The products of the OH-initiated oxidation mechanism of ethene have been studied as a function of temperature (between 250 and 325 K) in an environmental chamber, using Fourier transform infrared spectroscopy for end product analysis. The oxidation proceeds via formation of a peroxy radical, HOCH2CH2O2. Reaction of this peroxy radical with NO is exothermic and produces chemically activated HOCH2CH2O radicals, of which about 25% decompose to CH2OH and CH2O on a time scale that is rapid compared to collisions, independent of temperature. The remainder of the HOCH2CH2O radicals are thermalized and undergo competition between decomposition, HOCH2CH2O → CH2OH + CH2O (6), and reaction with O2, HOCH2CH2O + O2 → HOCH2CHO + HO2 (7). The rate constant ratio, k 6/k 7, for the thermalized radicals was found to be (2.0 ± 0.2) × 1025 exp[−(4200 ± 600)/T] molecule cm-3 over the temperature range 250−325 K. With the assumption of an activation energy of 1−2 kcal mol-1 for reaction 7, the barrier to decomposition of the HOCH2CH2O radical is found to be 10−11 kcal mol-1. A study of the Cl-atom-initiated oxidation of ethene was also carried out; the main product observed under conditions relevant to the atmosphere was chloroacetaldehyde, ClCH2CHO. Theoretical studies of the thermal and “prompt” decomposition of the oxy radicals were based on a recent ab initio characterization that highlighted the role of intramolecular H bonding in HOCH2CH2O. Thermal decomposition is described by transition state and the Troe theories. To quantify the prompt decomposition of chemically activated nascent oxy radicals, the energy partitioning in the initially formed radicals was described by separate statistical ensemble theory, and the fraction of activated radicals dissociating before collisional stabilization was obtained by master equation analysis using RRKM theory. The barrier to HOCH2CH2O decomposition is inferred independently as being 10−11 kcal mol-1, by matching both of the theoretical HOCH2CH2O decomposition rates at 298 K with the experimental results. The data are discussed in terms of the atmospheric fate of ethene.

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The present study concerns an experimental microkinetic approach of the photocatalytic oxidation (PCO) of isopropyl alcohol (IPA) into acetone on a pure anatase TiO2 solid according to a procedure previously developed. Mainly, the kinetic parameters of each surface elementary step of a plausible kinetic model of PCO of IPA are experimentally determined: natures and amounts of the adsorbed species and rate constants (preexponential factor and activation energy). The kinetics parameters are obtained by using experiments in the transient regime with either a FTIR or a mass spectrometer as a detector. The deep oxidation (CO2 and H2O formation) of low concentrations of organic pollutants in air is one of the interests of the PCO. For IPA, literature data strongly suggest that acetone is the single route to CO2 and H2O and this explains that the present study is dedicated to the elementary steps involving gaseous and adsorbed C3H(x)O species. The microkinetic study shows that strongly adsorbed IPA species (two species denoted nd-IPA(sads) and d-IPA(sads) due to non- and dissociative chemisorption of IPA, respectively) are involved in the PCO of IPA. A strong competitive chemisorption between IPA(sads) and a strongly adsorbed acetone species controls the high selectivity in acetone of the PCO at a high coverage of the surface by IPA(sads). The kinetic parameters of the elementary steps determined in the present study are used in part 2 to provide a modeling of macroscopic kinetic data such as the turnover frequency (TOF in s(-1)) of the PCO using IPA/O2 gas mixtures.

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Abstract. An environmental chamber/Fourier transform infrared system was used to determine the rate coefficient k• for the gas-phase reaction of OH with 2-methyl-3-buten-2-ol (MBO, (CH3)2C(OH)CH=CH2), relative to the rate of its reaction with ethylene (k2) and propylene (k3).

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