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Article Dans Une Revue Physical Chemistry Chemical Physics Année : 2020

Water does not catalyze the reaction of OH radicals with ethanol


Recent experiments suggested that water catalyzes the reaction of OH radicals with alcohols, while another work showed the opposite result. Here, we resolve this disagreement and show that heterogeneous oxidation systematically biased the work showing the catalytic effect and corroborate that water does not catalyze the reaction of OH with alcohols. Graphical abstract: Water does not catalyze the reaction of OH radicals with ethanol Methanol, CH3OH, is one of the most abundant oxygenated volatile organic compounds in the atmosphere.1,2 The major sources are direct emissions, but some oxidation pathways of methane also contribute to its abundance, especially in the remote troposphere.3–5 Ethanol, C2H5OH, is a less abundant volatile organic compound and its major source is also the direct emission into the atmosphere from living and decaying plants, with minor input from anthropogenic production. C2H5OH is a precursor of acetaldehyde, peroxyacetyl nitrate and secondary aerosols and has thus some impact on the composition of the atmosphere.6 The major decay pathway for both alcohols in the troposphere is their reaction with OH radicals. Rate constants for both reactions, OH + CH3OH → product + H2O (R1) OH + C2H5OH → product + H2O (R2) have been measured many times and are thought to be well known with the IUPAC committee estimating uncertainties of less than ±20%7 in the recommended values for k1 and k2. However, Jara-Toro et al. recently published two papers that showed that for CH3OH8, C2H5OH and the next-larger alcohol, n-propanol,9 the rate constant of their reaction with OH radicals increases with increasing relative humidity (RH). Their experiments showed that the rate constant increased quadratically with RH and was enhanced by around a factor of 2 when the measurements were carried out under high relative humidity (RH > 90%), far outside of the current estimated uncertainty of the rate constant. As earlier measurements were always carried out at low RH, such an increase might not have been observed in earlier experiments. Such an increase of the rate constant with RH would have a significant impact on the lifetime of alcohols in the troposphere, because high RHs are often encountered in tropical regions where the photochemical activity is high. Therefore, more recently we have carried out new experiments to investigate again the influence of water on the rate constant of reaction (R1)10 but could not experimentally show the increase of the rate constant with RH such as that observed by Jara-Toro et al.8,9 We carried out quantum chemical calculations which confirmed that the addition of water should not increase the rate constant of (R1): even though it was shown that water stabilizes the reactant complex and thus lowers the barrier, the increase in entropy makes more than up for this, and as a result no impact of water on the rate constant is expected at room temperature. The following table summarizes the results of different studies and the presence (×) or absence (−) of the catalytic effect.
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hal-02961092 , version 1 (08-10-2020)



Isabelle Weber, Hichem Bouzidi, Bianca Krumm, Coralie Schoemaecker, Alexandre Tomas, et al.. Water does not catalyze the reaction of OH radicals with ethanol. Physical Chemistry Chemical Physics, 2020, 22 (14), pp.7165-7168. ⟨10.1039/d0cp00467g⟩. ⟨hal-02961092⟩
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