluate three different approaches to account for zonal temperature asymmetries in the calculation of gas phase and heterogeneous chemical reaction rate constants and polar stratospheric cloud (PSC) surface area in a two‐dimensional chemistry transport model (2‐D CTM). The first method uses the daily (and monthly) averaged three‐dimensional (3‐D) temperature distribution derived from the MA CCM2 to calculate chemical and heterogeneous reaction rates at each 3‐D grid point, followed by zonal averaging (pseudo‐3‐D method). The second method uses 3‐D daily temperature statistics from the MA CCM2 to calculate the monthly averaged probability function (stochastic approach). The third method is based on a planetary wave superposition on the zonally averaged temperature (wave approach). The sensitivity of the gas phase reactions to the longitudinal temperature asymmetry is small, while the sensitivity of the heterogeneous reaction rates is comparable to the ozone response to aircraft emissions. All three methods of accounting for longitude temperature asymmetry give similar PSC morphologies in the southern hemisphere, in good agreement with climatological data and independent model calculations. In the northern hemisphere, where the CCM2 winter temperatures at high latitudes are known to be warmer than those observed, the PSCs predicted by the pseudo‐3‐D and wave methods are much scarcer than those observed or calculated by other authors using climatological temperatures. For the same reason, all other methods employed in the present study failed to predict any PSCs in the northern hemisphere.

Gas-Phase Oxidation Products of Biphenyl and Polychlorinated Biphenyls

Our laboratory recently measured the gas-phase reaction rate constants of polychlorinated biphenyls (PCBs) with the hydroxyl radical (OH) and concluded that OH reactions are the primary removal pathway of PCBs from the atmosphere. With the reaction system previously employed for kinetics, we have now investigated the products of these PCB−OH reactions. Experiments were carried out in either air or He as the diluent gas at approximately 1 atm in a 160-mL quartz chamber. Temperatures ranged between 318 and 363 K in order to enhance the vapor pressures of these less volatile compounds. OH was produced in situ by the photolysis of ozone in the presence of H2O. Reaction products from biphenyl, the three monochlorobiphenyls, and five dichlorobiphenyls were extracted from the chamber, derivatized by diazomethane, and analyzed by gas chromatographic mass spectrometry (GC/MS). Experiments gave benzoic acid and the appropriate chlorinated benzoic acids in significant product yields (8−17%).

Model calculations of stratospheric OBrO indicating very small abundances

We have used a one‐dimensional photochemical model to investigate the potential role of OBrO in stratospheric photochemistry. The OBrO lifetime against photolysis is likely to be very short (around a few s) which prevents any appreciable concentration during sunlit hours. This rapid photolysis also prevents the gas‐phase production of significant OBrO in the model during twilight, as possible precursors (e.g. BrO) are converted to their nighttime reservoirs. Using a range of possible gas‐phase production reactions, the maximum (nighttime) OBrO volume mixing ratio produced in the model in the lower stratosphere is around 0.01 × 10−12 (0.01 pptv). These model results contradict recent tentative nighttime balloon observations of large OBrO [Renard et al., 1997, 1998]. We have used our model results to put constraints on the rates of gas‐phase and heterogeneous reactions that would be necessary to produce appreciable amounts of OBrO in the stratosphere. These constraints show that if OBrO is indeed present in the nighttime stratosphere at the pptv level, our current understanding of atmospheric bromine chemistry is severely flawed.

Micro/macro modeling of CVD synthesis

We propose a CVD reaction scheme in which a source material undergoes a gas-phase reaction to produce an intermediate, and then the intermediate diffuses to the solid surface and changes into a solid film through a surface reaction. A series of simple Monte Carlo (SMC) codes has been developed to simulate the observed film shape on micro-trenches and holes. By using these codes, surface reaction rate constants were determined so as to reproduce the experimentally observed film shape. By means of a macro-scale reactive transport analysis of a hot wall tubular reactor, gas-phase reaction rate constants for single component systems were determined to simulate the experimental growth rate distributions. The composition and growth rate of Yttria stabilized Zirconia (YSZ) film, a solid solution of Yttria and Zirconia, were qualitatively explained by a sum of single component's growth rates. As an application of these reaction models, we simulated a rotating-disk CVD reactor under low pressure. The simulations based on a quasi three-dimensional model revealed that the susceptor rotation suppresses the buoyancy convection and forms steeper gradients in temperature and concentration near the susceptor uniformly over wide area. At higher temperatures, the growth rate increased with rotation speed, but at lower temperatures the growth rate decreased with increasing rotation speed because the reduced retention time in the high-temperature region suppressed the gas-phase reaction.

Chemical reaction rate sensitivity and uncertainty in a two‐dimensional middle atmospheric ozone model

The NASA Goddard Space Flight Center two‐dimensional (2‐D) model has been used to study the sensitivity of model ozone concentrations to input chemical reaction rates, and the uncertainty of the model‐calculated concentrations. Ozone sensitivity coefficients to changes in chemical reaction rates are defined as logarithmic partial derivatives of the ozone concentration with respect to the chemical reaction rates. These logarithmic derivatives are estimated using a finite difference technique. The ozone sensitivity coefficients to 96 gas phase chemical reactions in the 2‐D model show that the ozone concentration is sensitive to the rates of about 25 reactions. The magnitude of the ozone sensitivity coefficients varies from 0.05 to 0.9. The latitude‐altitude distributions of the ozone sensitivity coefficients to several reactions are presented. The uncertainty of the model‐calculated ozone concentration is evaluated using a guided Monte Carlo (GMC) method from a probability distribution function. The GMC method judiciously combines uncertainty estimates derived from the sensitivity information with Monte Carlo runs of the model. The uncertainty of the model ozone concentration due to uncertainties in gas phase reaction rates is calculated from published chemical rate uncertainties and varies from 10–20% in the lower stratosphere to 30–40% in the mesosphere. Details concerning the GMC method are discussed, and the latitude‐altitude distribution of the uncertainty of the model‐calculated ozone is presented.

Heterogeneous reactions on stratospheric background aerosols, volcanic sulfuric acid droplets, and type I polar stratospheric clouds: Effects of temperature fluctuations and differences in particle phase

Northern hemispheric ER‐2 (NASA) data from stratospheric aerosol measurements during background conditions, periods disturbed by the influence of Mount Pinatubo, and polar stratospheric cloud (PSC) type I events are used to study the heterogeneous reactions of ClONO2 with H2O and of HOCl and ClONO2 with HCl in comparison to the gas phase reaction rate of OH with HCl. To calculate the reaction rates, the measured data of pressure, temperature, water vapor, and aerosol surface are utilized together with recent laboratory results for the heterogeneous reactive uptake coefficients. Because observations are limited, the mixing ratios of the gas phase species entering these rate calculations (i.e., ClONO2, HOCl, HCl, and N2O5) are taken from a two‐dimensional model. It is found that in dense volcanic clouds at temperatures below 200 K the resulting heterogeneous reaction rates of chlorine activation can be of similar magnitude as the gas phase reaction rate. The heterogeneous rates in PSCs can exceed the gas phase rates by more than 2 orders of magnitude. For the ClONO2 and HOCl reactions the measured aerosol surfaces during the PSC events are treated both as liquid (e.g., ternary solution) droplets and as solid NAT to compare the effects of the different phases. The reaction rates on NAT are significantly lower than on liquid droplets. Indeed, this study shows that a transition from liquid ternary solutions to NAT is expected to reduce the rate of chlorine activation based on present chemical understanding and on observed aerosol surface areas. Additionally, the effect of temperature and surface area fluctuations on the heterogeneous reaction rates is discussed.

Reaction and Evaporation Rates of High Temperature Candidate Fluids from a TFOST Test

Five high temperature fluids have been evaluated in a thin-film oxidation sealed tube (TFOST) test at 300°C to assess their capability for high temperature application. Degradation maps were constructed for each individual lubricant based on the amount of fresh lubricant remaining. Liquid-phase and gas-phase reaction rates and evaporation rate were then calculated and plotted against test time to illustrate their temporal functionalities. Composite plots of the test fluids were made in both global-rate and specific-rate forms, which are then used jointly to explain a number of important mechanisms concerning lubricant behavior in this experimental system. In general, lubricant evaporation rates are an order of magnitude larger than their reaction rates, and chemical reaction rates in the gas phase are higher than those from the liquid phase. Presented at the 51st Annual Meeting in Cincinnati, Ohio May 19–23, 1996

Kinetic Study of Silicon Nitride Growth from Dichlorosilane and Ammonia

The model of silicon nitride film deposition in a batch furnace from dichlorosilane ( SiH2Cl2; DCS) and ammonia ( NH3) is presented. The gas-phase and surface reaction rates of DCS are estimated from thickness profiles along the gas stream direction, and surface reaction and DCS decomposition are found to be of Langmuir and Lindemann types, respectively. It means that at a normal deposition temperature ( ∼1000 K), some DCS molecules adsorb directly on the Si surface while others decompose into a more reactive intermediate, SiCl2. At higher temperatures, the reverse reaction of DCS decomposition and the subsequent reaction of SiCl2 with NH3 may play significant roles. Using the present reaction rates, we estimate the film thickness profiles across a 12-inch wafer.

Nonequilibrium effects in reactive systems; the effect of reaction products and the validity of the Chapman-Enskog method

The rates of gas phase reactions can be calculated from the averages of the appropriate reactive cross sections with the velocity distribution functions of the reacting species. The reactive process, especially for reactions with activation energy, removes translationally energetic species and the velocity distribution functions depart from Maxwellian. The rate coefficients can differ from the equilibrium rate calculated with the Maxwell-Boltzmann distribution. The extent of the departure of the distribution function from Maxwellian can be estimated from solutions of the Boltzmann equation with appropriate choices for the elastic and reactive collision cross sections. If there is a good separation in the elastic and reactive collision time scales, a steady solution of the Boltzmann equation can be obtained with a procedure analogous to the Chapman-Enskog method for transport coefficients. In the present paper, the nonequilibrium effects for model reactive systems of the type A + A ⇌ B + B with and without the reverse reaction, and the reaction A + C → products are examined with both a Chapman-Enskog method along with an explicity time-dependent solution for the irreversible reaction A + A → B + B. The main objectives are to study the effect of the inclusion of the products with and without a reverse reaction as well as the range of validity of the Chapman-Enskog method.

Environmental Chemistry (Gas and Gas−Solid Interactions): The Role of Physical Chemistry

This paper reviews two physical chemistry topics related to atmospheric chemistry. First, we describe semiempirical models, based on thermochemistry and transition state theory, which are employed to estimate gas phase reaction rates over a wide range of temperatures and pressures. We also review briefly the experimental techniques utilized for measurements of elementary reaction rate constants, which provide the primary input to these models. We then address chemical reactions which take place in the atmosphere on the surface of solid, ice-like aerosol particles, discussing some current views on the mechanisms of these reactions and describing some laboratory techniques for the study of these heterogeneous processes.

Direct Kinetics Study of the CH3C(O)O2+ NO Reaction Using Chemical Ionization Mass Spectrometry

A direct measurement of the CH3C(O)O2 + NO gas-phase reaction rate coefficient over the temperature range 200−402 K was made using chemical ionization mass spectrometric detection of the CH3C(O)O2 reactant. A significant temperature dependence was observed, and a temperature dependent expression of k(T) = (8.1 ± 1.3) × 10-12 exp{(270 ± 60)/T} cm3 molecule-1 s-1 was determined. The 298 K rate coefficient, k = (2.0 ± 0.3) × 10-11 cm3 molecule-1 s-1, agrees well with results from previous indirect measurements. NO2, CH3, and CO2 were positively identified as products originating from the reaction. The question of whether CH3 and CO2 are direct products of the reaction or result from the thermal decomposition of CH3C(O)O could not be answered. The 298 K rate coefficients for the reactions of SF6-, I-, and O3- with CH3C(O)O2 were measured to be ( ) × 10-10, ( ) × 10-10, and ≥2 × 10-10 cm3 molecule-1 s-1, respectively.

Radiative Ignition of Volatile Gases on a Vertical Fuel Plate

ABSTRACT The ignition phenomena of a solid fuel plate of polymethyl-methacrylate (PMMA), which is vertically positioned and exposed to a thermal radiation source, is numerically studied here. A two-dimensional transient model includes such various aspects as thermal decomposition of PMMA, gas phase radiation absorption, chemical reaction and air entrainment by natural convection. Whereas the previous studies consider the problem approximately in a one-dimensional geometry by neglecting the natural convection, the present model takes account of two-dimensional effect of radiation and air entrainment. The inert heating of the solid fuel is also taken into consideration. Radiative heat transfer is incorporated by the Discrete Ordinates Method (DOM) with local absorption coefficient evaluated with corresponding gas species concentrations. The thermal history of solid fuel plate shows a good agreement compared with experimental results. Despite of induced natural convective flow that takes away heat from the fuel surface, the locally absorbed radiant energy, which is eventually converted to the internal energy, is found to play an important role in the onset of gas phase ignition. The ignition is considered to occur when the rate of variation of gas phase reaction rate reaches its maximum value. Once the ignition takes place, the flame propagates downward. Then the effects of incident radiant intensity and gas phase absorption coefficient on the ignition are discussed.

Temperature Dependence of the Gas Phase Reaction Rates of CF3O with Methane, Ethane, and Isobutane

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTTemperature Dependence of the Gas Phase Reaction Rates of CF3O with Methane, Ethane, and IsobutaneC. Bourbon, C. Fittschen, J. P. Sawerysyn, and P. DevolderCite this: J. Phys. Chem. 1995, 99, 41, 15102–15107Publication Date (Print):October 1, 1995Publication History Published online1 May 2002Published inissue 1 October 1995https://doi.org/10.1021/j100041a028RIGHTS & PERMISSIONSArticle Views48Altmetric-Citations7LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (676 KB) Get e-Alerts

Reaction analysis for ZrO2 and Y2O3 thin film growth by low-pressure metalorganic chemical vapor deposition using β-diketonate complexes

A mathematical model was developed for low-pressure metalorganic chemical vapor deposition (LPMOCVD) of ZrO2 and Y2O3 film growth. Zr(DPM)4(tetrakis-2,2,6,6-tetramethyl-3,5-heptandionate zirconium (β-diketonate complex)) and Y(DPM)3 were used as source materials. The surface reaction rate constant (or the reactive sticking coefficient) was determined by comparing the experimentally observed step coverages on micro-scale trenches with those predicted by a simplified Monte Carlo simulation. A gas-phase reaction rate constant was taken as a disposable parameter to fit the growth rate distributions along the reactor tube by a diffusion reaction transport model. Arrhenius-type equations were proposed for both surface and gas phase reactions. For the surface reactions, the activation energies were 188 kJ/mol (T < 909 K) and 38 kJ/mol (T > 909 K) for ZrO2 and 133 kJ/mol (T < 870 K) for Y2O3. For the gas phase reactions, they were 140 and 123 kJ/mol for ZrO2 and Y2O3, respectively. The scanning electron microscopy (SEM) micrographs and X-ray diffraction (XRD) patterns revealed that the crystallographic orientation and morphology of the grown films depend on the growth temperature.

Proton transfer reaction studies of multiply charged proteins in a high mass-to-charge ratio quadrupole mass spectrometer

Proton transfer reaction of multiply charged ions at high mass-to-charge ratios were explored with a low frequency quadrupole mass spectrometer. This instrument enabled a qualitative comparison of proton transfer reaction rates at low charge states for ions generated by electrospray ionization (ESI) from different solution conformations and for disulfide-linked versus disulfide-reduced protein ions. Proton transfer reactions that efficiently reduced the number of charges for ESI-generated ions to approximately the number of arginines in the polypeptide sequence were observed. No significant differences in gas-phase reaction rates were noted between different solution conformers. Differences in reaction rates between “native” and disulfide-reduced proteins were much smaller than those observed below m/z 2000 with lower proton affinity reagents or by using lower reagent concentrations. These smaller differences in reaction rates are thought to reflect the reduced electrostatic contributions from widely spaced charge sites and thus, the reduced sensitivity to an ion's three-dimensional structure or “compactness.”

A Chemical Kinetic Mechanism for Atmospheric Inorganic Mercury

A chemical kinetic mechanism for the atmospheric chemistry of inorganic mercury has been developed. Computer simulations conducted with this mechanism suggest that the half-life of elemental mercury could be of the order of hours for reaction with Cl 2 in a nocturnal marine layer and for reactions with oxidants such as H 2 O 2 in the ambientatmosphere. However, large uncertainties exist in the gas-phase reaction rates of Hg(O) with Cl 2 , O 3 , and H 2 O 2 , and the overall half-life of Hg(O) due to chemical reactions in the atmosphere cannot be assessed with certainty without further laboratory kinetic data. In the presence of an atmospheric liquid phase, oxidation and reduction reactions lead toward an equilibrium between Hg(O) and Hg(II)

Modeling of silicon nitride deposition by RF plasma-enhanced chemical vapor deposition

A one-dimensional mathematical model for the deposition of silicon nitride thin solid films by plasma-enhanced chemical vapor deposition (PECVD) is developed. The electron density and temperature have been evaluated through the ambipolar theory, assuming the Druyvesteyn function as the electron energy distribution function. All the model parameters related to both reaction and mass transport are evaluated a priori. Gas-phase reaction rates have been estimated by means of thermochemical methods. The model predictions are satisfactorily compared with experimental values of deposition rate and composition of the film.

Modifications to the angle-dependent line of normals model for gas-phase reaction rate constants

This article addresses improvements that need to be made to the angle-dependent line of normals (ADLN) model.

A molecular dynamics investigation of energy transfer efficiency in collisions of diatomic molecules

Diatomic collisional energy transfer has been studied with the aim of aiding the development of accurate representation of the activation-deactivation mechanism in gas phase reaction rate theory. We seek to determine the dependence of the energy transfer efficiency on the parameters describing the colliding molecules, i.e. mass, vibrational frequency, intermolecular potential and initial energy or temperature. While weak van der Waals type interactions give rise to very poor energy transfer efficiency scaling up the interactions to the strength of weak chemical bonds increases the energy transfer efficiency dramatically up to nearly statistical limit values. The dependence on mass and vibrational frequency is found to be weaker. The interaction strength is resolved into two factors, hardness of encounter and attractive strength. Varying these factors independently by a modification of the intermolecular atomic pair potential, both are found to contribute strongly to the observed energy transfer efficiency. Two temperatures, 160 and 1500 K, are considered and the sensitivity to the intermolecular potential is found to be distinctly greater at the lower temperature. The average energy transferred per collision 〈Δ E〉 is obtained as a function of the internal energy of the target molecule at normal and high interaction strength.