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At least 163 records · Page 9

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NASA NTRS1979

Reaction of HO2 with O3 and the effect of water vapor on HO2 kinetics

The effects of temperature and water vapor concentration on the ratio of the rate constant of the reaction HO2 + O3 yields OH + 2(O2) to the square root of the rate constant for the reaction HO2 + HO2 yields H2O2 + O2 are determined. Photolysis of H2-O2-O3 mixtures at 253.7 nm was carried out with H2O pressures in the range 0 to 15 torr at a temperature range of -42.5 to 61 C along with 184.9 nm photolysis of H2O-O2-O3 mixtures. It is shown that the rate of O3 photolysis is suppressed by the addition of water vapor and it is suggested that this effect is realized in the HO2 + HO2 yields H2O2 + O2 reaction. The calculated expression for the temperature dependence of the rate constant ratio is found to be in good agreement with that calculated from separate rate constants. Rate constants determined for the reaction OH + HO2 yields H2O + O2 are found to be higher than those previously determined, presumably due to increased pressure, indicating that atmospheric models should take into account the possible pressure dependences of the reactions considered.

Demore, W. B.↗

NASA NTRS1979

Heterogeneous atmospheric reactions - Sulfuric acid aerosols as tropospheric sinks

The reaction probabilities of various atmospheric species incident on a bulk sulfuric acid surface are measured in order to determine the role of sulfuric acid aerosols as pollutant sinks. Reaction products and unreacted starting materials leaving a Knudsen cell flow reactor after collision at 300 K with a H2SO4 surface or a soot surface were detected by mass spectrometry. Significant collision reaction probabilities are observed on a H2SO4 surface for H2O2, HNO3, HO2NO2, ClONO2, N2O5, H2O and NH3, and on soot for NH3. Estimates of the contribution of heterogeneous reactions to pollutant removal under atmospheric conditions indicate that while aerosol removal in the stratosphere is insignificant (loss rate constants approximately 10 to the -10th/sec), heterogeneous reactions may be the dominant loss process for several tropospheric species (loss rate constant approximately 10 to the -5th/sec, comparable to photolysis rate constants).

Baldwin, A. C.↗

NASA NTRS1979

Pressure and temperature dependence kinetics study of the NO + BrO yielding NO2 + Br reaction - Implications for stratospheric bromine photochemistry

The reactivity of NO with BrO radicals over a wide range of pressure (100-700 torr) and temperature (224-398 K) is investigated using the flash photolysis-ultraviolet absorption technique. The flash photolysis system consists of a high-pressure xenon arc light source, a reaction cell/gas filter/flash lamp combination, and a 216.5 half-meter monochromator/polychromator/spectrography for wavelength selectivity. The details of the reaction and its corresponding Arrhenius expression are identified. The results are compared with previous measurements, and atmospheric implications of the reaction are discussed. The NO + BrO yielding NO2 + Br reaction is shown to be important in controlling the concentration ratios of BrO/Br and BrO/HBr in the stratosphere, but this reaction does not affect the catalytic efficiency of BrOx in ozone destruction.

Watson, R. T.↗

NASA NTRS1980

A laser flash photolysis-resonance fluorescence kinetics study of the reaction Cl/2P/ + CH4 yields CH3 + HCl

The technique of laser flash photolysis-resonance fluorescence is employed to study the kinetics of the reaction Cl(2P) + CH4 yields CH3 + HCl over the temperature range 221-375 K. At temperatures less than or equal to 241 K the apparent bimolecular rate constant is found to be dependent upon the identity of the chemically inert gases in the reaction mixture. For Cl2/CH4/He reaction mixtures (total pressure = 50 torr) different bimolecular rate constants are measured at low and high methane concentrations. For Cl2/CH4/CCl/He and Cl2/CH4/Ar reaction mixtures, the bimolecular rate constant is independent of methane concentration, being approximately equal to the rate constant measured at low methane concentrations for Cl2/CH4/He mixtures. These rate constants are in good agreement with previous results obtained using the discharge flow-resonance fluorescence and competitive chlorination techniques. At 298 K the measured bimolecular rate constant is independent of the identity of the chemically inert gases in the reaction mixture and in good agreement with all previous investigations. The low-temperature results obtained in this investigation and all previous investigations can be rationalized in terms of a model which assumes that the Cl(2P 1/2) state reacts with CH4 much faster than the Cl(2P 3/2) state. Extrapolation of this model to higher temperatures, however, is not straightforward.

Ravishankara, A. R.↗

NASA NTRS1981

Matrix spectroscopic studies of chlorine atom-ozone reaction products

A spectroscopic study is presented of the matrix-isolated products of the direct combination reaction of chlorine atoms with ozone and of the microwave discharge reaction of chlorine and oxygen. Infrared spectra in the wavenumber range 2000 to 200 per cm were recorded for reaction products trapped by the condensation of a matrix of solid argon. Evidence for the formation of the ClO radical as well as the ClOO and ClClO radicals during the direct chlorine-ozone reaction is obtained, however no evidence is found for the presence of asymmetric ClO3 under conditions thought favorable for its formation. The ClO molecule is also observed following the microwave discharge reaction of chlorine and oxygen in argon in the isotopic forms (Cl-35)(O-16), (Cl-37)(O-16), (Cl-35)(O-18), and (Cl-37)(O-18). It is noted that the vibrational frequency for the (Cl-35)(O-16) radical at 849.2 per cm is blue-shifted 5 per cm above its gas-phase fundamental.

Carter, R. O., III↗

NASA NTRS1982

A comprehensive model to determine the effects of temperature and species fluctuations on reaction rates in turbulent reacting flows

A principal element to be derived from modeling turbulent reacting flows is an expression for the reaction rates of the various species involved in any particular combustion process under consideration. A temperature-derived most-likely probability density function (pdf) was used to describe the effects of temperature fluctuations on the Arrhenius reaction rate constant. A most-likely bivariate pdf described the effects of temperature and species concentrations fluctuations on the reaction rate. A criterion is developed for the use of an "appropriate" temperature pdf. The formulation of models to calculate the mean turbulent Arrhenius reaction rate constant and the mean turbulent reaction rate is considered and the results of calculations using these models are presented.

Foy, E.↗

NASA NTRS1982

Rate constant for the reaction between OH and CS2 at 298 and 520 K

In an attempt to resolve discrepancies between published values of the rate constant for the reaction between the hydroxyl radical and carbon disulfide, the reaction has been studied in a discharge flow system by using resonance fluorescence for kinetic measurements and mass spectrometry for product analysis. On the basis of the measured rate constant for disappearance of OH, and measurements of the amount of carbonyl sulfide formed, it was estimated that for the reaction HO + CS2 yields HS + OCS, rate constant values are not greater than 3 x 10 to the -15th/cu cm per sec at 520 K and not greater than 7 x 10 to the -15th/cu cm per sec at 298 K, upper limits are specified because of the inability to isolate exclusively this reaction channel, and because of possible involvement of wall reactions. These results confirm the low values found for this rate constant in two very recent studies.

Leu, M.-T.↗

NASA NTRS1983

Pulsed laser photolysis study of the reaction between O(3P) and HO2

It is pointed out that bimolecular reactions involving two free radicals are of great interest because both reactants have unpaired electrons and hence could interact at distances longer than those typical of radical-molecule encounters. A method based on laser photolysis is being developed to produce selectively free radicals in the homogeneous gas phase. This is to be done in such a way as to isolate the reaction of interest and subsequently follow the course of the reaction using spectroscopic techniques. The present investigation is concerned with a study in which the rate coefficient for the reaction of O(3P) with HO2, has been measured at N2 pressures ranging from 10 to 500 torr, taking into account the reaction O(3P)+HO2 yields OH-O2. In the described study, O(3P) and HO2 were produced by cophotolysis of O3 and H2O2 in N2 at 248.5 nm using a KrF excimer laser.

Ravishankara, A. R.↗

NASA NTRS1984

Measured ion-molecule reaction rates for modelling Titan's atmosphere

Rate coefficients for several two- and three-body ion-molecule reactions involving hydrocarbons have been determined at thermal energies and above using drift tube-mass spectrometer techniques. The measured rates for clustering and breakup reactions involving CH5(+) and C2H5(+) ions in methane are found to be strongly temperature dependent in the range from 80 to 240 K. The equilibrium constants determined for these reactions differ somewhat from those of Hiraoka and Kebarle (1975). Rate coefficients for two-body reactions of CH5(+), C2H5(+), N(+), H(+), and D(+) ions with methane and/or ethane have been measured. The results indicate that the product yields of several of the fast ion-molecule reactions depend strongly on ion energy (temperature), and therefore previous room-temperature results may be of limited value for model calculations of Titan's atmosphere.

Dheandhanoo, S.↗

NASA NTRS1986

Kinetic studies of the reaction of the SO radical with NO2 and ClO from 210 to 363 K

The rates of the reactions of the SO radical with NO2 and ClO (significant in the upper atmosphere of earth and Venus) were determined in a discharge flow system near 1 torr pressure with detection of radical and molecular species using collision-free sampling mass spectrometry. The rate constants were obtained from the decay of SO radicals in the presence of an excess of NO2 and ClO. The NO2 reaction was examined between 210 and 363 K and found to be temperature invariant: SO + NO2 yields SO2 + NO; k1 = (1.37 + or - 0.10) x 10 to the -11th cu cm/s. In addition, the ClO reaction was observed to be independent of temperature between 248 and 363 K: SO + ClO yields SO2 + Cl; k2 = (3.22 + or - 0.48) x 10 to the -11th cu cm/s. A comparison was made with previous investigations of these reactions at room temperature and with other radical-radical reactions involving SO or ClO.

Brunning, J.↗

NASA NTRS1987

Product energy distributions and energy partitioning in O atom reactions on surfaces

Surface reactions involving O atoms are likely to be highly exoergic, with different consequences if energy is channeled mostly to product molecules or surface modes. Thus the surface may become a source of excited species which can react elsewhere, or a sink for localized heat deposition which may disrupt the surface. The vibrational energy distribution of the product molecule contains strong clues about the flow of released energy. Two instructive examples of energy partitioning at surfaces are the Pt catalyzed oxidations: (1) C(ads) + O(ads) yields CO* (T is greater than 1000 K); and (2) CO(ads) + O(gas) yields CO2* (T is approx. 300 K). The infrared emission spectra of the excited product molecules were recorded and the vibrational population distributions were determined. In reaction 1, energy appeared to be statistically partitioned between the product CO and several Pt atoms. In reaction 2, partitioning was non-statistical; the CO2 asymmetric stretch distribution was inverted. In gas reactions these results would indicate a long lived and short lived activated complex. The requirement that Pt be heated in O atoms to promote reaction of atomic O and CO at room temperature is specifically addressed. Finally, the fraction of released energy that is deposited in the catalyst is estimated.

Halpern, Bret↗

NASA NTRS1986

Reaction of N2O5 with H2O on carbonaceous surfaces

The heterogeneous reaction of N2O5 with commercially available ground charcoal in the absence of H2O revealed a physisorption process (gamma = 0.003), together with a redox reaction generating mostly NO. Slow HNO3 formation was the result of the interaction of N2O5 with H2O that was still adsorbed after prolonged pumping at 0.0001 torr. In the presence of H2O, the same processes with gamma = 0.005 are observed. The redox reaction dominates in the early stages of the reaction, whereas the hydrolysis gains importance later at the expense of the redox reaction. The rate law for HNO3 generation was found to be d(HNO3)/dt = k(bi)(H2O)(N2O5) with k(bi), the effective bimolecular rate constants, for 10 mg of carbon being (1.6 + or - 0.3) x 10 to the -13th cu cm/s.

Brouwer, L.↗

NASA NTRS1986

Reactions of chlorine nitrate with HCl and H2O

The kinetics of the reactions of chlorine nitrate with HCl and H2O are characterized using a static photolysis/Fourier transform infrared spectrophotometer apparatus. For the homogeneous gas-phase reaction with HCl, an upper limit for the rate constant of less than 8.4 x 10 to the -21st, and for the reaction with H2O, a limit of less than 3.4 x 10 to the -21st cu cm/molecule per s, were obtained at 296 + or - 2 K. The yield of HNO3 is almost unity in both cases, and no synergistic effect is noted between HCl and H2O. The kinetic behavior of the reaction with H2O is well described by simple first-order kinetics, while the behavior of the reaction with HCl is described in terms of the Langmuir adsorption isotherm.

Hatakeyama, Shiro↗

NASA NTRS1986

A kinetics study of the homogeneous and heterogeneous components of the HCl + ClONO2 reaction

The kinetics of the reaction HCl + ClONO2 to Cl2 + HNO3 were investigated at 298 K using a flow reactor with FTIR analysis to assess the importance of this reaction for stratospheric chemistry. The observed reaction was characteristic of a heterogeneous process; an upper limit of 5 x 10 to the -18th cu cm/molecule per s was obtained for the homogeneous gas phase rate constant. From calculations of the first order wall rate constant, estimates were made of the reaction rate on stratospheric aerosols. Because both HCl and ClONO2 need to be adsorbed on the particle surface, the reaction will be of negligible importance under most stratospheric conditions.

Friedl, Randall R.↗

NASA NTRS1988

The effect of Co alloying content on the kinetics of reaction zone growth in tungsten fiber reinforced superalloy composites

A Co-free modified superalloy similar in composition to Waspaloy is investigated in an effort to understand the effect of Co on reaction zone growth kinetics and verify the chemistry dependence of reaction zone growth in the matrix of tungsten fiber reinforced superalloy composites. The values of the parabolic rate constant, characterizing the kinetics of reaction zone growth, for the Waspaloy matrix and the C-free alloy as well as five other alloys from a previous study confirm the dependence of reaction zone growth kinetics on cobalt content of the matrix. The Co-free alloy composite exhibits the slowest reaction zone growth among all tungsten fiber reinforced composites studied to date.

Rodriguez, A.↗

NASA NTRS1989

Kinetics and product studies of the reaction ClO + BrO using flash photolysis-ultraviolet absorption

The reaction between BrO and ClO was studied over the pressure range 50-700 torr and temperature range 220-400 K, using the flash photolysis-ultraviolet absorption method described by Watson et al. (1979). In order to investigate the mechanism of the BrO + ClO reaction, the product branch reactions Br + Cl2O yielding ClO + BrCl and Cl2O + h(nu) yielding products were examined. The rate constant for the overall reaction and the Arrhenius expression for the Br + Cl2O reaction are given, as well as the quantum yield for the production of atomic oxygen from the Cl2O photolysis.

Sander, Stanley P.↗

NASA NTRS1990

Capture reactions on C-14 in nonstandard big bang nucleosynthesis

Nonstandard big bang nucleosynthesis leads to the production of C-14. The further reaction path depends on the depletion of C-14 by either photon, alpha, or neutron capture reactions. The nucleus C-14 is of particular importance in these scenarios because it forms a bottleneck for the production of heavier nuclei A greater than 14. The reaction rates of all three capture reactions at big bang conditions are discussed, and it is shown that the resulting reaction path, leading to the production of heavier elements, is dominated by the (p, gamma) and (n, gamma) rates, contrary to earlier suggestions.

Wiescher, Michael↗

NASA NTRS1991

Heterogeneous reactions with NaCl in the El Chichon volcanic aerosols

Previous investigations of the effects of the 1982 eruption of the El Chichon volcano could not explain all the observations of changes in O3, HCl, NO and NO2 simultaneously without proposing unproven chemical reactions. Since reactions between solid NaCl and gaseous ClNO3 and N2O5 rapidly produce photochemically active chlorine species and solid NaNO3 in laboratory experiments, it is suggested that these reactions could have occurred with the NaCl observed to be present in the El Chichon sulfuric acid areosols. As a consequence, it is predicted that HCl should increase substantially, while NO(x) should decrease, in agreement with the measurements after the eruption. Ozone should be only slightly affected by these reactions. Reactions between solid NaCl and the acids H2SO4 and HNO3 might prove to be important, but sufficient evidence regarding their efficiency and the pressure of HNO3 in the aerosols is lacking.

Michelangeli, Diane V.↗