Heterogeneous reactions of volatile organic compounds on oxide particles of the most abundant crustal elements: Surface reactions of acetaldehyde, acetone, and propionaldehyde on SiO2, Al2O3, Fe2O3, TiO2, and CaO

Abstract

The heterogeneous reactivity of volatile organic compounds (VOCs) on oxide particles has been investigated. Oxide particles composed of the most abundant elements present in the Earth's crust were used to identify potentially important reactions of VOCs on mineral dust. In particular, heterogeneous reactions of acetaldehyde, acetone, and propionaldehyde on α‐Al2O3, α‐Fe2O3, TiO2, CaO, and SiO2 particle surfaces have been investigated. Fourier transform infrared and UV/visible spectroscopic measurements show that these carbonyl compounds weakly and reversibly adsorbed on SiO2. However, on the more basic and acidic oxides investigated, the data show that these carbonyl compounds irreversibly adsorb and can, in part, react on the surface to form larger molecular weight compounds. The kinetics of the heterogeneous reaction of acetaldehyde, acetone, and propionaldehyde on α‐Al2O3, α‐Fe2O3, TiO2, CaO, and SiO2 were measured with a Knudsen cell reactor at a gas concentration of 6×1010 molecules/cm3. Using the Brunauer‐Emmett‐Teller surface area of the powdered sample, initial uptake coefficients were determined to be in the 10−4 to 10−6 range for the adsorption of carbonyl compounds on the oxides at 298 K. By using these values for the uptake coefficient, it is found that the heterogeneous loss of these trace volatile organic compounds is comparable to that due to photolysis and reaction with OH in the middle to upper troposphere. Although surface adsorption can occur, the measured second‐order pressure dependence to form higher molecular weight compounds suggests that these reaction products will less likely form under atmospheric conditions.