Carlos J. Cobos

Evaluated kinetic data for combustion modelling

This compilation contains critically evaluated kinetic data on elementary homogeneous gas phase chemical reactions for use in modelling combustion processes. Data sheets are presented for some 196 reactions. Each data sheet sets out relevant thermodynamic data, rate coefficient measurements, an assessment of the reliability of the data, references, and recommended rate parameters. Tables summarizing the preferred rate data are also given. The reactions considered are limited largely to those involved in the combustion of methane and ethane in air but a few reactions relevant to the chemistry of exhaust gases and to the combustion of aromatic compounds are also included.

Experimental and modeling study of the reaction C2F4 (+ M) ⇔ CF2 + CF2 (+ M).

The thermal dissociation reaction C2F4(+ M) → 2CF2(+ M) was studied in shock waves monitoring CF2 radicals by their UV absorption. The absorption coefficients as functions of wavelength and temperature were redetermined and are represented in analytical form. Dissociation rate constants as functions of bath gas concentration [M] and temperature, from previous and the present work, are presented analytically employing falloff expressions from unimolecular rate theory. Equilibrium constants are determined between 1200 and 1500 K. The data are shown to be consistent, with a C-C bond energy of 67.5 (±0.5) kcal mol(-1). High-pressure limiting rate constants for dissociation and recombination are found to be unusually small. This phenomenon can be attributed to an unusually pronounced anisotropy of the potential energy surface, such as demonstrated by quantum-chemical calculations of the potential energy surface.

Kinetics of the recombination reaction between F atoms and FCO radicals

Abstract The laser flash photolysis-absorption technique has been used to study the F + FCO + M → CF 2 O + M recombination reaction between 20 and 700 Torr of M = CF 4 at 296 K. The values k rec,0 = (5.6 ± 1.2) × 10 −29 [CF 4 ] cm 3 molecule −1 s −1 and k rec,∞ = (7.9 ± 1.1) × 10 −11 cm 3 molecule −1 s −1 were determined by extrapolation of the fall-off curve. An analysis of k rec,0 with respect to energy-transfer properties has been carried out. The measured k rec,∞ value is in reasonable agreement with the predictions of the simplified statistical adiabatic channel model. For the FCO + FCO → CF 2 O + CO reaction a rate coefficient of (2.0 ± 0.2) × 10 −11 cm 3 molecule −1 s −1 was determined.

Pulsed laser photolysis study of the reaction Cl+FSO3+M→FSO2OCl+M

Abstract The recombination reaction Cl+FSO3+M→FSO2OCl+M has been studied by ArF excimer laser photolysis-visible absorption spectroscopy in the pressure range 24–708 Torr of the bath gases He and CF4 at 297 K. For M = CF4 the limiting high-pressure rate constant is (6.0±0.7) × 10−11 cm3 molecule−1 s−1. Small fall-off effects are apparent for He diluent. An analysis of the in terms of the statistical adiabatic channel model leads to α/β=0.52 for the ratio of the looseness and Morse parameters. For Cl+FSO2OCl→Cl2+FSO3 a rate constant of (1.3±0.2) × 10−11 cm3 molecule−1 s−1 was determined.

Shock wave study of the thermal decomposition of CF3 and CF2 radicals.

The thermal dissociation reactions CF(3) + M --> CF(2) + F + M (reaction 1 ) and CF(2) + M --> CF + F + M (reaction 3 ) were studied behind shock waves. CF(2) radicals were monitored through their UV absorption. By working at very low reactant concentrations, the rate coefficients of the unimolecular processes could be derived. Reaction 1 was investigated between 1600 and 2300 K in the intermediate range of the falloff curves, at approximately 10 times larger bath gas pressures than employed in earlier work (Srinivasan, N. K.; Su, M.-C.; Michael, J. V.; Jasper, A. W.; Klippenstein, S. J.; Harding, L. B. J. Phys. Chem. A 2008, 112, 31). The combination of the two sets of data, together with theoretical modeling, allows one to construct falloff curves and to provide complete representations of the temperature and pressure dependences of the rate coefficients. Reaction 3 was studied in the limiting low-pressure range and, over the range 2900-3800 K, a rate coefficient k(3) = [Ar] 1.6 x 10(15) exp(-48,040 K/T) cm(3) molecule(-1) s(-1) was obtained. Representations of the rate coefficients over the full falloff curves were again derived by theoretical modeling.

Experimental and theoretical study of the recombination reaction F + FC(O)O + M → FC(O)OF + M

The kinetics of the F+FC(O)O+M→FC(O)OF+M recombination reaction (1) has been experimental and theoretically investigated. Both F atoms and FC(O)O radicals were generated by 193-nm laser flash photolysis of FC(O)OO(O)CF in mixtures with He or CF4 at 295 K. From the extrapolation of the fall-off curves measured from 55 to 790 Torr total pressure, the high pressure rate coefficient (2.1±0.3)×10−11 cm3 molecule−1 s−1 and the low pressure rate coefficients (1.6±0.3)×10−29[He], (3.3±0.7)×10−29[CF4] and (7.1±1.5)×10−29[FC(O)OO(O)CF] cm3 molecule−1 s−1 were determined. The rate coefficient for the reaction between F atoms and FC(O)OO(O)CF has been found to be (1.5±0.5)×10−15 cm3 molecule−1 s−1. The limiting rate coefficients were analyzed using the unimolecular reaction theory on an ab initio potential energy surface derived at the G3S level of theory. Structural properties, harmonic vibrational frequencies and heats of formation for the FC(O)O radical and for the cis and trans conformers of FC(O)OF calculated with the B3LYP/6-311+G(3df) functional are presented. The values −87.9, −103.4 and −104.6 kcal mol−1, respectively, for the heats of formation of the three molecules were obtained at 298 K from isodesmic energies computed at the G3//B3LYP/6-311++G(3df,3pd) level.

EXAFS and DFT study of the cadmium and lead adsorption on modified silica nanoparticles

Silica nanoparticles of 7 nm diameter were modified with (3-aminopropyl) triethoxysilane (APTES) and characterized by CP-MAS (13)C and (29)Si NMR, FTIR, zeta potential measurements, and thermogravimetry. The particles were shown to sorb successfully divalent lead and cadmium ions from aqueous solution. Lead complexation with these silica nanoparticles was clearly confirmed by EXAFS (Extended X-ray Absorption Fine Structure) with synchrotron light measurements. Predicted Pb-N and Pb-C distances obtained from quantum-chemical calculations are in very good agreement with the EXAFS determinations. The calculations also support the higher APTES affinity for Pb(2+) compared to Cd(2+).

Experimental and modelling study of the unimolecular thermal decomposition of CHF3.

Abstract The unimolecular thermal decomposition reaction CHF3 (+M)→CF2 + HF (+M) was studied in shock waves by monitoring the UV absorption of the forming CF2 radicals. The results of the present and previous experiments on the temperature and pressure dependence of the rate constants are analyzed in terms of unimolecular rate theory. Falloff curves (for 1500–1900 K) are represented in terms of fitted limiting low pressure rate constants k0 = [Ar] 1.1 × 1016exp(−53.0 kcal mol−1/RT) cm3mol−1s−1, limiting high pressure rate constants k∞ = 1.25 × 1015 exp(−75.8 kcal mol−1/RT) s−1 from quantum-chemical calculations, and center broadening factors Fcent = 0.170(± 0.04) including strong and weak collision contributions from unimolecular rate theory. With these results, approximate analytical expressions of the falloff curves well represent the measured and calculated rate constants over wide ranges of pressure and temperature.

Temperature and Pressure Dependence of the Reaction 2CF3 (+ M) ⇔ C2F6 (+ M)†

Limiting low- and high-pressure rate coefficients as well as full falloff curves have been modeled by unimolecular rate theory for the recombination reaction 2CF(3) (+ M) --> C(2)F(6) (+ M) and the reverse dissociation of C(2)F(6). The results are compared with experimental data from the literature. Although there are considerable discrepancies (up to a factor of 5) between various experimental data near 300 K and the database for high temperatures is still limited, we try to conclude on the temperature dependence of the high-pressure rate coefficient. We suggest that there is only a small and probably positive temperature coefficient of the latter quantity. The present theoretical modeling seems to be in agreement with this experimental result, but it is in disagreement with conclusions from earlier theoretical work. The difference is attributed to different empirical assumptions about the anisotropy of the potential. It is shown that nearly all previous experiments (except high-temperature shock wave and very low pressure pyrolysis/photolysis experiments) correspond to nearly limiting high-pressure conditions.

Kinetics of formation and unimolecular decomposition of the FS(O2)OOO radical

Abstract The kinetics and mechanism of the 193 nm laser flash photolysis of FSO 2 OF in the presence of O 2 and the bath gases N 2 and CF 4 were studied at 295 and 383 K. FSO 3 radicals and F atoms are formed in the primary photolytic process. Subsequently, the FO 2 radicals, produced by the combination of F with O 2 , react with FSO 3 leading to the intermediate trioxide FS(O 2 )OOOF. To explain the build-up of the roomtemperature, time-resolved absorption signals observed at 450 nm, the formation of the novel FS(O 2 )OOO radical, via F atom abstraction from the trioxide by FO 2 , is proposed. FS(O 2 )OOO then dissociates unimolecularly on a millisecond timescale into FSO 3 and O 2 . This radical is not formed by recombination of FSO 3 with O 2 . At 383 K, the faster thermal decomposition of FO 2 radicals precludes the formation of FS(O 2 )OOO.