Showing papers on "Elementary reaction published in 1995"

The Catalytic Oxidative Coupling of Methane

Abstract: One of the great challenges in the field of heterogeneous catalysis is the conversion of methane to more useful chemicals and fuels. A chemical of particular importance is ethene, which can be obtained by the oxidative coupling of methane. In this reaction CH4 is first oxidatively converted into C2H6, and then into C2H4. The fundamental aspects of the problem involve both a heterogeneous component, which includes the activation of CH4 on a metal oxide surface, and a homogeneous gas-phase component, which includes free-radical chemistry. Ethane is produced mainly by the coupling of the surface-generated CH radicals in the gas phase. The yield of C2H4 and C2H6 is limited by secondary reactions of CH radicals with the surface and by the further oxidation of C2H4, both on the catalyst surface and in the gas phase. Currently, the best catalysts provide 20% CH4 conversion with 80% combined C2H4 and C2H6 selectivity in a single pass through the reactor. Less is known about the nature of the active centers than about the reaction mechanism; however, reactive oxygen ions are apparently required for the activation of CH4 on certain catalysts. There is spectroscopic evidence for surface O− or O ions. In addition to the oxidative coupling of CH4, cross-coupling reactions, such as between methane and toluene to produce styrene, have been investigated. Many of the same catalysts are effective, and the cross-coupling reaction also appears to involve surface-generated radicals. Although a technological process has not been developed, extensive research has resulted in a reasonable understanding of the elementary reactions that occur during the oxidative coupling of methane.

Elementary reaction modeling of high-temperature benzene combustion

Abstract: We have developed an elementary reaction mechanism containing 514 reactions without adjusted parameters for the low-pressure flaming rich combustion of benzene. The starting point for the present mechanism is the benzene sub-mechanism of Emdee, Brezinsky, and Glassman. Key features of the mechanism are: accounting for pressure-dependent unimolecular and bimolecular (chemically activated) reactions using QRRK, inclusion of singlet methylene chemistry, and phenyl radical oxidation and pyrolysis reactions. The results are compared to the detailed molecule and free radical profiles measured by Bittner and Howard using a molecular beam mass spectrometer. In general, the present mechanism does a good job of predicting stable species and free radical profiles in the flame. The computed profiles of small free radicals, such as H-atom or OH, match the data quite well. The largest discrepancies between the model and experiment are phenyl radical and phenoxy radical concentrations.

Direct femtosecond observation of the transient intermediate in the α‐cleavage reaction of (CH3)2CO to 2CH3+CO: Resolving the issue of concertedness

Abstract: When a reaction involving two equivalent bonds has sufficient energy to break both of them, it can proceed by either a concerted or a stepwise mechanism. For Norrish type‐I and other reactions, this issue has been controversial since direct time resolution of the individual C–C cleavage events was not possible. Here, for the elementary α‐cleavage of acetone, we report on the femtosecond resolution of the intermediates using mass spectrometry. The results show the nonconcertedness of the reaction, provide the times for the primary and secondary C–C breakage, and indicate the role of electronic structure (σ*, antibonding impulse) and the vibrational motions involved.

Gas-phase reactions and energy transfer at very low temperatures

Abstract: Experimental studies of gas-phase chemical reactions and molecular energy transfer at very low temperatures and between electrically neutral species are reviewed. Although work of collisionally induced vibrational and rotational transfer is described, emphasis is placed on very recent results on the rates of free radical reactions obtained by applying the pulsed laser photolysis (PLP)–laser-induced fluorescence (LIF) technique in a CRESU (Cinetique de Reactions en Ecoulement Supersonique Uniforme) apparatus at temperatures as low as 13 K. These measurements demonstrate that quite a wide variety of reactions—including those between two radicals, those between radicals and unsaturated molecules, and even some of those between radicals and saturated molecules—remain rapid at very low temperatures. Theoretical efforts to explain some of these results are described, as is their impact on attempts to model the synthesis of molecules in interstellar clouds.

Detailed chemical kinetics model for supercritical water oxidation of C1 compounds and H2

Abstract: A detailed chemical kinetics model comprising 148 reversible elementary reactions for the supercritical water oxidation (SCWO) of methane, methanol, carbon monoxide and hydrogen was developed. Rate constants were taken from previous critical evaluations. The Lindemann model, at times modified with a broadening parameter, was used to account for the effects of pressure on the kinetics of unimolecular reactions. Model predictions were compared with published experimental SCWO kinetics data for 450--650 C and 240--250 atm. The model correctly predicted global reaction orders for all four fuels to within their uncertainties. In addition, the model correctly predicted that the global reaction orders for O{sub 2} during methanol and hydrogen oxidation were essentially zero, and that the O{sub 2} concentration had the greatest effect on the methane oxidation rate. The pseudo-first-order rate constants predicted by the model were consistently higher than the experimental values, but the global activation energies were predicted correctly for methane oxidation and for CO and H{sub 2} oxidation t high temperatures. The model`s predictions generally became worse as the temperature decreased toward the critical point of water. A sensitivity analysis revealed that fewer than 20 elementary reactions largely controlled the oxidation kinetics for the compounds studied. Nearly half ofmore » these reactions involved HO{sub 2}, which is an important free radical for SCWO. Quantitative agreement with the experimental methane conversions was obtained by adjusting the preexponential factors for three elementary reactions within their uncertainties. It could also be obtained by using the JANAF value (0.5 kcal/mol) for the standard heat of formation of HO{sub 2}, but this value is lower than other recently recommended values.« less

Observation of individual chemical reactions in solution.

Abstract: Discrete chemical reaction events occurring in solution have been observed by single photon detection of a bimolecular, chemiluminescent reaction. The reactants were generated from 9,10-diphenylanthracene in acetonitrile with potential pulses applied to an ultramicroelectrode. Electrogenerated radical ions of opposite sign react to yield the excited singlet state of the parent compound. The chemical reactions were restricted to a 20-femtoliter volume adjacent to the electrode by the use of rapid potential pulses. Individual chemical reaction events were stochastic and followed the Poisson distribution, and the interarrival time between successive reaction events was exponentially distributed.

Pyrolysis of methane in the presence of hydrogen

Abstract: The kinetics of methane pyrolysis were studied in a tubular flow reactor in the temperature range 1200 to 1500°C at atmospheric pressure. To avoid excessive carbon formation the reaction time was short and the methane feed was diluted with hydrogen. Ethene, ethyne, benzene and hydrogen were the main gaseous products. Ethane was observed as a product at very low conversions of methane. More than 90% selectivity was obtained for C2 products. The ratio of ethyne to ethene increased with increasing temperature. The yield of C2 products is limited by gas-phase equilibrium at lower temperatures. Formation of carbon was strongly depressed by hydrogen at higher temperatures. The maximum yield of ethyne was found to increase from about 10% to about 50% when the temperature was increased from 1200 to 1500°C, with hydrogen dilution H2: CH4 = 2: 1. A mechanistic reaction model was used to simulate the pyrolysis of methane at the actual conditions. A sensitivity analysis was performed to evaluate the elementary reactions which influence the formation and consumption of the species in the model system.

Period-doubling bifurcations and chaos in a detailed model of the peroxidase-oxidase reaction

Abstract: published in Advance ACS Abstmcrs, June 1, 1995. into the reaction scheme.7-" Although both types of models are capable of exhibiting complex dynamics, including chaos, the simple models have, up to now, outperformed the detailed models in reproducing the reaction's individual features, such as the waveform of the oscillations and their nonuniformity.I2 Furthermore, in the detailed models most of the rate constants used to obtain complex dynamics are not in accordance with the experimentally determined rate constants. Here we present a detailed model of the PO reaction which is capable of simulating the recent period doubling experiments of Geest et al.,I3 using the proper rate constants and the same conditions (enzyme concentration, 02 and NADH supply rates) as in the experiments. The model is a slight extension of a model recently proposed by Olson et al.'' under the name Urbanalator. The Model The model is built using the same elementary reaction steps as in previous detailed models such as the Yokota-Yamazaki (YY) ~cheme,~ the Fedkina-

Modeling of Nitrogen and Oxygen Recombination on Partial Catalytic Surfaces

Abstract: In connection with recombination coefficients derived from experimental data described in the literature, a reaction scheme including detailed rate expressions for O and N atom recombination on the surface of re-entry vehicles is established, consisting of elementary reaction steps. To validate the reaction mechanism derived, surface chemistry and fluid mechanical processes are coupled assuming a one-dimensional stagnation flow field. A quantitative agreement is achieved between recombination coefficients resulting from the numerical computations and those calculated from experiments. The temperature dependence of the recombination coefficient is explained by elementary reaction steps. Furthermore, the reaction scheme established is implemented in a two-dimensional Navier-Stokes code computing the re-entry flow around a simple geometry to show the importance ofa detailed modeling of surface reactions.

Elementary radical reactions and autoignition

Abstract: The kinetics of the elementary reactions involved in the low-temperature combustion of alkanes are reviewed. The reactions are centred on the alkyl radical and its decomposition, recombination and reactions with O2. A combination of theory, modelling and direct measurements, using especially laser flash photolysis, have played key roles in the characterisation of all three reaction types. A remaining area of uncertainty is the mechanism of the R + O2 reaction and the interplay of channels leading to the alkyl hydroperoxyl radical and to the conjugate alkene + HO2. A mechanism based on two electronic surfaces is proposed.This Polanyi memorial lecture was given when the 1994 Polanyi medal was presented to Professor M. J. Pilling at the 13th International Gas Kinetics Symposium in September 1994 in Dublin.

The decomposition of nitrous oxide at 1.5 ⩽ P ⩽ 10.5 atm and 1103 ⩽ T ⩽ 1173 K

Abstract: Reaction experiments on mixtures of N2O/H2O/N2 were performed in a variable pressure flow reactor over temperature, pressure, and residence time ranges of 1103–1173 K, 1.5–10.5 atm, and 0.2–0.8 s, respectively. Mixtures of approximately 1% N2O in N2 were studied with the addition of varying amounts of water vapor, from background to 3580 ppm. Experimentally measured profiles of N2O, O2, NO, NO2, H2O, and temperature were compared with predictions from detailed kinetic modeling calculations to assess the validity of a reaction mechanism developed from currently available literature thermochemical and rate constant parameters. Sensitivity and reaction flux analyses were performed to determine key elementary reaction path processes and rates. Reaction rate constants for the uni-molecular reaction, N2O N2 + O, were determined at various pressures in order to match overall experimental and numerical decomposition rates of N2O. The numerical model included a newly determined rate constant for N2O + OH HO2 + N2 with an upper limit of 5.66 × 108 cm3 mol−1 sec−1 at 1123 K. This is considerably smaller than presently reported in the literature. The experimentally observed rate of N2O decomposition was found to be slightly dependent on added water concentration. The rate constant determined for the elementary decomposition is strongly dependent on the choice of rate constants for the N2O + O N2 + O2 and N2O + O NO + NO reactions. In the absence of accurate data at the temperatures of this work, and based on these and other experiments in this laboratory, we presently recommend rate constants from the review of Baulch et al. The basis for this recommendation is discussed, including the impact on the rate constants derived for elementary nitrous oxide decomposition. The uncertainties in the rate constants as reported here are ±30%. The present mechanism was applied to previously reported high-pressure shock tube data and yields a high-pressure limit rate constant a factor of three larger than previously reported at these temperatures. The following expressions for the elementary decomposition reaction are recommended: k = 9.13 × 1014 exp (−57, 690/RT) cm3 mol−1 s−1 and k∞ = 7.91 × 1010 exp(−56020/RT) s−1. Simple Lindemann fits utilizing these parameters reproduce the pressure dependent rate constants measured here within ±25%. © 1995 John Wiley & Sons, Inc.

Addition of cyclopentane to slowly reacting mixtures of H2+ O2 between 673 and 783 K: reactions of H and OH with cyclopentane and of cyclopentyl radicals

Abstract: A number of elementary reactions involved in the oxidation of cyclopentane (CP) have been studied by adding CP to H2–O2 mixtures over the temperature range 673–783 K. Kinetic studies of the relative rates of consumption of CP and H2 have given values of k21=(1.11 ± 0.25)× 1010 and k22=(1.84 ± 0.32)× 109 dm3 mol–1 s–1 at 753 K. Combination with independent data and critical analysis gives k21= 3.65 × 102T2.5 exp(519/T) dm3 mol–1 s–1, which is recommended between 250 and 1500 K, with error limits of 50 Torr, very little ring rupture has been observed in the initial products. The formation of C2H4 has been discussed in detail, and a value of k32= 1013.14 ± 0.55 exp(– 17260/T) s–1 has been obtained over the temperature range 580–783 K. c-C5H9→ CH2= CHCH2CH2CH2(32) The cyclisation of C3, C4, C5 and C6 alken-1-yl radicals has been discussed and it has been concluded that the predominant reaction of hex-5-en-1-yl radicals between 600 and 1300 K and at 1 atm O2 pressure is cyclisation giving c-hexyl radicals. As the latter rapidly undergo successive oxidation to cyclohexene, cyclohexadiene and then benzene, it has been further concluded that such C6 cyclisations could be major sources of benzene and substituted benzenes in the oxidation of conventional fuels.

The Catalytic Oxidative Coupling of Methane

Abstract: One of the great challenges in the field of heterogeneous catalysis is the conversion of methane to more useful chemicals and fuels. A chemical of particular importance is ethene, which can be obtained by the oxidative coupling of methane. In this reaction CH4 is first oxidatively converted into C2H6, and then into C2H4. The fundamental aspects of the problem involve both a heterogeneous component, which includes the activation of CH4 on a metal oxide surface, and a homogeneous gas-phase component, which includes free-radical chemistry. Ethane is produced mainly by the coupling of the surface-generated CH radicals in the gas phase. The yield of C2H4 and C2H6 is limited by secondary reactions of CH radicals with the surface and by the further oxidation of C2H4, both on the catalyst surface and in the gas phase. Currently, the best catalysts provide 20% CH4 conversion with 80% combined C2H4 and C2H6 selectivity in a single pass through the reactor. Less is known about the nature of the active centers than about the reaction mechanism; however, reactive oxygen ions are apparently required for the activation of CH4 on certain catalysts. There is spectroscopic evidence for surface O− or O ions. In addition to the oxidative coupling of CH4, cross-coupling reactions, such as between methane and toluene to produce styrene, have been investigated. Many of the same catalysts are effective, and the cross-coupling reaction also appears to involve surface-generated radicals. Although a technological process has not been developed, extensive research has resulted in a reasonable understanding of the elementary reactions that occur during the oxidative coupling of methane.

Reactive collisions with excited-state atoms

Abstract: In the context of molecular reaction dynamics, the study of elementary reactions with excited species has developed rapidly in the last decades. This article reviews recent developments in the study of reactive collisions with excited-state atoms, under single-collision conditions provided by crossed-beam and low-pressure gas cell experiments. Application of high-resolution laser techniques allows selective electronic excitation of atomic reagents. This excitation is used to identify the key features of state-selected and spin-orbit effects, on both the reaction cross-sections and product state distributions. The polarized nature of the laser field is used to gain a detailed understanding of the reaction dynamics and stereodynamics, through the alignment of atomic orbitals. Several examples are discussed in the light of current theoretical models, a detailed analysis of the data leading to the identification of non-adiabatic processes taking place on excited potential-energy surfaces. New experimental developments will allow for an even deeper understanding of these elementary processes.

Ab initio molecular orbital studies of elementary reactions and homogeneous catalytic cycles with organometallic compounds

Abstract: Results of ab initio molecular orbital studies are presented for one elementary reaction and two full catalytic cycles of organo-transition metal complexes: (i) the oxidative addition of H-H, C-H, N-H, 0-H and Si-H bonds to CpRh(C0) in gas phase, (ii) the full catalytic cycle of hydroboration of olefin with Rh(PR3)2Cl, and (iii) the full catalytic cycle of hydroformylation of olefin with RhH(C0)2(PR3)2.

Investigation of the Reaction of 3CH2 with NO at high Temperatures

Abstract: The reaction 3CH2 + NO was studied behind incident shock waves over the temperature range 1100–2600 K by following the H and O atom formation with Atomic Resonance Absorption Spectroscopy (ARAS) and the OH radical formation with Laser Absorption. In the temperature range from 1100 to 2500 K the rate constant of the H atom formation formally attributed to a reaction channel giving HCNO+H (1 h) obtained on the basis of the experiments reported here is k1h = 10(12.4 ± 0.2).exp (-(25 ± 2) kJ mol−1/RT) cm3 mol−1 s−1. The rate constant of the channel leading to HCN + OH (1d) was determined to be k1d = 10(11.7 ± 0.3).exp (-(12 ± 2) kJ mol−1/RT) cm3 mol−1 s−1. The evaluation of the experimental profiles by means of computer simulation leads to an overall rate constant of about k1 = (1 ± 0.5)·1012cm3mol−1 s−1 without any noticeable temperature dependence within the error limits in the temperature range from 1200 to 2600 K. The contribution of a reaction channel leading directly or indirectly to O (or N) atoms was found to be less than 15% of the overall reaction at temperatures below 2000 K.

Simulation of the behavior of rich hydrogen-air flames near the flammability limit

Abstract: This paper presents an investigation of the behavior of rich hydrogen-air flames near the flammability limit. Ignition processes arc simulated using a computational model which involves the solution of the governing equations (for one-dimensional geometries) by implicit methods on an adaptive non-uniform grid, using detailed chemistry and a multi-species transport model. The reaction mechanism consists of 37 elementary reactions and 9 species. Calculations have been performed for different one-dimensional geometries. An investigation of the fundamental flammability limit {intrinsic to the combustion system itself), which is governed only by the physical and chemical processes in the gaseous mixture, is carried out by eliminating external factors such as heat loss, buoyancy, etc, in the calculations. Compulations have been performed for hydrogen-air mixtures of varying hydrogen content. Mixtures containing 75% or less hydrogen are found to be steadily propagating, whereas mixtures containing 82% o...

Determination of Rate Constants and Product Branching Ratios for the Reactions of CH3O2 and CH3O with Cl Atoms at Room Temperature

Abstract: Reactions of CH 3 O 2 and of CH 3 O radicals with Cl atoms have been investigated at 300 K using a discharge flow system with LIF and mass-spectrometric detection. CH 3 O 2 + Cl → OCl + CH 3 O (1a) → HCl+CH 2 O 2 (1b) CH 3 O + Cl → HCl + HCHO (2) Rate constants k 1 = (1.15 ± 0.3).10 -10 and k 2 = (1.00 ± 0.2).10 -10 , in units cm 3 molecule -1 s -1 , have been obtained observing pseudo-first-order decays of CH 3 O 2 and CH 3 O radicals, respectively. The branching ratio k 1a /k 1 = 0.51 ± 0.09 has been found from the determination of OCl yields. Formation of the Criegee intermediate CH 2 O 2 in reaction (1b) was inferred from CO 2 generation. A lower limit of 0.8±0.5 was derived for the CO 2 yield from unimolecular CH 2 O 2 decomposition. Possible reaction pathways for steps (1a) and (1b) are suggested on the basis of ab initio model calculations. The calculations include the estimation of structures and energies of intermediate CH 3 O 2 Cl collision complexes and the decomposition of CH 3 OOCl to yield products.

Electronic orbital alignment effects in the reaction Mg*(3p 1P1)+CH4→MgH+CH3

Abstract: We have measured the final state resolved far‐wing action spectra for the MgCH4 reactive collision system. The results show a dramatic ‘‘Π‐like’’ orbital alignment preference in the reaction channel. The reactive channel action spectra for different MgH rotational states in v=0 are identical, suggesting that the reaction follows from a single approach geometry, with the product rotational distribution determined by exit channel effects. Based on these observations and molecular orbital considerations, we propose that the reaction proceeds in η2 approach geometry through a triangular C–Mg–H transition state.

Thermochemistry and Reaction Mechanisms of Nitromethane Ignition

Abstract: The thermochemistry and reaction mechanisms of nitromethane initiation are modeled using detailed chemical kinetics. Initial conditions correspond to gaseous nitromethane at atmospheric and liquid-like densities and initial temperatures between 1100 and 2000 K. Global reactions as well as elementary reactions are identified for each of the two stages of ignition. The chemical steps to convert the nitro group to N 2 involve a complex set of elementary reactions. The time-dependence of the ignition steps (ignition delay times) as a function of temperature and pressure is used to determine effective activation energies and pressure dependencies to ignition. The ignition delay times range from several nanoseconds to tens of microseconds. At atmospheric conditions, the delay times for both ignition stages are in excellent agreement with observed experimental data. At the high densities, the ignition times at these elevated temperatures appear to be dominated by the same reaction mechanism that occurs for atmospheric gaseous nitromethane initiation. This is to be contrasted with lower temperature, condensed-phase ignition studies where it appears that solvent-assisted reactions dominate.

C. Spectroscopy of Excited States, Kinetics and Photodissociation. Fine Structure and Hyperfine Structure Effects in Unimolecular and Complex‐Forming Bimolecular Reactions

Abstract: Fine structure and hyperfine structure effects can influence the kinetics of unimolecular and complex-forming bimolecular reactions. The corresponding energetic effects are analyzed in the framework of the statistical adiabatic channel model (SACM). The reactions O+OH O2 + H, OH + CO H + CO2, C+ + OH products, C+ + NO products, and NO2 O + NO serve as examples.

Elementary reactions and thermodynamic effects in the conversion of propene over an acidic A1MFI

Abstract: The conversion of propene over H-A1MFI zeolites was investigated at various temperatures and subatmospheric pressure. The overall conversion scheme was cracked down into a set of well known reactions: namely a primary dimerization step, two secondary or tertiary disproportionation reactions involving intermediate carbenium ions within the C 6 –C 9 range and, in addition, a number of hydrogen transfer reactions. Quantitative analysis of the product distributions provided an evaluation of the relative weight of each step depending on the temperature. The variation of the overall conversion with temperature was interpreted in terms of a thermodynamically limited process above 573 K.

Reaction of N on a Ni(110) surface with H atoms

Abstract: The p(2×3) Ni(110)‐N surface structure was produced by the reaction of NO with highly excess H2 (1:150) on a Ni(110) surface at 650 K. The N atoms on Ni(110) surface were inactive for the hydrogenation with H2. They, however, reacted with H atoms in the presence of H2, and the formation of NH species was detected by high resolution electron energy loss spectroscopy (HREELS). In the temperature range between 300 K and 450 K, the amount of N on Ni(110) surface decreased by reacting with H atoms in the zero order kinetics with respect to the amount of N and the decreasing rate did not depend on the temperature. When the temperature is higher than a critical temperature of 500 K, however, the amount of N on the Ni(110) surface does not decrease even if the surface is exposed to H atoms. This critical temperature corresponds to the decomposition temperature of the NH species on Ni(110) surface determined by the HREELS. One one hand, the NH species were produced by the reaction, hydrogenation, of N with H atoms on the Ni(110) suface. On the other hand, it was reported that NH2 species were preferentially formed by the decomposition, dehydrogenation, of NH3 on the Ni(110) surface. Taking these facts into account, it was deduced that there is a rather high activation barrier in an elementary reaction path from NH to NH2 on the Ni(110) surface.

A Model of the Gas-Phase Chemistry of Boron Nitride CVD From BCl3 and NH 3*

Abstract: The kinetics of gas-phase reactions occurring during the CVD of boron nitride (BN) from BCl3 and NH3 are investigated using an elementary reaction mechanism whose rate constants were obtained from theoretical predictions and literature sources. Plug-flow calculations using this mechanism predict that unimolecular decomposition of BCl3 is not significant under typical CVD conditions, but that some NH3 decomposition may occur, especially for deposition occurring at atmospheric pressure. Reaction of BCl3 with NH3 is rapid under CVD conditions and yields species containing both boron and nitrogen. One of these compounds, Cl2BNH2, is predicted to be a key gas-phase precursor to BN.