Shock-tube study of the thermal decomposition of CH3CHO and CH3CHO + H reaction

Theoretical Studies on the Kinetics of Multi-Channel Gas-Phase Unimolecular Decomposition of Acetaldehyde

Theoretical kinetic studies are performed on the multichannel thermal decomposition of acetaldehyde. The geometries of the stationary points on the potential energy surface of the reaction are optimized at the MP2(full)/6-311++G(2d,2p) level of theory. More accurate energies are obtained by single point energy calculations at the CCSD(T,full)/augh-cc-pVTZ+2df, CBS-Q and G4 levels of theory. Here, by application of steady-state approximation to the thermally activated species CH₃CHO* and CH₂CHOH* and performance of statistical mechanical manipulations, expressions for the rate constants for different product channels are derived. Special attempts are made to compute accurate energy-specific rate coefficients for different channels by using semiclassical transition state theory. It is found that the isomerization of CH₃CHO to the enol-form CH₂CHOH plays a significant role in the unimolecular reaction of CH₃CHO. The possible products of the reaction are formed via unimolecular decomposition of CH₃CHO and CH₂CHOH. The computed rate coefficients reveal that the dominant channel at low temperatures and high pressures is the formation of CH₂CHOH due to the low barrier height for CH₃CHO → CH₂CHOH isomerization process. However, at high temperatures, the product channel CH₃ + CHO becomes dominant.

Shock Tube Measurement for the Dissociation Rate Constant of Acetaldehyde Using Sensitive CO Diagnostics

The rate constant of acetaldehyde thermal dissociation, CH3CHO = CH3 + HCO, was measured behind reflected shock waves at temperatures of 1273-1618 K and pressures near 1.6 and 0.34 atm. The current measurement utilized sensitive CO diagnostics to track the dissociation of CH3CHO via oxygen atom balance and inferred the title rate constant (k1) from CO time histories obtained in pyrolysis experiments of 1000 and 50 ppm of CH3CHO/Ar mixtures. By using dilute test mixtures, the current study successfully suppressed the interferences from secondary reactions and directly determined the title rate constant as k1(1.6 atm) = 1.1 × 10(14) exp(-36 700 K/T) s(-1) over 1273-1618 K and k1(0.34 atm) = 5.5 × 10(12) exp(-32 900 K/T) s(-1) over 1377-1571 K, with 2σ uncertainties of approximately ±30% for both expressions. Example simulations of existing reaction mechanisms updated with the current values of k1 demonstrated substantial improvements with regards to the acetaldehyde pyrolysis chemistry.

Products from pyrolysis of gas-phase propionaldehyde

A hyperthermal nozzle was utilized to study the thermal decomposition of propionaldehyde, CH3CH2CHO, over a temperature range of 1073-1600 K. Products were identified with two detection methods: matrix-isolation Fourier transform infrared spectroscopy and photoionization mass spectrometry. Evidence was observed for four reactions during the breakdown of propionaldehyde: α-C-C bond scission yielding CH3CH2, CO, and H, an elimination reaction forming methylketene and H2, an isomerization pathway leading to propyne via the elimination of H2O, and a β-C-C bond scission channel forming methyl radical and (•)CH2CHO. The products identified during this experiment were CO, HCO, CH3CH2, CH3CH═C═O, H2O, CH3C≡CH, CH3, H2C═C═O, CH2CH2, CH3CH═CH2, HC≡CH, CH2CCH, H2CO, C4H2, C4H4, and CH3CHO. The first eight products result from primary or bimolecular reactions involving propionaldehyde while the remaining products occur from reactions including the initial pyrolysis products. While the pyrolysis of propionaldehyde involves reactions similar to those observed for acetaldehyde and butyraldehyde in recent studies, there are a few unique products observed which highlight the need for further study of the pyrolysis mechanism.

Pyrolysis of Ethyl Iodide as Hydrogen Atom Source: Kinetics and Mechanism in the Temperature Range 950–1200 K

Ethyl iodide is a well known H atom precursor in shock tube experiments. In the present work, we study peculiarities, when C2H5I is used under conditions, where its decomposition is not longer fast compared to consecutive bimolecular reactions. On the basis of shock tube experiments with detection of H and I atoms by resonance absorption spectrometry, accompanied by quantum chemical (CCSD(T)/6-311G//CCSD/6-311G) and statistical rate theory calculations, we propose a small mechanism (5 reactions, 7 species) and kinetic data, which allow an adequate description of C2H5I pyrolysis as a H atom source down to temperatures between 950 and 1200 K at pressures ranging from 1 to 4 bar: C2H5I→C2H5 + I (1), k 1 = 9.9 × 1012 exp(−23200 K/T) s−1; C2H5 + M→C 2H4 + H + M (2), k 2 = 1.7 × 10−6 exp(−16800 KT) cm3 s−1 [D. L. Baulch et al., J. Phys. Chem. Ref. Data 34 (2005) 757]; C2H5I→C2H4 + HI (3), k 3 = 1.7 × 1013 exp(−26680 KT) s−1; H + HI→H2 + I (4), k 4 = 7.9 × 10−11 exp(−330 KT) cm3 s−1 [D. L. Baulch et al., J. Phys. Chem. Ref. Data 10(Suppl. 1) (1981) 1]; C2H5I + H→C2H5 + HI (5), k 5 = 7.0 × 10−9 exp(−3940 KT) cm3 s−1. The latter bimolecular abstraction step turned out crucial for an adaquate d escription of the hydrogen atom concentration-time profiles in the above mentioned temperature and pressure range for initial concentrations [C2H5I]0 > 2 × 1013 cm−3 corresponding to mole fractions > 1 ppm.

Towards cleaner combustion engines through groundbreaking detailed chemical kinetic models

In the context of limiting the environmental impact of transportation, this critical review discusses new directions which are being followed in the development of more predictive and more accurate detailed chemical kinetic models for the combustion of fuels. In the first part, the performance of current models, especially in terms of the prediction of pollutant formation, is evaluated. In the next parts, recent methods and ways to improve these models are described. An emphasis is given on the development of detailed models based on elementary reactions, on the production of the related thermochemical and kinetic parameters, and on the experimental techniques available to produce the data necessary to evaluate model predictions under well defined conditions (212 references).

Shock-Tube Study of the Reactions of Hydrogen Atoms with Benzene and Phenyl Radicals

The reactions of hydrogen atoms with phenyl radicals, H + C6H5 → products (1), and with benzene, H + C6H6 → products (2), have been studied behind reflected shock waves in the temperature range 1200–1350 K with argon as the bath gas. H-atom resonance absorption spectrometry at 121.6 nm was used as detection technique. Hydrogen atoms and phenyl radicals were produced by thermal decomposition of C2H5I and C6H5I, respectively. Low initial concentrations (~1012–1015 cm-3) were employed to suppress consecutive bimolecular reactions as far as possible.The rate coefficients were determined from fits of the H atom concentration-time profiles in terms of a small mechanism. For reaction (1), a temperature-independent rate coefficient k 1 = 1.3×10–10 cm3 s–1 was obtained at pressures around 1.3 bar. For the rate coefficient of reaction (2), the temperature dependence can be expressed as k 2(T) = 5.8×10–10 exp(–8107 K/T) cm3 s–1, and a pressure dependence was not observed between 1.3 and 4.3 bar. The uncertainties of k 1 and k 2 were estimated to be ±40%.