Joseph W. Bozzelli

Kinetic Analysis of Complex Chemical Activation and Unimolecular Dissociation Reactions using QRRK Theory and the Modified Strong Collision Approximation

A method to predict temperature and pressure-dependent rate coefficients for complex bimolecular chemical activation and unimolecular dissociation reactions is described. A three-frequency version of QRRK theory is developed and collisional stabilization is estimated using the modified strong-collision approximation. The methodology permits analysis of reaction systems with an arbitrary degree of complexity in terms of the number of isomer or product channels. Specification of both high and low pressure limits is also provided. The chemically activated reaction of vinyl radical with molecular oxygen is used to demonstrate the approach. Subsequent dissociation of the stabilized vinyl peroxy radical is used to illustrate prediction of dissociation rate coefficients. These calculations confirm earlier results that the vinoxy + O channel is dominant under combustion conditions. The results are also consistent with RRKM results using the same input conditions. This approach provides a means to provide reasonably accurate predictions of the rate coefficients that are required in many detailed mechanisms. The major advantage is the ability to provide reasonable estimates of rate coefficients for many complex systems where detailed information about the transition states is not available. It is also shown that a simpler 1-frequency model appears adequate for high temperature conditions.

Impact of SO2 and NO on CO oxidation under post-flame conditions

An experimental and theoretical study on the effect of SO2 on moist CO oxidation with and without NO present has been carried out. The experiments were performed in an isothermal quartz flow reactor at atmospheric pressure in the temperature range 800–1300 K. Inlet concentrations of SO2 ranged from 0 to 1800 ppmv, while the NO ranged between 0, 100, or 1500 ppm. SO2 inhibits CO oxidation under the conditions investigated, shifting the fast oxidation regime 20–40 K towards higher temperatures at 1500 ppm SO2. The inhibition is most pronounced at high O atom levels. The experimental data supported by model analysis suggest that SO2 primarily reacts with O atoms forming SO3, which is subsequently consumed mainly by reaction with O and HO2. Addition of NO significantly diminishes the effect of SO2. Since NO is usually present in combustion flue gases, the impact of SO2 on CO burnout in most practical systems is projected to be small. The H/S/O thermochemistry and reaction subset has been revised based on recent experimental and theoretical results, and a chemical kinetic model has been established. The model provides a reasonable overall description of the effect of SO2 and NO on moist CO oxidation, while the SO3/SO2 ratio is well predicted over the range of conditions investigated. In order to enhance model performance further, rate constants for a number of SO2 and SO3 reactions need to be determined with higher accuracy.

Thermal reactions of CH2Cl2 in H2/O2 mixtures: Implications for chlorine inhibition of CO conversion to CO2

The thermal decomposition of dichloromethane in hydrogen/oxygen mixtures and argon bath gas was carried out at 1 atm pressure in tubular flow reactors of varied surface-to-volume ratios. The degradation of dichloromethane plus intermediate and final product formation was analyzed from 873 to 1093 K, with average residence times of 0.1–2.0 s. A detailed kinetic reaction mechanism based upon fundamental thermochemical principles and Transition State Theory was developed and used to model our experimental results. Sensitivity analysis was used to determine important reactions effective in inhibiting CO conversion to CO2. The results indicate that the reaction: OH + HCl → H2O + Cl is a major cause of OH loss and this decrease in OH significantly reduces CO conversion by reaction with OH. Lower temperatures result from reduced CO reaction with OH, which increases the importance of HO2. Here, the reaction of HO2 + Cl to the HCl + O2 (termination) channel further inhibits combustion. A significant fraction of the CH2Cl2 conversion occurs through C2 chlorocarbon formation, which results from methyl and chloromethyl combination reactions.

CHEMACT: A Computer Code to Estimate Rate Constants for Chemically-Activated Reactions

Abstract CHEMACT is a computer code that uses the QRRK treatment of chemical activation reactions to estimate apparent rate constants for the various channels that can result in addition, recombination and insertion reactions. The working equations are developed, the necessary input file is described, and sources of required input data are discussed. Three applications of the code to representative examples of reactions relevant to combustion are presented. These include C2H5 + O2, H + C6H5Cl, and CH3 + OH. These reactions illustrate some of the expected behavior with different types of chemical activation and demonstrate the necessity of explicitly accounting for the effects of chemical activation in high temperature gas-phase reactions. Methods by which these results can be included in detailed kinetic mechanisms are discussed.

Ethanol Oxidation: Kinetics of the α-Hydroxyethyl Radical + O2 Reaction

Bioethanol is currently a significant gasoline additive and the major blend component of flex-fuel formulations. Ethanol is a high-octane fuel component, and vehicles designed to take advantage of higher octane fuel blends could operate at higher compression ratios than traditional gasoline engines, leading to improved performance and tank-to-wheel efficiency. There are significant uncertainties, however, regarding the mechanism for ethanol autoignition, especially at lower temperatures such as in the negative temperature coefficient (NTC) regime. We have studied an important chemical process in the autoignition and oxidation of ethanol, reaction of the alpha-hydroxyethyl radical with O2(3P), using first principles computational chemistry, variational transition state theory, and Rice-Ramsperger-Kassel-Marcus (RRKM)/master equation simulations. The alpha-hydroxyethyl + O2 association reaction is found to produce an activated alpha-hydroxy-ethylperoxy adduct with ca. 37 kcal mol(-1) of excess vibrational energy. This activated adduct predominantly proceeds to acetaldehyde + HO(2), with smaller quantities of the enol vinyl alcohol (ethenol), particularly at higher temperatures. The reaction to acetaldehyde + HO2 proceeds with such a low barrier that collision stabilization of C2O3H5 isomers is unimportant, even for high-pressure/low-temperature conditions. The short lifetimes of these radicals precludes the chain-branching addition of a second O2 molecule, responsible for NTC behavior in alkane autoignition. This result helps to explain why ignition delays for ethanol are longer than those for ethane, despite ethanol having a weaker C-C bond energy. Given its relative instability, it is also unlikely that the alpha-hydroxy-ethylperoxy radical acts as a major acetaldehyde sink in the atmosphere, as has been suggested.

Oxidation of the Benzyl Radical: Mechanism, Thermochemistry, and Kinetics for the Reactions of Benzyl Hydroperoxide.

Oxidation of the benzyl radical plays a key role in the autoignition, combustion, and atmospheric degradation of toluene and other alkylated aromatic hydrocarbons. Under relevant autoignition conditions of moderate temperature and high pressure, and in the atmosphere, benzyl reacts with O2 to form the benzylperoxy radical, and the further oxidation reactions of this radical are not yet fully characterized. In this contribution, we further develop the reaction chemistry, thermodynamics, and kinetics of benzyl radical oxidation, highlighting the important role of benzyl hydroperoxide and the benzoxyl (benzyloxyl) radical. The benzylperoxy + H reaction mechanism is studied using computational chemistry and statistical reaction rate theory. High-pressure limit rate constants in the barrierless benzylperoxy + H association are obtained from variational transition state theory calculations, with internal rotor contributions. The benzylperoxy + H reaction is seen to produce an activated benzyl hydroperoxide adduct that has 87 kcal mol(-1) excess energy over the ground state. We show that this activated adduct proceeds almost exclusively to the benzoxyl radical + OH across a wide range of temperature and pressure conditions. Minor reaction paths include benzyl + HO2, α-hydroxylbenzyl + OH, and benzaldehyde + H2O, each constituting around 1% of the total reaction rate at higher temperatures. Thermal decomposition of benzyl hydroperoxide, formed by hydrogen abstraction reactions in the benzylperoxy radical and at low temperatures in the benzylperoxy + H and benzyl + HO2 reactions, is also investigated. Decomposition to benzoxyl + OH is fast at temperatures of 900 K and above. The contribution of benzyl hydroperoxide chemistry to the ignition and oxidation of alkylated aromatics is discussed. Benzyl radical oxidation chemistry achieves the conversion of toluene to benzaldehyde, aiding autoignition via processes that either release large amounts of energy or form reactive free radicals through chain-branching.

Thermal Decomposition of the Benzyl Radical to Fulvenallene (C7H6) + H

We show that the benzyl radical decomposes to the C7H6 fragment fulvenallene (+H), by first principles/RRKM study. Calculations using G3X heats of formation and B3LYP/6-31G(2df,p) structural and vibrational parameters reveal that the reaction proceeds predominantly via a cyclopentenyl-allene radical intermediate, with an overall activation enthalpy of ca. 85 kcal mol(-1). Elementary rate constants are evaluated using Eckart tunneling corrections, with variational transition state theory for barrierless C-H bond dissociation in the cyclopentenyl-allene radical. Apparent rate constants are obtained as a function of temperature and pressure from a time-dependent RRKM study of the multichannel multiwell reaction mechanism. At atmospheric pressure we calculate the decomposition rate constant to be k [s(-1)] = 5.93 x 10(35)T(-6.099) exp(-49,180/T); this is in good agreement with experiment, supporting the assertion that fulvenallene is the C7H6 product of benzyl decomposition. The benzyl heat of formation is evaluated as 50.4 to 52.2 kcal mol(-1), using isodesmic work reactions with the G3X theoretical method. Some novel pathways are presented to the cyclopentadienyl radical (C5H5) + acetylene (C2H2), which may constitute a minor product channel in benzyl decomposition.

Variational Analysis of the Phenyl + O2 and Phenoxy + O Reactions

Variational transition state analysis was performed on the barrierless phenyl + O2 and phenoxy + O association reactions. In addition, we also calculated rate constants for the related vinyl radical (C2H3) + O2 and vinoxy radical (C2H3O) + O reactions and provided rate constant estimates for analogous reactions in substituted aromatic systems. Potential energy scans along the dissociating C-OO and CO-O bonds (with consideration of C-OO internal rotation) were obtained at the O3LYP/6-31G(d) density functional theory level. The CO-O and C-OO bond scission reactions were observed to be barrierless, in both phenyl and vinyl systems. Potential energy wells were scaled by G3B3 reaction enthalpies to obtain accurate activation enthalpies. Frequency calculations were performed for all reactants and products and at points along the potential energy surfaces, allowing us to evaluate thermochemical properties as a function of temperature according to the principles of statistical mechanics and the rigid rotor harmonic oscillator (RRHO) approximation. The low-frequency vibrational modes corresponding to R-OO internal rotation were omitted from the RRHO analysis and replaced with a hindered internal rotor analysis using O3LYP/6-31G(d) rotor potentials. Rate constants were calculated as a function of temperature (300-2000 K) and position from activation entropies and enthalpies, according to canonical transition state theory; these rate constants were minimized with respect to position to obtain variational rate constants as a function of temperature. For the phenyl + O2 reaction, we identified the transition state to be located at a C-OO bond length of between 2.56 and 2.16 A (300-2000 K), while for the phenoxy + O reaction, the transition state was located at a CO-O bond length of 2.00-1.90 A. Variational rate constants were fit to a three-parameter form of the Arrhenius equation, and for the phenyl + O2 association reaction, we found k(T) = 1.860 x 1013T-0.217 exp(0.358/T) (with k in cm3 mol-1 s-1 and T in K); this rate equation provides good agreement with low-temperature experimental measurements of the phenyl + O2 rate constant. Preliminary results were presented for a correlation between activation energy (or reaction enthalpy) and pre-exponential factor for heterolytic O-O bond scission reactions.

Kinetic study on pyrolysis and oxidation of CH3Cl in Ar/H2/O2 mixtures

Abstract The thermal decomposition of methyl chloride in hydrogen/oxygen mixtures and argon bath gas was carried out at 1 atmosphere pressure in tubular flow reactors. CH3Cl, intermediate, and final products were analyzed over the temperature range 1098 to 1273 K, with average residence times of 0.2 to 2.0 s. A detailed kinetic reaction mechanism based upon fundamental thermochemical and kinetic principles, Transition State Theory and evaluated literature rate constant data was developed to explain and understand the data. The model results show good fits to methyl chloride, intermediate, and final products species profiles with both temperature and time of reaction. The model also fits the data on HCl inhibited oxidation of CO presented in the companion paper by Roesler et al. Reactions showing high sensitivity to inhibition of CO oxidation were identified. The results indicate that the reaction OH + HCl → H2O + Cl is an important source of OH loss. This decrease in OH strongly effects CO burnout. The re...

Chloroform Pyrolysis: Experiment and Detailed Reaction Model

Abstract Chloroform was used as a model chlorocarbon system with high Cl/H ratio to investigate the thermal decomposition processes of chlorocarbons in pyrolytic reaction environments. The thermal decomposition of 1.0% chloroform in argon bath gas was studied to determine important chlorocarbon reaction pathways in pyrolysis. The reactions were studied in tubular flow reactors at a total pressure of 1 atm with residence times of 0.3-2.0 sec in the temperature range 535-800°C. A detailed kinetic reaction mechanism to describe the important features of products and reagent loss was developed. The mechanism is based on thermochemical principles and accurately describes the overall reaction process. We determine that the primary decomposition reaction pathway for chloroform is: CHCl3 → HCl +: CCl2 in the above temperature regime by combined experiment and modeling studies. High pressure limit rate constants obtained in this work for initially important decomposition of chloroform were determined as: A(1/sec.)...