Sándor Dóbé

Effect of the uncertainty of kinetic and thermodynamic data on methane flame simulation results

A method for assessing and comparing the impact of uncertainties in both kinetic and thermodynamic parameters on the predictions of combustion chemistry models has been developed. Kinetic, thermodynamic and overall uncertainty parameters are defined, which allow tracking the sources of uncertainties for a particular model result. The method was applied to premixed laminar methane-air flames using the Leeds Methane Oxidation Mechanism (K. J. Hughes et al., Int. J. Chem. Kinet., 2001, 33, 513). Heat of formation and rate coefficient data for species and elementary reactions, respectively, related to methane combustion were collected from several recent reviews and critically assessed error limits were assigned to them. Local rate coefficient sensitivities and heat of formation sensitivities were calculated for lean (φ = 0.62), stoichiometric (φ = 1.00) and rich (φ = 1.20) laminar atmospheric premixed methane-air flames. Uncertainties of flame velocity, maximum flame temperature and also the value and location of maximum concentration of radicals H, O, OH, CH and CH2 were obtained from the sensitivities and the uncertainties of thermodynamic and chemical kinetic data. The uncertainty of the calculated flame velocity is typically 2–5 cm s−1. Maximum flame temperature and concentration of H, O, and OH can be calculated accurately, while there is high uncertainty in the calculated maximum concentration of CH and CH2. The calculations have revealed that the uncertainty of the calculated flame velocity is caused mainly by errors of the input rate coefficients. This is the case also for the calculated concentration of CH and CH2. The uncertainty of the location of concentration maxima is also of kinetics origin and it is caused by the very same rate coefficients that affect flame velocity. Uncertainty of maximum adiabatic flame temperature and maximum concentration of H, O and OH originates mainly from errors of the input heat of formation data. In order to obtain good simulation results for methane flames, accurate heats of formation are required in particular for radicals OH, CH2(S), CH2, CH2OH, HCCO and CH2HCO. Simulation results could be improved by better knowledge of the reaction rate parameters for the reactions O2 + H = OH + O, O2 + H + M = HO2 + M, CO + OH = CO2 + H, H + CH3(+M) = CH4(+M), CH3 + OH = CH2(S) + H2O, C2H2 + OH = C2H + H2O and C2H2 + CH = C2H + CH2. This conclusion is somewhat surprising since at least the first three reactions are among the most frequently studied ones in chemical kinetics. The calculations demonstrate that all simulation results of chemical kinetic modelling studies should be accompanied by uncertainty information (e.g. standard deviation) for the model outputs to indicate which results are well supported by the model and which ones are merely nominal values that were obtained using the selected set of input parameters.

Kinetics of the reaction between methoxyl radicals and hydrogen atoms

The kinetics of the reaction of CH3O with H have been studied under pseudo-first-order conditions with an excess of H using an isothermal discharge-flow reactor. Three different CH3O sources were used and the decay of [CH3O] was monitored by laser-induced fluorescence (LIF) as a function of [H]. A second-order rate coefficient of (2.0 ± 0.6)× 1013 cm3 mol–1 s–1 was determined for reaction CH3O + H → products at room temperature and a slight positive temperature dependence was observed between 298 and 490 K. Formaldehyde formation was found to be the dominant reaction path (81 ± 12%). Further identified products were OH (7 ± 3%) and methanol (a few percent) which were produced by the decomposition and stabilization, respectively, of the initially formed bound adduct.

Features of the potential energy surface for the reaction of OH radical with acetone

The mechanism of the reaction of OH with acetone has been studied by quantum chemical computations. 21 stationary points (among them reactant complexes, reaction transition states, intermediate complexes and product complexes) have been characterised on the potential energy surface of the reaction. The MP2 method with 6-31G(d,p) basis set was employed for geometry optimisation. Electronic energies were obtained at the CCSD(T)/6-311G(d,p) level of theory. Hydrogen abstraction was found to occur through two complex mechanisms; no transition state for direct abstraction could be located. Minimum energy path analyses have revealed two distinct pathways which lead to CH3 (+CH3COOH) formation. One of them sets out the abstraction channel and proceeds via intermolecular complexes and the other one involves addition of OH to the carbonyl double bond and subsequent decomposition of the adduct hydroxy-alkoxy radical. The rate limiting steps involve large energy barriers and, consequently, these pathways do not explain the high methyl yields observed experimentally at and below room temperature. Characteristic for the reaction of OH with acetone is the existence of numerous hydrogen-bridged complexes on the potential energy surface that are stabilised by as much as 3.2–26.6 kJ mol−1 binding energy. Some properties of these complexes and their possible role in the molecular mechanism of the reaction are discussed.

Theoretical study of the reaction OH + acetone: a possible kinetic effect of the presence of water?

The effect of water on the molecular mechanism of the reaction of the OH radical with acetone in the homogeneous gas-phase has been studied by quantum chemical computations. The three-molecular reaction system of OH + acetone + H2O has been characterised using molecular parameters, electronic energies and Gibbs free energies computed for the stationary points of the potential energy surface. The MP2 method with a 6-31G(d,p) basis set was employed for geometry optimisation. The electronic energies were obtained at the MP4 and the CCSD(T) level of theory using the 6-311G(d,p) basis set. We have found that the presence of a water molecule changes significantly both the energy profile and free energy profiles of the reaction. A “water-assisted” reaction mechanism has been established in which both the H-abstraction channel and the CO-addition channel occur via intermolecular complexes and transition state structures that involve the water molecule. The activation free energy for the out-of-plane abstraction channel at low temperatures has been found to be significantly smaller than that for the “water-free” system indicating a possible catalytic rate enhancement effect. Abstraction is the predominant reaction route also for the water-assisted reaction as shown by the much larger activation free energy computed for the addition channel. In order to estimate atmospheric concentrations of some intermolecular complexes, we have validated our employed level of theory by computing the equilibrium constant of HO2 + H2O ⇄ HO2⋯H2O at three temperatures and compared them to the values derived from experiments available in the literature. Then, using our theoretical results, we have estimated the tropospheric concentration of OH⋯acetone⋯H2O complexes to be very small, but they are probably detectable under laboratory conditions.

Atmospheric Chemistry of 2,3-Pentanedione: Photolysis and Reaction with OH Radicals

The kinetics of the overall reaction between OH radicals and 2,3-pentanedione (1) were studied using both direct and relative kinetic methods at laboratory temperature. The low pressure fast discharge flow experiments coupled with resonance fluorescence detection of OH provided the direct rate coefficient of (2.25 ± 0.44) × 10(-12) cm(3) molecule(-1) s(-1). The relative-rate experiments were carried out both in a collapsible Teflon chamber and a Pyrex reactor in two laboratories using different reference reactions to provide the rate coefficients of 1.95 ± 0.27, 1.95 ± 0.34, and 2.06 ± 0.34, all given in 10(-12) cm(3) molecule(-1) s(-1). The recommended value is the nonweighted average of the four determinations: k(1) (300 K) = (2.09 ± 0.38) × 10(-12) cm(3) molecule(-1) s(-1), given with 2σ accuracy. Absorption cross sections for 2,3-pentanedione were determined: the spectrum is characterized by two wide absorption bands between 220 and 450 nm. Pulsed laser photolysis at 351 nm was used and the depletion of 2,3-pentanedione (2) was measured by GC to determine the photolysis quantum yield of Φ(2) = 0.11 ± 0.02(2σ) at 300 K and 1000 mbar synthetic air. An upper limit was estimated for the effective quantum yield of 2,3-pentanedione applying fluorescent lamps with peak wavelength of 312 nm. Relationships between molecular structure and OH reactivity, as well as the atmospheric fate of 2,3-pentanedione, have been discussed.

Direct kinetic study of reactions of hydroxyl radicals with alkyl formates

Abstract Rate coefficients were determined for the gas phase reactions of hydroxyl radicals with a series of alkyl formats. Experiments were carried out using the isothermal fast flow method coupled with resonance fluorescence detection. The obtained room temperature rate coefficients are (in 10−13cm3 molecule−1s−1 units): 1.83 ± 0.33 (methyl formate), 9.65 ± 0.43 (ethyl formate), 18.73 ± 0.83 (isopropyl formate), 6.77 ± 0.38 (tert-butyl formate) and 1.62 ± 0.13 (methyl chloroformate). These results are compared with the literature data. In addition estimations are made for the partial reactivities of the formate group and for the hydrocarbon groups adjacent to HC(O)O. Moreover, it has been found that the partial reactivity of the HC(O)O group (in reactions of OH with formates) is two orders of magnitude smaller than that of the HC(O) group (in reactions of OH with aldehydes). This has been explained using the results of ab initio calculations at the G3MP2//MP2(full)/6-31G(d) level of theory.

Laser spectrometry and kinetics of selected elementary reactions of the acetonyl radical

The laser induced fluorescence excitation spectrum (LIF) and the ultraviolet absorption spectrum (TA) of the acetonyl radical (CH3C(O)CH2) were remeasured by using the time-resolved fast discharge flow (DF) and laser flash photolysis (LP) experimental techniques (T = 298 K). The absorption cross section of σ(acetonyl, 207 nm) = (3.16 ± 0.61) × 10−18 cm2 molecule−1 was determined calibrated against the acetyl-peroxyl radical (CH3C(O)O2) in LP/TA measurements. The kinetics of the reactions of CH3C(O)CH2 with the open shell reaction partners O2 (1), NO (2), NO2 (3) and H (4) were studied by using the DF method with LIF detection of the acetonyl radical at 298 ± 1 K and 2.85 ± 0.05 mbar He pressure. The rate constants for the overall reactions were determined in units of cm3 molecule−1 s−1 to be k1 = (3.49 ± 0.51) × 10−13, k2 = (1.04 ± 0.19) × 10−11, k3 = (3.25 ± 0.65) × 10−11 and k4 ≥ 3 × 10−10 with 2σ accuracy given. The acetonyl radical was found to react similarly to alkyl radicals by comparison with literature results. A reduced reactivity was observed toward O2 and NO that might be attributed to the resonance stabilisation of the acetonyl radical. No such effect was observed for the NO2 and H atom reactions.

Theoretical enthalpy of formation of the acetonyl radical

The standard enthalpy of formation of the acetonyl radical (CH2COCH3) was theoretically estimated using several working chemical reactions, with four variants of theoretical approaches (levels) and four extended basis sets. Our best theoretical enthalpy of formation is ΔfH0298(CH2COCH3)=−32±4kJmol−1. This computed value corresponds to the bond dissociation energy of DH0298(H–CH2COCH3)=403±4kJmol−1, and to the resonance stabilization energy (RE0) and the intrinsic stabilization energy (SE0) of 16.5 and 14.2kJmol−1, respectively. These energies indicate a greater stabilization of the acetonyl radical than previously thought.

Formation of methoxy and hydroxymethyl free radicals in selected elementary reactions

The fast flow technique combined with laser-induced fluorescence (LIF) and laser magnetic resonance(LMR) detections have been used to obtain branching ratios for CH 3 O and CH 2 OH formation respectively, in various reactions. A detailed study of the product yields showed that reaction F+CH 3 ONO→ FNO+CH 3 O is a clean methoxy radical source at low temperature that gives CH 3 O with practically 100% efficiency. The branching ratios for CH 3 O and CH 2 OH formation in the reaction of F with CH 3 OH are k 1a /( k 1a + k 1b )=0.57±0.05 and k 1b /( k 1a + k 1b )=0.41±0.05, respectively, between 300 and 600 K independent of temperature; F+CH 3 OH→HF+CH 3 O (1a), F+CH 3 OH→HX+CH 2 OH (1b). The branching ratios for product radical formation in reactions X+CH 3 OH→HX+CH 3 O and X+ CH 3 OH→HX+CH 2 OH (where X=F, OH, and Cl) were studied between 298 and 612 K. The considerable decrease of exothermicity in the order of F, OH, and Cl reactions is expected to go with a decrease in reactivity and a simultaneous increase in selectivity, which is indeed experienced for reactions F+CH 3 OH and OH+CH 3 OH. However, the reaction Cl+CH 3 OH was found to be a “high reactivity high selectivity” process yielding CH 2 OH radicals with practically 100% yield up to about 500 K. The fast reaction and the relatively small temperature dependences of the branching ratios indicate that some specific effects govern the kinetics of this reaction. Combustion implications and utilization of the results in kinetic experiments are discussed.

Experimental and Theoretical Study on the OH-Reaction Kinetics and Photochemistry of Acetyl Fluoride (CH3C(O)F), an Atmospheric Degradation Intermediate of HFC-161 (C2H5F)

The direct reaction kinetic method of low pressure fast discharge flow (DF) with resonance fluorescence monitoring of OH (RF) has been applied to determine rate coefficients for the overall reactions OH + C2H5F (EtF) (1) and OH + CH3C(O)F (AcF) (2). Acetyl fluoride reacts slowly with the hydroxyl radical, the rate coefficient at laboratory temperature is k2(300 K) = (0.74 ± 0.05) × 10(-14) cm(3) molecule(-1) s(-1) (given with 2σ statistical uncertainty). The temperature dependence of the reaction does not obey the Arrhenius law and it is described well by the two-exponential rate expression of k2(300-410 K) = 3.60 × 10(-3) exp(-10500/T) + 1.56 × 10(-13) exp(-910/T) cm(3) molecule(-1) s(-1). The rate coefficient of k1 = (1.90 ± 0.19) × 10(-13) cm(3) molecule(-1) s(-1) has been determined for the EtF-reaction at room temperature (T = 298 K). Microscopic mechanisms for the OH + CH3C(O)F reaction have also been studied theoretically using the ab initio CBS-QB3 and G4 methods. Variational transition state theory was employed to obtain rate coefficients for the OH + CH3C(O)F reaction as a function of temperature on the basis of the ab initio data. The calculated rate coefficients are in good agreement with the experimental data. It is revealed that the reaction takes place predominantly via the indirect H-abstraction mechanism involving H-bonded prereactive complexes and forming the nascent products of H2O and the CH2CFO radical. The non-Arrhenius behavior of the rate coefficient at temperatures below 500 K is ascribed to the significant tunneling effect of the in-the-plane H-abstraction dynamic bottleneck. The production of FC(O)OH + CH3 via the addition/elimination mechanism is hardly competitive due to the significant barriers along the reaction routes. Photochemical experiments of AcF were performed at 248 nm by using exciplex lasers. The total photodissociation quantum yield for CH3C(O)F has been found significantly less than unity; among the primary photochemical processes, C-C bond cleavage is by far dominating compared with CO-elimination. The absorption spectrum of AcF has also been determined by displaying a strong blue shift compared with the spectra of aliphatic carbonyls. Consequences of the results on atmospheric chemistry have been discussed.